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State of the art of bioimplants manufacturing: part II

State of the art of bioimplants manufacturing: part II Adv. Manuf. (2018) 6:137–154 https://doi.org/10.1007/s40436-018-0218-9 1 1,2 Cheng-Wei Kang Feng-Zhou Fang Received: 17 November 2017 / Accepted: 14 March 2018 / Published online: 25 April 2018 The Author(s) 2018 Abstract The manufacturing of bioimplants not only crucial in this regard. Bioimplants are playing a dominant involves selecting proper biomaterials with satisfactory role in this regard. For example, a study conducted by bulk physicochemical properties, but also requires special Kurtz et al. [1] implies that by the end of 2030, the demand treatments on surface chemistry or topography to direct a for primary total hip and knee arthroplasties in the USA is desired host response. The lifespan of a bioimplant is also estimated to grow by 174% to 572 000 and 673% to 3.48 critically restricted by its surface properties. Therefore, million respectively. Given that such trend may continue developing proper surface treatment technologies has [2], it makes sense that the advances in bioimplant manu- become one of the research focuses in biomedical engi- facturing are required to support the production demand neering. This paper covers the recent progress of surface and expected growth. In the past few decades, the manu- treatment of bioimplants from the aspects of coating and facturing has essentially become the core of integration topography modification. Pros and cons of various tech- domain of the biomedical field. On the other hand, indus- nologies are discussed with the aim of providing the most tries are required to master cutting-edge manufacturing suitable method to be applied for different biomedical techniques that are suitable for commercial production. products. Relevant techniques to evaluate wear, corrosion Such needs, in turn, drive significant levels of research and and other surface properties are also reviewed. development in the manufacturing area. The present progress and development of bioimplants Keywords Bioimplant  Precision manufacturing  Surface forming technologies were comprehensively reviewed in treatment  Evaluation the first part of this paper. However, untreated bioimplants are prone to wear and corrosion, which are critical factors in the success of possessing optimal service life. The 1 Introduction physicochemical interaction between living tissues and biomaterial surfaces is another concern [3]. In other words, Addressing health issues such as osteoporosis and a satisfactory biocompatibility after implantation must be osteoarthritis, which are prevalent in an aging population, guaranteed. The host body normally responds to bioim- is a great challenge for modern society. Bioimplants are plants in nanoseconds after initial contact and the envi- ronment remains in a state of flux thereafter. It is obvious that an unsatisfactory biocompatibility is likely to result in serious consequences such as immunologic rejection over & Feng-Zhou Fang fengzhou.fang@ucd.ie time. Therefore, a broad range of surface treatment tech- nologies are being developed in order to enable the Centre of MicroNano Manufacturing Technology (MNMT- bioimplants to possess unique surface characteristics [4]. Dublin), University College Dublin, Belfield, Ireland At present, surface modification of coating on biomaterials State Key Laboratory of Precision Measuring Technology & is normally carried out prior to putting them into practical Instruments, Centre of MicroNano Manufacturing uses. Typical purposes of surface coating on bioimplants Technology, Tianjin University, Tianjin 300072, People’s include improving wear and corrosion resistance, achieving Republic of China 123 138 C.-W. Kang, F.-Z. Fang high osseointegration and enabling the desired degradation past years, a broad range of coating systems has been rate. developed, which generally falls into three categories: The surface topography of a bioimplant is an important physical, chemical and combined physical and chemical signalling modality in controlling the cell function and methods [4]. determines the biological reaction to the device [3]. Cell behaviours such as morphology, adhesion, orientation, 2.1 Plasma spraying migration and differentiation have all been observed to be related to the textures or patterns on the surface [5]. As the Plasma spraying, which is a subset of thermal spraying, biocompatibility of an implant is closely related to the takes advantage of the heat of ionized inert gas (plasma), response of cells in contact with the surface [3], the surface and sprays molten powders of metal or ceramic onto the topography modification aiming to define cells’ reaction objective biomaterials to form a protective layer. As almost has long been a research focus in the field of implantology. all kinds of materials can be melted in the plasma jet, this Theoretical analysis indicated that the ideal surface technique is quite versatile and has been widely applied in roughness (R ) for hard tissue implants is in a range of the electronic, petrochemical, medical and aerospace 1–10 lm[6]. Numerous in vivo and in vitro studies have industries. Plasma spraying presents many advantages supported that the roughness within this range exhibits the including a rapid deposition rate, thick deposits and also best in interlocking implant surface and mineralized bones low cost. More attractively, the objects can be kept in low [7, 8]. Particularly, the osseointegration was stimulated temperature during processing and the gas in the plasma considerably by the microscale roughened surfaces. flame can remain chemically inert, which helps to reduce Therefore, suitable surface modification technologies at the risk of thermal degradation [12]. Compared to other microscale are being carried out to achieve a positive coating processes, the plasma sprayed layers exhibit rela- influence on protein adsorption, cellular activity and tissue tively better coating properties [13]. response. Due to the ease of application, plasma spraying is the On the other hand, most joint implants are facing tri- first method to fabricate calcium phosphate coating on bology issues after long periods of use. For instance, the biomaterials [14]. The most commonly used material for artificial knee and hip joints would experience a great deal spraying is hydroxyapatite (HA), which can promote the of rolling and sliding contacts under cyclic loading during osseointegration after implanting an help the biodevices walking activities. Friction between joint prostheses nor- bond directly to the surrounding tissues. Evaluations on the mally leads to increased energy losses in the biomedical plasma sprayed HA coatings on titanium-based biomate- system and ultimately to wear [9]. The wear released debris rials showed that the new bone apposed directly on the would, in turn, induce physical pain and adverse immune coatings with satisfactory adhesion [15–17], and the overall responses. Instead of rectification by replacing the total bone recovery was found to be fairly quick [18]. The poor joint, surface treatment is believed to be a promising mechanical properties of HA coatings are likely to cause method to reduce the material friction coefficient, and thus brittle damages and delamination, hence the alteration of prolong the device lifetime [10, 11]. In this regard, surface structure is prone to occur. Aiming to address the issue, texturing is favoured for the ability to retain the desired many parametric studies on the spaying process were car- bulk attributes of biomaterials, meanwhile improving the ried out and followed by characterizations [19]. It was tribological properties required by different clinical proved that by using high spraying power, suitable me- applications. chanical properties and high bonding strength could be The scope of this paper is to review the typical surface achieved. This is due to a greater extent of coating melting treatment technologies for bioimplants in two aspects, i.e., resulted in a denser microstructure. The trade-off, however, surface coating and topography modification. A discussion is that a higher energy consumption is involved. Yang et al. of current challenges and perspectives will be given in the [20] produced plasma sprayed HA on Ti-6Al-4V with final section. various cooling conditions and substrate temperatures in order to have different residual stress values at the HA/ metal interfaces. The evaluation results revealed that the 2 Surface coating technologies interfacial residual stress played an important role in determining the bonding strength, where coatings with Surface coatings are currently of particular interests in lower residual stress were found to exhibit better adhesion. prolonging lifespan and enhancing performances of various Apart from temperature effects, the increased coating bioimplants. Such modification allows both suitable bio- thickness is believed to be another reason for the rise of compatibility and biofunctionality while preserving the residual stress [19]. It is also known from early reports that favourable bulk characteristics of the biomaterial. In the compared to a smooth substrate, a highly roughened 123 State of the art of bioimplants manufacturing: part II 139 substrate surface is beneficial to achieving a better bond contact length for the as-sputtered calcium phosphate strength [21]. implants was reported to be close to plasma sprayed The advantages of low cost and rapid deposition rate implants, respectively are (70.4±1.6)% and (78.6±4.9)%, make plasma modification process the mainstream for to be specific [22]. surface modification of biomaterials. The technology pro- One obvious drawback of sputter coated HA layer on the vides a flexible and environmentally friendly process that metallic substrate is the poor degree of crystallinity allows manufacturers to tailor the surface properties of the [22, 24, 27], which would increase the dissolution rate of biomaterial to suit specific needs. However, issues cited the coating in a human body [19]. Post-treatment of with the plasma-sprayed coatings include variation in bond annealing with controlled temperature and processing time strength between the coatings and the substrates, poor was used to crystallize the film. It was found that the adhesion at the interface and alterations in HA structure thermal process would make a change in the surface due to the coating process. In addition, to our best morphology and further contribute to the changes in crystal knowledge, there is no evidence showing that plasma structure [28]. Nevertheless, it should be noted that con- sprayed coating would prolong implants’ lifespan or ventional thermal treatment in the electric furnace increa- improve the reliability comparing to the uncoated implants. ses the potential possibility of cracks formation and may Knowing the problems of coating brought by plasma thus degrade the HA films [19]. In addition, since the spraying, numerous alternatives of deposition process were process involves high energy consumption and large costs, developed. improvement of economic efficiency must be taken into consideration for industrial applications. 2.2 Sputter coating Although the sputter coating technique is not currently used as a commercial deposition process by most bioim- Sputter coating technique is classified as a physical vapour plant vendors, its capabilities of producing uniform and deposition (PVD) method and shows great promises in dense coating with better adhesion strength make it a viable eliminating the issues associated with the plasma spraying alternative to plasma spraying for the application of HA process [22]. During the process, a gas plasma is utilized to coatings on bioimplants. eject materials from a negatively charged target. The material would be then deposited as a coating onto the 2.3 Ion-beam assisted deposition substrate material. From an industrial perspective, the technology is considered as a complex process since it The principle of ion implantation was first postulated in involves many parameters that control sputter deposition. 1906, but it was not until the 1990s that the technique was first applied as a coating technology for biomedical On the other hand, however, the availability of precisely varying parameters allows a large degree of control over implants [29]. In a typical ion implantation process, ions the growth and microstructure of the coating. Some early are accelerated through high graded potential difference reports showed that the initial sputtering using multi- and directed towards a substrate material. Due to the ion- component ceramic targets such as superconducting oxi- solid interactions, the energetic ion would get incorporated des, HA and other calcium phosphate materials would produce coatings whose chemistry was different upon deposition than the bulk target [22, 23]. Successful attempts have been made on depositing cal- cium phosphate layers on metallic biomaterials using radio frequency magnetron sputtering [24, 25]. The sputtered layers were observed to be more homogeneous than the plasma sprayed ones and the surfaces appeared to be very smooth [19]. Meanwhile, the adhesion strength of sputtered HA coating and its reliability have also been found to be higher than most plasma sprayed HA coatings. A com- parative study conducted by Ozeki et al. [26] indicated that the adhesion strength of the sputtered coating exceeded that of the plasma sprayed coating by more than 70%, 40%, and 30% after a period of 2 weeks, 4 weeks, and 12 weeks, respectively. In terms of biological responses, improved bond strength and the initial osseointegration rate were Fig. 1 Illustration of ion-solid interactions in an ion-beam assisted observed in sputtered HA coatings. The percent of bone deposition process [29] 123 140 C.-W. Kang, F.-Z. Fang into the substrate after losing all the energy [29]. Figure 1 confirmed the validity of calcium-ion implantation as a pre- illustrates the ion-solid interactions during an ion-beam treatment to endow the desirable bioactivity on porous Ti assisted deposition process [29]. As can be told from the for bone tissue engineering applications. Some other ? ? working principle, the penetration of the ion correlates with reports indicated that the ion implantation of Ca ,N and the level of energy. Therefore, by carefully controlling the F was helpful in promoting the anti-bacterial effect of ion beam energy to avoid deep penetration inside the various titanium surfaces [35]. substrate, modifications can be confined to the near-surface In general, ion implantation technique is useful in region, and hence significantly influence the surface char- improving the mechanical, chemical and biological prop- acteristics. Except for the ion beam energy, other important erties of biomaterials. The process is extremely control- parameters involved in ion implantation include ion spe- lable and can be accurately tailored in order to implant cies, fluence (or the total number of ions that bombard a different ions to form ultra-high purity coatings with surface) and beam current density or flux, which can all be excellent adhesion. Nevertheless, because the entire pro- adjusted to influence the ultimate effects on the substrate cess is conducted in a high vacuum and involves costly and achieve wide atomic intermixing zone [30]. steps such as beam extraction, beam focusing and beam An attractive feature of ion-beam assisted deposition is scanning, the soaring cost has hindered its widespread uses. that it offers independent and specific control of the At present, ion-beam based treatment is mostly applied in deposition parameters. Such feature enables the manufac- high-value-added products and limited in the regular pro- turing of gradual transition between the substrate material duction line. Besides the cost, it is disadvantageous for and the coating, thus a more durable bond can be achieved being inappropriate for components with complex geome- [31]. Rautray et al. [29] indicated that the adhesion prop- tries [4]. erties of ion-beam implanted and plasma sprayed coatings seemed to be similar, but the atomic intermixing interfacial 2.4 Conversion coating layer formed by ion dynamic intermixing contributed to a better bonding strength. In the case of fabricating HA Conversion coating, also referred as in situ grown coating, coating on a titanium substrate, ion-beam assisted deposi- is formed through specific reactions between materials and tion achieved a tensile bonding strength of 70 MPa, which environment. This technology is typically used in reactive exceeded that of 51 MPa associated with the plasma metallic materials, where an inorganic oxide layer is pro- spraying process. The existence of a transition structure at duced with the help of a chemical or electrochemical the HA/Ti interface consisting of amorphous HA, amor- process. As the conversion is formed in situ, the adhesion phous calcium phosphates and amorphous Ti phosphate of the coatings to the substrate is relatively strong. Passi- compounds were thought to be responsible for such phe- vation is one typical branch of conversion coating and nomenon. The formation of such a chemical bond was being used as a simple approach to protect reactive bio- thought to be attributed to the energetic ion bombardment materials, such as magnesium and its alloys. By simply process [29]. It was also reported that the ion-beam treat- immersing Mg-based biomaterials in a solution with a ment was capable of providing HA-coated titanium stable pH of 11 or higher, a passive layer of Mg(OH) in bioimplants with hardened surfaces, and thus improved the nanometric thickness can be formed within a short time wear resistance [29]. An essential element for the human period [4]. By means of adding mixtures of oxides or body, phosphorus, can be implanted on titanium-based hydroxides in the solution, a film of anti-corrosive metal biomaterials via ion-beam deposition. In this way, a com- phosphates could be formed as well. It is noted that pact TiP phase could be formed on the titanium surface. although the converted layer provides protection during the The new phase turned out to be helpful for strengthening initial phases of corrosion in a living body, the protective the corrosion resistance. The satisfactory biocompatibility ability is reported to be inadequate [4]. As a result, of phosphorus-ion implanted titanium was confirmed by researchers tended to develop innovative technologies to Krupa et al. [32]. In addition to above, it was also reported produce more stable and powerful conversion coatings. that the utilization of ion implantation was advantageous in The anodization process is favoured for its controlla- the aspects of avoiding stress shielding, enhancing fatigue bility on the coating thickness, and it is mainly used to resistance and improving fracture toughness for bioim- produce or thicken native oxide layers on metal materials. plants. From the perspectives of biological activities, ion- The coating thickness normally increases with the beam implantation provides the benefits of induction of increased applied voltage and the value is in a range of crystallinity and reduction in apatite dissolution rate [33]. 5–200 lm[36]. Numerous studies have proved that the Chen et al. [34] investigated the influence of calcium ion anodized layers are more stable and inhibit corrosion better deposition on the apatite-inducing ability of porous tita- than traditional chemical conversion layers [36–38]. When nium in a modified simulated body fluid. The results anodizing the metallic material above the breakdown 123 State of the art of bioimplants manufacturing: part II 141 voltage, porous layers can be formed with improved the corrosion resistance and weaken the adhesion strength. resistance to abrasion and corrosion [38]. Such technology Post-processing of thermal treatment was capable of is called plasma electrolytic anodization (PEO), as well as reducing the porosity effectively, but cracks caused by anodic spark deposition (ASD) and micro arc oxidation shrinkage were observed in the ceramic coating [43]. (MAO) [4]. Commercially, PEO has become the most HIP involves the pressing of ceramic granules at high applied protection method for Mg alloys [4]. Nevertheless, temperature and pressure, during which the operating the coating process would result in electric isolation, which temperature is normally above 2000 C and the working makes PEO inappropriate for further processing via electric pressure is usually maintained in a range of 100–320 MPa deposition [4]. Besides the utilizations described above, [59]. The most distinct merit of HIP is that it is capable of conversion coating techniques are sometimes performed as controlling the size and shape of the product and realizing a pre-treatment process to promote the expected adhesion high precision finish without further machining procedures of a deposition coating. [44]. This technique enables the fabrication of thick and dense coatings with a variable thickness. Besides, the HIP 2.5 Other methods treated coatings usually exhibit homogeneous material structure as well as a high uniformity of properties [44]. In addition to the above-mentioned methods, there are This method is particularly advantageous for reducing the several more techniques being used to create coatings for porosity and enhancing the mechanical properties of the bioimplants, such as electrophoretic deposition (EPD) ceramic coating. It has been used as a post-treatment by [39–43], hot isostatic pressing (HIP) [44, 45], pulsed laser many researchers to densify the plasma sprayed HA coat- ablation [46–49], sol-gel [50–53] and dip coating [54, 55]. ings [60]. Khor et al. [45] applied HIP in reducing the In an EPD process, particles in suspension are coated amount of micropores in a plasma sprayed HA coating on onto an electrode in the effect of an electric field [39]. This Ti alloy. A mercury intrusion porosimeter was utilized to method is popular in coating complex shapes and patterns. measure the pore size distribution of the hot isostatic The regulatable particle size and deposition conditions pressed samples. The results indicated that most of the allow EPD to have a high degree of control on the coating micropores were drastically reduced after HIP. As a result, results [40]. Commercially, EPD is generally regarded as the physical properties such as microhardness and bonding an economy coating process as it does not involve any strength were significantly improved. Another study con- costly equipment, and the instrument set-up is relatively ducted by Kameyama [61] employed HIP to implant easy. The possibility of stoichiometric deposition and encapsulated porous HA granules into a superplastic Ti- capability of achieving high purity material make this 4.5Al-3V-2Fe-2Mo alloy. They obtained a new hybrid method a limelight in biomedical applications [19]. Cur- biomaterial with both biological affinity and high rently, there is a rising trend of applying EPD to coat mechanical strength. Nevertheless, it can be easily told metallic biomaterials. Successful attempts have been made from the working principle that this technique is difficult to to achieve uniform thin ceramic coatings with good coat complex substrates. This is because the elevated mechanical properties on Ti and Mg alloys [41, 42, 56, 57]. temperature and pressure are simultaneously applied to the As EPD processes can be carried out at room temperature workpiece during the process. Thus, the thermal expansion or lower, the ceramic coatings are protected from the for- mismatches between the substrate and coating particles are mation of amorphous phases. A novel EPD process at room prone to occur. It should also be noted that the encapsu- temperature was proposed by Zhang et al. [43] to fabricate lation material was hard to be removed after pressing [62]. nanostructured HA coating. The experimental results Pulsed laser deposition (PLD) is a conceptually and showed that the coating’s bond strength was significantly experimentally simple yet highly versatile tool for fabri- improved up to 50–60 MPa. Moreover, the corrosion cating a wide range of thin films and multi-layered struc- resistance of the nanostructured HA coating was confirmed tures [46]. The physical phenomena of PLD are quite 50–100 times higher than conventional HA coatings. The complicated and it can be divided into three stages: laser feasibility of fabricating bioactive composite systems using radiation interaction with the target, dynamic of the abla- EPD was investigated by Kumar and Wang [58], where tion materials and deposition of the ablation materials with TiO powders were first coated on Ti-6Al-4V alloys, fol- the substrate, nucleation and growth of a thin film on the lowed by depositing an HA-TiO composite. In this way, substrate surface [46, 63]. Comparing to other deposition functionally graded coatings of HA-TiO -Ti systems can methods, PLD is easy-to-operate and allows the growth of be successfully obtained. Although the application of EPD coating at lower temperature [64]. The main advantage of has kept a consistently rising trend in the field of PLD is that the stoichiometry of the target can be retained biomedical manufacturing lately, it was reported that the in the deposition films [19]. This is attributed to the high presence of porosities on the deposited layer might lower rate of ablation that led all elements or compounds to 123 142 C.-W. Kang, F.-Z. Fang evaporate at the same time [46]. Another unique feature of applied this method to coat HA/TiO on a nontoxic TiZr PLD is that it can change the deposited material in situ, alloy for biomedical applications. The simulated body fluid which enables the fabrication of advanced materials. Since tests showed that the coated products exhibited excellent the first achievement of PLD deposited high-quality HA bone-like apatite-forming ability and were expected to be a thin film was made in 1992 [47], the progress of its tech- promising load-bearing implant material. In the past two nical improvement had been substantial and steady. In the decades, the sol-gel technique has been found to be of past two decades, HA coatings with diverse compositions value in enhancing the corrosion resistance of Mg-based and crystallinities have been successfully produced by PLD clinical implants, as well as retarding the degradation rate [48, 65]. The interest in PLD for use in biomedical appli- [53, 70]. Kim et al. [71] deposited fluor-HA on a titanium cations is from the ability to suffice the bioimplant with substrate through sol-gel, while different concentrations of good mechanical properties and biocompatibility. Previous F were incorporated during the process. The results researches have revealed that pulsed laser deposited HA proved that the tailoring of F solubility would change the coatings exhibit better interfacial adhesion and have minor dissolution rate of the coating layer. Despite all the undesirable phase under optimal conditions [46]. Arias advantages mentioned above, there are some considerable et al. [66] conducted micro-scratch tests to evaluate the drawbacks of this technique. Most obviously, cracks are adhesion properties of pulsed laser deposited HA coatings often observed in sol-gel coatings. It was reported that in on Ti substrates. The absence of detachment suggested that order to reduce cracking, the thickness of the coating had to good adhesion properties were obtained through PLD. The be retained under 0.5 lm[69]. Another point which needs authors also confirmed that both amorphous and crystal- to be considered is the thermal effects. Like other thermal lized HA coatings were produced during the PLD process. deposition methods, thermal expansion mismatch at the Therein the crystalline HA coating is superior in internal interface is always an issue. The limited wear resistance of cohesion while the amorphous coating could be more the product is also a major concern that comes with the sol- mechanically compatible with natural bone. The distin- gel coating. Last but not the least, sol-gel coatings suffer guished adhesion strength of PLD coatings on Ti-based from high permeability. Because of the requirements of biomaterials is believed to be attributed to the existence of post sintering and long processing time, sol-gel is presently an oxide layer [67]. A study conducted by Cotell [49] used to a lesser degree in the industrial processes. supported the hypothesis that there might be effects of Dip coating is usually compared with sol-gel technique epitaxy on the interface and hence contributed towards as they share similar processes [19]. The fundamental adhesion. Limitations of the PLD technique include procedures of a dip coating begin with immersing the bringing the film into contact with the particulates depo- substrate into the solution of the coating material. After sition, which can either be solved by employing filers or withdrawing, solvents and other accompanied chemical polishing target surface before each run. Another drawback reactions in the liquid film would be evaporated. A post- is the deposited layers have a lack of uniformity over a treatment of curing or sintering is required as a final step to large area of the plume owing to the angular distribution of form the anticipated coating, during which high temperate the ablation plume. Thus, rasterizing the laser beam across is normally involved. Compared to sol-gel technique, dip the rotation target was proposed to solve this issue [46]. coating is more time-efficient. It is reported that a complete Sol-gel has attained its reputed fame for being one of the transition can be completed in a couple of seconds if simplest techniques to manufacture thin films [51]. It has volatile solvents are used [72]. In addition to this, its simple attracted widespread interests in coating optical, magnetic, procedures and low cost in coating complex shapes make electronic and chemical components [50]. The typical sol- this method relatively popular in the industry. HA thin gel process involves the immersion of the substrate mate- films coated on metallic substrates via dipping exhibit high rial into a concentrated solution with a gel-like texture. surface uniformity and homogeneity. Mavis and Tas¸[54] Detailed procedures of the process were introduced by presented a series of recipes of HA dip-solution, by using some researchers [19, 68]. Apart from the ease of operation which highly porous coatings with over 30 MPa bonding and cost-effectiveness, this method is capable of producing strength could be deposited on the Ti-6Al-4V substrates. thin bond coatings with excellent adhesion, as well as thick Dip coating also introduces its profound advantages in layers with satisfactory corrosion resistance [69]. During slowing down the corrosion rate of Mg alloys. Gu et al. the process, high purity can normally be guaranteed. In [55] dipped coated chitosan on a group of Mg-Ca alloys addition to above, super high temperature processing is and tested their respective corrosion resistance in a simu- unnecessary, as the post-sintering process is only con- lated body fluid. The authors implied that this technique ducted in the rage of 200–600 C. Given the mentioned showed great promises of future adaptation for Mg sub- appealing features, there is a growing interest in coating strates in matching the implant corrosion rate with the bioimplants using the sol-gel technique. Wen et al. [52] tissue healing rate. 123 State of the art of bioimplants manufacturing: part II 143 biomaterial but also is favoured for its environmental 3 Surface topography generation friendliness. To meet the demands from an enhanced interaction Nevertheless, several studies have reported that blasting with the particle material other than the implant itself bears between the biomaterials and living body and simultane- ously reduce the risk of wearing, various methods were a potential risk of changing the surface composition [6]. Attention was typically paid to the alumina blasted applied to create microstructural surface features for bioimplants. Typical technologies in engineering substrate implants. Some researchers insisted that the remnants of the alumina particles could release aluminium ions into the at the micro and nanoscale will be reviewed in the fol- lowing sections. host body due to dissolution, and further cause inflamma- tory responses [6]. There are also some concerns that the Al ions would inhibit normal differentiation of the bone 3.1 Blasting marrow stromal cell and normal bone mineralization [78]. Although no statistically significant differences were found Blasting in biomedical engineering refers to an operation which propels a stream of abrasive particles against the between the implants blasted with Al O and other particle 2 3 materials [74, 79], the application of non-biocompatible substrate biomaterial under a high pressure. The process is used to remove surface contaminants or roughen the sur- particles for blasting remains controversial. As a result, the feasibility of using hydroxyapatite and beta-tricalcium faces in order to enhance the biomaterial’s reactivity after implantation [73, 74]. The alteration in the surface topog- phosphate particles in blasting was investigated. Benefited from the material features of biocompatibility, osteogenesis raphy is attributed to the plastic deformation. Although it is and resorbability, the bioceramic blasted surfaces exhibited difficult to precisely control the surface texture due to the a suitable bone-implant contact after implantation. Mean- numerous variables inherent in the blasting process, the while, other surface properties are reported to be compa- size of the particles can be adjusted to meet the roughness rable to those treated by conventional blasting procedures requirement. Considering that the particles need to be [80–82]. chemically stable, alumina, titania and hydroxyapatite particles are most commonly employed at the stage [6]. 3.2 Chemical etching Valverde et al. [75] showed that a wide variety of microtopography, ranging from minimally rough to Etching techniques performed on untreated biomaterial excessively rough surfaces, could be prepared by regulat- ing the variable factors during the blasting procedure. The surfaces have been used to form micro pits at sizes between 0.5 lm and 2 lm to enhance cell adhesion and osseointe- effects of blasting parameters on the surface roughness of Ti-6Al-4V were studied by Mohammadi et al. [76]. In their gration [6, 83]. In surface etching processes, chemical reagents are selectively applied on specific areas to remove study, two particle materials, i.e., Al O and SiO , were 2 3 2 materials and therefore form expected texturing. The employed with different sizes using different types of blasting systems. Through optimizing the processing con- etching on specific regions is generally achieved through masking, where the selected masking method determines ditions, an equivalent surface roughness of 3.51 lm was achieved. Their follow-up coating experiments confirmed the resolution of the texture features. Costa et al. [84] proposed the application of drop-on-demand inkjet printing that the substrate surface topography had a significant influence on the coating properties at the interface. Obvious for masking steel surfaces with subsequent chemical etching and ink stripping. It was proven to be a fast, ver- differences were observed between the HA coatings deposited on the substrates with and without blasting satile and highly feasible technique for texturing steel surfaces [84]. Figure 2 shows the typical steel surfaces treatments in terms of the tensile bonding strength. Arif- etched via inkjet printing [85]. vianto et al. [77] blasting treated 316L stainless steel using Strong acids such as HCl, H SO and HNO are com- steel slag balls, which were the residues from steelmaking 2 4 3 monly used in most etching processes. It is believed that processes and presently regarded as an industrial waste. The authors reported that both surface microhardness and higher concentrated acidic solutions normally generate better surface defect distributions while less aggressive irregularity of the stainless steel were increased after the treatment. It was also found that some bioactive elements mixtures would be conducive to a finer roughening [86]. In etching of titanium-based bioimplants, fluoric acid is such as Ca, Si and Mg were introduced by the slag balls. This study clearly indicated that the steel slag blasting was regarded as an alternative chemical reagent. Previous reports showed that HF can effectively dissolve the passi- a promising method for the surface modification of the medical grade 316L stainless steel. It is not only capable of vation TiO layer [6]. In addition, since titanium is very reactive to fluoride ions, the fluoride would be incorporated improving the mechanical properties and bioactivity of the into the created surface structures and form soluble TiF . 123 144 C.-W. Kang, F.-Z. Fang Fig. 2 Inkjet printing of steel surfaces with a parallel gaps and b chevron-like gaps [85] Such incorporation is beneficial for the osseointegration of [6, 92–94]. Through incorporating patterning techniques at implants [87]. As a result, HNO is usually mixed with HF submicron scale with subsequent sandblasting and etching to produce microscale surface structures on Ti-based processes, Zinger et al. [95] achieved desired titanium implants [88]. However, attention should be paid to fluo- surface with a combined micrometre and nanometre ride contaminations as they may induce an ambivalent structures, which showed an improved osteoblast ability. A response in the host tissue [6]. The risk of weakening metallurgic-mechanical analysis conducted by Pazos et al. mechanical properties is another concern brought by the [94] explained the advantage of blasting ? etching surface chemical etching. In the etching of titanium-based bioim- treatments in improving titanium material properties. The plants, hydrogen embrittlement triggered by the acid authors suggested that the decrement of fatigue endurance environment was reported, which might be the reason for induced by the acid etching could be counteracted by the the forming of micro-cracks on the implants’ surfaces and foregoing blasting process. The formation of a plastically ultimately led to a reduction in the fatigue resistance [83]. deformed layer and compressive residual stress contributed Selective infiltration etching (SIE) is a special surface to the strain-hardening, and therefore resulted in a better topography modification method, which coats target sam- fatigue behaviour. ples with special infiltration glass [89]. By heating the coated objects above the glass transition temperature, the 3.3 Laser-based techniques molten glass would diffuse between the grain boundaries and result in sliding, splitting and rearrangement of the As reviewed in part I of this paper, the improvement of surface grains. After cooling to room temperature, the glass osseointegration relied on roughening the surfaces of can be dissolved in an acidic bath, thus exposing the newly implants. In most cases, the selected surface regions of a created surfaces [90]. This technique is now being used in biomaterial are blasted to be roughened at microscale. transforming zirconia surfaces into dense, highly retentive However, one obvious drawback of this technique is that it and smooth nanoporous surfaces. A significantly higher can only produce randomized surfaces [96]. Such surface degree of osseointegration of the selective infiltration features may alter the near-surface mechanical and chem- etched zirconia implants was claimed by Aboushelib et al. ical properties, and thus cause mechanical degradation [91]. [97]. Moreover, it gives rise to the local concentrations of Presently, attempts are being made regarding the etching toxic elements such as Al after the surface modification process after a blasting step. Such technology integration is [97, 98]. Considering the above-mentioned issues, micro- designed for removing blasting induced surface damages grooving is now being explored as an alternative surface and simultaneously improving surface roughness charac- treatment approach to facilitate the bone-implant integra- teristics [6]. Many previous investigations have demon- tion for bioimplants [96]. Among all known micro-fabri- strated that a combined blasting and etching structuring cation methods, laser-based technologies showed method is of great help in producing superior quality of themselves to be the most advanced way in producing topographies with different scales at the same surface 123 State of the art of bioimplants manufacturing: part II 145 micro-grooves with optimal groove dimensions for cell surface regions to fabricate dent arrays, shows a great adhesion. potential on this aspect. Hu et al. [11] employed this Previous works have reported on the positive effect of technique to create micro-dimple patterns on Ti-6Al-4V laser-ablated micro-grooves on promoting the contact surfaces. Excellent tribological performances of the bio- guidance of cell alignment [96, 98–100]. For instance, a material were verified under various loads applied. The comparative experiment conducted by Chen et al. [101] effects of texture parameters on tribological behaviours revealed that the laser-irradiated Ti-6Al-4V surfaces with were also investigated in their researches. A higher dimple micro-grooves provided the best cell/surface interactions density was found to result in a lower friction coefficient. over polished and blasted ones. The migration and align- This is because the micro-dimples functioned as traps for ment of cells would not only enhance the osseointegration wear debris. A higher dimple density was more likely to but also reduce the extent of scar tissue formation during absorb more wear particles and therefore eliminated the wound healing [102]. More recently, Hsiao et al. [103] potential debris ploughing effect. In addition, it was developed an ultraviolet (UV) laser treatment system to believed that the micro-dimples might serve as fluid texture Ti-6Al-4V biomaterials. Major micro-groove reservoirs in the host body, which would help to retain structures and minor porosities were obtained simultane- body fluid as a lubricant and lead to less wear. ously. The following in vitro tests proved that the texture The major disadvantage of LST is that the laser ablation effectively offered a favourable environment for the may alter surface integrity. Previous reports have indicated osteogenic cells. Nevertheless, due to the factors such as that elevated temperatures encountered during ablation high energy outputs, top-hat intensity profiles and the high may change surface microstructures and form cracks. Such photon energy associated with the deep UV wavelengths, damages would drastically shorten the fatigue life of the the current UV laser based micro-grooving technologies material [10, 105]. For this reason, laser shock peening may introduce micro-cracks and induce heat-affected zones (LSP) process was proposed. LSP is capable of producing inside the grooves [96, 104]. Such damage would definitely micro dent arrays, and at the same time improving surface degrade the performances and reduce the lifespan of mechanical properties via inducing deep compressive bioimplants. Considering this, improving laser processing residual stress in the subsurface. A typical denting sche- techniques to produce durable laser-textured biomaterial matic of LSP is shown in Fig. 4 [10]. During the process, a surfaces is crucial [96]. Diode-pumped solid-state (DPSS) short, high-power laser pulse is applied (under a water laser technologies were developed to address the above blanket) to vaporize a sacrificial coating on the objective. issues. Fasasi et al. [96] proposed a nanosecond DPSS UV Selected surface areas would then undergo plastic defor- laser processing technique to optimize the groove geome- mation by the pressure waves [106]. The improved fatigue tries. By judiciously adjusting laser processing parameters, performance of Ti alloys offered by LSP processing was such as wavelength, pulse repetition rate and scan speed, claimed by Ruschau et al. [107]. Guo and Caslaru [10] satisfactory groove depths and widths of around 11 lm and demonstrated that LSP could efficiently manufacture mass 14 lm were obtained. Some scanning electron microscopic microscale dent arrays on Ti-6Al-4V alloy surfaces though (SEM) images of micro-grooved surfaces are shown in adjusting the laser power. Strain hardening and compres- Fig. 3. In addition, achievements on decreased groove sive residual stress in the centre area of the peened dents roughness and reduced heat-affected zones on Ti-6Al-4V contributed to the increment in microhardness. However, it were claimed by the authors. The absence of micro-cracks should be noted that all the laser texturing methods is also believed to be beneficial for cell attachment and reviewed above have been mainly applied in aeronautical spreading on the grooved structures. components for their increasing wear-resistance ability, but Apart from creating micro-grooved structures on rarely reported in improving the tribological properties of biomedical materials to improve the integration with sur- bioimplants. rounding tissues, laser-based texturing technologies are In summary, laser-based surface treatment methods are also applied to improve the tribological behaviour [10, 11]. favoured for the simple processing and effortless operation. Among various biomaterials, titanium and its alloys are Desired surface patterns can be fabricated through varying characterized by poor tribological properties, such as high the laser parameters. The laser micro-grooved metallic and unstable friction coefficient [11]. The rupture of pas- biomaterials exhibited better osseointegration and longer sive oxide layer would release wear debris to the host body lifetime in comparison with blasted or etched ones. More and lead to aseptic loosening of the implant. Therefore, importantly, a potential development tendency on great efforts have been made on introducing specific sur- enhancing tribological properties of bioimplant through face patterns on Ti and Ti alloys to enhance the tribological laser treatment is noteworthy. However, the high energy or performances. Laser surface texturing (LST), which uti- elevated temperate involved in the laser treatment process lizes high energy laser pulses to melt and vaporize specific is a major concern as it may alter the surface integrity after 123 146 C.-W. Kang, F.-Z. Fang Fig. 3 Typical SEM images of micro-groove surfaces produced by nanosecond DPSS UV laser processing [96] regarding working efficiency and accuracy. During a typ- ical EDM process, the removal of material is achieved through the high thermal energy generated by a series of high-frequency electrical sparks. Detailed working princi- ples could be found in Refs. [109, 110]. Figure 5 shows the typical EDM experimental setup [110]. In comparison to other surface treatment techniques, this technique does not require any pre-treatments on the objects’ surfaces. Since the electrode and workpiece are not directly contacted in EDM, issues triggered by mechanical stress during con- ventional contacting machining can be avoided. Another advantage of EDM is that such technology would introduce carbides on the workpiece surface, and hence enhance the surface properties such as hardness and wear and corrosion resistance [109]. Apart from above, what makes EDM attractive in biomaterial manufacturing is that a porous Fig. 4 Schematic of dent fabrication by laser shock peening [10] nanostructured oxide layer would be converted on the surface during the process. The layer thickness can be treatment. The relatively higher cost is another issue that intentionally controlled according to the requirement, so hinders the wide-spread of laser-based surface treatments. that a suitable biocompatibility could be achieved [111]. Furthermore, little work has been reported on the laser Several in vitro and in vivo studies have examined the surface structuring of bioceramics. This is a strong indi- improved osteoconductivity of metallic biomaterials whose cator that laser-based texturing on biomedical engineering surfaces were modified by EDM. Peng et al. [111] found requires further studies [108]. that a nanophase transition occurred on the titanium surface during EDM, which played a critical role in forming a thick 3.4 Electric discharge machining nanoporous TiO layer on the titanium surface. The porous structure is believed to be beneficial for enhancing the Electric discharge machining (EDM) was established to biocompatibility. In their following work, EDM was manufacture geometrically complex or hard material parts applied to produce a rough texture with pores and craters at which are difficult to be machined by conventional pro- nanoscale on the surface of Ti-6Al-4V alloys [112]. Again, cesses [109]. More recently, it has become favourable in the formation of nanoporous TiO layers was observed. producing nanostructured biocompatible surfaces [110]. The follow-up evaluations revealed that the EDM-func- The exploration of the destructive properties of electrical tionalized surfaces significantly increased the activities of discharges can be traced back to the 1940s, when the first surrounding cells in terms of adhesion, differentiation and attempt was made on vaporizing material from the diffi- proliferation. It was confirmed that an improvement of cult-to-machine metal surface [109]. Since then, EDM multiple osteoblast functions could be achieved by experienced a successive development in the past decades increasing the pulse durations. 123 State of the art of bioimplants manufacturing: part II 147 Fig. 5 Typical representation of the experimental setup for EDM process [110] As the present trend of surface treatment has been Previous studies have proved that the high-frequency switched from conventional machining to advanced tool-work interaction induced by ultrasonic vibration is of micro/nano-manufacturing, EDM has become favoured in great help in the manufacturing of various micro/nanos- offering nanoporous surfaces with enhanced mechanical tructures [115–118]. Some successful attempts of ultra- properties and biocompatibility. One major drawback of sonic-assisted machining have been made on different the EDM fabricated material is the low fatigue perfor- materials such as stainless steels [115, 119, 120], silicon mance brought by the recast layer [110]. Post-treatment carbide [121], glasses [120, 122], polymers [123], etc. such as blasting is required to address the issue. In general, Nowadays, an increasing number of studies have shown the application of EDM in biomanufacturing remains at the that the ultrasonic-assisted machining techniques are initial stage and the fulfilled results are confined to labo- valuable in achieving high precision textures on biomedical ratories. More works need to be done before EDM is rad- materials. For instance, a novel rotary ultrasonic texturing ically accepted by the biomedical industry. (RUT) technique was proposed by Xu et al. [124]. In their study, the ultrasonic vibration was integrated into a rotary 3.5 Other methods machining process. The combination of vibration, rotation and feed motion offers high-frequency periodic change. It In the past decade, the emergence of surface texturing was suggested that this new technique allowed manufac- technologies on biomaterials goes well beyond the above- turers to fabricate various fine surface structures by offer- mentioned ones. For example, Roy et al. [113] employed ing an additional processing freedom. the micro-drilling technique to manufacture micro-dimpled Electrochemical machining (ECM) is a relatively surface textures for ceramic-on-ceramic hip prostheses. In mature surface modification technique which enables the simulated hip joint condition, the dimpled workpieces removing metallic materials selectively by an electro- exhibited obviously improved tribological performances chemical reaction at the anode [125]. Its flexible machining compared to non-dimpled ones. Choudhury et al. [114] rate is attributed to the adjustable electric current [9]. revealed that plateau-honed technique was effective in Through years of development, the processing dimension reducing wear rate, friction coefficient and removing wear of ECM has reduced to microscale. Electrochemical debris from the contact interface of metal-on-metal hip micromachining (EMM) has replaced traditional ECM in joints. many places and been widely used in producing patterns on 123 148 C.-W. Kang, F.-Z. Fang stainless steel based hip prosthesis stems. Mask electro- chemical micromachining (TMEMM) is typical branch of EMM, which involves photolithography to produce micropatterns on the photoresist-coated substrates [126]. Lu and Leng [125] developed a jet electrochemical micromachining (Jet-EMM) to form micro-holes on the titanium-based bioimplants. The technique exhibits merits of producing patterns on curved surfaces and enables fea- tures with a high aspect ratio. Comparing to TMEMM, the equipment required for Jet-EMM is less complicated. The Fig. 6 Schematic of a pin-on-disk system, modified from Ref. [128] invention of the first rapid, mask-less EMM was claimed should be kept below 2 MPa when simulating the wear of by Sjo¨stro¨m and Su [127], where the surface patterns were ultrahigh molecular weight polyethylene. It should be created via a direct writing manner. During the process, a noted that this technology is a simplified model of normal microscaled single-tip in conjunction with short voltage walking, and simulation of the motion and loading in pulses moves across the substrate and the resolution was activity is very limited. In fact, there are three complicated kept in the submicron region. This technique is demon- articulation mechanisms that are involved in the motion of strated to be ideal for the fast fabrication of desired surface a tibiofemoral joint, namely gliding, pure rolling and patterns on metallic biomaterials. Microscaled grooves and rolling-slipping [129]. Investigations are required to elu- pits (around 50 lm in width and diameter) were success- cidate the complex environment of the host body and fully produced on titanium surfaces with high manufac- increased patient activities. turing efficiency. Note that although EMM is favoured for For a more precise evaluation of the tribological prop- its ability to handle complex geometries, its application is erties of orthopaedic joints, friction and wear experiments restricted to electrically conductive materials. are performed on a simulator. Compared to the pin-on-disk test, the simulators are capable of imitating more complex kinetics and kinematics of a human body in a physiological 4 Characterizations of bioimplant surfaces environment. Running in accordance with ISO 14242 and 14243, an array of hip and knee complexities can be The surface properties generally play a dominant role in evaluated. The simulator testing allows implementing determining the longevity of bioimplants, among which the various surface textures on the workpieces as well as a major concerns include the products’ wear and corrosion selection of lubricant, which ensures that the simulated resistance, mechanical properties of hardness and elastic condition is as practically similar as possible to the com- modulus, fabrication caused residual stress and surface plexities of the human anatomy. In a typical hip simulator, composition after surface treatments. Evaluation tests of a single joint force was normally applied in one axis, these aspects are necessary for guaranteeing the final pro- offering a shear stress pattern similar to that of the human duct acceptance. body. Bowsher and Shelton [130] added a vertically mounted torque cell on the hip simulator to measure the 4.1 Wear tests changes in joint friction. The photographs and schematics of the simulator can be seen in Fig. 7. Such design pro- Pin-on-disk wear test system has been widely used to vided a better understanding of the influence of patient measure the friction coefficient and characterize the wear activity level on the tribological performances. In the response of the manufactured bioimplants, especially arti- aspect of knee simulators, since the traditional uniaxial and ficial knee and hip joints. Figure 6 shows the schematic of two-axis simulators resulted in too low wear rate values, a typical pin-on-disk system. Briefly, the pin moves biax- Saikko et al. [131] proposed a three-axis wear model which ially with a normal load applied on top. Either cyclical or implemented anterior-posterior translation (APT), inward- non-cyclical translating programs can be employed. outward rotation (IOR) and flexion-extension (FE). The Parameters such as pin and disk materials, normal load, principle of the simulator is shown in Fig. 8. Such ball-on- cycle frequency, sliding speed and lubricant can be judi- flat contact design has been successfully applied to study- ciously selected to replicate the actual tribological condi- ing the basic wear and frictions of metal-polymer and tions. The friction coefficient is calculated from the applied ceramic-polymer knee pairs [132, 133]. Again, attention normal load and the measured friction force. According to should be paid to the fact that the enhanced walking cycle, a recent pin-on-disk test conducted by Saikko [128], in such as ascending the stairs, is more aggressive than the order to avoid unrealistically low wear and friction values standard walking cycle, which may increase approximately caused by protuberance formation, the contact pressure 123 State of the art of bioimplants manufacturing: part II 149 Fig. 7 Photographs and schematics showing a IRC MTS 8-station hip joint simulator, b physiological test setup, c fully constraining socket fixture, d partially constraining socket fixture, e location of horizontal torque cells, and f direction of torque measured [130] Scratch test has long been recognized as a useful tool in emulating an individual deformation or removal event at micro/nanoscale [135]. The feasibility of using this method to estimate the wear debris-induced surface damage was verified by Dearnley [136]. Through adjusting scratching parameters, scratched marks with similar dimensions compared to the abrasion damages produced in vivo were achieved. In their later stage study, scratches were per- formed on coated/uncoated metallic biomaterials of stain- less steel and Co-Cr-Mo alloy. The results proved that the samples with TiN coating contributed to a greater tolerance to the scratch because the hard film hindered the deepening of the scratch. 4.2 Corrosion tests The most common method to examine the corrosion behaviour of bioimplants is via electrochemical techniques, where the manufactured bioimplants are soaked into a simulated body fluid (SBF). During corrosion testing, the electrochemical corrosion potentials and currents are con- Fig. 8 Principle of the ball-on-flat contact knee simulator, proposed tinuously recorded so that the electrochemical activity of by Saikko et al. [131] the bioimplants can be obtained. It was suggested that a 0.89% NaCl aqueous solution with a constant temperature 25% in both anterior-posterior shear force and external- of 37 C could create an environment similar to human internal rotation [134]. Therefore, an enhanced walking body [136]. The SBF is buffered to maintain a physiolog- cycle program should be applied during the test to improve ical pH value slightly above 7 [137, 138]. Hank’spoten- the wear prediction accuracy. tiodynamic polarization (Tafel solution is also extensively 123 150 C.-W. Kang, F.-Z. Fang employed. The detailed information of its composition can achieved after treatment. This paper reviewed both coating be found in Refs. [138, 139]. A three-electrode cell is and morphology processing techniques, whose advantages usually used to carry out the electrochemical studies. and disadvantages were described in comparison with each Quantitative assessments of corrosion include electro- other. Among various coating methods, plasma spraying is chemical impedance spectroscopy (EIS), potentiodynamic currently the most commonly implemented technique, polarization (Tafel analysis), open circuit voltage (OCV) while the relatively high cost and complexity of process and electrochemical noise (ECN) [140]. Among them, EIS involved simulated researchers to look for alternatives. is recognized as one of the most accurate electrochemical Laser-based technologies present themselves to be the most methods [140]. This is due to the fact that EIS requires advanced way in producing micro-groove structures on minimum AC signals, hence avoiding the perturbation on bioimplant surfaces. However, few studies have reported the electrochemical system and meanwhile reducing the the laser-based texturing of bioceramic surfaces, indicating errors. Additionally, valuable mechanistic information can that the development of this technology on biomedical be offered by this technique as the data are obtained from engineering is in its infancy and requires further studies. both electrode capacitance and charge-transfer kinetics. In terms of evaluating the manufactured surfaces, although standards are available to assess the wear and 4.3 Assessment of other properties corrosion performances of orthopaedic devices, variation always exists in the methodology adopted by different In terms of assessing other surface properties, X-ray research groups. Thus, it is necessary to develop a unani- diffraction (XRD) has been proved as a feasible technique mous evaluation system. Note that the existing simulators to measure the residual stress by many studies [141, 142]. are limited in providing in vitro approximations. Design A recent study conducted by Roy et al. [113] confirmed optimizations are required to guarantee that the complex- that XRD was practical on measuring the residual stress on ities of human anatomy are as practically similar as bioceramics after surface treatment process. Due to a possible. potential possibility of introducing foreign materials to the Acknowledgements The authors would like to thank Dr J. Zhang, Dr bioimplants during the surface treatment, XRD testing and N. Yu and Dr X. Li at University College Dublin for their valuable energy dispersive spectroscopic (EDS) analyses are usually discussions. Acknowledgments are also extended to the support of the undertaken to detect the elemental composition of the Science Foundation Ireland (SFI) (Grant No. 15/RP/B3208) and the modified surfaces [143]. With respect to measuring the National Science Foundation of China (Grant Nos. 51320105009 & 61635008). microhardness and elastic modulus of manufactured bioimplants, indentation test has been proved to be a robust Open Access This article is distributed under the terms of the technique [113, 144]. To be specific, the indentation pro- Creative Commons Attribution 4.0 International License (http://crea cess involves penetrating a sharp diamond tip into the tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give surface of workpiece, meanwhile continuously recording appropriate credit to the original author(s) and the source, provide a the imposed force and corresponding indentation depth. link to the Creative Commons license, and indicate if changes were The recorded load-displacement curve is useful in provid- made. ing insights into the mechanical behaviour of the deformed material. 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State of the art of bioimplants manufacturing: part II

Advances in Manufacturing , Volume 6 (2) – Apr 25, 2018

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Engineering; Manufacturing, Machines, Tools; Control, Robotics, Mechatronics; Nanotechnology and Microengineering
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Abstract

Adv. Manuf. (2018) 6:137–154 https://doi.org/10.1007/s40436-018-0218-9 1 1,2 Cheng-Wei Kang Feng-Zhou Fang Received: 17 November 2017 / Accepted: 14 March 2018 / Published online: 25 April 2018 The Author(s) 2018 Abstract The manufacturing of bioimplants not only crucial in this regard. Bioimplants are playing a dominant involves selecting proper biomaterials with satisfactory role in this regard. For example, a study conducted by bulk physicochemical properties, but also requires special Kurtz et al. [1] implies that by the end of 2030, the demand treatments on surface chemistry or topography to direct a for primary total hip and knee arthroplasties in the USA is desired host response. The lifespan of a bioimplant is also estimated to grow by 174% to 572 000 and 673% to 3.48 critically restricted by its surface properties. Therefore, million respectively. Given that such trend may continue developing proper surface treatment technologies has [2], it makes sense that the advances in bioimplant manu- become one of the research focuses in biomedical engi- facturing are required to support the production demand neering. This paper covers the recent progress of surface and expected growth. In the past few decades, the manu- treatment of bioimplants from the aspects of coating and facturing has essentially become the core of integration topography modification. Pros and cons of various tech- domain of the biomedical field. On the other hand, indus- nologies are discussed with the aim of providing the most tries are required to master cutting-edge manufacturing suitable method to be applied for different biomedical techniques that are suitable for commercial production. products. Relevant techniques to evaluate wear, corrosion Such needs, in turn, drive significant levels of research and and other surface properties are also reviewed. development in the manufacturing area. The present progress and development of bioimplants Keywords Bioimplant  Precision manufacturing  Surface forming technologies were comprehensively reviewed in treatment  Evaluation the first part of this paper. However, untreated bioimplants are prone to wear and corrosion, which are critical factors in the success of possessing optimal service life. The 1 Introduction physicochemical interaction between living tissues and biomaterial surfaces is another concern [3]. In other words, Addressing health issues such as osteoporosis and a satisfactory biocompatibility after implantation must be osteoarthritis, which are prevalent in an aging population, guaranteed. The host body normally responds to bioim- is a great challenge for modern society. Bioimplants are plants in nanoseconds after initial contact and the envi- ronment remains in a state of flux thereafter. It is obvious that an unsatisfactory biocompatibility is likely to result in serious consequences such as immunologic rejection over & Feng-Zhou Fang fengzhou.fang@ucd.ie time. Therefore, a broad range of surface treatment tech- nologies are being developed in order to enable the Centre of MicroNano Manufacturing Technology (MNMT- bioimplants to possess unique surface characteristics [4]. Dublin), University College Dublin, Belfield, Ireland At present, surface modification of coating on biomaterials State Key Laboratory of Precision Measuring Technology & is normally carried out prior to putting them into practical Instruments, Centre of MicroNano Manufacturing uses. Typical purposes of surface coating on bioimplants Technology, Tianjin University, Tianjin 300072, People’s include improving wear and corrosion resistance, achieving Republic of China 123 138 C.-W. Kang, F.-Z. Fang high osseointegration and enabling the desired degradation past years, a broad range of coating systems has been rate. developed, which generally falls into three categories: The surface topography of a bioimplant is an important physical, chemical and combined physical and chemical signalling modality in controlling the cell function and methods [4]. determines the biological reaction to the device [3]. Cell behaviours such as morphology, adhesion, orientation, 2.1 Plasma spraying migration and differentiation have all been observed to be related to the textures or patterns on the surface [5]. As the Plasma spraying, which is a subset of thermal spraying, biocompatibility of an implant is closely related to the takes advantage of the heat of ionized inert gas (plasma), response of cells in contact with the surface [3], the surface and sprays molten powders of metal or ceramic onto the topography modification aiming to define cells’ reaction objective biomaterials to form a protective layer. As almost has long been a research focus in the field of implantology. all kinds of materials can be melted in the plasma jet, this Theoretical analysis indicated that the ideal surface technique is quite versatile and has been widely applied in roughness (R ) for hard tissue implants is in a range of the electronic, petrochemical, medical and aerospace 1–10 lm[6]. Numerous in vivo and in vitro studies have industries. Plasma spraying presents many advantages supported that the roughness within this range exhibits the including a rapid deposition rate, thick deposits and also best in interlocking implant surface and mineralized bones low cost. More attractively, the objects can be kept in low [7, 8]. Particularly, the osseointegration was stimulated temperature during processing and the gas in the plasma considerably by the microscale roughened surfaces. flame can remain chemically inert, which helps to reduce Therefore, suitable surface modification technologies at the risk of thermal degradation [12]. Compared to other microscale are being carried out to achieve a positive coating processes, the plasma sprayed layers exhibit rela- influence on protein adsorption, cellular activity and tissue tively better coating properties [13]. response. Due to the ease of application, plasma spraying is the On the other hand, most joint implants are facing tri- first method to fabricate calcium phosphate coating on bology issues after long periods of use. For instance, the biomaterials [14]. The most commonly used material for artificial knee and hip joints would experience a great deal spraying is hydroxyapatite (HA), which can promote the of rolling and sliding contacts under cyclic loading during osseointegration after implanting an help the biodevices walking activities. Friction between joint prostheses nor- bond directly to the surrounding tissues. Evaluations on the mally leads to increased energy losses in the biomedical plasma sprayed HA coatings on titanium-based biomate- system and ultimately to wear [9]. The wear released debris rials showed that the new bone apposed directly on the would, in turn, induce physical pain and adverse immune coatings with satisfactory adhesion [15–17], and the overall responses. Instead of rectification by replacing the total bone recovery was found to be fairly quick [18]. The poor joint, surface treatment is believed to be a promising mechanical properties of HA coatings are likely to cause method to reduce the material friction coefficient, and thus brittle damages and delamination, hence the alteration of prolong the device lifetime [10, 11]. In this regard, surface structure is prone to occur. Aiming to address the issue, texturing is favoured for the ability to retain the desired many parametric studies on the spaying process were car- bulk attributes of biomaterials, meanwhile improving the ried out and followed by characterizations [19]. It was tribological properties required by different clinical proved that by using high spraying power, suitable me- applications. chanical properties and high bonding strength could be The scope of this paper is to review the typical surface achieved. This is due to a greater extent of coating melting treatment technologies for bioimplants in two aspects, i.e., resulted in a denser microstructure. The trade-off, however, surface coating and topography modification. A discussion is that a higher energy consumption is involved. Yang et al. of current challenges and perspectives will be given in the [20] produced plasma sprayed HA on Ti-6Al-4V with final section. various cooling conditions and substrate temperatures in order to have different residual stress values at the HA/ metal interfaces. The evaluation results revealed that the 2 Surface coating technologies interfacial residual stress played an important role in determining the bonding strength, where coatings with Surface coatings are currently of particular interests in lower residual stress were found to exhibit better adhesion. prolonging lifespan and enhancing performances of various Apart from temperature effects, the increased coating bioimplants. Such modification allows both suitable bio- thickness is believed to be another reason for the rise of compatibility and biofunctionality while preserving the residual stress [19]. It is also known from early reports that favourable bulk characteristics of the biomaterial. In the compared to a smooth substrate, a highly roughened 123 State of the art of bioimplants manufacturing: part II 139 substrate surface is beneficial to achieving a better bond contact length for the as-sputtered calcium phosphate strength [21]. implants was reported to be close to plasma sprayed The advantages of low cost and rapid deposition rate implants, respectively are (70.4±1.6)% and (78.6±4.9)%, make plasma modification process the mainstream for to be specific [22]. surface modification of biomaterials. The technology pro- One obvious drawback of sputter coated HA layer on the vides a flexible and environmentally friendly process that metallic substrate is the poor degree of crystallinity allows manufacturers to tailor the surface properties of the [22, 24, 27], which would increase the dissolution rate of biomaterial to suit specific needs. However, issues cited the coating in a human body [19]. Post-treatment of with the plasma-sprayed coatings include variation in bond annealing with controlled temperature and processing time strength between the coatings and the substrates, poor was used to crystallize the film. It was found that the adhesion at the interface and alterations in HA structure thermal process would make a change in the surface due to the coating process. In addition, to our best morphology and further contribute to the changes in crystal knowledge, there is no evidence showing that plasma structure [28]. Nevertheless, it should be noted that con- sprayed coating would prolong implants’ lifespan or ventional thermal treatment in the electric furnace increa- improve the reliability comparing to the uncoated implants. ses the potential possibility of cracks formation and may Knowing the problems of coating brought by plasma thus degrade the HA films [19]. In addition, since the spraying, numerous alternatives of deposition process were process involves high energy consumption and large costs, developed. improvement of economic efficiency must be taken into consideration for industrial applications. 2.2 Sputter coating Although the sputter coating technique is not currently used as a commercial deposition process by most bioim- Sputter coating technique is classified as a physical vapour plant vendors, its capabilities of producing uniform and deposition (PVD) method and shows great promises in dense coating with better adhesion strength make it a viable eliminating the issues associated with the plasma spraying alternative to plasma spraying for the application of HA process [22]. During the process, a gas plasma is utilized to coatings on bioimplants. eject materials from a negatively charged target. The material would be then deposited as a coating onto the 2.3 Ion-beam assisted deposition substrate material. From an industrial perspective, the technology is considered as a complex process since it The principle of ion implantation was first postulated in involves many parameters that control sputter deposition. 1906, but it was not until the 1990s that the technique was first applied as a coating technology for biomedical On the other hand, however, the availability of precisely varying parameters allows a large degree of control over implants [29]. In a typical ion implantation process, ions the growth and microstructure of the coating. Some early are accelerated through high graded potential difference reports showed that the initial sputtering using multi- and directed towards a substrate material. Due to the ion- component ceramic targets such as superconducting oxi- solid interactions, the energetic ion would get incorporated des, HA and other calcium phosphate materials would produce coatings whose chemistry was different upon deposition than the bulk target [22, 23]. Successful attempts have been made on depositing cal- cium phosphate layers on metallic biomaterials using radio frequency magnetron sputtering [24, 25]. The sputtered layers were observed to be more homogeneous than the plasma sprayed ones and the surfaces appeared to be very smooth [19]. Meanwhile, the adhesion strength of sputtered HA coating and its reliability have also been found to be higher than most plasma sprayed HA coatings. A com- parative study conducted by Ozeki et al. [26] indicated that the adhesion strength of the sputtered coating exceeded that of the plasma sprayed coating by more than 70%, 40%, and 30% after a period of 2 weeks, 4 weeks, and 12 weeks, respectively. In terms of biological responses, improved bond strength and the initial osseointegration rate were Fig. 1 Illustration of ion-solid interactions in an ion-beam assisted observed in sputtered HA coatings. The percent of bone deposition process [29] 123 140 C.-W. Kang, F.-Z. Fang into the substrate after losing all the energy [29]. Figure 1 confirmed the validity of calcium-ion implantation as a pre- illustrates the ion-solid interactions during an ion-beam treatment to endow the desirable bioactivity on porous Ti assisted deposition process [29]. As can be told from the for bone tissue engineering applications. Some other ? ? working principle, the penetration of the ion correlates with reports indicated that the ion implantation of Ca ,N and the level of energy. Therefore, by carefully controlling the F was helpful in promoting the anti-bacterial effect of ion beam energy to avoid deep penetration inside the various titanium surfaces [35]. substrate, modifications can be confined to the near-surface In general, ion implantation technique is useful in region, and hence significantly influence the surface char- improving the mechanical, chemical and biological prop- acteristics. Except for the ion beam energy, other important erties of biomaterials. The process is extremely control- parameters involved in ion implantation include ion spe- lable and can be accurately tailored in order to implant cies, fluence (or the total number of ions that bombard a different ions to form ultra-high purity coatings with surface) and beam current density or flux, which can all be excellent adhesion. Nevertheless, because the entire pro- adjusted to influence the ultimate effects on the substrate cess is conducted in a high vacuum and involves costly and achieve wide atomic intermixing zone [30]. steps such as beam extraction, beam focusing and beam An attractive feature of ion-beam assisted deposition is scanning, the soaring cost has hindered its widespread uses. that it offers independent and specific control of the At present, ion-beam based treatment is mostly applied in deposition parameters. Such feature enables the manufac- high-value-added products and limited in the regular pro- turing of gradual transition between the substrate material duction line. Besides the cost, it is disadvantageous for and the coating, thus a more durable bond can be achieved being inappropriate for components with complex geome- [31]. Rautray et al. [29] indicated that the adhesion prop- tries [4]. erties of ion-beam implanted and plasma sprayed coatings seemed to be similar, but the atomic intermixing interfacial 2.4 Conversion coating layer formed by ion dynamic intermixing contributed to a better bonding strength. In the case of fabricating HA Conversion coating, also referred as in situ grown coating, coating on a titanium substrate, ion-beam assisted deposi- is formed through specific reactions between materials and tion achieved a tensile bonding strength of 70 MPa, which environment. This technology is typically used in reactive exceeded that of 51 MPa associated with the plasma metallic materials, where an inorganic oxide layer is pro- spraying process. The existence of a transition structure at duced with the help of a chemical or electrochemical the HA/Ti interface consisting of amorphous HA, amor- process. As the conversion is formed in situ, the adhesion phous calcium phosphates and amorphous Ti phosphate of the coatings to the substrate is relatively strong. Passi- compounds were thought to be responsible for such phe- vation is one typical branch of conversion coating and nomenon. The formation of such a chemical bond was being used as a simple approach to protect reactive bio- thought to be attributed to the energetic ion bombardment materials, such as magnesium and its alloys. By simply process [29]. It was also reported that the ion-beam treat- immersing Mg-based biomaterials in a solution with a ment was capable of providing HA-coated titanium stable pH of 11 or higher, a passive layer of Mg(OH) in bioimplants with hardened surfaces, and thus improved the nanometric thickness can be formed within a short time wear resistance [29]. An essential element for the human period [4]. By means of adding mixtures of oxides or body, phosphorus, can be implanted on titanium-based hydroxides in the solution, a film of anti-corrosive metal biomaterials via ion-beam deposition. In this way, a com- phosphates could be formed as well. It is noted that pact TiP phase could be formed on the titanium surface. although the converted layer provides protection during the The new phase turned out to be helpful for strengthening initial phases of corrosion in a living body, the protective the corrosion resistance. The satisfactory biocompatibility ability is reported to be inadequate [4]. As a result, of phosphorus-ion implanted titanium was confirmed by researchers tended to develop innovative technologies to Krupa et al. [32]. In addition to above, it was also reported produce more stable and powerful conversion coatings. that the utilization of ion implantation was advantageous in The anodization process is favoured for its controlla- the aspects of avoiding stress shielding, enhancing fatigue bility on the coating thickness, and it is mainly used to resistance and improving fracture toughness for bioim- produce or thicken native oxide layers on metal materials. plants. From the perspectives of biological activities, ion- The coating thickness normally increases with the beam implantation provides the benefits of induction of increased applied voltage and the value is in a range of crystallinity and reduction in apatite dissolution rate [33]. 5–200 lm[36]. Numerous studies have proved that the Chen et al. [34] investigated the influence of calcium ion anodized layers are more stable and inhibit corrosion better deposition on the apatite-inducing ability of porous tita- than traditional chemical conversion layers [36–38]. When nium in a modified simulated body fluid. The results anodizing the metallic material above the breakdown 123 State of the art of bioimplants manufacturing: part II 141 voltage, porous layers can be formed with improved the corrosion resistance and weaken the adhesion strength. resistance to abrasion and corrosion [38]. Such technology Post-processing of thermal treatment was capable of is called plasma electrolytic anodization (PEO), as well as reducing the porosity effectively, but cracks caused by anodic spark deposition (ASD) and micro arc oxidation shrinkage were observed in the ceramic coating [43]. (MAO) [4]. Commercially, PEO has become the most HIP involves the pressing of ceramic granules at high applied protection method for Mg alloys [4]. Nevertheless, temperature and pressure, during which the operating the coating process would result in electric isolation, which temperature is normally above 2000 C and the working makes PEO inappropriate for further processing via electric pressure is usually maintained in a range of 100–320 MPa deposition [4]. Besides the utilizations described above, [59]. The most distinct merit of HIP is that it is capable of conversion coating techniques are sometimes performed as controlling the size and shape of the product and realizing a pre-treatment process to promote the expected adhesion high precision finish without further machining procedures of a deposition coating. [44]. This technique enables the fabrication of thick and dense coatings with a variable thickness. Besides, the HIP 2.5 Other methods treated coatings usually exhibit homogeneous material structure as well as a high uniformity of properties [44]. In addition to the above-mentioned methods, there are This method is particularly advantageous for reducing the several more techniques being used to create coatings for porosity and enhancing the mechanical properties of the bioimplants, such as electrophoretic deposition (EPD) ceramic coating. It has been used as a post-treatment by [39–43], hot isostatic pressing (HIP) [44, 45], pulsed laser many researchers to densify the plasma sprayed HA coat- ablation [46–49], sol-gel [50–53] and dip coating [54, 55]. ings [60]. Khor et al. [45] applied HIP in reducing the In an EPD process, particles in suspension are coated amount of micropores in a plasma sprayed HA coating on onto an electrode in the effect of an electric field [39]. This Ti alloy. A mercury intrusion porosimeter was utilized to method is popular in coating complex shapes and patterns. measure the pore size distribution of the hot isostatic The regulatable particle size and deposition conditions pressed samples. The results indicated that most of the allow EPD to have a high degree of control on the coating micropores were drastically reduced after HIP. As a result, results [40]. Commercially, EPD is generally regarded as the physical properties such as microhardness and bonding an economy coating process as it does not involve any strength were significantly improved. Another study con- costly equipment, and the instrument set-up is relatively ducted by Kameyama [61] employed HIP to implant easy. The possibility of stoichiometric deposition and encapsulated porous HA granules into a superplastic Ti- capability of achieving high purity material make this 4.5Al-3V-2Fe-2Mo alloy. They obtained a new hybrid method a limelight in biomedical applications [19]. Cur- biomaterial with both biological affinity and high rently, there is a rising trend of applying EPD to coat mechanical strength. Nevertheless, it can be easily told metallic biomaterials. Successful attempts have been made from the working principle that this technique is difficult to to achieve uniform thin ceramic coatings with good coat complex substrates. This is because the elevated mechanical properties on Ti and Mg alloys [41, 42, 56, 57]. temperature and pressure are simultaneously applied to the As EPD processes can be carried out at room temperature workpiece during the process. Thus, the thermal expansion or lower, the ceramic coatings are protected from the for- mismatches between the substrate and coating particles are mation of amorphous phases. A novel EPD process at room prone to occur. It should also be noted that the encapsu- temperature was proposed by Zhang et al. [43] to fabricate lation material was hard to be removed after pressing [62]. nanostructured HA coating. The experimental results Pulsed laser deposition (PLD) is a conceptually and showed that the coating’s bond strength was significantly experimentally simple yet highly versatile tool for fabri- improved up to 50–60 MPa. Moreover, the corrosion cating a wide range of thin films and multi-layered struc- resistance of the nanostructured HA coating was confirmed tures [46]. The physical phenomena of PLD are quite 50–100 times higher than conventional HA coatings. The complicated and it can be divided into three stages: laser feasibility of fabricating bioactive composite systems using radiation interaction with the target, dynamic of the abla- EPD was investigated by Kumar and Wang [58], where tion materials and deposition of the ablation materials with TiO powders were first coated on Ti-6Al-4V alloys, fol- the substrate, nucleation and growth of a thin film on the lowed by depositing an HA-TiO composite. In this way, substrate surface [46, 63]. Comparing to other deposition functionally graded coatings of HA-TiO -Ti systems can methods, PLD is easy-to-operate and allows the growth of be successfully obtained. Although the application of EPD coating at lower temperature [64]. The main advantage of has kept a consistently rising trend in the field of PLD is that the stoichiometry of the target can be retained biomedical manufacturing lately, it was reported that the in the deposition films [19]. This is attributed to the high presence of porosities on the deposited layer might lower rate of ablation that led all elements or compounds to 123 142 C.-W. Kang, F.-Z. Fang evaporate at the same time [46]. Another unique feature of applied this method to coat HA/TiO on a nontoxic TiZr PLD is that it can change the deposited material in situ, alloy for biomedical applications. The simulated body fluid which enables the fabrication of advanced materials. Since tests showed that the coated products exhibited excellent the first achievement of PLD deposited high-quality HA bone-like apatite-forming ability and were expected to be a thin film was made in 1992 [47], the progress of its tech- promising load-bearing implant material. In the past two nical improvement had been substantial and steady. In the decades, the sol-gel technique has been found to be of past two decades, HA coatings with diverse compositions value in enhancing the corrosion resistance of Mg-based and crystallinities have been successfully produced by PLD clinical implants, as well as retarding the degradation rate [48, 65]. The interest in PLD for use in biomedical appli- [53, 70]. Kim et al. [71] deposited fluor-HA on a titanium cations is from the ability to suffice the bioimplant with substrate through sol-gel, while different concentrations of good mechanical properties and biocompatibility. Previous F were incorporated during the process. The results researches have revealed that pulsed laser deposited HA proved that the tailoring of F solubility would change the coatings exhibit better interfacial adhesion and have minor dissolution rate of the coating layer. Despite all the undesirable phase under optimal conditions [46]. Arias advantages mentioned above, there are some considerable et al. [66] conducted micro-scratch tests to evaluate the drawbacks of this technique. Most obviously, cracks are adhesion properties of pulsed laser deposited HA coatings often observed in sol-gel coatings. It was reported that in on Ti substrates. The absence of detachment suggested that order to reduce cracking, the thickness of the coating had to good adhesion properties were obtained through PLD. The be retained under 0.5 lm[69]. Another point which needs authors also confirmed that both amorphous and crystal- to be considered is the thermal effects. Like other thermal lized HA coatings were produced during the PLD process. deposition methods, thermal expansion mismatch at the Therein the crystalline HA coating is superior in internal interface is always an issue. The limited wear resistance of cohesion while the amorphous coating could be more the product is also a major concern that comes with the sol- mechanically compatible with natural bone. The distin- gel coating. Last but not the least, sol-gel coatings suffer guished adhesion strength of PLD coatings on Ti-based from high permeability. Because of the requirements of biomaterials is believed to be attributed to the existence of post sintering and long processing time, sol-gel is presently an oxide layer [67]. A study conducted by Cotell [49] used to a lesser degree in the industrial processes. supported the hypothesis that there might be effects of Dip coating is usually compared with sol-gel technique epitaxy on the interface and hence contributed towards as they share similar processes [19]. The fundamental adhesion. Limitations of the PLD technique include procedures of a dip coating begin with immersing the bringing the film into contact with the particulates depo- substrate into the solution of the coating material. After sition, which can either be solved by employing filers or withdrawing, solvents and other accompanied chemical polishing target surface before each run. Another drawback reactions in the liquid film would be evaporated. A post- is the deposited layers have a lack of uniformity over a treatment of curing or sintering is required as a final step to large area of the plume owing to the angular distribution of form the anticipated coating, during which high temperate the ablation plume. Thus, rasterizing the laser beam across is normally involved. Compared to sol-gel technique, dip the rotation target was proposed to solve this issue [46]. coating is more time-efficient. It is reported that a complete Sol-gel has attained its reputed fame for being one of the transition can be completed in a couple of seconds if simplest techniques to manufacture thin films [51]. It has volatile solvents are used [72]. In addition to this, its simple attracted widespread interests in coating optical, magnetic, procedures and low cost in coating complex shapes make electronic and chemical components [50]. The typical sol- this method relatively popular in the industry. HA thin gel process involves the immersion of the substrate mate- films coated on metallic substrates via dipping exhibit high rial into a concentrated solution with a gel-like texture. surface uniformity and homogeneity. Mavis and Tas¸[54] Detailed procedures of the process were introduced by presented a series of recipes of HA dip-solution, by using some researchers [19, 68]. Apart from the ease of operation which highly porous coatings with over 30 MPa bonding and cost-effectiveness, this method is capable of producing strength could be deposited on the Ti-6Al-4V substrates. thin bond coatings with excellent adhesion, as well as thick Dip coating also introduces its profound advantages in layers with satisfactory corrosion resistance [69]. During slowing down the corrosion rate of Mg alloys. Gu et al. the process, high purity can normally be guaranteed. In [55] dipped coated chitosan on a group of Mg-Ca alloys addition to above, super high temperature processing is and tested their respective corrosion resistance in a simu- unnecessary, as the post-sintering process is only con- lated body fluid. The authors implied that this technique ducted in the rage of 200–600 C. Given the mentioned showed great promises of future adaptation for Mg sub- appealing features, there is a growing interest in coating strates in matching the implant corrosion rate with the bioimplants using the sol-gel technique. Wen et al. [52] tissue healing rate. 123 State of the art of bioimplants manufacturing: part II 143 biomaterial but also is favoured for its environmental 3 Surface topography generation friendliness. To meet the demands from an enhanced interaction Nevertheless, several studies have reported that blasting with the particle material other than the implant itself bears between the biomaterials and living body and simultane- ously reduce the risk of wearing, various methods were a potential risk of changing the surface composition [6]. Attention was typically paid to the alumina blasted applied to create microstructural surface features for bioimplants. Typical technologies in engineering substrate implants. Some researchers insisted that the remnants of the alumina particles could release aluminium ions into the at the micro and nanoscale will be reviewed in the fol- lowing sections. host body due to dissolution, and further cause inflamma- tory responses [6]. There are also some concerns that the Al ions would inhibit normal differentiation of the bone 3.1 Blasting marrow stromal cell and normal bone mineralization [78]. Although no statistically significant differences were found Blasting in biomedical engineering refers to an operation which propels a stream of abrasive particles against the between the implants blasted with Al O and other particle 2 3 materials [74, 79], the application of non-biocompatible substrate biomaterial under a high pressure. The process is used to remove surface contaminants or roughen the sur- particles for blasting remains controversial. As a result, the feasibility of using hydroxyapatite and beta-tricalcium faces in order to enhance the biomaterial’s reactivity after implantation [73, 74]. The alteration in the surface topog- phosphate particles in blasting was investigated. Benefited from the material features of biocompatibility, osteogenesis raphy is attributed to the plastic deformation. Although it is and resorbability, the bioceramic blasted surfaces exhibited difficult to precisely control the surface texture due to the a suitable bone-implant contact after implantation. Mean- numerous variables inherent in the blasting process, the while, other surface properties are reported to be compa- size of the particles can be adjusted to meet the roughness rable to those treated by conventional blasting procedures requirement. Considering that the particles need to be [80–82]. chemically stable, alumina, titania and hydroxyapatite particles are most commonly employed at the stage [6]. 3.2 Chemical etching Valverde et al. [75] showed that a wide variety of microtopography, ranging from minimally rough to Etching techniques performed on untreated biomaterial excessively rough surfaces, could be prepared by regulat- ing the variable factors during the blasting procedure. The surfaces have been used to form micro pits at sizes between 0.5 lm and 2 lm to enhance cell adhesion and osseointe- effects of blasting parameters on the surface roughness of Ti-6Al-4V were studied by Mohammadi et al. [76]. In their gration [6, 83]. In surface etching processes, chemical reagents are selectively applied on specific areas to remove study, two particle materials, i.e., Al O and SiO , were 2 3 2 materials and therefore form expected texturing. The employed with different sizes using different types of blasting systems. Through optimizing the processing con- etching on specific regions is generally achieved through masking, where the selected masking method determines ditions, an equivalent surface roughness of 3.51 lm was achieved. Their follow-up coating experiments confirmed the resolution of the texture features. Costa et al. [84] proposed the application of drop-on-demand inkjet printing that the substrate surface topography had a significant influence on the coating properties at the interface. Obvious for masking steel surfaces with subsequent chemical etching and ink stripping. It was proven to be a fast, ver- differences were observed between the HA coatings deposited on the substrates with and without blasting satile and highly feasible technique for texturing steel surfaces [84]. Figure 2 shows the typical steel surfaces treatments in terms of the tensile bonding strength. Arif- etched via inkjet printing [85]. vianto et al. [77] blasting treated 316L stainless steel using Strong acids such as HCl, H SO and HNO are com- steel slag balls, which were the residues from steelmaking 2 4 3 monly used in most etching processes. It is believed that processes and presently regarded as an industrial waste. The authors reported that both surface microhardness and higher concentrated acidic solutions normally generate better surface defect distributions while less aggressive irregularity of the stainless steel were increased after the treatment. It was also found that some bioactive elements mixtures would be conducive to a finer roughening [86]. In etching of titanium-based bioimplants, fluoric acid is such as Ca, Si and Mg were introduced by the slag balls. This study clearly indicated that the steel slag blasting was regarded as an alternative chemical reagent. Previous reports showed that HF can effectively dissolve the passi- a promising method for the surface modification of the medical grade 316L stainless steel. It is not only capable of vation TiO layer [6]. In addition, since titanium is very reactive to fluoride ions, the fluoride would be incorporated improving the mechanical properties and bioactivity of the into the created surface structures and form soluble TiF . 123 144 C.-W. Kang, F.-Z. Fang Fig. 2 Inkjet printing of steel surfaces with a parallel gaps and b chevron-like gaps [85] Such incorporation is beneficial for the osseointegration of [6, 92–94]. Through incorporating patterning techniques at implants [87]. As a result, HNO is usually mixed with HF submicron scale with subsequent sandblasting and etching to produce microscale surface structures on Ti-based processes, Zinger et al. [95] achieved desired titanium implants [88]. However, attention should be paid to fluo- surface with a combined micrometre and nanometre ride contaminations as they may induce an ambivalent structures, which showed an improved osteoblast ability. A response in the host tissue [6]. The risk of weakening metallurgic-mechanical analysis conducted by Pazos et al. mechanical properties is another concern brought by the [94] explained the advantage of blasting ? etching surface chemical etching. In the etching of titanium-based bioim- treatments in improving titanium material properties. The plants, hydrogen embrittlement triggered by the acid authors suggested that the decrement of fatigue endurance environment was reported, which might be the reason for induced by the acid etching could be counteracted by the the forming of micro-cracks on the implants’ surfaces and foregoing blasting process. The formation of a plastically ultimately led to a reduction in the fatigue resistance [83]. deformed layer and compressive residual stress contributed Selective infiltration etching (SIE) is a special surface to the strain-hardening, and therefore resulted in a better topography modification method, which coats target sam- fatigue behaviour. ples with special infiltration glass [89]. By heating the coated objects above the glass transition temperature, the 3.3 Laser-based techniques molten glass would diffuse between the grain boundaries and result in sliding, splitting and rearrangement of the As reviewed in part I of this paper, the improvement of surface grains. After cooling to room temperature, the glass osseointegration relied on roughening the surfaces of can be dissolved in an acidic bath, thus exposing the newly implants. In most cases, the selected surface regions of a created surfaces [90]. This technique is now being used in biomaterial are blasted to be roughened at microscale. transforming zirconia surfaces into dense, highly retentive However, one obvious drawback of this technique is that it and smooth nanoporous surfaces. A significantly higher can only produce randomized surfaces [96]. Such surface degree of osseointegration of the selective infiltration features may alter the near-surface mechanical and chem- etched zirconia implants was claimed by Aboushelib et al. ical properties, and thus cause mechanical degradation [91]. [97]. Moreover, it gives rise to the local concentrations of Presently, attempts are being made regarding the etching toxic elements such as Al after the surface modification process after a blasting step. Such technology integration is [97, 98]. Considering the above-mentioned issues, micro- designed for removing blasting induced surface damages grooving is now being explored as an alternative surface and simultaneously improving surface roughness charac- treatment approach to facilitate the bone-implant integra- teristics [6]. Many previous investigations have demon- tion for bioimplants [96]. Among all known micro-fabri- strated that a combined blasting and etching structuring cation methods, laser-based technologies showed method is of great help in producing superior quality of themselves to be the most advanced way in producing topographies with different scales at the same surface 123 State of the art of bioimplants manufacturing: part II 145 micro-grooves with optimal groove dimensions for cell surface regions to fabricate dent arrays, shows a great adhesion. potential on this aspect. Hu et al. [11] employed this Previous works have reported on the positive effect of technique to create micro-dimple patterns on Ti-6Al-4V laser-ablated micro-grooves on promoting the contact surfaces. Excellent tribological performances of the bio- guidance of cell alignment [96, 98–100]. For instance, a material were verified under various loads applied. The comparative experiment conducted by Chen et al. [101] effects of texture parameters on tribological behaviours revealed that the laser-irradiated Ti-6Al-4V surfaces with were also investigated in their researches. A higher dimple micro-grooves provided the best cell/surface interactions density was found to result in a lower friction coefficient. over polished and blasted ones. The migration and align- This is because the micro-dimples functioned as traps for ment of cells would not only enhance the osseointegration wear debris. A higher dimple density was more likely to but also reduce the extent of scar tissue formation during absorb more wear particles and therefore eliminated the wound healing [102]. More recently, Hsiao et al. [103] potential debris ploughing effect. In addition, it was developed an ultraviolet (UV) laser treatment system to believed that the micro-dimples might serve as fluid texture Ti-6Al-4V biomaterials. Major micro-groove reservoirs in the host body, which would help to retain structures and minor porosities were obtained simultane- body fluid as a lubricant and lead to less wear. ously. The following in vitro tests proved that the texture The major disadvantage of LST is that the laser ablation effectively offered a favourable environment for the may alter surface integrity. Previous reports have indicated osteogenic cells. Nevertheless, due to the factors such as that elevated temperatures encountered during ablation high energy outputs, top-hat intensity profiles and the high may change surface microstructures and form cracks. Such photon energy associated with the deep UV wavelengths, damages would drastically shorten the fatigue life of the the current UV laser based micro-grooving technologies material [10, 105]. For this reason, laser shock peening may introduce micro-cracks and induce heat-affected zones (LSP) process was proposed. LSP is capable of producing inside the grooves [96, 104]. Such damage would definitely micro dent arrays, and at the same time improving surface degrade the performances and reduce the lifespan of mechanical properties via inducing deep compressive bioimplants. Considering this, improving laser processing residual stress in the subsurface. A typical denting sche- techniques to produce durable laser-textured biomaterial matic of LSP is shown in Fig. 4 [10]. During the process, a surfaces is crucial [96]. Diode-pumped solid-state (DPSS) short, high-power laser pulse is applied (under a water laser technologies were developed to address the above blanket) to vaporize a sacrificial coating on the objective. issues. Fasasi et al. [96] proposed a nanosecond DPSS UV Selected surface areas would then undergo plastic defor- laser processing technique to optimize the groove geome- mation by the pressure waves [106]. The improved fatigue tries. By judiciously adjusting laser processing parameters, performance of Ti alloys offered by LSP processing was such as wavelength, pulse repetition rate and scan speed, claimed by Ruschau et al. [107]. Guo and Caslaru [10] satisfactory groove depths and widths of around 11 lm and demonstrated that LSP could efficiently manufacture mass 14 lm were obtained. Some scanning electron microscopic microscale dent arrays on Ti-6Al-4V alloy surfaces though (SEM) images of micro-grooved surfaces are shown in adjusting the laser power. Strain hardening and compres- Fig. 3. In addition, achievements on decreased groove sive residual stress in the centre area of the peened dents roughness and reduced heat-affected zones on Ti-6Al-4V contributed to the increment in microhardness. However, it were claimed by the authors. The absence of micro-cracks should be noted that all the laser texturing methods is also believed to be beneficial for cell attachment and reviewed above have been mainly applied in aeronautical spreading on the grooved structures. components for their increasing wear-resistance ability, but Apart from creating micro-grooved structures on rarely reported in improving the tribological properties of biomedical materials to improve the integration with sur- bioimplants. rounding tissues, laser-based texturing technologies are In summary, laser-based surface treatment methods are also applied to improve the tribological behaviour [10, 11]. favoured for the simple processing and effortless operation. Among various biomaterials, titanium and its alloys are Desired surface patterns can be fabricated through varying characterized by poor tribological properties, such as high the laser parameters. The laser micro-grooved metallic and unstable friction coefficient [11]. The rupture of pas- biomaterials exhibited better osseointegration and longer sive oxide layer would release wear debris to the host body lifetime in comparison with blasted or etched ones. More and lead to aseptic loosening of the implant. Therefore, importantly, a potential development tendency on great efforts have been made on introducing specific sur- enhancing tribological properties of bioimplant through face patterns on Ti and Ti alloys to enhance the tribological laser treatment is noteworthy. However, the high energy or performances. Laser surface texturing (LST), which uti- elevated temperate involved in the laser treatment process lizes high energy laser pulses to melt and vaporize specific is a major concern as it may alter the surface integrity after 123 146 C.-W. Kang, F.-Z. Fang Fig. 3 Typical SEM images of micro-groove surfaces produced by nanosecond DPSS UV laser processing [96] regarding working efficiency and accuracy. During a typ- ical EDM process, the removal of material is achieved through the high thermal energy generated by a series of high-frequency electrical sparks. Detailed working princi- ples could be found in Refs. [109, 110]. Figure 5 shows the typical EDM experimental setup [110]. In comparison to other surface treatment techniques, this technique does not require any pre-treatments on the objects’ surfaces. Since the electrode and workpiece are not directly contacted in EDM, issues triggered by mechanical stress during con- ventional contacting machining can be avoided. Another advantage of EDM is that such technology would introduce carbides on the workpiece surface, and hence enhance the surface properties such as hardness and wear and corrosion resistance [109]. Apart from above, what makes EDM attractive in biomaterial manufacturing is that a porous Fig. 4 Schematic of dent fabrication by laser shock peening [10] nanostructured oxide layer would be converted on the surface during the process. The layer thickness can be treatment. The relatively higher cost is another issue that intentionally controlled according to the requirement, so hinders the wide-spread of laser-based surface treatments. that a suitable biocompatibility could be achieved [111]. Furthermore, little work has been reported on the laser Several in vitro and in vivo studies have examined the surface structuring of bioceramics. This is a strong indi- improved osteoconductivity of metallic biomaterials whose cator that laser-based texturing on biomedical engineering surfaces were modified by EDM. Peng et al. [111] found requires further studies [108]. that a nanophase transition occurred on the titanium surface during EDM, which played a critical role in forming a thick 3.4 Electric discharge machining nanoporous TiO layer on the titanium surface. The porous structure is believed to be beneficial for enhancing the Electric discharge machining (EDM) was established to biocompatibility. In their following work, EDM was manufacture geometrically complex or hard material parts applied to produce a rough texture with pores and craters at which are difficult to be machined by conventional pro- nanoscale on the surface of Ti-6Al-4V alloys [112]. Again, cesses [109]. More recently, it has become favourable in the formation of nanoporous TiO layers was observed. producing nanostructured biocompatible surfaces [110]. The follow-up evaluations revealed that the EDM-func- The exploration of the destructive properties of electrical tionalized surfaces significantly increased the activities of discharges can be traced back to the 1940s, when the first surrounding cells in terms of adhesion, differentiation and attempt was made on vaporizing material from the diffi- proliferation. It was confirmed that an improvement of cult-to-machine metal surface [109]. Since then, EDM multiple osteoblast functions could be achieved by experienced a successive development in the past decades increasing the pulse durations. 123 State of the art of bioimplants manufacturing: part II 147 Fig. 5 Typical representation of the experimental setup for EDM process [110] As the present trend of surface treatment has been Previous studies have proved that the high-frequency switched from conventional machining to advanced tool-work interaction induced by ultrasonic vibration is of micro/nano-manufacturing, EDM has become favoured in great help in the manufacturing of various micro/nanos- offering nanoporous surfaces with enhanced mechanical tructures [115–118]. Some successful attempts of ultra- properties and biocompatibility. One major drawback of sonic-assisted machining have been made on different the EDM fabricated material is the low fatigue perfor- materials such as stainless steels [115, 119, 120], silicon mance brought by the recast layer [110]. Post-treatment carbide [121], glasses [120, 122], polymers [123], etc. such as blasting is required to address the issue. In general, Nowadays, an increasing number of studies have shown the application of EDM in biomanufacturing remains at the that the ultrasonic-assisted machining techniques are initial stage and the fulfilled results are confined to labo- valuable in achieving high precision textures on biomedical ratories. More works need to be done before EDM is rad- materials. For instance, a novel rotary ultrasonic texturing ically accepted by the biomedical industry. (RUT) technique was proposed by Xu et al. [124]. In their study, the ultrasonic vibration was integrated into a rotary 3.5 Other methods machining process. The combination of vibration, rotation and feed motion offers high-frequency periodic change. It In the past decade, the emergence of surface texturing was suggested that this new technique allowed manufac- technologies on biomaterials goes well beyond the above- turers to fabricate various fine surface structures by offer- mentioned ones. For example, Roy et al. [113] employed ing an additional processing freedom. the micro-drilling technique to manufacture micro-dimpled Electrochemical machining (ECM) is a relatively surface textures for ceramic-on-ceramic hip prostheses. In mature surface modification technique which enables the simulated hip joint condition, the dimpled workpieces removing metallic materials selectively by an electro- exhibited obviously improved tribological performances chemical reaction at the anode [125]. Its flexible machining compared to non-dimpled ones. Choudhury et al. [114] rate is attributed to the adjustable electric current [9]. revealed that plateau-honed technique was effective in Through years of development, the processing dimension reducing wear rate, friction coefficient and removing wear of ECM has reduced to microscale. Electrochemical debris from the contact interface of metal-on-metal hip micromachining (EMM) has replaced traditional ECM in joints. many places and been widely used in producing patterns on 123 148 C.-W. Kang, F.-Z. Fang stainless steel based hip prosthesis stems. Mask electro- chemical micromachining (TMEMM) is typical branch of EMM, which involves photolithography to produce micropatterns on the photoresist-coated substrates [126]. Lu and Leng [125] developed a jet electrochemical micromachining (Jet-EMM) to form micro-holes on the titanium-based bioimplants. The technique exhibits merits of producing patterns on curved surfaces and enables fea- tures with a high aspect ratio. Comparing to TMEMM, the equipment required for Jet-EMM is less complicated. The Fig. 6 Schematic of a pin-on-disk system, modified from Ref. [128] invention of the first rapid, mask-less EMM was claimed should be kept below 2 MPa when simulating the wear of by Sjo¨stro¨m and Su [127], where the surface patterns were ultrahigh molecular weight polyethylene. It should be created via a direct writing manner. During the process, a noted that this technology is a simplified model of normal microscaled single-tip in conjunction with short voltage walking, and simulation of the motion and loading in pulses moves across the substrate and the resolution was activity is very limited. In fact, there are three complicated kept in the submicron region. This technique is demon- articulation mechanisms that are involved in the motion of strated to be ideal for the fast fabrication of desired surface a tibiofemoral joint, namely gliding, pure rolling and patterns on metallic biomaterials. Microscaled grooves and rolling-slipping [129]. Investigations are required to elu- pits (around 50 lm in width and diameter) were success- cidate the complex environment of the host body and fully produced on titanium surfaces with high manufac- increased patient activities. turing efficiency. Note that although EMM is favoured for For a more precise evaluation of the tribological prop- its ability to handle complex geometries, its application is erties of orthopaedic joints, friction and wear experiments restricted to electrically conductive materials. are performed on a simulator. Compared to the pin-on-disk test, the simulators are capable of imitating more complex kinetics and kinematics of a human body in a physiological 4 Characterizations of bioimplant surfaces environment. Running in accordance with ISO 14242 and 14243, an array of hip and knee complexities can be The surface properties generally play a dominant role in evaluated. The simulator testing allows implementing determining the longevity of bioimplants, among which the various surface textures on the workpieces as well as a major concerns include the products’ wear and corrosion selection of lubricant, which ensures that the simulated resistance, mechanical properties of hardness and elastic condition is as practically similar as possible to the com- modulus, fabrication caused residual stress and surface plexities of the human anatomy. In a typical hip simulator, composition after surface treatments. Evaluation tests of a single joint force was normally applied in one axis, these aspects are necessary for guaranteeing the final pro- offering a shear stress pattern similar to that of the human duct acceptance. body. Bowsher and Shelton [130] added a vertically mounted torque cell on the hip simulator to measure the 4.1 Wear tests changes in joint friction. The photographs and schematics of the simulator can be seen in Fig. 7. Such design pro- Pin-on-disk wear test system has been widely used to vided a better understanding of the influence of patient measure the friction coefficient and characterize the wear activity level on the tribological performances. In the response of the manufactured bioimplants, especially arti- aspect of knee simulators, since the traditional uniaxial and ficial knee and hip joints. Figure 6 shows the schematic of two-axis simulators resulted in too low wear rate values, a typical pin-on-disk system. Briefly, the pin moves biax- Saikko et al. [131] proposed a three-axis wear model which ially with a normal load applied on top. Either cyclical or implemented anterior-posterior translation (APT), inward- non-cyclical translating programs can be employed. outward rotation (IOR) and flexion-extension (FE). The Parameters such as pin and disk materials, normal load, principle of the simulator is shown in Fig. 8. Such ball-on- cycle frequency, sliding speed and lubricant can be judi- flat contact design has been successfully applied to study- ciously selected to replicate the actual tribological condi- ing the basic wear and frictions of metal-polymer and tions. The friction coefficient is calculated from the applied ceramic-polymer knee pairs [132, 133]. Again, attention normal load and the measured friction force. According to should be paid to the fact that the enhanced walking cycle, a recent pin-on-disk test conducted by Saikko [128], in such as ascending the stairs, is more aggressive than the order to avoid unrealistically low wear and friction values standard walking cycle, which may increase approximately caused by protuberance formation, the contact pressure 123 State of the art of bioimplants manufacturing: part II 149 Fig. 7 Photographs and schematics showing a IRC MTS 8-station hip joint simulator, b physiological test setup, c fully constraining socket fixture, d partially constraining socket fixture, e location of horizontal torque cells, and f direction of torque measured [130] Scratch test has long been recognized as a useful tool in emulating an individual deformation or removal event at micro/nanoscale [135]. The feasibility of using this method to estimate the wear debris-induced surface damage was verified by Dearnley [136]. Through adjusting scratching parameters, scratched marks with similar dimensions compared to the abrasion damages produced in vivo were achieved. In their later stage study, scratches were per- formed on coated/uncoated metallic biomaterials of stain- less steel and Co-Cr-Mo alloy. The results proved that the samples with TiN coating contributed to a greater tolerance to the scratch because the hard film hindered the deepening of the scratch. 4.2 Corrosion tests The most common method to examine the corrosion behaviour of bioimplants is via electrochemical techniques, where the manufactured bioimplants are soaked into a simulated body fluid (SBF). During corrosion testing, the electrochemical corrosion potentials and currents are con- Fig. 8 Principle of the ball-on-flat contact knee simulator, proposed tinuously recorded so that the electrochemical activity of by Saikko et al. [131] the bioimplants can be obtained. It was suggested that a 0.89% NaCl aqueous solution with a constant temperature 25% in both anterior-posterior shear force and external- of 37 C could create an environment similar to human internal rotation [134]. Therefore, an enhanced walking body [136]. The SBF is buffered to maintain a physiolog- cycle program should be applied during the test to improve ical pH value slightly above 7 [137, 138]. Hank’spoten- the wear prediction accuracy. tiodynamic polarization (Tafel solution is also extensively 123 150 C.-W. Kang, F.-Z. Fang employed. The detailed information of its composition can achieved after treatment. This paper reviewed both coating be found in Refs. [138, 139]. A three-electrode cell is and morphology processing techniques, whose advantages usually used to carry out the electrochemical studies. and disadvantages were described in comparison with each Quantitative assessments of corrosion include electro- other. Among various coating methods, plasma spraying is chemical impedance spectroscopy (EIS), potentiodynamic currently the most commonly implemented technique, polarization (Tafel analysis), open circuit voltage (OCV) while the relatively high cost and complexity of process and electrochemical noise (ECN) [140]. Among them, EIS involved simulated researchers to look for alternatives. is recognized as one of the most accurate electrochemical Laser-based technologies present themselves to be the most methods [140]. This is due to the fact that EIS requires advanced way in producing micro-groove structures on minimum AC signals, hence avoiding the perturbation on bioimplant surfaces. However, few studies have reported the electrochemical system and meanwhile reducing the the laser-based texturing of bioceramic surfaces, indicating errors. Additionally, valuable mechanistic information can that the development of this technology on biomedical be offered by this technique as the data are obtained from engineering is in its infancy and requires further studies. both electrode capacitance and charge-transfer kinetics. In terms of evaluating the manufactured surfaces, although standards are available to assess the wear and 4.3 Assessment of other properties corrosion performances of orthopaedic devices, variation always exists in the methodology adopted by different In terms of assessing other surface properties, X-ray research groups. Thus, it is necessary to develop a unani- diffraction (XRD) has been proved as a feasible technique mous evaluation system. Note that the existing simulators to measure the residual stress by many studies [141, 142]. are limited in providing in vitro approximations. Design A recent study conducted by Roy et al. [113] confirmed optimizations are required to guarantee that the complex- that XRD was practical on measuring the residual stress on ities of human anatomy are as practically similar as bioceramics after surface treatment process. Due to a possible. potential possibility of introducing foreign materials to the Acknowledgements The authors would like to thank Dr J. Zhang, Dr bioimplants during the surface treatment, XRD testing and N. Yu and Dr X. Li at University College Dublin for their valuable energy dispersive spectroscopic (EDS) analyses are usually discussions. Acknowledgments are also extended to the support of the undertaken to detect the elemental composition of the Science Foundation Ireland (SFI) (Grant No. 15/RP/B3208) and the modified surfaces [143]. With respect to measuring the National Science Foundation of China (Grant Nos. 51320105009 & 61635008). microhardness and elastic modulus of manufactured bioimplants, indentation test has been proved to be a robust Open Access This article is distributed under the terms of the technique [113, 144]. To be specific, the indentation pro- Creative Commons Attribution 4.0 International License (http://crea cess involves penetrating a sharp diamond tip into the tivecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give surface of workpiece, meanwhile continuously recording appropriate credit to the original author(s) and the source, provide a the imposed force and corresponding indentation depth. link to the Creative Commons license, and indicate if changes were The recorded load-displacement curve is useful in provid- made. ing insights into the mechanical behaviour of the deformed material. 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Advances in ManufacturingSpringer Journals

Published: Apr 25, 2018

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