TY - JOUR
AU - Kumar, Vikram, S
AB - Physicists have long struggled to understand the quirky material vanadium dioxide (VO2). At temperatures >67 °C this metal oxide acts as a conductor. Cool it to a moderate temperature, however, and VO2 quickly rearranges its interior structure to become an insulator. And we are talking quick: Some have estimated the transition to happen in a trillionth of a second in a controlled environment. As researchers have increased their understanding of this inorganic compound, which changes so abruptly with respect to both its structure and its conductivity, some have looked for ways—from electronics to optics—to take advantage of the uniqueness of VO2. We review a new application of VO2 and its potential clinical impact: as a microactuator. What Is the Innovation? Scientists are no strangers to actuators. Piezoelectrics, differential thermal expansion, and shape-memory alloys—all are used to explore ways to transduce an external stimulus into mechanical action. The investigators in a new proof-of-concept study have proposed that VO2 be the next actuator possibility on everyone's list. While studying the compound's dual-phase transition, a team of researchers led by Junqiao Wu at the University of California, Berkeley, came to realize that the structure moves quite dramatically during its phase transition: a 100-μm VO2 wire consistently shrank by about 1 μm in length. According to the researchers, this 1% change in length is much greater than the approximately 0.005% observed with regular thermal-expansion materials. Given the extent of the shrinkage, the speed of the transition, and the energy produced in the metal–insulation phase transition, the researchers calculated that a work density of 7 J/cm3 was “theoretically possible by using VO2 as the driving material.” The output of the muscles of the body appears to be a miniscule 0.008 J/cm3 by comparison. Kevin Wang Open in new tabDownload slide Open in new tabDownload slide “We are excited by the sheer amplitude of it,” says third-year graduate student Kevin Wang, author on the report and part of the team that continues to work on the project. “Normally, things that are not hinged do not bend that much.” The team therefore used lithography and etching processes to manufacture a VO2-based, chromium-cladded bimorphic microactuator only a few hundred nanometers thick. “The devices exhibit superior performance by nearly all metrics,” the authors write in their December 2012 Nano Letters report on the topic. “They offer the highest displacement-to-length ratio in the sub-100 μm length scale.” The team's proof-of-concept design was tested with thermal, electrothermal, and photothermal stimuli—under both ambient and aqueous conditions. Because the devices were lithographically fabricated, the design allowed the team to “achieve different geometries,” something, the team writes, that has not previously been seen with VO2 microscale actuator designs. How Does It Work? When VO2 goes through its phase transition, its lattice structure encounters a change along the 3 axes, 2 of which expand while the third shrinks. This axial shrinking is used in the microactuator, and it can be used to convert a stimulus into motion. The team found that to capitalize—and capture—this shrinkage requires a film of chromium and VO2 be laid down on a silicon substrate with the silicon dioxide layer on the top (silicon dioxide/silicon substates). Silicon is a key part in the fabrication, according to the report. When the VO2 film is deposited, the atoms arrange as a texture on the surface of the amorphous silicon, with the shrinking axis lying in the plane of the substrate. If such alignment did not occur, the group explains, the motion of the shrinking axis would be canceled out by the expansion along the other 2 axes, and no energy would be converted into mechanical motion. The team reported that their careful fabrication allowed a displacement of 36 μm for a 60-μm bimorph (a displacement-to-length ratio of 0.6), and as much as 73 μm for an 80-μm bimorph (a ratio of approximately 0.9). Existing technology has been limited to a displacement-to-length ratio of 0.4. Through repeated increasing of the temperature from 0 °C to 80 °C, the microactuator can be made to repeatedly furl and unfurl (in part because the team uses a thin multicrystal VO2 film grown on substrates, the temperature of the phase transition can be shifted and extended anywhere between 65 °C and 80 °C; Fig. 1). Submit this microactuator to an electric current (as small as 1.4 V), and it can go through “tens of thousands of actuation cycles in air without noticeable degradation,” write the authors. The figure shows an actuator palm structure in the closed state at room temperature to 65 °C (left panel) and in the open state at 80 °C (right panel). Fig. 1. Open in new tabDownload slide Fig. 1. Open in new tabDownload slide Moreover—and perhaps one of the most fanciful parts of the design—the fabrication techniques allow actuators to be made into multiple shapes, from bimorphs to a comb-like structure (Fig. 2) to a radial design that resembles a palm with 8 symmetrically extending fingers (Fig. 3). This last design can be made so that the fingers clench and unclench individually or all at once—much like a claw grabbing prizes in a penny arcade game. A comb-like array of parallel cantilevers. Fig. 2. Open in new tabDownload slide Each cantilever can bend with a displacement nearly equal to its length when the temperature changes. Fig. 2. Open in new tabDownload slide Each cantilever can bend with a displacement nearly equal to its length when the temperature changes. An 8-fingered palm structure at room temperature. Fig. 3. Open in new tabDownload slide The palm is closed at room temperature but opens when the temperature increased from 67 °C to 80 °C. Fig. 3. Open in new tabDownload slide The palm is closed at room temperature but opens when the temperature increased from 67 °C to 80 °C. Where Can This Technology Fit in the Laboratory? The team is not short on ideas for actuator applications, from the ordinary—optics, microfluidics, drug delivery—to what may seem fantastical—thermal energy harvesting, or a robotic-like insect that crawls on the floor on actuator-powered legs. None of such applications have moved beyond the proof-of-concept stage, however. We checked in with Sam Sia at Columbia University to get his take on this technology. Sia got his start at George Whiteside's laboratory at Harvard and became famous for his work with microfluidics for point-of-care devices. Samuel Sia Open in new tabDownload slide Open in new tabDownload slide “This is a very nice piece of scientific work demonstrating that a new alloy material can achieve different force characteristics than traditional materials,” said Sia. “To make real impact, applications will have to be identified which can benefit from such microactuators.” The team believes that one such application is microfluidics. In aqueous solutions, the tiny device appears to have response speeds >17 ms. The team suggests the product might be used for “high-speed” microfluidics. Because of the device's lack of hinges, it provides a tight seal, and because the device can be made to open and shut, it allows scientists greater control over valves, pumps, and liquid flow through channels. Perhaps another foreseeable use is making good on one of the actuator's most interesting geometries—fabrication into claws. One can envision such a “catch-and-release” actuator being used in nanoparticle drug therapy, according to Wang. “When closed, the actuator could be used to seal stuff in, trapping a drug particle within the claw's grasp,” says Wang. “And then it could be used for controlled delivery.” For example, it could be opened for 30 s to release some of what is in its grasp, be shut, and then be opened again later. To be of such use would require the actuator creators to do something that is on their short-term to-do list and is prominent in Sia's analysis of the device. “On the near-term horizon, it may be interesting to see if such actuators can improve micromanipulators which are being used to study cell behavior and also in reproductive health,” says Sia. To work in a biological system would require that activation temperatures be reduced to biologically suitable temperatures. This goal may actually be within reach: Over the last decade, teams have shown that tungsten doping can push the critical VO2 temperature to as low as −13 °C (3). Such results would help this proof-of-concept technology move from science project to a real scientific—and clinical—tool. " Author Contributions:All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article. " Authors' Disclosures or Potential Conflicts of Interest:No authors declared any potential conflicts of interest. © 2013 The American Association for Clinical Chemistry This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Micromanipulation through Chemical Play
JF - Clinical Chemistry
DO - 10.1373/clinchem.2013.203455
DA - 2013-05-01
UR - https://www.deepdyve.com/lp/oxford-university-press/micromanipulation-through-chemical-play-iFL9oSZ0jm
SP - 862
VL - 59
IS - 5
DP - DeepDyve
ER -