Building machines that can detect gravitational waves has brought wider industrial benefits. Sue Bowler looks at two of the spin-offs: gravimeters and nanokicking for cell research. The first gravitational wave detection, in 2015, marked decades of persistent and determined effort in science, technology, engineering and IT. That 50-year effort is now beginning to bring a return on the considerable long-term research investment in terms of its impact on other fields: advances in fabrication, measurement and associated technologies are already spreading to other sciences and to medicine and industry. 1 View largeDownload slide Technology developed to detect gravitational waves is finding applications in industry. (LIGO/T Pyle) 1 View largeDownload slide Technology developed to detect gravitational waves is finding applications in industry. (LIGO/T Pyle) The technology that has detected gravitational waves is simple, in principle, but not in practice. LIGO uses interferometers to measure the distortions of spacetime as a gravitational wave passes – a strain of 10−21. The LIGO interferometers measure the distances travelled by light in two perpendicular directions using lasers within the 4 km long arms of each interferometer. Achieving any detection demands a full undertsanding of the ever-present noise, whether that noise is seismic, thermal, quantum or technical in origin. There are two interferometers in the US, allowing identification of signals originating outside the complex machinery of either detector. Having two or more detectors also means that noise signals affecting only one instrument can be isolated, whether internal or external in origin. The system is designed so that seismic noise dominates at low frequencies, thermal noise from atomic motion at intermediate frequencies and shot noise – variations within the laser beam – at higher frequencies, giving the detectors their maximum sensitivity between 40 Hz and 6 kHz. There is also electronic noise – glitches – from the many components and the more than 200 000 monitoring channels needed to operate these complex machines. While there is considerable potential for applications of LIGO technologies, two areas are already benefiting: gravimeters and bone grafts. Building better gravimeters The LIGO detector environments are monitored to high precision. Livingston in Louisiana, for example, is in a swampy region. Changes in the water levels around the detector subtly alter the mass distribution in the subsurface, for example, and noise arising from such changes in the gravity vector locally have to be measured and removed from the data. But, as Giles Hammond of the University of Glasgow points out: “What is noise to us is signal to somebody else.” Detecting small variations in the local gravitational field opens up a range of applications in fields from resources to renewable energy. Hammond is using the suspension technology developed for seismic isolation in the LIGO interferometers to build better gravimeters, for use in hydrocarbon exploration, civil engineering and natural hazard mitigation. The gravitational field changes as a result of variations in subsurface density. This can arise in real time from movement of molten rock below a volcano as a precursor to eruption, for example; spatial variations can indicate the location of mineshafts, ore bodies or sinkholes in the subsurface. Most gravimeters currently in use are based on a mass on a spring: they measure the change in the local gravitational attraction, g. While sensitive, these instruments are also delicate and expensive, costing about £70 000 each; it is not feasible to consider using a network of 10 or 20 of them to monitor an active volcano, for example. But inertial accelerometers, found in modern mobile phones, offer another way. Smartphones already work as simple seismometers, thanks to their microelectromechanical systems (MEMS). Hammond is part of the team developing a MEMS gravimeter between 10 000 and 100 000 times more sensitive than a phone, to measure g to 40 parts per billion in an integration time of 1 second (Middlemiss et al. 2016). The MEMS gravimeter uses the same “mass on a spring” technology, but uses gravitational wave technology to combine a relatively large mass with soft springs, to boost the sensitivity. Hammond works on stabilizing the suspension system for the LIGO mirror masses; for the gravimeter he has applied fused silica technology to give the MEMS device very high stability. The new gravimeter is etched from a single silicon wafer and consists of a test mass of 0.02 mg, held on three geometric “anti-springs”, 7 μm thick silicon flexures that work like springs but get softer at high loads (figure 2). The stability of the MEMS inertial accelerometers means that they can read variations in g of ∼250 μGal (to 300 parts per billion), enough to detect Earth tides, which change the planet's radius by between 30 and 40 cm per day. 2 View largeDownload slide The MEMS device, etched from a single piece of silicon, consists of a central oblong proof mass suspended from three arched springs only 7 μm in width. It is the first MEMS accelerometer that can be classed as a gravimeter, thanks to its stability. 2 View largeDownload slide The MEMS device, etched from a single piece of silicon, consists of a central oblong proof mass suspended from three arched springs only 7 μm in width. It is the first MEMS accelerometer that can be classed as a gravimeter, thanks to its stability. The Glasgow team is working on making the device suitable for fieldwork and autonomous operation. They hope for a device about the size of an apple, able to log data autonomously in the field, and cheap enough to deploy by the dozen. Their prototypes have been tested in a lift and on Scottish hills. Their small size and mass makes them also suitable for use in small satellites. Clyde Space Ltd is interested in using them for altitude control in CubeSats, for example. They are also being examined by QinetiQ for underwater mapping, and to track reservoir drainage in pumped storage systems to within a few metres. The MoD's Defence Science and Technology Laboratory is investigating their use on drones for aerial surveys. Good vibrations The measurement technology needed to build and operate the LIGO interferometers also has uses in the medical field, in experimental work examining how fine-scale vibration can promote cell growth. Stuart Reid, professor of biomedical engineering at the University of Strathclyde, and formerly at the University of the West of Scotland, says: “Having spent 15 years working in astrophysics and gravitational wave detection with LIGO, it is amazing to see technology arising that could revolutionize key aspects of tissue engineering and regenerative medicine. The team is hard at work to get the technology ready for the first human trials, and to get devices into other labs around the UK and further afield.” The gravitational wave input comes from the precision measurement technology developed for LIGO mirror coatings. In the 1960s, biological research found that cells responded to the physical shape of the environment in which they grew. Intriguingly, cells typically 100 μm across appeared to use or sense features that were smaller than the cell itself, perhaps only 100 nm in size. The cell membrane appeared to “ruffle”, says Reid, as the cells appeared to probe the surface at the nanoscale. This raised the question of whether changing the environment by nanoscale vibrations could therefore change cell behaviour. A bioscience team at the University of Glasgow, led by cell biologist Matthew Dalby, wondered if LIGO measurement technology could help. Reid and Dalby, together with bioengineer Manual Salmerón-Sanchez and their research team, found that “nanokicking” – exposing adult stem cells to 20 nm scale vibrations at 500 Hz – produced a shape change (figure 3). “The cells were reaching out, stretching and grabbing the surface around them,” says Reid. “The biologists were very excited by this, because it looked like the cells were forming new bone in response to the hard shapes around them.” And they were doing so faster than usual. 3 View largeDownload slide Fluorescent image of mesenchymal stem cells. The stretched-out structure of the cell is indicative of differentiation to osteoblasts (bone building cells). Nucleus is shown in blue with the cytoskeleton shown in red. (Prof. M Dalby, Univ. Glasgow) 3 View largeDownload slide Fluorescent image of mesenchymal stem cells. The stretched-out structure of the cell is indicative of differentiation to osteoblasts (bone building cells). Nucleus is shown in blue with the cytoskeleton shown in red. (Prof. M Dalby, Univ. Glasgow) The boosted growth rates were something of a big deal. Bone cells can be cultured for grafts using engineered scaffolds – expensive – or by chemical means – potentially toxic. Using measurement techniques developed for the gravitational wave project, Reid and his team were able to show that it worked in 3D too, producing mineralized structures that looked like healthy bone. How it works is another matter, and a subject of active research. Reid suspects it may be to do with pressure and the effects of vibration on folded proteins. The success of these experiments led to the establishment of a company, NanoKick Technologies, to develop the ideas. It is currently focusing on developing bioreactors with a view to producing bone grafts for victims of landmines, in association with the landmine charity Find a Better Way. They hope to use a bone scaffold to give a larger volume, filled with stem cells undergoing nanokicking to grow bone for grafts (Tsimbouri et al. 2017). There is also the possibility of using the techniques for drug discovery and pre-clinical testing – thanks to curiosity, enterprise and the precision engineering needed for LIGO. If you were intending either to build a better gravimeter, or grow better bone grafts, it is unlikely that you would start by trying to detect gravitational waves. But both the applications described here have come about as a result of trying to do something new, and recognizing opportunities to use the developments more widely. Gravitational wave discoveries are already changing the astrophysical landscape and will do so much more in the future; gravitational wave technology looks set to do the same. REFERENCES Middlemiss R Pet al. 2016 Nature 531 614 CrossRef Search ADS PubMed Tsimbouri P M 2017 Nature Biomedical Engineering 1 758 CrossRef Search ADS © 2018 Royal Astronomical Society
Astronomy & Geophysics – Oxford University Press
Published: Apr 1, 2018
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