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(Nanowerk News) Think about your favorite t-shirt, the one you’ve worn a hundred times, and all the abuse you’ve suffered. You’ve washed it more times than you remember, poured it over, stretched it, crumbled, maybe even burned it resting on the stove once.
We put a lot into our clothes and if the smart fabrics of the future will survive everything we give them, their components will have to be durable.
Now, researchers at Harvard John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering have developed an ultra-sensitive and seriously resilient strain sensor that can be incorporated into soft robotic fabrics and systems.
The research is published in Nature (“Conformal ultra-sensitive and resilient strain gauges for soft machines”).
?? Current soft strain gauges are very sensitive but also very fragile, ?? said Oluwaseun Araromi, an associate researcher in Materials Science and Mechanical Engineering at SEAS and the Wyss Institute and first author of the paper. ?? The problem is that we are working in an oxymoron paradigm ?? high-sensitivity sensors are generally very fragile and very strong sensors are usually not very sensitive. So, we had to find mechanisms that could give us enough of each property. ??
Eventually, the researchers created a design that looks and behaves very much like a Slinky.
?? A Slinky is a solid rigid metal cylinder but if you change it to this spiral shape, it becomes extensible, ?? Araromi said. ?? This is essentially what we have done here. We started with a rigid bulk material, in this case carbon fiber, and molded it in such a way that the material became elastic. ??
The pattern is known as a serpentine meander, because its sharp ups and downs are reminiscent of a snake’s crawl. The patterned conductive carbon fibers are then sandwiched between two predetermined elastic substrates.
The overall electrical conductivity of the sensor changes as the edges of the molded carbon fiber come out of contact with each other, similar to how the individual coils of a slinky detach from each other when pulled both ends. This process occurs even with small amounts of effort, which is key to the sensor’s high sensitivity.
Unlike today’s highly sensitive stretchable sensors, which are based on exotic materials such as silicon or gold nanowires, this sensor does not require special manufacturing techniques or even a clean room. It could be made using any conductive material.
The researchers tested the sensor’s resilience by stabbing it with a scalpel, hitting it with a hammer, hitting it with a car and throwing it in the washing machine ten times. The sensor came out unscathed from every test.
To demonstrate its sensitivity, the researchers incorporated the sensor into a fabric sleeve and asked a participant to make several hand gestures, including a fist, open palm, and pinch motion. The sensors detected the small changes in the subject’s forearm muscle through the tissue, and a machine learning algorithm was able to successfully classify these gestures.
?? These mechanical resilience and robustness features put this sensor in a whole new field, ?? Araromi said.
Such a case could be used in anything from virtual reality simulations and sportswear to clinical diagnostics for neurodegenerative diseases such as Parkinson’s disease.
Harvard’s Office of Technology Development has filed to protect the intellectual property associated with this project.
?? The combination of high sensitivity and resilience are clear advantages of this type of sensor, ?? said Robert Wood, Charles River professor of engineering and applied sciences at SEAS and senior author of the study. ?? But another aspect that differentiates this technology is the low cost of the constituent materials and assembly methods. Hopefully, this will lower the barriers to getting this technology widespread in smart textiles and beyond.
?? We are currently exploring how this sensor can be integrated into clothing thanks to the intimate interface with the human body it provides, ?? says Conor Walsh, Paul A. Maeder Professor of Engineering and Applied Sciences at SEAS and co-author of the study. “This will enable exciting new applications by being able to make biomechanical and physiological measurements throughout a person’s day, not possible with current approaches.”
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