Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences and the Wyss Institute for Biologically Inspired Engineering have developed a resilient strain sensor that can be embedded into textiles or soft robotic systems.
The idea of e-textiles, or smart textiles/smart fabrics, is to weave electronics and sensors into ordinary clothing to monitor health conditions, power portable devices or monitor biometric conditions of military soldiers and the clothing involved could include grafting sensors to t-shirts or jackets, pants or hats.
A problem surrounding e-textiles is that clothing gets dirty. People stretch t-shirts, spill food on pants and stuff them in a hamper bound for a washer and dryer. If smart textiles are to work, they will have to survive all the same types of abuse normal clothing endures.
"Current soft strain gauges are really sensitive but also really fragile," said Oluwaseun Araromi, a research associate at Harvard SEAS. "The problem is that we're working in an oxymoronic paradigm — highly sensitivity sensors are usually very fragile and very strong sensors aren't usually very sensitive. So, we needed to find mechanisms that could give us enough of each property."
In essence, what researchers created looks and behaves like the children’s toy Slinky.
"A Slinky is a solid cylinder of rigid metal but if you pattern it into this spiral shape, it becomes stretchable," Araromi said. "That is essentially what we did here. We started with a rigid bulk material, in this case carbon fiber, and patterned it in such a way that the material becomes stretchable."
The overall electrical conductivity of the sensor changes as the edges of the patterned carbon fiber come out of contact with each other, similar to the way the individual spirals of a slinky come out of contact with each other when you pull both ends.
Current stretchable sensors use exotic materials such as silicon or gold nanowires. Harvard’s sensor does not use special manufacturing techniques or even a clean room and could be made using any conductive material.
To test the sensor, researchers stabbed it with a scalpel, hit it with a hammer, ran it over with a car and washed it 10 times. The sensor came through unscathed.
Researchers then embedded the sensor in a fabric arm sleeve and asked a researcher to make different gestures with their hands including a fist, open palm and pinching motion. The sensors were able to detect changes in the subject’s forearm muscle through the fabric and a machine learning algorithm was able to classify these gestures.
A sleeve such as this could be used in anything from virtual reality and sportswear to clinical diagnostics for diseases.
"The combination of high sensitivity and resilience are clear benefits of this type of sensor," said Robert Wood, the Charles River Professor of Engineering and Applied Sciences at SEAS. "But another aspect that differentiates this technology is the low cost of the constituent materials and assembly methods. This will hopefully reduce the barriers to get this technology widespread in smart textiles and beyond."
The full research can be found in the journal Nature.