Their discovery is reported in the journal ACS Sensors where they demonstrate the ability to measure a wide range of pressure – from the light touch of a fingertip to being driven over by a forklift.
Fabric coated with the sensing technology could be used in future ‘smart garments’ where the sensors are slipped into the soles of shoes or stitched into clothing for detecting human motion. Carbon nanotubes give the light, flexible, breathable fabric coating impressive sensing capability. When the material is squeezed, large electrical changes in the fabric are easily measured.
“As a sensor, it’s very sensitive to forces ranging from touch to tons,” says Erik Thostenson, an Associate Professor in the Departments of Mechanical Engineering and Materials Science and Engineering.
Nerve-like electrically conductive nanocomposite coatings are created on the fibres using electrophoretic deposition (EPD) of polyethyleneimine functionalised carbon nanotubes.
“The films act much like a dye that adds electrical sensing functionality,” explains Thostenson. “The EPD process developed in my lab creates this very uniform nanocomposite coating that is strongly bonded to the surface of the fibre. The process is industrially scalable for future applications.”
Researchers are said to be able to add these sensors to fabric in a way that is superior to current methods for making smart textiles. Existing techniques, such as plating fibres with metal or knitting fibre and metal strands together, can decrease the comfort and durability of fabrics, reports Thostenson, who directs UD’s Multifunctional Composites Laboratory. The nanocomposite coating developed by Thostenson’s group is flexible and pleasant to the touch and has been tested on a range of natural and synthetic fibres, including Kevlar, wool, nylon, Spandex and polyester. The coatings are just 250-750 nm (about 0.25-0.75% as thick as a piece of paper) and would only add about a gram of weight to a typical shoe or garment. The materials used to make the sensor coating are inexpensive and relatively eco-friendly, since they can be processed at room temperature with water as a solvent.
Exploring future applications
One potential application of the sensor-coated fabric is to measure forces on people’s feet as they walk. This data could help clinicians assess imbalances after injury or help to prevent injury in athletes. Thostenson’s research group is collaborating with Jill Higginson, Professor of Mechanical Engineering and Director of the Neuromuscular Biomechanics Lab at UD, and her group as part of a pilot project funded by Delaware INBRE. Their goal is to see how these sensors, when embedded in footwear, compare to biomechanical lab techniques such as instrumented treadmills and motion capture.
Sagar Doshi, a doctoral student in mechanical engineering at UD, is the lead author on the paper. He worked on making the sensors, optimising their sensitivity, testing their mechanical properties and integrating them into sandals and shoes. He has worn the sensors in preliminary tests, and so far, the sensors collect data that compares with that collected by a force plate, a laboratory device that typically costs thousands of dollars.
“Because the low-cost sensor is thin and flexible the possibility exists to create custom footwear and other garments with integrated electronics to store data during their day-to-day lives,” Doshi notes. “This data could be analysed later by researchers or therapists to assess performance and ultimately bring down the cost of healthcare.”
This technology could also be promising for sports medicine applications, post-surgical recovery, and for assessing movement disorders in paediatric populations.
Thostenson’s research group also uses nanotube-based sensors for other applications, such as structural health monitoring.
“We’ve been working with carbon nanotubes and nanotube-based composite sensors for a long time,” reports Thostenson, who is affiliated faculty at UD’s Center for Composite Materials (UD-CCM). Working with researchers in civil engineering his group has pioneered the development of flexible nanotube sensors to help detect cracks in bridges and other types of large-scale structures.
“One of the things that has always intrigued me about composites is that we design them at varying lengths of scale, all the way from the macroscopic part geometries, an airplane or an airplane wing or part of a car, to the fabric structure or fibre level. Then, the nanoscale reinforcements like carbon nanotubes and graphene give us another level to tailor the material structural and functional properties. Although our research may be fundamental, there is always an eye towards applications. UD-CCM has a long history of translating fundamental research discoveries in the laboratory to commercial products through UD-CCM’s industrial consortium.”