Creating smart textiles with carbon nanotube-based sensors

Engineers at the University of Delaware (UD) are developing next-generation smart textiles by creating flexible carbon nanotube composite coatings on a wide range of fibres, including cotton, nylon and wool. Fabric coated with this sensing technology could be used in future ‘smart garments’, according to the researchers, with the sensors slipped into the soles of shoes or stitched into clothing for detecting human motion.

Nerve-like electrically conductive nanocomposite coatings are created on the fibres using electrophoretic deposition (EPD) of polyethyleneimine functionalised carbon nanotubes. According to Associate Professor Erik Thostenson, who directs UD’s Multifunctional Composites Laboratory, “The films act much like a dye that adds electrical sensing functionality.

“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.”

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, with Assoc Prof Thostenson and co-author Sagar Doshi reporting in the journal ACS Sensors the ability to measure a wide range of pressure — from the light touch of a fingertip to being driven over by a forklift.

“As a sensor, it’s very sensitive to forces ranging from touch to tons,” said Assoc Prof Thostenson.

These sensors can be added to fabric in a way that is superior to current methods for making smart textiles, with Assoc Prof Thostenson noting that existing techniques — such as plating fibres with metal or knitting fibre and metal strands together — can decrease the comfort and durability of fabrics. By contrast, the nanocomposite coating 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 to 750 nm thick — about 0.25 to 0.75% as thick as a piece of paper — and would only add about a gram of weight to a typical shoe or garment. What’s more, 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.

One potential application of the sensor-coated fabric is to measure forces on people’s feet as they walk — data which could help clinicians assess imbalances after injury or help to prevent injury in athletes. Specifically, Assoc Prof Thostenson’s research group is collaborating with Professor Jill Higginson, 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.

“One of our ideas is that we could utilise these novel textiles outside of a laboratory setting — walking down the street, at home, wherever,” said Assoc Prof Thostenson. During lab testing, people know they are being watched, but outside the lab, behaviour may be different.

Associate Professor Erik Thostenson demonstrates how a sensor could be placed inside a shoe to measure foot pressure. Image credit: Kathy F Atkinson.

Co-author Doshi, a doctoral student in mechanical engineering, has worked on optimising the sensors’ sensitivity, testing their mechanical properties and integrating them into sandals and shoes. Having worn the sensors himself in preliminary tests, he can confirm that 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 said. “This data could be analysed later by researchers or therapists to assess performance and ultimately bring down the cost of health care.”

The technology could also be promising for sports medicine applications, post-surgical recovery and for assessing movement disorders in paediatric populations. The latter application would be particularly useful for Robert Akins, Director of the Center for Pediatric Clinical Research and Development at the Nemours/Alfred I. duPont Hospital for Children and an affiliated professor at UD, who noted, “It can be challenging to collect movement data in children over a period of time and in a realistic context.

“Thin, flexible, highly sensitive sensors like these could help physical therapists and doctors assess a child’s mobility remotely, meaning that clinicians could collect more data, and possibly better data, in a cost-effective way that requires fewer visits to the clinic than current methods do,” said Akins.

Top image caption: Sagar Doshi (left) and Associate Professor Erik Thostenson test an elbow sleeve outfitted with one of their novel sensors. Image credit: Kathy F Atkinson.