While the coating of fibres with conductive compounds is a regular trick used in academia to craft flexible pressure piezoresistive sensors (with resistance decreasing when pressure is applied), the researchers toughened up the game with a specially developed deposition process that maintains the fibres’ original compliance yet makes very durable and reliable pressure sensors even under repeated flex.
Instead of merely blending a conductive compound into a polymer used to coat the fibres or to draw them, the paper “Thin and Flexible Carbon Nanotube-Based Pressure Sensors with Ultrawide Sensing Range” published in the ACS Sensors journal details a scalable electrophoretic deposition (EPD) process, whereby functionalized multi-walled carbon nanotubes (positively charged in a water-based dispersion) are deposited under a direct current electric field to grow a uniform coating on a fabric strapped to the cathode.
More specifically, the nanotubes are functionalized with the dendritic polyelectrolyte polyethyleneimine (PEI) which forms covalent bonds with the oxide groups on the nanotube surface as well as functional groups on the fibres’ surfaces, the paper reports. This PEI functionalization also acts as a polymer matrix, creating a porous, flexible, and electrically conductive nanocomposite film less than 750nm thick, on the treated textile.
This tight chemical integration means that pressure sensors built with such coated fibres are not subject to physical damage even under repetitive flex or high pressure loads, unlike alternative solutions.
The authors report a very wide range of pressure sensing, ranging from 0.0025 to 40 MPa, outperforming any other reported fabric-based sensors, with a sensor response fairly linear at low pressures and only becoming nonlinear at high pressures.
As the paper explains, the initial linear response comes from the compressibility of the fabric at low pressures (putting more fibres into contact), while at higher pressures, the piezoresistive response of the actual carbon nanotubes come into play, with nanotube to nanotube tunnelling resistance decreasing locally under local compression at the fibre-fibre crossovers.
Comparative tests performed with carbon fibre-based and dip-coated fabric sensors illustrated how physical damage permanently affected other sensors’ output, with a sharp resistance transition near 2 MPa, with increasing resistance despite increasing pressure. The researchers also tested their fabric sensors under cyclic loads, up to 550 cycles from 0 to 5.2 MPa, observing a highly repeatable sensing response for all cycles, without any permanent resistance change.
Due to their very wide sensing range, the same fabric piezoresistive sensor implemented in a glove or a shirt could be used to detect pressures in the tactile range (<10 kPa), or it could be fitted in a sole to make measurements in the body weight range (about 500 kPa). The author ran over one sample with a forklift truck to test their sensor at up to 40 MPa.
The water-based process can be applied to many types of fibres and textiles and the sensors’ realtime response make them suitable for many applications such as gait analysis (when integrated in footwear) or body movement analysis.
“We have filed for a patent with the help from the resources that our University provides. Since there are huge potential applications for these sensors in healthcare, biomedical devices and the sporting industry among, we are contemplating commercialization either spinning out a startup and/or working with other companies to conduct further R&D specific for an application and then ultimately license the technology”, lead author of the paper Sagar Doshi told eeNews Europe.
“We have also been contacted by people from the aviation and the automobile industry” he added.
University of Delaware – www.udel.edu