A Breathable and Washable Smart Fabric for Pressure Sensing ^†

The skin is an important organ that informs human beings of small perceptual changes in the body, resulting from various pathologies. Smart textiles appeared as promising substrates that can envelop the skin to monitor health, and among different transduction methods, be used as interlocutors to detect changes in the body, such as pressure. However, the skin is so sensitive to external stimuli that it can be affected by them to the point of cutaneous wound development. That said, a balance needs to be struck between thermophysiological/sensorial comfort requirements and the electrical properties of smart textiles. Therefore, this study aims to functionalise a 100% cotton woven fabric and grant it the ability to detect changes in body pressure, maintaining substrate characteristics such as touch and permeability. The piezoresistive behaviour was chosen as the means of obtaining an electromechanical response, using graphene nanoplatelets (GNPs) as the conductive nanomaterial. Three different concentrations (2%, 3%, and 5% w/v) of GNPs were tested to dope both sides of the textile substrate using a knife-coating technique.

Cotton woven fabrics with and without GNPs were characterised by Field Emission Scanning Electron Microscopy, Ground State Diffuse Reflectance, and Raman. For the electrical properties, the samples were submitted to tests of the linear electrical surface resistance of textiles, according to the standard DIN EN 16812, two-point method. To evaluate the piezoresistive behaviour, a Shimadzu-AG-IS universal testing machine was used, to exert a 1-mm compression on the Z-axis for 10 cycles. Regarding the comfort properties of the samples, tests of friction coefficient, air, and water vapour permeability were carried out. In addition, the samples were subjected to 20 washing cycles, under the ISO 6330: 2012 standard and their surface linear electrical resistance was analysed.

The work developed resulted in fabrics being functionalised with GNPs, whose coating process changed, but did not significantly compromise, the touch and breathability of the fabric itself, important requirements for the comfort of a garment. From 3% GNPs, the resistance values of semiconductor materials were identified in the order of kΩ and hΩ. The electrical resistance decreased with the increase in displacement (mm) for all GNP concentrations. Such behaviour can be explained by the increase in the contact surface between the GNPs and the polymer matrix when pressure is applied, thus reinforcing the conductive network, which results in a reduction in electrical resistance. In other words, all three samples showed piezoresistive behaviour, and the sample with 5% GNPs reached a superior Gauge Factor of 15.75. These results are promising and indicate that the functionalised substrates can be applied in a pressure sensor system for health monitoring.