Investigation of changes in the electrical properties of novel knitted conductive textiles during cyclic loading

Abstract

Abstract-Combining stainless steel with polyester fibres adds an attractive conductive behaviour to the yarn. Once knitted in such a manner, fabrics develop sensing properties that make the textiles, also known as e-textiles, suitable for smart/wearable applications. Structural deformations of the fibres (e.g. stretching) will cause changes in the conductivity of the fabric. This work investigates changes in the electrical properties exhibited by four knitted conductive textiles made of 20% stainless steel and 80% polyester fibres during cyclic loading. The samples were preconditioned first with 500 hundred cycles of unidirectional elongation and, after a rest interval, tested again for repeatability at the same conditions. In both cases the electrical behaviour stabilises after a few tens of cycles. In particular the repeatability test exhibited a considerably smaller settling time and a larger resistance due to the mechanical stabilisation and the loosening of the fabrics, respectively. It was found that the current provided to the fabrics affects the resistance measurements by decreasing the resistance value at which the samples become electrically stable. The reported findings present a valid method for the electrical characterisation of conductive textiles for use in further studies as a wearable technology.

Fatigue Testing of Wearable Sensing Technologies: Issues and Opportunities

“…To meet these desired properties, polymers including polytetrafluoroethylene (PTFE) [ 13 ], polyurethane acrylate (PUA) [ 80 ], polydimethylsiloxane (PDMS) [ 81 , 85 , 89 , 100 ], polyimide (PI) [ 82 , 83 ], poly(3,4-ethylenedioxythiophene) (PEDOT) [ 82 , 96 , 99 ], poly(styrenesulfonate) [ 82 ], silicone [ 88 , 90 ], thermoplastic urethane [ 99 , 103 ], ionogels [ 97 ], and hydrogels [ 21 , 91 ], knitted and woven textiles [ 76 , 90 , 95 , 98 , 104 ], overlock stitched textiles [ 78 ], conductive multifilaments that can be incorporated into textiles [ 92 ], polymer coated textiles [ 96 ], sensors embedded in apparel [ 12 ], carbon nanotubes (CNTs) [ 20 , 75 , 77 , 79 , 87 , 93 , 105 ], graphene [ 30 , 84 , 94 , 106 ], or combinations thereof are often used in conjunction with conductive materials.…”

“…Most studies that assess the fatigue of wearable stress sensors employ high-amplitude, low-cycle fatigue (LCF) of ≤ 10,000 cycles [ 107 , 108 ] that typically does not result in failure of the sensors [ 1 , 11 , 12 , 13 , 21 , 76 , 78 , 79 , 80 , 81 , 84 , 85 , 87 , 88 , 89 , 90 , 92 , 93 , 94 , 95 , 96 , 97 , 98 , 103 , 104 ]; therefore, the number of cycles to failure ( N f ) are unknown. To reiterate, the purpose of such studies is to demonstrate “proof of concept” for a given sensor, not to necessarily identify the mechanical properties of the materials used in the fabrication of the sensor; however, for physiologically based applications, such as cardiovascular monitoring or measuring the typical range of motion for a joint, the sensors need to withstand low-amplitude, high-cycle fatigue (HCF) [ 109 ].…”

“…Additionally, sensors utilizing thermoplastic polymers may begin to incubate cracks in early cycles that propagate into microstructurally small cracks (MSC) in later cycles, which could further propagate into the long crack regime [ 119 , 120 ], resulting in a drift of the signal or catastrophic failure of the sensor. In coated piezoelectric textiles, interfacial debonding of the conductive materials and fabric could occur at some point, while textiles knitted with conductive filaments may experience residual loop stretch [ 95 , 104 ] or fiber pullout during cycling. Additionally, the material and electrical fatigue properties of the sensors can be affected by their loading histories and exhibit both time and strain dependency [ 11 , 12 , 20 , 30 , 75 , 76 , 78 , 79 , 80 , 82 , 83 , 84 , 85 , 87 , 89 , 90 , 91 , 92 , 94 , 95 , 96 , 97 , 100 , 103 ].…”

“…Fibers constructed of AISI 316L (low-carbon) stainless steel and polyester were knitted into one of four jersey fabrics with varying wales, courses, stitch densities, and loop lengths to create stretch sensors [ 95 , 104 ]. Each sensor was subjected to six total tensile tests.…”

Comparative analysis of knitted pressure sensors

“…This research showed that sensors could be used for applications where mechanical movement caused stretching. Other study [14] has been carried out with four knitted conductive textiles during 500 cyclic loadings. It was discovered that after an initial preconditioning round of testing the stabilisation of the electrical behaviour was faster and more repeatable.…”