E-Textiles Network Webinar – Stretchable Inks for Wearable Applications
The inks used to create printed electronics for wearable technology, especially if it is to be attached directly to garments, must be able to bend and stretch to conform to the wearers body and to accommodate the movement of the wearer. The performance of a conductive ink consisting of a thermoplastic polyurethane and a blend of functionalised nano carbons developed for application to high stretch garments has been fully characterised from its viscoelastic properties as an ink through to the impact of stretching and flexing the printed ink film.
The inks were subjected to full rheological testing including shear flow measurements and viscoelastic measurements using Small Amplitude Oscillatory Shear (SAOS). Extension tests were undertaken using a Hounsfield Tensile testing machine whilst the effect of this extension on the electrical resistance of the sample was simultaneously monitored. The inks were strained to max extensions and cyclic strains to 10% and 100%, compared with the nominal strains of 15 – 30% typically seen in the normal distortions of clothing.
The addition of the nano carbons to the resin increases the elastic properties of the inks and has a direct impact on the printability. The electrical conductivity of the ink is unaffected by flexing without stretch. The inks survived to maximal strains greater than 300% which is far greater than typically experience in wearable applications. Following an initial stretch cycle that increased the resistance of the ink the ink and substrate exhibited consistent resistance change with extension. This repeatable performance would allow for predictability of ink performance in wearable applications. It also suggests the potential for use as a low resistance strain sensor. Beyond this point there is an increasing loss of conductive paths as the material permanently deforms and there is an exponential increase in resistance. The ink was pulled to maximum extension, becoming non-conductive at nearly 350% strain.
Recent work to develop hybrid inks which contain a blend of other conductive materials including flake and nano particle silver conductive inks, as well as silver coated copper flakes, was also discussed.
Recording of webinar: