Nondrying, Sticky Hydrogels for the Next Generation of High-Resolution Conformable Bioelectronics
Francisca M. Carvalho, Pedro Alhais Lopes, Manuel Reis Carneiro, Arménio C. Serra, Jorge F. J. Coelho, Anı́bal T. de Almeida, Mahmoud Tavakoli
Abstract
This work addresses two well-known problems of tissue-interfacing hydrogels, that is, rapid drying and skin adhesion. These are considered as the main barriers against the wider application of hydrogels in many fields, including wearable bioelectronics. We demonstrate for the first time a hydrogel with a double solvent that is softer than the skin (compression Young’s modulus = 3.6 kPa), yet highly stretchable (>500%), nondrying, skin-adhering, with a skin–electrode impedance lower than the gold-standard Ag/AgCl electrodes, skin-friendly, and transparent. This combination of properties has not been demonstrated before but is necessary for widespread use of hydrogel electrodes. Various formulations of the double-network glycerol–polyacrylamide hydrogel are fully characterized in order to enhance skin adhesion, softness, liquid content retention, and signal-to-noise ratio. We then demonstrate a double-layer stretchable e-textile architecture, which embeds a large number of these electrodes, as a multinode gateway for high-resolution bidirectional data exchange between the body and bioelectronics and show proof-of-concept applications in functional neuromuscular electrostimulation of the forearm and wearable monitoring of brain, heart, and facial expressions.