Biodegradable microneedle-based electrodes for electrophysiological measurements

Biopotential recordings such as electroencephalogram (EEG) and electrocardiogram (ECG) generally use wet gel electrodes to ensure a low coupling impedance at the electrode/tissue interface. This set-up is long and tedious and may lead to non-robust signals because of the gel that can leak or dry out. We propose to replace wet gel electrodes with initially dry hydrogel microneedle (MN)-based electrodes capable of piercing the insulating outer layers of the skin and reach the conductive interstitial fluid located in the dermis. Interestingly, purely safe and biodegradable polymer MN tips are able to self-degrade after measurement, increasing safety in the event of microneedle breakage into the skin. We fabricated biocompatible and biodegradable hydrogel-based MN patches made of the cross-linked carboxymethylcellulose (CMC) polymer, rigid and electrically insulating in the dry state, and able to swell once in contact with the ion-conducting interstitial fluid. A metal transduction layer was integrated on the back of the MN patches to obtain the wearable measuring MN-based electrodes. The swelling and ion-conducting capacity of the MN patches were demonstrated. The electrical measurement capability of the MN-based electrodes was assessed using a simple lab-made skin phantom representing the mechanical and electrical properties of the dermis and epidermis. In this proof-of-concept, superior measurement quality was demonstrated with MN-based electrodes in comparison to those of standard wet gel electrodes without any skin preparation. The biodegradable hydrogel-based MN electrodes could therefore offer easy use, patient comfort and safety, and record biopotentials for several hours.