Study of electrical conductivity, biocompatibility and degradation effect of copper electrodes with hydrogel coatings for use in flexible electronics

Most basic flexible electronic structures include the development of micro-electrodes, micro-resistors, micro-heaters and interconnecting lines between them to the signal conditioning circuitry over a flexible substrate. Biocompatibility of these metallic elements is thus a prerequisite for successful realization of any wearable or implantable devices as it determines the degradation effects of the material and the living cells or tissues at the target site. Present paper attempts to test the biocompatibility of a micro-electrode array (MEA) patch made of copper for use in Electrical Stimulation (ES) applications using various simulated body solutions like simulated body fluid (SBF), simulated sweat (SS), and physiological saline (PS) solution. Fabricated MEA structures also demonstrated excellent mechanical stability and good electrical continuity for up to 750 bending cycles highlighting its high flexibility. The paper also aims to observe the degradation study of the fabricated electrodes with coatings like polyvinyl alcohol (PVA) hydrogel and commercially available electrode hydrogel with and without the application of ES. Optimization of PVA coating thickness over the electrodes for promoting effective electrical conduction has also been reported through simulation studies. The results obtained demonstrated the effectiveness of PVA hydrogels over commercial ones. However, the absence of an adhesive layer over the PVA sheets decreased its compatibility period. Tests with applied ES retained electrically stable structures for up to only 24 h compared to a large stability time of ~ 5 days for those without any ES under the test solutions indicating its possible usage for disposable type wearable applications over skin surface.