Recent advances in vitro diagnostics on human skin or organ surfaces rely on the use of sensors. Wearable diagnostic and therapeutic devices are ubiquitous to health care, where technology can improve patient’s life. Due to the high aspect ratio and excellent electrical conductivity, silver nanowires (AgNWs) have been widely explored for flexible/stretchable conductors that can accommodate large deformations, make intimate contact with curvilinear surfaces (such as human skin) and maintain high conductivity under large strain. A plethora of wearable devices using AgNWs have been demonstrated in the past few years, such as strain (motion) sensors, pressure sensors, touch sensors, and electrophysiological electrodes. However, biocompatibility of silver nanowires to human skin is largely unknown, which is critical for these wearable devices as many of them are in contact with skin for long time for long-term, continuous health and wellness monitoring. Lack of biocompatibility in a skin-attachable sensor, can result in complications including: cell cytotoxicity, skin irritation, skin sensitization, skin chronic inflammation and dermatitis.
We fabricated a highly conductive and stretchable conductor with AgNWs embedded in the surface layer of poly(dimethylsiloxane) (PDMS). Assessment of the materials biocompatibilities on skin is currently performed with the use of animals. Alternative non-animal related in vitro methods are demanded by Animal welfare and EU legislation. We have developed novel in vitro biocompatibility test methodology for screening of skin-wearable AgNW/PDMS sensors. These silver sensor components were assessed in vitro using two skin cell lines and 3-dimensional tissues. Three characterized in vitro assays were used to measure the cytotoxicity, irritation and sensitization of the skin-attachable sensor materials. These in vitro assays could meet the FDA requirement, as well as minimize the usage of live animals. We also characterized these AgNW/PDMS sensors to ensure that their electrical conductivities were performed functional for different applications, such as ECG and hydration sensors, without eliciting any undesirable local or systemic effects, including cytotoxicity and skin irritation.