Along with the increasing demands for wearable health care and biomedical applications, there has been extensive research on the development of various stretchable electronic devices with mechanical stability even under deformations caused by body or skin movements. Also, energy storage devices which can be integrated with the stretchable active sensor devices have been actively studied for the practical utilization of wearable electronics without any wire-connected external power sources. Among various candidates for integrated energy storage devices, high performance solid-state micro-supercapcitors (MSCs) have been extensively investigated. Furthermore, integration of those MSCs with photovoltaic devices is expected to provide the power for the sensors, leading to the wirelessly driven skin-attachable health monitoring sensor system.

In this work, we report on the fabrication of stretchable array of high performance solid-state MSCs which can be charged via integration with Si-based commercial solar cell so that those can power the integrated strain sensor. The planar MSC consists of electrodes of potentiostatically deposited polypyrrole on spray coated multi-walled carbon nanotubes film and gel-type electrolyte of LiCl/polyvinyl alcohol with redox additive of 1-Methyl-3-propylimidazolium iodide. The fabricated MSC shows areal capacitance 5.2 mF cm^-2 at a current density of 100 µA cm^-2. The MSC retains 80% of initial capacitance after 5000 charge/discharge cycles at a current density of 100 µA cm^-2. A strain sensor was fabricated with a composite film of fragmentized graphene foam and PDMS. Such fabricated MSCs, strain sensor, and the solar cell were integrated on a single deformable polymer substrate with embedded stiff platforms of SU-8 via long serpentine interconnections of Ti/Pt metal film for mechanical stability under stretching. After 1000 repetitive biaxial stretching/releasing cycles by 30%, there appeared no noticeable change in the charge/discharge behaviors. Furthermore, the photo-charge/discharge characteristics and electrochemical performance remained stable under biaxial stretching of 30 %. With the energy stored in MSCs using the solar cell, the integrated strain sensor could detect the externally applied strain and the arterial pulse after attachment of the whole integrated system onto the wrist of the author.

This work successfully demonstrates the potential application of our stretchable self-charging power/sensor system to skin-attachable health monitoring devices.