Conformable on-skin devices for thermo-electro-tactile stimulation: Materials, design, and fabrication

Conformable electronics is an emerging and innovative research field investigating functional materials and electronic devices capable of adhering and conforming to non-planar surfaces such as human skin. Conformable devices find applications of high economical and scientific interest in healthcare, human-machine interfaces, wearable electronics, robotics, and the internet of things. Compared to the more widely used rigid and bulky electrodes often employed in wearable platforms, the former offers multiple advantages such as imperceptibility, low cost, and the possibility of continuous use, being able to follow the multiple deformations of curvilinear living tissues. In recent years, much attention has been paid to the development of soft and skin-wearable sensors, but an additional area of great interest is certainly that of conformable electrodes capable of providing some kind of stimulation. This review provides an overview of the most attractive and innovative conformable devices developed to stimulate the human body through thermal, electrical, mechanical, and optical stimuli. In particular, it focuses on the functional materials employed, the fabrication techniques involved, and the design solutions proposed to improve the performance of conformable stimulation devices.