A UV-responsive pressure sensitive adhesive for damage-free fabrication of an ultrathin imperceptible mechanical sensor with ultrahigh optical transparency

The development of epidermal electronics, which provides precise measurement of various human physiological signals without reducing the quality of life, has encountered a serious delay in its practical application because of potential damage experienced during fabrication. Such damage occurs mainly because fabrication of the epidermal device is typically carried out after attaching an ultrathin or stretchable film onto a stiffer carrier substrate, and eventually peeling it from the carrier. Substrates used in epidermal electronics suffer from very low stiffness to bending and stretching, resulting in severe strain during stripping from the carrier substrate, where devices formed on its surface are also subjected to significant damage. When a device is fabricated using an adhesive with an excessively low adhesion strength, however, some parts of the soft film can be prematurely peeled from the carrier during device fabrication. We propose a UV-responsive transparent pressure-sensitive adhesive (PSA) to fabricate ultrathin devices without process-induced damage. We synthesised an acryl-based PSA and mixed it with UV-curable oligomers. The soft UV curable PSAs contract and become rigid when the oligomers in the PSAs are cured by UV irradiation. This dramatically reduces the adhesion strength to less than 1% of the initial value and consequently enables peeling from the carrier without deteriorating the properties of the devices formed on the ultrathin film. In this case, it is not necessary to fabricate the device by using more materials than necessary considering the performance degradation in advance. Using this method, we developed a transparent bending-induced pressure sensor, with light transmittance higher than 93.5% at 550 nm.