Strain Insensitive Pressure Sensor Integrated on Selectively Photo-crosslinkable Substrate

<br/> Soft, stretchable, and wearable electronic devices have been studied for their potential use in various applications that cannot be achieved with conventional rigid devices. Stretchable electronics, as an advanced technology for flexible devices, have great potential to be used in various wearable devices and sensors, regarding human skins or tissue’s stretchable property. However, it is still insufficient for commercial use due to lack of durability and performance.<br/> A common method of manufacturing stretchable electronic devices is to design all constituent materials to be stretchable. Another approach is to design the substrate so that the strain is concentrated on the interconnects and strain-free or limited strain on the unit area. The former approach can provide high integration, but there is a limit to making all elements of a device stretchable. While stretchable substrates (SEBS or PDMS) can be stretched up to 100%, it is very challenging for other elements such as electrodes and active layers to be stretched along the substrate. Even an organic semiconductor that can be significantly stretchable than an inorganic material, it is not easy to withstand deformation of more than tens of %. The latter approach, "rigid island" structure allows the integration of existing rigid electronic components with-out deformation. In particular, the development of stretchable printed interconnects has made it possible to successfully integrate active electronic devices located on rigid polymer islands into stretchable devices.<br/> Herein, we introduce a simple method to fabricate a photo-crosslinkable, transparent elastomer substrate for stretchable electronics and its application to physical sensors. Acrylate based substrate was formed with copolymers. With photo-initiator, acrylate functional group can be crosslinked by UV irradiation and by adjusting the amount of crosslinker, we could control its crosslinking density and modulus. The modulus of substrate increased to 8900%. Additional crosslinking provided 38000% higher modulus than that prior to UV irradiation. The substrate can be selectively crosslinked by exposure to UV light in order to form a rigid region using a patterned mask. Rigid island pattern shape and size can be designed without restraint owing to its simple crosslinking mechanism. 20 mm pattern of rigid/soft area pattern was stretched 100% to confirm its stretching property, rigid region was only stretched 30% while soft area was stretched 210%.<br/> Rigid island structures with repetitive rigid-soft-rigid systems can facilitate the development of capacitive strain/pressure sensor by forming electrodes only on the rigid islands. Rigid island can work as a dielectric layer of the capacitor, sensor sensitivity to strain and pressure are dependent to modulus of the rigid island. We designed pressure insensitive strain sensor (PISS) and strain insensitive pressure sensor (SIPS) by this system. For SIPS, electrospun dielectric layer was introduced to increase pressure sensitivity while maintaining low strain sensitivity.