Sustainable bioelectronics fabrication through photo-induced swelling of green hydrogels

Electrical circuit manufacture for flexible electronics is a very specialized printing process in which electrically functional inks are printed onto a substrate. In almost all cases, the substrate assumes a passive role in ink distribution, which has been the conventional methodology used up until now. Herein we have discovered that a sodium carboxymethyl cellulose (CMCNa) hydrogel substrate demonstrates heightened susceptibility to UV photo-irradiating and because of molecular-level bond lability that leads to a macroscopic improved swelling (“writing” action). The localized photo-activated events lead to temporary 3D contours on the hydrogel substrate where conductive ink is held in valleys to allow the formation of conductive traces. A self-distribution of ink in the valleys is achieved which, moreover, is a type of mask-based photolithography or digital image generation. The process can be employed for polymeric inks such as PEDOT:PSS to obtain ink patterns without need of complex inkjet printers or other conventional printers. The drying causes recession of the temporary swollen hydrogel contours and returns the surface to flattened format. The process works at lower ink solids of 0.125 % and has shown that 1.15 J/mm² of UV energy is capable of creating an electrically isolated conductive pattern. Initial water content of the system plays an important role in which 20 g/g of absorbed water/substrate is sufficient for acceptable pattern generation.