3D Printing of Transparent and Conductive Heterogeneous Hydrogel–Elastomer Systems

A simple approach to 3D extrusion printing of soft, stretchable electrical devices integrating a conductive hydrogel and a dielectric elastomer with sub-millimeter resolution was studied. The hydrogel precursor consisted of a concentrated aqueous solution of a hygroscopic salt, a compatible rheological modifier, and UV initiated polymerization/crosslinking compounds. Lithium chloride (LiCl) was selected as the hygroscopic salt in a compromise between vapor pressure and ionic conductivity, and its concentration was set above peak conductivity, but below saturation in an aqueous solution. The PDMS was a UV curing formulation to allow for rapid setting during the printing process. Printing was performed using an extrusion 3D printer comprised of a precision positioning system, an ink extrusion system, and a hardware/software interface to control location and rate of material extrusion relative to the sample stage. The relative humidity (RH) of the environment was fixed at 43% by bubbling nitrogen through a saturated solution of potassium carbonate. Oxygen displacement was needed to prevent inhibition of the free-radical polymerization in the hydrogel precursor. In situ UV curing was used to partially set the extruded ink and prevent extensive spreading. By tuning the rheological behavior of the hydrogel precursor and performing oxygen plasma treatments of PDMS surfaces, it is possible to print and integrate hydrogels directly with PDMS at sub-millimeter resolution. This capability was demonstrated by the fabrication and functional verification of an ionic cable and a resistance-based strain sensor.