Qualification of a Swellable Hydrogel-Silicone Bimorph for Implantable Actuation

To improve the structural adaptability and biological suitability of cochlear implant electrode carriers, this study systematically develops and evaluates novel 3D-printed, swellable hydrogel-silicone composites. A vinylphosphonic acid-acrylamide hydrogel powder (VPA-AAm) was pre-washed, milled, and sieved, incorporated into a silicone matrix, and 3D-printed before rapid thermal curing, followed by microstructural, swelling, and in vitro WST-1 cytotoxicity analyses. Pre-washing minimized particle agglomeration and facilitated high-resolution printing. Incorporating the hydrogel induced complete macroscopic opacity at 25 wt.% and localized microporosity during rapid thermal curing. The non-percolating morphology influenced swelling and mass transport through the composite, providing a mechanistic link between water uptake and the observed cell-viability response. While a 35 wt.% loading was associated with matrix degradation and cell viability below the 70% threshold, the 30 wt.% composition achieved the highest water absorption of approximately 400-460%, consistent with a favorable balance between osmotic swelling and matrix elasticity. This swelling behavior was associated with controlled release characteristics and non-cytotoxic extract responses at intermediate hydrogel loading. Consequently, the 30 wt.% composite represents a promising material combination for implantable medical devices, balancing swelling performance, structural stability, and favorable in vitro cell viability under the investigated extraction conditions.