Targeted Micro-Phase separation – A generic design concept to control the elasticity of extrudable hydrogels

Hydrogels are ubiquitous in nature and technology. Controlling their mechanical properties and under-standing their complex microstructure is critical e.g. for 3D bioprinting or tissue engineering applications. Here a generic design concept for tuning the elasticity of extrudable gels at given polymer or particle concentration is presented. Targeted micro-phase separation leading to micro-heterogeneities (1–100 µm) yields high gel strength allowing for extrusion of uniform filaments with high shape accuracy as long as the heterogeneity length scale is small compared to the extruded filament diameter (>500 μm). Micro-mechanical and structural heterogeneity was enhanced in alginate hydrogels by accelerating crosslinking kinetics, corresponding to gel elasticity variation of more than two orders of magnitude (17 Pa to 2300 Pa), enabling filament extrusion (1046 µm) with high shape fidelity. Introducing poly(vinylalcohol) into gelatin gels resulted in more heterogeneous materials with a 2-fold increase in elasticity (951 Pa to 1993 Pa) and thinner filaments (908 µm to 590 µm). Higher ionic strength in Laponite-based hydrogels induced nanoparticle aggregation, leading to higher elasticity (857 Pa to 2316 Pa) enabling smooth filament extrusion. Eliminating the often tacitly assumed hydrogel uniformity on the microscale provides additional degrees of freedom to achieve high gel strength without increasing polymer, particle or crosslinker concentration.