We hypothesize that the microstruct

We hypothesize that the microstructural differences between the single- and trisolvent fibers are a result of the interplay between solvent evaporation and polymer condensation. In both cases, inks begin asparticles homogeneously dispersed within elastomer-rich solutions (Fig. 2E). Immediately upon extrusion (t = 0), local particle density in- creases as a result of shearing forces (51). Once exposed to air, most of the DCM rapidly evaporates (t1). In the single-solvent system, composed only of DCM, this results in rapid precipitation of all dissolved elastomer homogeneously throughout the fiber volume, resulting in a crater-web microstructure (Fig. 2F), which is characteristic of extreme “solvent popping” commonly observed in paints and other coatings when the suspending solvent evaporates too quickly. This leaves solid surface films that trap remaining solvent beneath, which eventually vaporizes and forcefully emerges through the solidified surface in the form of popping bubbles. The resulting thin elastomeric webbing presents concentrated regions of high stress and results in brittle structures that are unable to absorb significant loads without failing. In the trisolvent system, however, the two additional low-volatility solvents slow elastomer precipitation, permitting it to preferentially coat particles (t1). Enough elastomer pre- cipitates onto the particles to form robust interparticle bridges, whereas the inability for the spherical HA particles to densely pack results in interparticle pores (Fig. 2G). Over the course of several minutes (t2), the remaining minority solvents evaporate, slowly precipitating the last of the solubilized material onto previously precipitated elastomer (52). The retention of the two minor solvents after initial DCM evap- oration also immediately enables adjacent fibers and layers to fuse during 3D printing. This likely mitigates interlayer delamination and results in monolithic objects that can be handled immediately after being 3D-printed.