3D Printing of Polyvinyl Alcohol Hydrogels (PVA) Enabled by Aqueous Two-Phase System (ATPS)

The synthesis of PVA hydrogels (PVA-Hy) requires a high-concentration alkali solution (<i>e.g.</i>, sodium hydroxide, NaOH, 4.2 M), and the rapid physical crosslinking of PVA makes it challenging to ensure layer-to-layer adhesion for 3D printing. This work demonstrated the three-dimensional (3D) printing of PVA hydrogels in benign conditions (NaOH, 0.3 M) using a two-phase system (ATPS). Salting out of PVA to form ATPS allowed temporal stabilization of 3D-printed PVA structures while it was physically crosslinked by moderate alkaline conditions. Crucially, the layer-by-layer printing of PVA was enabled by delayed reaction at low alkaline concentrations. To verify this principle, we studied the feasibility of direct ink write (DIW) 3D printing of PVA inks (5 – 25% w/w, m = 0.1 – 20 Pa s, and MW = 22000 and 74800) in aqueous embedding media offering three distinct chemical environments: (1) salts for salting out (<i>e.g.</i>, Na₂SO₄), (2) alkali hydroxides for physical crosslinking (<i>e.g.</i>, NaOH), and (3) mixture of salt and alkali hydroxide. The presence of the salt did not compromise the stretchability and durability of PVA-Hy. Overall, our method demonstrated a unique concept of embedded 3D printing enabled by ATPS for temporary stabilization of the printed structures to facilitate 3D fabrication.