Fabrication of thiolated HA hydrogel microspheres

The thiolated HA (ThHA) precursor was prepared by dissolving the ThHA (HyStem; Biotime, Inc.) at 1.2% (w/w) in a custom buffer containing 100 mM calcium chloride and OptiPrep to adjust solution density (CosmoBioUSA) with a pH of 7.0 (adjusted with 15 mM HEPES). The solution viscosity was measured as described above. Core-shell constructs were produced as described above. However, crosslinking was achieved by chemical crosslinking, versus the free radical photopolymerization with MeHA microspheres. Briefly, the precursor solution formed core-shell constructs upon contact with the alginate bath, containing 0.4% PEGDA (MW 3.4 kDa; Laysan Bio, Inc.), which was stirred gently for 5 min.

Crosslinking proceeded by diffusion of the smaller 3.4 kDa PEGDA molecule into the core, through the alginate shell. The bath was then diluted by half with DPBS, which reduced the solution pH to 7.4. Stirring continued for an additional 30 min to crosslink the HA precursor within the core. The microspheres were collected, and the alginate shells were removed as described above.