Alginate-Based Hydrogel Blends with Uncrosslinked Polymers: Multiscale Structure and Its Relationship with Viscoelastic Properties

The multiscale architecture of alginate–gelatin–hyaluronic acid (AGH) hydrogels was elucidated using a synergistic approach of swelling tests, mechanical characteristics, TEM, and SAXS; it was demonstrated that, in AGH, a crosslinked alginate network coexists with non-interconnected cavities composed of uncrosslinked polymer strands. While swelling analysis yields a mesh size of ξS = (12 ± 3) nm corresponding to the alginate network, mechanical measurements reveal an effective mesh size ξM–eff = (44 ± 2) nm, reflecting the combination of the viscoelastic network and cavity contributions. SAXS identifies two rod-like crosslink-site populations (∼1 and ∼4 nm); the latter was directly validated by TEM, and these populations were found in the swollen and dried states to be independent of alginate content. These findings establish the porosity of the cavities, ϕ, as the key structural parameter to predict AGH mechanical properties from those of pure alginate, providing a framework for designing complex bio-based composites.