Advanced formulation of methacryl- and acetyl-modified biomolecules to achieve independent control of swelling and stiffness in printable hydrogels

Biobased hydrogels are considered to mimic native extracellular matrix due to their high water content and are considered as adequate matrices for cell encapsulation. However, the equilibrium degree of swelling (EDS) and stiffness of simple hydrogel formulations are typically confined: Increasing polymer concentration results in increasing stiffness and simultaneously decreasing EDS. The aim of this contribution was to decouple this standard correlation between polymer content, stiffness and EDS as well as the assembly of hydrogels with graded composition of hydrogels by layer-wise printing. We investigated two sets of formulations, which consisted of three different compositions with increasing total biopolymer concentration (10.6%, 11.5%, 13.0%). Within these compositions the amount of gelatin methacryloyl acetyl (GMA) was constant (10%), whereas the proportion of methacrylated hyaluronic acid and chondroitin sulfate increased. In the first set of formulations GMA with one fixed degree of methacryloylation (DM) was used, whereby the storage modulus (G') increased from ~10 to ~25 kPa and the EDS decreased from ~700 to ~600%. In the second set of formulations we gradually lowered the DM of the GMA in parallel to increase of polymer concentration and achieved an increase of both, G' from ~11 to ~18 kPa and EDS from ~690 to ~790%. By dispensing these compositions, we created a glycosaminoglycan-graded hydrogel. We proved the cytocompatibility of the dispensing process, the used photoinitiator lithium phenyl-2,4,6-trimethylbenzoylphosphinate, and layer-wise UVA irradiation. Glycosaminoglycan gradient was proved stable for 28 d, encapsulated chondrocytes were viable and produced new matrix.