3D Extrusion-Printed Alginate–Gelatin Hydrogel Modified with Nanoscale Hydroxyapatite: A Comprehensive Understanding of Process Science and Evaluation of the Antimicrobial Property

In developing hydrogel scaffolds for the soft tissue regeneration, a number of inorganic or carbonaceous fillers are embedded in alginate/gelatin-based hydrogel while manufacturing shape fidelity compliant constructs using three-dimensional (3D) extrusion printing. Among the spectrum of nanofillers, nanohydroxyapatite (nHAP), due to its intrinsic bioactivity, could promote mineralization and interaction with host tissues while conferring superior mechanical properties (strength and elastic modulus). Against this backdrop, this study demonstrates the effectiveness of nHAP reinforcement in tuning several clinically relevant properties such as rheological properties, mechanical properties, swelling, degradation, and antimicrobial properties. At higher concentrations of nHAP (0.75%) in the hydrogel matrix (3A5G0.75H), a 3.13-fold increment in the compressive strength was observed, with the gel stability window and the thermal stability of the cross-linked graft being extended to greater than 40 and 143 °C, respectively. This study demonstrated the printability of the nHAP-reinforced hydrogel ink by fabricating the matrix-shaped graft of dimension 20 mm in diameter and 10 mm in thickness, and the buildability was established by making the bulk-sized construct up to 62 layers (20 mm in height) with a well-maintained pore interconnectivity, as demonstrated using micro-CT analysis. Interestingly, the CFU study revealed a 2.9- and 1.5-fold improvement in the reduction of bacterial adhesion for 3A5G0.75H with respect to <i>Escherichia coli</i> and <i>Staphylococcus aureus</i> bacteria. Cell culture studies on the 3D printed scaffolds w.r.to NIH 3T3 fibroblast cell line demonstrated a consistent increase in cell viability and pronounced filopodial extensions, confirming the cytocompatibility of 3A5G0.75H scaffolds and their ability to support cellular growth during an <i>in vitro</i> culture. Taken together, the present study uncovers a process science-based understanding of the 3D buildability and biophysical properties of different concentrations of nHAP-reinforced hydrogel inks with clinically relevant properties.