Controlling the Bioprinting Efficiency of Alginate–Gelatin by Varying Hydroxyapatite Concentrations to Fabricate Bioinks for Bone Tissue Engineering

This study systematically investigated how nanohydroxyapatite (nHA) loading (0–5% w/v) modulates rheology, print fidelity, bulk mechanics, swelling, and cytocompatibility of alginate–gelatin (Alg–Gel) composite bioinks intended for extrusion-based bioprinting in bone tissue engineering. Six formulations (7% w/v alginate + 8% w/v gelatin with 0–5% w/v nHA) were prepared and assessed for viscoelastic response (frequency sweeps), shear recovery after high-strain disruption (time-sweep recovery; ~66–81% recovery after 10 s depending on nHA content), and shear-thinning behavior at 37°C. Printability was quantified using standardized zig-zag filament tests and a six-layer grid construct under optimized conditions (37°C cartridge, 20G nozzle with 0.58 mm ID, 20 mm/s, 200 kPa), revealing formulation-dependent tradeoffs between viscosity, recovery, and filament accuracy; intermediate nHA (notably ~3% w/v) balanced extrusion stability and shape fidelity. Bulk mechanical properties were evaluated by uniaxial compression of printed/crosslinked cylinders, showing an nHA-dependent increase in apparent compressive modulus from ~11 ± 2 kPa (0% nHA) to ~25 ± 9 kPa (5% nHA). Swelling decreased with higher nHA loading, consistent with increased network reinforcement. Pre-osteoblastic MC3T3-E1 cells were embedded and printed (≈3 × 10^6 cells/mL), with Live/Dead imaging indicating >70% viability at day 1 across formulations and generally maintained or improved viability by day 7 (except the highest nHA condition). Overall, the work provides quantitative composition–property–performance correlations and identifies an intermediate nHA range as a practical design space for bone-mimetic, cytocompatible Alg–Gel bioinks.