Optimization of Mechanical Stiffness and Cell Density of 3D Bioprinted Cell-Laden Scaffolds Improves Extracellular Matrix Mineralization and Osteocyte-Like Cell Organization for Bone Tissue Engineering

Abstract

Bioprinting is an emerging technology in which cell-laden biomaterials are precisely dispersed to engineer artificial tissues that mimic aspects of the anatomical and structural complexity of relatively soft tissues such as skin, vessels, and cartilage. However, reproducing the highly mineralized and cellular diversity of bone tissue is still not easily achievable and is yet to be demonstrated. Here, an extrusion-based 3D bioprinting strategy is utilized to fabricate 3D bone-like tissue constructs containing osteoblast- and osteocyte-like cell organization. A simple and low-cost bioink for 3D bioprinting of bone-like tissue is prepared based on two unmodified FDA-compliant polymers (alginate and gelatin) and combined with human mesenchymal stem cells (hMSCs). To form 3D bone-like tissue and bone cell phenotype, the influence of different scaffold stiffness and cell density of 3D bioprinted cell-laden porous scaffolds on osteogenic differentiation and bone-like tissue formation was investigated over time. Our results showed that soft scaffolds (0.8%alg, 0.66 ± 0.08 kPa) had higher DNA content, enhanced ALP activity and stimulated osteogenic differentiation than stiff scaffolds (1.8%alg, 5.4 ± 1.2 kPa). At day 42, significantly more mineralized tissue was formed in soft scaffolds than in stiff scaffolds (43.5 ± 7.1 mm3 vs 22.6 ± 6.0 mm3). Importantly, immunohistochemistry staining demonstrated more osteocalcin protein expression in high mineral compared to low mineral regions. Additionally, cells in soft scaffolds exhibited osteocyte-related gene expression at day 42. Furthermore, the results showed that cell density in 15M cells/ml can promote cell-cell connections at day 7 and mineral formation at day 14, while 5M cells/ml had the significantly higher mineral formation rate than 15M cells/ml from day 14 to day 21. In summary, this work reports the formation of 3D bioprinted bone-like tissue using a simple and low-cost cell-laden bioink, which was optimized for stiffness and cell density, showing great promise for bone tissue engineering applications.