Mesenchymal Stem Cell-Laden Nanofibril-Reinforced Injectable and Self-Healing Alginate Dialdehyde/Gelatin Hydrogels with Enhanced Mechanical and Chondrogenic Performance In Vitro.

Injectable hydrogels offer a minimally invasive approach for cartilage repair, enabling precise defect filling and in situ scaffold formation; however, balancing mechanical strength with stem cell viability and chondrogenic differentiation remains challenging. Here, alginate dialdehyde-gelatin (ADAG) hydrogels are reinforced with cellulose nanofibrils (CNFs) to enhance mechanical performance while retaining injectability, rapid self-healing, and chondroinductive properties. FTIR spectroscopy confirms efficient Schiff base cross-linking between ADA and gelatin, with CNF incorporation preserving chemical integrity. SEM imaging revealed a porous, interconnected architecture, with the 9:1 (ADAG: CNF, v/v) hydrogel exhibiting a more homogeneous and compact network structure, similar to ADAG. Optimization identifies 9:1 and 8:2 ratios as maintaining crosslinking density, while 7:3 ratio slightly disrupts uniformity but retains fast gelation (<5 min) and smooth injectability. CNF reinforcement markedly enhanced mechanical properties, as evidenced by increased storage modulus (0.007 MPa) and compressive modulus (0.006 MPa). The hydrogels exhibit high self-healing efficiency (98%) and support robust proliferation of human bone marrow-derived mesenchymal stem cells. Biochemical and gene-expression analyses show enhanced glycosaminoglycan and collagen deposition, upregulation of chondrogenic markers (SOX9, COL2A1, ACAN), and low COL1A1 expression. Overall, CNF-reinforced ADAG hydrogels combine cytocompatibility, mechanical resilience, injectability, and chondroinductive potential, highlighting suitability for stem cell-mediated cartilage tissue engineering applications.