Electroactive nanocomposite hydrogels for bone repair: integration of nanohydroxyapatite-decorated carbon nanotubes

Advances in bone tissue engineering emphasize functional biomaterials to overcome graft limitations. Hydrogels present minimally invasive alternative for delivering bioactive cues and cells to promote bone regeneration. Incorporating nanomaterials enhances their mechanical, biological, and therapeutic potential.

This study developed an injectable nanocomposite hydrogel with improved mechanical strength and osteoinductive properties. Methacrylated gellan gum (GGMA) was synthesized via glycidyl methacrylate modification. Carbon nanotubes (CNTs) were coated with nano-hydroxyapatite (nHAp) through Ca²-mediated nucleation and phosphate addition to mimic hydroxyapatite self-assembly on collagen fibrils.

Three hydrogel formulations—GGMA, GGMA–CNT, and GGMA–nHApCNT—were evaluated. Cationic gelation formed hydrogels, which were analyzed for rheology, structure, bioactivity, antioxidant and antibacterial properties, angiogenic potential, conductivity, and hemocompatibility. Human adipose-derived stem cells were encapsulated and cultured under basal and osteogenic conditions. Assessments included viability, proliferation, metabolic activity, alkaline phosphatase, and gene expression.

NMR and spectroscopy confirmed GGMA methacrylation and nHAp formation on CNTs. nHAp functionalization enhanced hydrogel’s mechanical properties. Micro-CT revealed that GGMA–nHApCNT presented 0.117 µm wall thickness, 0.042 µm pore size, and 9.08% porosity. Injectability tests showed CNTs increased injection force (6.6 N), whereas nHApCNT reduced it (5.6 N vs. 6.1 N for GGMA), indicating improved injectability.

Bioactivity assays showed enhanced mineralization in GGMA–nHApCNT. Both CNT-based hydrogels exhibited ~25% antibacterial activity, and while GGMA–nHApCNT retained antioxidant properties, a 30% decrease was observed. Conductivity tests confirmed all hydrogels were conductive, with GGMA–nHApCNT highest at 0.029 S/m. CAM assays demonstrated superior angiogenesis in GGMA–nHApCNT.

Cell studies showed GGMA–nHApCNT promoted higher viability (20% > GGMA, 80% > GGMA–CNT), greater cell spreading, and stronger osteogenic gene expression. Hemocompatibility was favorable for GGMA and GGMA–nHApCNT, with <1% hemolysis, unlike GGMA–CNT (5%).

Altogether, GGMA–nHApCNT hydrogels combine improved stability, bioactivity, conductivity, and cell response. The reduced injection force supports minimally invasive delivery, and the multifunctional properties suggest strong potential for bone regeneration. This nanocomposite hydrogel platform addresses current scaffold limitations and informs the design of next-generation biomaterials in regenerative medicine.