Alkaline shear-thinning micro-nanocomposite hydrogels initiate endogenous TGFβ signaling for in situ bone regeneration

Recruiting endogenous stem cells to bone defects without stem cell transplantation and exogenous factor delivery represents a promising strategy for bone regeneration. Herein, we develop an alkaline shear-thinning micro-nanocomposite hydrogel (10-MmN), aiming to alkaline-activate endogenous TGF beta 1 and achieve in situ bone regeneration. It contains polyethyleneimine (PEI)-modified gelatin, laponite nanoplatelets (LAP), a bicarbonate buffer with a pH of 10, and gelatin microspheres (MSs). PEI-modified gelatin plays a pivotal role in hydrogel fabrication. It endows the system with sufficient positive charges, and forms a shear-thinning nanocomposite matrix in the pH 10 buffer (10-mN) with negatively charged LAP via electrostatic gelation. For biological functions, the pH 10 buffer dominates alkaline activation of endogenous serum TGF beta 1 to recruit rat bone marrow stem cells through the Smad pathway, followed by improved osteogenic differentiation. In addition, MSs are incorporated into 10-mN to form 10-MmN, and function as substrates to provide good attachment sites for the recruited stem cells and facilitate further their osteogenic differentiation. In a rat critical-sized calvarial defect model, 10-MmN exhibits excellent biocompatibility, biodegradability, hydrogel infusion and retention in bone defects with flexible shapes and active bleeding. Importantly, it repairs similar to 95% of the defect areas in 3 months by recruiting TGF beta R2(+) and CD90(+)CD146(+) stem cells, and promoting cell proliferation, osteogenic differentiation and bone formation. The present study provides a biomaterial-based strategy to regulate alkalinity in bone defects for the initiation of endogenous TGF beta signaling, which can be extended to treat other diseases.

Automated summary

Recruiting endogenous stem cells and activating TGF beta 1 can lead to in situ bone regeneration without stem cell transplantation. A newly developed alkaline shear-thinning micro-nanocomposite hydrogel showed excellent biocompatibility and biodegradability, and promoted cell proliferation, osteogenic differentiation, and bone formation in a rat bone defect model.