Nanoclay Promotes Mouse Cranial Bone Regeneration Mainly through Modulating Drug Binding and Sustained Release

Nanoclay (Nanosilicates, NS) is appearing as an intriguing 2D nanomaterial for bone tissue engineering with multiple proposed functions, e.g., intrinsic osteoinductivity, improving mechanical properties, and drug release capacity. However, the mechanism of NS for in vivo bone regeneration has been hardly defined so far. This knowledge gap will significantly affect the design/application of NS-based biomaterials. To determine the role of NS in osteoblastic differentiation and bone formation, we used the mouse calvarial-derived pre-osteoblasts (MC3T3-E1) and a clinically-relevant mouse cranial bone defect model. Instead of a hydrogel, we prepared biomimetic 3D gelatin nanofibrous scaffolds (GF) and NS-blended composite scaffolds (GF/NS) to determine the essential role of NS in critical low-dose (0.5 μg per scaffold) of BMP2-induced cranial bone regeneration. In contrast to "osteoinductivity", our data indicated that NS could enable single-dose of BMP2, promoting significant osteoblastic differentiation while multiple-dose of BMP2 (without NS) was required to achieve similar efficacy. Moreover, our release study revealed that direct binding to NS in GF scaffolds provided stronger protection to BMP2 and sustained release compared to GF/NS composite scaffolds. Consistently, our in vivo data indicated that only BMP2/NS direct binding treatment was able to repair the large mouse cranial bone defects after 6 weeks of transplantation while neither BMP2, NS alone, nor BMP2 released from GF/NS scaffolds was sufficient to induce significant cranial bone defect repair. Therefore, we concluded that direct nanoclay-drug binding enabled sustained release is the most critical contribution to the significantly improved bone regeneration compared to other possible mechanisms based on our study.