Focused Ultrasound-Mediated Release of Bone Morphogenetic Protein 2 from Hydrogels for Bone Regeneration

An ultrasound-responsive hydrogel system was developed that provides on-demand release when stimulated by focused ultrasound (fUS). Diels–Alder cycloadducts crosslinked polyethylene glycol (PEG) hydrogels and underwent a retrograde Diels–Alder reaction when exposed to fUS. Four-arm and eight-arm furan-based Diels–Alder hydrogel compositions were used to evaluate the link between the crosslinking density and the fUS-induced release and retention rates. PEG crosslinked with glutaraldehyde was also used as a non-Diels–Alder control hydrogel. By increasing the exposure time and the amplitude of fUS, the Diels–Alder-based hydrogels exhibited a correlative increase in the release of the entrapped BMP-2. Real-time B-mode imaging was used during fUS to visualize the on-demand degradation of the crosslinking matrix for the release of BMP-2. When monitored with a thermocouple, the increase in temperature observed was minimal in the area surrounding the sample during fUS stimulation, indicating fUS to be an external stimulus which could be used safely for spatiotemporally controlled release. PEG hydrogels were characterized using nuclear magnetic resonance, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and compression testing. PEG degradation byproducts were evaluated for cytocompatibility in vitro. Overall, this study demonstrated that Diels–Alder-based PEG hydrogels can encapsulate BMP-2, undergo a retrograde reaction when externally stimulated with fUS, and release active BMP-2 to induce differentiation in human mesenchymal stem cells.