Comparison of micro- vs. nanostructured colloidal gelatin gels for sustained delivery of osteogenic proteins: Bone morphogenetic protein-2 and alkaline phosphatase

Colloidal gels have recently emerged as a promising new class of materials for regenerative medicine by employing micro- and nanospheres as building blocks to assemble into integral scaffolds. To this end, physically crosslinked particulate networks are formed that are injectable yet cohesive. By varying the physicochemical properties of different particle populations, the suitability of colloidal gels for programmed delivery of multiple therapeutic proteins is superior over conventional monolithic gels that lack this strong capacity for controlled drug release. Colloidal gels made of biodegradable polymer micro- or nanospheres have been widely investigated over the past few years, but a direct comparison between micro- vs. nanostructured colloidal gels has not been made yet. Therefore, the current study has compared the viscoelastic properties and capacity for drug release of colloidal gels made of oppositely charged gelatin microspheres vs. nanospheres. Viscoelastic properties of the colloidal gelatin gels were characterized by rheology and simple injectability tests, and in vitro release of two selected osteogenic proteins (i.e. bone morphogenetic protein-2 (BMP-2) and alkaline phosphatase (ALP)) from the colloidal gelatin gels was evaluated using radiolabeled BMP-2 and ALP. Nanostructured colloidal gelatin gels displayed superior viscoelastic properties over microsphere-based gels in terms of elasticity, injectability, structural integrity, and self-healing behavior upon severe network destruction. In contrast, microstructured colloidal gelatin gels exhibited poor gel strength and integrity, unfavorable injectability, and did not recover after shearing, resulting from the poor gel cohesion due to insufficiently strong interparticle forces. Regarding the capacity for drug delivery, sustained growth factor (BMP-2) release was obtained for both micro- and nanosphere-based gels, the kinetics of which were mainly depending on the particle size of gelatin spheres with the same crosslinking density. Therefore, the optimal gelatin carrier for drug delivery in terms of particle size and crosslinking density still needs to be established for specific clinical indications that require either short-term or long-term release. It can be concluded that nanostructured colloidal gelatin gels show great potential for sustained delivery of therapeutic proteins, whereas microstructured colloidal gelatin gels are not sufficiently cohesive as injectables for biomedical applications.