Enhanced osteoinduction by controlled release of bone morphogenetic protein-2 from biodegradable sponge composed of gelatin and β-tricalcium phosphate

Biodegradable gelatin sponges at different contents of β-tricalcium phosphate (β-TCP) were fabricated to allow bone morphogenetic protein (BMP)-2 to incorporate into them. The in vivo osteoinduction activity of the sponges incorporating BMP-2 was investigated, while their in vivo profile of BMP-2 release was evaluated. The sponges prepared had an interconnected pore structure with an average pore size of 200 μm, irrespective of the β-TCP content. The in vivo release test revealed that BMP-2 was released in vivo at a similar time profile, irrespective of the β-TCP content. The in vivo time period of BMP-2 retention was longer than 28 days. When the osteoinduction activity of gelatin or gelatin–β-TCP sponges incorporating BMP-2 was studied following the implantation into the back subcutis of rats in terms of histological and biochemical examinations, homogeneous bone formation was histologically observed throughout the sponges, although the extent of bone formation was higher in the sponges with the lower contents of β-TCP. On the other hand, the level of alkaline phosphatase activity and osteocalcin content at the implanted sites of sponges decreased with an increase in the content of β-TCP. The gelatin sponge exhibited significantly higher osteoinduction activity than that of any gelatin–β-TCP sponge, although every sponge with or without β-TCP showed a similar in vivo profile of BMP-2 release. In addition, the in vitro collagenase digestion experiments revealed that the gelatin–β-TCP sponge collapsed easier than the gelatin sponge without β-TCP incorporation. These results suggest that the maintenance of the intrasponge space necessary for the osteoinduction is one factor contributing to the osteoinduction extent of BMP-2-incorporating sponges.