The use of designer scaffolds to deliver biologically active osteogenic growth factors such as recombinant human bone morphogenetic protein-2 (rhBMP-2) to the sites of tissue regeneration in for example orthopaedics, has tremendous therapeutic implications. The aims of this study were to generate biomimetic biodegradable porous osteogenic scaffolds using a supercritical fluid process to encapsulate rhBMP-2, and to examine the ability of the scaffolds to promote human osteoprogenitor differentiation and bone formation in vitro and in vivo.

The rhBMP-2 encapsulated in Poly(-lactic acid) (PLA) scaffolds (100ng/mg PLA) were generated using an innovative supercritical fluid mixing method. The bioactivity of rhBMP-2 encapsulated PLA scaffolds were confirmed by induction of the C2C12 promyoblast cell line into the osteogenic lineage as detected by alkaline phosphatase expression. No induction of alkaline phosphatase-positive cells was observed using blank scaffolds. BMP-2 released from encapsulated constructs promoted adhesion, migration, expansion and differentiation of human osteoprogenitor cells on 3-D scaffolds. Enhanced matrix synthesis and cell differentiation on growth factor encapsulated scaffolds was observed following culture of human osteoprogenitors on explants of chick femoral bone wedge defects in an ex vivo model of bone formation developed using the chick chorioallantoic membrane model. In vivo studies using diffusion chamber implantation and subcutaneous implantation of human osteoprogenitors on rhBMP-2 encapsulated scaffolds showed morphologic evidence of new bone matrix and cartilage formation in athymic mice as assessed by x-ray analysis, immunocytochemistry and birefringence. These studies provide evidence of controlled release of BMP-2 from biodegradable polymer scaffolds initiating new bone formation in vivo.

The generation of 3-D biomimetic structures incorporating osteoinductive factors such as BMP-2 indicates their potential for de novo bone formation that exploits cell-matrix interactions and, significantly, realistic delivery protocols for growth factors in musculo-skeletal tissue engineering.