Autogenous bone grafting limitations have motivated the development of Tissue-Engineered (TE) biomaterials that offer an alternative as bone void fillers. However, the lack of a blood supply within implanted constructs may result in avascular necrosis and construct failure¹. The aim of this project was to investigate the potential of novel TE constructs to promote vascularisation and bone defect repair using two distinct approaches. In Study 1, we investigated the potential of a mesenchymal stem cell (MSC) and endothelial cell (EC) co-culture to stimulate pre-vascularisation of biomaterials prior to in vivo implantation². In Study 2, we investigated the potential of TE hypertrophic cartilage to promote the release of angiogenic factors such as VEGF, vascular invasion and subsequent endochondral bone formation in an in vivo model.

Collagen-only (Coll), collagen-glycosaminoglycan (CG) and collagen-hydroxyapatite (CHA) scaffolds were fabricated by freeze-drying³, seeded with cells and implanted into critical-sized calvarial and femoral defects in immunocompetent rats. In Study 1, Coll and CG scaffolds were initially seeded with ECs, allowed to form capillary-like networks before the delayed addition of MSCs and continued culture prior to calvarial implantation. In Study 2, CG and CHA scaffolds were seeded with MSCs and cultured under chondrogenic and subsequent hypertrophic conditions to form a cartilage pre-cursor prior to calvarial and femoral implantation in vivo.

MicroCT and histomorphometry quantification demonstrated the ability of both systems to support increased bone formation compared to controls. Moreover, the greatest levels of bone formation were observed in the CG groups, notably in those containing cartilage tissue (Study 2). Assessment of the immune response suggests the addition of MSCs promotes the polarisation of macrophages away from inflammation (M1) towards a pro-remodelling phenotype (M2).

We have developed distinct collagen-based systems that promote vascularisation and ultimately enhance bone formation, confirming their potential as advanced strategies for bone repair applications.

Gene-activated scaffolds have shown potential in localised gene delivery resulting in bone tissue regeneration. In this study, the ability of two gene delivery vectors, polyethyleneimine (PEI) and nano-hydroxyapatite (nHA), to act as carriers for the delivery of therapeutic genes when combined with our collagen-nHA (coll-nHA) scaffolds to produce gene-activated scaffolds [1, 2], was determined. In addition, coll-nHA-dual gene scaffolds containing both an angiogenic gene and an osteogenic gene were assessed for bone healing in an in vivo Wistar rat calvarial defect model. When cells were applied to the coll-nHA scaffolds under osteogenic conditions in vitro, the dual scaffolds exhibited significantly superior osteogenic potential when analysed using microCT, calcium quantification and histology compared to single-gene scaffolds and gene-free controls. When the dual scaffolds were assessed in vivo, the nHA dual scaffold outperformed all other groups as early as 4 weeks post-implantation as determined using X-ray, microCT, quantification of new bone volume, histology and vessel formation. This research has demonstrated the potential of using novel coll-nHA scaffolds for therapeutic gene therapy while also being capable of simultaneously delivering numerous genes. This study underlines the effect of specifically tailoring gene-activated scaffolds for bone regeneration applications.