Bioactive coatings on 3D printed scaffolds for bone regeneration: Translation from in vitro to in vivo models and the impact of material properties and growth factor concentration.

Bone tissue engineering is a rapidly advancing field that seeks to develop new functional bone tissue, harnessing materials for application in bone defects which may fail to heal without intervention, as seen in critical-sized bone defects. The material properties must be developed, tailored and optimised as the environment progresses, through increasing animal size and complexity, of the target bone defect site. This study has examined the potential of a poly(caprolactone) trimethacrylate (PCL-TMA) 3D-printable scaffold with select bioactive coatings to function as a scaffold to augment bone formation. Three bioactive coatings were examined, i) elastin-like protein (ELP), ii) poly (ethyl acrylate) (PEA), fibronectin (FN) and bone morphogenetic protein-2 (BMP-2) applied sequentially (PEA/FN/BMP-2) and iii) both ELP and PEA/FN/BMP-2 coatings applied concurrently. The PCL-TMA scaffold construct was observed to be a robust scaffold material and the bioactive coatings applied were found to be biocompatible, with a significant osteogenic response from human skeletal cell populations observed in vitro . The PCL-TMA scaffold and bioactive coatings supported angiogenesis and displayed excellent biocompatibility following evaluation on the chorioallantoic membrane (CAM) assay. Biocompatibility was confirmed, however, no significant bone formation was detected, following examination of heterotopic bone formation in the murine subcutaneous implantation model, whereas extensive mineralisation was observed in the positive control material of collagen sponge with BMP-2. The absence of bone formation on the PCL-TMA scaffolds, in vivo , was potentially a consequence of the method of action of the applied coatings, the surface area of the scaffold construct for BMP-2 binding and the necessity of an appropriate in vivo environment to facilitate skeletal cell ingress, warranting future examination in an orthotopic bone defect model of bone tissue repair. The current studies demonstrate the development of a range of innovative scaffold constructs with in vitro efficacy and clearly illustrate the importance of an appropriate in vivo environment to validate in vitro functionality prior to scale up and preclinical application. ### Competing Interest Statement The authors have declared no competing interest.