Evaluation of a Large Volume Composite Biomaterials System for an In Vivo Bioreactor
Repairing craniofacial defects remain a challenge because these defects are generally large and plagued by infection, scar tissue, or other complications. A bioreactor could be a viable solution to producing clinically relevant shapes and sizes of bone tissues by harnessing the body's own capability to generate vascularized tissues. This project aimed to create a large in vivo bioreactor capable of growing bone tissue to be used in the repair of craniofacial defects. A biodegradable polymer scaffold was designed using a 70:30 mixture of biodegradable poly(ethylene glycol)-co-(L-lactic acid) diacrylate (PEG-PLLA-DA) to non-degradable poly(ethylene glycol) diacrylate (PEG-DA). Ceramic particles were added to increase osteogenesis, and porosity was created using salt leaching. Pores were loaded with fibrin and a concentrated layer of Platelet-Derived Growth Factor-BB (PDGF-BB) was added to create a gradient distribution of growth factor throughout the scaffold. The scaffold was scaled up to 3cm x 1.5cm x1cm and formed in a poly (methyl methacrylate) (PMMA) 5-sided chamber. The bioreactor was tested in a pig model by implanting against the periosteum isolated from the rib and calvaria. After 8 weeks the chamber was harvested and the material in the tissue was analyzed using scanning electron microscopy (SEM) imaging and histological analysis. Results showed the material was comprised of primarily residual scaffold without any evidence of tissue interaction. The tissue interface of the scaffold was analyzed and it was determined that pores were not exposed on the outer surface of the scaffold, generating a poor tissue-material interface. The scaffold was modified to reduce degradation time by removing all PEG-DA and the non-porous interface layer was removed to expose a porous interface to the tissue. The redesigned scaffold was tested in the pig model, and after 8 weeks the bioreactors were harvested. This time no residual scaffold was found in the bioreactor, but the harvested material was completely comprised of red blood cells with no tissue growth. The scaffold materials were examined to analyze activity of the bioreactor. The growth factor was tested to evaluate activity following release in a fibroblast proliferation assay. It was found that the growth factor increased cell proliferation, thus verifying activity. The gradient distribution of protein was verified by using fluorescently labeled protein. The scaffolds were tested to determine if the ceramic particles or the depth of scaffold impacts the formation of a protein gradient. It was found that a protein gradient exists from day 1 and is maintained out to 20 days in all test groups. The surgical model was evaluated by implanting a chamber with morsellized bone graft (MBG) as a positive control, known to grow and retain tissue in bioreactors for up to 24 weeks. After 8 weeks the MBG chambers were harvested and found to be empty, indicating the surgical model is causing an issue with the ability to accurately test the bioreactor. Future studies might include changing the implant location from the periosteum to the muscle fascia, or using a sheep model instead of a pig model as sheep have been successfully used in this model.