Andrea E. Hafeman

The Effects of rhBMP-2 Released from Biodegradable Polyurethane/Microsphere Composite Scaffolds on New Bone Formation in Rat Femora

Scaffolds prepared from biodegradable polyurethanes (PUR) have been investigated as a supportive matrix and delivery system for skin, cardiovascular, and bone tissue engineering. While previous studies have suggested that PUR scaffolds are biocompatible and moderately osteoconductive, the effects of encapsulated osteoinductive molecules, such as recombinant human bone morphogenetic protein (rhBMP-2), on new bone formation have not been investigated for this class of biomaterials. The objective of this study was to investigate the effects of different rhBMP-2 release strategies on new bone formation in PUR scaffolds implanted in rat femoral plug defects. In the simplest approach, rhBMP-2 was added as a dry powder prior to the foaming reaction, which resulted in a burst release of 35% followed by a sustained release for 21 days. Encapsulation of rhBMP-2 in either 1.3-micron or 114-micron PLGA microspheres prior to the foaming reaction reduced the burst release. At 4 weeks post-implantation, all rhBMP-2 treatment groups enhanced new bone formation relative to the scaffolds without rhBMP-2. Scaffolds incorporating rhBMP-2 powder promoted the most extensive new bone formation, while scaffolds incorporating rhBMP-2 encapsulated in 1.3-micron microspheres, which exhibited the lowest burst release, promoted the least extensive new bone formation. Thus our observations suggest that an initial burst release followed by sustained release is better for promoting new bone formation.

Characterization of the Degradation Mechanisms of Lysine-derived Aliphatic Poly(ester urethane) Scaffolds

Characterization of the degradation mechanism of polymeric scaffolds and delivery systems for regenerative medicine is essential to assess their clinical applicability. Key performance criteria include induction of a minimal, transient inflammatory response and controlled degradation to soluble non-cytotoxic breakdown products that are cleared from the body by physiological processes. Scaffolds fabricated from biodegradable poly(ester urethane)s (PEURs) undergo controlled degradation to non-cytotoxic breakdown products and support the ingrowth of new tissue in preclinical models of tissue regeneration. While previous studies have shown that PEUR scaffolds prepared from lysine-derived polyisocyanates degrade faster under in vivo compared to in vitro conditions, the degradation mechanism is not well understood. In this study, we have shown that PEUR scaffolds prepared from lysine triisocyanate (LTI) or a trimer of hexamethylene diisocyanate (HDIt) undergo hydrolytic, esterolytic, and oxidative degradation. Hydrolysis of ester bonds to yield α-hydroxy acids is the dominant mechanism in buffer, and esterolytic media modestly increase the degradation rate. While HDIt scaffolds show a modest (<20%) increase in degradation rate in oxidative medium, LTI scaffolds degrade six times faster in oxidative medium. Furthermore, the in vitro rate of degradation of LTI scaffolds in oxidative medium approximates the in vivo rate in rat excisional wounds, and histological sections show macrophages expressing myeloperoxidase at the material surface. While recent preclinical studies have underscored the potential of injectable PEUR scaffolds and delivery systems for tissue regeneration, this promising class of biomaterials has a limited regulatory history. Elucidation of the macrophage-mediated oxidative mechanism by which LTI scaffolds degrade in vivo provides key insights into the ultimate fate of these materials when injected into the body.

Local Delivery of Tobramycin from Injectable Biodegradable Polyurethane Scaffolds

Infections often compromise the healing of open fractures. While local antibiotic delivery from PMMA beads is an established clinical treatment of infected fractures, surgical removal of the beads is required before implanting a bone graft. A more ideal therapy would comprise a scaffold and antibiotic delivery system administered in one procedure. Biodegradable polyurethane (PUR) scaffolds have been shown in previous studies to promote new bone formation in vivo, but their potential to control infection through release of antibiotics has not been investigated. In this study, injectable PUR scaffolds incorporating tobramycin were prepared by reactive liquid molding. Scaffolds had compressive moduli of 15–115 kPa and porosities ranging from 85–93%. Tobramycin release was characterized by a 45–95% burst (tuned by the addition of PEG), followed by up to 2 weeks of sustained release, with total release 4–5-times greater than equivalent volumes of PMMA beads. Released tobramycin remained biologically active against Staphylococcus aureus, as verified by Kirby–Bauer assays. Similar results were observed for the antibiotics colistin and tigecycline. The versatility of the materials, as well as their potential for injection and controlled release, may present promising opportunities for new therapies for healing of infected wounds.

Injectable Polyurethane Composite Scaffolds Delay Wound Contraction and Support Cellular Infiltration and Remodeling in Rat Excisional Wounds

Injectable scaffolds present compelling opportunities for wound repair and regeneration because of their ability to fill irregularly shaped defects and deliver biologics such as growth factors. In this study, we investigated the properties of injectable polyurethane (PUR) biocomposite scaffolds and their application in cutaneous wound repair using a rat excisional model. The scaffolds have a minimal reaction exotherm and clinically relevant working and setting times. Moreover, the biocomposites have mechanical and thermal properties consistent with rubbery elastomers. In the rat excisional wound model, injection of settable biocomposite scaffolds stented the wounds at early time points, resulting in a regenerative rather than a scarring phenotype at later time points. Measurements of wound length and thickness revealed that the treated wounds were less contracted at day 7 compared to blank wounds. Analysis of cell proliferation and apoptosis showed that the scaffolds were biocompatible and supported tissue ingrowth. Myofibroblast formation and collagen fiber organization provided evidence that the scaffolds have a positive effect on extracellular matrix remodeling by disrupting the formation of an aligned matrix under elevated tension. In summary, we have developed an injectable biodegradable PUR biocomposite scaffold that enhances cutaneous wound healing in a rat model.

Local injection of lovastatin in biodegradable polyurethane scaffolds enhances bone regeneration in a critical‐sized segmental defect in rat femora

Statins, a class of naturally‐occurring compounds that inhibit HMG‐CoA reductase, are known to increase endogenous bone morphogenetic protein‐2 (BMP‐2) expression. Local administration of statins has been shown to stimulate fracture repair in in vivo animal experiments. However, the ability of statins to heal more challenging critical‐sized defects at the mid‐diaphyseal region in long bones has not been investigated. In this study, we examined the potential of injectable lovastatin microparticles combined with biodegradable polyurethane (PUR) scaffolds in preclinical animal models: metaphyseal small plug defects and diaphyseal segmental bone defects in rat femora. Sustained release of lovastatin from the lovastatin microparticles was achieved over 14 days. The released lovastatin was bioactive, as evidenced by its ability to stimulate BMP‐2 gene expression in osteoblastic cells. Micro‐computed tomography (CT) and histological examinations showed that lovastatin microparticles, injected into PUR scaffolds implanted in femoral plug defects, enhanced new bone formation. Furthermore, bi‐weekly multiple injections of lovastatin microparticles into PUR scaffolds implanted in critical‐sized femoral segmental defects resulted in increased new bone formation compared to the vehicle control. In addition, bridging of the defect with newly formed bone was observed in four of nine defects in the lovastatin microparticle treatment group, whereas none of the defects in the vehicle group showed bridging. These observations suggest that local delivery of lovastatin combined with PUR scaffold can be an effective approach for treatment of orthopaedic bone defects and that multiple injections of lovastatin may be useful for large defects. Copyright