(261g) Polyurethane-Hydroxyapatite Scaffolds for Bone Regeneration

Bone is the second most transplanted tissue after blood; however, transplantation of autologous tissue is limited by the availability of donor tissue (e.g., iliac crest) and its harvest is associated with pain, donor site morbidity, and increased surgical time. While ceramics (e.g., hydroxyapatite (HAp)) are attractive alternatives that have been shown stimulate bone formation, because they are brittle, difficult to shape into the size of the bone defect, and non-degradable they are typically used in granular form as bone void fillers. One strategy to leverage the osteogenicity of HAp in regeneration of a patient-specific bone defect is to incorporate it within a porous, degradable polymer scaffold. To this end, considerable effort has been made to fabricate porous scaffolds from degradable polyesters (e.g., polycaprolactone (PCL), poly(lactic-co-glycolic acid) (PLGA)) that incorporate HAp particles. PCL and PLGA exhibit high moduli (0.4–1 GPa), that is comparable to mineralized bone tissue, but deform plastically and are known to creep under load. Therefore, in this project we are fabricating degradable scaffolds for bone regeneration from segmented polyurethane elastomers. Polyurethanes are generally considered to be biocompatible, have seen application in catheters, dialysis membranes, pacemaker leads, and breast implants, and are being developed for cardiac and cartilage tissue regeneration.

In this project a segmented polyurethane was synthesized by a two-step process: first reacting 2000 Da PCL diol with hexane diisocyanate, and then chain-extending this prepolymer with putrescine (Figure 1a). Here, the PCL segments confer degradability and elasticity, while the polyurethane groups act as physical crosslinks to confer mechanical stability and toughness. NMR analysis of the polymer demonstrated complete reaction of the isocyanate groups and a roughly 1:1 ratio of urethane and urea groups (indicative of a high molecular weight linear polymer). TGA analysis showed the thermoplastic polymer decomposes above ~330°C, while DSC indicated a thermal transition at 45-60°C that can be ascribed to melting of PCL (Figure 1b,c). This indicates that the polymer microphase separates and PCL-rich regions form crystalline domains. Finally, tensile testing revealed a bulk elastic modulus of 8.4 ± 1.9 MPa and elastic deformation up to ~20% strain (Figure 1d,e). This means that the elastomer cannot withstand physiologic stresses (and therefore must be used in conjunction with orthopedic plates and/or fixators). Nevertheless, it is significantly stiffer than the 34 kPa gels on which Engler et al. demonstrated spontaneous osteoblastic differentiation of mesenchymal stem cells (MSCs) (Cell, 126:677-689; 2006). Thus, this polymer is sufficiently stiff to facilitate bone formation, and withstand physical handling and surgical implantation.

Currently, we are confirming that our polymer supports MSC proliferation and are developing a protocol for processing our segmented polyurethane into porous foam scaffolds by solvent casting/porogen leaching. Here, our plan is to form foams with extensive pore interconnectivity and internal surface to facilitate vascular infiltration for new tissue formation when implanted in a bone defect model. Our next steps will be 1) to incorporate HAp into polyurethane foams and films, which we hypothesize will increase their mechanical properties, and 2) to confirm their osteogenicity by characterizing osteoblastic differentiation of MSCs in vitro.