John P. Fisher

Photocrosslinking characteristics and mechanical properties of diethyl fumarate/poly(propylene fumarate) biomaterials

The development of tissue engineered materials for the treatment of large bone defects would provide attractive alternatives to the autografts, allografts, non-degradable polymers, ceramics, and metals that are currently used in clinical settings. To this end, poly(propylene fumarate) (PPF), a viscous polyester synthesized from diethyl fumarate (DEF), has been studied for use as an engineered bone graft. We have investigated the photocrosslinking of PPF dissolved in its precursor, DEF, using the photoinitiator bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO) and low levels of ultraviolet light exposure. A three factor, 2 x 2 x 4 factorial design was developed, studying the effects of PPF number average molecular weight, BAPO initiator content, and DEF content upon photocrosslinking characteristics and mechanical properties. Uncured DEF/PPF solution viscosity fell over three orders of magnitude as DEF content was increased from 0% to 75%. The exothermic photocrosslinking reaction released low levels of heat, with no more than 160J/g released from any formulation tested. As a result, the maximum photocrosslinking temperature remained below 47 degrees C for all samples. Both sol fraction and swelling degree generally increased with increasing DEF content. Compressive mechanical properties were within the range of trabecular bone, with the strongest samples possessing an elastic modulus of 195.3 +/- 17.5 MPa and a fracture strength of 68.8 +/- 9.4MPa. Finally, the results indicate that PPF crosslinking was facilitated at low DEF precursor concentrations, but hindered at higher precursor concentrations. These novel DEF/PPF solutions may be preferred over pure PPF as the basis for an engineered bone graft because they (1) exhibit reduced viscosity and thus are easily handled, (2) form polymer networks with compressive strength at fracture suitable for consideration for trabecular bone replacement, and (3) may be readily fabricated into solids with a wide range of structures.

Synthesis and properties of photocross-linked poly(propylene fumarate) scaffolds

The photocross-linking of poly(propylene fumarate) (PPF) to form porous scaffolds for bone tissue engineering applications was investigated. PPF was cross-linked using the photoinitiator bis(2,4,6-trimethylbenzoyl) phenylphosphine oxide (BAPO) and exposure to 30 min of long wavelength ultraviolet (UV) light. The porous photocross-linked PPF scaffolds (6.5 mm diameter cylinders) were synthesized by including a NaCl porogen (70, 80, and 90 wt% at cross-linking) prior to photocross-linking. After UV exposure, the samples were placed in water to remove the soluble porogen, revealing the porous PPF scaffold. As porogen leaching has not been used often with cross-linked polymers, and even more rarely with photoinitiated cross-linking, a study of the efficacy of this strategy and the properties of the resulting material was required. Results show that the inclusion of a porogen does not significantly alter the photoinitiation process and the resulting scaffolds are homogeneously cross-linked throughout their diameter. It was also shown that porosity can be generally controlled by porogen content and that scaffolds synthesized with at least 80 wt% porogen possess an interconnected pore structure. Compressive mechanical testing showed scaffold strength to decrease with increasing porogen content. The strongest scaffolds with interconnected pores had an elastic modulus of 2.3 ± 0.5 MPa and compressive strength at 1% yield of 0.11 ± 0.02 MPa. This work has shown that a photocross-linking/porogen leaching technique is a viable method to form porous scaffolds from photoinitiated materials.

Direct-Write Construction of Tissue-Engineered Scaffolds

A computer-controlled xyz dispensing system called the Biological Architecture Tool (BAT) has been extensively tested in the creation of multilayered and three-dimensional biological objects: tissue scaffolds and plain and patterned cellular-array slides. The BAT dispensing system has proven its versatility and reliability in tissue engineering and biological experiments. The potential employments of modified versions of the xyz dispensers for in vivo minimally invasive surgery and other in vitro aspects of biological and medical research are discussed.

Immunohistochemical characterization of guided bone formation by a biodegradable tissue engineering scaffold in a healing tooth socket of a rabbit model

We have developed a novel, biocompatible, biodegradable tissue engineering scaffold that has been shown to facilitate bone formation in vivo. The process of bone formation within this scaffold, however, is unknown at a molecular level. To study this process, a rabbits four healing incisor sockets (two mandibular and two maxillar) that remain after tooth extraction were used as a bone defect model. One socket was left empty, while the remaining three were filled with crosslinked polymer networks formed from either the hydrophobic polymer poly(propylene fumarate) (PPF), the hydrophilic oligomer oligo(poly(ethylene glycol) fumarate) (OPF), or PPF with adsorbed transforming growth factor - /spl beta/1 (PPF+TGF-/spl beta/1). At 4 days, 1 week, 2 weeks, 4 weeks, 8 weeks, and 16 weeks both the mandible and maxilla were removed and prepared for histological analysis. Using immunohistochemical techniques, frozen sections were stained for the presence of TGF-/spl beta/1, platelet derived growth factor (PDGF), fibroblast growth factor - 2 (FGF-2), vascular endothelial growth factor (VEGF), and bone morphogenetic protein-2 (BMP-2). Results indicate the effect of differing biomaterial properties upon bone formation as described by the spatial and temporal development of those growth factors thought to be intimately involved in the process.