(184a) Blends of L-Tyrosine Based Polyurethanes and Polyphosphate for Biomedical Applications

Novel L-tyrosine based polyurethanes and polyphosphate polymers with tunable material properties have been developed by employing tools of molecular design and synthetic chemistry. L-tyrosine is a non-essential amino acid with a phenolic hydroxyl group, which makes it possible to use derivatives of tyrosine dipeptides as a motif to generate monomers that serve as important building blocks for these polymers. Desaminotyrosyl hexyl ester (DTH), a diphenolic monomeric dipeptide molecule developed using a carbodiimide mediated condensation reaction is an important monomer for the synthesis of both polyurethane and polyphosphate polymers. The soft segments in these polyurethanes are either polycaproloctone diol (PCL) or poly (ethylene glycol) (PEG) and the hard segment diisocyanates are non-toxic and biocompatible. In case of the polyphosphate polymers, a polymeric backbone containing alternating peptide bonds and phosphoester bonds is created by reacting DTH with suitable dihalophosphates. The polymers so obtained have different functional groups within the polymeric backbone thereby leading to different chemical structures and thus highly diverse physico-chemical properties and degradation rates. The polyurethanes in general are semi-crystalline in nature and exhibit relatively slow degradation rates whereas polyphosphates are amorphous materials that are susceptible to very rapid hydrolytic degradation. Based on an evaluation of these physico-chemical properties, the polyurethanes seem to be more suitable for tissue engineering applications whereas the polyphosphates are suitable for development of short term drug delivery devices. However, a better application base for these materials may be developed if a step wise transition in the material properties could be obtained. Blending of polymers to obtain a cohesive miscible blend with retention of desirable properties from both parent polymers appears to be the most practical approach to attain this objective. Blends of these novel polyurethanes and polyphosphates are studied in order to develop a family of materials with a wide range of properties that are pertinent to biomedical applications. The thermal, mechanical, morphological and degradation properties of these blends are investigated. Thermal degradation studies performed using PCL-CHMDI-DTH and PEG-CHMDI-DTH polymers and their blends with polyphosphate shows that the range over which the thermal degradation occurs increases upon addition of polyphosphates. This increase was found to be dependent on the weight percent of polyphosphate present within the blend. DSC thermograms of blends of showed the primary thermal transitions belonging to both the parent polymers. A shift in the thermal transitions was also observed indicating an increased or decreased phase compatibility between the two polymers within the blend. Dynamic swelling studies performed using these blends showed an increased water uptake with an increased polyphosphate concentration for both polyurethanes, however; the effect of polyphosphate concentration on swelling was much more significant for PEG-based polyurethane. Contact angle studies suggested a decrease in the surface hydrophobicity with an increase in the polyphosphate concentration within the blends. Finally, a study of surface morphology revealed a smooth surface in case of all films but at higher phosphate concentrations, the films were found to develop fine cracks which may be due to the brittle nature of the polyphosphate. Further physico-chemical characterization of these blends is currently underway and we expect to accomplish it soon. Thus, by using composition of the parent polyurethanes and amount of polyphosphate present within the blend as changeable parameters, the material properties of the final material can be easily tuned to suit the application under consideration. These blends seem to possess suitable properties for easy processing and fabrication and maybe applied for both drug delivery device and tissue engineering scaffold applications.