Bulk Polymerization and Structure-Property Relationship of Medical Grade Polyurethane

Polyurethanes as biomaterials have been used in the augmentation and repair of the human body with great success. The choice of material for a particular application often hinges on the body\u27s response to the polymer, the mechanical and thermal properties of the polymer and the stoichiometric ratio of active ingredients used in the synthesis. A large number of polyurethanes used in medical applications are polyester-based and cannot be used in long-term implant applications because they are very susceptible to hydrolysis. Other medical-grade polyurethanes, which are suitable for implants are polyether-based which might to some extent, suffer from environmental stress cracking (ESC), thus adversely affecting the mechanical properties of the polymer in the long run. Such degradation is due to enzymatic attack on the polymer, enzymes that are secreted by macrophages, which are part of the body\u27s immune response to foreign substances. Once initiated, micro-cracks can propagate and lead to potentially catastrophic device failure. What seems to be emerging as the answer to some of these problems are polycarbonate-based polyurethanes that have demonstrated resistance to environmental stress cracking. Polyurethanes have found applications in the domains of medical devices, drug delivery, heart valves, catheters and the most dramatic and successful development is in the artificial hip prosthesis. Most of these applications depend on the excellent properties that can be achieved with controlled polymerization processes and their methodology. Polycarbonate-based polyurethane is considered to be a block co-polymer consisting of alternating hard and soft segments. Its bulk properties are generally attributed to the nature and extent of phase separation. The unique aspect of a polyurethane, is that it can be tailor-made with specific properties solely by altering the ratio of the main components of the polymer\u27s molecular structure (crystalline and amorphous segments) without the need for catalysts, plasticizers, additives, or reinforcing agents. In this study, linear polyurethanes with a range of formulations based on soft-to-hard segment ratios were synthesized in bulk using Diphenylmethane-4,4\u27-diisocyanate (MDI), Butandiol (BD) as chain extender, and Polycaprolactone diol (PCL) with varying molecular weights. The variations in polyurethane properties that can be obtained during synthesis were characterized using Thermogravimetric Analysis (TGA), Fourier Transform Infrared spectroscopy (FTIR), Nuclear Magnetic Resonance spectroscopy (NMR) and Differential Scanning Calorimetry (DSC). Biocompatibility tests were investigated for some polyurethane samples in order to check if these polyurethanes were suitable for the desired medical application such as hip prosthesis. The results were analyzed using both Scanning Electron Microscopy (SEM) for the polyurethane samples and Optical Microscopy (OM) for the biological tissues. The main finding was that some polyurethane samples with uniform distribution between hard and soft segments had the lowest adverse biological effect. Other samples caused severe irritation in the tissue and a lot of surface erosion in the polymer sample. This can be traced to the polyurethane composition and the curing process used in the preparation of these samples. It is concluded from experimental results that the properties of polyurethane samples depend on their structure, which can vary according to their composition. It was found that two sets of tested polyurethane samples were biocompatible and one set was not