Understanding Polyurethane in Biomedical Applications: Safety and Structure

Polyurethanes are a versatile class of polymers that find extensive use in biomedical applications. Notably, recent observations indicate that patients with polyurethane implants have not reported cases of cancer, which raises questions about the safety of the materials used in medical devices. Manufacturers often opt for MDI (methylene diphenyl diisocyanate), a material considered relatively safe, as opposed to other aromatic diisocyanates that may introduce potential carcinogenic risks.

While the carcinogenic properties of degradation products from diisocyanate-based polyurethanes remain understudied, researchers are exploring alternatives to mitigate these concerns. One proposed solution is the hydrogenated version of MDI, known as HMDI. This modification aims to reduce the potential carcinogenic risks associated with MDI, although it comes with trade-offs in mechanical and in vivo characteristics. In general, the aromatic variants of polyurethanes are preferred for their rigidity, which is attributed to the delocalization of π electrons across aromatic rings.

The flexibility of aliphatic diisocyanates, such as those featuring cyclohexane moieties, is a double-edged sword. While this flexibility can facilitate certain polymer characteristics, it often leads to poorer mechanical properties due to reduced hard segment ordering. Additionally, polymers derived from aromatic diisocyanates tend to exhibit yellowing when exposed to light, a phenomenon linked to the formation of chromophores called di-quinones. Although MDI-based polyurethane manufacturers assert that this transformation does not affect biological responses or mechanical properties, the scientific community continues to seek clarity on this issue.

The choice of the macroglycol used in synthesizing polyurethanes significantly influences the mechanical characteristics of the final product. For instance, the ability of macroglycol moieties to achieve close molecular packing directly impacts the rigidity of the polyurethane. Research indicates that poly(tetramethylene adipate) glycol provides superior structural regularity, leading to higher mechanical strength in the synthesized polyurethanes.

Beyond mechanical considerations, the selection of macroglycol also hinges on its chemical stability. Resistance to hydrolysis and chemical degradation is essential, particularly in biomedical applications where longevity and performance are critical. Polyester-urethanes, known for their favorable mechanical properties due to hydrogen bonding capabilities, were among the first polyurethanes to find application in the field of medicine.

Overall, the exploration of polyurethanes in biomedical applications underscores the importance of balancing safety, mechanical performance, and chemical stability. As research continues, the development of safer and more effective polyurethane formulations remains a priority for manufacturers and medical professionals alike.