Elsevier

Biomaterials

Volume 26, Issue 35, December 2005, Pages 7457-7470
Biomaterials

Understanding the biodegradation of polyurethanes: From classical implants to tissue engineering materials

https://doi.org/10.1016/j.biomaterials.2005.05.079Get rights and content

Abstract

After almost half a century of use in the health field, polyurethanes (PUs) remain one of the most popular group of biomaterials applied for medical devices. Their popularity has been sustained as a direct result of their segmented block copolymeric character, which endows them with a wide range of versatility in terms of tailoring their physical properties, blood and tissue compatibility, and more recently their biodegradation character. While they became recognized in the 1970s and 1980s as the blood contacting material of choice in a wide range of cardiovascular devices their application in long-term implants fell under scrutiny with the failure of pacemaker leads and breast implant coatings containing PUs in the late 1980s. During the next decade PUs became extensively researched for their relative sensitivity to biodegradation and the desire to further understand the biological mechanisms for in vivo biodegradation. The advent of molecular biology into mainstream biomedical engineering permitted the probing of molecular pathways leading to the biodegradation of these materials. Knowledge gained throughout the 1990s has not only yielded novel PUs that contribute to the enhancement of biostability for in vivo long-term applications, but has also been translated to form a new class of bioresorbable materials with all the versatility of PUs in terms of physical properties but now with a more integrative nature in terms of biocompatibility. The current review will briefly survey the literature, which initially identified the problem of PU degradation in vivo and the subsequent studies that have led to the field's further understanding of the biological processes mediating the breakdown. An overview of research emerging on PUs sought for use in combination (drug+polymer) products and tissue regeneration applications will then be presented.

Section snippets

Chemistry of PUs

The nature of PU chemistry is central to understanding why some PUs undergo degradation faster than others. For this reason a brief review of the material chemistry is needed prior to discussing PU degradation. Segmented PUs can be represented by three basic components described by the following general form:P-(D(CD)n-P)n,where P is the polyol, D is the diisocyanate and C is the chain extender. Typically, the polyol, or the so-called soft segment, is an oligomeric macromonomer comprising a

Molecular reactions between biological agents and PUs

Early observations relating to the biodegradation of PUs raised much controversy around the pathways leading to the process. This resulted in good part from the lack of understanding associated with the molecular mechanisms contributing to the cleavage of the polymer chains. It was the work of Ken Stokes, which initially provided the most comprehensive theory of in vivo biodegradation in his description of ESC [2], [4], [5], [6], [7]. This explanation of the biodegradation process was adopted

PU degradation and inflammatory cells

As highlighted in the previous section in the description of EB, it is the white blood cell and more specifically the MDM that has emerged as the predominant cell type that is orchestrating the damage in biodegradation processes [44]. But it must be clearly stated that the macrophage takes its cue from the presence of the material and its unique chemistry. Work by Marchant and colleagues quantitatively determined the cellular composition of the inflammatory exudates surrounding the implanted

Design of biodegradable PUs for the in vivo environment

While traditionally investigators have used PUs as long-term implant materials [92] and have attempted to shield them from the biodegradation processes that were described in the previous sections of this review, more recent work by investigators has utilized the flexible chemistry and diverse mechanical properties of PU materials to design degradable polymers for applications as varied as neural conduits [93] to bone replacements [94]. These materials have for the most part taken advantage of

Summary

With knowledge, new opportunity is almost always found. This review of literature presented biomedical PUs from their early conceptual design through events that at one time cast the future of PUs into doubt. However, through careful analysis of the EB mechanisms a better understanding of the materials and their behaviour in vivo has been acquired. While new and more bioresistant formulations of PUs have been conceived the area of biodegradable PUs is poised to make significant strides within

Acknowledgements

The authors wish to thank the editors of the Biomaterials journal and the Board of Directors for the Canadian Biomaterials Society for having provided the authors with this unique opportunity to convey International and Canadian developments occurring in the area of PU biodegradation.

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