Polyurethanes in Biomedical Applications

Nina M.K. Lamba, Kimberly A. Woodhouse, Stuart L. Cooper

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351423373
• Edition: 1
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

Polyurethanes in Biomedical Applications studies the use of polyurethanes in implanted medical devices. This analysis describes the concepts of polymer science, the manufacture of polyurethanes, and the biological responses to implant polyurethanes, reflecting the developments in biomaterials science and the interdisciplinary nature of bioengineering.

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1 Introduction

I. Biomaterials

A biomaterial is a nonviable material used in a medical device, intended to interact with biological systems.¹ Biomaterials may be used singularly to replace or augment a specifie tissue, or in combination to perform a more complex function, e.g., in organ replacement.² Biomedical materials include metals, ceramics, pyrolytic carbon materials, composites and polymers. Of these groups, polymers represent the largest class.

There are three fundamental properties that a biomaterial should possess; mechanical strength, a functional characteristic, and biocompatibility.³ The functional characteristic is required so that the material has the specific property to perform the required task. Mechanical strength is required to retain an adequate level of performance. Biocompatibility is considered in more detail in the next section.

II. Biocompatibility

The interactions between polymers and the biological environment are not yet fully understood. The definition of biocompatibility has evolved over the years from one of biological inertness, to acceptance that some degree of interaction between the biomaterial and the host is inevitable and may actually be beneficial. A widely accepted definition of biocompatibility is taken to be the ability of a material to perform with an appropriate host response in a specific application.¹ This definition recognizes that the degree of biocompatibility required of a biomaterial depends very much on the application.

Biocompatibility can be considered in terms of blood compatibility (hemocompatibility) and tissue compatibility (histocompatibility). Blood compatibility is less well defined than biocompatibility, and there is no widely accepted definition.⁴ It is often defined as what should not occur, including thrombosis, destruction of formed elements, and complement activation. Histocompatibility encompasses the lack of toxicity, and excessive tissue growth around an implant. It is unlikely that there will be one biomaterial that will work equally well in all applications. This is due to the large range of mechanical and functional requirements of biomaterials, as determined by the specific application and the host tissues.

III. Biomedical Polymers

A large number of polymers have been used in biomedical applications. During the 1930s, materials available for medical use were limited to those that were naturally available. Developments in polymer science opened up the variety of materials that were available. Since the mechanical properties of these synthetic polymers resembled those of biological tissues more closely than, for example, metals, wood and glass, they were readily introduced as biomaterials. Another contributory factor to the increased use and application of biomaterials was the development of antibiotics, which improved the survival rates of patients, increasing the need for prosthetic devices. Advances in surgery and medicine have been responsible for, and will continue to create a demand for biomaterials, both with respect to the need for new materials and their scope of application.

TABLE 1
Use of plastics in medical and pharmaceutical applications

Polymers that are used in medical applications include naturally occurring materials such as natural rubber and cellulose. Synthetic biomaterials include silicone rubber (SR), Polyvinylchloride (PVC), Nylon®, polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET) and of course the polyurethanes. The current amounts of polymeric materials used in medical and pharmaceutical applications are presented in Table 1.⁵

Plasticized PVC is by far the most common polymeric biomaterial, due to its use in disposable medical devices, including blood storage bags, infusion sets and blood tubing. Polyurethanes account for very little of the total amount of polymers used for medical and pharmaceutical applications. This does not reflect the diversity of their applications and their relative success as a biomaterial. Polyurethane elastomers combine excellent mechanical properties with good blood compatibility, which has favored their use and development as biomaterials, particularly as components of implanted devices.

IV. Organization

The first part of this book is designed to introduce the reader to concepts in polymer science, Polyurethane chemistry, and the mechanical and surface properties of polyurethane elastomers. Chapter 2 explores the synthesis of polyurethane elastomers, their chemistry, and methods of bulk and surface modification. Chapter 3 contains a review of polymer processing, relevant to the fabrication of medical devices and a survey of biomedical polymers. It must be noted that in recent years, a number of these materials have been withdrawn from the medical implant marketplace and are no longer commercially available. Despite this, consideration of the chemistry, properties and performance has been given to these materials throughout the book, as they constitute some of the most widely studied polyurethanes for biomedical applications. Chapter 4 contains an examination of the microphase structure of the polyurethanes, and consideration of their physical properties. Characterization of the mechanical properties of the polyurethanes also is discussed. Chapter 5 contains an review of the surface characterization and properties of polyurethanes.

The second half of the book contains an introduction to the biological systems that contribute to the host response to implanted biomaterials. After a brief discussion of the biological assessment of materials, Chapter 7 presents a review of the biological interactions with polyurethanes, including plasma protein adsorption, blood compatibility and soft tissue interactions of polyurethanes. Chapter 8 discusses the biodegradation of polyurethanes, reviewing the current understanding of toxicity of polyurethanes and their stability in the biological environment. Calcification of polyurethane devices also is considered. Chapter 9 reviews the more common applications and performance of polyurethanes as medical devices. Cardiovascular, artificial organs, tissue augmentation and genito-urinary applications are reviewed. A discussion on the future direction of polyurethanes and the field of biomaterials in general completes the book.

References

1. Williams D. F., Ed. Definitions in Biomaterials. Elsevier Science Ltd., 1987.

2. Friedman D. W., Orland P. J. and Greco R. S. “History of Biomaterials.” In Implantation Biology —The Host Responses and Biomedical Devices, Ed. Greco R. S. CRC Press, Boca Raton, FL, 1994: 1.

3. Courtney J. M., Lamba N. M. K., Gaylor J. D. S., Ryan C. J. and Lowe G. D. O. “Blood-contacting biomaterials: bioengineering viewpoints.” Artif. Organs, 19: 852, 1995.

4. Ratner B. D. “The blood compatibility catastrophe.” J. Biomed. Mater. Res., 27: 283, 1993.

5. Szycher M. “Medical/Pharmaceutical markets for medical plastics.” In High Performance Biomaterials, Ed. Szycher M. Technomic, Lancaster, PA, 1991: 3.

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2 The Chemistry of Polyurethane Copolymers

I. Introduction

Polymers are a class of high molecular weight materials, with a structure that is characterized by “building blocks” of repeat units or monomers. The monomers react together to form long chains of repeating chemical units. The polymer chains that result may be linear or form a branched or three-dimensional network. Polymers can be classified in a number of ways, for example, according to whether they are of natural origin, or synthetic. Naturally occurring polymers include polysaccharides, cellulose, silk and natural rubber. Common synthetic polymers include polyethylene, polystyrene, Polyvinylchloride, polyesters, polytetrafluoroethylene, polycarbonates, and the polyurethanes. Polymers also can be classified according to chemical composition, chemical structure, physical state, thermal behavior and application. Classification on the basis of chemical composition of the polymer considers the elemental composition and types of monomer residues within the chain. The chemical structure considers the stereoregularity of the polymer, and the placement of side chains, which also is referred to as the tacticity of a polymer. The physical structure of the material also can be used, to classify materials as crystalline or amorphous, indicating the state of order of the molecules, or to indicate whether or not the polymer chains are branched as opposed to linear. The thermal behavior of the polymer also can categorize polymers as either thermoplastic or thermosetting, which is an important consideration in processing. The ultimate application of the material also can be used to classify polymers.

Other criteria for polymer classification include the number or type of repeat units that are present in a polymer chain. In some cases polymers are composed of a single repeat unit, and such materials are referred to as homopolymers. Examples of homopolymers are Polyvinylchloride and polyethylene. Heteropolymers, or copolymers as they often are called, contain more than one monomer residue within the polymer chains. Different types of heteropolymers exist, and are depicted in Figure 1. The precise sequence of the individual monomers influences the chemistry and physical nature of the material. Alternating copolymers consist of alternating residues. Block copolymers consist of several units of one type followed by a number of units of another residue within the main chain of the polymer. Graft copolymers consist of blocks of one residue as a side chain on a backbone of a different polymer. There are many excellent texts on polymeric materials, encompassing polymer synthesis and structure-property relationships of polymers, in which aspects of polymer structure, classification and chemistry are discussed in detail; the reader is directed towards these books for further information.^1-7

Polyurethanes are a family of heterogeneous polymers; they contain the urethane linkage (Figure 2), analogous to the carbamate group in organic chemistry, within the polymer chains. Uretha...

Table of contents

• Cover Page
• Title Page
• Copy Right Page
• Contents
• 1. Introduction
• 2. The Chemistry of Polyurethane Copolymers
• 3. Fabrication and Processing of Polyurethanes
• 4. Structure and Physical Characterization of Polyurethanes
• 5. Surface Characterization of Polyurethanes
• 6. Introduction to Host-Biomaterial Interactions
• 7. Protein, Cellular and Soft Tissue Interactions with Polyurethanes
• 8. Degradation of Polyurethanes
• 9. Polyurethanes in Biomedical Applications
• 10. Summary and Future Perspectives
• List of Abbreviations
• Index