Coatings for Biomedical Applications

Mike Driver

• Publisher: Woodhead Publishing
• Year: 2012
• Print ISBN: 9781845695682
• eBook ISBN: 9780857093677
• Topic: Physical Sciences
• Subtopic: Medical Technology & Supplies

The biomaterials sector is rapidly expanding and significant advances have been made in the technology of biomedical coatings and materials, which provide a means to improve the wear of joints, change the biological interaction between implant and host and combine the properties of various materials to improve device performance. Coatings for biomedical applications provides an extensive review of coating types and surface modifications for biomedical applications.The first part of the book explores a range of coating types and their biomedical applications. Chapters look at hydrophilic, mineral and pyrolytic carbon coatings in and ex vivo orthopaedic applications and finally at surface modification and preparation techniques. Part two presents case studies of orthopaedic and ophthalmic coatings, and biomedical applications including vascular stents, cardiopulomonary by-pass equipment and ventricular assist devices.With its clear structure and comprehensive review of research, Coatings for biomedical applications is a valuable resource to researchers, scientists and engineers in the biomedical industry. It will also benefit anyone studying or working within the biomedical sector, particularly those specialising in biomedical coatings.- Provides an extensive review of coating types and surface modifications for biomedical applications- Chapters look at hydrophilic coatings for biomedical applications in and ex vivo, mineral coatings for orthopaedic applications, pyrolytic carbon coating and other commonly-used biomedical coatings- Presents case studies of orthopaedic and ophthalmic coatings, and biomedical applications including vascular stents, cardiopulomonary by-pass equipment and ventricular assist devices

Part I

Coating types and applications

1

Hydrophilic coatings for biomedical applications in and ex vivo

P. Wyman, DSM Biomedical Materials, the Netherlands

Abstract:

Hydrophilic coatings are applied to a wide range of surfaces of biomaterials. This chapter addresses the need for coatings in both in- and ex-vivo settings for both blood-contacting and non-blood-contacting applications, with illustrations of the coating chemistry used in each setting. Applications for non-fouling surfaces in diagnostics, lubricious surfaces on cardiovascular devices, and both lubricious and antimicrobial hydrophilic surfaces for urological applications are presented. Processes using both photochemical grafting and addition cure technologies to generate hydrophilic surfaces are outlined, and a selection of polymers commonly employed in commercially available coating systems are considered and discussed in the context of the application area.

Key words

hydrophilic polymer coatings

non-fouling surfaces

medical devices

in-vitro diagnostics

blood contact

1.1 Introduction

Hydrophilic coatings for biomedical application, and more specifically for medical devices, serve numerous purposes. This chapter focuses on applications relevant to medical and medical-related devices, with occasional reference to other applications.

The features and chemistry of common polymers are explored, including covalently and non-covalently bound layers and interpenetrating networks. The relative merits of each approach, along with the advantages and disadvantages of a particular polymer, are illustrated. The chapter covers the application areas relevant to hydrophilic coatings and provides some background and highlights of the favoured chemistries in each of these areas; they are split into in-vivo blood contact and non-blood contacting and ex-vivo, the division reflecting the regulatory requirements in each application area.

Section 1.2 explores the polymers and chemistries used to generate hydrophilic surfaces and considers the most commonly used materials. Section 1.3 on ex-vivo coatings evaluates the use of polyethylene glycol (PEG), and especially PEG functional colloidal particles, for non-fouling applications. Section 1.4 on in-vivo coatings is split into two areas; Section 1.4.1 on non-blood-contacting applications highlights the area of coatings for urology catheters, with a feature on Foley catheters, and the requirements for antimicrobial hydrophilic coatings; and Section 1.4.2 on in-vivo blood-contacting coatings focuses on hydrophilic coatings for guidewires and balloon catheters. Section 1.5 serves to review the discussion and leads into future trends.

Within this introduction, an overview of the surfaces encountered and indeed coated, with a rough order of difficulty for obtaining a high-integrity coating, are surveyed and the need for surface coatings is discussed.

Aside from hydrophilic coatings, there is an equally large body of work on hydrophilic polymers, or more specifically, hydrogels, as materials in their own right. In the biomedical field, these are often applied to tissue engineering and cell culture scaffolds (Drury and Mooney, 2003; Hoffman, 2002; Lee and Mooney, 2001; Lutolf et al., 2003; Nguyen and West, 2002) and, not surprisingly, this class of materials shares a good deal of polymer chemistry with hydrophilic coatings, although mechanical and bulk properties become the focus rather than adhesion, and coatability, surface properties and bio-compatibility share significant overlap (Anseth et al., 1996; Novikov et al., 2003; Stammen et al., 2001). Hydrogels other than those relevant to coatings are not covered in this chapter.

1.1.1 Background to hydrophilic coating development and needs

Surprising as it may seem, and despite the availability of suitable coating technology, the application of hydrophilic coatings to medical devices is a relatively recent field. It is interesting to consider, for instance, the evolution of the indwelling urinary catheter, in its most basic form a piece of curved tubing inserted into the bladder through the urethra. Urinary catheterization dates back to the 5th century BC, when the use of bronze tubes was apparently common (Moog et al., 2005); fortunately bronze has antimicrobial properties. Whether or not a lubricant was used to aid insertion is not well documented and the procedure was no doubt rather painful (under certain circumstances it could lead to coma), although one suspects that oil of some description must have been used.

In current practice, hydrophilic gels such as hydroxyethylcellulose (HEC) are often applied to devices to aid insertion. More recently still, although initially available from the 1980s (Montagnino, 2000), a shift towards self-lubricating devices, i.e. those with a hydrophilic coating, has emerged, with a full range of such devices now available on the market.

As analytical technology has evolved and microfluidics has emerged as a valuable tool for in-vitro diagnostics (IVD), there is also an increasing demand for hydrophilic surfaces in such applications, either to simply aid wetting, induce capillary flow or to specifically prevent protein adsorption or blood clotting. This becomes especially relevant where an analytical technique exists to determine the presence or extent of a medical condition, whether from a blood, urine or saliva sample, biopsy, or other source of analyte but where the signal source is weak and may be lost into or onto the surface of the sampling device, storage container or the device itself. Miniaturisation brings the benefits of, for instance, smaller samples and more facile automation, but inevitably increases the surface-area-to-volume ratio and therefore increases the need for all contact surfaces to have correct properties for the given application.

General properties of hydrophilic surfaces

It is a general perception that hydrophilic surfaces are non-biofouling, and although this is at least partially true for certain uncharged or charge neutral (e.g. zwitterionic) hydrophilic surfaces, many may be considered low fouling, and only in a few cases non-fouling (see Section 1.2). Such low fouling features may, however, with a simplistic interpretation, be sufficient to limit cell adhesion or blood platelet activation, and would appear to enhance biocompatibility.

Reduced protein fouling of a surface is likely to reduce both bacterial and mammalian cell adhesion (Magin et al., 2010) and this can be a significant advantage; however, this presents a challenge where a cell adhesive surface is desired for bio-integration or simply to improve biocompatibility. This is addressed by the promotion of cell adhesion achieved by, for instance, expression of cell adhesive proteins such as fibronectin and collagen, or adhesion-promoting peptides, RGD being one of the most common. This effect can also be mimicked; for instance, lysine provides a suitable surface for certain cell types, as do certain amino functional surfaces (Rimmer et al., 2007; Wang et al., 2003). However, such approaches are always in the balance, wherein the desired cell type must attach and proliferate more effectively than any species that could result in infection and eventual biofilm formation (Subbiahdoss et al., 2010), such a feat being far easier in a laboratory than on an implanted device.

Blood compatibility is a specific case requiring biocompatibility and is a complex issue (Gorbet and Sefton, 2004; Ríhová, 1996), not helped by the efficiency with which the blood clotting cascade performs. Numerous strategies exist to tackle this challenge (Tanzi, 2005), including biomimetic approaches such as mimicking cell membranes, enhancing the surface affinity for, or pre-conditioning with, serum proteins such as albumin, promoting cell adhesion, inclusion or immobilisation of anticoagulants such as heparin, and simply enhancing surface hydrophilicity, although most of these approaches are based on an underlying hydrophilic surface.

Often, the result of a surface treatment or coating that produces a hydrophilic surface, and such hydrogel-type layers, is a good degree of biocompatibility, be it by initial adsorption of proteins or other species from the contact environment, or through an intrinsic low or non-fouling ability. The binding of species to a surface exposed to an in-vivo environment is strongly dictated by not only the binding affinity, b...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Coating types and applications
• Part II: Case studies
• Index