Surface Modification of Biomaterials

Methods Analysis and Applications

Rachel Williams

• Publisher: Woodhead Publishing
• Year: 2010
• ISBN:
  9780857090768
• Topic: Medicine
• Subtopic: Biotechnology in Medicine

The surface modification of biomaterials plays a significant role in determining the outcome of biological-material interactions. With the appropriate modification a material's surface can be tailored to improve biocompatibility, adhesion and cell interactions. Consequently surface modification is vital in the development and design of new biomaterials and medical devices. Surface modification of biomaterials reviews both established surface modifications and those still in the early stages of research and discusses how they can be used to optimise biological interactions and enhance clinical performance.Part one begins with chapters looking at various types and techniques of surface modification including plasma polymerisation, covalent binding of poly (ethylene glycol) (PEG), heparinisation, peptide functionalisation and calcium phosphate deposition before going on to examine metal surface oxidation and biomaterial surface topography to control cellular response with particular reference to technologies, cell behaviour and biomedical applications. Part two studies the analytical techniques and applications of surface modification with chapters on analysing biomaterial surface chemistry, surface structure, morphology and topography before moving onto discuss modifying biomaterial surfaces to optimise interactions with blood, control infection, optimise interactions with soft tissues, repair and regenerate nerve cells, control stem cell growth and differentiation and to optimise interactions with bone.The distinguished editor and international team of contributors to Surface modification of biomaterials have produced a unique overview and detailed chapters on a range of surface modification techniques which will provide an excellent resource for biomaterials researchers and scientists and engineers concerned with improving the properties of biomaterials. It will also be beneficial for academics researching surface modification.

• Reviews both established surface modifications and those still in the early stages of research and how they can be used to optimise biological interactions and enhance clinical performance
• Studies analytical techniques and applications of surface modification with chapters assessing biomaterial surface chemistry, surface structure, morphology and topography
• Discusses modifying biomaterial surfaces to optimise interactions with blood and soft tissues and also to repair and regenerate nerve cells and control infection

Part I

Surface modification techniques

1

Surface modification of biomaterials by plasma polymerization

E.J. Szili, R.D. Short and D.A. Steele, University of South Australia

J.W. Bradley, University of Liverpool, UK

Abstract:

The use of low pressure, low power plasma established in an atmosphere of precursor monomer(s) affords the deposition of functionalized films. Such films can retain the functional group and molecular structure of the starting compound(s) in an ultrathin ‘plasma polymer’ deposit. A basic introduction to plasma and the modulation of some of the important physical and chemical properties will identify some of the key relationships between the plasma physics and the chemistry of the resultant plasma polymer films. There then follows an examination of where plasma polymerized films have found application in the area of biomaterial preparation and, in summary, identification of the likely direction of future developments.

Key words

biomaterials

biomolecules

cell adhesion

plasma polymerization

1.1 Introduction

The use of low pressure and, most recently atmospheric, low power plasma established in an atmosphere of precursor monomer(s) affords the deposition of functionalized films. Such films can retain the functional group and molecular structure of the starting compound(s) in an ultrathin ‘plasma polymer’ deposit. Most commonly, surfaces are functionalised with oxygen (O), nitrogen (N), and O and N functional groups such as carboxylic acids, alcohols, ethers, amines and amides. This chapter outlines this surface engineering technology and highlights the importance that it has developed. In the first instance a basic introduction to plasma, how it is generated, and the modulation of some of the important physical and chemical properties is provided. In doing so, some of the relationships between the physics of low pressure, low power (radio-frequency) plasma discharges of O and N functionalized monomer(s) and the chemistry of the resultant plasma polymer films will be explored. There then follows, using selected examples, an examination of where plasma polymerized films have found, or have the potential to find biomedical application. We have structured this section according to the physiochemical characteristics of the plasma polymer that is being exploited; although rarely is control over one character manipulated to the exclusion of all others. In summary, we identify the likely direction of future developments.

1.2 An overview of plasma and plasma polymerization

Recent years have seen an increased interest in the preparation of functional, organic ultrathin films deposited by plasma. This technology is seen as the method of choice for modifying the surfaces of materials for biomedical and tissue engineering applications where the bulk mechanical properties have dictated the choice of material; however, the surfaces of such materials continue to elicit an undesirable biological-biomaterial response (Griesser et al., 1994; Chan et al., 1996; Daw et al., 1998; Barry et al., 2005; Siow et al., 2006; Zelzer et al., 2008). This is especially true as emerging technologies, which take advantage of events that occur at the nanometre and/or micrometre scale-length have developed (Morgenthaler et al., 2008; Robinson et al., 2008; Walker et al., 2009). For example, in devices utilizing microfluidic channels, the ratio of surface to bulk material is extremely high, hence the property of the surface in relation to the efficacy of the technology is paramount. Thus, in microfluidics, surface functionalization of microchannels can be utilized to both inhibit unwanted surface events and promote others (Hiratsuka et al., 2004; Sibarani et al., 2007). One particular feature of the plasma deposition of organic films, often referred to as ‘plasma polymerization’, that makes these technologies particularly attractive is that such films can be deposited onto a wide range of different substrates without the need for special surface preparation prior to deposition. This provides a substantial advantage over other techniques where specific substrate chemistry is required, e.g. gold for thiols or oxidized surfaces for silanes; however, substantial commercial development of this technique has not yet followed: poor understanding of plasma systems, their operation, the chemical processes responsible for plasma polymer film growth coupled with the blanket description of these surfaces as biocompatible, based mainly upon short term in vitro studies, have restricted the field.

1.3 Plasma generation and system design

Historically the scientific investigation of plasmas began in the late 19th century when Sir William Crookes described a DC discharge in an evacuated column as ‘the fourth state of matter’; yet, it was Irving Langmuir in 1929, who first defined an ionized gas using the term ‘plasma’. However, the first industrial application of plasma processing occurred with the development of the modern integrated circuit first developed in the 1950s. Today anisotropic plasma etching allows patterning of integrated circuits (ICs) and the semiconductor industry, with its desire for ever smaller features on microchips, has proven to be one of the main driving forces behind the research into plasma processing (Manos and Flamm, 1989; Flamm, 1996; Chang and Coburn, 2003). This research has, in turn, led to the almost ubiquitous implementation of plasma and plasma processing technology throughout the manufacturing sector. In addition to IC manufacture plasma coatings find application for modified adhesion – both increasing and decreasing this property, scratch, corrosion and wear resistance (Wohlrab and Hofer, 1995; Villermaux et al., 1996; Benitez et al., 2000; Forch et al., 2007). Low temperature plasma surface modification is particularly attractive when applied to polymeric materials where the opportunity to modify surface properties such as wettability, adhesion, permeability and bio-compatibility without thermal damage exists (Liston et al., 1993; Chan et al., 1996).

Generation of low temperature plasma is achieved by use of DC or AC fields at reduced pressures, typically in the region of 1 to 100 s of mTorr (0.133 to 10 s Pascals), such that ionization can occur at reasonable power inputs. Methods of excitation include DC, radio frequency (RF at 13.56 MHz) and microwave (MW at 2.45 GHz) and this power source may be coupled directly, with the electrodes within the plasma chamber, indirectly with the electrodes external to the chamber or by combination (Roth, 1995). For DC plasma the coupling is typically conductive between two electrodes and, depending on the application, a range of conducting materials may be used for electrode fabrication. With RF sources, the power, coupled to the electron current, can be capacitive or inductive. RF power coupling offers some significant advantages over DC and AC sources for industrial applications:

• RF plasma can process insulating materials without sputtering of the electrodes and so, can be used for deposition from organic monomers.

• Since the RF power is deposited in the plasma by displacement rather than particle currents, it is easier to couple through the chamber wall resulting in less ion and electron bombardment of electrodes.

• In general, RF-generated plasma are more stable with electrons with higher temperatures for the same densities than an equivalent DC or AC plasma. This can be beneficial where an increased number of free radicals, plasma-chemical reactions or dissociation and ionization reactions are desired.

When considering the preparation of plasma polymer films, much of the previous research has been performed with RF power, typically 13.56 MHz, in glass reactor vessels with external coil-configurations, or bands to couple the power, although many studies with capacitively coupled, internal electrodes have also been undertaken (Griesser and Zientek, 1993; O’Toole et al., 1995; Beck et al., 1996; Dai et al., 1997; Alexander and Duc, 1998). Early designs were based on glass tubes which were either purpose made with ports for the vacuum, monomers...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Surface modification techniques
• Part II: Analytical techniques and applications
• Index