Surfaces and Interfaces for Biomaterials

Pankaj Vadgama

• Publisher: Woodhead Publishing
• Year: 2005
• Print ISBN: 9781855739307
• eBook ISBN: 9781845690809
• Topic: Technology & Engineering
• Subtopic: Chemical & Biochemical Engineering
• Index: Technology & Engineering

Given such problems as rejection, the interface between an implant and its human host is a critical area in biomaterials. Surfaces and interfaces for biomaterials summarises the wealth of research on understanding the surface properties of biomaterials and the way they interact with human tissue.The first part of the book reviews the way biomaterial surfaces form. Part Two discusses ways of monitoring and characterising surface structure and behaviour. The final two parts of the book look at a range of in vitro and in vivo studies of the complex interactions between biomaterials and the body. Chapters cover such topics as bone and tissue regeneration, the role of interface interactions in biodegradable biomaterials, microbial biofilm formation, vascular tissue engineering and ways of modifying biomaterial surfaces to improve biocompatibility.Surfaces and interfaces for biomaterials is a standard work on how to understand and control surface processes in ensuring biomaterials are used successfully in medicine.- Complete coverage on the fundamentals of surface structure and forming to biological and clinical outcomes- Includes reviews of key surface analytical techniques- Edited by a renowned expert and written by an international team of authors

Part I

Forming methods

1

Fundamental properties of surfaces

P. Weightman; D.S. Martin The University of Liverpool, UK

1.1 Introduction

Surface scientists occasionally observe that while God created solids, the Devil created surfaces. This phrase encapsulates the fact that while surfaces often dominate the behaviour of materials they are very difficult to study. Surfaces usually lack the high symmetry and purity of the interior of a solid and are often strongly influenced by adsorbed impurities from their environment. It is hard to overstate the significance of the Devil’s work since surfaces are very important. Phenomena experienced in everyday life such as corrosion, adhesion, adsorption, friction and lubrication all occur at surfaces. More intimately, the crucial role played by surfaces in biocompatibility gives them an importance in the design of materials used in dentistry, contact lenses and medical implants such as hip joints and knee replacements. Industrial processes that occur at surfaces have a great impact on our lives and include crystal growth, semiconductor device manufacture and heterogeneous catalysis. Surface properties will also have a dominant influence on the emerging field of nanotechnology. The control of surface properties is thus essential to the function of a wide variety of materials. Clearly, then, a primary aim of surface scientists is to obtain a sufficient understanding of surfaces to make it possible to control surface properties.

The field of surface science concerns fundamental, nanoscale investigations of surface phenomena that are both scientifically important and technologically relevant. The subject is relatively new since the experimental study of clean surfaces was delayed for many years by the crucial limitation that in a pressure in excess of 10⁻⁶ mbar a clean surface adsorbs a monolayer of impurities within a few seconds. Thus experimental work on clean surfaces could not begin until the development in the 1960s of ultra-high vacuum (UHV) techniques, which reach pressures of 10⁻¹⁰ mbar or less. This technological advance led to a rapid increase in surface studies and a proliferation of experimental techniques of which there is space here to give only the briefest of descriptions of some of the most important.

The advances made in the understanding of surfaces in UHV conditions in the last forty years raise two questions. Firstly, to what extent are the surfaces that we now understand representative of the field in general? Secondly, does an understanding of the behaviour of surfaces in UHV provide a good guide to the behaviour of surfaces in the ambient conditions in which we find them in everyday life? The second of these questions is particularly pertinent to the role of surfaces in biomaterial applications and it will be addressed later in this chapter. The first question requires a rough estimate of how many surfaces are worthy of study since there are an infinite number of ways of terminating single crystals, to say nothing of the number of possible surfaces of amorphous materials. As a rough guide we note that Wyckoff’s six volume classification¹ lists ~7000 crystal structures. If we assume that each crystal structure has three important crystal faces and that it is appropriate to seek to understand the interactions that occur between each of these faces with the ten most important gases then we have a target for surface science of understanding ~200,000 surfaces. So far, the structures of ~1000 surfaces have been determined, about 1% of the target. We have a good understanding of the behaviour of the known surface structures in UHV and some understanding of the factors that are important in catalysis. In addition surface science has made major contributions to the considerable progress that has been made in the design and controlled growth of semiconductor systems. However, as will be made clear later, we are only just beginning to develop an understanding of surfaces in ambient conditions. The study of surfaces thus has a long way to go, particularly in addressing issues that are important in the real world.

1.2 Experimental considerations

We begin with a brief account of the importance of UHV and a description of single crystal surfaces. There are many experimental probes capable of detailed investigations of surfaces and interfaces, however, we have space to give only a brief overview of some of the most commonly used surface techniques. A more extensive introduction to these and many other techniques may be found in the books by Woodruff and Delchar,² Zangwill,³ Prutton⁴ and Venables.⁵

1.2.1 Ultra-high vacuum

As indicated in the introduction, the experimental investigation of the fundamental properties of surfaces have mostly taken place in UHV where the pressure is typically 10^-1 mbar – thirteen orders of magnitude lower than atmospheric pressure. A UHV environment is required to prepare a well-defined clean surface and maintain it for a sufficient time for experimental studies. In addition to surface preparation, a good vacuum is also a prerequisite for many of the experimental probes used to study surfaces since these probes are often based upon controlling the trajectories of electrons and ions. Vacuum technology has developed pumping systems capable of maintaining a UHV environment within stainless steel chambers for indefinite periods of time.

Experiments are not exclusively performed on clean surfaces in UHV and the recent development of experimental probes that are capable of operating in a non-UHV environment is giving rise to an increasing trend of experimental studies of surfaces in ambient and liquid environments.

1.2.2 Crystal surfaces and surface preparation

The majority of experimental surface studies have been performed on single crystals in order to simplify the atomic and electronic structure of the surface. Crystal surfaces can be prepared so as to consist of relatively large flat terraces made up of atoms of similar atomic coordination, with relatively few atoms associated with defect sites such as steps. The majority of single crystals grow in one of four basic structures: simple cubic (SC), face centred cubic (FCC), body centred cubic (BCC) or hexagonal close packed (HCP). When a single crystal is terminated by a surface, then, depending on the angle of the termination, different atomic arrangements are exposed. These different surfaces are described by the Miller indices and an introduction to this system of classification of crystal structures and surfaces can be found in ref. 6, which also explains the Wood notation that is used to describe the symmetry of surfaces. For FCC and BCC crystals, surface planes are defined by three integers. The three ‘low index’ surfaces, (110), (111) and (100) created from the FCC and BCC structures are shown in Figs 1.1 and 1.2, respectively. The figures show that different crystal planes have different atomic densities and hence differences in free energy at the surface. The free energy of a surface is an important determinant of its behaviour as will be discussed later in considerations of crystal growth. Free energy consideration also influences the natural cleavage planes of crystalline materials and cleavage along a non-preferred direction often results in a rough morphology composed of small areas of energetically preferred faces known as faceting.

1.1 Surface structures created from cleavage of the FCC structure: (a) (110), (b) (111), and (c) (100). For the surface structures, the unit cells are shown: light spheres = 1 st layer atoms, dark spheres = 2nd layer atoms.

1.2 Surface structures created from cleavage of the BCC structure: (a) (110), (b) (111), and (c) (100). For the surface structures, the unit cells are shown: light spheres = 1st layer atoms, dark spheres = 2nd layer atoms, darkest spheres = 3rd layer atoms.

The cleavage of single crystals in UHV is one of the easiest ways of producing a clean surface. However, it can only be applied to the natural cleavage planes and these are not always the most important surfaces of a material. A variety of ways have been developed for producing clean surfaces on crystal faces that cannot be obtained by cleaving. These often involve bombarding the surface with argon ions in UHV to remove impurities followed by annealing to remove the structural damage. However, there is no cleaning procedure that wor...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Contributor contact details
• Preface
• Part I: Forming methods
• Part II: Measurement, monitoring and characterisation
• Part III: Surface interaction and in-vitro studies
• Part IV: Surface interactions and in-vivo studies
• Part V: Appendices
• Index