Materials for Biomedical Engineering. Mohamed N. Rahaman

of the material, its production process and the environment to which it is exposed, the thickness of this surface layer can vary from a few nanometers to hundreds of nanometers. Due to their lower coordination with surrounding atoms, the outermost surface atoms have a higher energy than atoms in the interior and, consequently, they have a higher reactivity and mobility. Ions, molecules, and contaminants from the ambient environment can also adsorb physically or chemically onto the surface. Together, these factors commonly lead to a surface composition that is different from that of the interior.

Figure 5.1 Various types of surface characteristics: (a) rough or smooth; (b) compositionally inhomogeneous with depth or composed of different functional groups; (c) chemically and structurally homogeneous or inhomogeneous; (d) crystalline or amorphous.

      Except for the noble metals such as gold, silver, and platinum, atoms at the surface of clean metals react with chemisorbed oxygen molecules to form a metal oxide surface layer upon exposure of the metal to a dry atmosphere containing oxygen, even for a short time. The surface oxide layer formed under such conditions is often very thin, ~2–3 nm for titanium (commercial purity) and stainless steel (316L) used as implants. Autoclaving and thermal sterilization, however, often lead to an increase in the thickness of the surface oxide layer, to ~5–10 nm for titanium, for example.

      In an atmosphere containing moisture, such as common ambient conditions, all materials are typically covered with ~2–3 layers of adsorbed water molecules. In the case of most metals and ceramics, the outermost more reactive metal atoms react with adsorbed water molecules to form surface hydroxyl (OH) groups in order to increase their coordination. Thus, the surface of metals and ceramics is normally composed of chemically bonded OH groups on top of which are ~2–3 layers of physically adsorbed water molecules. The surface of polymers that have undergone some oxidation during their processing may contain a limited amount of OH groups but, generally, the surface of most polymers are covered with physically adsorbed H₂O molecules.

      The aqueous physiological fluid in vivo contains a variety of components such as inorganic ions, amino acids, proteins, and substances released by cells. Upon implantation, the surface of a biomaterial becomes rapidly covered with positive or negative ions and water molecules, which subsequently influences its interaction with the other components of the physiological fluid. Depending on the scale of observation, the surface of a biomaterial can show a variety of structural features, such as a crystalline or amorphous structure, and topography (roughness or smoothness). These structural features can also influence the interaction of the surface with the physiological environment.

      The inherent composition of the material itself and its exposure to the environment can, in some cases, result in surface characteristics and properties that are not optimal for the intended application. In these cases, a variety of techniques are available to modify the surface of the material in order to achieve a more desirable composition (Chapter 13) and topography (Chapter 21).

Overall, then, this chapter will discuss the following topics:

       Surface properties of biomaterials related to surface energy, composition, charge, and topography

       Physical and chemical principles underlying these surface properties and the techniques used to measure them.

      Detailed descriptions of equipment and procedures are not covered because they are well provided elsewhere, such as in texts devoted more specifically to surface characterization or manuals provided by the equipment manufacturers.

5.2 Surface Energy

The outermost atoms at the surface of a material have a higher energy when compared to atoms in the interior due to their lower coordination and disrupted bonding (Figure 5.2). This excess energy is referred to as the surface energy, defined as the work required to create a unit area of surface. Surface energy (J/m²) and surface tension (N/m) have different units but they are numerically equal. These two terms are often used interchangeably but, largely, surface energy has been more commonly associated with a solid whereas surface tension has been associated with a liquid. The difference in units stems from the way in which the terms are defined. Surface tension is defined mechanically in terms of a pressure difference Δp across a curved surface by the Laplace equation

(5.1)

where γ_s is the surface tension, and r₁ and r₂ are the principal radii of curvature of the surface. According to Eq. (5.1), the units of γ_s are N/m. On the other hand, surface energy γ_s is defined thermodynamically as the surface energy per unit area, giving units of J/m².

Figure 5.2 Illustration of lower coordination and disrupted bonding of outermost atoms at the surface of a crystalline material that gives rise to a surface energy.

      Unless a material is in an ideal vacuum, its surface will be in contact with another medium such as a vapor (gas) phase or a liquid phase, which will influence the bonding at the solid surface and, thus, its surface energy. Consequently, the surface energy γ_s of a solid commonly refers to the energy of the solid–vapor interface, that is, the solid surface in contact with the appropriate vapor (gas) phase such as air. It is often designated γ_sv to signify this, where the subscript sv refers to the solid–vapor interface. Similarly, the surface energy (surface tension) of a liquid is the interfacial energy of the liquid–vapor interface, designated γ_lv. At room temperature, γ_sv of many synthetic polymers are in the range ~20–50 mJ/m². In comparison, many metals and ceramics show much higher γ_sv values, in the range ~0.2–2 J/m², while a few metals show γ_sv values higher than 2 J/m². The low γ_sv for polymers is related to their weak van der Waals intermolecular bonds whereas the higher γ_sv for metals and ceramics is related to their strong interatomic bonding.

Surface energy has a significant influence on reactions that take place at the surface of a material. As there is a thermodynamical driving force to lower its energy, a material with a high surface energy will tend to encourage adsorption of substances from its surroundings if this will lead to a reduction in energy. In this sense, the surface energy of a solid has often been discussed in terms of the degree of contact between a drop of liquid and the solid surface (Figure 5.3). The change in the Gibbs free energy dG when the area A of the drop in contact with the solid increases by an infinitesimal amount dA is given by

(5.2)

where γ_sv,