Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 162

Surface properties are important because, together with bulk properties (Chapter 4), they determine the performance of biomaterials in vivo. Upon implantation, it is the surface of the biomaterial that is first encountered by the physiological environment. The interaction between the surface and components of the physiological fluid, such as water molecules, inorganic ions, amino acids and proteins, strongly influences the response of cells and, thus, the biocompatibility of the implant. In the healing of bone defects using nondegradable implants, for example, if the surface properties and characteristics of the implant are not selected properly, cells can form fibrous tissue around the implant instead of new bone. Consequently, the ability of the implant to integrate with host bone is limited, jeopardizing a successful outcome of the procedure (Chapter 23).

The surface of a material consists of a thin layer with properties and characteristics that are different from those of its interior due to a variety of physical and chemical factors (Figure 5.1). Depending on the composition 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.