Materials for Biomedical Engineering. Mohamed N. Rahaman

of adsorbed ions and molecules

       Development of a surface electrostatic charge

       Mobility of side groups attached to polymer chains and their structural reorientation at the surface.

      Apart from the noble metals such as gold, silver, and platinum, the surface of metals, exposed even for a short time to an environment containing oxygen (such as air) is typically composed of a thin layer of a metal oxide. This is due to the reaction of the surface atoms with chemically adsorbed oxygen molecules, which can be written

      (5.11)

      The driving force for the reaction is the free energy change associated with formation of the metal oxide but the growth of the oxide layer is controlled by kinetic factors such as time and temperature. Similarly, ceramics such as silicon nitride (Si₃N₄) that are not composed of a metal oxide, often referred to as nonoxide ceramics, commonly develop a thin oxide surface layer when exposed to an environment containing oxygen.

In an environment containing moisture, such as common ambient conditions of relative humidity 40–50%, the surface of all materials is covered with two to three layers of physically adsorbed water molecules. The outermost, lower‐coordinated metal atoms at a metal oxide surface react with the adsorbed water molecules to form surface hydroxyl (OH) groups in order to increase their coordination (Figure 5.8). On top of this hydroxylated surface, the water molecules are physically adsorbed by hydrogen bonds. Consequently, prior to implantation, most metals and ceramics used as biomaterials have a surface com posed of OH groups attached to the outermost metal atoms on top of which are physically adsorbed H₂O molecules. The presence of these surface OH groups strongly influences the development of an electrostatic charge on the surface of the biomaterial in an aqueous medium in vitro and in the physiological medium in vivo (Section 5.4).

Figure 5.8 Illustration of the formation of an oxide surface layer on a clean metal exposed to an oxidizing atmosphere and a hydroxylated surface upon exposure to a moist atmosphere.

      Depending on the nature of the surrounding medium, the polymer chains at the surface of some polymers can undergo structural reorientation of their side groups, which results in a change in surface composition. The thermodynamic driving force for this structural change is the reduction in energy due to a lower interfacial energy between the polymer surface and the medium. However, the rate at which this structural reorientation can occur is a kinetic problem, related to factors such as the flexibility of the chain backbone and the nature of the side groups. Typically, structural reorientation of the side groups occurs faster for polymers composed of single bonds between carbon atoms in the chain backbone and flexible, less bulky side groups. Surface structural reorientation in different polymers has been estimated to take place in time scales ranging from minutes to months.

The occurrence of surface structural reorientation has been studied for polyhydroxyethyl methacrylate (PHEMA), a biomaterial used for contact lenses, which has both hydrophilic and hydrophobic side groups (Figure 5.9). In air, the hydrophobic side groups are placed preferentially at the surface, resulting in a low surface energy. However, in an aqueous medium, the side groups can reverse their arrangement, presenting the hydrophilic (polar) side groups preferentially at the surface to interact with water molecules of the aqueous medium by hydrogen bonding and polar mechanisms.

Figure 5.9 A model for reorientation of polymer surface functional groups during dehydration of polyhydroxyethyl methacrylate (PHEMA) hydrogels. The proportion of methyl and hydroxyethyl groups depicted at the polymer surface reflects qualitative changes in surface composition.

      Source: From Chen et al. (1999) / with permission of American Chemical Society.

      5.3.1 Characterization of Surface Chemistry

In view of the significant role that surface chemistry plays in determining the performance of materials in biomedical applications and engineering applications as well, several techniques are available to study and characterize the surface chemistry of materials. The most common techniques are Auger electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS) also referred to as electron spectroscopy for chemical analysis (ESCA), and secondary ion mass spectroscopy (SIMS) (Table 5.1). AES, XPS, and SIMS are truly surface sensitive techniques in that they provide chemical information about the outermost layer (1–2 nm). They can also provide information from much deeper layers, commonly by sequential or simultaneous removal of surface layers by sputtering the surface with an ion beam and analyzing the sputtered surface, a mode of analysis referred to as depth profiling. A problem with these and most other surface analysis techniques is that they require an ultrahigh vacuum, necessitating the use of dry specimens that often differ in surface chemistry from that of biomaterials implanted in vivo. As noted earlier, in practice, biomaterials are often exposed to the ambient environment for a limited or prolonged period and, consequently, their surface is typically covered with physically and chemically adsorbed water and contaminants such as hydrocarbons.

Table 5.1 Common methods for the surface chemical analysis of materials.

Measurement parameter	Auger electron spectroscopy (AES)	X‐ray photoelectron spectroscopy (XPS)	Secondary ion mass spectrometry (SIMS)
Incident particle	Electrons (1–20 keV)	X‐rays (1.254 keV; 1.487 keV)	Ions (He+, Ne+, Ar+) (100 eV–30 keV)
Emitted particle	Auger electrons (20–2000 eV)	Photoelectrons (20–2000 eV)	Sputtered ions (~10 eV)
Element range	>Li (Z = 3)	>Li (Z = 3)	>H (Z = 1)
Detection limit	10−3	10−3	10−6–10−9
Depth of analysis	2 nm	2 nm	1 nm
Lateral resolution	>20 nm	>150 μm	50 nm–10 mm