Materials for Biomedical Engineering. Mohamed N. Rahaman

modes of analysis. In static SIMS, an ion beam of low intensity is used so that analysis is confined to the outermost layers of the surface. In comparison, in dynamic SIMS, a high intensity beam is used to erode successive atomic layers at a relatively rapid rate. Static SIMS is more relevant to the application of biomaterials as it provides information about the outermost surface atoms. While SIMS has the advantages of high spatial resolution, high sensitivity for qualitative elemental analysis and the ability to provide a detailed analysis of the chemical composition of the surface, quantitative analysis of the surface composition is often difficult.

5.4 Surface Charge

      As noted earlier, prior to implantation in the physiological environment, the surface of most polymers is normally covered with physically adsorbed water molecules. In comparison, most metals and ceramics have a surface composed of OH groups attached to the outermost metal atoms, on top of which are physically adsorbed H₂O molecules. The physiological fluid in vivo, on the other hand, can be approximated as an aqueous medium of homeostatic temperature 37.4 °C and pH 7.4, which contains a variety of ions, small molecules such as amino acids, macromolecules such as proteins, and substances released by cells. Upon implantation, the surface of a biomaterial acquires a positive or negative charge due to adsorption of ions or molecules from the aqueous medium or dissociation of certain surface functional groups, depending on the surface chemistry of the biomaterial.

      5.4.1 Surface Charging Mechanisms

The surface of most metals and ceramics used as biomaterials, normally hydroxylated due to chemically adsorbed water molecules, acquire a surface charge typically by adsorption of hydrogen (H⁺) ions, equivalent to hydronium (H₃O⁺) ions, or hydroxyl (OH⁻) ions (Figure 5.13). These ions are often referred to as the charge‐determining ions for these materials. In an acidic medium (lower pH), preferential adsorption of H⁺ ions leads to a positively charged surface, whereas in a basic medium (higher pH), a negatively charged surface is formed due to preferential adsorption of OH⁻ ions. At some intermediate pH, called the point of zero charge (PZC), there is a balance between adsorption of H⁺ and OH⁻ ions, which leads to an electrically neutral surface. The PZC depends on the acidity or basicity of the surface composition. The more acidic oxides such as SiO₂ have a lower PZC whereas the more basic oxides such as MgO have a higher PZC.

Figure 5.13 Production of surface charge on a hydroxylated metal oxide surface by adsorption of ions from an “acidic” or “basic” solution.

      The PZC can be measured from acid–base titrations but, often, it is easier to measure the zeta ( ζ ) potential corresponding to the electrostatic potential at a small distance from the surface (a few tenths of a nanometer). The pH at which the measured ζ potential is zero is referred to as the isoelectric point (IEP). Upon implantation in the physiological environment, then, a material whose IEP is lower than ~7.4, such as a more acidic metal oxide, will have a negative surface charge and electrostatic potential, whereas one having an IEP higher than ~7.4, such as a more basic metal oxide, will have a positive surface charge and potential.

Some polymers that contain ionizable surface groups can acquire a charge by dissociation, such as dissociation of the H atom in the carboxyl (C=O)OH group, resulting in a negatively charged surface (Figure 5.14). In comparison, the presence of amine (NH₂) groups can lead to a positively charged surface due to adsorption of H⁺ ions from an aqueous medium. For a given composition, the extent of dissociation or adsorption and, thus, the magnitude of the surface charge depends on the pH of the aqueous medium. The dissociation of H⁺ from the (C=O)OH group, for example, often starts at a pH of ~3 and is essentially completed at a pH of ~7 to 8 when a large fraction of the H atoms at the surface has dissociated. In comparison, NH₂ groups are essentially neutral at pH higher than ~10 but the fraction at the surface that is protonated increases at lower pH.

Figure 5.14 Production of negative or positive surface charge on surface composed of functional groups, as exemplified by the carboxyl and amine groups.

The presence of ionizable functional groups at the surface of a material is not a prerequisite for the acquisition of a surface charge. Polymers devoid of such functional groups can acquire a surface charge in aqueous media by dispersion forces, that is, van der Waals forces of attraction between the material and ions present in the aqueous medium. These attractive forces are similar in nature to the van der Waals forces between molecules described in Chapter 2. Ions in aqueous solutions are normally hydrated, that is, they are surrounded by an adsorbed layer of water molecules. Attraction between the atoms at the material's surface and the hydrated ions, positive or negative depending on the surface composition, leads to adsorption and, thus, to the production of a charged surface (Figure 5.15). The aqueous medium of the physiological environment, for example, contains a variety of ions such as H⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, OH⁻, Cl⁻, HCO₃ ⁻, HPO₄ ²⁻, and SO₄ ²⁻, and a variety of biomolecules (Chapter 14). The ensuing zeta potential of the biomaterial reflects the nature of the ions or molecules adsorbed at its surface.

Figure 5.15 Production of surface charge on a surface devoid of functional groups by van der Waals attraction of ions in solution.

Overall, then, in an aqueous medium, the surface of metals and ceramics acquire an electrostatic charge by adsorption of H⁺ or OH⁻ ions (Figure 5.13) whereas polymers composed of certain surface functional groups can also acquire a surface charge by, for example, dissociation or adsorption of H⁺ ions (Figure 5.14). Adsorption of water molecules and ions of the opposite charge, called counterions, leads to the formation of an ion atmosphere that is spread out in the solution. In colloid chemistry, this ion atmosphere is referred to as an electrical double layer (Figure 5.16). It consists of a rather tightly bound layer of counterions, referred to as the Stern layer, and a more diffuse layer in which the positive and negative ions migrate freely. The electrostatic potential at the boundary between the Stern layer and the more diffuse layer approximates the zeta potential. As noted earlier, polymers that do not possess appropriate surface functional groups acquire a surface charge by adsorption of hydrated ions from the solution (Figure 5.15).

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