about the chemical bonding (or oxidation state) of the surface atoms as well. As the incident beam is composed of X‐rays, XPS suffers from fewer problems related to electrostatic charging of the specimen and damage to the sample surface when compared to AES that relies on the use of an incident electron beam. Consequently, XPS is often a preferred technique for surface characterization of ceramics that are typically electrically insulating and polymers that are typically insulating and have a low hardness as well. However, as they provide similar information about elemental composition, XPS and AES tend to be used in a complementary manner.
A common mode of using XPS is to perform a survey scan over a wide binding energy range (typically 0–1000 eV) to provide a qualitative analysis of the surface composition. The fractional concentration of the elements present at the surface can be determined from the area of the major peaks of each element using software. Then, information about the chemical bonding or oxidation state of the relevant atoms can be determined from higher resolution scans of the relevant peaks and measuring their chemical shift, that is, the change in their binding energy. A variation in the number of valence electrons or the types of bonds that they form results in a change in the binding energy of the innermost electrons and, thus, to a chemical shift.
As an example, Figure 5.11 shows an XPS survey spectrum of commercial purity titanium that, prior to analysis, was subjected to treatments commonly used for titanium implants, such as machining, cleaning with various solvents, and steam autoclaving (at 135 °C for 20 minutes) (Lausmaa 1996). The major peaks corresponding to Ti and O reveal an oxidized surface. The minor peak corresponding to C is often due to contamination and, in many cases, to carbon present adventitiously in the spectrometer. Except for Ti and O, the peaks corresponding to C and the minor elements were found to disappear or to decrease significantly after sputtering off a few nanometers of the surface, indicating that these elements were present mainly as impurities on the surface. A peak at ~459 eV, which, from reference spectra, corresponds to Ti in the compound TiO2, dominates the high‐resolution spectrum of the Ti 2p peak (Figure 5.12). A smaller peak at ~454 eV corresponds to Ti in the underlying metal. Overall, then, the surface oxide layer on this titanium specimen corresponds to TiO2.
Figure 5.11 XPS survey spectrum for an autoclaved titanium dental implant.
Source: From Lausmaa (1996) / with permission of Elsevier.
Figure 5.12 XPS high‐resolution spectrum of the Ti 2p peak for a machined titanium implant.
Source: From Lausmaa (1996) / with permission of Elsevier.
The alloy Ti6Al4V also sees considerable use as a biomaterial. When subjected to the same treatments, its XPS spectrum is similar to that of commercial purity titanium but, in addition, it often shows a small amount of Al2O3. Typically, the concentration of Al in the surface oxide layer is approximately the same as that in the interior of the alloy.
Secondary Ion Mass Spectroscopy (SIMS)
SIMS consists of bombarding a surface with a primary beam of Ar, Ne, or He ions and analyzing the emitted ions and ion clusters in a mass spectrometer. As the emitted ions and ion clusters are characteristic of the surface, SIMS provides information about the chemical composition of the surface. Some information on the chemical bonding of the atoms can also be extracted by analyzing the composition of emitted ions and ion clusters.
SIMS can be used in two distinct 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
