Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 176

Zeta potential measurements are often used to characterize the surface charge and surface potential of fine particles in a suspension using techniques that track the motion of the particles when an electric field is applied between two electrodes in the suspension. Although zeta potential measurements of macroscopic solids are more difficult to perform, they are more relevant to biomaterials used in their macroscopic solid form. This is because the zeta potential of a macroscopic solid is typically different from that of fine particles of the same nominal composition. The zeta potential of macroscopic solids is commonly determined from measurements of their streaming potential. Flow of an aqueous liquid over the charged surface of a material leads to shearing and shifting of the ions adsorbed at the surface. This leads to an electrokinetic effect called the streaming potential. The surface charge can be determined from the measured surface potential and the use of theoretical equations, but this is often not necessary because the zeta potential is commonly used as a measure of extent of the surface charge.

As an example, Figure 5.17 shows streaming potential data for the zeta potential as a function of pH for three biomaterials, PEEK, a titanium alloy (Ti6Al4V), and silicon nitride (Si₃N₄), the same materials described in Figure 5.7 (Bock et al. 2017). The pH of the aqueous liquid in these measurements was controlled using 0.1 M HCl solution at pH values in the range 3–5.5 and 0.1 M NaOH solution at pH between 5.5 and 10.0. In this medium, PEEK, Ti6Al4V, and Si₃N₄ have an IEP of 3.9, 4.4, and 4.5, respectively, and, at a pH of 7.4 (equal to the homeostatic pH of the physiological medium), a negative zeta potential of −50 (extrapolated by nonlinear regression), −15, and −45 mV, respectively. As PEEK has no ionizable functional groups, its negative zeta potential at the homeostatic pH is presumably due to preferential adsorption of negative ions present in the medium, such as Cl⁻ and OH⁻. The IEP of Ti6Al4V is in the measured range for TiO₂ (~4–6) and, thus, the negative ζ potential at the homeostatic pH is most likely due to preferential adsorption of OH⁻ ions at TiOH groups (Figure 5.13). As the surface of Si₃N₄ is composed of both, SiOH and NH₂ groups, the IEP does not correspond to that (2–3) commonly observed for SiO₂. Instead, the higher IEP results from deprotonation and protonation reactions, respectively, at the SiOH and NH₂ groups. Presumably, deprotonation at SiOH dominates at pH values above ~4.5, giving a negative surface charge and potential.

Figure 5.17 Zeta potential as a function of pH, as measured by the streaming potential method, for silicon nitride (Si₃N₄) as fabricated, and machined surfaces of Ti6Al4V and polyether ether ketone (PEEK).

Source: From Bock et al.(2017) / with permission of John Wiley & Sons,