Materials for Biomedical Engineering. Mohamed N. Rahaman

Figure 5.16 Illustration of the electrostatic charge distribution surrounding a solid surface upon introduction of the solid into an aqueous solution, shown for a solid that develops a negatively charged surface. The vertical dashed line corresponds approximately to the plane of the zeta potential.

      5.4.2 Measurement of Surface Charge and Potential

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,

      5.4.3 Effect of Surface Charge

      As the surface of a biomaterial rapidly develops an electrostatic surface charge and an associated electrical double layer upon implantation in the physiological environment, it is realistic to assume that this surface charge system should influence subsequent interactions with the physiological environment. Depending on their composition, the components of the physiological fluid such as ions, amino acids, and proteins can be positively charged, negatively charged, polar or nonpolar (Chapter 3). Consequently, we might expect varying degrees of electrostatic interaction between these components and the charged surface. This interaction should influence, for example, the type of ions and molecules adsorbed at the surface and the conformation of adsorbed proteins, which, in turn, should influence the response of cells. In practice, a correlation between the surface charge of a biomaterial and its interaction with the physiological environment has been difficult to establish. While this is due to a variety of reasons, a major factor is the difficulty in separating the true effect of surface charge from the contributions of other surface properties such as wettability (contact angle) and surface topography.

      

5.5 Surface Topography

When applied to materials, the term topography commonly refers to the roughness or smoothness of a material's surface. It has been well established that surface roughness can have beneficial effects on the response of certain cells in vitro and in vivo when compared to a smooth surface of the same material (Schwartz et al. 2008). As several biomaterials used in dental and orthopedic surgery are composed of metals, ceramics, and polymers that are neither degradable nor bioactive, surface roughness plays a key role in improving their interaction with cells and tissues in vivo. Nowadays, most dental implants composed of titanium or its alloy Ti6Al4V are designed with some degree of surface roughness to improve their integration with host bone, although there is some debate about the optimal nature of the surface roughness.

Surface topography of materials can be accidentally or deliberately introduced (Figure 5.18). Accidental features normally result from the fabrication process or from subsequent treatments such as abrasion, machining or grinding, and consist typically of marks, lines or groves. They are commonly random and variable over the surface, which presents difficulties in studying and interpreting their interaction with cells in vitro and in vivo. In comparison, deliberate topographical features, often composed of ordered spikes, grooves, or pores, can be created using a variety of controllable techniques, such as photolithography, electron beam lithography, or laser interference lithography. When this deliberately introduced topography has an ordered pattern, it is often referred to as surface texture. Although accidentally introduced topographical features are more common in biomaterials, much information on understanding cell response to topography has been achieved from deliberately introduced surface features.

Figure 5.18 Examples of surface topography accidentally introduced (a, b) or deliberately introduced (c–e) in biomaterials. (a) Machined surface of polyether ether ketone (PEEK); (b) machined surface of Ti6Al4V; (c) sand‐blasted surface of Ti6Al4V. (a–c)

      Source: From Bock et al. (2017).

      (d) Hemispherical depressions in titanium formed by photolithography. (e) Micro‐pillars on polyurethane produced by lithography. Source: From Xu and Siedlecki (2012).

The effect of topography on cell response has been shown to depend on