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 the scale and geometry of the surface roughness, and on the type of cell as well. Consequently, an understanding of the interaction between surface topography and cell response is essential in creating implants with the desired surface topography. The scale of roughness that has the greatest effect on cell response is approximately equal to the cell dimension or smaller (Figure 5.19). This is because a surface will be perceived as approximately smooth by cells if the scale of roughness is larger than the cell diameter. Additionally, cells can respond differently to a “spiky” surface when compared to a smoothly undulating surface even though the peak to valley distance of the roughness is the same (Xu et al. 2017).
Figure 5.19 Schematic illustrating surface roughness parameters that can have a strong influence on cell response: scale of roughness spacing relative to the cell dimension and spikiness of roughness feature.
5.5.1 Surface Roughness Parameters
Surface roughness is commonly quantified in terms of the vertical height of the surface relative to a mean or reference line, defined such that the area between the roughness profile and the mean line is the same above and below the line. While a variety of parameters are used to quantitate the topography of materials, the average roughness Ra and the root mean square roughness Rq are most often used. Ra is the arithmetic mean of the average values of the vertical deviation from the mean line of the roughness profile whereas Rq is the square root of the arithmetic mean of the square of the vertical deviation from the mean line. As these two parameters are primarily concerned with the relative departure of the roughness profile in the vertical direction only, they do not provide information about the slopes, shapes, and sizes of the surface asperities or about their spacing. It is possible for surfaces of widely different profiles to have approximately the same Ra or Rq values. In general, these two parameters are more useful for characterizing surfaces of the same type that are produced by the same method.
5.5.2 Characterization of Surface Topography
Characterization of surface topography commonly starts with examining the surface of one or more representative specimens of the material in a scanning electron microscope (SEM) (Figure 5.18). Qualitative information about topographical features such as surface roughness at a microscale or nanoscale, the waviness and spacing of the surface roughness, and the presence of surface flaws can be obtained using this technique. Thereafter, other techniques, classified into two broad categories, depending on whether a component of the measuring instrument makes contact with the specimen surface (contact type) or not (noncontact type), can be used to obtain topographical data such as surface roughness parameters. As a contact‐type technique has the potential for damaging a soft surface, applied loads on the contacting component (for example, a stylus) should not exceed the hardness of the surface. The major techniques used for biomaterials are profilometry and atomic force microscopy (AFM), both of which can be used in a contact or noncontact mode.
