Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 178

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.