Materials for Biomedical Engineering. Mohamed N. Rahaman
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Название: Materials for Biomedical Engineering
Автор: Mohamed N. Rahaman
Издательство: John Wiley & Sons Limited
Жанр: Химия
isbn: 9781119551096
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Figure 5.23 Schematic curve of force versus separation between the tip and specimen surface, showing the repulsive and attractive regions corresponding to contact mode and noncontact mode imaging.
Profilometry
Noncontact optical profilometry, based on the principle of interference between two light beams, is a widely used technique for characterizing the topography of materials. The reflected beams from two parallel plates placed normal to an incident beam interfere and result in the formation of fringes (lines) whose spacing is a function of the spacing of the two plates. If one of the plates is a reference plate and the other is a specimen surface whose roughness is to be measured, the fringe spacing can be related to the surface roughness (Figure 5.24). In white light interferometry, the incident beam is composed of all wavelengths in the visible spectrum, giving interference fringes of various colors in the spectrum. Profilometers based on white light interferometry have a lateral resolution of ~0.5–1.0 μm and a vertical resolution of ~1 nm. Consequently, this technique has a lower resolution than AFM.
Figure 5.24 Schematic illustrating the principle of optical interferometry.
Figure 5.25 shows a comparison of SEM, contact mode AFM and white light interferometry images of the same nominal material, Si3N4 in its as‐fabricated condition, a biomaterial used in spine repair. SEM shows a surface composed of elongated hexagonal grains, 0.2–2 μm in cross section, that protrude to various lengths, less than ~5 μm, in a random manner from the surface. Consequently, the roughness varies in scale from approximately a fraction of a micrometer to a few micrometers. Roughness parameters Ra and Rq obtained from AFM are 0.34 and 0.43 μm, respectively, values consistent with the SEM image. In comparison, Ra and Rq values obtained from white light interferometry are 0.64 and 0.83 μm, respectively. Whereas these values are almost twice those found from AFM, they are also consistent with the SEM image. One factor that could contribute to the difference between the AFM and interferometry values is that although the Si3N4 was fabricated using the same procedure, different regions of the material were imaged in the two techniques.
Figure 5.25 Topography of as‐fabricated silicon nitride obtained by (a) SEM, (b) AFM, and (c) optical profilometry.
Source: From Bock et al. (2017) / with permission of John Wiley & Sons.
5.5.3 Effect of Surface Topography on Cell and Tissue Response
Although the mechanism of interaction is complex, beneficial effects of nanoscale to microscale surface topography on the response of cells in vitro and in vivo have been reported for several biomaterials, as discussed in Chapter 21. These effects have been well demonstrated for Ti and its alloy Ti6Al4V. Titanium and Ti6Al4V see considerable use as implants in dental and orthopedic surgery, and their microtopography can be varied using common surface treatments such as machining, grit blasting, and acid etching. When compared to smoother machined surfaces (Ra ≈ 0.2 μm), microrough grit‐blasted surfaces of Ti6Al4V (Ra ≈ 3 μm) showed a better ability to stimulate the differentiation of osteoblastic cells in vitro (Schwartz et al. 2008). Additionally, when implanted in the vertebrae of sheep in vivo, implants with the microrough surface showed better bone formation around the implants and integration with host bone when compared to implants with the smoother machined surface.
The addition of microtopography, as noted earlier, is now a key design feature to improve outcomes associated with nondegradable biomaterials used as implants in medical and dental surgery. Adding nanoscale surface features to smooth (machined) surfaces using surface modification techniques has been reported to enhance the beneficial effects observed for microrough surfaces. For example, the formation of vertical TiO2 nanotubes (outer and inner diameter ~100 and ~80 nm, respectively, and height ~250 nm) on commercial purity Ti surfaces, has been shown to improve bone formation around the implants and integration with host bone in rabbit tibiae in vivo when compared to machined titanium surfaces (Bjursten et al. 2010).
For the same material, different treatments such as grinding, polishing, and grit blasting can lead to changes in other surface characteristics, such as surface chemistry, surface energy, contact angle and wettability, and not just in the surface topography. Consequently, the effects of surface topography alone often cannot be separated statistically from the contributions of these other surface properties and characteristics. This creates an additional level of complexity in understanding the mechanism by which surface topography influences the response of cells and in designing the optimal surface topography for a specific application.
5.6 Concluding Remarks
Surface properties and characteristics, together with bulk properties, have a strong influence on the performance of biomaterials in vivo. In this chapter, we discussed important surface properties relevant to biomaterials, including surface chemistry, surface energy, surface topography, and surface charge.
Surface chemistry is the most important surface property because it influences the behavior of a biomaterial in any given environment. We discussed a variety of techniques for characterizing surface chemistry of a material, but it should be noted that because most of these techniques require the use of an ultrahigh vacuum, the measured surface chemistry is often different from that of biomaterials implanted in vivo.
Surface energy is difficult to measure for many materials and, consequently, it is often considered more simply in terms of the extent to which a liquid will wet and spread over a surface. The contact angle between a liquid (water) droplet and a solid surface is often taken as a measure of the wettability of the system. Overall, hydrophilic materials (low contact angle) show good wetting and spreading by water whereas hydrophobic materials (high contact angle) show poor wetting by water.
When placed in an aqueous environment, such as the physiological fluid, biomaterials develop a surface charge that can influence subsequent adsorption of ions and molecules from the medium.
Certain surface topographical features, such as certain roughness features, have been shown to have beneficial effects on the response of certain cells in vitro and in vivo, a topic СКАЧАТЬ