Materials for Biomedical Engineering. Mohamed N. Rahaman

surfaces of Ti6Al4V (R_a ≈ 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 that is discussed further in Chapter 21.

Problems

      1 5.1 A dental implant composed of titanium (Ti), approximated as a cylinder of diameter 5.0 mm and length 8.0 mm, has a surface layer of thickness 3 nm composed of titanium dioxide (TiO2). Determine the fractional volume of the TiO2 surface layer, that is, the volume of the TiO2 layer relative to the total volume of the implant.

      2 5.2 Distinguish between the terms surface tension and surface energy.

      3 5.3 Determine the contact angle of a droplet of deionized water on ultrahigh molecular weight polyethylene (UHMWPE), given that the densities of water and UHMWPE are 1.0 and 0.94 g/cm3, respectively, the surface tension of water is 73.0 mN/m, and the surface energy of UHMWPE is 36 mJ/m2.

      4 5.4 The contact angle of a droplet of deionized water on a microrough surface of silicon nitride was observed to decrease from 66° ± 9° to 30° ± 9° in 30 minutes. Suggest an explanation for the decrease in contact angle.

      5 5.5 Explain the significance of the critical surface tension in relation to biomaterials.

      6 5.6 In an X‐ray photoelectron spectrum of titanium, is the adventitious C 1s peak expected to be at a higher or lower binding energy than the O 1s peak? Is the Ti 2s peak expected to be at a higher or lower binding energy than the Ti 2p peak? Explain.

      7 5.7 The composition of the oxide surface layer on silicon nitride (Si3N4) can change with depth. Briefly describe a method for studying the change in surface composition with depth and, in general terms, the expected change in surface composition.

      8 5.8 Explain how the following biomaterials develop a surface charge when implanted in the physiological environment: (a) titanium, (b) silicon nitride, and (c) PEEK.

      9 5.9 Briefly explain the type of surface topographical features that are expected to influence the response of cells.

References

      1 Bjursten, L.M., Rasmusson, L., Oh, S. et al. (2010). Titanium dioxide nanotubes enhance bone bonding in vivo. Journal of Biomedical Materials Research. Part A 92: 1218–1224.

      2 Bock, R.M., Jones, E.N., Ray, D.A. et al. (2017). Bacteriostatic behavior of surface modulated silicon nitride in comparison to polyetheretherketone and titanium. Journal of Biomedical Materials Research. Part A 105: 1521–1534.

      3 Chen, Q., Zhang, D., Somorjai, G., and Bertozzi, C.R. (1999). Probing the surface structural arrangement of hydrogels by sum‐frequency generation spectroscopy. Journal of the American Chemical Society 121: 446–447.

      4 Fox, H.W. and Zisman, W.A. (1950). The spreading of liquids on low‐energy surfaces. I. Polytetrafluoroethylene. Journal of Colloid Science 5: 514–531.

      5 Fox, H.W. and Zisman, W.A. (1952). The spreading of liquids on low‐energy surfaces. III. Hydrocarbon surfaces. Journal of Colloid Science 7: 428–442.

      6 Girifalco, L.A. and Good, R.J. (1957). A theory for the estimation of surface and interfacial energies. I. Derivation and application to interfacial tension. The Journal of Physical Chemistry 9: 904–909.

      7 Lausmaa, J. (1996). Surface spectroscopic characterization of titanium implant materials. Journal of Electron Spectroscopy and Related Phenomena 81: 343–361.

      8 Marmur, A. (2003). Wetting on hydrophobic rough surfaces: to be heterogeneous or not to be. Langmuir 19: 8343–8348.

      9 Ratner, B.D. (2013). Surface properties and surface characterization of biomaterials. In: Biomaterials Science: An Introduction to Materials in Medicine, 3e (eds. B.D. Ratner, A.S. Hoffman, F.J. Schoen and J.E. Lemons), 34–55. New York: Elsevier.

      10 Schwartz, Z., Raz, P., Zhao, G. et al. (2008). Effect of micrometer‐scale roughness of the surface of Ti6Al4V pedicle screws in vitro and in vivo. The Journal of Bone and Joint Surgery. American Volume 90: 2485–2498.

      11 Xu, L.C. and Siedlecki, C.A. (2012). Submicron‐textured biomaterial surface reduces staphylococcal bacterial adhesion and biofilm formation. Acta Biomaterialia 8: 72–81.

      12 Xu, L.C., Wo, Y., Myerhoff, M.E., and Siedlecki, C.A. (2017). Inhibition of bacterial adhesion and biofilm formation by dual functional textured and nitric oxide releasing surfaces. Acta Biomaterialia 51: 53–65.

Further Readings

      1 Gittens, R.A., Olivares‐Navarrete, R., Schwartz, Z., and Boyan, B.D. (2014). Implant osseointegration and the role of microroughness and nanostructures: lessons for spine implants. Acta Biomaterialia 10: 3363–3371.

      2 Schwartz, Z., Lohmann, C.H., Oefinger, J. et al. (1999). Implant surface characteristics