Materials for Biomedical Engineering. Mohamed N. Rahaman

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Название Materials for Biomedical Engineering
Автор произведения Mohamed N. Rahaman
Жанр Химия
Серия
Издательство Химия
Год выпуска 0
isbn 9781119551096



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target="_blank" rel="nofollow" href="#ulink_52df14f9-ac57-57ab-b39e-a027a160e999">Figure 5.5 Zisman plot for polymethyl methacrylate (PMMA) using various liquids.

      5.2.2 Measurement of Contact Angle

      While a variety of methods can be used to measure the contact angle of a liquid on a solid (Ratner 2013), the sessile drop technique is easy to perform and, thus, finds considerable use. In this technique, a drop of the appropriate liquid, for example deionized water, is placed on a flat surface of the material according to a standard procedure and the contact angle is determined from images of the drop using automated equipment (Figure 5.3). The surface tension γlv of liquids used in contact angle measurements to determine γcr or to estimate γsv of a material (Eq. (5.6)) can be measured using simple techniques such as capillary rise of a liquid. However, this is often not necessary because the surface tension values of pure liquids at an appropriate experimental temperature such as room temperature are given in reference tables.

      where θ is the contact angle for a smooth surface of the same composition and Ρ is the roughness ratio defined as the true area of the rough surface relative to its nominal cross sectional area. In comparison, when wetting of the rough surface is inhomogeneous, that is, when air is trapped between the drop and the rough surface, the apparent contact angle θCB is given by

Schematic illustration of homogeneous wetting (a) and heterogeneous wetting (b) of a rough surface illustrated for a hydrophobic liquid.

      For homogeneous wetting, Eq. (5.9) predicts that a rough surface will decrease the contact angle of a hydrophilic material ( θ < 90°), that is, the hydrophilicity of the material will increase. In comparison, the contact angle of a hydrophobic material ( θ > 90°) will increase with surface roughness, that is, the hydrophobicity of the material will increase. For a greater amount of air trapped between the liquid and the rough surface, f decreases and, consequently, the apparent contact angle is higher. Subsequently, if the liquid slowly infiltrates the areas of trapped air, the contact angle is predicted to decrease with time, eventually becoming smaller than the contact angle of a smooth surface of the same composition. Thus, the measured contact angle of a rough surface can vary, depending on the time.

Schematic illustration of images showing contact angle of a deionized water drop on (a) machined surface of polyether ether ketone (PEEK), (b) machined surface of Ti6Al4V, (c) surface of as-fabricated silicon nitride (Si3N4), and (d) surface of as-fabricated Si3N4 after an oxidation treatment.

      Source: From Bock et al. (2017).

      5.2.3 Effect of Surface Energy

      Apart from its effect in controlling hydrophilicity or hydrophobicity, surface energy by itself has not played a major role in the applications of biomaterials. This is due to a variety of factors. One factor is that the true effect of surface energy is often difficult to separate from contributions from the effects of other surface properties such as surface charge and surface topology. Another factor is the difficulty in correlating surface energy with the interactions of biomaterials with the physiological environment. Often, surface energy is determined from experiments performed under certain ideal conditions that are different from practical conditions in vitro and in vivo. Prior to surface energy measurement, the biomaterial is often subjected to grinding and polishing, and thermal treatment to remove physically and chemically adsorbed water. These treatments can lead to a surface that is different from that of the implanted biomaterial due to adsorption of moisture and impurities such as hydrocarbons from the environment or from the container used for packaging. Standard liquids such as deionized water or, in some cases, phosphate buffered saline used experimentally are also different from the aqueous medium of the physiological environment which contains a variety of ions, small molecules, and proteins.

      The surface chemistry of a material influences its behavior in any given environment and, thus, forms the most important surface property of biomaterials. Surface chemistry influences several characteristics of a biomaterial, such as

       Chemical reactivity toward its environment as, for example, oxidation