Materials for Biomedical Engineering. Mohamed N. Rahaman
such as silicon nitride (Si3N4), and in the network structure of glasses (Chapter 3). It contributes to the bonding in some oxides such as Al2O3 and high melting point metals such as tungsten and tantalum.
Covalent‐bonded ceramics find some use as biomaterials. Examples include silicon nitride used as implants in spinal repair, a few nondegradable glasses in some medical devices and a few bioactive glasses used in healing bone defects and skin wounds (Chapter 7). Carbon materials such as diamond and graphite are often classified as ceramics but their properties are more relevant to industrial applications (Chapter 3). Pyrolytic carbon is an important material in the manufacture of heart valves whereas diamond‐like carbon (DLC) has been investigated as coatings for articulating bearings in hip and knee implants. More recently discovered allotropes of carbon such as fullerenes, carbon nanotubes, and graphene, have been the subject of an enormous amount of investigations for use in technological applications and are now receiving considerable interest for potential biomedical applications.
Covalent Bonding in Polymers
Covalent bonding occurs in synthetic and natural polymers, linking atoms in the backbone of the long‐chain molecule, often referred to as a macromolecule. The macromolecules in the synthetic polymer polyethylene, a material used as articulating bearings in hip and knee implants, has the simplest structure, built up of –H2C–CH2– repeating units (Figure 2.10). These macromolecules show strong covalent bonds between the carbon atoms in the chain backbone and a tetrahedral arrangement of the covalent bonds at each carbon atom in the chain resulting from sp3 hybridization of the electron orbitals of the carbon atom. The single bond between the carbon atoms in the chain backbone allows one carbon atom to rotate more or less freely, relative to a neighboring carbon atom and this, coupled with weak intermolecular forces between neighboring chains, often lead to a random arrangement of the chains in three dimensions. On the other hand, if the repeating unit has a simple structure as, for example, in polyethylene, some regions of the macromolecule can arrange themselves in an ordered three‐dimensional pattern. When formed under appropriate conditions, such as cooling sufficiently slowly from the molten state, polyethylene is composed of crystalline regions interspersed with amorphous regions, a structure often described as semicrystalline (Chapter 3).
Figure 2.10 Illustration of covalent bonds linking carbon atoms (C) in the chain backbone of the polyethylene molecule and between the carbon atoms and hydrogen (H) atoms.
Other synthetic polymers have a structure that is more complex than polyethylene but similar principles are applicable (Chapter 8). Consequently, most solid polymers produced by conventional methods show low strength and low elastic modulus, become soft or molten at low to moderate temperatures, and are insulating electrically and thermally. On the other hand, alignment of the polymer chains in a certain direction and hydrogen bonding, a strong form of intermolecular bonding (Section 2.5.2), can lead to high strength and high elastic modulus in the direction of alignment as, for example, in fibers of the synthetic polymer nylon.
2.4.3 Metallic Bonding
The metallic bond is the dominant bond in metals and their alloys. Because the metal atoms have low electronegativity, the valence electrons tend to leave the parent atom, making the atom a cation, and form a “sea” of rather free electrons that surrounds the ions (Figure 2.1c). The valence electrons are shared by the cations and are not localized at any one cation. Bonding arises from the strong attraction between the cations and the sea of delocalized electrons. Consequently, the bond has no directionality and the cations arrange themselves to form simple densely packed crystalline structures in solids (Chapter 3). The packing of the cations in the solid can be visualized in a simple manner as the ordered packing of tiny ball bearings.
Except when there is a covalent contribution, as in the high‐melting point metals such as tungsten and tantalum, the metallic bond is somewhat weaker than the ionic or covalent bond. Thus, many metals have a lower strength, lower elastic modulus, and lower melting point than most ceramics (Figure 1.5). The sea of mobile electrons is responsible for the high electrical conductivity of metals and this sea of electrons, together with a greater ability of the cations to undergo thermal vibration, is responsible for their high thermal conductivity. Another attractive property of metals is their ductility, the ability to deform when subjected to sufficiently high mechanical stresses instead of fracturing in a brittle manner, characteristic of ceramics. This ductility is due to the ease with which the metallic bond can be broken and reformed when compared to the ionic or covalent bond, coupled with the ease with which the sea of electrons can adjust itself to accommodate the change in position of the cations (Chapter 3). Metals typically show moderate hardness that is lower than that of most ceramics due to their ability to deform in a ductile manner. Corrosion resistant metals find considerable use as biomaterials in a variety of devices such as fracture fixation plates, implants for total joint repair, and dental implants because of their attractive mechanical properties, and electrodes in pacemakers and neural stimulators because of their high electrical conductivity (Table ).
To summarize at this stage, features of the three types of primary bonds are illustrated in Figure 2.11.
Figure 2.11 Schematic comparison of the attractive interactions in the ionic, covalent, and metallic bond.
2.5 Secondary Bonds
Although considerably weaker than the primary bonds (Table 2.2), secondary bonds still have a significant effect on the properties of some materials. Secondary bonds are typically intermolecular bonds. Intermolecular bonding between neighboring chains, for example, is responsible for polyethylene and many other polymers existing as a solid at room temperature. Without intermolecular bonding between its polar molecules, water would boil at approximately −80 °C instead of existing as a liquid at room temperature. Proteins are the most versatile macromolecules in living organisms and they serve crucial functions in essentially all biological processes. As described in Section 2.6, intermolecular bonding in these macromolecules controls the shape (conformation) that these macromolecules take up and, thus, their functions. Secondary bonds are often divided into two main types: van der Waals bonds and hydrogen bonds.
2.5.1 Van der Waals Bonding
Van der Waals bonding is the weakest type of intermolecular bonding. It exists between all atoms and molecules regardless of what other interactions might be present. There are three types of van der Waals bonds, classified