Materials for Biomedical Engineering. Mohamed N. Rahaman

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. Diamond and graphite are not used as biomaterials. On the other hand, more recently discovered allotropes of carbon such as fullerenes, carbon nanotubes and graphene, have been the subject of an enormous amount of investigation for use in technological applications and are now receiving considerable interest for potential biomedical applications.

Diamond is often considered prototypical of strongly bonded covalent solids. It consists of a cubic unit cell in which the atoms are located at the corners of a tetrahedron due to sp³ hybridization of the valence electron orbitals of the carbon atoms (Figure 3.12). While the properties of diamond are very attractive due to its strong covalent bonds, they are more suitable to industrial applications. Diamond is also difficult and expensive to produce industrially.

Figure 3.12 Tetrahedral arrangement of covalent bonded carbon atoms in diamond.

      Diamond‐like carbon, abbreviated DLC, is a term used to describe a variety of amorphous carbon materials that contain varying amounts of hydrogen. Commonly deposited as a film or coating, DLC has properties that are dependent on the method and carbonaceous precursors used in its production. A high degree of sp³ bonding and a low amount of hydrogen enhance the diamond‐like properties of these materials. Based on their attractive properties such as high hardness, high wear resistance, low coefficient of friction, chemical inertness and biocompatibility, DLC has been investigated for orthopedic applications, such as coatings on articulating metal bearings in hip and knee implants to reduce the amount of wear particles and to improve the corrosion resistance of the metal bearings (Dearnaley and Arps 2005). DLC has also elicited interest for use in cardiovascular applications, such as coatings on stents and heart valves.

      DLC coatings for orthopedic applications typically have a high degree of sp³ bonding (approximately 80–85%) and a low amount of hydrogen (less than 1 atomic percent). However, the use of hard coatings on articulating metal bearings has not proved successful in clinical applications due to their unpredictable performance in critical stress conditions. The coating can fracture and chip off when subjected to high local stresses due to wear particles between the articulating surfaces or by inadvertent contact with hard components such as the metal rim of the acetabular cup, resulting in catastrophic wear rates.

Although graphite is not used as a biomaterial, the basic building block of its structure, composed of a planar array of hexagonally arranged carbon atoms (Figure 3.13), is common to a variety of graphitic materials. In each plane, one s orbital and two p orbitals form three sp² orbitals that have a trigonal planar arrangement in which each atom is bonded by σ bonds to three neighboring atoms (Chapter 2). The remaining one electron per carbon atom is delocalized, residing in p orbitals that are perpendicular to the planar arrangement. Neighboring p orbitals overlap, forming π bonds, allowing these delocalized electrons to move more easily within each plane. Thus, each bond in the hexagonal arrangement has partial double bond character. Graphite has a three‐dimensional structure that shows strong covalent bonding within each plane and weak intermolecular bonding between neighboring planes. These structural characteristics are responsible for the well‐known properties of graphite, such as lubricity and electrical conductivity, which are suitable for engineering applications but not relevant for biomedical applications.

Figure 3.13 (a) Basic building block of graphite composed of a planar array of hexagonally arranged carbon atoms; (b) three‐dimensional arrangement of planar arrays in graphite.

Pyrolytic carbon is used in the manufacture of mechanical heart valves (Chapter 24), where it is deposited as a thick coating (~25–500 μm) on graphite to produce valve leaflets (Figure e). In addition to adequate mechanical properties, such as strength, wear resistance, and durability, pyrolytic carbon is highly resistant to blood clotting and causes little damage to blood cells. This combination of properties makes pyrolytic carbon‐coated graphite suitable for use as heart valve leaflets. Pyrolytic carbon belongs to a family of carbon materials called turbostratic carbons that have a similar structure to graphite. However, instead of the flat hexagonal arrays that are stacked and held together by weak interlayer bonding as in graphite, pyrolytic carbon, and other turbostratic carbons are composed of layers that are disordered, resulting in wrinkles or distortions within layers. These disordered layers give pyrolytic carbon improved durability and a smoother surface when compared to graphite.

      Fullerenes, Graphenes, and Carbon Nanotubes

Fullerenes, graphenes, and carbon nanotubes share the same basic building block as graphite, a monolayer of hexagonally arranged carbon atoms. Graphene is the name given to a two‐dimensional flat monolayer of carbon atoms arranged in a hexagonal lattice (Figure 3.14a). The stacking of these flat monolayers gives the three‐dimensional structure of graphite. Graphene is a one‐atom thick, two‐dimensional crystal, often of size less than 1 mm (Geim and Novoselov 2007). The term graphenes is used in a more general sense to describe two‐dimensional crystals composed of one, two or a few (3–10) monolayers that are each distinguishable by their electronic structure. Structures thicker than ~10 monolayers are typically described as thin films of graphite.

Figure 3.14 Arrangement of carbon atoms in (a) graphene, (b) single‐walled carbon nanotube, and (c) buckminsterfullerene, C₆₀.

      The structure of carbon nanotubes can be viewed as graphene rolled to form a tubular geometry. Carbon nanotubes may consist of single tubes (Figure 3.14b), called single‐walled carbon nanotubes (SWNTs), or concentric tubes called multi‐walled carbon nanotubes (MWNTs). SWNTs have a diameter from ~1 nm to a few tens of nanometers and lengths of hundreds of nanometers to a few millimeters (Iijima and Ichlhashi 1993). As the long‐range periodicity of the atomic arrangement is retained along the axial direction of the tube, SWNTs may be viewed as one‐dimensional crystals.

Fullerenes are large molecules with a structure that can be viewed as a monolayer composed of hexagonal and pentagonal arrangements of carbon atoms which has been used to form a cage‐like geometry (Giacolone and Martin 2006). The most commonly investigated fullerene is the buckminsterfullerene, C₆₀, also called a buckyball, composed of 60 carbon atoms joined together to form 20 hexagons and 12 pentagons (Figure 3.14c). This arrangement of hexagons and pentagons, called a truncated icosahedron, has a pattern similar to a soccer ball. The carbon atoms in the pentagons are bonded by single bonds whereas the bonds in the hexagons consist of resonant double bonds. Although classified as a molecule, the C₆₀ fullerene has a diameter (0.7 nm) close to the lower limit of nanoparticles. When dispersed in a solvent, fullerenes often form aggregates