Mohamed N. Rahaman

Materials for Biomedical Engineering


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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.

Schematic illustration of 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.

Schematic illustration of (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.

      Fullerenes, Graphenes, and Carbon Nanotubes

Schematic illustration of 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.