© 2003 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume IIA: Biocompatibility, Landes Bioscience, Georgetown, TX, 2003

15.3.3.1 Vitreous or Glassy Carbon

Vitreous, polymeric or glassy carbon is a type of graphite formed by the decomposition of hydrocarbon gases on smooth surfaces (such as glazed porcelain) at temperatures above 650 ^oC. When examined by STM, this material shows atomic lattices with many relatively ordered defects and patchlike carbon crystallites with sizes of 3-15 nm [792]. The crystallites form surface domains that may differ in surface properties due to different orientations of the crystallites [792].

Adsorption of serum proteins onto glassy carbon has been lightly studied. Amorphous carbon exposed to solutions of fibrinogen, to modified fibrinogen lacking the alpha chain protuberance, or to serum albumin, flowing at a shear rate of 135 sec^-1, adsorbed all 3 proteins to form a film [793]. During the adsorption process, individual fibrinogen molecules retained their trinodular structure and adsorbed randomly until a monolayer formed [793]. Adsorption of bovine serum albumin onto glassy carbon takes place in several steps. The structure of the adsorbed layers is different for various serum shear rates at the surface [794]. Kinetics of adsorption of serum albumin onto the surface of glassy carbon electrodes is highly accelerated by application of positive potential, suggesting an electrostatic interaction between the negatively charged albumin molecules and the positively polarized electrode [795]. In this study, adsorption of albumin was irreversible if the albumin solution was simply diluted; albumin formed a monomolecular layer on the electrode surface [795]. Glassy carbon electrodes are widely employed in biosensors [4822-4830].

Microporous glassy carbon has good biocompatibility in rats [796]. Minimal tissue response is seen to the presence of glassy carbon, and glassy carbon bars aged in vivo for 5 months undergo no weakening [797]. Tissue reactivity to vitreous carbon was studied in dogs and the material was found to be quite inert. There were no inflammatory reactions or sensitivity changes in dog tissues and no unusual changes in the hemopoietic or enzyme systems [798]. Glassy carbon implants placed intraorbitally in rabbits for up to 150 days produced no intolerance reaction or deviation of blood parameters, showing only a typical “foreign body” (Section 15.4.3.5) reaction [4821].

Most studies have found good biocompatibility of glassy carbon implants in bone. For example, vitreous carbon inserted into the mandible and iliac crest of the rabbit pelvic bone for up to 1 year excited very little tissue reaction and did not appear to be degraded [799]. The material was well tolerated and showed no movement of known contaminants from the implants into the surrounding tissue [782]. In another experiment [800], porous vitreous carbon cylinders were implanted intra-articularly into rabbits in the metaphysis of the femur opposite from the patella. There was new bone growth into the implants from the surrounding bone, with new bone in the pores reaching a maximum of 45% of pore volume after 12 weeks and no adverse tissue responses [800]. Only in one study did vitreous carbon implanted in rabbit mandibular tissues for 0.5-3 months elicit fibrous connective tissue capsule formation, multinucleated phagocytic cells, a mild inflammatory infiltrate, and reactive bone [801].

Glassy carbon also appears biocompatible with teeth. In one experiment [802], conical vitreous carbon endosteal implants were placed in premolar extraction sites in dogs and then allowed to heal for 2-8 weeks prior to restoration with a gold crown. Gingival tissues healed routinely and showed mild irritation similar to tissues adjacent to teeth, with normal sulcular depths. Bone formation was observed within grooves in the implant surface, providing retention and stabilization. Normal bone remodeling occurred adjacent to the implant sites. The resulting interlocking between tissues and implant appeared to function effectively as a bacterial seal. No inflammatory responses, foreign body reactions, or infections were observed. Glassy carbon is generally considered to have good biocompatibility [904], despite the relatively high failure rate of vitreous carbon dental implants in humans [905] which seems largely due to mechanical factors such as brittleness [903].

Glassy carbon materials placed in the middle ear have been less successful. Vitreous carbon implants in the mastoid bulla (middle ear) of gerbils for 1-13 months were well tolerated [803]. But 9 months after vitreous carbon and glazed carbon fiber reinforced carbon were placed in the middle ears of rats and guinea pigs, 40% of the implants had been extruded and 8% had elicited inflammatory responses which would undoubtedly result in extrusion, with only 52% of the implants remaining in situ [804]. Vitreous carbon ossicular chain prostheses implanted in the middle ear of rabbits during tympanoplastic procedures often produced extensive inflammation of the middle ear mucosa along with formation of an increasing fibrous capsule [805]. There was also a permanent foreign body reaction at the implant surface and missing formation of new bone or contact with bone [805].

Vitreous carbon particles 11 microns in size implanted into the triceps surae muscle of Wistar rats produced no muscle tissue necrosis or exudative reaction during the acute phase (up to 1 week). During the chronic phase, the particles induced only a modest inflammatory infiltration of fibroblasts and phagocytes [826].

Last updated on 30 April 2004