Biomedical Engineering Reference
In-Depth Information
the literature. It is still not entirely understood why such a small number of
atomic layers of oxide can sustain continuous corrosion reaction for
extended period of time, while only a few more create such a dramatic
reduction in performance. Major research efforts are needed in the near
future to fundamentally understand the mechanism for the enhanced
performance.
8.5
Using ultra-thin ceramic films in tissue engineering
Porous polymers have gained increased interest in the field of tissue
engineering. This is attributed to their unique physicochemical properties,
such as an interconnected pore structure, large surface area, and small pore
size. A good scaffold should give structural support, provide the
physicochemical signals to control cellular interactions, and also provide
sites for cell attachment, migration, and tissue in-growth (Hutmacher, 2000;
Liu and Ma, 2004). Bone graft substitutes should consist of highly porous
structures, which can induce high bioactivity and permit tissue in-growth
and thus anchor the prosthesis with the surrounding bone to prevent the
loosening of implants (Hutmacher, 2000; Liu and Ma, 2004). Owing to the
poor bioactivity and mechanical properties of pure porous polymers, many
strategies have been developed to improve the bioactivity and mechanical
properties of these low-cost materials. Ceramics such as alumina, titania,
and zirconia have excellent biocompatibility and bone bonding (Piconi and
Maccauro, 1999; Warashina et al., 2003). The inclusion of a bioactive
ceramic phase in polymer substrates can reinforce the porous structures of
the polymer and enhance the bioactivity and subsequent tissue interaction.
Most porous polymer/ceramic composites are produced via incipient
wetting methods. Solvent-based methods, however, have the risk of leaving
potentially toxic organic solvent residues and, as such, regulations often
inhibit their usage commercially. Also, a potential negative effect of
nanoparticle-containing scaffolds is the possibility of the migration of
nanoparticles within the body and their distribution via the blood stream,
leading to pathologies with unknown consequences (nanopathologies)
(Gerhardt et al., 2007). A novel process to produce reinforced porous
structures with enhanced biocompatibility and improved tissue interaction is
to coat inside the pores as well as on the surface of the porous polymers with
ultra-thin ceramic films, while maintaining the original porous structure of
the substrates. The ceramic films can be coated on some dense substrates by
traditional methods such as CVD, but it poses a great challenge in coating
inner surfaces of pores as the coating area is partially enclosed in the
material's matrix and has a high ratio of curvature. Films grown by CVD
will block the pores of the porous polymers. In addition, typical CVD
processes require high operating temperatures, which are much higher than
