MEDICAL DEVICES BASED ON BIOINSPIRED CERAMICS - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 354

properties and better biological responses [Hench, 1993; Yamamuro, 1990]. The
remarkable biomechanical properties of human tissues to be replaced or repaired
are based in their hierarchic structure, which is an organized assembly of struc-
tural units at increasing size levels that provides optimum fl uid transfer and self-
healing [Sikavitsas, 2001]. The new generation of materials for medical implant
must mimic the smart hierarchical structures presented in nature.
Bioinspired materials take advantage of the knowledge that nature has been
optimising by evolution over millions of years. Advanced ceramics can now take
inspiration from the most complex naturally organised chemical and biological
structures. In this context, wood-based SiC ceramics (bioSiC) have been a matter
of interest in the last decade [Ota, 1995; Byrne, 1998; Byrne 1997; Greil, 1998,
1998a, 1999a; Shin, 1999; Martínez, 2000, 2000a, 2003]. This bioinspired material is
fabricated via the pyrolysis and infi ltration of natural wood-derived preforms,
have tailorable properties with numerous potential applications. The experimen-
tal studies conducted to date on the development of materials based on biologi-
cally derived structures indicate that these materials behave like ceramic materials
manufactured by conventional approaches. These structures have been shown to
be quite useful in producing porous or dense materials having various microstruc-
tures and compositions. To further improve the fi xation and osteointegration
performance, the bioinspired ceramics can be coated with a bioactive layer of
hydroxyapatite, substituted apatites, or silica-based glasses.
This chapter shows that a new generation of bioderived ceramics can be
developed and successfully used as a base material for medical implants, because
they mimic the natural structure of bone. In vivo implantation experiments dem-
onstrate the excellent biocompatibility of this new material, and how it gets colo-
nized by the hosting bone tissue due to its unique interconnected hierarchic
porosity. Specifi c plant species can be used as templates on which innovative
transformation processes, acting at molecular level, can modify the chemical com-
position maintaining the original biostructure. Utilizing these transformation
processes and building on the outstanding mechanical properties of the starting
lignocellulosic templates, it is possible to develop light-weight and high-strength
scaffolds for bone substitution with a suitable structure-morphology for optimum
biomechanical performance, which opens the door to a whole new generation of
biomedical applications.
11.3 BIOINSPIRED SiC BASED CERAMICS
11.3.1 Processing
The melt infi ltration method to produce biomimetic SiC-based ceramics follows
the general fl ow chart in Figure 11.1. The fi nal SiC product resembles the hard-
wood and softwood structures, shown in Figure 11.2 [Wheeler, 2001]. Therefore, a
porous ceramic with a specifi ed structure and density can be made by choosing
the appropriate wood precursor.