Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 15

dissolve these initial calcium and silicate ions nor do they form a BLM layer unless their
surfaces are functionalized before immersion (Kokubo 1991). As with metals, the osseointe-
gration of BLM coated ceramics occurs more rapidly than on noncoated surfaces due to the
enhanced conductive properties of the mineral layer (Liu, Ding, and Chu 2004).
Polymers
Biomineralized metals, glasses, and glass ceramics bond well to bone and serve as good
implant materials. However, these materials possess high elastic moduli compared to
bone, which can cause resorption of the host bone tissue (Kokubo 1996; Nagano et al. 1996).
Furthermore, metals and many ceramics can only be used as prosthetic implants since
they are not bulk biodegradable and thus do not allow replacement of the material by new
tissue over time. Polymers are good tissue engineering substitutes and possess advantages
such as biodegradability, improved flexibility in controlling structure, composition, and
properties, as well as the ability to be molded into different shapes to fit the wound or
defect site.
Natural polymers are readily available, inexpensive, and nontoxic. Raw silk, fibrinogen,
and collagen have been incubated in SBF solutions to obtain mineralized biopolymers
that can be used as biomimetic bone analogs (Takeuchi et al. 2003; Wei et al. 2008; Girija,
Yokogawa, and Nagata 2002). However, since these polymers are protein-based, they may
elicit undesired immunological responses when placed in vivo. Alternative natural mate-
rials are polysaccharide-based systems, several of which have been developed and used as
biomineralization templates. Chitosan microparticles functionalized with Si-OH groups
are made bioactive by soaking in SBF and can be used as injectable biomaterials and pro-
tein/drug delivery systems (Leonor et al. 2008). Another example is a cornstarch-ethylene
vinyl alcohol (SEVA-C) polymer blend that is made bioactive by its ability to induce bone-
like apatite formation in SBF after the introduction of carboxylic acid functional groups on
the surface (Leonor et al. 2007).
While synthetic polymers possess most of the same advantages as biopolymers, they
also offer more control in tailoring their synthesis and degradation properties to suit spe-
cific applications. Biodegradable synthetic polymers such as poly-l-lactic acid (PLLA),
poly-glycolic acid (PGA) and poly-lactic- co -glycolic acid (PLGA) break down into nontoxic
natural acid metabolites over time (Murphy, Kohn, and Mooney 2000a). Apatite coatings
or polymer/apatite composite materials can compensate for this acidic release by the dis-
solution of basic calcium phosphate and maintain pH within physiological ranges (Linhart
et al. 2001).
BLM coatings have been produced on a variety of polymers including polyvinyl chlo-
ride, poly(tetrafluoroethylene), nylon 6, poly(ethylene terephthalate), alkanethiols, and
polyhydroxyalkanoates (Tanahashi and Matsuda 1997; Tanahashi et al. 1994; Misra et al.
2006). Biomimetic mineral layers formed on electrospun nanofiber poly(ε-caprolactone)
meshes support proliferation of Saos-2 osteogenic sarcoma cells up to 2 weeks in culture,
demonstrating potential to regenerate bone ECM (Araujo et al. 2008). Continuous and uni-
form BLM layers can be produced throughout the porous structure of 3-D PLGA scaf-
folds (Murphy, Kohn, and Mooney 2000a; Segvich et al. 2008). These mineralized scaffolds
exhibit superior mechanical properties, as seen by a 5-fold increase in compressive modu-
lus (Murphy, Kohn, and Mooney 2000a). They also support higher bone marrow stromal
cell adhesion through well-distributed fibrillar contacts, and when used to transplant bone
marrow stromal cells, form a higher bone volume fraction in comparison to nonmineral-
ized polymer scaffolds (Leonova et al. 2006; Kohn et al. 2005). Other in vivo studies using