Bone-Like Mineral and Organically Modified Bone-Like Mineral Coatings - Biological and Biomedical Coatings: Processing and Characterization - page 17

creating bone mimics that attempt to recreate the structural hierarchy of natural bone and
can be also be used to elucidate the roles of organic molecules in biomineralization.
There are several approaches to create calcium phosphate-organic hybrids. The two
main methods to produce organic/BLM hybrids with bioactive molecules discussed in
this section are adsorption and coprecipitation. For materials intended as protein deliv-
ery systems, each of these approaches has advantages depending on the release profile
desired. Adsorption can be used when a quick transient release of the biomolecule is
desired, or to recruit cells to the material surface initially. Coprecipitation is more useful
in creating gradual release profiles, where the desired molecule is needed over a longer
period (Figure 1.2).
Adsorption of Proteins to BLM Surfaces
Adsorption of proteins to mineral surfaces is the simplest method of forming organic/
inorganic hybrids containing biologically active molecules and involves incubating the
mineralized substrate in a protein solution, allowing the protein to associate itself with
the mineral. The mechanism of adsorption is thought to be via electrostatic interactions
between the protein and the apatite, and hence the protein-mineral bond created by
adsorption is not strong compared to the covalent attachment of proteins to apatite created
by other techniques. Adsorption of protein onto biomimetic apatite does not cause any
change in the morphology of the mineral (Luong et al. 2006; Liu et al. 2001), since adsorp-
tion is a surface phenomenon and does not result in protein integration into the mineral
structure (Figure 1.3).
Coating substrates with BLM enhances specificity of protein adsorption. For example,
apatite formed on titanium from saturated Ca-P solutions selectively adsorbs proteins
from serum and in higher amounts compared to plasma-sprayed hydroxyapatite coatings
(Wen, Hippensteel, and Li 2005). Similarly, bioactive glass coated with BLM specifically
adsorbs high molecular weight proteins such as fibronectin when incubated in serum
(El-Ghannam, Ducheyne, and Shapiro 1999). This enhanced specific adsorptive capability
has been attributed to the highly nanoporous structure and high surface area and surface
roughness of BLM coatings (Wen, Hippensteel, and Li 2005; Murphy et al. 2005). Increased
surface adsorption of proteins onto BLM is especially advantageous for in vivo applica-
tions since the adsorbed serum protein layer that forms on implants is vital for cell migra-
tion and attachment.
Generally, the release kinetics of an adsorbed protein from a material surface involve a
burst profile typified by a fast initial spike in release followed by a more gradual release
over time. The amount of protein adsorbed and subsequently released is dependent on the
characteristics of the protein (most importantly charge and conformation) and the min-
eral (surface area, charge). Depending on charge, size, and electrostatic interaction with
the BLM surface, different proteins exhibit different affinities to, and therefore different
release kinetics from BLM coatings. While TGF-β, Nell-1 and osteocalcin are released grad-
ually over time, BSA exhibits a more characteristic burst release profile (Lee et al. 2009;
Wen, Hippensteel, and Li 2005; Krout et al. 2005). BLM coatings may reduce the extent of
burst release that occurs with some adsorbed proteins, making BLM coatings more useful
therapeutic agent carriers. However, it is important to fully characterize the protein being
used to understand the influence of its properties on adsorption and release.
Coprecipitation of proteins along with the mineral is another way to control burst
release as the protein is physically incorporated into the mineral and distributed spatially
throughout the mineral layer, as compared to surface localization seen with adsorption.