Page 399 - Biomedical Engineering and Design Handbook Volume 1, Fundamentals

376 BIOMATERIALS

Not only is the method of protein incorporation an integral part of developing an effective delivery
system, but the interaction between the biological factor and mineral is also important. Biological
factors can alter nucleation, growth, and biomineral properties (e.g., crystal phase, morphology, crystal
growth habit, orientation, chirality) (Wen et al., 1999; Azvedo et al., 2005; Liu et al., 2003; Uchida
et al., 2004; Combes et al., 1999), changing the osteoconductive capacity of the mineral. When
organic constituents are introduced into the mineralizing solution, the dynamics of mineralization
change due to changes in pH, interactions between the biological factor and ions in solution, and
interactions with the substrate. These dynamics can enhance or inhibit the heterogeneous deposition
of mineral onto the substrate.
Following coprecipitation, the release of biological factors and resultant biological responses are
influenced by many variables, including the concentration of the factor, the expression of the recep-
tors that are affected by the presence of the factor, the physical characteristics of the delivery sub-
strate and mineral/organic coating, and the site of implantation. Release kinetics can be controlled
via diffusion of the biological factor, dissolution/degradation of the carrier and/or osmotic effects.
For delivery systems based on coprecipitation of a biological molecule with a biomineral, the disso-
lution mechanisms of mineral are the most important.
Mineral dissolution is controlled by factors associated with the solution (pH, saturation), bulk solid
(solubility, chemical composition), and surface (adsorbed ions, phases). The apatite that is typically
formed from a supersaturated ionic solution is carbonated (Murphy et al., 2000a; Shin et al., 2007). The
presence of carbonate in an apatite lattice influences crystallinity and solubility (Tang et al., 2003; Ito
et al., 1997; Krajewski et al., 2005). The dissolution rate of carbonated HA depends on pH, and occurs
with the protonation of the carbonate or phosphate group to form either carbonic acid or phosphoric
acid (Hankermeyer et al., 2002). Thus, when experimental conditions change, the dissolution proper-
ties of mineral and release kinetics of any biomolecules incorporated into the mineral also change.
Apatite that has protein simply adsorbed to its surface undergoes a burst effect, releasing most of
the protein within the first 6 hours, whereas less than 1 percent of the protein incorporated within
bonelike apatite is released after 5 days (Liu et al., 2001). With coprecipitation, a small burst occurs
due to a small amount of protein that is adsorbed to the surface. The resultant sustained release is
hypothesized to be due to the incorporation of protein within the apatite matrix, rather than just a
superficial association (Liu et al., 2001). The affinity a protein has for apatite influences the disso-
lution rate of the mineral and, therefore, the release rate. Since protein release is proportional to
apatite dissolution, the possibility of temporally controlling the release profile, as well as developing
multifactor delivery systems is possible due the ability to spatially localize the protein within the bio-
mimetically nucleated mineral (Luong et al., 2006).
In addition to trying to control cell function via biomolecular incorporation within apatite, another
strategy is to present biomolecules on a biomimetic surface. While the objective of coprecipitation is to
control spatial and temporal release of biomolecules, the objective of presenting peptides with confor-
mational specificity on a material surface is to recruit a population of cells that can initiate the early
stages of bone regeneration. Proteins, growth factors, and peptides have been ionically or covalently
attached to biomaterial surfaces to increase cell adhesion, and ultimately the amount of bone growth.
While specific proteins that enhance cell adhesion have been identified, proteins, in general, are subject
to isolation and prone to degradation (Hersel et al., 2003). Proteins can also change conformation or ori-
entation because they possess sections with varying hydrophobicities that address cellular functions
other than adhesion. On the other hand, peptides can effectively mimic the same response as a protein
while being smaller, cheaper, and less susceptible to degradation. Peptides have a greater potential for
controlling initial biological activity, because they can contain specific target amino acid sequences and
can permit control of hydrophilic properties through sequence design (Ladner et al., 2004).
Identification of cell recognition sequences has motivated the development of bioactive materials
that can recruit a desired cell population to adhere to a material surface via specific integrin-mediated
bonding. One peptide sequence that interacts with a variety of cell adhesion receptors, including
those on osteoblasts, is the RGD (Arg-Gly-Asp) sequence. Other peptide sequences have been designed
to mimic sections of the ECM proteins bone sialoprotein, osteopontin, fibronectin, statherin, elastin,
and osteonectin (Fujisawa et al., 1996, 1997; Gilbert et al., 2000; Simionescu et al., 2005). Peptide
sequences with preferential affinity to HA and bonelike mineral have been discovered using phage
display libraries (Segvich et al., 2008b).