Protein-based Biointerfaces to Control Stem Cell Differentiation - Biointerfaces: Where Material Meets Biology - page 13

and bone sialoprotein, 80,81 osteocalcin 82 and salivary statherin. 83 Moreover,
following this approach, phage display technology has been employed for
discovering new peptides able to bind ceramic surfaces. 84,85 Modular pep-
tides have been reported for functionalizing several calcium phosphate-
based materials, such as HA, 80,86 HA-titanium, 87 b-TCP, 88 HA-polymer

d n 3 r 4 n g | 1

composites, 89 native bone grafts 90 and allografts. 91
In a series of recent studies, Rohanizadeh and his team investigated
protein adsorption onto hydroxyapatite (HA) surface, factors affecting this
phenomenon and techniques to modulate it. Using bovine serum albumin
(BSA) and cytochrome c as model proteins, the effect of HA crystallinity on its
protein adsorptive capacity was investigated by this team. 1 They concluded
that regardless of the total surface charge of protein, the adsorption of
proteins onto HA was directly influenced by HA crystallinity, where higher
crystallinity resulted in a lower protein adsorption rate. The crystallinity of
HA also affected proteins release profile, in which HA particle with higher
crystallinity showed lower protein release kinetics. In addition, the study
demonstrated that the surface charge of HA particles influence protein ad-
sorption, where BSA, an acidic protein, was adsorbed at higher rate on a
negatively charged surface compared to cytochrome c, a basic protein.
In another study, the team altered the HA surface charge by immobiliza-
tion of different amino acids during HA precipitation. 2 Four amino acids
with different isoelectric points were used in this study: neutral (serine, Ser;
and asparagines, Asn), acidic (aspartic acid, Asp) and basic (arginine, Arg). In
order to evaluate the anity of amino acid-functionalized HA (AA-HA) to
proteins, BSA and lysozyme were used respectively as an acidic and basic
model protein. The protein adsorption onto the surface of AA-HA depended
on the surface charges of HA particles, whereby BSA demonstrated higher
anity towards positively charged Arg-HA. Alternatively, lysozyme, a posi-
tively charged protein showed greater tendency to attach to negatively
charged Asp-HA. The AA-HA particles that had higher proteins adsorption
demonstrated a lower protein release rate. Thus, the results demonstrated
that selective immobilization of amino acid onto HA could provide a strategy
to tailor the adsorptive capacity of surface to a specific protein.
The team further investigated the effects of side chain length of amino
acids on protein adsorption onto amino acid-treated HA. 3 The results
showed that immobilization of amino acids with longer side chains de-
creased the crystallinity and increased the negative value of the surface
charge of HA particles. This could be due to the steric hindrance of a 'bulky'
side chain, disturbing the arrangement of HA crystals, due to lowered
crystallinity of AA-HA. Furthermore, manipulation of external factors,
namely pH and ionic strength, during proteins adsorption can significantly
improve HA binding anity to proteins. The protein adsorption rate in
AA-HA increased with decreasing the pH, while reverse trend obtained in
unmodified HA.
HA consisting of carboxyl rich (COO-) groups on its surface was also
synthesized by Boccaccini an his team. 4 HA particles were precipitated in

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