BIOMIMETIC MATRICES FOR INTEGRIN-MEDIATED CELL ADHESION - Biologically-Responsive Hybrid Biomaterials - page 241

adhesion to thin films of poly- ō -caprolactone (PCL). Similarly, El-Amin and
colleagues [46] found that human osteoblasts expressed higher levels of the
osteoblast marker protein, osteocalcin, and adhered to PLAGA at a rate
significantly higher than on PLA polymers at 3, 6 and 12 h. This adhesion was
confirmed by higher expression levels of the integrin subunits α 1 , α 2 , α 3 , α 4 , α 5 ,
α 6 , and β Ӏ , with α 2 β 1 and β 5 showing the greatest difference in levels between the
two surfaces. Studies carried out on metallic implants, such as Titanium (Ti),
show increased osteoblast collagen synthesis, but decreased bone formation, or
alkaline phosphatase (ALP) activity and calcium deposition, on type-III collagen
Ti coatings as compared to non-coated substrates. [47]. These studies are
representative of the myriad of studies conducted on protein adsorption, integrin
expression, and subsequent cell behavior, and indicate the inherent complexity of
mimicking interactions.
The most important aspect of surface bioactivation is preservation of the
protein biological activity after attachment to the bio-inert surface. Upon
adsorption, proteins frequently undergo conformational changes from protein
native structures to denatured states [41]. This event changes the available
binding sequences for cell adhesive integrins and thus affects cell adherence
ability to biomaterials. Protein denaturing can be prompted by solution related
factors such as concentration. For instance, MacDonald et al. [48] demonstrated
that at low concentrations protein conformation of human plasma FN coated on
commercially pure Ti scaffolds had a small globular structure (similar to that in
solution), while at higher concentrations it appeared as a thin cross-linked
monolayer. Rico et al. confirmed this phenomenon on polymer surfaces as well
[49]. Also, protein denaturation can occur because of specific surface properties
such as hydrophobicity, electric charge, topography, and chemistry. As
demonstrated by García et al. [50], different substrates change FN
conformations, which alter the quantity of bound C2C12 myoblast integrin α 5 -
and β 1 -subunits and affects myoblast proliferation and differentiation. Martínez
et al. [51] elucidated the effects of crystallinity, showing how FN tended to
expand its arms and form a protein network on smooth poly-L-lactide (PLLA)
substrates, unlike the maintenance of its globular structure on semicrystalline
PLLA. Thus, investigating the effects of these surface physiochemical properties
on the conformation, density, and spatial distribution of the intervening adsorbed
protein layer and ensuing integrin binding is critical in understanding how
various cell types will adhere, proliferate, and differentiate on an implanted
material [29,52].