Biomedical Engineering Reference
In-Depth Information
lysosomal-associated membrane protein (LAMP)-derived peptides and lipidic
anchors [134]. Other chemoseletive strategies have been used to modify thiol-
functionalized surfaces with bromoacetyl-containing RGD cyclopeptides [135]
and maleimide-functionalized surfaces with thiol-containing RGD peptides
[136-138].
5.2.
Effects of immobilized peptide concentration and distribution
Biomimetic scaffolds for tissue engineering can be fabricated by chemically
immobilizing small peptides to the surfaces of various biomaterials. The cellular
responses to these modified biomaterials are dictated not only by the type of
biomimetic peptide molecule immobilized to the biomaterial surface, but also by
the ECM ligand concentration, spatial distribution, orientation and receptor-
ligand affinity [5].
Early studies that investigated the cellular response to various concentrations
of adsorbed cell adhesive ECM proteins demonstrated that a minimum surface
concentration of FN and VN was needed for sufficient cell spreading, ranging
from approximately 100 fmol/cm
2
to 1300 fmol/cm
2
[139-141]. For surfaces
immobilized with RGD peptides, cellular attachment and spreading appeared to
demonstrate a sigmoidal increase as a function of increasing RGD surface
concentration [142-146]. Thus, a minimum surface density is needed to elicit
certain cellular responses. Massia et al. [99] performed quantitative studies to
examine the cellular response to GRGDY immobilized onto polymer-modified
glass substrates via the N-terminal glycine residue. A minimal amount of RGD
peptide (1 fmol/cm
2
) was found to be sufficient for fibroblast cell spreading,
while at least 10 fmol/cm
2
was needed for
ŋ
V
Ȳ
3
integrin binding, focal contact
formation and cytoskeletal organization of F-actin stress fibers. At this surface
concentration, approximately 60 RGD ligands/mm2 were available for binding to
cell-surface integrins, demonstrating the biological potency of these synthetic,
EMC-derived oligopeptides for use in biomimetic scaffold fabrication [29].
Drumheller et al. [146,147] also examined the cellular response to RGD- and
YIGSR-containing peptides grafted at various concentrations within cross-linked
poly(acrylic acid) hydrogels via a PEG linker molecule. Fibroblast cells required
a peptide concentration of 12 pmol/cm
2
for cell spreading and 66 pmol/cm
2
for
focal contact formation on these interpenetrating networks. While higher RGD
surface densities are generally associated with enhanced cell adhesion and
spreading, there is a point at which greater peptide immobilization may impede
cellular responsiveness. Neff et al. [148] demonstrated that maximum fibroblast
proliferation occurred on polystyrene surfaces immobilized RGD peptides at an
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