Biomedical Engineering Reference
In-Depth Information
each case. Chondrocytes have shown a strong binding affinity for RGD, PECAM, YGYYGDALR,
FYFDLR, and RLD/KRLDGS with weak binding for KQAGDV and LDV [
426
].
Functional peptides are not restricted to a set number of amino acids. Long sequences corre-
sponding to specific attachment proteins have been used successfully to modify scaffold materials.
However, the key sequence (e.g., RGD) must be accessible to cellular integrins to allow binding.
Sequences such as G
RGD
[
437
], G
RGD
SP [
438
], and CGGNGEP
RGD
TYRAY [
439
] all use
the RGD sequence to facilitate cell binding, but the peptides are modeled on different proteins
(the latter is from bone sialoprotein). Shorter peptide sequences are often preferred because of their
versatility. For example, GRGD can be synthesized onto the end of hydrophilic linker chains that are
attached to an underlying bulk material, thereby allowing cell seeding on scaffolds that are otherwise
unattractive [
437
].
While most studies involving peptides have been conducted in monolayer, their use is not
restricted to two-dimensions. A number of researchers have been investigating how peptides can be
incorporated into three-dimensional scaffolds for use in tissue engineering [
440
-
442
]. Peptides can
be incorporated into hydrogel scaffolds or grafted onto the exposed surfaces of porous scaffolds. As
in monolayer, cells are expected to bind to available peptides, thereby altering cellular proliferation,
migration, and differentiation. Alginate modified with RGD has been shown to promote cell adhe-
sion, spreading, and chondrocytic differentiation [
443
]. However, alternative studies using adhesion
peptides and PEG hydrogels showed a reduction in proliferation and protein synthesis [
444
,
445
].
These discrepancies could be caused by differences in peptide densities, which are more difficult to
control in three-dimensions than in two-dimensions.
As with full proteins, micropatterning techniques can be applied using peptide sequences to
create specific designs on material surfaces [
446
-
449
]. These experiments typically focus on the
attachment properties of cells since only a small portion of the protein structure is actually present.
Integrin binding reactions can be investigated in a controlled environment using this experimental
setup. Additionally, micropatterning can be used to control the geometry of single cells, allowing
investigations of cytoskeletal structures. Interactions between different cell populations have also
been studied using monolayer micropatterning [
450
]. Similar to protein stamping, peptide-patterned
regions allow cell attachment whereas the rest of the surface does not. Theoretically, if different
peptides are patterned in specific regions, then only cells expressing matching integrins will be able
to bind, creating a surface with segregated populations based on cell phenotype. This approach would
be interesting for co-culture experiments, assuming the populations are different enough to possess
distinct integrin profiles.
Micropatterning can control cell morphology by restricting the available surface binding sites.
Cell shape has been shown to influence whether a cell will proliferate, die, or differentiate [
449
]. For
some cell types, a spread/flat morphology promotes proliferation while a severely restricted, rounded
morphology promotes apoptosis and cell death. However, patterned surfaces that fall in between and
promote neither growth or death have been shown to induce cell differentiation [
448
]. This state is
different for each cell type and possibly each individual cell since peptide spacing would be largely
