Biomedical Engineering Reference
In-Depth Information
in the prior group showed no effect on cell attachment while the latter group had markedly reduced
attachment and spreading. While these findings are not directly applicable to chondrocytes, vit-
ronectin could still be important in modulating the attachment of cells in osteochondral constructs.
Other experimental studies investigating vitronectin have shown that it competes better than most
proteins when adsorbing to surfaces in the presence of serum [
431
]. Additionally, good adhesion
between vitronectin and chondrocytes has been observed, possibly through the
α
5
β
1
,α
V
β
5
, and
α
V
β
3
integrins.
Cartilage matrix protein (CMP), not to be confused with cartilage oligomeric matrix protein
(COMP), is a less extensively studied molecule that is expressed almost exclusively in cartilage [
432
].
CMP binds to aggrecan and collagen type II, and chondrocytes attach to it via the
α
1
β
1
integrin dur-
ing adhesion. When used as a coating material, CMP enhanced both cell attachment and spreading
on surfaces [
433
]. The addition of collagen type II to the CMP coating showed even more im-
provement in these characteristics. Because CMP is specific to cartilage tissue, it might be a more
appropriate protein to target for coating purposes although issues such as ease of production and
cost would certainly be important factors.
Functional cartilage constructs will have to be designed as three-dimensional structures, but
preliminary experiments can still be conducted in monolayer to investigate areas such as cell-surface
interactions. Micro- and nano-technologies now allow for precise control on protein placement on
a variety of surfaces. Technologies such as soft lithography and self-assembled monolayers allow
protein stamping on materials that restrict the attachment of cells to specific regions [
434
]. Custom
designs incorporating multiple types of proteins are feasible using these techniques, making possible
a wide variety of experiments at the cellular level. Many researchers are investigating cell-surface in-
teractions using these patterning techniques, which are hoped to be translatable to three-dimensions
in the future.
A major difficulty with using proteins to facilitate adhesion and migration of cells within
scaffolds is that the protein-surface interaction is typically transient. Unless the protein is cross-
linked to the material, a state that can dramatically affect its biological activity, proteins will eventually
disassociate from the scaffold. For short-term applications such as cell seeding, this is not a problem.
For longer-term needs, the base scaffold material should be hydrophobic, which increases the affinity
proteins have for the surface. While two-dimensional surfaces can be rigidly designed to control
where cells attach, three-dimensional scaffolds are more difficult. Choosing an appropriate base
material is the best starting point if the experiment relies on protein coating over longer time frames.
An attractive alternative to protein coating is to use only the amino acid sequences, or peptides,
involved in cell-surface binding. This approach allows for permanent modification of a scaffold since
the peptides are chemically bonded to the material rather than just adsorbed. The number of binding
sites and their location can be controlled during fabrication, and these parameters do not change,
provided the scaffold does not degrade or otherwise alter its structure dramatically. Cells attach to
protein-coated surfaces through amino acid recognition sequences located in the macromolecular
structure. Peptides are simply short amino acid sequences derived from the larger protein. After
