CHARACTERIZATION OF THE ADHESIVE INTERACTIONS BETWEEN CELLS AND BIOMATERIALS - Micro and Nanotechnologies in Engineering Stem Cells and Tissues - page 166

is an important part of immune system activation. ICAM-1 has recently been used as
a way to attract stem cells to an area of injury on the endothelium. By coating the
surface of mesenchymal stem cells with antibodies to ICAM-1, Ko et al. 31 were able
to successfully target the interaction between the stem cells and the endothelium.
ICAM-2 is expressed constitutively on platelets and endothelial cells but unlike
ICAM-1 is not affected by cytokines. ICAM-4 is expressed on erythrocytes.
VCAM is a transmembrane protein that contains six or seven extracellular Ig
domains. Similar to ICAM-1, VCAM can also function as a ligand for integrins and
is responsive to cytokines. 29 VCAM can be expressed on both vascular and
nonvascular cells and is an important mediator in some cell signaling pathways.
Junctional adhesion molecules (JAMs) are also type I transmembrane proteins that
contain two extracellular Ig domains and are found in the tight junctions of
endothelial and epithelial cells. 32 JAM proteins are known to form homodimers
and have an important role in the trafficking of leukocytes.
Research in biomaterial development for tissue engineering applications aims to
mimic the native adhesive interactions that are mediated by integrins, cadherins, and
immunoglobulins. Controlling the interactions between cells and the underlying
substrate offers a means to control the downstream effects of cell adhesion, which
includes cell spreading, proliferation, and migration. In the following section,
modifications that intend to optimize these interactions are discussed.
7.3 OPTIMIZATION OF CELLULAR ADHESION THROUGH
BIOMATERIAL MODIFICATION
Degradable polymeric scaffolds are typically used in vitro and in vivo in the field of
tissue engineering and serve as a temporary matrix that can be seeded with cells to
promote healing, proliferation, and differentiation at an injury site. Polymeric
scaffolds must meet certain criteria before being used: they must have degradation,
mechanical, adhesive, and biocompatible properties that will result in proper
healing and regeneration of tissue at the implant site. 33 Some of the commonly
used natural and synthetic polymers are discussed briefly in this chapter, but for
more thorough information on the properties these materials, please see Tables 7.2
and 7.3 for a list of materials as well as suggested references for further
information.
Several factors play a role in how cells adhere and respond to biomaterials. On a
basic level, the hydrophilicity of a material has an effect on cell adhesion. In a study
completed by Schakenraad et al., 34 several commonly used polymers were tested,
and the results showed that those with a higher degree of hydrophilicity better
supported cell adhesion than those that were hydrophobic. Although cells may prefer
a hydrophilic polymer in vitro, biomaterials always exist in vivo in the presence of a
protein solution. In the native environment, cells rarely interact with biomaterials
directly but instead interact with an adsorbed protein layer on the surface of a
material. Protein adsorption is also related to the hydrophilicity of a material. Highly
hydrophilic materials resist adsorption and therefore resist cell adhesion in vivo.