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Encyclopedia of Physical Science and Technology EN016J-783 August 1, 2001 10:58

822 Tissue Engineering

biodegradable extracellular matrix materials, when used TABLE III Ligands Used for Chemisorption of Protein
deliberately, must be such that the cell-generated matrix to Surfaces
has sufficient time to form and the mechanical integrity Type of primary
of the tissue is maintained at all times. In the end, there Ligand surface
are no foreign materials left in the patient to cause adverse
long-termimmunereactionsorharborbacterialinfections. Silanating reagents Glass, silicon
Alkane thiols Gold
Carboxylic acids Alumina
2. Optimization of Surface Chemistry Sulfonyl halides, Synthetic polymers a
carbonyldiimidazole, succinimidyl
Cells do not usually directly attach to artificial substrates,
chloroformate, succimidyl esters
but rather to extracellular matrix proteins which are
a Dacron and PTFE require chemical treatment in order to
physically adsorbed (i.e., by virtue of hydrophobic and
create free alcohol and carboxylic groups prior to derivatization.
electrostatic interactions) or chemically attached (i.e.,
via covalent bonds) to the surface. Many polymers are
processes are available depending on the type of surface
highly hydrophobic and do not favor protein adsorption.
to be modified (Table III). The first step involves using
Increasing substrate wettability to a certain point (water
a reactive chemical that bonds to the surface and has a
◦
◦
contact angle of 60 to 80 ), such as by using ionized gas,
free functional group that easily reacts with free thiol, hy-
increases protein adsorption and is commonly used for
droxyl, carboxyl, or amine groups on proteins. This step
preparing tissue-culture-grade polystyrene Petri dishes.
often requires harsh chemical conditions, while the sec-
This process only modifies the surface of the material,
ond step, which involves conjugation of the protein, can be
and thus minimally affects its bulk mechanical properties.
done under physiological conditions. This approach is also
Highly hydrophilic surfaces are also not favorable to
suitabletograftsmalladhesivepeptides(e.g.,RGD)which
protein adsorption. In addition, if negatively charged,
otherwise would not stably bind to surfaces by physisorp-
they may cause repulsive electrostatic interactions with
tion (Table IV). Furthermore, physisorption sometimes
the cells, the latter of which usually display a negative
leads to unexpected changes in protein activity, probably
surface charge due to the presence of negative sialic
due to denaturation on the surface. For example, adsorbed
acid residues on their surface glycocalyx. Conversely,
fibrinogen activates and binds to platelets, unlike solution-
coating surfaces with positively charged materials, such
phase fibrinogen in normal plasma or blood.
as poly-L-lysine, has been used to promote cell adhesion
to the surface. Physisorption of proteins for which cells
do not express any adhesion receptor, such as albumin, is TABLE IV Compounds Used to Promote or Prevent
also commonly used to prevent cell adhesion. Cell Adhesion to Surfaces
Physisorbed proteins are not stably bound and can be
Pro-adhesive Anti-adhesive
displaced by other proteins. This especially occurs in com-
plex media such as plasma, where fibrinogen physisorp- Extracellular matrix proteins Polyethylene glycol
tion may occur within seconds, following by displacement Collagen Albumin
of more slowly diffusible but “stickier” proteins (this is Fibronectin Polyvinyl alcohol
sometimes called the Vroman effect). When the surface is Vitronectin Cellulose acetate
transferred to a different medium after protein coating, the Laminin Agarose
type and amount of physisorbed proteins will change until Sulfonate residues
reaching equilibrium with the proteins in solution above Adhesive peptide sequences a
the surface. The time scale for desorption can extend over RGD (from collagen)
several hours, thus physisorption can be useful to control YIGSR, IKVAV (from laminin)
the initial attachment of cells at the time of seeding. Over REDV (endothelial-specific)
a period of several days of culture, however, virtually all Adhesion molecules
cells will have secreted significant quantities of their own Intercellular adhesion
extracellular matrix onto the substrate, and the initial sur- molecule-1 (ICAM-1)
face properties of the material often become irrelevant. Vascular cell adhesion
Because physisorption is notoriously nonselective, co- molecule-1 (VCAM–1)
valent modification of substrates or chemisorption is used Platelet cell adhesion
molecule-1 (PCAM-1)
if it is necessary to provide more control over the type,
Sialyl Lewis X
density, and distribution of adhesive protein on the sur-
face of the material. For this purpose, several chemical a Single-letter amino acid abbreviations.