Biomedical Engineering Reference
In-Depth Information
1.2.2.3 Electropolymerization of Comonomer Mixtures of Biomimetic Tyrosine and
Tyrosine-Containing Peptides Forms Thin Films Possessing Specific Cell
Attachment/Localization Properties
One of the potential uses of the thin-film electropolymerization strategy we consider is to
combine monomers to form copolymerized product films that increase the resulting film
specificity for biosensor applications. At the Center for Intelligent Biomaterials, we have
pursued this approach. We copolymerized cell recognition peptides containing an added
terminal tyrosine amino acid with tyrosine and phenolic-based monomers to form films
upon the electrode surface. The films formed have the peptide pendant in solution and are
thus available for recognition by a given cell type's specific surface receptors in a subse-
quent step involving cell addition and binding. One example of the specific cell recognition
films formed was the following. Electropolymerizing of the two monomers, tyrosineamide
and the terminal tyrosine found in the tetrapeptide arginine-glycine-aspartic acid-tyrosine
(Arg-Gly-Asp-Tyr or RGDY), was carried out as shown in the upper panel of Figure 1.24
(83). The net decrease in the oscillating
f evident with each synthesis cycle of voltage
sweep along the time axis corresponds to the addition of film mass to the electrode surface
of the quartz crystal microbalance (QCM) device used to make these measurements. The
Arg-Gly-Asp tripeptide portion of the tetrapeptide monomer is the biological recognition
sequence found within fibronectin and vitronectin. These proteins are located within the
extracellular matrix (ECM) that binds to surface integrin protein sites on normal endothe-
lial cells (ECs) found in vivo located on the interior surface of blood vessels (84). In the
lower panel of Figure 1.24, there is a significant
f decrease observed following the addi-
tion of ECs to the QCM device that is due to cells binding to the electropolymerized film.
As Figure 1.25 also clearly illustrates, ECs do bind to these biomimetic electropolymerized
films in substantial numbers, exhibiting normal growth properties and possessing a normal
attached and spread light microscopic phenotype. However, this occurs only when the
RGDY recognition sequence is electropolymerized into these copolymer films at suffi-
ciently high concentration (the 1:3 monomer ratio case for example) to provide a sufficient
number of stable attachment sites. We believe that this electropolymerization film forming
strategy could be utilized to build into those films different binding specificity for particu-
lar cell types. Films formed in this way could be tailored to specific biosensor designs for
different cell types since they possess a cassettelike quality where only the recognition
sequence portion of the peptide, not the electropolymerized tyrosine, would need to be
varied to achieve each different desired cell binding result. The only requirement for this
cassette system, for any particular application, would be prior knowledge of the peptide
recognition sequence for that specific cell type.
1.2.3
Piezoelectric-Based Biosensors
In this signal transduction mechanism, any surface mass change is sensitively detected by
an alteration in the resonant frequency of an oscillating crystal. The piezoelectric effect has
served as the basis for many devices and biosensors. One of the most frequently used
devices for biosensor creation has been the QCM. In this device, an AT-cut quartz crystal
of varying thickness operates at its resonant frequency, ranging between about 5 and 20
MHz, driven by an AC oscillator circuit operating through electrodes placed on either face
of the crystal. In Figure 1.26, we present a schematic of an electrochemical quartz crystal
microbalance (EQCM). This is a variant of the QCM technique that allows the mass
changes measured by the standard QCM technique at the upper electrode, to be combined
with electrochemical processes at that electrode, which also serves as a working electrode
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