Biomedical Engineering Reference
In-Depth Information
mutants of that cell specific for a particular cytoskeletal target. In this way, drugs could be
screened for their differential effect upon the particular target within the biological context
of these two cell types as the biological components of the biosensor. Finally, clinical cell
biopsies might be capable of being screened or diagnosed where these biopsy-derived cells
would represent the biological element in cell QCM biosensors.
1.2.3.3 Quartz Crystal Microbalance Cell Biosensor—Removing Cells Yields Intact
Extracellular Matrix: A Natural Intelligent Biomaterial With Potential for
Creating a Smart Bandage
Many normal cells, including ECs, synthesize an ECM underneath themselves, to which
they remain stably attached in vivo. Not only is the ECM a defined structural protein
matrix for cell attachment, but it also serves as a repository for many important biological-
signaling factors (102). These protein and peptide signaling factors are stored in the ECM
in vivo in a stable form until they are needed to be quickly mobilized in response to par-
ticular biological demands, such as
repair
needed for injuries or surface wounds. These
properties of the ECM impart to them some of the aspects of intelligent materials or sys-
tems. It appears that in the case of an injury such as a wound, this abnormal local state is
sensed by the ECM at the wound site. In response, the stored biological factors are
released to initiate a cascade of cellular and biochemical processes that result in repair of
the wound (103). However, the multiple mechanism(s) involved in this process and how
the ECM carries out its particular evolved functions is not well understood.
1.2.3.3.1 Isolating and Studying the Extracellular Matrix—A Natural Intelligent
Biomaterial
In an early study of ours, normal ECs attached to their underlying ECM on a conducting
ITO electrode surface were stimulated by small positive and negative electrochemical
potentials (104). We demonstrated that the cells' growth rate and appearance were signif-
icantly affected by these different treatments. The negative potential was stimulatory, and
the positive potential was inhibitory to cell growth. One possible explanation for these
effects was the differential release of biologically active peptide or protein factors from the
ECM during the different electrochemical stimulations. Given the importance of the ECM
in vivo as a natural biomaterial possessing intelligent properties, we therefore decided to
study it devoid of attached cells after first isolating it carefully and intact upon the cell
QCM biosensor surface. In these studies, the cell QCM biosensor comprised either ECs or
human breast cancer cells (MCF-7), but primarily the former. We studied the relative con-
tributions of the cells and their underlying ECM to the measured QCM
R
shift val-
ues. The detachment of the ECs from their ECM was carried out using a simple
nonproteolytic method involving EGTA complexation of Ca
2
. This resulted in disruption
of the Ca
2
-dependent bonds between the cells' integrin receptors and their binding sites
in the underlying ECM. With the cell ECM biosensor, we were able to follow the disrup-
tion of the cells' integrin-ECM linkages and thereby monitor the process of cellular
detachment (105,106). Representative cell QCM biosensor data for ECs undergoing this
process are presented in Figure 1.37. We could apportion the total frequency decrease of
the biosensor into contributions from cell attachment to the ECM as well as from the mass
of intact underlying ECM following cell removal. That the frequency shift remaining after
EC removal does correspond to the native ECM remaining on the QCM surface was con-
firmed in fluorescence antibody staining experiments by the presence of fibronectin pro-
tein, one of the principle structural components of intact ECM.
During the process of cell detachment, we observed an interesting and novel transient
increase in visoelastic behavior expressed in the cell QCM biosensor output as a transient
f
and