Biomedical Engineering Reference
In-Depth Information
connected to the outside by input and outputs that pierce through the chip. Specifically, the
FC technique exploits the microfabricated electrodes in these microchannel/microcham-
bers to sense minute variations in impedance that are caused by morphological or electro-
physiological changes within cells.
To this end, Song and co-workers recently presented a microfluidic ECIS-based flow
cytometer that could be used to identify the differentiation states of single stem cells [24].
Specifically, they utilized a novel dual microprobe design, which not only allowed for
high-throughput screening, but also allowed for ease of ECIS measurements owing to the
electrode configuration used. For their demonstration, Song
et al
. characterized the
differentiation state of a mouse embryonic carcinoma cell line, P19. In particular, these cells
were chosen because they are originally derived from an embryonic tetracarcinoma in mice
and are known to readily differentiate into neuronal cells in the presence of retinoic acid [25].
First, to demonstrate that their microfluidic device could be used to measure impedance
signals they performed three experiments where 20 μm polystyrene beads, undifferentiated
P19 cells, or a mixed sample were flowed through the device with a measurement frequency
of 50 kHz. In the case of the beads alone, it was observed that there was a negative and
positive spike when the bead passed through the bottom micropore and the top micropore,
respectively. Similar spikes were observed in the undifferentiated P19-cell case with smaller
amplitude. In the case of the mixed sample, two populations were observed through imped-
ance measurements: a larger spike which corresponded to the polystyrene, and a smaller
spike corresponding to undifferentiated P19 cells. When trying to identify undifferentiated
P19 cells from differentiated P19 cells, the size and electrical behavior of these populations
were comparable when observed in the resistance-dominant domain. However, using differ-
ences in the membrane capacitance [26], which could be observed at 1 MHz, even when a
mixture of undifferentiated and differentiated P19 cells was introduced, two distinct peaks
were observed corresponding to the different populations.
Altogether, FC represents a powerful and advantageous technique when compared
to conventional end-point protocols to quantify proliferation and/or differentiation.
Specifically, not only is FC able to provide noninvasive real-time monitoring of stem cell
self-renewal and differentiation, but it is also able to do this in a label-free and single-cell
fashion.
Electrochemical Methods
Electrochemical sensors have been used for a wide variety of applications owing to their
many advantages, including their high sensitivity and selectivity as well as their fast and
noninvasive nature [27]. In particular, a number of cell properties can be measured using
electrochemical techniques, including volume, concentration, electrical, and morphological
parameters, which are all important when monitoring biomass, sterilization control,
quantitative evaluation of drug effects (e.g., on cancer), and, more recently, the self-renewal
and differentiation of stem cells in a high-throughput, noninvasive, and label-free method
on various surfaces including nanomaterials. In this section, we will cover electrochemical
cyclic voltammetry-based sensors.
Cyclic voltammetry (CV) is a type of potentiodynamic electrochemical measurement
and is the most widely used technique to acquire qualitative and quantitative information
about electrochemical reactions and has recently been applied to the measurement of
reactions in living cells [28]. In particular, to perform CV, a voltage is swept between two
values (from
V
1
to
V
2
) at a fixed rate and when the voltage reaches
V
2
the scan is reversed
and swept back to
V
1
. The result is a current/voltage plot where, during the forward
