Single-Cell Platforms for
Microbiomechanics
Minh Guong Nguyen
Biomedical Engineering (BME)
University of California, Irvine (UCI)
Mentor: Prof. William C. Tang, Department of BME, UCI
Graduate student: Yu-Hsiang Hsu, Department of BME, UCI
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ABSTRACT
It is known that physical changes in an individual cell can play an important role as indicators of
healthy and abnormal cell activities. Nano-biomechanics aims to illustrate nanotechnology’s
increasing contribution to the understanding of mechanical aspect of physiological behaviors at
the cellular levels. Due to the fact that many of the mechanical properties of a cell are defined by
cytoskeleton morphology, which can be represented by viscosity and stiffness, it is hypothesized
that viscosity and stiffness measurements of the cytoskeleton can be used to produce enhanced
understanding of diseases and can have a significant impact on the future of medicine. The current
research holds promises in improving parallel drug screening, cancerous cell identification and
qualification, as well as the studies of single-cell physiologies. The approach in this research is to
develop a micro-platform with massive arrays of micro chambers, each instrumented with a
resonant transducer capable of interrogating the mechanical properties of a cell at the micron scale.
We utilized piezoelectric thin films to serve as the resonant transducers to interrogate the
mechanical properties of cytoskeleton. The preliminary result indicates that the first prototype
devices were able to detect the presence or absence of different media with a shift in the impedance
versus frequency characteristic curves. These promising results will leap to further investigation of
cellular activities with improved future generation of the prototype devices. The ultimate goal is to
integrate a massive array of microfluidic chambers instrumented with this piezoelectric
transducers and signal processing circuits.
.
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KEY TERMS

Cell mechanics: physical changes in an individual cell such as: stiffness, viscosity,
migration and etcetera

Cytoskeleton: a system of protein filaments including intermediate filaments,
microtubules, and actin filaments

Microbiomechanics: the study of biological systems at the micro- and nano-scales
with mechanically-derived modalities

Piezoelectric transducer: a transducer that converts mechanical to electrical signals

Quartz crystal microbalance (QCM): measures the change in frequency of a
piezoelectric transducer when it is interrupted by a small mass or any other tiny objects
INTRODUCTION
Each year, Technology Review, a
magazine associated with the Massachusetts
Institute of Technology, identifies the ten
most important technologies. This year the
list includes nanotechnology [1]. The Microand Nano-Technologies for Implantable
Devices including the “Sing-Cell Platforms
for Microbiomechanics” research at the
University of California, Irvine under Prof.
William C. Tang’s guidance is a part of the
state-of-the-art research project.
Figure 1: The eukaryotic cytoskeleton. Actin
filaments are shown in red, microtubules in green,
and the nuclei are in blue [3].
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Due to the fact that physical changes in an individual cell can play an important role in
assisting the cause of diseases, nano-biomechanics contributes to the field to advance the
understanding and treatment of diseases [2]. One of the most important components of a cell is its
cytoskeleton, which is made up of a system of protein filaments (including intermediate filaments,
microtubules, and actin filaments) in the cytoplasm of a cell (Fig. 1) [3]. The cytoskeleton provides
mechanical strength, maintains cell shape, cell division and cell migration of the cell. Therefore,
many of the mechanical properties of a cell are defined by the cytoskeleton morphology, including
viscosity and stiffness. Following this fact, it is hypothesized that viscosity and stiffness
measurements of the cytoskeleton can be used to deduce the different morphological behaviors of
cell, which in turn, are directly driven by cellular activities (Fig. 2) [4].
Intermediate filaments
protect cells and tissues from disintegration
by mechanical stress
Microtubules
essential component of cell
division
Actin filaments
responsible for cell
migration
Figure. 2: Three types of protein filaments and their functions [4]
Until now, the quartz crystal microbalance (QCM) was used to monitor the attachment and
spreading of cells when they make contact with the resonator surface (Fig. 3) [5]. However, since
the QCM cannot detect a single cells mechanical properties, the result is not precise. Currently
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there is no device that can measure the
overall
changes of the cytoskeleton,
researchers at Prof. Tang’s lab are currently
working on the “Single-Cell Platforms for
Microbiomechanics” project, which aims at
establishing
the
critical
engineering
feasibility that will lead to a micro-platform
with massive arrays of micro chambers,
Figure 3: Sketch of the QCM experimental setup.
Cells are seeded to the surface of a quartz resonator
[5].
each instrumented with a resonant transducer which will be able to measure single cell’s
mechanical changes.
METHODS AND MATERIALS
As mentioned before, by implementing piezoelectric transducer to serve as the resonant
transducer, it can be used as a tool in the field of biology to investigate any kind of cell morphology
related to its cytoskeleton.
Experimental setup
Our piezoelectric transducer
Our device
The piezoelectric transducer has a micro scale
which is only 1.5 µm in thickness and 200 µm in
diameter. Connected to our device is a 15 µm thin line
top electrode which delivers the signals of our device to
Thin electrode
Top electrode
Figure 4: Our piezoelectric transducer
zoom-in
a 1 mm square top electrode (Fig. 4). The top electrode
is a place where a tip from a probe makes contact with
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the devices in order to deliver signals to our devices and receive the signals back.
The probe
When two tips of the probe make contact
with our devices, they are considered a bridge to
connect the signal from the impedance analyzer to
our devices, and then our device’s signal will be
returned back to the impedance analyzer to
ground
top electrode
interpret the data. One tip connects to the 1 mm
square top electrode while the other tip connects
to the ground (Fig. 5). The tip cannot connect
directly to our devices because this will damage
Figure 5: Our piezoelectric transducer taken
by naked eyes.
our devices (which are only 1.5 µm in thickness and 200 µm in diameter). Working with the probes,
especially with the tips, is the most time consuming process due to the difficulty in which the tips
must move to different targets in order to make contact with the top electrodes of different devices.
Impedance Analyzer 4395A (100 KHz to 500 MHz)
The Agilent 4395A is an
impedance analyzer that is connected
to the probe to receive the signal of our
devices from the top electrodes. This
impedance analyzer is complicated to
operate and extremely sensitive to a
single
noise
environment.
in
This
the
working
step
in
our
Figure 6: Our experimental setup
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experimental setup also requires a lot of patience in order to perform a good calibration to test our
devices’ signals. Finally, a computer connected to the impedance analyzer will collect our
devices’ data (Fig. 6).
Testing
Our devices are expected to have a curve of impedance vs. frequency similar to the curve
of a reference piezoelectric material. Impedance analysis of our devices is performed at 801 points,
equally spaced data points in a frequency range (to be chosen by users). Graph 1 below shows the
results of our different devices when being tested. The labels on the right hand side represent
different devices according to their colors plotted in the graph. All of the impedance vs. frequency
curves are very consistent. The valley of the curve is called the resonance frequency and the peak
of the curve is called the anti-resonance frequency. This looks very close to the reference of the
piezoelectric transducer materials which proves that our devices are working (Graph 1).
Measurement
The testing results have proven that our devices are working; however, it is not known if
Graph 1: Impedance vs. Frequency of our devices
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the device is sensitive to a tiny 0.1 µL water droplet on top of its surface. Therefore, another test is
set up for the measurement when the device is treated with a 0.1 µL water droplet.
First, one of our devices is tested and data is recorded. Then, by using a micro-pipette and a
probe, a 0.1 µL water droplet is placed on the top of that device to compare signal’s difference to
the device when not treated with a water droplet. Working with a micro-pipette is a very
challenging procedure to perform. Since a 0.1 µL water droplet is such a tiny amount, which itself
doesn’t weight enough to be pushed out by the micro-pipette, a small portion of the droplet must
make contact with the device in order to be pushed out from the micro-pipette. However, if the
water droplet is pushed forward to make contact with the device, it is extremely easy to damage the
device since it is so delicate (200 µm in diameter and 1.5 µm thickness).
Despite the obstacle, after successful placing a water droplet on top of the device, two tips
of the probe are connected to the device’s top electrodes. By using AC impedance analysis in a
frequency range from 33 MHz to 40 MHz, we have explored the different signals of the device
when it is treated with/without a 0.1 µL water droplet.
RESULTS
Results indicate that the device is sensitive to a tiny weight of a 0.1 µL water droplet.
Reviewing graph 2 below, a blue curve represents our device when is treated without a 0.1 µL
water droplet. A red curve represents our device when treated with a 0.1 µL water droplet. The red
curve has a distortion compared to the blue curve. In order words, the red curve has a lower
resonance frequency and wider bandwidth compared to the blue curve (Graph 2). To interpret our
devices data in term of mechanical properties to electrical properties, it is necessary to describe the
experimental situation through appropriate physical models. For illustration, it is critical to know
which parts of the cellular device contribute to the signal’s response [6]. The lumped element
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Impedance vs. Frequency
460
440
Impedance (Ω)
420
400
Without
water
380
360
With water
340
320
33.0
34.0
35.0
36.0
37.0
38.0
39.0
40.0
Frequency (MHz)
Graph 2: Comparison of our device when treated with and without water
Butterworth-Van-Dyke (BVD) equivalent circuit, which consists of a capacitor Co parallel by a
series combination of an inductance L, a resistor R, and a capacitor C can be used as an example
[3]. The combination of L, R, and C is associated with the mechanical properties of the
piezoelectric transducer. The static capacitance Co is associated with the electrical properties of
Fig. 8: Lumped-element equivalent circuit
to model the electrical characteristics when
the device is applied more load [6]
Fig. 7: Butterworth-Van-Dyke
(BVD) equivalent circuit [6]
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the piezoelectric transducer (Fig. 7) [6]. If a load is applied on our devices, the circuit can be
re-modeled for adjustment. Fig. 8 is another example of the circuit if a load is applied.
To further interpret the device’s impedance vs. frequency, graph 1 can be zoomed-in,
which now becomes graph 3. Since the increasing of frequency leads to the decreasing of the
capacitor’s impedance, the descending curve represents the domination of the capacitor C based
on the BVD circuit. In contrast, the increasing of frequency leads to the increase of the inductor’s
impedance L, the increasing curve represents the domination of the inductor based on the BVD
circuit. The frequency doesn’t affect the impedance of the resistor R (Graph 3).
Impedance vs. Frequency
Impedance of
Resistor:
ZR = R
Impedance (Ω)
Impedance of
Inductor:
ZL = j ω L
Impedance of
Capacitor:
Zc =
1
j C
ω = 2 () (f)
Frequency (MHz)
Graph 3: The graphs of impedance vs. frequency of our devices zoom-in
DISCUSSION
Since the quality factor QM is a measure of the quality of our resonant system, it is necessary to
calculate the Q factor of the devices.
2
fa
QM 
2
2
2  f r Z r C f a  f r

where QM is the quality factor
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
fa is the anti-resonance frequency
fr is the resonance frequency
Zr is the impedance at resonance frequency
C is the static capacitance [7]
Also, the percent change in frequency is also important and needs to be calculated to analyze the
data. The percent change in frequency can be calculated on the formula below:
% change in frequency =
f r withoutwater   f r withwater 
*100
f r (withwater)
Table 1: Table of the data and calculation results
Our
device
Without
water
(Air)
With
water
Resonance
frequency
fr (MHz)
Antiresonance
frequency
fa (MHz)
Impedance
at
resonance
frequency
Zr (Ω)
Static
capacitance
(pF)
Viscosity
(cP)
3.6144
3.8141
350.17
25
0.0185
Frequency
shift (%)
Quality
factor
4.933
9.16
0.26
3.6050
3.8366
333.74
25
0.982
Quality
factor shift
(%)
4.519
When a tiny load is applied on our devices, the frequency shift down is indicated in the red curve
(Graph 2). The percentage of frequency shift is 0.26 (Table 1). This has proven that the frequency
shift is related to the weight of water. When our device is measured without any media on top of
its surface, we actually measure the impedance of the air. The viscosity of air (0.0185 cP) is 9
times lower than the viscosity of water (0.982 cP), but the quality factor of air is higher than water
(Table 1). This has proven that the quality factor is related to viscosity; somehow the quality factor
is inversely proportional to the viscosity. In fact, the quality factor shift is 9.16%.
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Problem
Our
experiment
is
Impedance vs. Frequency
510
extremely sensitive even to a
single noise.
Since there is
always noise in the environment,
the
major
problem
of
our
Impedance (Ω)
490
W11-0
W6-1
W6-0
WA6-REF
W10-0
470
450
430
410
research is to find a way to get rid
390
23
24
of the noise. If the noise is too
high, it will drown out the
25
26
27
Frequency (MHz)
Fig. 4: Bad graph of impedance vs. frequency
device’s signal. Graph 4 below shows the distortion in the data collected when it is interfered with
by noise. With these curves, it is impossible to find the result of the percent change in frequency
and quality factor. Many techniques have been tried to get rid of the noise such as: turning off all of
the lights, computers, and even the AC system in the lab. However, these attempts did not reflect
better results. Finally after spending three weeks to find where the background noise was coming
from, a conclusion was reached that the noise source came from the probe, since the probe is
originally designed not to be tested at a high frequency. This probably is one of the reasons that,
sometimes, the data reflects a very distorted signal from the devices. To overcome this obstacle, a
new probe was already bought and it is on the way to UCI. With this new probe, a better signal
from the device should be acquired.
ACKNOWLEDGEMENTS
We are greatly indebted to Prof. William C. Tang for his expert mentorship, as well as to
Yu-Hsiang Hsu and John Lin for their guidance and assistance with the experiments. We are also
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grateful to Said M. Shokair, Edward M. Olano, UROP staff, and UROP Fellows for their support
and helpful discussions. In addition, we would like to express our appreciation to National Science
Foundation that has been financially supported and given us an opportunity to be part of this
exciting research.
WORK CITED
[1] Fitzgerald, Michael. “Nanobiomechanics”. Technology Review. March/April 2006.
[2] Online posting. http://www.wctgroup.eng.uci.edu/
[3] “Cytoskeleton.” Whikipedia. 9 May 2006. http://en.wikipedia.org/wiki/Cytoskeleton
[4] Alberts, Bruce, et al. Essential Cell Biology. 2nd ed. New York & London: Garland
Science, 2004.
[5] Jing Li, Christiane Thielemann, Ute Reuning, and Diethelm Johannsmann. “Monitoring of
integrin-mediated adhesion of human ovarian cancer cells to model protein surfaces by
quartz crystal resonators: evaluation in the impedance analysis mode.” BioSensors &
BioElectronics 20 (2005): 1333-1340.
[6] Joachim Wegener, Jochen Seebach, Andreas Janshoff, and Hans-Joachim Galla. « Analysis of
the Composite Response of Shear Wave Resonators to the Attachment of Mammalian
Cells.» Biophysical Journal. Volume 78. June 2000: 2821-2833.
[7] “Piezo Ceramics Tutorial 15 of 15.” MorganElectrodeCeramics. 30 August 2006.
http://www.morganelectroceramics.com/tutorials/piezoguide15.html
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