INSTITUTE OF PHYSICS PUBLISHING

SUPERCONDUCTOR SCIENCE AND TECHNOLOGYSupercond. Sci. Technol. 16 (2003) 1536–1539

PII: S0953-2048(03)66444-4

Measurement of the signal from acultured cell using a high-Tc SQUID
Saburo Tanaka, Nobuaki Tanaka, Miyuki Natsume,
Masashi Uchida, Miyuki Oshita, Zarina Aspanut, Toshihiko Ekiand Sachiko Yoshida
Toyohashi University of Technology, Hibarigaoka Tempaku-cho Toyohashi Aichi, 441-8580,
Japan

Received 17 July 2003, in final form 15 September 2003
Published 13 November 2003
Online at stacks.iop.org/SUST/16/1536Abstract
Stem cells in developing tissues give rise to the multiple specialized cell
types that make up the heart, lung, skin, and other tissues. For scientists, it isimportant to know the stages of stem cell development. We propose a new
method to obtain knowledge of the cell development stages by using ahigh-Tc SQUID magnetometer. In the first step of the research, we used
Wister rat myocardial cells incubated for 12–14 days in a culture dish as asample, which were not derived from stem cells. With the cells we tried tomeasure the signal using a SQUID magnetometer. We were able to measure
the magnetic signals, which may be generated by ion transport through thecell membrane. A beat signal with a period of 0.6 s was observed. The
peak-to-peak value was about 400 pT. This showed good agreement with theresult of microscopic observation.

1. Introduction
Research on stem cells is advancing knowledge about how
an organism develops from a single cell and how healthy cells
replace damaged cells in adult organisms. This promising areaof science is also leading scientists to investigate the possibilityof cell-based therapies to treat disease, which is often referred
to as regenerative or reparative medicine. Stem cells areimportant for living organisms for many reasons. Stem cells
in developing tissues give rise to the multiple specialized cell
types that make up the heart, lung, skin, and other tissues.
When unspecialized stem cells give rise to specialized cells, theprocess is called differentiation. Scientists are just beginningto understand the signals inside and outside cells that trigger
stem cell differentiation [1].
For scientists, it is important to know the stages of stem
cell development. Growing cells in a laboratory is known
as cell culture. Stem cells are isolated by transferring the innercell mass into a plastic laboratory culture dish that contains anutrient broth known as a culture medium. Scientists areusing a patch clump method, which includes a microscope, amicroprobe and microvoltmeter to investigate the development
stages of the cell. Figure 1 shows the patch clump method. It ispossible to discover the growth stage by reading the voltmetercorresponding to the membrane potential of the cell, which0953-2048/03/121536+04$30.00

Figure 1. Patch clump method. It is possible to find the growthstage by reading the voltmeter corresponding to the membranepotential of the cell. Using this method, much skill is required toinsert the micropipette.

voltage is the order of several tens of millivolts. In thismethod, much skill is required to insert the micropipette.
Also, this method is not applicable to the tissue for cellbased regenerative therapies because the stem cell is fragileand easily broken by touching or sting into the surface [2–5].
Therefore, non-invasive methods are desired.
We propose a new method to discover the cell
development stages by using a high-Tc SQUID magnetometer.
This method is perfectly invasive and does not break the cell.
This technology must also be applicable to screening andtesting new drugs and toxins. In this paper, we describe
the system for the measurement and the results using rat
myocardial cells. We note that the cells used here are notderived from stem cells at present.

© 2003 IOP Publishing Ltd Printed in the UK1536

Measurement of the signal from a cultured cell using a high-Tc SQUID

Figure 2. Mechanism for the generation of the membrane potentialof the live tissue. There are some pumps such as Na+/K+ ATPase
and Ca2+ ATPase, which are pumping each ion to keep ion balance.
As a result, the membrane potential is generated.

2. Principles

Figure 3. Spectral density of flux noise in the SQUID including theelectronics. The white noise is about 20 µφ 0 Hz−1/2, whichcorresponds to 0.36 pT Hz−1/2.

Figure 2 shows the mechanism for the generation of themembrane potential of the live tissue. There are somepumps such as Na+/K+ ATPase and Ca2+ ATPase, which
are pumping each ion to keep ion balance. As a result,the membrane potential is generated. For the myocardial
cells, the membrane potential temporarily becomes positiveby spontaneous excitation. Therefore if a cell sample is placedon a sensitive sensor, it is possible to measure the magnetic
field generated by the current.
Suppose there is a live cell tissue in a culture dish,then drugs or chemicals are dropped into the dish and thechange of the signal is monitored by the SQUID. As a result,
we can find whether the drugs are effective for the tissue. If thecardiac tissue is used as a sample, it is possible to investigate
the effectiveness of the drug to the heart, for example.

3. Experimental details
3.1. Cell samples
We used the tissue samples of neonatal Wister rats. All theanimals were kept in a temperature-controlled room with a
12 h light–dark cycle, and had free access to water and astandard laboratory diet. All the experiments were carried out
in compliance with guidelines on the care and use of laboratory
animals from Toyohashi University of Technology. Cardiac
muscle tissues were taken from 1–3-day-old Wister rats under
CO2 anaesthesia and were sliced into small pieces. They
were cultured in a plastic laboratory culture dish at 32 ◦ C for
12–14 d. The inner surface of the culture dish was coatedwith collagen and contained 2 mL of HMEM-based culture
medium. The size of the cultured cell array was about 500 µmin diameter and 50 µm in height.
The cells were attached to the bottom of the culture dishafter three days of culture. After four days, an unstable
heartbeat motion in part of the cellular structure, which showedsystole and diastole, could be confirmed by microscopic
observation. The cells cultured for 12–14 d were used forour experiments. At this stage, it could be seen that the wholecellular structures beat periodically in the culture dishes.

Figure 4. Cross-sectional view of the cryostat. This is speciallydesigned for a SQUID microscope. The minimum separation
between the SQUID and the sapphire window is 200 µm.

geometric size is 5.5 × 5.0 mm2. The effective area of theSQUID is 0.11 mm2. The spectral density of the flux noise isshown in figure 3. The white noise was about 20 µφ 0 Hz−1/2.
The cryostat was specially designed for a SQUID microscopeas shown in figure 4. The SQUID magnetometer was installedin the cryostat. The minimum separation between the SQUID
and the sapphire window is 200 µm. A more detailed
description can be found elsewhere [6–9].
All of the experiments were performed inside amagnetically shielded room (MSR), with a shielding factor of−50 dB at dc. Figure 5 shows a schematic layout of the systemand the data acquisition system. The SQUID was operated
in a flux-locked loop with a flux modulation frequency of256 kHz. The output voltage of the SQUID electronics ispassed through a low pass filter at a frequency of 5 kHz andalso through a 60 Hz band elimination filter. The signal voltage
is coupled to an A/D interface board and stored in a PC.

3.2. Measurement system

3.3. Measurement

The dc SQUID magnetometer is made of Y1Ba2Cu3O7−y thinfilm. The junctions utilized in the SQUID are of the stepedge type. The SQUID type is a dc large washer type, whose

Figure 6 shows the cell sample in the plastic culture dish.
The dish on the left contains a cell sample and that on theright has no samples for comparison. When the sample was1537

S Tanaka et al

Figure 5. A schematic layout of the system. The SQUID wasoperated in a flux-locked loop with a flux modulation frequency of256 kHz. The sample in the plastic culture dish was placed on thetop of the vacuum window. The signal output was coupled to an
A/D interface board and stored in a PC.

Figure 6. Photograph of the cultured dishes: left, containing a cellsample; right, blank (for control).

Figure 7. Time domain signal trace of the culture dish without cells(blank). A noise level of 30–40 pT was found in the period.

measured, it was placed on the top of the vacuum windowof the cryostat. Then the magnetic signals were recorded.
The total spacing between the cell sample and the SQUID
magnetometer is approximately 1.8 mm.

4. Results and discussion
First, a culture dish without cells (blank) was measured forthe control; the result is shown in figure 7. We found1538Figure 8. Time domain signal trace of a rat myocardial cell. Threesignals were obtained at different times. Periodicities of the threesignals are identical. A beat signal with a period of 0.6 s can be
seen. The peak-to-peak value is about 400 pT. The beat oscillationis continued for about 8–10 s and stopped for 10 s. This wave set
could be periodically observed.

a background noise level of 30–40 pT. The external noise
source is microphonic noise due to LN2 bubbling. Secondly,
a cultured cell in the dish was measured. Figure 8 shows
three signal traces obtained at different times. A beat signal
with a period of 0.6 s can be seen. The peak-to-peak value
is about 400 pT. The beat oscillation was continued for
8–10 s and stopped for 10 s even if any stimulation was given.
This wave set could be observed. These magnetic signals
may be generated by ion transport through the cell membrane.
This phenomenon showed good agreement with the result ofMeasurement of the signal from a cultured cell using a high-Tc SQUID

microscopic observation. The cells in the culture dish wereincubated for 12–14 d and started to form a network in whicha coherent excitation took place. This networking may cause
cells to give coherency. We believe this result has not beenreported to date.
We think some stimuli are needed to perform themeasurement for membrane potential of cultured stem cells,
which is our goal. However, this technology using a
myocardial cell is applicable to screening and testing new
drugs and toxins in the near future.

5. Conclusion
We have proposed a new method to discover stem cell
development stages by using a high-Tc SQUID magnetometer.
As the first step of the experiment, we used rat myocardial
cells incubated for 12–14 d in a culture dish, which were
not derived from stem cells. For the measurement of themagnetic signals from the cells, a SQUID magnetometer
was used. As a result, the magnetic signal from the cellular
structure could be detected. The peak-to-peak value was about
400 pT. The beat oscillation was continued for 10 s and stopped
for 10 s even if any stimulation was given. These magnetic
signals may be generated by ion transport through the cellmembrane. This phenomenon showed good agreement withthe result of microscopic observation. The cells in the culture

dish were incubated for 12–14 d and started to form a networkin which a coherent excitation took place. This networking
may cause cells to give coherency. This result is a first step torealize a system for measurement of the membrane potentialof stem cells. Although some stimuli are needed to performthe measurement for membrane potential of stem cells and any
toxins, this technology using a myocardial cell is applicable toscreening and testing new drugs in the near future.

Acknowledgment
This work was supported in part by the Cooperation of
Innovative Technology and Advanced Research in Evolutional
Area (CITY AREA).

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[8] Tanaka S 2001 IEEE Trans. Appl. Supercond. 11 665–8[9] Tanaka S 2002 Supercond. Sci. Technol. 15 146–91539