MED64 Key
technology
Technical advantages
Patents
History
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-Low-impedance planner
microelectrodes -
The use of MEAs (Multi-electrode
arrays) has been growing in
popularity over the last several
years. Among MEA systems, the
key technology unique to the
MED64 system is its “low-impedance”
planar microelectrode coated
with platinum black. This
low-impedance planar
microelectrode enables recording
from diverse preparations
including acute slices as well
as cultures and explants. It
allows beginners to
electrophysiology to easily
acquire clean, high quality
signals and perform
sophisticated experiments. The
development of the planar
microelectrodes for MEAs
required solving a major
problem. Electrode impedance is
inversely proportional to its
surface area but surface area
cannot be too large. This
dilemma was solved by coating
the planar microelectrodes with
platinum black using special
method. This technology
increases the surface area
100-200 times (greater than
standard plating methods) and
reduces the electrode impedance
to only 7-10 kohm at 1 kHz
(for 50x 50 µm electrode, 40
kohm for 20 x20 µm electrode),
which results in following
benefits.
1.
Low-noise and stable
recording
2.
Superior signal-to-noise
ratio
3.
Excellent electrical
stimulation
4.
No need
for
pre-amplifier
Close up views of electrode's
surface
Impedance of Pt. black electrode
(50 x 50 µm) on the MED probe
(red)
compared with the completely
flat electrode (50 x 50
mm) (blue)
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1. Low noise
There are two types of noise to
be considered in a typical
electrophysiology experiment,
exogenous noise and Johnson
noise. Both types of noise
decrease as the
electrode-impedance becomes
smaller.
Low exogenous
noise
One representative example of
exogenous noise is hum noise.
The lower the electrode
impedance, the less susceptible
the system is to exogenous
noise. Extracellular recordings
with conventional glass
electrodes often require a
special environment such as
Faraday cage and a vibration
isolation table. The MED64
system does not usually require
a Faraday cage but can be
installed on a stable table.
Experiments are conducted
everyday under stable low-noise
level.
Fig.1: Experiment using the
MED64 system
MED probe and
connector are placed in the
incubator.
Low Johnson
noise
Johnson noise is intrinsic to
the system itself and can be
determined by the amplitude of
the baseline noise. Unlike
exogenous noise, the level of
Johnson noise correlates with
the electrode’s impedance and
can NOT be improved by users.
Low electrode-impedance
will decrease the Johnson noise.
The measured noise level (Vn) of
the MED64 system (50 x 50 µm
electrodes) is as low as 2-3µV
RMS at 0.1-10kHz. (Noise levels
below 1µV RMS can be achieved
applying the 3kHz low-pass 9
pole Bessel filter included in
the MED64 Mobius software).
(0.02mV, 20 msec / div.)
Fig.2: noise level obtained with
saline (Bandwidth of 0.1-10kHz)
2. High signal to noise ratio
The signal to noise ratio is the
ratio of signal compared to the
noise. Obviously, higher quality
of signals could be recorded
with higher signal to noise
ratio. High signal to noise
ratio is achieved by enlarging
signal level or lowering noise.
With planner microelectrodes at
Multi-electrode array, it is
impossible to enlarge the signal
level. (In order to enlarge
signal level, the electrode
which point’s size is around
several micro meters needs to be
inserted deep into the middle of
slices.) Thus, lowering the
noise level is the only method
to achieve high signal to noise
ratio. The MED64’s Johnson noise
is very small due to its
low-impedance microelectrodes
High-quality signals can be
recorded even from acute slices.
Click here for
Publication list
of acute slices
Fig.5: Raw data from acute hippocampus slice (0.5mV, 10ms/div)
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3. Effective
stimulation and recording of
good electrically evoked responses
One of the capabilities of the
MED64 system excels at is
recording electrically-evoked
activity. As mentioned
previously, the Pt. black
electrode has a large surface
area which is associated to high
capacitance and low impedance.
As a result, high current
stimulation can be applied and
good evoked responses are
recorded without interference of
stimulus artifacts.
High current
stimulation
Fig.6 shows the mechanism of
extracellular electrical
stimulation by planar
microelectrodes in the MED64
system. The stimulus current
flows from the selected
electrode to reference
electrodes. This current changes
the field potentials, which
results in the hyperpolarization
or depolarization of cellular
membranes. The voltage changes
in the field potentials (Vf)
corresponds to the stimulator
output current (Is) as seen in
the right figure of Fig.6.
Ce:
Capacitance of electrical
double-layer capacitor
(electrode)
Re: Resistance of electrode
Rs: Resistance of solution with slice
Is: Stimulator output current
Vs: Stimulator output voltage
Vc: Voltage of Ce
Vf: Voltage of Rs (Voltage change in the field)
Fig.6. Stimulation by the
MED64's planar microelectrode
The applied stimulus
current charges the “electrical double-layer
capacitor” (Ce) which is formed at the
interface of electrode, which
causes the voltage of Ce (Vc) to
change. However, when the
absolute voltage (Vc) rises
above a certain limit (around 1V
for materials which are usually
used for MEAs), the electrode
will reach electrolysis and
release H2 bubbles. This causes
the stimulation efficacy to drop
dramatically, introduces
extremely large stimulus
artifacts, harm cells, and
damages electrodes. Thus, the
stimulus current (Is) should not
go beyond this level.
The (Vc) is defined by
Vc = Is * t
Vc: Voltage of Ce /
Is: Stimulation out put current
Ce
T: Pulse duration /
Ce: Capacitance of electrical
double layer capacitor
(electrode)
The higher the electrode’s
capacitance (Ce) is, the lower
its voltage (Vc) is. This also
means that the higher the
capacitance of the electrode is,
the greater the tolerance to
current (Is) is.
The capacitance of MED64 probe’s
microelectrodes coated with Pt.
black is as high as 50,000 pF for
50 x 50µm electrodes (22,000 pF
for 20 x 20µm electrode). The
value is much higher than any
other material used for MEAs.
For example, the (Vc) at the
stimulus current of 20µA for the
MED64’s microelectrode is much
lower than the one for another
material. (See Fig.7). This high
capacitance electrodes enables
stimulation with current
amplitudes as high as
200µA/0.1ms.
Fig.7: Comparison of (Vc) for
different types of
microelectrodes when 20
mA stimulus current is
applied
Small stimulus artifact
When the planar electrodes
at MEAs is used for electrical
stimulation, the pulses of
stimulus current are recorded at
other channels due to crosstalk.
This results in the so-called
“stimulus artifact”. The
stimulus artifact can persist
even after stimulus pulse
returns to zero volts and
interfere with the recording of
signals as seen in figure 8
This problem is mainly due to
following factors:
1) Usually, the maximum input
voltage for the amplifier is set
to several milli-volts. However,
when recording with MEAs, the
amplitude of the incoming
stimulus artifact exceeds these
levels, which causes the
amplifier to saturate
momentarily making data
acquisition impossible. This
delay due to the amplifier’s
momentary saturation becomes
larger in proportion to the
stimulator output voltage (Vs).
Fig.8: Prolonged stimulus
artifact
2) The electrode often needs
longer time to discharge than to
charge. (Refer to the (Vc) at
Fig.9) This discrepancy becomes
greater as the voltage of Ce (Vc)
becomes larger.
As described in the previous
chapter, the (Vc) at the MED64
electrodes can be kept very
small because of its high
capacitance. Moreover, the (Vs), can be
kept small by lowering the
electrode’s impedance (Refer to
the (Vs) and (Vc) at Fig.9) As a
result, the prolongation of the
stimulus artifact can be kept
small thanks to the
low-impedance and
high-capacitance electrodes at
the MED64 system
Fig.9: Comparison of voltages
between two different
electrodes.
(Vs) is smaller for
lower-impedance electrode and (Ce)
is smaller for higher
capacitance electrode
The stimulus artifact can return
to 0 volt within 0.5 m seconds
at non-stimulated channels with
the MED64 system even with
current stimuli as high as 100mA
as seen in figure 10, and rarely
interferes with recorded signals
(0.1mV, 20ms /div.)
(0.1mV, 0.2ms /div)
Fig.10: The stimulus artifact
with the MED64 system when a 100
mA
current pulse is applied.
Hardware-implemented approaches
such as electronic blanking
circuitry can solve the problem
only due to the first factor by
amplifier’s saturation.
Low-impedance and
high-capacitance electrodes can
be a good approach to minimize
the duration of the stimulus
artifact.
Finally, the underlying reason
for this artifact problem is
capacitive crosstalk. Thus, it
is the important and fundamental
solution to minimize the cross
talk. Lowering the electrode
impedance is one way of
achieving this.
4. No need for
pre-amplifier.
Another benefit which is unique
to the MED64 system is that the
preparation can be placed in a
humidified incubator during
recordings, which works very
well for long-term recording and
precise control of the temperature. This feature is also
enabled by its low-impedance
electrodes. With higher
impedance electrodes, a pre-amplifier
needs to be located close to
the electrode to minimize
exogenous noise and prevent
signal attenuation, under some
circumstances. However, it is
not necessary with the MED64’s
low-impedance microelectrode.
The MED probe can be connected
to the amplifier via a two meter
long cable and the MED connector
with no active circuitry. This
system configuration allows you
more flexibility for your
experiments.
