 |
||||
 |
 |
 |
||
 |
A low-noise multi-electrode array system for in vitro extracellular electrophysiology |
|||
 |
||||
 |
||||
KEY TECHNOLOGIES
MED64 Key technology
Technical advantages
Patents
History
Low-impedance planar 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.
- Low-noise and stable recording
- Superior signal-to-noise ratio
- Excellent electrical stimulation
- No need for pre-amplifier
 |
 |
|
Close up views of electrode's surface |
||
 Impedance of Pt. black electrode on the MED probe (Red: 50 x 50 µm, Green: 20 x20 µm) compared with completely flat ITO electrode (Blue: 50 x50 µm) |
||
Back to TOP
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)
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.
|
 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 x t | Vc: Voltage of Ce / Is: Stimulation output 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/0.2ms. |
 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:
|
 Fig.8: Prolonged stimulus artifacts |
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 100µA 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 µA 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.
Back to TOP
The novelty and originality of the research and development carried out at the Corporate Research Division of Panasonic. in Osaka, Japan, has been recognized with the award of the following patents in the US, Europe, Japan, Korea and Taiwan. The technology described therein has been incorporated into the components of the MED64 system, making these the only legitimate products with the large area microelectrodes indispensable for effective stimulation.
MED System
|
MED Probe
|
Back to TOP
| Among various studies pursuing multi-site recording from brain circuits, planar microelectrode arrays have been shown to be appropriate for recording responses of dissociated cultured neurons obtained from fetal and neonatal rat brains (Pine, 1980; Gross et al., 1982)*. However, it was recognized that the networks of dissociated cultured neurons might respond differently from the neural organization of intact brains. | Pine, J., J. Neurosci. Meth., 1980; 2, 19-31 Gross, G. W., Williams, A. N. and Lucas, J. M., J. Neurosci. Meth., 1982; 5: 13-22.
|
|
Application of planar microelectrode arrays to acute brain tissue was reported by Novak and Wheeler in 1988, where they showed that their array could both stimulate and record from multiple sites from hippocampal slice preparations. Although their work definitely showed that planar microelectrode arrays could be useful for studies of brain tissue, stimulation by their array electrodes seemed somewhat ineffective, and their recordings were limited to 2-4 hours.
|
Novak, J. L. and Wheeler, B. C., J. Neurosci. Meth., 1988; 23: 149-159 | |
The MED systems approach to achieving both effective stimulation and long periods of recording time is based on two key properties:
|
||
| An array of 64 planar microelectrodes was developed, each having a size of 50 x 50 µm, arranged in an 8 by 8 pattern (the MED probe, USP 5,810,725). The material of the insulating layer of the MED probe, which directly contacts the brain tissue, was carefully chosen. The coating material and method of preparation for the surface of the MED probe were also studied. These technologies, together with electronics designed exclusively for the MED probe (USP 5,563,067), enabled multi-channel recordings of field EPSPs induced by stimulation of array electrodes for more than six hours (Oka et al.). | Oka, H., Shimono, K., Ogawa, R., Sugihara, H., & Taketani, M., J. Neurosci. Meth., 1999 ; 93: 61-67 |
Back to TOP
 |
HOME PRODUCTS APPLICATIONS PUBLICATIONS SUPPORT CONTACT ABOUT US |
|