Comparing Cognionics Dry with Conventional Wet Sensors

 

Dry ECG and EEG systems have long been an area of active research and development. We are often asked what makes the Cognionics dry sensor unique and how do dry systems compare against wet systems. The answer is complex because real world dry sensors are not self sufficient - they must be paired with optimized supporting mechanics and electronics to realize a practical device.

Key Components of a Dry System

Conventional wet systems rely on electrolytic gels to penetrate hair, contact the skin, and provide a clean conductive path. The gel also serves as a buffer to 'fill-in' gaps formed between the sensor and skin during application and subject movement. 

A dry sensor platform requires innovation in all aspects of the system - sensors, mechanics, and electronics - in order to obtain low-noise recordings.

With no conductive gel, dry systems are subject to several challenges. First, the sensor must be specially designed to directly touch scalp or skin, even through thick hair. Second, the sensor must remain securely in place in order to minimize artifacts and noise. Finally, the electronics must tolerate high contact impedances - 100-200x higher than wet - while rejecting noise and interference. Therefore, a complete dry solution always involves not just the sensors but mechanics and electronics as well.
 
Sensors

Cognionics systems are equipped with two electrode options. The  Drypad Sensor is a flat cushioned sensor suitable for use on bare skin such as the chest or forehead. The Flex Sensor is designed to brush through hair with modest pressure while retaining the ability to flatten for safety and comfort.

Both sensors contain patent-pending materials and construction techniques to minimize contact impedances and noise without the use of electrolytic gels.
 

Mechanics

A good harness for dry electrodes must have the ability to apply just the right amount of pressure - too little and artifacts from motion overwhelm the signal, a little too much and the system generates pain.  This is a complex undertaking due to the many variations in human body shape.  Classic EEG 'stretch' caps and rigid harnesses are not well suited for dry electrodes because they contain no provision for individually adjusting sensor pressure. 

Our EEG headsets contain precision contact adjustments for fine-tuning the pressure exerted on the dry sensors to achieve good coupling while avoiding over tightening. 
 

Electronics

By their nature, dry electrodes exhibit higher contact impedances due to the lack of gel and skin abrasion to remove the outer layers of the skin. Advanced electronics design is necessary to compensate for the high impedance electrodes to adequately reject noise and interference. Cognionics systems utilize a proprietary combination of active shielding, high input impedance amplifiers and noise cancellation to enable low-noise acquisition with dry electrodes. The amplifier front-ends are designed to be robust against artifacts including offsets, movements and 50/60 Hz pickup.

In addition to the low-noise, high impedance amplifiers, Cognionics systems also contain a variety of advanced circuitry to support researchers. All systems are equipped with a patented real-time impedance monitor to gauge sensor contact quality. For precision timing experiments such as evoked potentials, our systems support an ultra-low latency link

to transfer markers with 1 millisecond accuracy without wires. The high density systems are also equipped with a custom Bluetooth implementation to transfer higher data rates than what is normally possible. Finally, researchers have full access to the raw data stream - for readings, data, and triggers.
 

Evaluating Dry Systems and Data Quality

Quantifying signal quality from an ECG or EEG system has always been challenging due to the lack of objective standards, the semi-random nature of the source signal and the inherent variability in human subjects. Dry systems add additional factors related to the sensor and mechanics and pure electronics specifications do not adequately cover the effects of real-world sensor-to-skin contact noise. Ultimately the question of whether or not a dry system is 'good' is answered by seeing if the dry system can deliver the same signal as the wet system.

In our experience, we have determined that it is necessary to evaluate dry systems on two levels. The first level involves usability - e.g., does the EEG headset fit on different heads with sensor contact across the scalp? Unlike wet systems, contact onto different head shapes is not a given since there are no gels to fill gaps between harness and scalp.

Simultaneous recording between wet (Ag/AgCl conductive gel) and dry electrodes (Flex, through hair) showing the high signal quality of Cognionics sensor systems. Bipolar montage to avoid the influence of noise from the mastoid reference.

The second level involves examining the signal from the dry sensor against the current gold standard - a wet sensor. In the above dataset, data was recorded at the exact same time from our dry Flex sensors and gel Ag/AgCl sensors. To simulate a wet sensor overlaid directly on a dry sensor, two wet sensors were placed adjacent to the dry sensor (while taking care not to contaminate the gel) and the wet signals were averaged. The example demonstrates the high raw signal quality of the Cognionics dry system as the wet and dry signals are virtually indistinguishable. 

Evoked potentials (P300 and AEP) measured with both wet and dry systems showing very high correlation in the time averaged response.

For a more quantitative test, the above example measures the correlation between time averaged evoked responses from both wet and dry systems. The close agreement (r>0.9) shows that the dry system measures the same signal as wet electrodes. Furthermore, the test validates the timing accuracy of Cognionics wireless triggering system, compared to the wet system which utilized a standard wired connection.

For more information regarding dry sensors and the technology, please refer to our  peer reviewed publications and papers.

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