Electrode Information

Adhesion

When you seal a box with shipping tape, high adhesion levels are a must—just as low adhesion levels are needed for sticky notes.

Electrode adhesion must strike a balance between “too sticky” and “not sticky enough.” Too sticky, and electrode removal is difficult, mildly painful and in a few cases, may result in some degree of skin removal. When an electrode is not sticky enough, it can lift from the user’s skin, which detrimentally affects the therapy and can result in burns (arcing).

There is a complete absence of industry standards for hydrogel electrode adhesion. Additionally, until now there have been no tests for hydrogel electrode adhesion. Hydrogel manufacturers conduct adhesion tests on hydrogel—but the tests are only conducted on the gel and not on the complete electrode assembly. Furthermore, the tests have no relativity—meaning no comparison to other electrodes.

Our Adhesion Testing Process

H.E.C.A.T. (Hydrogel Electrode Comparison & Analysis Tool) allows us to perform adhesion peel tests under controlled and measurable conditions. The tests are objective and consist of applying a fresh electrode to a clean stainless steel plate. The plate with the electrode attached is placed into a holding fixture and connected to the fittings of a digital force gauge.

The electrode is peeled away from the stainless steel plate and a force measurement is recorded. The test is conducted once on a clean stainless steel plate. Many electrodes are tested and the results are averaged. The measurements are in kilograms of force (KGF).

For comparison reasons, a cohort of various brands of electrodes are assembled and tested. Each electrode’s “pull” value (in KGF) is measured a predetermined number of times and the average of these readings recorded. Once all the brands have been tested and their average pull force determined, the arithmetic mean of all brands is determined—this establishes the true “middle ground” of adhesion amongst the candidates. Each individual brand is then ranked based on its deviation from the middle line. The electrode with adhesion closest to the mean is ranked the highest, while the electrodes with the most extreme values are ranked the lowest. In the event of a tie, we show a bias toward the electrode that has less than the median adhesion level.

This methodology is consistent with our premise that adhesion levels at the extremes are undesirable, and low adhesion levels can result in unsafe therapies. Adhesion tests are conducted on fresh (out-of-the-bag) electrodes, also after accelerated dry-out in an environment chamber and when necessary after fouling in the re-apply tests. Adhesion tests are designed to be used for the comparison of different electrode brands. We do not use the KGF data to suggest any standard for the amount of adhesion an electrode should have.

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Electrical Resistance

Electrical resistance is typically represented in OHMS (Ω) and is an easy measurement to obtain. Unfortunately, it is not measured in the same fashion by all manufacturers. Some manufacturers measure only the carbon dispersion pad, completely ignoring the resistance created by the pin connector, lead wire or most importantly, the electrode’s hydrogel. Others measure only the electrical resistance of the hydrogel. Add to this the fact that some electrodes are tested using AC voltage and others tested with DC voltage, which yields completely different resistance measurements.

As a result of the dramatic dissimilarities in testing methods, comparisons between electrodes on the basis of electrical resistance or impedance are a waste of time.

Our Electrical Resistance Testing Process

H.E.C.A.T.’s (Hydrogel Electrode Comparison & Analysis Tool) testing protocols provide a consistent and reliable measurement of a hydrogel’s electrical resistance. H.E.C.A.T tests the complete electrode assembly, end to end. Similar to the electrical conductivity testing, 324 points on the hydrogel surface are sampled and electrical resistance values are determined. Low resistance values are desirable, and the electrode with the lowest electrical resistance (OHMS) is assigned the highest comparison value.

Due to the resistance/capacitance nature of hydrogel, electrical resistance measurements are problematic. A preferred method of determining an electrode’s electrical performance is by measuring its electrical conductivity efficiency.

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Moisture Content

Moisture content in a hydrogel electrode is vital to its electrical performance, adhesion and re-use. Most electrode manufacturers produce hydrogels of varying formulas, and one of the variables is moisture content. Most fresh, out-of-the-bag electrodes have high (99% to 100%) moisture content levels and their electrical performance and adhesion are reflective of those high levels.

Keeping in mind our premise that the best electrodes should combine low cost with high performance—as well as high performance over time—an electrode’s resistance to dry-out is a critical performance category. It is not enough to have high moisture content levels for fresh electrodes; it is also desirable for an electrode to maintain high moisture content levels throughout the electrode’s useful life. In fact, lower moisture content levels lead to reduced adhesion, which is typically the catalyst for electrode replacement. Therefore, dry-out resistance as evidenced by moisture content percentage is a desirable characteristic in a hydrogel electrode.

Our Moisture Content Testing Process

Unfortunately, no industry standard exists for moisture content or dry-out resistance. But by using H.E.C.A.T. (Hydrogel Electrode Comparison & Analysis Tool), we are able to perform a test for these attributes. H.E.C.A.T. measures an electrode’s moisture content at 100 points on the hydrogel surface. These 100 points can be averaged and compared to fresh, out-of-the-bag moisture content readings, readings after various stages of accelerated dry-out, and during re-apply (fouling) tests. The moisture content readings are particularly helpful in determining dry-out resistance.

Using H.E.C.A.T.’s unique capabilities, dry-out resistance can be determined by obtaining an average moisture content (MC) from a fresh electrode and then subjecting the electrode to 12 hours in a heated, dehumidified environment chamber after which another 100-point average MC is obtained. The difference between the fresh MC reading and the accelerated dry-out MC reading can be used to determine which electrodes offer the greatest dry-out resistance. The lower the drop in MC (dry-out resistance), the higher the comparison value to be assigned to the electrode.

When you consider the important role Moisture Content plays in electrical performance and adhesion life, you can appreciate the direct relationship between dry-out resistance and overall electrode performance.

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Electrical Conductivity

Currently, the hydrogel electrode industry has adopted the concept of electrical resistance as a measurement of an electrode’s electrical performance. Initially, electrical resistance appears to be an easy-to-understand concept; the general idea is that an electrode’s electrical resistance is the inverse of its conductivity.

Conductivity is a measurement of how efficient, in terms of percentage, an electrode is at conducting electricity from the stimulation device (TENS, IF, NMES, etc.) to the user’s skin; this is what really counts in terms of therapy effectiveness. Electrical conductivity can be difficult to measure, and requires an understanding of electrical concepts and measuring techniques as well as sophisticated test equipment.

Our Electrical Conductivity Testing Process

H.E.C.A.T. (Hydrogel Electrode Comparison & Analysis Tool) is uniquely capable of taking thousands of conductivity measurements across an electrode’s surface in a single second. Comparisons between the voltage supplied to the subject electrode and the resulting throughput voltage are made and efficiency percentages are determined. As an example, consider a stimulation device that outputs 10 volts AC. It is connected to an electrode and the voltage that reaches the user is 8.5 volts AC. This simple example results in an efficiency percentage of 85%.

Once an electrode’s efficiency percentage is calculated, comparisons between electrodes can be made. Electrodes with higher throughput efficiencies are desirable because the efficiency translates into high therapy intensities, less strain on the stimulation device and reduced battery consumption. It is important to know that the 15% of electrical current that does not reach the user is actually wasted. When there is an efficiency of 85%, there is wastage of 15%.

Electrical conductivity is dramatically impacted by an electrode’s hydrogel moisture content. As the hydrogel dries out (loses moisture), the electrical conductivity efficiency drops as well. Electrodes with high intrinsic electrical efficiency and dry-out resistance will always provide superior performance.

H.E.C.A.T. uses a patented system that measures electrical throughput at 324 locations on an electrode’s surface. Each point is typically read several times across the duration of the entire test to ensure a distributed pattern of measurements. Calculations are made and an overall electrical conductivity efficiency percentage is determined.

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Electrical Dispersion

Electrical dispersion in an electrode is similar to a lawn sprinkler’s watering pattern. Poor dispersion of the water leads to dry spots on the lawn as well as flooded areas, both of which can damage the grass.

Unfortunately, poor dispersion in an electrode can have serious efficacy and safety implications. Too much electrical current in one area results in a “hot spot” and could lead to skin burns. Too little current in an area will reduce the effectiveness of the therapy.

Even dispersion of electrical current is a desirable performance characteristic. What is sought are low levels of deviation from the average voltage readings at any given point on the electrode. From H.E.C.A.T.’s (Hydrogel Electrode Comparison & Analysis Tool) 324 voltage-sensing points, data is obtained and a mean (average) voltage is determined. Standard deviation calculations are then performed and the electrodes with the lowest deviation value are considered superior.

Prior to the introduction of H.E.C.A.T., such measurements were not taken and comparisons between models of electrodes based on electrical dispersion were not made.

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Physical Strength Tests

Our physical tests are designed to destroy an electrode in some fashion and measure the amount of force required for the destruction. Destruction is the opposite of construction, and that is our goal—to pull apart the electrode and gauge the strength of its construction. We conduct four physical tests:

  • Lift by Lead Wire
  • Pad Tear
  • Lead Wire Pullout
  • Connector-to-Lead Wire Pullout

There are no strength standards for hydrogel electrodes today. Manufacturers are aware of inherent strength limitations in their electrodes—and guard against user actions that will place strain on their product. Case in point: the universal admonition that users should “never remove electrodes from the skin by lifting with the lead wire”. Our physical strength tests are for quality comparison only. Physical strength in an electrode is important—but not as vital as electrical conductivity, dispersion or moisture content. Physical strength tests simply illustrate which electrode manufacturers utilize high-quality materials and construction methods, and which ones do not.

Lift by Lead Wire

We begin by placing a fresh electrode on a piece of phenolic plastic (printed circuit board material) and leave it adhered to the phenolic plastic for a period of 24 hours to establish a significant bond between the electrode and the phenolic plastic. We then peel the electrode from the phenolic plastic substrate by attaching the electrode’s lead wire to the fittings on a force gauge. The force measurement is irrelevant because some electrodes fail prematurely due to poor construction or material failures and others “hold on” longer before failing. This test is not focused on the force required to create failure. Instead, it seeks to create failure at the electrode pad-to-lead wire connection and gauge the nature of the failure. Accordingly, a value is assigned to the test subjects based on the degree of destruction. This is the value assignment logic: 0 = Total failure: Either a ripping of the electrode pad or complete removal of the lead wire from the electrical pad 1 = Heavy partial failure: Major delamination, heavy pad distortion and lead wire almost pulled out 2 = Light partial failure: Minor delamination, minor pad distortion and minor lead wire stretching 3 = No failure: No delamination, no lead wire movement and no pad distortion

Pad Tear

In this test, the amount of force required to tear the pad is a direct measurement of the pad’s strength. We place the electrode pad into a fixture designed to shear the pad. The electrode pad is subjected to shearing forces and, once the pad tears, the maximum force required to tear the pad is recorded. In comparing the subject electrodes, the greatest force required to tear the pad receives the highest assigned value.

Lead Wire Pullout

This test measures the electrode assembly’s resistance to forced removal of the lead wire from the lead wire pad. This is different from the lift-by-lead wire test (you can see by viewing the lead wire pullout test and comparing it to the lift-by-lead wire test). In this test, the electrode pad is held around the pad’s perimeter by a fixture. The lead wire is attached to the force gauge, and pulling forces are exerted to the point of failure. The force required to cause failure is recorded and the greater the force, the higher the assigned comparison value.

Connector-to-Lead Wire Pullout

For this test, a lead wire is held tight in a clamping fixture and the connector is likewise held in a clamping fixture. Force is applied to pull the lead wire from the connector. Failure is not always observed at the connector. Typical connector failure is observed when the lead wire pulls out of the connector—but there are times when the lead wire breaks before pulling out the connector. In this instance, the force required to cause the failure is recorded and the type of failure (lead wire-to-connector pullout or lead wire break) is noted. Physical strength tests are designed to provide objective mechanical force data that can be quite useful in a side-by-side comparison of electrode brands.

Accelerated Dry-Out

electrode graphic - low cost - performance - performance over time One of the three main criteria for determining the best-value electrode in the world is “performance over time.” Electrodes begin degrading as soon as they are exposed to air. The process is slow, with minimal effect at first. However, the longer an electrode is exposed, the more its performance diminishes. As the electrode dries out, its electrical conductivity decreases, its electrical dispersion becomes less uniform, and the adhesion becomes unstable.

The Process

To effectively measure an electrode’s performance over time, it is necessary to force it to dry out in a process we call “accelerated dry-out.” This is accomplished with an environment chamber equipped with a computer-controlled dehumidifier.

espec environment chamber
Espec Environment Chamber allows us to simulate various environmental conditions.

We place the subject electrodes on a turntable inside the chamber. The turntable guarantees even airflow across all of the electrodes.

electrode turn table
Custom testing fixtures like this 2″ electrode tuntable allow even testing of all units under test.

Using H.E.C.A.T. (Hydrogel Electrode Comparison & Analysis Tool), we perform “baseline” tests on all subject electrodes before placement in the chamber. The baseline tests are:

  • Electrical conductivity
  • Electrical resistance
  • Electrical dispersion
  • Moisture content
  • Adhesion

The electrodes are then placed in the environment chamber, which is set to a temperature of 68 degrees Fahrenheit with a humidity level of 30%.

A timer is set for three hours. At three-, six-, nine- and 12-hour intervals, the electrodes are removed one at a time for H.E.C.A.T. testing. Comparing the baseline test results to the 12th hour results yields important data on the subject electrodes:

A timer is set for three hours.  At three-, six-, nine- and 12-hour intervals, the electrodes are removed one at a time for H.E.C.A.T. testing. Comparing the baseline test results to the 12th hour results yields important data on the subject electrodes:

  • Resistance to dry-out
  • Adhesive degradation

We conducted 12-hour and 120-hour accelerated dry-out processes on the subject electrodes.

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