TENS Under the Hood: Charge Balancing
This is the sixth in a series of posts that explore how TENS devices work. Our intention is to go “under the hood” of these useful pain relief devices to help you understand their operation. We explore key technical specifications and operating principles. We hope this information is valuable when deciding on a TENS device for your particular needs.
As was covered in a previous post (TENS Under the Hood: Stimulation Pulse), TENS devices send electric current through electrodes on the skin in the form of short pulses. The primary physiological effect of these pulses is to trigger impulses in nearby nerves that travel to the spinal cord to initiate pain relief. Another biological effect of current flow across skin is the movement of charged molecules within the skin. If not carefully managed, this can result in skin irritation. Read on to learn about charge balancing and why it is important for protecting your skin during TENS therapy.
Skin pH and Electrical Stimulation
Skin pH is normally between 4.7 and 5.7, so it is mildly acidic (i.e., pH below 7). When electricity flows across the skin, it causes charged molecules such as hydrogen ions (H+) and hydroxide ions (OH–) to move to towards one or the other electrode. If this process continues, then the skin area accumulating hydrogen ions becomes more acidic (i.e., lower pH) and that aggregating hydroxide ions becomes more alkaline (i.e., higher pH). If the pH deviates from the normal range, then the user may experience skin irritation, which can be severe if the pH shifts are large.
There are two strategies to mitigate this potential side effect of TENS therapy. The first is to limit stimulation to a short time period (e.g., 15-30 minutes) with a long interval (e.g., hours) between treatment sessions. This allows buildup of ions to diffuse away before altering the pH. In the second approach, the TENS device balances current flow to minimize accumulation of ions.
The figure below shows the two basic types of stimulation pulses: monophasic and biphasic.
In the monophasic case, current flows in the same direction with each pulse (shown as from electrode A to B). Since current is always flowing into one electrode and out the other, hydrogen ions will accumulate under one electrode and hydroxide ions will accumulate under the other. Even though ion accumulation is small for each pulse, the cumulative effect over an hour of stimulation can be large enough to disrupt skin pH and cause skin irritation. As a result, TENS devices using monophasic stimulation should only be used for a short time with a substantial wait before resuming stimulation.
In the biphasic case, the current flows in one direction during the first phase of the pulse (shown as from electrode A to B) and in the opposite direction in the second phase (i.e., shown as electrode B to A). If the charge in each phase (i.e., shaded area) is the same, then there is no net current flow. So even over the millions of individual pulses occurring during hours of TENS treatment, there should be no net current flow and thereby no significant change in skin pH. In practice, because the circuits generating the first and second phase usually have small differences, there may be a residual mismatch that results in a small net current flow during each pulse. Although biphasic stimulation is safer than monophasic, this residual current flow can lead to adverse changes in skin pH. This is particularly true when stimulation is nearly continuous as in the case of wearable devices.
Quell® utilizes an improvement over traditional biphasic stimulation that addresses the mismatch issue described above. It is called phase alternation and is shown in the figure below.
The direction of current flow alternates with each sequential pulse so that mismatches in the current generating electronics are balanced and no ion accumulation occurs. This is the most sophisticated charge balancing approach and is only found in Quell because of the need for circuits that rapidly switch current flow direction.
If you would like to read more posts in the TENS Under the Hood series, please click here.
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