Transcutaneous Electrical Nerve Stimulation (TENS) devices apply electrical currents to a particular area of the human body in order to suppress pain. The most common form of TENS is called conventional TENS. In conventional TENS, electrodes are placed on the skin within, adjacent to, or proximal to, the area of pain. Electrical stimulation is then delivered to the user through the electrodes, with the electrical stimulation being in the form of low intensity (typically less than 100 mA), short duration (typically 50-400 μsec) pulses at frequencies typically between about 10 and 200 Hz.
TENS electrodes typically utilize hydrogels to create a stable low-impedance electrode-skin interface to facilitate the delivery of electrical current to the user so as to stimulate peripheral sensory nerves. A minimum electrode-skin contact area must be maintained in order to ensure that the stimulation current and power densities (power intensity per unit contact area) remain below safe thresholds so as to avoid skin irritation and, in the extreme, thermal burns.
A significant safety concern for traditional TENS use is the potential for “electrode peeling” (i.e., where the electrodes of the TENS device unintentionally separate from the skin of the user) that results in an increased current and power density due to decreased electrode-skin contact area. Increased current and power density could lead to painful stimulation and, in the extreme, thermal burns. The U.S. Food and Drug Administration (FDA) has published draft guidelines on TENS devices that require a warning against the use of such devices during sleep due to the risk of unintended electrode peeling [Food and Drug Administration, Draft Guidance for Industry and Staff: Class II Special Controls Guidance Document: Transcutaneous Electrical Nerve Stimulator for Pain Relief, Apr. 5, 2010].
Poor sleep quality is one of the major causes of morbidity in patients suffering from chronic pain [Fishbain D A, Hall J, Meyers A L, Gonzales J, Mallinckrodt C. Does pain mediate the pain interference with sleep problem in chronic pain? Findings from studies for management of diabetic peripheral neuropathic pain with duloxetine. J Pain Symptom Manage. December 2008; 36(6):639-647]. It is, therefore, desirable that patients have the option of receiving TENS therapy during sleep. In fact, several studies have shown that TENS therapy can improve sleep quality (see, for example, Barbarisi M, Pace M C, Passayanti M B, et al. Pregabalin and transcutaneous electrical nerve stimulation for postherpetic neuralgia treatment. Clin J Pain. September 2010; 26(7):567-572). For these reasons, it would be advantageous to provide automated means to measure electrode-skin contact area in real-time so that TENS devices can be used during sleep for pain relief while safeguarding users from unintended electrode peeling. In particular, when a substantial reduction in electrode-skin area is detected, TENS stimulation should be halted or reduced in order to prevent excessive current or power density over the remaining electrode-skin contact area, thereby preventing painful stimulation and, in the extreme, thermal burns.