Reducing the sensation of electrical stimulation with dry electrodes by using an array of constant current sources

Highlights

• An 8 × 8 dry stimulation electrode array with individual constant current sources is proposed.

• The array has been compared with a single dry electrode for perception of sensitivity.

• It was found that the array allows higher stimulation intensity for the same sensation.

Abstract

Hydrogel electrodes are commonly used for functional and other electrical stimulation applications since the hydrogel layer has been shown to considerably reduce the perception of stimulation compared to dry electrodes. However, these hydrogel electrodes must be changed regularly as they dry out or become contaminated with skin cells and sweat products, thus losing their adhesiveness and resistive properties. Dry electrodes are longer lasting but are more uncomfortable due to unequal current distribution (current hogging). We hypothesise that if current through a dry electrode is equally shared amongst an array of small sub-electrodes, current hogging and thus the sensitivity perceived due to stimulation will be reduced. We constructed an 8 × 8 array of millimetre sized dry electrodes that could either be activated as individual current sources, or together as one large source. A study was performed with 13 participants to investigate the differences in sensation between the two modes of operation. The results showed that 12 out of 13 participants found the new (distributed-constant-current) approach allowed higher stimulation for the same sensation. The differences in sensation between single and multiple sources became larger with higher intensity levels.

Introduction

The application of electrical current to stimulate nerves for functional and therapeutic purposes is well established [1], [2]. Electrodes play a major role in the success of stimulation since the efficacy of intervention, avoidance of tissue injury and the associated discomfort are all determined by the stimulation waveform and type of electrode used [2]. Surface electrodes are the most commonly used electrode types in typical functional electrical stimulation (FES) application for correction of foot drop caused by damage to the brain or spinal cord. Guiraud et al. reported that implanted FES devices for gait restoration have been restricted to experimental concepts, and have very little follow-up data [3]. The size, shape, material and placement of surface electrodes determines how effectively the underlying muscles and nerves are stimulated with the least amount of discomfort [4]. Good surface electrodes should be comfortable during use, easy to apply, stay in place for at least a day, re-usable, cost effective and reliable [5].

In the past, carbon–rubber electrodes were commonly used. However, these require the application of electrode gel which can be messy and inconvenient. Therefore, low-cost self-adhesive hydrogel electrodes are currently use as standard. As the resistivity of the hydrogel layer increases, the stimulation-induced discomfort decreases [6]. Though high resistivity hydrogel electrodes possess most of the desired properties required for good electrodes, they have poor reusability. Using old, dried out and dirty electrodes increases the chances of causing skin irritation, reduces self-adhesiveness and increase electrode-tissue impedance. Regular replacement of these electrodes increases the costs of therapy, especially when more sophisticated and costly electrodes are required [8].

Taking these issues into consideration, dry electrodes appear attractive for long-term applications. However, dry electrodes may cause pain or discomfort when high intensity electrical stimulation is applied. At low current intensities, stimulation evokes a sensory reaction without muscle contraction; as the current intensity is increased in order to evoke a muscle contraction, this sensory response increases and can cause pain and skin irritation [9]. Hair follicles, sweat pores and other structures beneath the skin form paths of low resistance for the current passing through the electrodes and thereby cause uneven current densities (“current hogging”). It is thought that the local high current densities due to current hogging lead to the greater pain associated with surface stimulation [6]. We hypothesise that if current can be more evenly distributed across the stimulated area (thus avoiding current hogging) then stimulation will be more comfortable. One way to achieve this even distribution is to use a high impedance hydrogel electrode [6]; however, Cooper et al. conducted a study on the properties of high resistivity hydrogel samples and concluded that they became contaminated with skin products and lost their desired properties if they were used for several days [7], causing significant problems in long term applications. An alternative approach to achieve equal distribution of the current within the electrode is to use multiple constant current sources, each connected to one of an array of small, adjacent mini electrodes.