Wearable neural prostheses. Restoration of sensory-motor function by transcutaneous electrical stimulation.

In this article, we focus on the least invasive interface: transcutaneous ES (TES), i.e., the use of surface electrodes as an interface between the stimulator and sensory-motor systems. TES is delivered by a burst of short electrical charge pulses applied between pairs of electrodes positioned on the skin. Monophasic or charge-balanced biphasic (symmetric or asymmetric) stimulation pulses can be delivered. The latter ones have the advantage to provide contraction force while minimizing tissue damage.

The influence of electrode size on selectivity and comfort in transcutaneous electrical stimulation of the forearm.

Transcutaneous electrical stimulation (TES) is a technique to artificially activate motor nerves and muscles. It can be used for rehabilitation or the restoration of lost motor functions, e.g., in subjects with brain or spinal cord lesions. Apart from selectively activating motor nerves and muscles, TES activates sensory fibers and pain receptors, producing discomfort and pain. Clinicians try to minimize discomfort by optimizing stimulation parameters, electrode location, and electrode size. There are some studies that found optimal electrode sizes for certain stimulation sites (e.g., gastrocnemius), however the underlying effects why certain electrode sizes are preferred by patients is not well understood. We used a TES model consisting of a finite element (FE) model and a nerve model to assess the influence of different electrode sizes on the selectivity and the perceived comfort for various anatomies. Motor thresholds calculated using the TES model were compared with motor thresholds that were obtained from measurements performed on the forearm of ten human volunteers. Results of the TES model indicate that small electrodes (0.8 x 0.8 cm(2)) are more comfortable for thin fat layers (0.25 cm) and superficial nerves (0.1 cm) and larger electrodes (4.1 x 4.1 cm(2)) are more comfortable for thicker fat layers (2 cm) and deeper nerves (1.1 cm) at a constant recruitment.

A model for transcutaneous current stimulation: simulations and experiments.

Complex nerve models have been developed for describing the generation of action potentials in humans. Such nerve models have primarily been used to model implantable electrical stimulation systems, where the stimulation electrodes are close to the nerve (near-field). To address if these nerve models can also be used to model transcutaneous electrical stimulation (TES) (far-field), we have developed a TES model that comprises a volume conductor and different previously published non-linear nerve models. The volume conductor models the resistive and capacitive properties of electrodes, electrode-skin interface, skin, fat, muscle, and bone. The non-linear nerve models were used to conclude from the potential field within the volume conductor on nerve activation. A comparison of simulated and experimentally measured chronaxie values (a measure for the excitability of nerves) and muscle twitch forces on human volunteers allowed us to conclude that some of the published nerve models can be used in TES models. The presented TES model provides a first step to more extensive model implementations for TES in which e.g., multi-array electrode configurations can be tested.

New multi-channel transcutaneous electrical stimulation technology for rehabilitation.

Transcutaneous (surface) electrical stimulation (TES) is a widely applied technique for muscle atrophy treatment, muscle force training, endurance training, pain treatment, functional movement therapy, and the restoration of motor functions. We present a new TES technology based on a multi-channel stimulation approach, which allows us to perform real-time spatial and temporal variations of the electrical current density on the skin surface and in deeper tissue layers. This new approach can generate a better muscle selectivity and improved muscle activation patterns compared to state of art TES systems, which operate with predetermined electrode positions. In simulations using a finite element model (FEM) of the distal arm we could show that the nerve activation in the muscle layer is not significantly influenced by the structure of the multi-channel electrode, if the gap between elements is less than 2 mm. Experiments in healthy volunteers allowed us to measure the selectivity of single finger activations. We could also show in stroke subjects that this novel multi-channel approach was able to generate selective finger and wrist extension movements that were strong enough to overcome flexion hyperactivity. For future applications in rehabilitation a full integration of the stimulation hardware into a garment sleeve would be helpful. Once fully integrated, this new technology has a high potential to increase the ease of use, stimulation and wear comfort. It is able to improve muscle selectivity compared to state of the art TES systems, and allows the implementation of a variety of new applications for the medical and consumer market.