Abstract
Drop foot is a problem that can affect people following, for example, a stroke or onset of multiple sclerosis. In some individuals, it can be corrected by means of electrical stimulation of the peroneal nerve using surface (skin) electrodes. However, placing the electrodes in the right position is often described by patients as a time consuming and tedious task. Implantable solutions have been developed to address this problem, although the current state of the art indicates that surface stimulation is more suitable for patients with non-stable neurological conditions. Consequently, a number of researchers have developed approaches based on surface electrode arrays, yet problems delaying their clinical acceptance still remain. The key objective of this research was the exploration and implementation of novel solutions for surface arrays. This led to the design and fabrication of an open-loop array based system and its evaluation with healthy volunteers and individuals (patients) presenting with drop foot. Finite element based software was first used as a tool for modelling the current density distribution throughout the stimulated tissue using two preliminary array-gel combinations. The modelling results suggested that one of the configurations would produce greater selectivity and uniform patterns of current density distribution. Subsequently, both arrays were fabricated and their electrical performance tested against a commercial surface electrode of similar dimensions. The results showed that the arrays gave similar trends to those produced by the conventional one, and confirmed the selection of the array-gel combination for further testing with volunteers. This array was fabricated on a flexible printed circuit board, and then embedded in a self adjustable bandage. Microcontroller based hardware and software was developed to drive the array. This allowed creation and fine steering of virtual electrodes (VEs), as well as data visualisation and recording using a remote PC. In addition, it provided the patient with an operating interface together with an audiovisual environment to guide the task of electrode positioning. The array prototype was first evaluated with a group of 12 healthy volunteers while seated. Angular information from the ankle and foot were recorded using a twin-axis flexible goniometer placed on the lower leg. The data was used to chart the electrically induced angular motion obtained at each VE position throughout the array. The charts revealed that adequate (functional) responses were obtained by all the participants for one or more VE position and size, but that the distribution of these on the array varied between subjects. The intra-subject repeatability of the responses to stimulation during two different sessions was also assessed. The results showed that although the distribution of functional responses in the array was not highly repeatable, some of the VEs overlapped suggesting the likelihood of reappearance in future sessions. This suggests the feasibility of defining templates of functional sub-regions of the array to streamline the positioning process. The final stage of the project was a preliminary evaluation of the system with a group of 5 patients who are current users of stimulators to correct drop foot. The objective was to obtain preliminary feedback from them on the use of the array operating interface to find functional responses for various sizes of VEs while seated. A questionnaire was designed for this purpose and the data revealed that most of the patients not only found the right position for at least one of the VEs in an easy manner, but also found the stimulation to be similar or more comfortable than that experienced when using their conventional system. In summary, the results from these pilot studies with healthy and impaired volunteers demonstrated that it is feasible and relatively easy to find functional responses when using the system. Future work will be focused on the development of a self-contained portable stimulator/array sleeve that can be worn on the shank. However, additional studies involving a larger number of patients, a validated questionnaire, and the assessment of the response during walking would be necessary prior to clinical use of the system.