Institute For Biomedical Engineering (IBE)

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Browsing Institute For Biomedical Engineering (IBE) by Author "Dreyer, Rita Liezl"

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    Investigating the effect organic tissue has on the electromagnetic waves when targeting the visual cortex
    (Stellenbosch : Stellenbosch University, 2023-03) Dreyer, Rita Liezl; Van der Merwe, J.; Van den Heever, Dawie; Stellenbosch University. Faculty of Engineering. Institute of Biomedical Engineering.
    ENGLISH SUMMARY: The concept of using electromagnets to stimulate the visual cortex to elicit a phosphene response in the visual field is not novel. Instead, it is the basis of one of the prefered non-invasive neurostimulation methods to address vision loss. The problem, however, lies in the lack of effectiveness and specificity of electromagnetic-based prostheses. The current trend is to improve the design of micro-scale magnetic coils. The effect of the organic tissue, explicitly human organic layers over the visual cortex, on electromagnetic waves has not been explored yet. This study aimed to investigate how organic tissues (skin, facia, skull and meninges) affect electromagnetic waves targeting the visual cortex. This effect should be considered when calibrating electromagnetic-based neurostimulation devices. This study focused explicitly on the maximum field strength, RMS (root-mean-square) value of an alternating current (AC) waveform and the difference between the input and output frequencies. The study tested individual layers of organic tissue and the amalgamation of the layers, including a section of the visual cortex, at 5, 10, 15, 20, and 25 Hz. A bloodless pig's head proved to be a reliable test material since it is physiologically and anatomically similar to human tissue. A fluxgate was used as a sensor to detect changes in all three axes, as the visual cortex is a three-dimensional area. To minimise external interference, the experiment was conducted in a wooden box on the premises of the South African National Space Agency (SANSA), as it has buildings that have minimal electromagnetic interference. The frequency and sinusoidal wave format were controlled by a signal generator, while a DC power supply controlled the current and power. A single pig's head was dissected, with each layer placed in a plastic ziplock bag. The individual layers were tested by placing them between the electromagnet and the fluxgate on a wooden frame held up by a wooden jig. Each test was conducted three times while keeping the distance constant. Temperature control was considered; however, due to electromagnetic interference, the set-up was removed. The experiment was conducted instead at a room temperature of 30 °C. The results showed, unexpectedly, that organic tissue generally increased rather than decreased the magnetic field (maximum field strength, RMS). This trend was observed in the z-, y- and x-axes, where the z-axis had the highest magnetic strength, and the x-axis had the lowest. The effect of the organic tissue was dependent on the input frequency and the type of organic tissue used. The results observed in each axis were independent of the other axes. Only organic tissues of the skin, skull, and brain had an impact on the frequency. One could hypothesize that variations in the output frequencies for these particular organic tissues were caused by the resonance frequencies of these tissues. The findings of this thesis can help to calibrate electromagnetic-based visual prostheses in order to increase the effectiveness and specificity of the devices. However, further studies on a human model should be conducted for more precise calibration.