A Power Efficient Multichannel Neurostimulator Based on the Ultra High Frequency Technique for Transcranial Direct Current Stimulation Applications

Abstract

Transcranial direct current stimulation (tDCS) is a noninvasive technique, allowing for the reversible modulation of activity in particular brain regions. TDCS has obtained much scientific interest and it promises many potential benefits to the patients.

However, tDCS that is performed today is almost the same with the method that was used 20 years ago (applying 2 mA current, during a 20 min session, using two large surface sponge electrodes). The tDCS module of the future must be characterized by increased portability, battery life and focality.

Many commercially available devices have very low power efficiency, leaving space for the design of low power consumption tDCS devices. Power efficient tDCS modules will also need lower battery capacity and thus lighter batteries, increasing the portability of the system. Regarding focality, there is increased interest from the researchers and physicians for multichannel devices that use small diameter electrodes. These devices can increase the focality and the accuracy of the delivered currents offering more targeted therapies.

In this thesis, the realization of a novel, low power, multichannel stimulation module, made with discrete components, which uses the ultra high frequency (UHF) technique for tDCS applications is implemented. With this approach, the technological benefits of the UHF stimulation technique, regarding increased multichannel power efficiency, are derived, combined with a cost effective, low scale production method. Moreover, contrary to previous integrated circuit (IC) realizations, current control feedback is added to the system.

In this thesis, three prototypes are fabricated, with the last one being an eight channel module that can be supplied from a 3.5 V battery and has a very linear relationship between the selected DAC’s codes and the output delivered current and, at the same time, being able to stimulate a wide range of loads (0.148 - 10.11 kΩ) up to 2 mA. Furthermore, the employed novel boost technique shows 40.57% maximum improvement of the power efficiency, compared to the use of a conventional buck-boost converter. Moreover, the feedback system shows significant robustness, achieving only 7.6% output current divergence for 6731% change of the output load’s impedance. The module has 4 μΑ resolution, which is translated to 0.2% of the maximum delivered current. Except from the high resolution, the system also has a fast transient response, which is less than 2.1 ms. Additionally, when one channel is active, the stimulator shows 43.84% maximum power efficiency. The aforementioned power efficiency is 23.49% higher than the maximum efficiency of state of the art adaptive voltage current source implementations. Additionally, the multichannel system was tested in real life scenarios and its efficiency was compared to a fixed voltage current source module. The system achieved 37.57%, 45.47% and 11.59% power efficiency improvements for two, four and eight channels respectively.

Hence, a novel, multichannel module, with current feedback, is created that offers both high accuracy and improved multichannel power efficiency. The proposed system offers significant benefits compared to the existing solutions. Therefore, the system can be used for future implementations of power efficient multichannel tDCS devices.