Towards a new generation of brain–machine interface: an electro-optic approach

Abstract

This thesis presents an investigation into the design and fabrication of an electro-optic device for transducing small biological voltage signals into the optical domain using a chiral smectic C* liquid crystal. Liquid crystals provide an interesting solution to the problem of measuring voltages in biological systems because they afford several unique advantages over traditional electrical measurement methods. Firstly, the device is passive, requiring no external power to operate. Secondly, by adopting an electro-optic approach, signals transduced into the optical domain are decoupled from the electrical environment from which they originate. This has major implications for input-referred noise in the measurement system. In traditional electrical regimes, impedance increases as electrode size decreases, with a detrimental effect on the signal-to-noise ratio, placing limits on electrode size and array density. Additionally, optical measurement of bio-signals eliminates the need for wiring, a drawback of typical passive multi-electrode arrays, the channel count of which is typically limited to 256 channels mainly due to the space needed for connectorisation. Eliminating wiring allows an electro-optic device with a similar footprint to traditional devices to achieve a much larger channel count. In this work, I demonstrate a prototype with 323 optical electrodes, and in practice higher channel counts of up to 10,000 should pose no issues. To this end, I present a novel scalable fabrication process for such devices, focused mainly on the substrate that contains the optrode array and the processes that were developed to successfully produce a functioning prototype. This results in a new type of optical measurement of biological signals with potentially much a higher channel-count and electrode density than electrical counterparts. Several related experimental works are also presented that contribute to the overall understanding of the technology, particularly in relation to noise, accuracy and sensitivity of the liquid crystal used for biological signal transduction. The device is presented in an architecture suitable for use in an in vitro setting so that tissue samples can be measured for electrical activity and propagation. Some additional experimental work, involving distributed sensing systems, opens the door to further work in developing an in vivo application with important ramifications for the development of a new generation of brain–machine interface.