Benchtop and bedside validation of a low-cost programmable cortical stimulator in a testbed for bi-directional brain-computer-interface research

Abstract

IntroductionBi-directional brain-computer interfaces (BD-BCI) to restore movement and sensation must achieve concurrent operation of recording and decoding of motor commands from the brain and stimulating the brain with somatosensory feedback.MethodsA custom programmable direct cortical stimulator (DCS) capable of eliciting artificial sensorimotor response was integrated into an embedded BCI system to form a safe, independent, wireless, and battery powered testbed to explore BD-BCI concepts at a low cost. The BD-BCI stimulator output was tested in phantom brain tissue by assessing its ability to deliver electrical stimulation equivalent to an FDA-approved commercial electrical cortical stimulator. Subsequently, the stimulator was tested in an epilepsy patient with subcortical electrocorticographic (ECoG) implants covering the sensorimotor cortex to assess its ability to elicit equivalent responses as the FDA-approved counterpart. Additional safety features (impedance monitoring, artifact mitigation, and passive and active charge balancing mechanisms) were also implemeneted and tested in phantom brain tissue. Finally, concurrent operation with interleaved stimulation and BCI decoding was tested in a phantom brain as a proof-of-concept operation of BD-BCI system.ResultsThe benchtop prototype BD-BCI stimulator's basic output features (current amplitude, pulse frequency, pulse width, train duration) were validated by demonstrating the output-equivalency to an FDA-approved commercial cortical electrical stimulator (R2 > 0.99). Charge-neutral stimulation was demonstrated with pulse-width modulation-based correction algorithm preventing steady state voltage deviation. Artifact mitigation achieved a 64.5% peak voltage reduction. Highly accurate impedance monitoring was achieved with R2 > 0.99 between measured and actual impedance, which in-turn enabled accurate charge density monitoring. An online BCI decoding accuracy of 93.2% between instructional cues and decoded states was achieved while delivering interleaved stimulation. The brain stimulation mapping via ECoG grids in an epilepsy patient showed that the two stimulators elicit equivalent responses.SignificanceThis study demonstrates clinical validation of a fully-programmable electrical stimulator, integrated into an embedded BCI system. This low-cost BD-BCI system is safe and readily applicable as a testbed for BD-BCI research. In particular, it provides an all-inclusive hardware platform that approximates the limitations in a near-future implantable BD-BCI. This successful benchtop/human validation of the programmable electrical stimulator in a BD-BCI system is a critical milestone toward fully-implantable BD-BCI systems.

Convergence of Implantable Bioelectronics and Brain–Computer Interfaces

“…109 The electrical stimulation method has the advantage of a fast response time (∼400 ms). 110 This method has the disadvantage that it can affect the patient's tissue because the electric current spreads to the tissue adjacent to the nerve or stimulated cells. In addition, if the device's insulation is not good or leaks, the current can also affect the cells around the device.…”

“…The lengths of the microfibers increased from ≈0.5 to 5 cm (Figure A) . The electrical stimulation method has the advantage of a fast response time (∼400 ms) . This method has the disadvantage that it can affect the patient’s tissue because the electric current spreads to the tissue adjacent to the nerve or stimulated cells.…”

Power Budget of a Skull Unit in a Fully-Implantable Brain-Computer Interface: Bio-Heat Model

“…Note that PCB's multilayered structure causes its thermal conductivity to be anisotropic. Based on our benchtop BCI prototypes [11], [12], we assumed a two-layer PCB and applied the formulas in [49] to obtain…”

“…There, these signals are D/A converted and sent to a sensory cortex grid, where cortical stimulation is delivered to elicit leg sensation. Our group has developed multiple preliminary benchtop prototypes of this system [11], [12], including custom integrated circuits (ICs) for ECoG recording [13], [14], [15] and wireless communication [16].…”

A Real-Time Non-Implantation Bi-Directional Brain–Computer Interface Solution Without Stimulation Artifacts

“…A bidirectional BCI is a device that capable of reading and writing data from the brain, enabling a complete linkage between the brain and an external device [4]. For instance, a bidirectional BCI can detect an epileptic seizure and stimulate the brain to prevent it [5], or provide tactile feedback to improve the control of a robotic arm [6]. Bi-directional BCIs hold significant promise in medical and cognitive enhancement domains.…”