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, 8 (4), 046021

A Wirelessly Powered and Controlled Device for Optical Neural Control of Freely-Behaving Animals

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A Wirelessly Powered and Controlled Device for Optical Neural Control of Freely-Behaving Animals

Christian T Wentz et al. J Neural Eng.

Abstract

Optogenetics, the ability to use light to activate and silence specific neuron types within neural networks in vivo and in vitro, is revolutionizing neuroscientists' capacity to understand how defined neural circuit elements contribute to normal and pathological brain functions. Typically, awake behaving experiments are conducted by inserting an optical fiber into the brain, tethered to a remote laser, or by utilizing an implanted light-emitting diode (LED), tethered to a remote power source. A fully wireless system would enable chronic or longitudinal experiments where long duration tethering is impractical, and would also support high-throughput experimentation. However, the high power requirements of light sources (LEDs, lasers), especially in the context of the extended illumination periods often desired in experiments, precludes battery-powered approaches from being widely applicable. We have developed a headborne device weighing 2 g capable of wirelessly receiving power using a resonant RF power link and storing the energy in an adaptive supercapacitor circuit, which can algorithmically control one or more headborne LEDs via a microcontroller. The device can deliver approximately 2 W of power to the LEDs in steady state, and 4.3 W in bursts. We also present an optional radio transceiver module (1 g) which, when added to the base headborne device, enables real-time updating of light delivery protocols; dozens of devices can be controlled simultaneously from one computer. We demonstrate use of the technology to wirelessly drive cortical control of movement in mice. These devices may serve as prototypes for clinical ultra-precise neural prosthetics that use light as the modality of biological control.

Figures

Figure 1
Figure 1. Design and implementation of a wirelessly powered and controlled headborne optical neural control device
(a), Block diagram of the device, which comprises three core modules (power, which contains the supercapacitor and antenna, optics, which holds up to 16 LEDs in two banks of 8 each, and motherboard, which contains the microcontroller and power circuitry), as well as one optional module (radio module, which mediates on-line updating from a computer or laptop). (b), Schematic side view, and (c), three-dimensional representations of, the modules that make up the device, with numbers indicating some of the key parts used. The optional radio module contains an antenna (1), radio chipset (2), and motherboard-docking connector (3). The power module contains an antenna for power reception (4), power rectifier chip (5), supercapacitor (6), and motherboard-docking connector (7). The motherboard module contains a microcontroller (9), an LED power supply (11), and connectors for docking the power (8), radio (10), and optics modules (12); the ten pins on connector 12 that are not used by the optics module are used for post-device assembly programming of the microcontroller (e.g., as seen in Figure 2(b), which shows a motherboard + radio connected to a laptop through a USB-connected base station board (the red board) in the fashion required for reprogramming). The optics module, which is cemented to implanted skull screws and acts as the support for the detachable three remaining components of the device, comprises a connector that docks to the motherboard module (14), a copper thermal sink that also serves as an isolated ground for the LEDs (15), one or more LEDs (16), and a temperature sensor (not shown, mounted on bottom of the copper post-machining of thermal sink, see Supplementary Files for schematic), as well as the LED multiplexer (attached to the side of the connector, see Supplementary Files) (d), Angled, and (e), Side view of the detachable portion of the device (radio, power, motherboard; optics module is not shown), with penny shown for scale. (f), headborne electronics unit affixed to subject via low insertion force connector between optics module (surgically affixed) and motherboard module. Supercapacitor removed to show connector interface between motherboard and radio modules.
Figure 2
Figure 2. A simple wireless power and communications interface for operation of headborne optical neural control devices on the awake behaving mouse
(a), Photograph of an arena equipped with power transmitter coil (a 120 kHz LC tank circuit), containing a mouse equipped with a headborne optical neural control device, schematic showing operation with a computer with a base station attached via USB port (detailed photograph of base station in (b). When powered up, the microcontroller automatically initializes the radio module if attached, wirelessly connecting to the base station to receive instruction from the experimenter; if no radio module is attached, it can operate in open-loop fashion. (b), Photograph of a USB-connected wireless interface board (red) docked to laptop, and equipped with a copy of a headborne device (green) to serve as the transceiver for wireless communication (collectively the base station). (c), Guidelines for usage of wireless headborne optical control devices for typical neuroscience experiments involving pulse trains of light delivery. i, Schema defining various properties of pulse trains (i.e., within train duty cycle = PW/(1/PR), and within-train average power = A * PW/(1/PR), for assistance with visualizing typical protocols for device operation. ii, Plot of the range of typical protocols for device operation, expressed as a function of within-train duty cycle and within-train average power, assuming that the between-train pause (i.e., ITI – 2 * TD) is at least 3 seconds. Any point under the purple curve is easily achievable. The reason is that because the power antenna continuously receives 2 W, the device can run indefinitely with a time-averaged power of 2 W; the device can exceed 2 W using the supercapacitor’s excess capacity, but with reduced duty cycle, and never crossing the device maximum peak power of 4.3 W. In addition, there is no hit in device performance with between-train pauses of less than 3 seconds if 2 W or less is consumed; if more than 2 W is consumed, because the supercapacitor’s excess capacity is needed, some time (up to 3 seconds, depending on how much energy is used) will be required to recharge the supercapacitor in between the LED-on periods that drain the supercapacitor. (d) Unilateral optogenetic control of motor cortex neurons in a freely behaving Thy1-ChR2 mouse, which expresses ChR2 in layer 5 pyramidal cells (right), eliciting reliable drive of mouse rotation compared to the no-light condition before stimulation (n = 9 trials across 2 subjects, positive value indicates CCW rotation, * indicates p < 0.01, paired t-test). When the LEDs were turned off (left), no rotation was apparent. p>0.5, paired t-test).

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