Listening to Brain Microcircuits for Interfacing With External World-Progress in Wireless Implantable Microelectronic Neuroengineering Devices: Experimental systems are described for electrical recording in the brain using multiple microelectrodes and short range implantable or wearable broadcasting units

Abstract

Acquiring neural signals at high spatial and temporal resolution directly from brain microcircuits and decoding their activity to interpret commands and/or prior planning activity, such as motion of an arm or a leg, is a prime goal of modern neurotechnology. Its practical aims include assistive devices for subjects whose normal neural information pathways are not functioning due to physical damage or disease. On the fundamental side, researchers are striving to decipher the code of multiple neural microcircuits which collectively make up nature's amazing computing machine, the brain. By implanting biocompatible neural sensor probes directly into the brain, in the form of microelectrode arrays, it is now possible to extract information from interacting populations of neural cells with spatial and temporal resolution at the single cell level. With parallel advances in application of statistical and mathematical techniques tools for deciphering the neural code, extracted populations or correlated neurons, significant understanding has been achieved of those brain commands that control, e.g., the motion of an arm in a primate (monkey or a human subject). These developments are accelerating the work on neural prosthetics where brain derived signals may be employed to bypass, e.g., an injured spinal cord. One key element in achieving the goals for practical and versatile neural prostheses is the development of fully implantable wireless microelectronic "brain-interfaces" within the body, a point of special emphasis of this paper.

Fig. 1

A silicon-based cortical microelectrode array (inset); implanted for intracortical neural microcircuit recording via a percutaneous connection to a skull mounted pedestal connector (main figure schematic).

Fig. 2

(Upper left): Location of the arm area in the cortex; (lower left): typical action potentials of neural spikes; (right panel): local neural landscape with a single, needle-like recording microelectrode in the vicinity of a neuron (cell body size ~20 μm).

Fig. 3

Photo of a subject patient in a human clinical pilot trial, operating a cursor on a display screen by direct cortical “thought-to-action” control (after [24], [62] and courtesy of BrainGate2.org). The implanted multielectrode array is connected via a skin penetrating wirebundle to a head-mounted stage for analog signal amplification. This stage is tethered to other signal processing (digital) and neural signal decoding electronics in the subject’s vicinity, and the operation is supervised by trained technical personnel.

Fig. 4

Schematic and geometry of a wireless head-mounted modular system for recording neural signals from monkeys. The constituent elements that are housed in an aluminum can, attached to the animal’s skull, are show in the right photographic inserts (from bottom to top: MEA with its wire bundle, mounting hardware, the microelectronic circuitry on a PCB, and the aluminum enclosure, respectively). After [38], with permission.

Fig. 5

Upper traces: Video snapshots of a monkey reaching for food outside its home cage. Lower traces: bursts of neural cell firing activity from one channel (microelectrode) recorded synchronously via a wireless link outside the cage (from [39] with permission).

Fig. 6

The design concept for a fully implantable, transcutaneous, wireless brain communication interface for primates, composed of the cortical module (with ASIC analog amplifiers and multiplexing integrated onto the multielectrode array) and the cranial module (integrating A/D converter, command/control, and telemetry ICs and components). The images show the placement of the microsystem within a subject’s head. The entire microsystem is mounted on a single flexible polymer substrate which embeds five planar conducting microwires for electrically interconnecting the cortical and cranial units. An external receiver and inductive power supply unit is shown as a compact head proximity unit (see Fig. 9 for details). (Courtesy of Braingate2.org and from [49], [50] with permission).

Fig. 7

Upper trace: Block diagram of the active device distribution within the implantable microsystem. Lower trace: cross-sectional photographic view of a fully encapsulated device, showing the location of the key components within the cortical and cranial units.

Fig. 8

Upper trace: Full photographic image of the implantable microsystem (displayed as though viewed from the direction of the skull/brain). The cortical front end’s flexibility is shown by gravity pull (for realistic animation of the flexibility of the cortical–cranial tether, see [57] at www.braingate2.org/sensors.asp). The ground reference wire is visible, but the receiving planar inductive coil on the backside of the cranial module is not. The polymer encapsulant shows holes which are presently used to fix the cranial module onto the skull via Ti-screws to prevent motion from impact by a monkey in its cage. Lower trace: Transmitted IR laser from the top of a monkey’s head (restrained), with an implanted microsystem under operation, imaged by a night vision camera.

Fig. 9

Upper trace: Schematic of the external-to-head-unit in an exploded view, displaying the IR photoreceiver and RF inductive power coils, respectively. Lower trace: a photograph of the PCB layout of the unit, where the primary RF coil and the photodiode are “co-centric,” together with the latter’s preamplifier circuit [59].

Fig. 10

Upper trace: Recordings from a monkey implanted with a fully wireless neural recording microsystem, where neural signals exit transcutaneously by the infrared link, and power to the system is delivered via inductive coupling. Lower trace: Raster plots correlating spiking activity relative to timing of the hand grasp movement of another monkey trained to sit in a chair and grasp (in this “hybrid” device the power to the implant was delivered percutaneously).