Wireless, Ultra Low Power, Broadband Neural Recording Microsystem
Wireless Recording for Neuroprosthetic Application
Overview
We have built a wireless implantable microelectronic device for transmitting cortical signals transcutaneously. The
device is aimed at interfacing a microelectrode array cortical to an external computer for neural control applications. Our
implantable microsystem enables presently 16-channel broadband neural recording in a non-human primate brain by
converting these signals to a digital stream of infrared light pulses for transmission through the skin. The implantable
unit employs a flexible polymer substrate onto which we have integrated ultra-low power amplification with analog
multiplexing, an analog-to-digital converter, a low power digital controller chip, and infrared telemetry. The scalable 16channel microsystem can employ any of several modalities of power supply, including via radio frequency by induction,
or infrared light via a photovoltaic converter. As of today, the implant has been tested as a sub-chronic unit in nonhuman primates (~ 1 month), yielding robust spike and broadband neural data on all available channels.
The Substrate
Currently we use flexible Kapton (polyimide), laminated on either side for mechanical strength of the metal traces.
Kapton provides excellend flexibility, and relatively low water uptake. Kapton has been used for many years in the
retinal prosthetic realm, allowing extemely miniaturized electronics to be placed in the tight constraints of the eye.
Behind either end of the substrate, we attach a rigid plating (alumina) to allow proper wedge-bonding conditions. This
also provides slight mechanical resistance to bending of the substrate around the electronics.
The connecting tether of the substrate has been made as thin as possible to allow greatest flexibility of the transcranial
region of the implant. Slight movements of the brain in relation to the skull can cause damage from electrodes dragging
through cortex, and this must be minimized.
A recent system is imaged on the right. The green material is the Kapton substrate. iPhone is underneathe for
comparisson.
The Array/Amplifier
The array/amplifier integration process was developed at Brown by Yoon-Kyu Song from the need to directly mount the
ASIC amplifier design (in-house by William Patterson) onto the back of the array, allowing maximal miniaturization and
nearest amplification to the input signal (the neurons). At Brown, we use a "flip-chip" bonder to bring the two
components together and cure the joining silver epoxy.
The Telemetry
The ADC receives a clock and a start-of-conversion signal from the digital controller IC that also supplies the channel
address to the amplifier circuit. The controller is a custom integrated circuit built in the AMIS 0.5-micron process through
MOSIS and has been described in more detail elsewhere. The controller has two other functions. First, it multiplexes the
ADC data with a periodic synchronization code word that replaces the data from one channel. The external electronics
that receive the neural data uses this unique code word to find the beginning of the serial data for the first channel.
Second, the controller converts the multiplexed data into the drive current for a low-current, high-efficiency, verticalcavity surface-emitting laser diode (VCSEL), which produces a peak optical output power of 2 mW for the optical
telemetry. The VCSEL occupies less than 1 mm2 of substrate area. The controller derives its clock from either the RF
inductive power loop or from modulation on the DC power source depending on the supply mode. The controller IC
contains a comparator that regenerates the digital clock from a small sinusoidal signal separated from the appropriate
source in the power module. Total system power consumption is approximately 12 mW in the present version including
all parts of the implanted system.
Encapsulation
While much is understood about the issues regarding soft, polymeric electrical insulation of electronics, little is known
about how these issues can be solved. We have previously developed a small scale testing station to evaluate the
major constituents of device failure due to encapsulation breakdown; these are surface adhesion strength of the
material, electric driving force over the material, and temperature of the material in question. While this setup was
extremely useful in the guidance of our research, it does not provide that statistical significance we need to make
founded conclusions about the dynamics of the physical system we are dissecting. To achieve a greater degree of
certainty we are currently in the process of developing a system with the capability and capacity to test 100 units
concurrently. This system has been designed and is currently being fabricated in our labs to provide the scientific
community with hard evidence as to how polymeric insulators (specifically, polydimethylsiloxane and Parylene C) will
behave in biological environments. Spending the time and effort to thoroughly dissect this issue is essential to providing
an implant that is safe for the patient/subject and functional for research and eventually human implant.
The Brown Neurocard (BNC)
Development and testing of a fully implantable neural microsystem is a multi-stage process, requiring rigorous
performance evaluation and validation at each step. Our development pathway towards the final goal of a fully
implantable (presently 16-channel) system had four steps: (A) evaluation at the benchtop level via immersion in
physiologic saline solution (mimicking the conductivity of brain tissue) and „pseudospike‟ electrical current injection; (B)
building a printer circuit board (PCB) version of the microsystem (“Neurocard”) for external mounting atop a primate
skull, to validate the system component performance by coupling this external unit to passive microelectrode array
implants with skull-mounted connectors, (C) In-vivo testing during acute surgery in rodents (rats, whose anatomical
dimensions only permit the insertion of the cortical “front panel”), and finally (D) surgical techniques for microsystem
implant into a monkey with wireless transcutaneous signal transmission, with online reliability and animal safety
monitoring.
Implanting passive microelectrode arrays into non-human primates is relatively routine; however, integrated constructs
such as the one described here require a different set of surgical parameters to be "mastered‟ by neurosurgeons. One
key issue is to ensure that microelectrodes reach their required target area and depth (latter with submillimeter
precision), while carrying the additional electronic payload and a mechanically different tether and associated force
loads. A practical approach to decouple the evaluation of microsystem electronic performance from surgical, anatomical
and neurophysiological implant complications is to move the active electronics to an external platform and use input
from existing implanted passive arrays. Such an approach has been adapted recently for neuroscientific studies in
freely moving monkeys. We have pursued this strategy and developed a small printed circuit board (PCB) bearing all
the active microelectronics rigidly connected to a skull-mounted pedestal connector, and can be used in conjunction
with standard passive implants in monkeys. The neural signals extracted from the board (by wire or wirelessly via IR)
can then be directly compared in quality to those acquired from the same animal using a standard commercial (rack-
mounted) neural signal acquisition system. The figure above shows a block diagram of the test system (a) and a
photographic image of the “Brown Neurocard” (b).
The Results
The present status of our work underway is summarized the figure here showing an example of ongoing experiments
where the full microsystem is being implanted into the head of macaque monkeys. The night vision camera shows the
spot at which the IR laser beam exits through the skin. This implant also includes an accompanying electrical
feedthrough for in-situ comparison between the wireless (IR) and wired telemetry. The implant was placed in a monkey
subchronically (for a period of ~30 days), and analyzed post-explant for functionality and performance. The figure
shows neural pseudospike waveforms recorded on a single channel of the explanted microsystem, and verifies that
there was no change in system performance through the duration of the implant as well the handling during the actual
implant and explant surgeries. This durability is another initial indicator of the resilience of the packaging. Work is
underway to correlate the neural signals to specific task related behaviors.
Related publications:
•
Active Microelectronic Neurosensor Arrays for Implantable Brain Communication Interfaces. IEEE Transactions on
Neural Systems and Rehabilitation Engineering. 2009.
•
A Brain Implantable Microsystem with Hybrid RF/IR Telemetry for Advanced Neuroengineering Applications.
EMBC. 2007
•
A microelectrode/microelectronic hybrid device for brain implantable neuroprosthesis applications. Biomedical
Engineering. 2004.
Recent conferences and talks:
•
International Conference on Solid State Circuits, San Francisco, January 29-31
•
Northeast Bioengineering Conference, Providence, April 4-5
•
Neuro-IC ’08 Conference, UCLA, Los Angeles, May 28
•
•
•
NIH Conference on Neural Prosthetics, June 16-18
IBM Physical Sciences Colloquium, Yorktown Heights, November 7
International Conference on Electron Devices, San Francisco, December
•
Conference on Lasers and Optoelectronics (CLEO), San Jose, May
•
•
Congress of Neurological Surgeons (CNS), Orlando, May
Society for Neuroscience Annual Meeting, Washington DC, November
On Wireless Power for the Brain Implantable Chip
Currently we are striving to make our BIC device fully implantable. To make this possible, we’re adopting IR- RF data
power telemetry for the next version of BIC device. As shown in the following figure, internal device consists of
microelectrode array with amplifier on front-end and ASIC, ADC etc on the back end panel. Also on fully implantable
internal unit, there is a coil printed on the back-end of a substrate which receive power, and VCSEL (850nm), mounted
in the middle of the coil, sends data with full bandwidth through skin. To pick up those signals and feed the implantable
device, external unit consists of primary coil and IR detector. While the concept of the whole system is straightforward,
there are several challenging issues on RF/IR data transmission when the device is actually implanted.
Design Issues: RF coils on external and internal unit should align well together to deliver maximum power, while the
external part should be minimally obtrusive and wearable easily as shown in cochlear implant. The device for freely
moving animal model may be different from the future clinical application, but both should be optimized appropriately.
Transmission efficiency: Optical data has loss of power when it passes through skin due to scattering and absorption
(and small but certain amount is due to back reflection). As the photodiode should be in the midst of the primary coil, the
optical pickup is subject to picking up the RF output especially when the output of PD is tens (or less than)of mV level.
This requires the proper optical model of scattering through tissue, since the total area of scattered light is related to the
size of photodiode, and the size of PD is related to the time constant (speed of the system).
Biocompatibility: How big the temperature change of tissue will be near the implanted device when RF power is
applied, and what will be the tolerable range? How can the system detect and shut down as soon as possible when
induced power of implanted device by any reason? These are the region of biocompatibility that one might actually have
during the clinical trial, and should be proved perfectly before any possible clinical application.
These and other possible issues that can come up with the implementation should be solved, and we’re
working on these for the next version of BIC.