Wireless, Ultra Low Power, Broadband Neural Recording Microsystem
Wireless Recording for Neuroprosthetic Application
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 16-channel 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 non-human primates (~ 1 month), yielding robust spike and broadband neural data on all available channels.
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 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 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, vertical-cavity 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.
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 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.
• 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
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.