Wireless Implantable Microsystem

Needle-type probes for recording and stimulation featuring 64 electrodes and 8 channels have been developed and also incorporated in a 3D-elec-trode array [Fig. 8.12(a)] for in vivo applications at the Engineering Research Center for Wireless Integrated MicroSystems, at the University of Michigan [16, 17].

Silicon micromachined electrode arrays permit the long-term monitoring of neural activity in vivo as well as the insertion of electronic signals into neural ensembles or networks at the cellular level. Such electrode arrays are facilitating significant advances in the understanding of the nervous system, and merged with on-chip circuitry, signal processing, microfluid-ics, and wireless interfaces, they are forming the basis for a family of neural prostheses for the possible treatment of disorders such as blindness, deafness, paralysis, severe epilepsy, and Parkinson's disease.

Wireless operation of implantable systems is key to their successful deployment in clinical applications. Wires, typically used for power and data transfer between the implant and the outside world, are a primary source of infection, failure, manufacturing cost, and discomfort to the patient. Wireless transmission of power and data circumvents all of these problems. Power and data signals can be transmitted using electromagnetic radio frequency (RF), infrared, or acoustic energy; however, wireless telemetry based on RF transmission between two closely coupled coils is most commonly used. A typical wireless interface must satisfy several basic requirements. First, sufficient power has to be transmitted to the implant to enable operation of its circuitry and, in the case of stimulation, deliver the respective charge to the tissue. Second, the telemetry technique used must feature a sufficient range. This requirement depends on the application, but a range of a few centimeters is adequate for most prosthetic applications. Third, the wireless link should provide a high data transfer rate (bandwidth). This

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Figure 8.12 (a) 3-D 1024-site, 128-channel neuroelectronic interface, (b) A 64-site 8-channel recording probe, shown on a U.S. nickel. The probe dissipates 0.8 mW while providing a gain of 40 dB over a bandwidth of 10 Hz to 10 kHz with an input noise of less than 10 |iV. (c) A schematic of the recording amplifier is shown at the right, (d) Block diagram of an inductive RF telemetry link for an implantable microsystem, (e) Recordings from a guinea pig cochlear nucleus using a multisite, multishank, multiplexed, buffered probe. The shanks providing the top two signal traces were not in the neural tissue and show the noise of the recording system without any background neural noise (Reprinted from [16] with permission).

requirement is also application-dependent, although in most emerging recording and stimulating systems, bandwidths in excess of 1-2 Mb/s are needed. Fourth, the telemetry approach chosen should be immune to most environmental conditions and should be able to pass through tissue. Finally, the wireless link should be adaptable so it can satisfy the needs of different applications.

For many neuroscience applications, the desired structure of a wireless implantable microsystem would be similar to that shown in Fig. 8.12(a). Here, a 3-D array of electrodes interfaces with the tissue and resides in a platform that rests on the cortical surface. The platform supports electronics for signal processing (e.g. spike identification and event recognition) and for wireless communication with the outside world. To avoid significant heating of the tissue, the in vivo power dissipation should be limited to 20-30 mW, depending on the size of the array.

Figure 8.12(b) shows a typical 64-site recording probe. The probe dissipates 0.8 mW and has a circuit area of 4.3 mm2 in 3 ^m CMOS technology. The amplifier must provide a stable gain and limit the high-frequency response to around 12 kHz to prevent aliasing in the multiplexer. It must also AC-couple the signal to minimize offsets and suppress the unstable DC potential of the site. It must provide these features while not increasing the overall system noise, occupying very little area, and dissipating less than 10 mW to avoid appreciable heating of the tissue. Temperature rises of more than 2°C would damage the surrounding neurons. The capacitively coupled recording amplifier shown in Fig. 8.12(c) provides these features. A 10 -pF input capacitor and a 100 fF feedback capacitor set the midband gain at 40 dB. A diode-connected subthreshold nMOS transistor in the feedback loop sets the lower band-pass corner at less than 10 Hz, while a Miller capacitor in the operational amplifier sets the high-frequency limit. The equivalent-input noise is less than 10 ^V [86], and the amplifier [Fig. 8.12(c)] dissipates less than 100 ^W from 1.5 V supplies.

Figure 8.12(d) shows the diagram of a wireless telemetry link. Power and program data are transmitted via an inductive link to an implanted antenna (receiver). The implanted circuitry generates a DC power supply from the RF carrier, demodulates program data from the carrier, and generates a clock signal with which to operate the electronics. The data from the probe are digitized and transmitted back to the outside world over a second data link, often at a higher frequency. The external circuitry receives the transmitted data from the implant and reconstructs the transmitted signals.

Figure 8.12(e) shows neural activity as recorded with a multiplexed buffered probe in a guinea pig cochlear nucleus, driven by white noise bursts.

Channels 3 and 4 show neural activity with a high SNR; no neural activity is recorded on the first two channels because the corresponding sites were not in the tissue due to the convexity of the cortical surface. These channels (which were nonetheless in the fluid) show the noise of the system without any background neural activity. For a bandwidth from 300 Hz to 3 kHz, the overall noise level of the system is less than 9 pV.

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