Elsevier

Microelectronics Journal

Volume 45, Issue 12, December 2014, Pages 1585-1594
Microelectronics Journal

RFID transceiver for wireless powering brain implanted microelectrodes and backscattered neural data collection

https://doi.org/10.1016/j.mejo.2014.08.007Get rights and content

Abstract

The developments in micro-nano-electronics, biology and neuro-sciences make possible interfaces between the human brain and the environment. Implantable and smart microprobes have been proposed that are able to transmit neural data at the outside world in RFID mode. In this paper a high resolution RFID reader, collecting neural data from implanted electrodes while powering the tag is proposed. The system gives power to the implanted tag, using a class E power amplifier (PA) and in between receives the data by an asynchronous demodulation. The technology used is a standard 65 nm CMOS TSMC. Simulations shown here, reveal an average power consumption of the overall system of 65 mW with a supply of 1.2 V and a BER less than 10−5.

Introduction

The wireless sensor technology had significant impact in improving health care, both from life-style and economic point of views, by allowing monitoring of and even administering drugs remotely to elder patients suffering of chronic diseases. Advanced signal processing algorithms and circuits allow monitoring of patients even when they are on-the-move. In the last ten years the development of ultra-low-power implantable electronics enables new monitoring methodologies of biological signal. Recent research results in Brain-to-Machine Interfaces (BMI) that go way beyond the state-of-the-art: the possibility of introducing electrodes inside the brain for monitoring single or for clusters of neurons on the cortex area are making it possible to control a prosthetic device or to help patients who are affected by a stroke, Parkinson, Alzheimer and other degenerative syndromes [1], [2], [3], [4]. The possibility of investigating neuronal plasticity opens a new potential for curing drug and alcohol addictions and depressive syndromes [12], [14]. To this end several micro-fabricated solutions for the implantable multi-electrode array and more recently [5], [6], [7] for integrated readout directly connected by capton or other flexible materials to the array [6], have been proposed and in-vivo tested. Still unsolved is the problem of how to wirelessly power the implanted electrodes and at the same time efficiently collect the detected neural data and make them available for driving a prosthetic device or any bio-corrective signal (bio-feedback or bio-stimulation). The problem is associated with the requirement to eliminate batteries and find a effective solution for powering the implant for a long period of time; the presence of batteries in fact not only enlarges the size of the implant but also decreases the lifetime of the implant itself. The approach proposed makes possible a system to monitor the neural signals through a miniaturized and implantable unit and power it and collecting the wireless data by a reader. The implantable unit can be made by using micro-fabricated electrodes, integrated with their interfaces (fully integrated transponder chip or TAG) or it can be realized by connecting the probe in flip-chip configuration or via capton to the readout electronics (transponder on PCB). In both the transponder solutions, the architecture proposed here is capable of wireless powering and reading the neural data coming from one of those systems (integrated or discrete transponder) through an RFID reader.

In fact to overcome the problems associated with using a battery or a wired link [6], we propose here a passive telemetry link for power and data transfer between the implantable microsystem and the external reader. The link operates at 300 MHz but can be adapted to operate at the industrial-scientific-medical (ISM) frequency band of 13.56 MHz. This paper describes the reader architecture and working principle and shows simulation results from the implementation in a standard 65 nm CMOS technology. Section 2 provides a systematic overview of the system and the architecture of the reader. Section 3 describes the circuit details of the power amplifier implemented as a cascoded class E configuration reaching an efficiency of 90% at 300 MHz. Section 4 presents the demodulation scheme. Section 5 deals with the comparator used here as at the same time rail to rail amplifier and digitizer. Section 6 presents simulation results and noise analysis. Section 7 finally concludes this paper.

Section snippets

The reader architecture

Fig. 1 illustrates the reader architecture. Realization of the system illustrated in Fig. 1 is driven by the specification given by the implanted TAG described in [5]. In particular the decision on the frequency of the powering signal is obliged by the requested power at the sensing node and the antenna size. Power delivered to the node is maximized by careful selection of the wireless transmission frequency and by the power amplifier design (Section 2.1). To enable robust multi-node

Cascoded class E power amplifier

The power amplifier is the block of the overall transceiver contributing to the most significant power consumption. For this reason, many research has recently focused on switched power amplifiers [8], particularly on classes E and F ones, as class D PAs are commonly believed to be not suited for RF operations [7], [8], [9].

The main advantage of class F PA is the low peak voltage and rms current, that is very beneficial from the device stress point of view. However, although class-F PAs feature

The demodulator

Many communication schemes for implantable devices have been described in implantable device transmission data: amplitude, frequency and phase modulation and ranging from binary to higher order encoding in complexity [9]. One common justification for constant amplitude modulation is the need for constant power flow since on-chip energy storage may be infeasible. However many designers choose phase or frequency shift keying modulation over amplitude modulation. The disadvantage, however, is the

The latched comparator

The full swing amplification has been achieved with a track-and-latch (TL) comparator. In a TL comparator the clock sets the comparator in the latch mode. While tracking, some internal nodes of the comparator follow the input and the output is not available. For making a decision, the clock forces the comparator to the latch mode and usually the input is internally disabled. The transition of the clock is fast relative to the bandwidth of the input signal and the input signal or noise does not

Simulation results and noise analysis

The reader circuitry has been designed in CMOS 65 nm TSMC technology, 1.2 V supply and the simulation results are shown in Fig. 11, Fig. 12. In Fig. 11, on the top the modulated signal on the antenna, the signals at the output of the comparator and the signal of the TAG (on the bottom) are shown.

Fig. 12 shows the signal at the output of the envelope detector on the top and on the bottom the signals at the input of the comparator after the bypass capacitor [15], [16]. The average signal is at the

Conclusion

Although a lot of work has been published in the field of wireless communications and wireless powering for biomedical implants, there is still a lack in the definition of neural data reader wireless powering and in the mean time collecting the data, without need of large battery itself. A high resolution reader, powering neural implanted electrodes while collecting data coming from the transponder has been described here. The technology used is a standard 65 nm CMOS TSMC with 1.2 V supply

Acknowledgments

This work has been carried out under the framework of the research activities of Prof. Jan Rabaey and his group at the Berkeley Wireless Research Center BWRC.

References (16)

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