An Asymmetric RF tagging IC for Ingestible Medication Compliance Capsules

Hong Yu, Chun-Ming Tang and Rizwan Bashirullah Dept. of Electrical and Computer Engineering, University of Florida, Gainesville, FL, 32611, USA

Abstract — This paper presents a feasibility study of a

low power electronic RF tagging device and a printed capsule antenna for medication compliance monitoring. RF transponders attached directly to the outer surface of a standard sized capsule can potentially serve as a cost-effective method of validating medication compliance via electronic detection of an ingested pill inside the digestive tract. The electrical radiation characteristics of a small biocompatible antenna are analyzed inside a human body model for various FCC telemetry bands using the finite difference time domain (FDTD) method. Based on this study, a novel asymmetric RF tagging IC is reported. The device is powered by low frequency AC signals to create externally detectable RF bursts in the 915MHz ISM band. A test chip was fabricated in 130nm CMOS technology and experimentally validated inside a phantom solution that mimics the human torso.

Fig 1. A prototype capsule antenna.

Index Terms — RFID, Telemetry, Medication compliance

I. INTRODUCTION

Medication compliance is the degree to which a medication is taken according to a prescribed treatment and is usually measured in terms of percent of doses taken over a given interval. It is estimated 125,000 people die of treatable ailments because of poor adherence. In addition, a tenth of hospital admissions are associated with noncompliance at a healthcare services expense of approximately $15.2 billion annually [1]. Clearly, there is a present need for new technologies that can replace directly observed therapy (DOT), which is the only known reliable method of determining medication compliance. Motivated in part by the need for new alternatives for medication compliance, the primary objective of this work is to investigate the feasibility of small electronic transponders as potential means for low cost and reliable detection schemes of orally ingestible electronic pills (e-pills). With continued advances in RF biotelemetry, it is envisioned that an external monitoring point-of-care device or a body-worn electronic sensor can be used to detect the presence of the pill in the stomach or GI-tract after ingestion.

Fig. 1 shows a prototype capsule antenna printed directly onto the surface of a 00-sized gelatin-based capsule using silver colloidal inks [2]. These antennas can be made of conductive bio-compatible coatings by incorporating a metal, which can dissolve, such as silver, under a temporary protective layer such as polyglycolic

acid, or by incorporating particles that are non-toxic by virtue of being non-absorbable. Thus, the substrate for the antenna and the drug delivery device are the same and the volume reserved for the medication remains unchanged. The medication capsule can therefore house on its outer surface an electronic compliance device. The RF tagging IC in this system-on-a-capsule can be made thin, mechanically compliant and small under a biocompatible protective sealant and excreted via the GI track. This paper presents an RF capsule tagging device for medication compliance monitoring. Section II provides an analysis of the radiation characteristics of a small prototype antenna inside a human-body model to understand the electrical properties of human tissue and frequency dependent absorption of electromagnetic waves. A prototype RF tagging IC fabricated in 130nm CMOS is described in Section III. The IC was assembled in a PTFE prototype capsule and experimentally validated in phantom solution to mimic the electrical properties of the human torso. Experimental results are presented in section IV, followed by concluding remarks in Section V.

II. RADIATION FROM AN INGESTED CAPSULE ANTENNA

Signal propagation inside the human body has been studied extensively [3]. In order to capture the interactions of radiating elements inside the body, we have evaluated the field distributions of an electrically small antenna in various FCC regulated frequency bands using the finite difference time domain (FDTD) method and a complete electrical model of an average American male with 23 different tissue types [4]. Simulations were carried out to calibrate against theoretical benchmarks of well

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characterized loop and dipole antennas, and to determine the appropriate setup (i.e. mesh size, cell size, time steps, radiation boundaries, padding cells and source type) for accurate and rapid convergence.

The small zigzag antenna shown in Fig. 1 has a measured return loss of -14.4dB at 5.78GHz and was used in the FDTD model to determine the radiation characteristics for a range of FCC regulated frequency bands spanning 400-2500MHz. The antenna was placed in the stomach longitudinally (from left to right) slightly off the body axis. For each simulation, the antenna was first designed to resonate at the desired frequencies by forcing a single cell sized gap in the antenna port and inserting a complex source impedance of the appropriate value, which was found via Gaussian source excitation simulations. Near field contour plots in the FCC approved 402 to 405MHz band for Medical Implant Communications Service (MICS), 608-614 MHz band for Wireless Medical Telemetry Service (WMTS), and the 902-928MHz and 2.4-2.483GHz bands for worldwide Industrial-Scientific-Medical (ISM) are shown in Fig. 2. Our simulation results show that the radiated field intensity of the capsule antenna, when normalized to 0 dBm input power, is strongest in the anterior location

slightly to the left of the stomach for the 915MHz ISM band. It is interesting to note that despite higher absorption of electromagnetic energy at frequencies above 1GHz due to water content, the radiation field intensity at the 2.4GHz ISM band is comparable to that of the 915MHz ISM band; this is largely due to improved radiation efficiency of the zig-zag dipole antenna at higher frequencies.

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(a) (b) (c) (d) Fig. 2. Normalized contours of radiated fields from a capsule antenna inside a human body torso (cross-sections shown) operating at (a) MICS 402-405MHz, (b) WMTS 608-614MHz, (c) ISM 902-928MHz and (d) ISM 2.4-2.483GHz FCC frequency bands.

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Fig. 3. RF IC capsule transponder architecture.

III. RFIC CAPSULE TRANSPONDER

A. Electronic-burst (E-burst) System Architecture Fig. 3 shows a block diagram of the proposed RF tagging E-burst device. The transponder is powered by applying a 13.56MHz signal to a pair of pads in direct contact with the communication medium. The human body can be considered as having electrical pathways modeled as a network comprised of resistors and capacitors, where cells have an electrical analog to capacitors, and the aqueous space around the cells are modeled as resistors [5]. Thus, low frequency pads on the transponder pill are used to capture transmitted power through this impedance network and energize the microchip; this link also provides downlink command functions. A second antenna designed for UHF frequency operation uplinks to an external detector. An input RF multiplier comprised of ten differential RF-DC converter stages powers the chip and supplies current to a storage capacitor CS until the supply voltage exceeds a programmed value set by the power level detector. The power detector is used to monitor the storage capacitor voltage and signals the device ready for transmission. An output transmitter generates short bursts at approximately 915MHz when both the power level and correct downlink data has been received. The frequency of the RF burst is controlled by a 6-bit digital word programmed by the user via the downlink data stream. A 3-bit cyclic-redundancy-check (CRC) code is used for

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Fig. 5. Modulator schematic.

PCBTag

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Fig. 6. Prototype RF capsule tagging (a) packaging and assembly, and (b) die photo.

data integrity and the TX burst is generated only after the CRC has been verified.

B. Demodulator An envelope detector and a clock-data recovery module are used to demodulate data packets comprised of 20 bits and 1.315ms in duration (Fig. 4). A pulse width modulation (PWM) scheme is used to encode data with gaps between pulses of approximately 3.25µs. A 3-stage RF-DC converter is used to extract the envelope which is compared against a low-pass filtered reference to generate the recovered pulses. These pulses are subsequently converted to PWM data using a toggle register. Since the pulses occur at approximately 30% or 70% of the bit-time duration, the full-rate non-return-to-zero (NRZ) clock edge information is readily available. The NRZ data stream is recovered by integrating the PWM waveform with a charge pump circuit, and sampling the analog waveform with a latched comparator.

C. Modulator Fig. 5 shows a schematic of the data modulator. It is comprised of a voltage controlled oscillator (VCO) followed by a buffer stage to the antenna pads. The VCO is implemented with cross-coupled nMOS pair to operate under small voltage headroom. Six bits of digital control are used to tune the oscillation frequency from approximately 790MHz to 938MHz. The modulator and buffer stage provides roughly -25dBm of output power (to 50 Ohm load) when activated. A selectable pMOS switch is used to enable the modulator and generate RF bursts when the voltage on the storage capacitor is larger than 1.5V and the downlink data packet is correctly received.

IV. EXPERIMENTAL RESULTS

A. Packaging and Assembly In order to experimentally verify the E-burst transponder a prototype capsule enclosure was fabricated using a cylindrical PTFE substrate, as shown in Fig. 6a. The IC was first mounted and wirebonded onto a custom made 2-layer printed circuit board. The assembled board was then flip-attached onto the PTFE capsule with the IC facing down into the cavity to provide a nearly cylindrical outer finish. The antenna was attached to backside connections exposed on the PCB to access and electrically connect to the IC ports. A 915MHz antenna along with the low

frequency pads were directly printed onto a flexible substrate and wrapped around the cylinder for the initial prototype. Fig. 6b shows the transponder die photo fabricated in 130nm CMOS process. The IC measures 800x1500 µm2.

B. Human Phantom Preparation Phantoms are solutions that exhibit electrical properties that are approximately equivalent to biological tissue and are used herein to evaluate the E-burst device performance. Phantom preparations vary in complexity and accuracy and the simplest formulation used in our experiments employs sodium chloride (NaCl) and distilled water; however, improved gel based phantoms consisting of polysaccharide gel, NaCl, aluminum powder and Sucrose can also be used. In the present experiments, the loading of the phantoms was designed to approximate that of an adult human back to model the torso. The phantom solution was placed in a Rubbermaid container measuring 40x40x25cm3. The relative concentrations of NaCl in roughly 4 liters of distilled water were varied until the loading of an external surface coil in proximity to an adult

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Fig. 7. Experimental setup of the E-burst RF tagging IC measured inside a phantom solution.

human back was approximately equal to the loading generated by the sample solution.

Table I. Summary of tagging performance Technology UMC 130nm CMOS

Down/Up link frequency 13.56MHz/915MHz Down link data rate 15kb/s

Burst power -25dBm (50Ω) Burst Duration 400ns

VCO tuning range 790MHz-938MHz 13.56 MHz 402 MHz RF input power 1 2.56dBm -3.5dBm

Chip area 1500µm x 800µm 1On-chip probed measurements (w/o input matching).

C. Measurement Results The experimental setup and measured results are shown in Fig. 7. A low frequency RF source is used to activate the E-burst chip inside the phantom solution. Low frequency transmit pads were inserted directly into the phantom solution. The modulated AC signal is verified using a second set of low frequency pads connected to two independent channels of an oscilloscope. While the voltages generated in each probe, labeled In+ and In-, are nearly in phase, a sufficiently large differential voltage (In+ - In-) is induced across the probes to power the tag. As shown in Fig. 7, RF bursts generated by the prototype E-burst chip from inside of the phantom solution were captured by a vector signal analyzer with a built-in demodulation function (Agilent N9020A). The receiving antenna was placed outside of the phantom container, and the prototype device was immersed in the center of the phantom solution (~20cm from the sidewalls). A periodic downlink data packet activates the transponder at 1.315ms intervals, generating RF bursts of approximately 400ns in duration and a measured signal level of -76dBV (at the receiving antenna).

V. CONCLUSION

An E-burst RF capsule tagging system is proposed for medication capsule monitoring. The RF tag utilizes an asymmetric powering and communication scheme to circumvent problems associated with signal attenuation inside the human body and poor radiation efficiency of electrically small antennas. Since the power levels required to activate a tag are orders of magnitude larger than what is detectable externally, we employ an asymmetric RF link to energize the transponder at low

frequency and transmit RF bursts at higher frequencies. The proof-of-concept device was fabricated in 130nm CMOS, operates in the 13.56MHz/915MHz ISM bands and was experimentally validated in phantom solution.

ACKNOWLEDGEMENT

This work is supported in part by the NSF CAREER (0547057) award. The authors thank UMC for chip fabrication support.

REFERENCES [1] K.C. Farmer, “Methods for measuring and

monitoring medication regimen adherence in clinical trials and clinical practice,” Clin. Ther., v.21, no. 6, pp. 1074-90, 1999.

[2] H. Yu, G. Irby, D. Peterson, M. Nguyen, G. Flores, N. Euliano, R. Bashirullah, “Printed capsule antenna for medication compliance monitoring,” IEE Elec. Lett. v.43, no. 22, pp. 1179-1181, Oct. 2007.

[3] C. Gabriel, S. Gabriel, E. Corthout, "The dielectric properties of biological tissues: I. Literature survey", Phys. Med. Biol. v.41, pp. 2231-2249, 1996.

[4] www.remcom.com [5] R. Liedtke, Fundamental of Bioelectrical Impedance

Analysis, RJL Systems, www.rjlsystems.com, 1998.

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