ISSCC 2014 / SESSION 30 / TECHNOLOGIES FOR NEXT-GENERATION SYSTEMS / 30.3
30.3

Organic-Transistor-Based 2kV ESD-Tolerant Flexible
Wet Sensor Sheet for Biomedical Applications withWireless Power and Data Transmission Using
13.56MHz Magnetic Resonance

TXdata, RC oscillator 3, and the reference RC oscillator 1. In this measurement,
the reference RC oscillator 1 is 10Hz, RC oscillator 2 (= Sensor 1) is notoscillating, which corresponds to “Dry” (= no urination), and RC oscillator 3(= Sensor 2) is 5Hz, which corresponds to “Wet” (= urination). Figure 30.3.3(d)shows a successful toggle operation of the multiplexer.

Hiroshi Fuketa1,2, Kazuaki Yoshioka1,2, Tomoyuki Yokota1,2,Wakako Yukita1,2, Mari Koizumi1,2, Masaki Sekino1,2, Tsuyoshi Sekitani1,2,
Makoto Takamiya1,2, Takao Someya1,2, Takayasu Sakurai1,2

Figure 30.3.4(a) shows a circuit schematic of the RC oscillator with the pseudoCMOS inverters [3]. The RC oscillator converts the resistance between
electrodes (RMEA) to the frequency of OSC. Fig. 30.3.4 (b) shows the measured
waveform of OSC. Figure 30.3.4(c) shows the measured dependence of theoscillation period on RMEA. In this work, the target range of RMEA is from 2MΩ to10MΩ because RMEA in cotton immersed in normal saline is several megaohms.
The oscillation period linearly depends on RMEA with an offset. The sensitivity of
the resistance sensor is 3.5%/MΩ, though such sensitivity is not required in theFWSS which detects the presence or absence of liquid.

University of Tokyo, Tokyo, Japan, 2JST/ERATO, Tokyo, Japan

1A wet sensor, which detects the presence or absence of liquid, is an important
tool for biomedical, nursing-care, and elderly-care applications such as thedetection of blood in bandages, sweat in underwear, and urination in diapers. Awet sensor should be a thin, mechanically flexible, large-area, and low-cost
device with wireless power and data transmission, because constant monitoringwith a rigid and wired wet sensor placed on human skin is annoying. Moreover,
the wet sensor should be disposable from a hygiene perspective. In order tomeet these requirements, an organic transistor based flexible wet sensor sheet(FWSS) with wireless power and data transmission using 13.56MHz magnetic
resonance is developed to detect urination in diapers.
Figure 30.3.1 shows a photograph of the developed 78mm x 53mm FWSS. In its
actual implementation, the FWSS is embedded in the cotton of a diaper, although
FWSS is placed on the surface of the diaper in Fig. 30.3.1 for clarity. The FWSS
is a passive transponder that is wirelessly powered by a reader and sends sensor
data to the reader. In the FWSS, organic circuits fabricated on a 12.5μm-thickflexible polyimide film are stacked on 40mm square coil on 12.5μm-thick flexible
PCB.
Figure 30.3.2 (a) shows a circuit schematic of the FWSS and reader. The reader
is expected to be attached to the pants near the diaper. The reader wirelesslytransmits power between the coils (L1 and L2) via magnetic resonance at
13.56MHz. In this work, instead of conventional electromagnetic induction [1,2],
magnetic resonance is used to increase the distance between the reader andFWSS. The typical power supply voltage (VDD) of the FWSS is 2V. In the FWSS,
except for L2 shown in Fig. 30.3.1, all the circuits including capacitors areintegrated in the organic transistor film. In the FWSS, the resistance between
two electrodes is measured to detect the presence or absence of liquid. Actually,
the resistance is converted to the frequency of TXdata by an RC oscillator (Fig.
30.3.4) in the wet sensor. Then, TXdata performs the load modulation in thetransponder and the demodulated signal is RXdata. Fig. 30.3.2 (b) shows themeasured waveforms of V2 and TXdata in the FWSS and RXdata in the reader.
The 10Hz frequency of TXdata is successfully demodulated in the reader.
The design challenges of the FWSS are as follows. 1) The power consumptionof the battery-operated reader is large because the reader must transmit a maximum power to deal with the loss of transmission efficiency due to the bendingof the flexible coil (L2) as well as change in the distance between L1 and L2. 2)
ESD protection is essential in the FWSS because the electrodes can directly
come in contact with wet human skin. ESD protection in organic transistors,
however, is difficult because they are fabricated on an insulating film. To solvethese problems, we propose the following two solutions. 1) Adaptive amplitudecontrol (AAC) decreases the power consumption of the reader by reducing theamplitude by up to 92%. 2) ESD protection circuits with organic Schottky diodes
using copper phthalocyanine (CuPc) are used to achieve 2kV ESD tolerance.
Both AAC and ESD protection are essential technologies in wireless and flexible
sensors for biomedical and elderly-care applications.
Figure 30.3.3(a) shows a circuit schematic of the wet sensor, including a fourinput multiplexer and three RC oscillators. The four inputs are toggled with apreamble for the data receiver in the reader. The reference RC oscillator is usedin the proposed AAC and the other two RC oscillators are used as the wetsensors. In AAC, as shown in Fig. 30.3.2 (a), the amplitude of the reader (V1) isadaptively controlled by the adaptive amplitude controller to maintain thefrequency of the reference oscillator at a target value, which regulates VDD of theFWSS. Figures 30.3.3(b) and (c) show a circuit schematic and a photograph ofthe four-input multiplexer based on a pseudo-CMOS [3], respectively. The
pseudo-CMOS is used to obtain a high gain in pMOS-only organic transistors.
To demonstrate the multiplexer, Fig. 30.3.3(d) shows measured waveforms of490• 2014 IEEE International Solid-State Circuits Conference

In Fig. 30.3.5, the change in the transmission efficiency due to the bending of theflexible coil (L2) as well as the changes in distance between L1 and L2 isdiscussed. Figure 30.3.5(a) shows the measurement setup used to investigate
the relationship between the input voltage (VIN) in the reader and the outputvoltage (VOUT) in the FWSS. The bending (B) of the flexible coil (L2) and thedistance (D) between L1 and L2 are varied. Figure 30.3.5(b) shows a photographof the coil L1 and the bent coil L2. Figure 30.3.5(c) shows the measured D
dependence of VIN/VOUT at B of 0mm and 17mm. D of 40mm is equal to thediameter of L1 and L2. The required VOUT for the FWSS is constant. Therefore, VIN
should be increased as D and B increase. In the conventional design, a constant
VIN (= 9.7 x VOUT) is used to guarantee the required VOUT in the worst case of
D=40mm and B=17mm. A constant VIN, however, is usually a waste of theenergy, because the required VIN is smaller than 9.7 x VOUT in most cases, as
shown in Figure 30.3.5(c). Therefore, AAC is proposed to track the minimum VIN,
which reduces the power consumption of the battery-operated reader. AAC
reduces VIN by up to 92%. To clarify the effect of B on VIN, Figure 30.3.5(d)shows the measured D dependence of VIN at B=17mm / VIN at B=0mm extracted
from Fig. 30.3.5(c). By changing B from 0mm to 17mm, VIN should be increased
by a factor of more than 1.5 when D is larger than 10mm. Therefore, AAC ismore effective in applications where the coil is bent.
Figure 30.3.6(a) shows a schematic of the circuit developed for ESD protection.
Instead of an organic pMOSFET, a vertical organic Schottky diode [4] is used as
the diode for ESD as well as a rectifier, because the Schottky diode has large
current drivability and a superior frequency characteristic compared with aMOSFET. Figures 30.3.6(b) and (c) show a cross section and a photograph ofthe organic Schottky diode using CuPc, respectively. The ESD tolerance isevaluated in accordance with the ESD standard IEC 61000-4-2 with a MiniZapESD simulator [5]. Figure 30.3.6(d) shows the ESD measurement steps and themeasured results. Without the ESD protection, the ESD tolerance is below 0.5kV.
In contrast, with the ESD protection, the ESD tolerance is above 2kV, which
successfully satisfies level 1 of IEC 61000-4-2. One of the issues of adding ESD
protection to the signal pad is bandwidth degradation. Figure 30.3.6(e) shows
the measured frequency dependence of the gain of the source follower with andwithout the ESD protection. By adding the ESD protection, the -3dB bandwidthis reduced from 500Hz to 300Hz, which is not a problem in the FWSS because
the oscillation frequency is less than 4Hz, as shown in Fig.30.3.4(c). Figure30.3.7 shows a micrograph of the organic full-wave rectifier and RC oscillator,
and a summary of key features.
References:[1] L. Yan, et al., “A 3.9mW 25-Electrode Reconfigured Thoracic Impedance/ECG
SoC with Body-Channel Transponder,” ISSCC Dig. of Tech. Papers, pp. 490-491,Feb. 2010.
[2] J. Yoo, et al., “A 5.2mW Self-Configured Wearable Body Sensor NetworkController and a 12μW 54.9% Efficiency Wirelessly Powered Sensor for
Continuous Health Monitoring System,” ISSCC Dig. of Tech. Papers, pp. 290291, Feb. 2009.
[3] K. Ishida, et al.,“Insole Pedometer With Piezoelectric Energy Harvester and2V Organic Digital and Analog Circuits,” ISSCC Dig. of Tech. Papers, pp. 308309, Feb. 2012.
[4] Y. Ai, et al., “14 MHz Organic Diodes Fabricated Using PhotolithographicProcesses,” Appl. Phys. Lett. 90, 262105 (2007).
[5] Thermo Scientific, MiniZap ESD tester, Accessed Sep. 2013,<https://static.thermoscientific.com/images/D20078~.pdf>.

978-1-4799-0920-9/14/$31.00 ©2014 IEEE

ISSCC 2014 / February 12, 2014 / 2:30 PM

Figure 30.3.1: Developed flexible wet sensor sheet (FWSS).

Figure 30.3.2: (a) Circuit schematic of FWSS and reader. (b) Measuredwaveforms of load modulation.

Figure 30.3.3: (a) Circuit schematic of wet sensor. (b) Circuit schematic,(c) photograph, and (d) measured waveforms of four-input multiplexer.

Figure 30.3.4: RC oscillator. (a) Circuit schematic. (b) Measured waveform.
(c) Measured dependence of oscillation period on RMEA.

Figure 30.3.5: Bending of flexible coil (L2). (a) Measurement setup.
(b) Photograph. (c) Measured D dependence of VIN/VOUT. (d) Measured D
dependence of VIN at B=17mm / VIN at B=0mm.

Figure 30.3.6: ESD protection. (a) Circuit schematic. (b) Cross section and(c) photograph of organic diode. (d) ESD measurement. (e) Bandwidthdegradation caused by ESD protection.

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Figure 30.3.7: Micrographs and key features.

• 2014 IEEE International Solid-State Circuits Conference

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