ISSCC 2015 / SESSION 16 / EMERGING TECHNOLOGIES ENABLING NEXT-GENERATION SYSTEMS / 16.4
16.4

Energy-Autonomous Fever Alarm Armband
Integrating Fully Flexible Solar Cells, Piezoelectric
Speaker, Temperature Detector, and 12V Organic
Complementary FET Circuits

Hiroshi Fuketa1,2, Masamune Hamamatsu1,2, Tomoyuki Yokota1,2,Wakako Yukita1,2, Teruki Someya1,2, Tsuyoshi Sekitani2,3,
Makoto Takamiya1,2, Takao Someya1,2, Takayasu Sakurai1,2
University of Tokyo, Tokyo, Japan,JST/ERATO, Tokyo, Japan,
3
Osaka University, Osaka, Japan
12

Three key requirements for wearable healthcare and biomedical devices are themechanical flexibility, the wireless interface, and the energy autonomy, because
unobtrusive and maintenance-free devices are needed for the constantmonitoring of vital human health data. Previously reported flexible healthcare
and biomedical devices, however, requires wired connection [1,2] or wirelesspower transmission [3,4]. A flexible energy autonomous healthcare device witha wireless interface, a fever alarm armband (FAA) integrating fully flexible solarcells, a piezoelectric speaker, a temperature detector, and 12V organic
complementary FET (CFET) circuits is presented here. The system is also
noteworthy for sound generation, with organic circuits driving the speaker.
Figure 16.4.1 shows a photograph of FAA with the size of 30x18cm2 and its use
case. All components in FAA are mechanically flexible. FAA is looped around anupper arm of a patient in a hospital room, and the temperature detector monitors
the underarm temperature of the patient. A 220μm thick amorphous silicon solarcells (PowerFilm P3-37) attached outside of the upper arm generate the power(e.g. 3mW at 1500lx). When high fever is detected, a 52μm thick piezoelectric
speaker with polyvinylidene difluoride (PVDF) makes a sound to alarm a nurse.
Organic CFET circuits and the temperature detector are fabricated on a 50μm
thick flexible polyimide film, and the solar cells and the speaker are attached onit.
Figure 16.4.2 shows a circuit schematic of FAA. An active voltage limiter (AVL)
regulates the power supply voltage (VDD) of the FAA, because VDD changes
depending on both the illuminance for the solar cells and the power
consumption of the FAA. When the measured temperature is higher than thepreset threshold temperature (Tt), the ring oscillator (RO) starts oscillation andthe speaker makes a sound. Two design challenges of the FAA are as follows. 1)
In the temperature detector, variable Tt by users is required, because the needed
Tt depends on the patient. 2) The voltage regulation of VDD is required, because
the output voltage change of the solar cells due to the variation of the indoor
lighting is large (e.g. 10V at 500lx and 20V at 3000lx) and the overvoltage breaks
down the organic transistors. In the organic electronics, however, the voltage
regulation is difficult, because a voltage reference and Zener diodes are notavailable. To solve these problems, the following two solutions are proposed inthis paper. 1) A voltage controlled threshold temperature tuning (VCTTT)
enables the tuning of Tt. 2) AVL is the world’s first voltage regulation in theorganic electronics.
Figures 16.4.3 (a) and (b) show a circuit schematic and the measured
temperature dependence of the temperature detector, respectively. When thetemperature is increased, the resistance of a resistive-type flexible thermal
sensor increases by more than 104 times at Tt, and EN changes from high to low.
In the proposed VCTTT, the gate voltage (VG) of p-type transistor operating in thesub-threshold region is varied to tune Tt. When VG is varied from 10V to 12V, Tt
changes from 36.5°C to 38.5°C, which indicates 1°C/V tunability. The control ofVG could be achieved by selectively cutting the wires of a resistor ladder, thougha VG generator is not implemented in the FAA.
In the FAA, a careful choice of the frequency of the sound is required, because
the frequency is determined by both RO and the speaker. Figure 16.4.4(a) shows
the measured VDD dependence of the RO frequency. The maximum frequency of
RO is 6kHz, because the maximum VDD (VMAX) is 15V due to the breakdown
voltage of the transistors. In contrast, the speaker does not make a sound below
1.8kHz. Therefore, the target frequency of the sound is selected to be 4.7kHz,
because the speaker itself has the maximum sound pressure level (SPL) at 5kHz
within a range of 1.8kHz to 6kHz. The corresponding target VDD for RO is 12V.
Figure 16.4.4 (a) also shows VDD dependence of SPL of the speaker at RO296• 2015 IEEE International Solid-State Circuits Conference

frequency measured with a sound level meter (RION NL-32) at 30cm distance.
The minimum VDD (VMIN) is defined as 9V at SPL of 10dB corresponding to thebackground noise. Figure 16.4.4(b) shows the measured waveform of the input
signal of the speaker (“Out” in Fig. 16.4.2) at 12V and 4.7kHz. The power
consumption of RO and the driver is 2.2mW. Figures 16.4.4 (c) and (d) show themeasured spectra of SPL without and with the speaker, respectively. Figure
16.4.4(c) shows background noise in the measurement room. As shown in Fig.
16.4.4(d), the speaker successfully makes a sound with SPL of 28dB at 4.7kHz
and the SNR is 18dB.
Figures 16.4.5(a) and (b) show circuit schematics of the proposed AVL and aconventional diode clamp for comparison, respectively. AVL is a kind of a shunt
regulator, and is composed of a constant voltage generator, two CFET inverters,
and a power transistor (M5). The constant voltage (V1) and a logical thresholdvoltage of the inverter (~VDD/2) are compared, and the output of the inverter (V2)is varied to regulate VDD. The essence of the constant voltage generator isstacked diode-connected transistor (M1) with small W/L and OFF transistor (M2)with large W/L. When the channel length modulation effect of the transistors isneglected, V3 is constant without a dependence on VDD, because M2 is a constantcurrent source with the OFF current and VDS (=VGS) of M1 is determined by theOFF current. To increase the constant voltage V3 to VDD/2 (=6V), M3 and M4 areadded, and V1=2xV3. Figure 16.4.5(c) shows the measured VDD dependence of V1and V2 in AVL. When VDD is within 7V to 15V, V1 stays around 6.5V andΔV1/ΔVDD=6% due to the channel length modulation. Figure 16.4.5(d) shows themeasured VDD dependence of ID of AVL and the conventional diode clamp. In Fig.
16.4.5(d), the maximum ΔID/ΔVDD of the diode clamp and AVL is 63μA/V and121μA/V, respectively, which indicates that AVL increases ΔID/ΔVDD by 1.9 times.
The impact of AVL on the operational illuminance range of FAA is shown. Figures
16.4.6(a), (b) and (c) show the measured illuminance dependence of VDD without
the voltage limiter, with the conventional diode clamp, and with the proposed
AVL, respectively. Two lines in each figure show with and without activating theload circuits (RO and the driver) in Fig. 16.4.2. The maximum and minimum
operational illuminance are determined by VMAX (=15V) and VMIN (=9V),
respectively. Without the voltage limiter in Fig. 16.4.6(a), the operational
illuminance range is 300lx. By adding the conventional diode clamp in Fig.
16.4.6(b), the range is 1900lx. In the proposed AVL in Fig. 16.4.6(c), the range
is 2200lx. AVL simultaneously achieves the minimum illuminance of Fig.
16.4.6(a) and the maximum illuminance of Fig. 16.4.6(b) thanks to the sharp ID
control shown in Fig. 16.4.5(d). Thus, AVL increases the operational illuminancerange from 300lx to 2200lx by 7.3 times.
Figure 16.4.7 shows micrographs of the organic circuits and a summary of theorganic CFET. The organic semiconductors for p-type and n-type transistors are
DNTT [4] and PDI-8CN2 [5], respectively. The minimum gate length is 10μm.
Acknowledgment:
The authors thank Dr. Zhouyu Ji of Institute of Microelectronics of Chinese
Academy of Sciences for valuable discussion.
References:[1] J. Viventi, et al., “Flexible, Foldable, Actively Multiplexed, High-Density
Electrode Array for Mapping Brain Activity in Vivo,” Nature Neurosciences, vol.
14, pp. 1599–1605, Dec. 2011.
[2] H. Fuketa, et al., “1μm-Thickness 64-Channel Surface Electromyogram
Measurement Sheet with 2V Organic Transistors for Prosthetic Hand Control,”
ISSCC Dig. of Tech. Papers, pp. 104-105, Feb. 2013.
[3] D.H. Kim, et al., “Epidermal Electronics,” Science, vol. 333, pp. 838-843,
Aug. 2011.
[4] H. Fuketa, et al., “Organic Transistor Based 2kV ESD Tolerant Flexible Wet
Sensor Sheet for Biomedical Applications with Wireless Power and Data
Transmission Using 13.56MHz Magnetic Resonance,” ISSCC Dig. of Tech.
Papers, pp. 490-491, Feb. 2014.
[5] L. Li, et al.,“High-Performance and Stable Organic Transistors and Circuitswith Patterned Polypyrrole Electrodes,” Advanced Materials, vol. 24, pp. 21592164, Apr. 2012.

978-1-4799-6224-2/15/$31.00 ©2015 IEEE

ISSCC 2015 / February 24, 2015 / 2:45 PM

Figure 16.4.1: Developed fever alarm armband.

Figure 16.4.2: Circuit schematic of fever alarm armband.

16
Figure 16.4.3: Temperature detector. (a) Circuit schematic. (b) Measured
temperature dependence of output voltage with proposed voltage controlled
threshold temperature tuning (VCTTT).

Figure 16.4.4: Circuits to make sound. (a) Measured VDD dependence of RO
frequency and sound pressure level. (b) Measured waveform of Out in Fig.
16.4.2. (c) & (d) Measured spectra of sound pressure level.

Figure 16.4.5: (a) Proposed active voltage limiter (AVL). (b) Conventional
diode clamp. (c) Measured VDD dependence of V1 and V2 in AVL. (d) Measured
VDD dependence of ID.

Figure 16.4.6: Measured illuminance dependence of VDD, (a) without voltage
limiter, (b) with conventional diode clamp, and (c) with proposed AVL.

DIGEST OF TECHNICAL PAPERS •297

ISSCC 2015 PAPER CONTINUATIONS

Figure 16.4.7: Micrographs of organic circuits and summary of organic CFET.

• 2015 IEEE International Solid-State Circuits Conference

978-1-4799-6224-2/15/$31.00 ©2015 IEEE