Using Piezoelectric Elements to Convert Bio-Mechanical Pulsations into

Electrical Energy for Energy Harvesting

ISAO NAKANISHI, HIROYUKI NAKAMURA, MASAYA JYOUKI, YUUMA HATAMOTO

Tottori University,

4-101 Koyama-minami, Tottori-shi,680-8552,

JAPAN

Abstract: On the Internet of Things (IoT) where all electronic devices are connected to the internet, a secure power

supply is necessary. Energy harvesting technology is attracting attention as ”enabling technology” that expands

the use and opportunities of IoT utilization. This technology harvests energy that dissipates around us, in the

form of electromagnetic waves, heat, vibration, etc. and converts it into easy-to-use electric energy. Alternating

current signals can be extracted from bio-mechanical pulsations using skin-attached piezoelectric elements with a

protrusion, as we reported previously. However, we found that the maximum voltage of the extracted signal was

unstable and low. This study demonstrates that piezoelectric elements can be stacked to stabilize and increase

the maximum voltage, and that the extracted signal can be used to charge a capacitor, after rectification using a

voltage-doubler rectifier circuit with Schottky barrier diodes.

Key-Words: Energy Harvesting, Piezoelectric Element, Pulsation, Protrusion, Schottky Barrier Diode, Voltage

Doubler Rectifier Circuit.

Received: September 14, 2023. Revised: August 12, 2024. Accepted: September 11, 2024. Published: October 9, 2024.

1 Introduction

In recent years, smartphones, tablet terminals, and

even sensors and home appliances such as televisions

and audio equipment have become connected to the

Internet. We are now living in an Internet of Things

(IoT) world in which many kinds of electronic devices

are connected through the Internet.

However, connecting all these electronic devices

to the internet requires a corresponding power

supply. Primary batteries cause disposal-related

environmental problems. Energy harvesting is an

alternative that collects and utilizes the unused

energy in our surroundings, [1]. Photovoltaic-power

generation is a well-known energy-harvesting

technology. Floor power generation is also a form

of energy harvesting. Piezoelectric elements are

embedded in the floor, and electricity is generated

when people walk on it. There are other methods that

use heat, vibration, moving, radio waves, pressure

differences, bioderived materials, etc.

This study focuses on energy harvesting using

bio-mechanical signals, [2], [3], [4], [5]. In

particular, various attempts have been made to

convert human body fluctuations into electrical

energy using piezoelectric elements, [6], [7], [8],

[9]. However, fluctuations based on human actions

generate electricity only when an action is performed.

On the other hand, the life-long heartbeat could

provide a semipermanent energy source; however,

it requires surgery. If the heart’s motion would be

sensed outside the body, a semi-permanent energy

source could be obtained.

One possible way is to use piezoelectric elements

to convert the skin fluctuations caused by the

pulsating cardiovascular system to alternating current

(AC) signals, which are then rectified, and used to

charge a capacitor. Research from this perspective

has largely focused on device manufacturing, [10].

Its effectiveness has only been verified using artificial

pressure and not in the human body.

In our previous study, [11], a piezoelectric element

was attached to the human body, and it was confirmed

that the pulsations could be converted to AC signals.

However, the peak voltage was approximately 200

mV. To charge the capacitor, it is necessary to rectify

the AC signal using a diode-based rectifier circuit,

with a diode threshold voltage of approximately 600

mV; therefore, a higher maximum voltage is required.

We attempt to increase the output voltage by

stacking the piezoelectric elements. Furthermore,

by introducing the lower threshold-voltage diode and

rectifier circuit, the capacitor can be charged.

2 Pulsation-Energy Extraction Using

Piezoelectric Elements

The purpose of this study is to convert bio-mechanical

pulsation energy into electrical energy using

piezoelectric elements. The piezoelectric element is

placed in close contact with the skin; the mechanical

pressure is converted into an AC signal rectified into

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Fig1: Piezoelectric element used

Fig2: Piezoelectric element taped to the wrist

Fig3: Relationship between the piezoelectric

element and the skin

a positive-only signal by a rectifier circuit, and the

rectified signal charges a capacitor. Because this

technology generates electrical energy, no additional

power source is required during the entire process.

2.1 Previous Work, [11]

A piezoelectric element is a passive device based

on the piezoelectric effect which generates a voltage

when pressure is applied. The piezoelectric elements

used in this study are shown in Fig. 1 (K2512BS1 of

the THRIVE KINEZ series).

The piezoelectric component and electrode are

made of flexible materials and can be placed close

to the skin. The piezoelectric element outputs an

electric voltage proportional to the change in applied

pressure; therefore, no voltage is generated under

constant pressure. Because the piezoelectric element

in Fig. 1 had no pre-attached lead wires, they were

attached by soldering, and the entire device was

covered with an insulation tape. The device was

Fig4: Protrusion used

Fig5: The protrusion taped to the wrist, [11]

Fig6: The protrusion and the piezoelectric

element on the skin viewed from the side

then fixed to the wrist with medical tape (hereinafter

referred to as ”tape”), at the point where the radial

artery passes close to the skin. Fig. 2 shows the

location, and Fig. 3 shows the relationship between

the element and the skin.

The output signal from the piezoelectric element

was observed using an oscilloscope. Consequently,

an output signal, corresponding to the pulsations,

was observed; however, the maximum voltage was

approximately 40 mV. The reason for this was

believed to be that the pulsations did not reach

the piezoelectric element sufficiently. Therefore, a

protrusion (Fig. 4) was inserted between the skin and

piezoelectric element.

This is similar to pressing the wrist with a fingertip

when examining a pulse. We used a disposable

electrode for cardiac electrocardiography with the

adhesive pad removed, leaving only the metallic part.

If the part of the protrusion that touches the skin is

sharp, it is painful; therefore, a rounded electrode was

used. (This is not mandatory; alternates can be used.)

Fig. 5 illustrates the placement and fixing (by

tape) of the convex portion of the protrusion against

the skin. They were then taped over the top. Fig. 6

shows the side view.

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Fig7: An example of the extracted waveform

obtained by inserting a protrusion (20 mV/div,

ms/div), [11]

Fig8: Maximum voltage (mV) in 10

measurements

The waveform shown in Fig. 7 was obtained when

the output signal was observed using an oscilloscope.

The period of this signal was approximately 0.7 s,

which is almost the same as that of a heartbeat;

therefore, the obtained signal was judged to be from

pulsation. The maximum voltage of the extracted

signal increases to approximately 80 mV.

2.2 Stabilization of Output Signal

The measurements were repeated 10 times. The tape

used to fix the protrusion and piezoelectric element

was changed for each measurement. The maximum

values of the obtained voltage signals, and their

average value, are shown in Fig. 8. Although the

pulsations could be transduced, there was a large

variation in the measurements, as shown in the figure,

and a stable output could not be obtained.

The cause of this variation was attributed to the

difficulty in maintaining a constant force while fixing

the tape. Fixing the protrusion and piezoelectric

elements, separately, increased the variation.

Therefore, we first fixed the piezoelectric element

and protrusion with insulating tape, as shown in Fig.

9, and then fixed them to the wrist with a single piece

Fig9: Piezoelectric element with protrusion fixed

with insulation tape

Fig10: Maximum voltage (mV) in 10

measurements after devising

of tape. In addition, because the force of the fixation

varies with the length of the tape, the tape length was

standardized to 20 cm and the wrist angle was fixed.

Fig. 10 shows that the output measurement results

were more uniform than those in Fig. 8.

3 Attempts to Increase Output

The piezoelectric element converts the

bio-mechanical pulsation pressure signal into a

voltage signal; which must pass through a diode

rectifier circuit. However, the diodes have a

threshold voltage (approximately 600 mV for silicon

devices), even in the forward direction, and no

output is generated unless the voltage exceeds this

threshold. As discussed in the previous section, the

output-signal voltage was approximately 100 mV

and could not be rectified at this level.

However, piezoelectric elements with higher

outputs were not available to us; therefore, we

attempted to increase the output-signal level using

multiple piezoelectric elements. The wrist area

where the pulsation can be sensed is small, and

even if multiple piezoelectric elements were placed

side by side, none of them would detect the

pulsation. Therefore, multiple piezoelectric elements

were stacked on top of each other.

3.1 Two3iezoelectric(lements6tacked

As shown in Fig. 11, the two piezoelectric elements

with protrusions were placed directly above each

other. The piezoelectric element closest to the skin

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Fig11: Two piezoelectric elements on the skin

viewed from the side

Fig12: Maximum voltage (mV) of two

piezoelectric elements

Fig13: Force applied to one piezoelectric element

was labeled 1, and the element on top of it was 2. The

measurements were performed five times. The results

are shown in Fig. 12. The piezoelectric elements

were able to output average maximum voltage of

56 mV and 101 mV for piezoelectric elements 1

and 2, respectively. The average maximum voltage

of Piezoelectric Element 2 was equivalent to that

of a single piezoelectric element, whereas that of

Piezoelectric Element 1 was approximately half of

that value.

The reasons for this are discussed below: Fig. 13,

Fig. 14, and Fig. 15 show the force applied to a

single piezoelectric element, and the forces applied

to Piezoelectric Element 2 and 1, when stacked. In

these figures, the tape is omitted. In the case of

Piezoelectric Element 2 in Fig. 14, the force pressing

down on the skin with the tape from above and

the pulsation force from below through Piezoelectric

Element 1 are the same as those in the case of the

Fig14: Force applied to piezoelectric element 2

when two piezoelectric elements are stacked.

Fig5: Force applied to piezoelectric element 1

when two piezoelectric elements are stacked

Fig16: Illustration of why the output voltage of

Piezoelectric Element 1 reduces

single piezoelectric element in Fig. 13. This suggests

that the average output value of Piezoelectric Element

2 is equivalent to that of one piezoelectric element.

However, in the case of Piezoelectric Element 1 in

Fig. 15, the force of the pulsation pushing up the

skin is the same as that of Piezoelectric Element 2,

but the force to be fixed by the tape is concentrated

at one point through the protrusion of Piezoelectric

Element 2; thus, a larger force is applied from the

top than in the case of one piezoelectric element. The

force applied by the tape is constant, and as mentioned

earlier, the piezoelectric element only reduces the

response range of the piezoelectric element to the

pressure and does not generate voltage at constant

pressures.

This is illustrated in Fig. 16. The

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Fig17: Arrangement of three piezoelectric

elements

pressure-response capability of both piezoelectric

elements was finite and identical, as indicated by

the bold boxes in the figure. The constant force

required to fix them is indicated by the colored area

in the figure. The force required to fix Piezoelectric

Element 1 is larger than that required to Piezoelectric

Element 2, as shown in (b). This limited the range

that detected the pulsation and the output voltage was

low.

3.2 Three3iezoelectric(lements6tacked

As shown in the previous section, when two

piezoelectric elements are stacked, and the

protrusions are placed such that their positions

overlap, the output voltage of one of the piezoelectric

elements decreases. Therefore, we shift the positions

of the protrusions.

Although the force applied to the lower

piezoelectric element should strictly be the same

even if the positions of the protrusions are shifted,

the piezoelectric element and electrode are made

of a flexible material; therefore, the constant force

applied to Piezoelectric Element 1 (by the fixing tape)

is distributed and weaker than when it is sandwiched

between the upper and lower protrusions. In addition,

even if the output values of the two piezoelectric

elements are equal, their combined value does not

exceed the threshold voltage of the diode.

Therefore, the number of piezoelectric elements

is increased to three. Each of them are fixed

individually with tape. When we tried to fix them

with a single piece of tape, the pulsation did not

evenly reach the three piezoelectric elements because

the stacked piezoelectric elements tilted.

Fig. 17 shows the arrangement of the piezoelectric

elements. Piezoelectric Element 1 was fixed with

tape, and Piezoelectric Elements 2 and 3 were

then taped to both ends of Piezoelectric Element 1,

avoiding the protrusion at the center of Piezoelectric

Element 1. A side view of their arrangement is shown

in Fig. 18.

Fig18: Fixing arrangement of three piezoelectric

elements

Fig19: Maximum voltage of three piezoelectric

elements

Fig20: Half-wave rectification circuit using a

Schottky barrier diode

The output of each of the three piezoelectric

elements was measured five times. The results are

presented in Fig. 19. The average maximum voltage

of the three piezoelectric elements were 117 mV, 120

mV, and 131 mV for Piezoelectric Element 1, 2, and 3

respectively. The maximum voltage of each of these

elements was almost equivalent to that of a single

piezoelectric element, and the total output exceeded

350 mV.

4 Rectifier Circuit and Capacitor

Energy Storage

Stacking the three piezoelectric elements and fixing

them with tape after shifting the protrusion positions,

an output signal exceeding 350 mV was obtained.

Then, we attempted to rectify the signal and charge

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Fig21: The waveform before rectification (100

mV/div, 200 ms/div)

Fig22: The waveform after rectification (100

mV/div, 200 ms/div)

the capacitor using the rectified signal. However, a

general-purpose silicon diode has a forward threshold

voltage of approximately 600 mV. The maximum

voltage obtained in the previous section does not

exceed that value; therefore, rectification is not

possible without modification.

Therefore, Schottky barrier diodes (1N5819),

which typically have a low threshold voltage of

150–450 mV, were selected. As shown in Fig. 20,

the rectification waveform was observed across a 1

MΩload resistor, using an oscilloscope.

The waveforms before and after the rectification

are shown in Fig. 21 and Fig. 22, respectively. It was

confirmed that the rectification was achieved by using

stacked three piezoelectric elements and a Schottky

barrier diode.

Next, we replaced the resistor with a capacitor and

verified whether the capacitor could be charged. The

circuit diagram is shown in Fig. 23. An electrolytic

capacitor with a 1 µF capacitance was used.

The capacitor was charged when a large pulsation

occurred but was discharged before the next large

pulsation occurred; as a result, electricity was not

stored in the capacitor. Increasing the capacitance to

10 µF gave the same results.

The next point of interest was the negative voltage

Fig.23: Half-wave rectification circuit terminating

in a single capacitor

Fig.24: Capacitor charging using a

voltage-doubler rectifier circuit

generated during the discharge period. For general

diodes, the reverse (leakage) current is extremely

small; however, it is not negligible for Schottky

barrier diodes. This was observed in Fig. 22.

Therefore, it was assumed that the capacitor charged

in the forward direction was discharged by the leakage

current in the reverse direction.

Therefore, we used a diode-bridge (full-wave)

rectifier circuit to eliminate discharge effect.

However, the rectified waveforms could not be

obtained because in the full-wave rectifier circuit,

rectification is performed by two diodes in series (in

both the forward and reverse directions) resulting in

double the diode threshold voltage. The maximum

voltage of the extracted signal did not exceed the

threshold.

Therefore, a voltage-doubler rectifier circuit was

used. The corresponding circuit diagram is shown

in Fig. 24. This circuit requires only one-diode

threshold voltage because it is rectified with a single

diode in the forward or reverse direction.

The results are shown in Fig. 25. It was confirmed

that the capacitor was gradually charged and the

voltage reached approximately 6 mV.

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Fig.25: The waveform of a voltage doubler

rectifier circuit (2 mV/div, 500 ms/div)

5 Conclusions

This study converted the mechanical energy of

pulsations into electrical energy using piezoelectric

elements. The AC voltage signals thus obtained using

commercially available piezoelectric elements were

rectified and used to charge a capacitor. The process

summary follows:

• A protrusion was inserted between the

piezoelectric element and the skin for effective

pulsation detection.

• The transducer output voltage was increased by

using three piezoelectric elements; they were

stacked by not aligning the position of the

protrusions, and each piezoelectric element was

individually fixed with tape so that the pulsation

could be applied evenly.

• Schottky barrier diodes with low threshold

voltages were used for rectification.

• A voltage-doubler rectifier circuit

was used to eliminate the

forward-threshold-voltage-doubling issue of

diode-bridge circuits.

• Finally, capacitor charging was accomplished.

In energy-harvesting, this is the first time that the

pulsations in the human body have been converted

into electrical energy using piezoelectric elements.

In the absence of piezoelectric elements that can be

directly used in pulsation–electricity transduction, the

piezoelectric-element-stacking method used in this

study could be effective. If a pulsation extraction

output that exceeds twice the threshold voltage of

a general-purpose diode can be obtained using a

single piezoelectric element, it can be easily rectified

by connecting a full-wave rectifier circuit to the

general-purpose diode.

In this study, it was confirmed that the capacitor

could be charged, but examining the stability of the

results obtained and supplying the charge to electronic

devices remain issues.

References:

[1] S. Priya, and D. J. Inman, Energy Harvesting

Technologies. Springer, USA, 2009.

[2] T. Starner, Human-powered wearable

computing, IBM Systems Journal, Vol. 35,

Nos. 3&4, 1996, pp.1-12.

[3] C. Dagdeviren, Z. Li, and Z. L. Wang, Energy

harvesting from the animal/human body for

self-powered electronics, Annual Review of

Biomedical Engineering, Vol. 19, 2017, pp.

85-108.

[4] C. G. Núñez, L. Manjakkal, and R. Dahiya,

Energy autonomous electronic skin, npj Flexible

Electronics, Vol. 3, No. 1, 2019, pp. 1-24.

[5] R. Hesham, A. Soltan, and A. Madian, Energy

harvesting schemes for wearable devices, Int. J.

Electron. Commun. (AEÜ), Vol. 138, 2021, pp.

1-11.

[6] J. L. González, A. Rubio and F. Moll, Human

powered piezoelectric batteries to supply power

to wearable electronic devices, Int. J. Soc.

Mater. Eng. Resour., Vol. 10, No.1, 2002, pp.

34-40.

[7] Q. Zheng, B. Shi, Z. Li, and Z. L. Wang, Recent

progress on piezoelectric and triboelectric

energy harvesters in biomedical systems,

Advanced Science, Vol. 4, 2017, pp. 1-23.

[8] H. Liu, J. Zhong, C. Lee, S. Lee and L. Lin, A

comprehensive review on piezoelectric energy

harvesting technology: Materials, mechanisms,

and applications, Applied Physics Reviews, Vol.

5, 2018, pp. 1-35.

[9] N. Sezer and M. Koç, A comprehensive review

on the state-of-the-art of piezoelectric energy

harvesting, Nano Energy, Vol. 80, 2021, pp.

1-25.

[10] S. Yoon and Y. Cho, A Skin-attachable Flexible

Piezoelectric Pulse Wave Energy Harvester,

Journal of Physics, Conference Series, 557

012026, 2014, pp. 1-5.

[11] I. Nakanishi and H. Nakamura, Extraction of

a Pulse Wave Using a Piezoelectric Element

Toward Energy Harvesting, Proc. of 2022 IEEE

the 4th Global Conference on Life Sciences

and Technologies (LifeTech2022), 2022, pp. 616

-618.

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Contribution of Individual Authors to the

Creation of a Scientific Article (Ghostwriting

Policy)

Isao Nakanishi involved in all aspects of this study

except for the experiments and wrote this article.

Hiroyuki Nakamura produced the

devices and organized and executed the

experiments and considered their results.

Masaya Jyouki and Yuuma Hatamoto helped with

some of those.

Sources of Funding for Research Presented in a

Scientific Article or Scientific Article Itself

No funding was received for conducting this study.

Conflicts of Interest

The authors have no conflicts of interest to declare

that are relevant to the content of this article.

Creative Commons Attribution License 4.0

(Attribution 4.0 International , CC BY 4.0)

This article is published under the terms of the

Creative Commons Attribution License 4.0

https://creativecommons.org/licenses/by/4.0/deed.en

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