Science - USA (2021-07-16)

SCIENCE sciencemag.org 16 JULY 2021 • VOL 373 ISSUE 6552 279

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SCIENCE

to its normal activity. Proteins

seem to drive the piezoelectric

effect inside a human body.

The basic building blocks

of proteins are amino acids,

which have electric dipoles

derived from their polar side

groups. For example, g-glycine

and D-alanine are amino acid

materials that have a strong

piezoelectric response ( 7 ). The

piezoelectric effect in amino

acids is obtained by a change

in the molecule electric dipole

amplitude in response to either

an applied mechanical force

or an electric field. Artificially

made biocompatible piezoelec-

tric materials also exist, such as

polyvinylidene fluoride (PVDF)

and its copolymers ( 8 ).

Yang et al. make an important

contribution by developing a

new method for scaling flexible

piezoelectric glycine thin films.

These films are self-assembled

by evaporating a solvent from

a glycine-polyvinyl alcohol solu-

tion. The films are also biocom-

patible and degradable.

Biocompatible piezoelectric materials

can be fabricated and designed for mul-

tiple purposes inside a human body. These

include monitoring local dynamic pressure

changes ( 9 ), such as heartbeats, breathing,

blood flow, and intraocular and cranial

pressure, and forming physical movements

inside the human body, such as muscle

activities ( 10 ). Another purpose is promot-

ing the healing of injuries by induced local

electric fields, such as in the case of local

growth of neurons and accelerated repair of

injured bones ( 11 ).

Flexible polycrystalline piezoelectric thin

films are more suitable than bulk size crys-

tals for applications inside a human body

because of the dynamic nature and flexibility

of human organs. The thin films have a pro-

nounced piezoelectric response when grown

with a preferred polar crystallographic ori-

entation in-vertical to the film plane.

Using the piezoelectric effect to replace

human muscle functionality by applying

a mechanical force requires a source of

electric energy. The optimal energy source

would be the human body itself, which

provides a mechanical energy that can

be converted into an electrical energy by

the piezoelectric materials. These devices

are called piezoelectric energy harvesters,

which can be attached as flexible thin films

to local sites inside the human body.

The piezoelectric harvesters can gener-

ate enough electric energy to operate the

device even from tiny mechanical move-

ments at extremely low frequencies ( 12 ),

such as heartbeats, blood flow, contraction

and expansion of lungs, walking, and eye

blinking. The piezoelectric energy harvest-

ers can be extremely flexible, lightweight,

and positioned close to the piezoelectric

actuator within the human body. The elec-

tric energy supplied by the piezoelectric

harvester should be accumulated in a bio-

compatible electric capacitor that enables

a controlled release of electric energy on

demand to the piezoelectric actuator ( 13 ).

Biocompatible capacitors can be made of

piezoelectric thin films by using their di-

electric properties.

Implanted microchips, developed for

controlled drug release at local sites in-

side a human body ( 14 ), can, in principle,

be used in a piezoelectric device. The im-

planted microchip controls the entire op-

eration of the device, which includes the

sensor, actuator, electric capacitor, and en-

ergy harvester (see the figure). The opera-

tions order of the microchip begins with a

signal received from the piezoelectric sen-

sor that indicates the misfunctioning of a

certain muscle. Then, an electric pulse is

sent through the electric capacitor to the

piezoelectric actuator to apply a mechani-

cal force to fix the problem. Finally, a feed-

back electric signal is received from the

sensor that communicates the resulting ef-

fect. These operations should continue un-

til the problem is fixed. In this

way, the device operates au-

tonomously where immediate

intervention is needed without

any external interference or ex-

ternal power supply.

An external human body

communication to the im-

planted microchip can be

achieved by using a wireless

communication ( 15 ). The pur-

pose of such a communication

is to receive real-time data on

the implanted piezoelectric

device activity and transmit

operational commands to the

microchip. A wireless commu-

nication to a human body from

an external device requires a

substantial power source. It

suffers from poor transmis-

sion through biological tis-

sues. It also needs a relatively

large antenna, which limits the

minimal size of the implant-

able microchip and prevents

implementation in organs such

as brain, heart, and spinal cord

that could be damaged by the

radiation energy.

Extensive research activities are cur-

rently done on every aspect of biocompat-

ible piezoelectric sensing, actuating, and

energy harvesting. A major task would

be to integrate them into an autonomous

biocompatible device implanted on a local

site inside a human body that optimally

functions without any external interven-

tion, replacing the functionality of a local

muscle for the normal operation of a hu-

man organ. j

REFERENCES AND NOTES

1. F. Yang et al., Science 373 , 337 (2021).


2. M. T. Chorsi et al., Ad v. M a t e r. 31 , 1802084 (2018).


3. W. G. Cady, Piezoelectricity: Volume One: An Introduction
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4. E. Fukada, Ferroelectrics 60 , 285 (1984).


5. E. Fukada, H. Ueda, Jpn. J. Appl. Phys. 9 , 844 (1970).


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7. V. V. Lemanov, S. N. Popov, G. A. Pankova, Phys. Solid
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8. J. Jacob, N. More, K. Kalia, G. Kapusetti, Inflamm. Regen.
   38 , 2 (2018).


9. Y. Zhang, F. Zhang, D. Zhu, Mater. Horiz. 2 , 133 (2015).


10. K. Kapat, Q. T. H. Shubhra, M. Zhou, S. Leeuwenburgh,
    Adv. Funct. Mater. 30 , 1909045 (2020).


11. A. H. Rajabi, M. Jaffe, T. L. Arinzeh, Acta Biomater. 24 , 12
    (2015).


12. M. T. Todaro et al., IEEE Trans. NanoTechnol. 17 , 220
    (2018).


13. H. Li et al., Adv. Sci. 6 , 1801625 (2019).


14. A. E. M. Eltorai, H. Fox, E. McGurrin, S. Guang, BioMed
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15. B. D. Nelson, S. S. Karipott, Y. Wang, K. G. Ong, Sensors
    20, 4604 (2020).

10.1126/science.abj0424

Implanted

device on

muscle

Testing the implant

Piezoelectric thin films

implanted on mouse

muscles allow for testing

for biocompatibility and

operation, ultimately

targeted for use in humans.

Automatic control

Piezoelectric films that actuate, sense, and harvest energy combined with a

biocompatible microchip and capacitor create a self-contained system that works

automatically without an external interface.

Piezoelectric

thin film

Microchip

Piezoelectric

actuator

Dielectric

capacitors

Piezoelectric

sensor

Piezoelectric

energy harvester

Biocompatible piezoelectrics

Piezoelectric implants require a sensor to identify a problem in a muscle, apply

corrections with actuation, monitor the change, and signal back to a microchip that

controls this process. Ideally, the microchip is also implanted, and the energy for the

processing is harvested from the local physical movements as well as vibrations.

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