# Piezopolymers to transform motion into electrical energy for chemical-free disinfection

Chaoqi Wang<fnref n="1" />,<fnref n="2" /> · Weilong Kong<fnref n="3" /> Ⓓ · Ya Liu<fnref n="3" /> Ⓓ · Eric Lichtfouse<fnref n="3" /> Ⓓ · Jie Han<fnref n="1" />,<fnref n="2" /> Ⓓ

The ever-moving world has an endless supply of untapped motion energy. Like solar radiation, mechanical energy is very abundant in the natural and built environments. In nature, motions occur at various frequencies, for example, in tides, winds, noise, vibrations, even our breathing. Nonetheless, those natural forces are difficult to harness due to their low frequencies and often intermittent availability. Piezocatalysis is the application of an external force at appropriate magnitudes and frequencies to induce a piezopotential, which can create an electrical field and, in turn, fuel electrochemical reactions. While this phenomenon has been observed in many materials, including minerals and synthetic polymers, the scale-up is often limited by the need for periodic external forces. Scientists and engineers, therefore, should look beyond their laboratory gadgets and carefully observe the environment to design smart piezocatalytic microsystems, and then test them in the real world. Here, we present possible future applications in chemical-free disinfection and redox reactions, antifouling, wearable devices, ship hull plating, jewelry cleaning, dental care, athletic shoes, air conditioners, and more (Fig. 1, Table 1).

## Piezocatalysis transforms mechanical energy into electrical energy

Piezocatalysis refers to catalytic reactions induced by the piezoelectric effect, which converts mechanical energy into electrical energy to drive redox reactions. A common device using this effect is the piezoelectric gas lighter, where the user applies thumb pressure on a crystal to induce a high-voltage electric spark that ignites the gas stove. Mechanistically, piezoelectric materials deform under external forces, inducing structural changes in their crystal lattice that, in turn, generate non-zero dipole moments [1]. This polarization effect separates positive and negative charges on the sides of the material and establishes an internal polarized electric field [2]. The carriers produced, whether they are generated via the ‘energy band theory’ or the ‘screening charge effect theory,’ i.e., the two mainstream mechanistic explanations currently under debate, mediate the piezocatalytic reaction [3].

Compared to traditional photochemical, electrochemical, and photocatalytic reactions, piezocatalysis eliminates the need for light availability in photochemical systems or the need for electrical energy supply and conductivity in electrochemical systems. This makes piezocatalysis particularly useful where light access is difficult and electrical supply or conductivity is not possible, by enabling spontaneous utilization of mechanical energy to induce in situ redox reactions. In environmental chemistry, piezocatalysis has recently demonstrated promising applications in green synthesis, water treatment, hydrogen ($H_2$) production, and carbon dioxide ($CO_2$) utilization [4–10].

## Piezocatalytic polymers

Since the discovery of the piezoelectric effect by Pierre Curie and Jacques Curie in 1880 [11], scientists have discovered a wide array of solid-state piezoelectric materials such as quartz

<footnote>☒ Eric Lichtfouse
eric.lichtfouse@icloud.com</footnote>

<footnote>☒ Jie Han
jiehan@xjtu.edu.cn</footnote>

<footnote n="1">¹ Technology Innovation Center for Land Engineering and Human Settlements, Shaanxi Land Engineering Construction Group Co., Ltd. and Xi’an Jiaotong University, Xi’an Jiaotong University, Xi’an 712000, People’s Republic of China</footnote>

<footnote n="2">² School of Human Settlements and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China</footnote>

<footnote n="3">³ State Key Laboratory of Multiphase Flow in Power Engineering, International Research Center for Renewable Energy, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China</footnote>
**Fig. 1** Potential applications of piezocatalytic polymers that harness wasted mechanical energies to enable safe and chemical-free disinfection. Unlike ultraviolet disinfection or photocatalysis, piezocatalysis does not rely on light irradiation to activate and, therefore, can be used on rough surfaces, textured objects, and complex environments that are difficult for light to access. See Table 1 for explanations on these potential scenarios of use in real-life settings

Applications of piezocatalytic polymers

<figure><img src="image_1.png" /><figcaption>Medical care</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>Air conditioning</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Water purification</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Grab handles</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Athletic shoes</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Ship surfaces</figcaption></figure>

and ceramics [12]. Recent discoveries of piezoelectric materials include organic polymers, e.g., polytetrafluoroethylene, polyvinylidene fluoride, and *n*-nylons [13–15]. Piezopolymers have allowed the development of new applications because polymers are by nature flexible and amenable to processing. Polytetrafluoroethylene, polyvinylidene fluoride, and *n*-nylons are chemically inert, inexpensive, and manufactured on an industrial scale. They are widely used in consumer products and industrial settings, such as filter membranes, breathable waterproof fabrics, and ski top sheets [16–18]. Polyvinylidene fluoride and *n*-nylons are certified for use in food contact products [19, 20], and are amenable to additive manufacturing (3D printing), making it possible to make rapid prototypes to test them in various applications [21–23]. Notably, nylon 11 can be entirely made from renewable sources on an industrial scale [24]. The application of piezopolymers in environmental piezocatalysis, however, remains under-explored and is attracting growing research interest.

Composite piezocatalysts

Although inorganic piezocatalysts display good activities in general, in bulk-size form they often display limited processing potential due to both their low specific surface area and non-uniform mechanical stress transfer. This is challenging in applications demanding high processability. Conversely, while nanosized inorganic piezocatalysts show high activities with improved versatility in laboratory studies [25–29], the difficulties with catalyst recovery and potential secondary pollution due to their small sizes remain major obstacles for their adoption in real-world applications [30].

These drawbacks could be circumvented using either polymeric or polymer-based composite piezocatalysts. Indeed, the flexibility, strength, and durability of piezopolymers, such as polyvinylidene fluoride and nylon 11, allow for versatile designs that are tailored to particular scenarios [30–32]. Moreover, polymeric piezocatalysts can mix with inorganic piezocatalysts to form composite piezocatalysts that bear the advantages of both materials. Composite piezocatalysts have been tested in laboratories for degrading water pollutants and for producing hydrogen (H₂) by splitting water molecules [32–35]. However, to be
<table><caption>**Table 1** Potential applications of piezocatalytic polymers or composites for in situ chemical-free disinfection</caption><thead><tr><th></th><th>Scenarios of potential use</th><th>Items</th><th>Mechanical energy</th></tr></thead><tbody><tr><td>Public transport and logistics</td><td>Aircrafts</td><td>Washing basin, water faucet, flush button, door handle, armrest, cabin baggage lever, food trays, other high-touch surfaces; pressurized air vents in the cabin</td><td>Vibrations (persistent with engines operating)</td></tr><tr><td></td><td>Metro and buses</td><td>Handrails, grab handles</td><td>Vibrations (when operating)</td></tr><tr><td></td><td>Taxis and other for-rent passenger vehicles</td><td>Cabin filters, air vent grille; door handles, window handles / buttons</td><td>Vibrations and deformations (when operating)</td></tr><tr><td></td><td>Elevators and escalators</td><td>Buttons, handrails</td><td>Vibrations (when operating)</td></tr><tr><td></td><td>Trucks and shipping containers for transporting refrigerated or frozen foods (or other perishable items)</td><td>Interiors of shipping containers and truck cargo boxes; levers and handles</td><td>Vibrations</td></tr><tr><td>Enclosed public spaces</td><td>Schools, libraries, grocery stores, airports, health-care premises</td><td>Air filters in air conditioners, ventilation systems and air purifiers; air vent grilles</td><td>Vibrations and deformations (when operating)</td></tr><tr><td>Public amenities</td><td>Vending machines (refrigerated models)</td><td>Door handles, buttons</td><td>Vibrations (when compressors are running)</td></tr><tr><td></td><td>Ambulances</td><td>Interiors, high-touch surfaces</td><td>Vibrations (when operating)</td></tr><tr><td>Furniture, home appliances, and car accessories</td><td>Upholstery furniture with irremovable covering fabrics and cushioning</td><td>Mattresses, couches; automotive seating, car seats</td><td>Deformations, vibrations (automotive seating and cat seats, when operating)</td></tr><tr><td></td><td>Refrigerators and freezers</td><td>Interior walls, food trays, door handles, and buttons</td><td>Vibrations (when compressors are running)</td></tr><tr><td></td><td>Microfiber mops, robot mops</td><td>Microfiber pads on the mop head</td><td>Deformations (when operating)</td></tr><tr><td></td><td>Microwave ovens</td><td>Interiors, door handles, buttons</td><td>Vibrations (when operating)</td></tr><tr><td></td><td>Ultrasonic humidifiers</td><td>Water tank</td><td>Ultrasonication</td></tr><tr><td>Personal care</td><td>Toothbrushes, including manual, electrical, and sonic toothbrushes</td><td>Bristles (filaments)</td><td>Deformations (all), vibrations (electrical and sonic toothbrushes), and ultrasonication (sonic toothbrushes)</td></tr><tr><td></td><td>Band-aids, wound dressings</td><td>Fabrics, liquid-absorbing pads</td><td>Intermittent deformations</td></tr><tr><td></td><td>Splints and braces</td><td>Fabrics and plastics</td><td>Intermittent deformations</td></tr><tr><td></td><td>Dental braces</td><td>Custom-made plastic braces (3D printing is possible using n-nylons)</td><td>Intermittent cyclic deformations (when chewing), vibrations (when speaking)</td></tr><tr><td></td><td>Face masks and respirators</td><td>Non-woven layers</td><td>Cyclic deformations (when breathing)</td></tr><tr><td></td><td>Vibrators and massagers</td><td>Moving parts and handles</td><td>Vibrators (when operating)</td></tr><tr><td>Wearables</td><td>Athletic shoes</td><td>Insoles and fabrics</td><td>Cyclic deformations</td></tr><tr><td></td><td>In-ear headphones and arbuds</td><td>Rubber inserts, filter grills</td><td>Vibrations (from sound and music playing)</td></tr><tr><td>Music instruments</td><td>Guitars, violins, and orchestral instruments</td><td>Nylon strings and plastic picks (guitars), other parts prone to microbial growth</td><td>Vibrations (when in use)</td></tr></tbody></table>
adopted in real-world settings, it is necessary to extend
their use beyond laboratory piezocatalytic systems, which
are generally driven by ultrasound [34, 35]. In the follow-
ing sections, we propose real-life scenarios that can be
adapted as in situ piezocatalysis systems to harness the
mechanical energies that are otherwise wasted and dis-
sipated as heat.

Municipal water supply pipelines

Turbulent water flows inside pipelines are a reliable and
powerful energy source to drive piezocatalysis, given their
constant occurrence in the urban water infrastructure. For
instance, when the Reynolds number exceeds a certain
threshold, e.g., above 4000, the water flow transitions
from laminar to turbulent state, which is characterized by
irregular motions of the flow. This generates both viscous
shear stress, mainly in the near-wall region, and turbulent
shear stress, i.e., Reynolds stress, which is prominent at
core turbulent regions [36]. These forces can drive inter-
mittent or even continuous piezocatalytic reactions in the
pipelines.

Turbulent flows constantly generate shear stresses in the
pipelines due to their random and disordered movements.
They also occur at certain points such as elbows, valves,
and joints along the pipelines where substantial mechani-
cal energy is lost. These locations often experience pressure
surges caused by fluid inertia due to sudden changes in the
flow velocity. A common manifestation of these forces is
the ‘water hammer,’ which is a loud and sometimes recur-
ring noise heard by utility operators when valves are closed
quickly. The roughness of pipe interiors, especially in aged
pipelines, increases turbulence and elevates the magnitudes
of the shear stresses. In water mains, i.e., municipal water
supply pipelines, all these mechanical energy losses are col-
lectively referred to as ‘head losses,’ which require huge
amounts of energy and expensive pumping equipment to
overcome, e.g., to lift water to storage facilities or treatment
plants, often in a cascade manner. If this wasted energy could
be harnessed to drive piezocatalytic reactions, the charges
and reactive intermediates generated from these reactions
could reduce biofilm growth on pipe interiors, valves, joints,
and, at the same time, disinfect the flowing water.

Filtration membranes

Fouling of filtration membranes, particularly biofouling,
is a major challenge for utility operators [37]. Biofouling
involves microorganisms attaching to the membrane surface
via extracellular polymeric substances, forming biofilms that

reduce membrane flux and support further attachment, then
growth of microorganisms on the membrane surface [38].
Due to the strong adhesive properties of extracellular poly-
meric substances, biofilms are more resistant to conventional
chemical and physical cleaning than salts and organics accu-
mulated on the membrane surface. The formation of biofoul-
ing significantly increases maintenance costs for membrane
systems. This issue could be solved using piezopolymers.

Indeed, membrane deformation and turbulent shear
stresses could be used to drive piezopolymers. While
no study has reported the use of piezocatalytic effects to
achieve fouling resistance on industrial-grade membranes,
researchers have evidenced surface antifouling properties
of piezocatalytic ceramic membranes [39]. Here, pressure
pulses were converted into piezocatalytic effects using a
manganese(III) oxide (Mn₂O₃) / barium titanate (BaTiO₃)
membrane with pore sizes of 130–151 nm. This membrane
exhibited antifouling activity against various foulants,
including organic molecules, oil droplets, proteins, bacteria,
and inorganic colloids. Reactions were mediated by near-
surface reactive oxygen species and dielectrophoretic forces.

Ceramic membranes generally lack the flexibility, tensile
strength, and processing capabilities exhibited by polymeric
membranes. Consequently, the vast majority of membranes
employed in water treatment applications, including micro-
filtration, ultrafiltration, nanofiltration, and reverse osmo-
sis, are fabricated from synthetic polymers. For membrane
systems highly susceptible to biofouling, utilizing piezo-
polymers for membrane fabrication or incorporating piezo-
polymers into the membrane surface represents a potentially
viable solution. For instance, polyvinylidene fluoride and
*n*-nylons possess inherent elasticity and processability with
good chemical inertness, which enables their application in
a wide range of filtration systems. Indeed, membrane discs
and filters made with polyvinylidene fluoride and polyte-
trafluoroethylene are already widely utilized in laboratory
settings [40–45], while hollow-fiber polyvinylidene fluoride
membranes, better suited for industrial applications, have
also achieved industrial-scale production [16]. Nevertheless,
the piezocatalytic effects generated by these membranes dur-
ing operation and their potential implications for reducing
membrane fouling remain largely unexplored.

Stirring rods and blades

Mechanical stirring can effectively drive piezocatalytic reac-
tions [46]. When the rotational speed of the stirrer reaches
a critical threshold, complex flow patterns, including radial
and axial flows, develop within the liquid medium, gener-
ating vortices and jets. The resulting turbulence, produced
by the combined action of these flow patterns, applies
mechanical stress to the stirrer through shear stress and pressure fluctuations, thereby initiating the piezocatalytic reaction. This in situ piezocatalysis offers a substantial advantage for chemical and biological research because it eliminates the need for additional chemical reagents or energy inputs to drive catalytic reactions, while simultaneously ensuring thorough mixing of the reaction system and avoiding any contamination from external sources.

Researchers can also use turbulence-induced piezocatalysis to drive redox reactions at the interface between stirring rods or blades and the liquid medium. Indeed, laboratory magnetic stirrers and mechanical stirrer blades are coated with polytetrafluoroethylene, which exhibits piezocatalytic properties. However, due to the typically small surface area-to-volume ratio of conventional stirring rods or blades, the resulting piezocatalytic effect during stirring is often subtle and easily overlooked [47]. To harness the piezocatalytic effect during stirring, the design of stirring rods or blades can be modified. For instance, stirring rods or blades can be fully encapsulated with polytetrafluoroethylene or polyvinylidene fluoride, followed by surface treatments, such as roughening or pore formation to enhance reactivity. Furthermore, designing sets of stirring rods or blades in a Christmas-tree configuration would ensure effective mixing while simultaneously increasing the contact area between the piezoelectric material and the liquid. This kind of design also induces higher turbulence during stirring, thereby increasing piezocatalytic reaction rates.

## Aquariums and ships

The continuous influx of air into aquariums generates bubbles in the water, which, through their constant collapse during ascent, provide the mechanical energy to drive piezocatalytic reactions. This is a means to clean the water and inhibit algal growth without introducing chemical disinfectants, thereby ensuring safety for aquatic organisms. This chemical-free antifouling strategy may be extended to surface antifouling applications on ship hull plating. The frictional forces and deformations of the hull plating resulting from its relative motion to the surrounding water could theoretically drive piezocatalytic reactions, preventing the adhesion of organisms like barnacles, mollusks, algae, and microbes. This, in turn, would reduce hull drag and associated carbon emissions, and prevent the transregional spread of marine organisms. Compared to traditional copper-based antifouling coatings, which are routinely used on ship hulls [48], this approach potentially offers an environmentally benign alternative.

## Ultrasonic cleaner

Those who have experience with cleaning fine jewelry or eyeglasses would know this trick. Even the tiny corners and inner parts where dirt and grease accumulate can be cleaned after a few minutes of cleaning in an ultrasonic bath, without using detergents. With piezocatalysts, this could be brought to the level of deep cleaning by generating reactive oxygen species to break down the dirt and disinfect the microbes growing in the cavities and corners. Those who use ultrasonic cleaners in their daily routine for cleaning glassware in chemistry laboratories can also benefit from piezocatalysis [49]. The stainless-steel mesh, which is often used as a holder to suspend glassware in the water bath in the ultrasonic cleaner [50], can be replaced with a plastic mesh made by piezopolymers, e.g., polyvinylidene fluoride or polytetrafluoroethylene, to generate reactive oxygen species and achieve enhanced cleaning and disinfection.

## Dental care

A majority of the commercial best-selling manual toothbrushes sold in the US market have their bristles made by nylon 6/12 filaments, followed by polybutylene terephthalate [51, 52]. These polymers can be replaced with odd-numbered *n*-nylons to generate reactive oxygen species during toothbrushing for enhanced cleaning or tooth whitening. As with nylon 6/12 or polybutylene terephthalate bristles, the feeling of the bristles, namely soft, medium, or hard, can be altered by changing the thickness of the filaments to compensate for the change of the bristle material [51]. The same type of concept can be brought to dental floss, which is typically made of nylon or polyester filaments [53]. It is worth noting that even with the same material, piezocatalytic effects can be more pronounced with electrical toothbrushes or 'sonic' toothbrushes, which are designed for enhanced cleaning [54]. Most electrical toothbrushes have oscillating-rotating heads. Sonic toothbrushes, on the other hand, generate ultrasonic waves while in use. With piezocatalytic bristles, the high-frequency oscillation or sonication provides an ideal source of periodic stresses to the bristles and effectuates redox reactions.

## Insoles

Most athletic shoes use foamed plastic pads as insoles to absorb moisture and to enhance cushioning. These insoles also provide the 'hot beds' for the growth of bacteria and fungi, which generate the decaying smell over time. Insoles are often designed as detachable pads, so they can
be removed and washed after use. In reality, many wear-
ers ignore this step, which would result in the build-up of
the bad smell after repeated wearing. Among the wearable
garments, insoles are some of the common items that are
most prone to bacterial and fungal growth, so they provide
a venue for adapting piezocatalysis in real-life settings. The
periodic forces applied to insoles during walking or running
can drive piezocatalytic reactions on flexible piezopolymer
insoles.

Air filters and ducts

The prevalence of respiratory illnesses, particularly the
COVID-19 and influenza, has drawn increased attention to
air disinfection in enclosed public spaces. However, exist-
ing ventilation, air conditioning, and air purification systems
often lack air disinfection capabilities. While some air puri-
fiers employ ultraviolet irradiation or photocatalysis for air
disinfection, these technologies often entail complex system
designs and increased maintenance. Although air purifica-
tion systems utilizing high efficiency particulate air (HEPA)
filters can block fine particulates in air, this results in the
accumulation of pathogens and droplets on the filter media.
Certain microbes, such as mold and bacteria, along with
their spores, can proliferate on the filter media and be rein-
troduced into the indoor environment via the filtered air [55].

Airflows generated by air conditioners and air purifiers
typically maintain steady flow rates. However, internal fil-
tration components induce intense turbulence in the airflow
and cause vibrations in filters. In addition, airflows passing
through air ducts and outlet grilles also generate shear stress
and vibrations. Theoretically, various types of air filters,
including high efficiency particulate air (HEPA) filters in
ventilation systems, mesh filters in air conditioners, cabin
filters in passenger vehicles, and the outlet grilles and air
ducts, could be made by or coated with piezopolymers. This
would enable the capture of the vibrational energy generated
by air filtration and turbulent airflows to drive piezocatalytic
reactions, generating reactive oxygen species to degrade cap-
tured microbes and organic matter within the filters while
disinfecting the passing air.

Biomedical applications

Ultrasound imaging based on the piezoelectric effect is
already widely utilized in medical diagnostics. In addition,
piezoelectric biosensors, most notably quartz crystal bio-
sensors, have been used for pathogen surveillance owing
to their label-free detection, real-time monitoring, and high
sensitivity [56–58]. Recently, piezocatalysis demonstrated

several promising applications in medical implants and
cancer therapy. Non-toxic and chemically inert piezocata-
lysts, introduced into the body in the form of nanoparticles
or implants, can generate reactive oxygen species in a con-
trolled manner through localized application of mechanical
stress, e.g., vibrations. For instance, piezoelectric coatings
on medical implants can generate reactive oxygen species
under mechanical stress to inhibit bacterial colonization
[59]. Sonodynamic therapy, on the other hand, employs
piezocatalytic nanoparticles activated by ultrasound to pro-
duce reactive oxygen species that selectively target cancer
cells. This remains an active area of research, with preclini-
cal studies yielding encouraging results [60, 61]. Despite
these applications being in early stages, recent advancements
in piezocatalytic materials, particularly piezopolymers and
polymer-ceramic piezocomposites with excellent process-
ability and biocompatibility, will likely expand their use in
biomedical applications.

Overall, the charge carriers and reactive intermediates generated through piezocatalysis provide a safe and versatile alternative to chemical disinfectants or ultraviolet disinfection (Fig. 1, Table 1). Theoretically, any surface or object experiencing continuous or intermittent mechanical stress represents a potential venue for piezocatalysis. This would address the drawbacks of conventional disinfection methods such as inducing microbial resistance to chemical disinfectants and ultraviolet irradiation [62–66].

**Funding** This work was funded by the Shaanxi Provincial Department of Science and Technology ‘Scientist + Engineer’ Team Building Project (2023KJT-198) and the Technology Innovation Center for Land Engineering and Human Settlements, Shaanxi Land Engineering Construction Group Co., Ltd. and Xi’an Jiaotong University (Grant No. 201912131-D9).

**Competing interest** YL, EL, and JH declare that they are Editors of Environmental Chemistry Letters.

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