*Review* **Recent Advances in Lubricant-Based Triboelectric Nanogenerators for Enhancing Mechanical Lifespan and Electrical Output**

**Seh-Hoon Chung 1, Jihoon Chung 2,\* and Sangmin Lee 1,\***


**Abstract:** A triboelectric nanogenerator (TENG) is a noteworthy mechanical energy harvester that can convert mechanical energy into electricity by combining triboelectrification and electrostatic induction. However, owing to the nature of its working mechanism, TENGs have critical limitations in mechanical and electrical aspects, which prevent them from being utilized as primary power sources. To overcome these limitations, several studies are turning their attention to utilizing lubricants, which is a traditional method recently applied to TENGs. In this review, we introduce recent advances in lubricant-based TENGs that can effectively enhance their electrical output and mechanical lifespan. In addition, this review provides an overview of lubricant-based TENGs. We hope that, through this review, researchers who are trying to overcome mechanical and electrical limitations to expand the applications of TENGs in industries will be introduced to the use of lubricant materials.

**Keywords:** triboelectric nanogenerator; energy harvesting; lubricant liquid; output enhancement; mechanical lifespan

#### **1. Introduction**

As the number of portable electronics and Internet of Things (IoT) devices has increased drastically, the need for on-site energy generation has been in the spotlight to power these devices individually and extend their battery life. Energy-harvesting technologies can convert ambient energy, such as solar [1–3], wind [4–8], wave [9–12], and radio frequency [13–15], into electricity that can provide sufficient energy for small electronic devices. Among these energy-harvesting technologies, harvesting mechanical energy has great potential to effectively power portable and small electronics because it is not affected by external environment such as weather conditions. In order to harvest mechanical motion, energy harvesters that utilizes electromagnetic, piezoelectric effect has been utilized [16,17]. Triboelectric nanogenerators (TENGs) are one of mechanical energy harvesters that can generate electricity by combining triboelectrification and electrostatic induction [18–23]. Owing to their light weight [24], high electrical output [25,26], and availability of raw materials, recent research has focused on developing various TENG designs and structures to effectively collect energy from multiple mechanical sources, such as vibration energy [27–30] and rotation energy [31–34]. Currently, TENGs charge commercial electrical devices and lithium-ion batteries used in building selfpowered systems [35–38] and are enhanced for utilization in industrial applications [39].

However, the working mechanisms of TENGs are critically limited in their mechanical and electrical aspects, restricting the use of TENGs as primary power sources. From a mechanical perspective, TENGs have two materials placed in contact to cause triboelectricity, leading to frictional wear. Frictional wear is one of the constant limitations of TENGs because they generate electrical output through mechanical input [40]. Due to the nature of triboelectricity, surface friction from two or more material is necessary to generate surface

**Citation:** Chung, S.-H.; Chung, J.; Lee, S. Recent Advances in Lubricant-Based Triboelectric Nanogenerators for Enhancing Mechanical Lifespan and Electrical Output. *Nanoenergy Adv.* **2022**, *2*, 210–221. https://doi.org/10.3390/ nanoenergyadv2020009

Academic Editors: Ya Yang and Zhong Lin Wang

Received: 18 April 2022 Accepted: 13 May 2022 Published: 19 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

charge and requires consistent or periodical contact due to degradation of surface charge over time. Particularly, various TENGs utilizing micro-/nano-structures and polymer dielectric materials for surface charge enhancements are more vulnerable to frictional wear owing to their poor mechanical properties [41,42]. Frictional damage on a TENG surface leads to a decrease in electrical output due to surface structure failure as well as device failure when mechanical damage is accumulated.

An electrical limitation is imposed when the surface charge potential of the material is higher than the breakdown potential of air, as the surface charge is lost through field emission and ionization of air [43–45]. Since more and more studies report higher surface charge materials with surface micro and nanostructures, this electrical limitation is becoming restriction factor for developing high power TENG [46,47]. With the introduction of the upper limitation of surface charge due to air breakdown, there have been studies in favor and opposing the use of this phenomenon to enhance the electrical output of TENGs [48–50]. However, the cause of this limitation remains unresolved, as the primary potential difference of TENGs is governed by the surface charge, which is still limited by air breakdown. Therefore, to overcome these limitations, several studies are turning their attention to utilizing lubricants, a traditional method recently applied to TENGs, to overcome both limitations.

In this review, we highlight recent advances in lubricant-based TENGs that can effectively enhance both the electrical output and mechanical lifespan of TENGs. The review will focus on overviewing lubricant-based TENGs, their working mechanisms, their various designs, and an output comparison with conventional TENGs. Especially, the reason for lubricant based TENGs can overcome mechanical and electrical limitation of TENG, and experimental data from various studies will be discussed. Finally, the different perspectives regarding lubricant-based TENGs, and challenges associated with their use are discussed.

#### **2. Lubricant-Based TENGs**

When mechanical input is applied to a conventional TENG, two dielectric materials come into contact and cause triboelectrification [51–57]. The surface charge generated by the friction between the two dielectric materials is then transferred to the electrodes through electrostatic induction [58–63]. Contact between the dielectric materials is inevitable during this process, leading to frictional wear. The frictional wear gradually decreases the output of TENG by damaging the surface structure fabricated on the surface and eventually fails after the mechanical damage is accumulated. Therefore, utilizing lubricants between the frictional surfaces is an effective approach to reducing the frictional force. Moreover, lubricating oils such as transformer oil and vegetable oil are also called as dielectric liquid which can be used as insulating material by its high breakdown voltage [64]. As the surface charge of dielectric materials in TENGs can be released to the atmosphere, utilizing the lubricant at TENGs can increase the electrical output. By these effects, recently, various lubricant materials combined with mechanical designs have been introduced to overcome the mechanical limitations of TENGs (Figure 1). As sliding motion-based TENGs are more vulnerable to friction failure, lubricant materials were more actively utilized in these than in other TENGs, such as horizontal contact-separation modes.

As shown in Figure 1a, a lubricating liquid can be applied to a sliding motion-based TENG system [65]. The lubricant liquid on the TENG surface forms a thin layer that can effectively decrease the frictional force between the electrode and polytetrafluoroethylene (PTFE) surface. The main electrical potential difference is generated by friction between the PTFE and polystyrene (PS) surfaces, and the TENG generates an amplified current of over 1 mA because the dielectric liquid acts as a switch during operation. Furthermore, field emission occurs when the PTFE contacts the electrode surface, and electrons can flow directly from the PTFE surface to the electrode. Hence, the lubricant liquid enhanced the electrical output of the TENG by suppressing air breakdown. Various TENGs utilizing other lubricant liquids, such as oleic acid, have been introduced (Figure 1b). In a previous study, oleic acid and PS were dissolved in N,N-dimethylformamide, and then spin-coated

on the conductive polyimide (PI) surface to be paired with nylon-11 surface [66]. In this work, the negative surface charge is generated on oleic acid and PS solution and positive surface charge is generated on nylon surface. The surface charge of PS and nylon is induced to the copper electrode which is underlying each material. The vertical movement between these materials generate electric potential difference which leads to generating electrical output. A similar structure is shown in Figure 1c, where squalene liquid was applied to the TENG surface [67]. The squalene liquid, which was chosen to be most effective through experiment with various liquids, provides a thin layer that can effectively lower friction and increase the surface charge, enhancing electrical output. The main surface charge is generated on the PI surface of the TENG and generates electrical output through horizontal movement. It is also possible to utilize liquids such as hexadecane, as shown in Figure 1d, where the nylon surface was coated with hexadecane to form a hexadecane-containing sandwich structure [68]. In this work, traditional triboelectric material consisting of PTFE and nylon was chosen to generate negative and positive surface charge, respectively. A total of 5 mL of hexadecane was brushed to the surface to form a hexadecane-containing sandwich structure.

**Figure 1.** Various designs for lubricant-based TENGs. Schematic of lubricant-based TENG using: (**a**) dielectric lubricant (Reprinted with permission from ref. [65]. 2021, Elsevier); (**b**) oleic acid (Reprinted with permission from ref. [66]. 2020, Elsevier); (**c**) liquid lubricant (Reprinted with permission from ref. [67]. 2020, Wiley); (**d**) hexadecane sandwich structure (Reprinted with permission from ref. [68]. 2021, Elsevier); (**e**) rolling cylinder structure (Reprinted with permission from ref. [69]. 2021, Wiley); (**f**) ball-bearing structure (Reprinted with permission from ref. [70]. 2022, Elsevier).

Recent research has focused on combining mechanical design with lubrication to enhance the mechanical lifespan of TENGs. As shown in Figure 1e, a TENG coated with a non-polar liquid lubricant consists of an outer cylinder substrate, multiple rolling electrodes, a PTFE cylinder, and aluminum plate electrodes placed on the inner surface of the cylinder substrate [69]. The rolling electrodes rotating inside the cylinder substrate have rolling friction, which is considerably lower than sliding friction. The main electrical potential

difference is generated through PTFE and rolling aluminum electrode in this study. The electrical potential accumulated in the rolling aluminum electrode is then transferred to the plate electrode as the rolling electrode rotates due to external mechanical input. In addition, the entire TENG system was submerged in a liquid lubricant, which enhanced the mechanical lifespan. With the lubricant-applied design, the aluminum surface of this TENG had no frictional damage even after 72 h of continuous operation. In addition, as shown in Figure 1f, a ball-bearing TENG with a semi-solid lubricant such as grease was introduced. A commercial semi-solid lubricant (multipurpose lubricant, Super Lube®), which can be used under a wide range of environmental conditions, was applied to the TENG surface [70]. The main electrical potential difference is generated through PTFE and steel ball located inside the substrate casing, and through commercial semisolid lubricant, it can effectively lower friction between each surface; additionally, rolling friction significantly lowered the friction force compared with sliding friction.

#### **3. Working Mechanism of Lubricant-Based TENGs**

The lubricant liquid on the TENG surface can enhance the mechanical and electrical performance. From a mechanical perspective, lubricant liquid can effectively decrease the frictional wear of TENGs in various condition. There are four different lubrication regimes which are called as boundary lubrication, mixed lubrication, full film lubrication and elastohydrodynamic lubrication. Each regimes show different result of frictional damage and it is decided by several factors such as dynamic viscosity of the lubricant liquid, entrainment speed, normal load per the length of contact, and the contact condition [71]. Normally the contact surfaces are less damaged by the full film lubrication which is also called as hydrodynamic lubrication. By full film lubrication, a sufficient amount of lubricant liquid between two surfaces can form a fluid film that can minimize the frictional wear between them [72]. Due to the fluid film, the two contact surfaces can be separated, thus the frictional wear effectively decreases. Furthermore, through elastohydrodynamic lubrication, the frictional wear from rolling friction can be decreased [73]. Even though rolling friction cause less wear than sliding friction, it can also be improved by using lubricant liquid. As both the coefficient of rolling friction by the roughness of the rolling surface and the contact surface, and normal force, cause friction force, the wear of the rolling surface and contact surface also occurs. Moreover, in real life, the slip between the rolling surface and contact surface can be also occurred. If the accurate lubricant liquid is used, due to the elastohydrodynamic lubrication, lubrication liquid can reduce the frictional damage of various devices such as bearing or gear. Additionally, lubricant liquid can also reduce heat from the friction and prevent the damage from the wear particles [74]. Hence, lubricant liquid has numerous advantages to be utilized for the TENGs. Considering that TENGs, especially those that harvest sliding motion, are constantly exposed to frictional contact, combining a lubricated surface and low-friction mechanical design is essential for a longer lifespan and expanding the application of TENGs to primary power sources.

Along with the mechanical advantages of applying a lubricant to TENGs, the electrical output can be enhanced by lubricating the TENG surface. As the electrical output of TENGs is governed by the amount of surface charge on the dielectric material, enhancing the surface charge is an important factor in increasing the total electrical output. However, as the air breakdown voltage is commonly known to be 3 × 106 V/m [75], dielectric surfaces with electrical potentials higher than this value can cause field emission and air breakdown, where electrons on the dielectric surface can escape to the air. Due to the electrons escaping from the material surface, the surface charge of the material surface is restricted as well. This leads to an upper limitation of the surface charge when the TENG operates in an atmospheric environment, resulting in reduced electrical output [43]. As more and more studies are reporting high surface charge materials and device structures to be utilized in TENG, overcoming this restriction is becoming important [76–78]. Surfaces under a lubricant liquid can avoid field emission and air breakdown because the lubricant liquid tends to have a higher breakdown voltage than air [79]. As shown in Figure 2a, under atmospheric conditions, electrons can escape to air owing to field emission and air breakdown, which would limit the maximum surface charge. This indicates that by utilizing lubricant, dielectric surface can withhold more surface charge compared to when it is exposed to air. Moreover, the liquid lubricant can be polarized due to the surface charge, resulting in transferring the charge to the electrode by electrostatic induction. As shown in Figure 2b, the voltage and current measured increased when the steel sphere was sliding over a polyvinylidene fluoride (PVDF) surface and polyalphaolefin (PAO) 4 was applied between them [80]. PAO 4 fills the microscale gap between the PVDF and steel sphere surfaces. Hence, the air breakdown at the contact interface was inhibited, and the triboelectric charge of PVDF was preserved because of the low polarity and high dielectric constant of PAO 4.

**Figure 2.** Working mechanism of lubricant-based TENGs: (**a**) schematic of a liquid lubricant suppressing air breakdown (Reprinted with permission from ref. [65]. 2021, Elsevier); (**b**) effect of adding lubricant on dielectric surface and schematic of liquid lubricant suppressing air breakdown in a microscale gap (Reprinted with permission from ref. [80]. 2022, Elsevier); (**c**) comparison of polar and non-polar liquid transferring surface charge to the electrode (Reprinted with permission from ref. [69]. 2021, Wiley); (**d**) accumulated charge on the dielectric material transfers to the electrode, producing amplified output (Reprinted with permission from ref. [65]. 2021, Elsevier).

Electrostatic induction plays an important role in the working mechanism of the TENGs for transferring the surface charge to the electrode. However, when the liquid is in contact with the surface of the dielectric material, it forms an electrical double layer (EDL), which screens the surface charge of the solid material. This means that the surface charge is reduced by EDL and surface charge cannot be transferred to the electrode by electrostatic induction. Hence, the electrical output is suppressed as the electrical potential difference between the electrode and dielectric material with the surface charge is reduced. As shown in Figure 2c, when polar liquids, such as water and ethanol, come into contact with the solid surface with the surface charge, the charge is screened by oriented liquid molecules [81]. The characteristic distance where the charge is screened by contacting liquid is called the Debye length (λD). When a strong polar liquid such as water comes into contact with the dielectric surface, λ<sup>D</sup> can be <20 nm [82]. Even ethanol, which has a lower polarity compared with water, has λ<sup>D</sup> around 38 nm. This indicates that a liquid with a lower λ<sup>D</sup> will screen the surface charge even with nano- to micro-scale gaps, resulting in a substantial decrease in the electrical output by reducing the charge induced to the TENG electrodes. This would result in a decrease in electrical output. In contrast, non-polar liquids have a higher λ<sup>D</sup> over 1 μm, which is significantly higher than that of polar liquids; therefore, a higher surface charge can be induced on the electrode through the polarization of the non-polar liquid molecules without electrical screening.

By lubricant liquid effectively suppressing air breakdown and inducing more charge, it can also open a new working mechanism for TENG by introducing a combination of lubricant suppressing air breakdown and non-lubricated surfaces inducing air breakdown. Figure 2d shows the extended working mechanism of the lubricant-based TENG, which utilizes the accumulated charge due to the non-polar lubricant liquid. When the PTFE plate slides across the PS surface, negative and positive charges are generated on the PTFE and PS surfaces, respectively, because of the triboelectric effect between the two surfaces. As the PTFE plate comes in contact with the electrode, electrons on the PTFE surface are emitted to the plate electrode and electrons from the counter electrode are emitted into the air, owing to the field emission. As field emission occurs on both electrodes, it can produce a high electrical output. In this working mechanism, the TENG is able to produce high electrical output through a combination of lubricant suppressing air breakdown on the solid surface, and the air-exposed surface inducing air breakdown to allow more electrons to flow between the two plate electrodes. As the PS surface reverses its sliding motion, a contrasting electrical output is produced, owing to the reverse field emission. As shown in this figure, lubricant materials are being actively studied in the TENG field, and new working mechanisms to enhance the electrical output of TENG have yet to be discovered.

#### **4. Performance of Lubricant-Based TENGs and Relevant Parameters**

Figure 3a,b show the mechanical advantages of utilizing lubricants in TENGs. As shown in Figure 3a, the microscopic photograph suggests that the TENG electrode surface remained undamaged in a lubricated environment even after continuous operation for 72 h [69]. This study also reports that the surface under non-lubricated environment have shown surface damages with noticeable scratches under microscope. In addition to reducing the frictional force and wear, the TENG surface may be subjected to less thermal damage during operation. Figure 3b shows the photograph of the TENG surface, and the thermal image during operation which shows the thermal condition and surface damage of the ball-bearing TENG [70]. When a semi-solid lubricant was applied to the surface, the TENG showed only a 1.7 ◦C increase in temperature after 55 h of continuous operation, and there was no noticeable electrode damage except for the dent marks from the rotating ballbearing spheres. In contrast, when no lubricant was applied to the surface, the operating temperature rises to a maximum of 69.2 ◦C after 1 h of operation. In addition, it showed noticeable damage to the electrode with metal powder wear from the electrode surface compared with a ball-bearing TENG with a semi-solid lubricant. The lower operation temperature shows that the friction force is much less in lubricated condition compared to non-lubricated condition. Through a lower operation temperature, the materials can have less damage from mechanical motion as well. In terms of electrical output, Figure 3c,d show the transferred charge and current output, respectively, depending on the presence of a

lubricant liquid on the surface. The transferred charge and current of the lubricated TENG were more than twice compared with those of the TENG operated in air. As mentioned in the previous paragraphs, this is result from combination of suppressing air breakdown and increasing the Debye length through using non-polar liquid lubricant.

**Figure 3.** Mechanical and electrical performance of lubricant-based TENGs: (**a**) microscopic photograph of electrode surface <**i**> before and <**ii**> after 72 h of continuous operation of the TENG (Reprinted with permission from ref. [69], 2021, Wiley); (**b**) photograph of the electrode surface and the thermal image of TENG during operation (Reprinted with permission from ref. [70]. 2022, Elsevier); (**c**) transferred charge; (**d**) current output of TENG with and without liquid lubrication (Reprinted with permission from ref. [67]. 2020, Wiley).

To further enhance the mechanical lifespan and electrical output, future studies are required to optimize the lubricants specialized for TENG applications. One of the important steps for optimizing lubricant materials is a quantitative study of various liquid lubricants. Many studies are continuing this effort to provide guideline for selecting appropriate lubricant materials to be utilized in various applications. As shown in Figure 4a,b, a recent study showed that liquid lubricants such as squalene, paraffin oil, and PAO 10 have higher electrical outputs than TENG operated under dry conditions [83]. In this work, TENG operating with liquids such as olive oil, rapeseed oil, plurial A 500 PE, PEG 200, water have shown considerably lower electrical output. This study also reported that the relative permittivity and viscosity of lubricant is the key factor to increase output of TENG according to the experimental result. In addition, in other studies, lubricant liquids such as mineral oil and silicone oil show high electrical output, whereas castor oil, water, and ethanol show relatively low output (Figure 4c,d). Overall, various studies have shown that synthetic non-polar liquids such as squalene, mineral oil, silicone oil, and hexadecane have a higher electrical output, whereas polar liquids such as ethylene glycol, water, and ethanol tend to show low electrical output. As shown in Figure 2, the polar liquids screen the surface charge and lead to a decrease in the output. In addition, considering that organic oils such as rapeseed, olive, and castor oils are mixtures of various compounds, they contain polar molecules such as glycerol that would lower the electrical output of TENGs. As shown in Figure 4e–h, a study on the electrical output and friction coefficient depending on PAO 4, perfluoropolyether (PFPE), glycerol, and ethanol, respectively. As shown in the plots, the

electrical output increases and friction coefficient decreases when using a non-polar liquid such as PAO 4 and PFPE, whereas the electrical output decreases and friction coefficient increases when using polar liquid such as glycerol and ethanol. Considering that there are vast number of synthetic and natural oils are used for lubrication, there must be further studies on these materials as well as effect of these lubrication materials when lubrication materials are used for a longer extension of time.

**Figure 4.** Electrical output depending on various liquids applied on the surface of TENGs: (**a**,**c**) Voltage and (**b**,**d**) current output of TENGs depending on various liquids (Reprinted with permission from ref. [69]. 2021, Wiley. And reprinted with permission from ref. [83]. 2021, Elsevier). Measured current and friction coefficient when (**e**) PAO 4, (**f**) PFPE, (**g**) glycerol, and (**h**) alcohol was applied on the surface of TENGSs.

#### **5. Summary and Perspectives**

This review introduces the current strategies and an overview of lubrication-based TENGs. As the air breakdown effect limits the electrical performance and induces frictional wear affecting the mechanical lifespan of TENGs, the use of lubricants has been actively studied to overcome these limitations. These studies have shown that a lubricant liquid applied to the TENG surface can effectively increase the mechanical lifespan and electrical output by lowering the friction coefficient and suppressing air breakdown. Previous studies have shown working mechanism of lubricant-based TENGs can generate high electrical output through suppressing air breakdown and non-polar liquid with high Debye length, which would effectively transfer the surface charge through electrostatic induction. Various mechanical designs and working mechanisms have been developed to further decrease the friction force and enhance the electrical output. For further enhancement of lubricant-based TENGs, optimization of lubricant liquids and mechanical components should be considered at the design level, and additional quantitative studies are required as follows:


We hope that, through this review, researchers who are trying to overcome mechanical and electrical limitations for expanding the applications of TENGs in industries will be introduced to the use of lubricant materials. We believe that constant research efforts and innovations in lubricant-based TENG have great potential for utilizing TENGs as a primary energy source for existing electronics.

**Author Contributions:** Conceptualization, S.-H.C., J.C. and S.L.; writing—original draft preparation, S.-H.C.; writing—review and editing, J.C. and S.L.; supervision, J.C. and S.L.; project administration, S.L.; funding acquisition, J.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education(2021R1A6A3A03040052) and the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT)(No. 2021R1A4A3030268).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Review* **Electromechanical Nanogenerators for Cell Modulation**

**Zhirong Liu 1,2,†, Zhuo Wang 1,2,† and Linlin Li 1,2,\***


**Abstract:** Bioelectricity is an indispensable part of organisms and plays a vital role in cell modulation and tissue/organ development. The development of convenient and bio-safe electrical stimulation equipment to simulate endogenous bioelectricity for cell function modulation is of great significance for its clinical transformation. In this review, we introduce the advantages of an electromechanical nanogenerator (EMNG) as a source of electrical stimulation in the biomedical field and systematically overview recent advances in EMNGs for cell modulation, mainly including cell adhesion, migration, proliferation and differentiation. Finally, we emphasize the significance of self-powered and biomimetic electrostimulation in cell modulation and discuss its challenges and future prospects in both basic research and clinical translation.

**Keywords:** triboelectric nanogenerator; piezoelectric nanogenerator; electromechanical conversion; self-powered; cell modulation

#### **1. Introduction**

Bioelectricity acting as an endogenous biophysical factor can modulate a myriad of cell behaviors, such as cell cycle, adhesion, proliferation, migration and differentiation, and further regulate important biological processes, such as embryogenesis and tissue regeneration [1–3]. Enlightened by endogenous bioelectricity, biomimetic electrical stimulation has been widely employed to regulate cell activities and offers widespread application potential for biomedical therapeutics [4]. At the same time, a series of electrical stimulation devices have been developed for in vitro and in vivo cell electrostimulation [5]. The commercial electrical stimulator is the most widely used device in basic biomedical research because its electrical parameters can be finely tuned from a wide range [6]. However, its bulky size brings a lot of inconvenience and causes poor patient compliance for clinical applications. Additionally, the long-distance wire connection between the stimulator and the target may increase potential safety risks. As another kind of commonly used electrical stimulator, the implantable battery is small and safe enough for in vivo applications; however, due to the limited battery capacity, regular battery replacement is required for long-term electrostimulation, which is expensive and increases the risk of postoperative infection [7].

To meet the requirements of small size, safety and long-term electrical stimulation at the same time, various new types of energy harvesters have been developed, which can collect energy from the surrounding environment or organisms and convert it into electricity [5]. Depending on the energy source, energy harvesters can be divided into three types: (i) environmental energy harvesters, including photovoltaic cells and pyroelectric nanogenerators [8]; (ii) mechanical energy harvesters, including triboelectric nanogenerators (TENG), piezoelectric nanogenerators (PENG) and electromagnetic generator (EMG) [1]; (iii) biochemical energy harvesters, including biofuel cells and non-pulley potential collectors [9,10]. Among them, several kinds of electromechanical nanogenerators (EMNGs) have drawn extensive attention in biomedical fields due to the existence of

**Citation:** Liu, Z.; Wang, Z.; Li, L. Electromechanical Nanogenerators for Cell Modulation. *Nanoenergy Adv.* **2022**, *2*, 110–132. https://doi.org/ 10.3390/nanoenergyadv2010005

Academic Editors: Ya Yang and Zhong Lin Wang

Received: 27 December 2021 Accepted: 3 March 2022 Published: 7 March 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

277

abundant mechanical energy in organisms from tissue to cell level, such as cell activities, heartbeat, limb movement and respiration. An EMG is a kind of efficient, well-established and versatile mechanical energy harvester, and some high-frequency EMGs have been reported for in vitro cell electroporation [11–13]. However, an EMG only has superior performance at high frequencies and high dimensions [14], so they are less efficient in collecting low-frequency, disordered and weak mechanical energy in living organisms. Therefore, this review mainly focuses on recent advances in TENGs and PENGs for cell modulation (Figure 1). For a TENG, its basic working modes and its working mechanism are introduced. For a PENG, the methods used to generate a surface piezopotential for cell electrostimulation are discussed, including speaker, ultrasonic wave, magnetic field and cell/tissue activities. Then, the applications of EMNGs on cell modulation are elucidated, including cell proliferation, adhesion, migration, differentiation, etc. Finally, we discuss the challenges and scope for development of EMNGs for cell modulation in the future.

**Figure 1.** EMNGs driven by biomechanical energy for cell modulation, including cell proliferation, adhesion, migration and differentiation.

#### **2. TENGs for Cell Modulation**

*2.1. Working Mechanism of TENGs*

TENGs can collect abundant biomechanical energy from human bodies and convert it into electricity due to the coupling effect of triboelectrification and electrostatic induction [15–18]. Since its invention in 2012, the TENG mainly presents four basic operating modes, including the vertical contact–separation mode, lateral sliding mode, single electrode mode and freestanding triboelectric layer mode [19] (Figure 2a). Therefore, the structure of a TENG can be flexibly designed according to various applications. As their working principles are similar, herein the contact–separation mode is taken as an example to explain its working principle in detail (Figure 2b). Two films with different triboelectric properties are placed face to face as the friction layers and then metal is attached to their back as the electrode layer. In the original state, there is no induced charge (Figure 2b(i)). When the two friction layers are in contact with each other, an equal amount of opposite triboelectric charge is generated on the contact surface due to the triboelectric effect (Figure 2b(ii)). When the two friction layers are separated, it generates a potential difference between the two electrodes due to electrostatic induction. Thus, the electrons flow from the top electrode to the bottom electrode through the external circuit, thereby generating instantaneous current (Figure 2b(iii)), and then finally, it reaches equilibrium when the two friction layers are completely separated (Figure 2b(iv)). When the two friction layers make contact again, the induced charges flow back through the external circuit to compensate for the potential difference (Figure 2b(v)). The two friction layers continue to make contact and separate in order to generate an alternating current [20,21]. Due to the widespread existence of triboelectricity, TENGs have a wide range of material choices, from natural to

synthetic materials, which can meet the requirements for biocompatibility and flexibility in biomedical applications [22–24]. The combinations of friction layer materials that are commonly used in biomedical TENG can be divided into metal–polymer and polymer– polymer combinations [25]. For biomedical applications, especially implantable medical devices, it is necessary to select materials with good biocompatibility, low toxicity and potential biodegradability. Therefore, noble metals (Au, Ag and Pt) and transition metals (Mg, Fe and Zn) are selected as the metal friction or electrode layers. Polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), polyimide (Kapton), polydimethylsiloxane (PDMS) and polypropylene are common polymer friction layers for TENGs. Specifically, conductive polymers, such as polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PE-DOT), polythiophene (PTh) and polyaniline (PANi), can act as both polymer friction layers and electrodes. In the process of the contact and separation of two friction layers, their ability to gain and lose electrons depends on the triboelectric properties of the friction layer materials. According to the triboelectric sequence of common friction materials [26,27], the farther the distance between the two materials in the list, the greater the amount of charge transfer during the contact–separation process. In addition, the surface micro–nano structure, chemical modification or electron injection of the friction layer materials can increase its effective contact area and surface charge density, thus greatly improving the output performance of the TENG [28–30]. To meet the needs of short-term electrostimulation and avoid secondary surgery, biodegradable and bioabsorbable TENGs have also been designed using biodegradable or photothermal-tuned degradable materials [31–33], such as Mg, poly (caprolactone) (PCL), polylactic acid (PLA), poly(lactic-co-glycolide acid) (PLGA), poly (vinyl alcohol) (PVA), chitosan, cellulose, chitin and silk fibroin.

**Figure 2.** (**a**) The four fundamental working modes of a TENG: vertical contact–separation mode; lateral sliding mode; single electrode mode; and freestanding triboelectric layer mode. (**b**) The working principle of the TENG in contact–separation mode.

#### *2.2. TENGs for Cell Modulation*

Due to the above advantages, the TENG has developed rapidly in biomedical applications. Early research mainly focused on biomechanical energy harvesting, self-powered health monitoring and tissue-level electrical stimulation [22,34]. Recently, TENG-based electrostimulation has been used for cell modulation, mainly for promoting neural/osteogenic differentiation and directing cell migration and proliferation for wound healing.

#### 2.2.1. TENGs for Nerve Repair

Neurons are electrical excitable cells from neural tissues, amongst others. It has been proven that electrostimulation can improve the synaptic function of neurons, thereby promoting nerve regeneration and functional recovery [2]. For this application, a biodegradable TENG was designed that can be implanted into an animal for the electrical stimulation of nerve cells and gradually degrade after finishing its work [31]. The employed TENG has a multilayered structure, including the friction layers, the electrode layers and the encapsulation structure. Two biodegradable polymers are assembled together as the friction layers, and there is a 200 nm spacer layer between the two friction layers. The electrode layer is prepared by depositing a 50 nm magnesium (Mg) film on the back of the friction layer. The fabricated TENG is then encapsulated to improve its stability in the surrounding physiological environment. When applying this TENG to stimulate cells seeded on complementary micrograting electrodes (10 V/mm, 1 Hz), the nerve cell growth can achieve directional growth, which is of great significance for neural repair. Guo et al. combined a wearable TENG and electroconductive microfibers to construct a self-powered electrostimulation system for the neural differentiation of stem cells (Figure 3a,b) [35]. The prepared TENG worked based on the continuous contact and separation of the upper aluminum (Al) electrode and the bottom Kapton film attached to a copper (Cu) electrode. A 3 M foam (2 mm) was bound between the poly(methyl methacrylate) (PMMA) substrate and the upper electrode as a stress buffer layer to increase its output stability. The output of the TENG could reach about 300 V and 30 μA when triggered by human walking (Figure 3c). The electroconductive microfibers were composed of poly(3,4-ethylenedioxythiophene) (PEDOT) and reduced graphene oxide (rGO), which can enhance the proliferation ability of mesenchymal stem cells (MSCs) and show a good neural differentiation tendency. Importantly, after 21 days of the continuous stimulation of the stem cells grown on the conductive hybrid microfibers by the TENG, the gene and protein expressions of both Tuj-1 (neuron-specific maker) and GFAP (neurogliocyte-specific maker) were significantly higher than those without electrical stimulation (Figure 3d,e). This work confirmed the feasibility and potential of TENGs in the field of neural differentiation and repair.

**Figure 3.** TENGs for nerve repair: (**a**) the micromorphology of the rGO−PEDOT microfiber; (**b**) the structural design of the TENG; (**c**) the output current of the TENG driven by normal human walking. The nucleus (blue), neuron-specific maker Tuj1 (red) and neurogliocyte-specific maker GFAP (green) of the cells (**d**) without electrostimulation and (**e**) with TENG-induced electrostimulation for 21 days. Reprinted with permission from ref. [35], Copyright 2016, American Chemical Society.

#### 2.2.2. TENG for Bone Repair

Bone is a natural composite of piezoelectric materials, which mainly comprises a piezoelectric collagen matrix and hydroxyapatite crystals [36]. In the process of bone repair and regeneration, the electric signal is a vital factor for promoting the osteodifferentiation of stem cells and osteoblast proliferation [37,38]. Due to the abundant mechanical energy during joint movement, TENGs can effectively collect this energy and then convert it into electricity for bone repair. It has been confirmed that the electrostimulation of TENGs can promote osteoblast attachment, proliferation and differentiation. For example, Tian et al. developed a self-powered electrical stimulator for bone repair, consisting of an implantable TENG as the power source, a rectifier and an interdigital electrode [39]. When the flexible TENG was implanted on the SD rat's femur, it could successfully convert the biomechanical energy of the rat's daily exercise into electricity. Applying the electricity generated by the TENG to the cells through the interdigital electrode could effectively promote cell adhesion, spreading and proliferation. Moreover, the osteogenic differentiation level of the cells increased by 28.2% after stimulation for 12 days. Shi et al. utilized a TENG to load and accumulate negative charges on the surface of an anodized titanium implant for promoting the osteogenesis of MC3T3-E1 preosteoblast cells [40]. The prepared TENG worked based on the vertical contact–separation mode (Figure 4a). A PTFE film with a nanorod structure and a titanium (Ti) foil with a nanotube structure were used as the triboelectric layers. A thin cooper (Cu) film was magnetron sputtered on the outer face of the PTFE friction layer as an electrode and a Ti foil was employed as another electrode. Then, PTFE tape and PDMS were used to encapsulate the TENG for waterproofing. The output of the TENG could reach up to 12 V, 0.15 μA and 5.3 nC (Figure 4b) and its output voltage could be well maintained after 1 × 106 cycles. The prepared TENG could convert the biomechanical energy of human daily movement into electricity for constructing a long-term and stable negatively charged implant surface, thereby inhibiting bacterial adhesion and biofilm formation. Moreover, the negatively charged implant could promote preosteoblast adhesion and the osteogenic differentiation of MC3T3-E1 cells (Figure 4c). Yao et al. designed an implantable and bioresorbable TENG that could be attached to living tissue and generate bidirectional electric pulses (Figure 4d) [41]. An island–bridge magnesium (Mg) layer served as both the bottom electrode and a triboelectric layer. PLGA with a micropyramid structure was used as another triboelectric layer and the Mg electrode with the island–bridge structure was coated on the top of the PLGA layer, serving as the top electrode. The micropyramid structure of the PLGA could improve the output of the TENG due to the increase in contact area. The serpentine geometry and island–bridge structure of the electrode layer could effectively improve the robustness of the structure and reduce the modulus of the flexible device. Thus, it could be pasted to irregular tissues and withstand large strains. When applying the electrostimulation on a pair of dressing electrodes, the generated electric field could activate relevant growth factors (Figure 4e,f). The enhanced secretion of the fibroblast growth factor (FGF1) and vascular endothelial growth factor (VEGF) could accelerate vascularization for nutrient supply and metabolic transportation. More transforming growth factor and bone morphogenetic protein could promote cell differentiation and accelerate bone formation and mineralization (Figure 4g). In general, the electrostimulation through the TENG could regulate the bone microenvironment, promote rapid bone regeneration and synergistically increase bone strength and mineral density (Figure 4h).

**Figure 4.** TENGs for bone repair: (**a**) the surface microstructure and photograph of the prepared TENG; (**b**) the output voltage and current of the TENG; (**c**) alizarin red staining showing the calcium deposits of MC3T3-E1 after TENG electrostimulation for 21 days. Reprinted with permission from ref. [40], Copyright 2020, Elsevier. (**d**) The schematics of the implantable self-powered bone fracture electrostimulation device, consisting of a TENG as a power source and a pair of dressing electrodes for applying electrostimulations to the fracture; (**e**) the output voltage of the TENG driven by a mechanical linear motor (top) and on a rat (bottom); (**f**) the simulated electrical field distribution of the dressing electrodes inside a tissue under the voltage of 4 V; (**g**) the immunohistochemistry (IHC) score of the expressed growth factors in different groups; (**h**) the three-point bending flexural stress measurement for the different groups after 6 weeks. N, normal group; I, dressing electrodes with TENG electrostimulation; S, dressing electrodes without TENG electrostimulation; F, no implanted device. n.s. and \*\*\* represent non-significant and *p* < 0.001, respectively. Reprinted with permission from ref. [41], Copyright 2021, the Authors. Published by PNAS.

#### 2.2.3. TENGs for Wound Healing

In addition, the endogenous electric field also plays key roles in wound healing. Generally, skin wounds have a transepithelial potential of 15–60 mV, which serves as a directional cue to direct cell proliferation and migration to the wound site for wound healing [42]. Thus, electrostimulation is an attractive auxiliary method for wound repair (Figure 5a). However, it is still largely limited in clinical applications due to the inconvenience of its implementation. To solve this problem, a wearable electrostimulation bandage was designed to accelerate the healing of skin wounds [43,44]. It was composed of a wearable TENG as a power supply and a pair of dressing electrodes, both of which were integrated in a bandage [43]. The TENG was prepared by overlapping a Cu (electropositive material) electrode layer with a Cu/PTFE (electronegative material) film on the PET substrate. After wrapping the bandage on the chest of the rat, the TENG could collect the mechanical energy of the rat's breathing to generate electricity. When the rat was under deep anesthesia, the output voltage was only ~0.2 V at a rate of 30 times per min, which corresponded to a slow and shallow breathing pattern. After the rat recovered from anesthesia, the output voltage reached ~1.3 V at a rate of 40 times per min, corresponding to the calm and stable state of the rat. When the rat moved normally, the output voltage reached the highest level of ~2.2 V at a rate of 110 times per min (Figure 5b). Under the electric field induced by the TENG (2 V cm<sup>−</sup>1, 1 Hz), fibroblasts exhibited an obvious proliferation and alignment. After 6 h of the electric field stimulation, cells continued to proliferate and differentiate along the direction of the electric field (Figure 5c). Fibroblasts play a crucial role in the healing process of skin wounds. Initially, cells migrate to the wound, proliferate and interact with surrounding cells. After that, fibroblasts differentiate and generate ECM, glycoproteins, adhesive molecules and various cytokines, which then replace the provisional fibrin-based matrix and accelerate wound repair. Under the treatment of the electrostimulation bandage, a full-thickness rectangular skin wound was quickly closed within 3 days, while the normal shrinkage-based healing process needs 12 days. Li et al. also confirmed the promoting effect of TENG electrostimulation on tissue repair. Their prepared TENG was implantable and its degradation rate could be tuned in vivo by near-infrared (NIR) light irradiation [33]. The biodegradable TENG consisted of three parts: an Au deposited, hemisphere array-structured layer served as both the triboelectric layer and the bottom electrode; a biodegradable polymer was another triboelectric layer; and a thin Mg film was deposited on the back as the top electrode. Another biodegradable polymer doped with Au nanorods (AuNRs) was applied as the bottom substrate, which endowed the TENG's ability to respond to NIR irradiation so that the biodegradation process could be rationally controlled (Figure 5d). The open-circuit voltage (Voc) and short-circuit current (Isc) of the TENG reached 28 V and 220 nA, respectively. After implantation in the subdermal region on the back of the SD rats, the Voc was about 2 V. With NIR treatment, the output of the TENG quickly dropped to 0 within 24 h and the device was largely degraded in 14 days (Figure 5e,f). Applying the output of the TENG to stimulate fibroblasts (100 mV mm−1) could significantly accelerate the migration of cells across the scratch, which was essential for the wound healing treatment (Figure 5g,h).

**Figure 5.** TENGs for wound healing: (**a**) the mechanism of wound healing under the endogenous electric field; (**b**) the output voltage of the TENG driven by different frequencies of breathing in rats; (**c**) the morphology of the NIH 3T3 cells after TENG electrical stimulation of different times. Reprinted with permission from ref. [43], Copyright 2018, American Chemical Society. (**d**) The structure design of the photothermal-controlled biodegradable TENG; (**e**) the output of the TENG with NIR irradiation; (**f**) a micro-CT image of the implanted TENGs over time. Fluorescence microscope images of fibroblast L929 cell migration (**g**) without electrostimulation and (**h**) with TENG electrostimulation. Reprinted with permission from ref. [33], Copyright 2018, Elsevier.

#### 2.2.4. TENGs for Drug Delivery and Cardiac Pacing

In addition to the aforementioned applications, TENGs have also been employed to construct self-powered electroporation systems to improve the permeability of cell membranes for drug delivery or drug release. Electroporation utilizes high-intensity electric fields to create transient nanopores in plasma membranes to introduce biomolecules into cells. Liu et al. integrated a TENG with a silicon nanoneedle array electrode to build a self-powered nanoelectroporation system for facilitating efficient intracellular drug delivery while minimizing cellular damage (Figure 6a) [45]. In this system, the TENG acted as a self-powered and stable electrostimulation source to provide electrical pulses for electroporation, which made the system more convenient in practical applications. On the other hand, a silicon nanoneedle array was employed to replace the traditional planar electrode for nanoelectroporation, which could only generate high local electrical fields in the nanoneedle– adherent cell interface, thereby reducing cell damage (Figure 6b). This system could deliver a variety of exogenous species, such as small molecules, siRNA and biomacromolecules, into different types of cells. Depending on the delivered biomolecule and cell type, the delivery efficiency ranged from 50% to 90% and cell viability was above 94% (Figure 6c). On this basis, the same group further designed a high-throughput electroporation system based on a TENG and silver nanowire-modified foam electrodes [28]. Cell suspension could continuously flow through the foam electrodes in the electroporation channel to achieve electroporation under the action of the TENG's electrical pulses (Figure 6d). The system could achieve high cell processing throughput of up to 10<sup>5</sup> cells/min while ensuring high cell delivery efficiency and viability. Yang et al. also designed a self-powered gene electrotransfection system that consisted of a TENG and a nanowire array electrode [46]. The employed TENG was based on the vertical contact–separation mode, which could harvest the mechanical energy of the simple human tapping motion and convert it into electricity for electrotransfection. Two CuO nanowire array meshes were coaxially placed as electrodes, which could greatly amplify the local electric field strength to enhance the transfection efficiency and reduce cell damage. The system could achieve a high-efficiency siRNA electrotransfection of 95% for MiaPaCa-2 cells and 84% for K562 cells, and then downregulate the expression of targeted mutation genes, thereby significantly inhibiting cell proliferation and anti-apoptosis ability. Zhao et al. loaded doxorubicin (DOX) into red blood cells (D@RBC), which could create transient pores in the cytomembrane through TENG electroporation and release DOX on demand to kill tumor cells for cancer therapy (Figure 6e–g) [47]. These results illustrate that TENG-based self-powered systems have great potential for both fundamental biological research and clinical applications.

**Figure 6.** TENGs for drug delivery: (**a**) a schematic illustration of the TENG-driven electroporation system based on a nanoneedle array electrode; (**b**) the simulated electrical field distribution of the nanoneedle array with an applied voltage of 20 V; (**c**) a fluorescence image of MCF-7 after delivering dextran-FITC (green). Reprinted with permission from ref. [45], Copyright 2019, WILEY-VCH. (**d**) A schematic illustration of the TENG-driven high-throughput electroporation system. Reprinted with permission from ref. [28], Copyright 2020, American Chemical Society. (**e**) A schematic illustration to show the controlled release of DOX from RBC driven by the TENG; (**f**) SEM images of D@RBC under the electric field; (**g**) the viabilities of HeLa cells in the D@RBC and D@RBC under the electric field groups (live cells are green and dead cells are red). Reprinted with permission from ref. [47], Copyright 2019, WILEY-VCH.

Cardiomyocytes are also electrically active cells and their action potentials contribute to the spontaneous beating of the cells. Rhythmic action potentials propagate continuously in multicellular cardiac tissue and travel through the myocardium to induce myocardial contraction [1]. Therefore, electrical stimulation has been used in heart failure treatments and myocardial reconstruction. Inspired by this, Ouyang et al. demonstrated a self-powered symbiotic pacemaker based on an implantable TENG [48]. The VOC of the implantable TENG was as high as 65.2 V. The energy collected from each heartbeat was about 0.495 μJ, which was above the endocardial pacing threshold energy (0.377 μJ). Thus, this self-powered symbiotic pacemaker successfully corrected sinus arrhythmia and prevented disease progression on a large animal model.

#### **3. PENGs for Cell Modulation**

PENGs can convert mechanical motion into electric energy based on piezoelectric materials and piezoelectric effects. A PENG is composed of piezoelectric materials, positive and negative electrodes and flexible substrates. Among them, the piezoelectric material is the core component of a PENG and is the basis for generating electricity.

#### *3.1. Piezoelectric Materials and Piezoelectricity*

Piezoelectric material is a kind of dielectric material with a non-centrosymmetric structure, which can generate and accumulate charges with opposite signs on its surface under the action of mechanical stress or strain [49]. According to compositions, piezoelectric materials can be divided into three categories: (i) inorganic piezoelectric materials; (ii) organic piezoelectric materials; and (iii) composite piezoelectric materials.

Inorganic piezoelectric materials mainly include piezoelectric single crystals, piezoelectric ceramics and piezoelectric semiconductors. Quartz, lithium niobate, lithium gallate, lithium germanate and lithium tantalate are several common piezoelectric single crystals. Piezoelectric ceramics often have strong piezoelectricity and high dielectric constants, such as barium titanate (BaTiO3), lead zirconate titanate (PZT), lead zinc niobium (PZN), lead metaniobate and lead barium lithium niobate. Generally, piezoelectric ceramics containing lead have high piezoelectric coefficients, but the toxicity of lead limits their application in the biomedical field. Currently, most piezoelectric semiconductors have wurtzite structures and have both piezoelectricity and semiconducting properties, such as ZnO, GaN, AlN and BN. The piezoelectricity of inorganic materials originates from the non-centrosymmetric crystal structure. Taking wurtzite-structured ZnO as an example (Figure 7a,b), positive Zn2+ and negative O2<sup>−</sup> have tetrahedral coordination in an unstrained hexagonal ZnO crystal structure. So, the centers of the anions and cations overlap each other and there is no polarization. When the crystal is under a mechanical stress, the displacement of Zn2+ and O2<sup>−</sup> inside the crystal causes the positive and negative charge centers to shift in opposite directions, resulting in a dipole moment in the unit cell [50]. Due to the continuous superposition of the dipole moments, a macroscopic piezoelectric potential (piezopotential) is generated in the ZnO crystal (Figure 6b) [51].

Compared to inorganic piezoelectric materials, although the piezoelectric properties of organic piezoelectric materials are relatively poor, their advantages of good biocompatibility, processability and high flexibility can satisfy the requirements for biomedical applications, especially for wearable and implantable devices. Organic piezoelectric materials can be divided into natural and synthetic piezopolymers. Collagen, chitosan, cellulose and chitin are widely studied natural piezoelectric polymers. PVDF and its trifluoroethylene (TrFE) copolymer (P(VDF–TrFE)), poly-L-lactic acid (PLLA) and polyhydroxyalkanoates (PHAs) are representative synthetic piezopolymers. The piezoelectricity in organic materials arises from their non-centrosymmetric molecular arrangement and orientation. Figure 6c depicts the piezoelectric effect of PVDF ((CH2–CF2)n). PVDF has five crystal phases, in which α-phase with trans−gauche−trans−gauche conformation (TGTG ) and a β-phase with all trans conformation (TTTTT) are predominant [52,53]. The dielectric property of PVDF is caused by the difference in electronegativity between F and H atoms that leads to a

dipole moment is generated along the F→H direction [54]. For α-phase PVDF, there is no piezoelectricity since the chains are arranged in the unit cell, resulting in a net cancellation of the dipole moments. For β-phase PVDF, since the parallel orientation of the dipole moments leads to the overlay of the electric dipole moments, it has the highest piezoelectricity [55] (Figure 7c). Composite piezoelectric materials are often obtained by dispersing piezoelectric nanoceramics into a polymer matrix, which combines the advantages of inorganic piezoelectric materials and piezoelectric polymers, showing good flexibility, high piezoelectric coefficients and processability. Piezoelectric materials with high mechanical and piezoelectric properties are the basis for the fabrication of highperformance PENGs.

**Figure 7.** (**a**) An atomic model of a wurtzite structure ZnO and a schematic diagram of a stressinduced electric dipole moment. Reprinted with permission from ref. [50], Copyright 2012, WILEY-VCH. (**b**) Piezopotential distribution along a ZnO nanowire under axial stretch or compress. Reprinted with permission from ref. [51], Copyright 2009, American Institute of Physics. (**c**) The molecular structure of α-phase and β-phase PVDF; (**d**) the working principle of the PENG.

#### *3.2. Working Mechanism of PENGs*

In 2006, Wang et al. first developed a zinc oxide (ZnO) nanowire-based PENG to collect tiny vibrational energy [56]. In 2010, a PENG that was based on single ZnO nanowire was successfully implanted in a live rat to harvest mechanical energy from its breath and heartbeat, with an output of around 3 mV and 30 pA [57]. After that, all kinds of piezoelectric materials and structures have been applied to fabricate implantable PENGs, including PVDF, P(VDF–TrFE), ZnO nanowire arrays, PZT, etc. [58–64]. Traditionally, a PENG is composed of an intermediate piezoelectric layer and two metal electrodes in a sandwich structure. Figure 7d demonstrates the electrical generation principles of a typical PENG. At the beginning, without the action of external force, the charge centers of the anions and cations overlap each other, so there is no polarization in the piezoelectric material. When mechanical stress is applied, the piezoelectric material is compressed and deformed, so that the anions and cations inside the crystal are displaced and the positive and negative charge centers are separated to generate a surface piezopotential. The electrons flow through the external circuit to achieve charge balance, thereby generating piezoelectric current output. When the external force is removed, the piezopotential gradually decreases and disappears and the electrons flow back to rebalance the charge. During the cyclic compression–release process, mechanical energy is continuously converted into electrical energy [65,66]. The selection of piezoelectric materials that have excellent performance is the key to improving the output of PENGs. In addition, the crystallinity of piezoelectric

materials and the arrangement direction of dipoles can be further improved by means of annealing, stretching and high-voltage polarization, thereby achieving efficient mechanical energy harvesting [66].

#### *3.3. PENGs for Cell Modulation*

A PENG is an energy harvesting device that converts mechanical energy into electrical energy using the piezoelectric effect. Therefore, its application is similar to that of a TENG and it too can be used as a self-powered electrical stimulation source. For example, Zhang et al. proposed a biomechanical energy-driven shape–memory PENG for promoting MC3T3-E1 cell proliferation and orientation and osteogenic differentiation [67]. Jin et al. designed a tribo/piezoelectric hybrid nanogenerator to promote the long-term proliferation and migration of Schwann cells and regenerate the myelination of nerve fibers and neuromotor function reconstruction [68]. In addition to being an electrical stimulation source, some piezoelectric materials are also employed as biological scaffolds [54]. Therefore, piezoelectric biomaterials can be designed for in situ cell-scale electrical stimulation, avoiding wire connections and electrode implantation. External mechanical force or deformation is an essential precondition for piezoelectric biomaterials to generate piezoelectric potential (piezopotential) for cell electrostimulation. The methods commonly used to apply mechanical force are acoustic waves, magnetic fields and cell traction.

#### 3.3.1. Acoustic Wave-Driven PENGs for Cell Modulation

An acoustic wave is a mechanical wave that is generated by the vibration of a sound source. Acoustic pressure, i.e., the pressure change caused by sound waves, is the main source of mechanical force. According to the frequency, acoustic waves can be divided into four categories: infrasound (<20 Hz); audible sound (20–20 kHz); ultrasound (20 kHz–1 GHz); and hypersound (>1 GHz). Wang et al. fabricated electrospun poly(vinylidene fluoride-trifluoroethylene) (P(VDF–TrFE)) nanofibers with a d31 of 16.17 pC/N [69]. A lab-designed speaker was employed to generate the mechanical vibration to deform the nanofibers. Under the dual effects of the nanofiber morphology and electrical stimulation (0.75 V, 22.5 nA), L929 fibroblast cells aligned perfectly along the direction of the nanofibers and the cell proliferation rate increased by 1.6-fold.

An ultrasonic wave is a commonly used mechanical force source in the field of biomedicine because of its high tissue penetration and good directionality [70] and its ultrasonic power, frequency and period can be adjusted precisely. In the ultrasonic process, acoustic pressure and ultrasonic cavitation effects play major roles in driving the deformation of piezoelectric materials [71]. When the sound pressure gradually reaches a certain value, the tiny bubble cores in the liquid expand rapidly and then suddenly close to generate the shock wave. The ultrasonic cavitation is a series of the dynamic processes, such as expansion, collapse and generation, of microjets. Wan et al. demonstrated an ultrasound-mediated cell sheet harvesting based on a piezoelectric polyvinylidene fluoride (PVDF)/barium titanate (BaTiO3, BTO) composite film (Figure 8a) [72]. Under the ultrasound stimulation (1 MHz, 0.8 W cm<sup>−</sup>2), the output voltage and current of the PVDF/BTO film could reach ~100 mV and 0.19 nA, respectively. The adsorption and conformation of fibronectin (FN) play an important role in regulating early cell material adhesion. After ultrasound treatment for 20 s, the surface potential of the PVDF/BTO film was more negative, thereby decreasing the FN adsorption to manipulate cell adhesive behavior. On the other hand, the generated piezopotential under ultrasound stimulation could regulate the secondary structure of adhesive protein fibronectin (FN) from β-sheet to α-helix, thereby weakening the interfacial protein interaction and further regulating the cell adhesion behavior (Figure 8b,c). In addition, ultrasound assisted piezoelectric biomaterials have also been employed for inducing neural differentiation [73–76]. Marino et al. prepared piezoelectric tetragonal BTO nanoparticles that could be attached to the cell membrane [76]. Under ultrasound stimulation, the generated piezopotential activated voltage-gated membrane channels, allowed calcium and sodium to flow in and then mediated the enhancement of

neurite outgrowth. Liu et al. designed a biohybrid multifunctional micromotor composed of magnetic Fe3O4 nanoparticles, piezoelectric BTO nanoparticles and *S. platensis* [74]. The micromotor was actuated and steered magnetically using a low-intensity rotating magnetic field (Figure 8d,e). Due to its diameter (≈0.8 μm) being smaller than the size of neural stemlike cells, this micromotor could reach the desired position at the single cell level. After reaching the targeted neural stem-like cell, the in situ piezopotential of BTO was generated by an ultrasound stimulation, which activated the Ca2+ channels and thereby induced the neuronal differentiation of the targeted cells (Figure 8f,g). Additionally, piezoelectric nanoparticles can be endocytosed by the cells to achieve non-destructive intracellular electrical stimulation for promoting osteogenic differentiation. Ma et al. synthesized piezoelectric nylon-11 nanoparticles with uniform morphology that could be easily endocytosed using dental pulp stem cells (DPSCs) [77]. Under ultrasound conditions, piezoelectric stimulation generated by nylon-11 nanoparticles could efficiently promote the osteogenic differentiation of stem cells through electromechanical conversion in a non-invasive way. Ultrasound assisted piezoelectric biomaterials can also regulate proinflammatory macrophage polarization [78], which is critical for antitumor immunity. After ultrasound treatment on *β*-PVDF, a significant upregulation of the mRNA levels of proinflammatory (M1) markers was observed, including TNF-*α*, IL-1*β* and MCP-1 (Figure 8h–j). In addition, the intensity of intracellular M1 marker iNOS was higher than that on tissue culture plates (TCPs), while anti-inflammatory (M2) marker Arg-1 was significantly reduced (Figure 8k,l). These results prove that the piezoelectricity of *β*-PVDF obviously promotes M1 polarization.

#### 3.3.2. Magnetic Field-Driven PENGs for Cell Modulation

Magnetic fields have also been employed to induce piezopotential by rationally designing magneto-electric (ME) composite materials. ME materials comprise a class of multifunctional nanostructures that are capable of strongly coupling magnetic and electric fields. The ME effect is defined as the electrical polarization of a substance in response to a magnetic field (positive effect) or the magnetization change of a substance under the action of an electric field (converse effect) [79]. ME materials include single phase and composite materials, within which the composites that are composed of a ferromagnetic phase and a piezoelectric phase have lager ME effects at room temperature [80]. Under the action of the magnetic field, the magnetic material is deformed and the strain is further transferred to the closely connected piezoelectric material to generate piezopotential. This phenomenon is the magneto-electric coupling in two-phase ME materials. The properties of ME composite materials can be regulated by changing the phase materials, component sizes and phase volume ratios [81]. Core shell-structured ME materials have attracted considerable attention in the design of drug delivery systems, which are capable of controlling drug release via external magnetic fields. For example, Nair et al. designed CoFe2O4@BaTiO3 ME nanoparticles that could generate piezopotential by applying a low alternating current magnetic field to allow controlled on-demand drug release [82]. The application of ME composites for the electrical stimulation of cells and tissues has been intensively studied in tissue engineering. Mushtaq et al. designed a soft hybrid nanorobot that mimicked an electric eel, based on the magneto-electric coupling effect [83]. These bionic nanoeels consisted of three parts: a flexible and elongated piezoelectric PVDF tail connected to a polypyrrole nanowire, which was decorated with nickel rings for magnetic actuation (Figure 9a). Upon the rotating magnetic fields, the nanoeels displayed different swimming modes, including tumbling, wobbling and corkscrewing (Figure 9b). At the same time, the piezoelectric soft tail was driven to deform, causing its electric polarization. As magnetic fields can achieve deep tissue penetration and can control the movement of magnetic materials with high precision, they can be used to achieve targeted local electrical stimulation. Dong et al. introduced a highly integrated multifunctional soft helical microswimmer based on CoFe2O4@BiFeO3 (CFO@BFO) core shell ME nanoparticles, which could realize targeted neuronal cell delivery, on-demand localized neuron electrostimulation and post-delivery enzymatic degradation (Figure 9c) [84]. Under a rotating magnetic field, the

helical microswimmer rotated around its long axis and moved in translation (Figure 9d). After reaching the target position, an alternating magnetic field was applied to induce the magnetostriction of the CFO core and then the pressure was transferred to the BFO shell to generate piezopotential, thus promoting the neuronal differentiation of the cells (Figure 9e–g). The prepared microswimmer could be degraded by the enzymes in the ECM produced by the cells. Fang et al. prepared a stretchable carbon porous nanocookies@conduit (NC@C) using 3D printing technology, in which the NC was composed of rGO, mesoporous silica and carbon layers with an excellent magneto-electric effect. The prepared NC@C could encapsulate the neuron growth factor (NGF) and achieve on-demand release under the control of a magnetic field. At the same time, it could electrically stimulate cells to effectively induce cell proliferation and neuronal differentiation in vitro and could further improve myelin layers and guide axonal orientation in vivo (Figure 9h–j) [85]. In addition, ME materials have also been developed for deep brain stimulation, bone regeneration, skeletal muscle tissue regeneration and more [85–88]. These materials inspire new approaches to targeted cell therapies for traumatic injuries.

**Figure 8.** Ultrasound-driven PENGs for cell modulation: (**a**) a schematic diagram of the cell detachment regulated by piezopotential; (**b**) the contents of fibronectin α-helix and β-sheet under different conditions (A, FN in PBS; B, FN on the PVDF/BTO film; C, FN under ultrasound; D, E, FN on the PVDF/BTO film under ultrasound); (**c**) fluorescence microscopy images of cells remaining on the PVDF/BTO composite film after ultrasound. Reprinted with permission from ref. [72], Copyright 2021, Elsevier. (**d**) An illustration of a highly controllable micromotor for inducing the neuronal differentiation of targeted cells; (**e**) SEM images of *S.platensis*@Fe3O4@tBaTiO3; (**f**) representative time-lapse Ca2+ imaging of PC12 cell stimulated by *S.platensis*@Fe3O4@tBaTiO3 with ultrasound; (**g**) a fluorescent image of differentiated PC12 cells stimulated by *S.platensis*@Fe3O4@tBaTiO3 with ultrasound (targeted cell indicated with a red arrow). βIII-tubulin is stained in green and the nuclei are in blue. Reprinted with permission from ref. [74], Copyright 2020, WILEY-VCH. (**h**) A representative SEM image of THP-1 cells on *β*-PVDF for 24 h; (**i**) the output voltage of *β*-PVDF with ultrasound stimulation; (**j**) the relative mRNA expression of the M1 markers of TNF-*α*, IL-1*β* and MCP-1 in different groups; (**k**,**l**) immunofluorescence staining images of M1 marker iNOS and M2 marker Arg-1 after culturing for 3 days on TCP and *β*-PVDF with ultrasound. \* *p* < 0.05, \*\* *p* < 0.01, \*\*\* *p* < 0.001 vs. the TCP control group, ### *p* < 0.001 vs. the *β*-PVDF group. Reprinted with permission from ref. [78], Copyright 2021, The Authors. Published by WILEY-VCH.

**Figure 9.** Magnetic field-driven PENGs for cell modulation: (**a**) a SEM image showing the hybrid nanoeels; (**b**) a time-lapse image showing the swimming behavior of the hybrid nanoeels, including tumbling, wobbling and corkscrewing. Reprinted with permission from ref. [83], Copyright 2019, WILEY-VCH. (**c**) A schematic diagram of the degradation process of the soft microswimmers and the neuronal differentiation of SH-SY5Y cells under a magnetic field; (**d**) a microswimmer loaded with cells driven by a rotating magnetic field. Fluorescent images of the cells on the (**e**) control and (**f**) soft microswimmers under magnetic stimulation. The nuclei are stained in blue and the neuronal specific protein GAP43 in green. (**g**) Box-and-whisker plots of the fluorescent intensities representing the level of GAP 43 expressed in SH-SY5Y cells (\*\*\* *p* < 0.001). Reprinted with permission from ref. [84], Copyright 2020, WILEY-VCH. (**h**) NC@C under magnetic field treatment promoting magneto-electric conversion into release growth factor and inducing neuron cell differentiation. Sciatic nerve defects harvested from (**i**) autograft and (**j**) NGF-NC@C+ magnetic field. Reprinted with permission from ref. [85], Copyright 2020, Springer Nature.

#### 3.3.3. Cell Traction-Driven PENGs for Cell Modulation

There are abundant mechanical forces in cell activities, such as cell spreading, migration, contraction and cardiomyocyte beating. Thus, within the cellular microenvironment, cells continuously exert mechanical forces on the extracellular matrix (ECM). Similarly, the cells can also exert mechanical forces on the piezoelectric biomaterials. Murillo

et al. demonstrated that the electromechanical interaction between living cells and a ZnO nanosheet-based PENG induced a local electric field that could modulate cell activity, such as stimulating the cell motility and activating the calcium channels to induce intracellular calcium transients, depending on the cell type (Figure 10a,d) [89]. The effective piezoelectric coefficient of the ZnO nanosheets was 4–6 pm V−1, according to a piezoresponse atomic force microscope. Under the weak force of cells (0.1–10 nN), ZnO nanosheets could generate a piezopotential in the range of 0.5–50 mV. The number of macrophages grown on ZnO nanosheets with a traveling distance exceeding 150 μm was twice of that grown on the control substrate, indicating that the electromechanical interaction of the ZnO nanosheet-based PENG increased the motility of macrophages and facilitated long-distance displacement (Figure 10c). In addition, 64% of the SaOS-2 cells grown on the ZnO nanosheets presented increases in [Ca2+]i. By contrast, only 6% of the cells grown on glass coverslips showed increases in [Ca2+]i, with low amplitudes of Ca2+ transients (Figure 10d). These results indicate that the cell adhesion forces could bend the ZnO nanosheets and induce a local electric field to stimulate the cells and alter their activities. Zhang et al. designed a piezoelectric PVDF with a nanostripe array structure for the neuron-like differentiation of mesenchymal stem cells (MSCs) [90]. The traction force of the living cells on the surface of the nanostriped PVDF could cause the deformation of the PVDF stripes, thereby generating a local piezopotential to provide continuous electrical stimulation to the living cells. According to the simulation, there was a piezopotential from 34 μV to 3.4 mV when cell traction forces increased from 0.1 to 10 nN (Figure 10e,f). However, the piezopotential induced by the cell force (10 nN) on flat PVDF film was only 960 nV, which was insignificant for the physiological activities of the cells. Thus, the nanotopography of the PVDF film could increase the generated piezopotential in response to cell migration and traction and it provided a stronger signal to stimulate the differentiation of the stem cells. Unlike the spindle-shaped and flat cell morphology in the control group, the MSCs on the nanostriped PVDF formed a neuron-like morphology, including highly refracted cell bodies and elongated nanostriped PVDF. Moreover, the mRNA and protein expression levels of neuronal marker Tuj-1 were significantly increased, indicating that the generated piezopotential of the nanostriped PVDF had a positive effect on the neuron-like differentiation of MSCs (Figure 10g). Inspired by the biophysical cues of ECM, Li et al. developed an electromechanical coupling 3D bio-nanogenerator composed of GO/PEDOT/Fe3O4/PAN fibers (GO, graphene oxide; PEDOT, poly(3,4-ethylenedioxythiophene); PAN, polyacrylonitrile), in which piezoelectric PAN was used as the electromechanical conversion unit [91]. The unique 3D structure of the bio-nanogenerator provided an ECM-like microenvironment for cell growth. It could also generate piezopotential up to millivolts through cell inherent force, thereby providing in situ electrostimulation for the adherent cells. Liu et al. prepared nanofibers with a suitable stiffness that was analogous to that of collagen using electrospinning technology [92]. Interestingly, the obvious mechanical deformation of the nanofibers was only observed after cell adhesion and mature focal adhesion formation (Figure 10h,i). Based on these dynamic mechanical forces in the cell microenvironment, a smart piezoelectric PVDF scaffold was designed that could generate piezopotential by cell traction force after cell adhesion, activate the transmembrane calcium channels for extracellular Ca2+ influx and promote the neuron-like differentiation of stem cells (Figure 10j,k). Since the deformation of the piezoelectric PVDF scaffold only occurred after cell adhesion, the on-demand electrical stimulation was only realized in the differentiation stage, thereby avoiding the inhibitory effect of early electrical stimulation on cell adhesion and spreading. In addition, Liu et al. designed a biodegradable piezoelectric PLLA that could generate piezopotential under joint load. This batteryless electrostimulation could promote protein adsorption and cell migration or recruitment, as well as induce endogenous TGF-β, thereby improving cartilage formation and cartilage regeneration. Rabbits with severe osteochondral defects regenerated hyaline cartilage and achieved complete cartilage healing after 1 to 2 months of self-driven electrical stimulation therapy [93]. This in situ electrical stimulation based on PENGs paves the way for smart scaffold design and future bioelectronic therapies.

**Figure 10.** Cell traction-driven PENGs for cell modulation: (**a**) cell forces could bend the ZnO nanosheets of the PENG; (**b**) the micromorphology of the cells on the nanosheets; (**c**) the length of the trajectory represented the macrophage movement; (**d**) the quantification of activated Saos-2 cells with intracellular Ca2+ concentration change (\* *p* < 0.05). Reprinted with permission from ref. [89], Copyright 2017, WILEY-VCH. (**e**) Cells grown on the nanostriped PVDF; (**f**) a simulation of the nanostriped PVDF generating a piezopotential of 3.4 mV when strained by a tangential force of 10 nN; (**g**) the percentage of Tuj-1 positive cells and GFAP positive cells (\* *p* < 0.05). Reprinted with permission from ref. [90], Copyright 2019, WILEY-VCH. (**h**) A schematic diagram of the cell traction triggered, on-demand electrostimulation for neuron-like differentiation; (**i**) cell traction caused deformation along the nanofiber; (**j**) the cells grown on PVDF nanofibers with obvious transient calcium activity; (**k**) the morphology of the cells grown on the PVDF after differentiation. Reprinted with permission from ref. [92], Copyright 2021, WILEY-VCH.

#### **4. Summary and Perspectives**

In summary, EMNGs have shown promising applications in self-powered cell modulation, with impressive progress ranging from promoting cell migration, orientation and proliferation to regulating cell adhesion and differentiation (Table 1). Due to the advantages of convenience, good biosafety and patient compliance, EMNGs provide a promising approach for the clinical transformation of electrical stimulation. Generally, TENGs as an electrostimulation source need to cooperate with bioelectrodes to exert electrical stimulation to cells. By contrast, PENGs can achieve in situ wireless electrical stimulation while simultaneously acting as a biological scaffold. However, there are still great challenges and broad spaces for both basic research and clinic applications. Since the mechanical energy in the human body is weak and disordered, the structure of the nanogenerators needs to be reasonably designed and optimized to efficiently harvest the surrounding mechanical energy. Additionally, TENGs and PENGs can be hybridized with EMGs to combine the high voltage of the TENGs/PENGs and the high current of EMGs to realize multimodal electrostimulation, which is of great significance for broadening the application of EMNGs

in the biomedical field. At the same time, this combination also poses some challenges for EMG technology, such as miniaturization, flexibility and system integration, etc. [14]. The long-term stability and safety of EMNGs in vivo still need to be verified. The current research is mainly focused on the effect of the open circuit voltage of the EMNG on the cells. In fact, the cell microenvironment is a complex electrophysiological environment. The voltage actually sensed by the cell depends not only on the output of the EMNG, but also on the electrical property of the culture medium and cell membrane [89]. Clarifying the actual voltage sensed by cells is more instructive for future research. In addition, the output of the EMNG is often a pulse wave, which is different from the square wave, sine wave and other waveforms produced by traditional electric stimulators. Thus, the electrostimulation conditions of the EMNG need to be further optimized and its underlying mechanism also needs to be explored. Additionally, the application of EMNGs can be extended to other electroactive cells, such as cardiomyocytes. The contractile force (20–140 nN) of cardiomyocytes is much higher than the cell traction force (1–10 nN) [90,94], which can be utilized to drive EMNGs more efficiently. Research at the cellular level is more conducive to revealing the regulatory mechanism of EMNG-based electrostimulation, with the ultimate goal of tissue function regulation and disease treatment. Both TENGs and PENGs have the advantages of small size, flexibility and good biocompatibility. To date, many wearable and implantable EMNGs have been reported that can efficiently harvest mechanical energy from living organisms, such as from joint motion, heartbeat and respiration, which have laid the foundation for further bioelectronic implants, especially for bones, hearts and muscles with abundant mechanical activities [93,95]. With the current progress and huge development space, EMNGs show tremendous potential for cell modulation and biomedical therapeutics.



**Table 1.** *Cont.*

**Author Contributions:** Conceptualization, L.L. and Z.L.; writing—original draft preparation, Z.L. and Z.W.; writing—review and editing, L.L.; supervision, L.L.; project administration, L.L.; funding acquisition, L.L. and Z.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA16021100), the National Nature Science Foundation (No. 82072065, 81471784), the National Key R&D project from Minister of Science and Technology, China (2016YFA0202703), the National Youth Talent Support Program and the China Postdoctoral Science Foundation (No. BX2021299, 2021M703166).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## MDPI St. Alban-Anlage 66 4052 Basel Switzerland Tel. +41 61 683 77 34 Fax +41 61 302 89 18 www.mdpi.com

*Nanoenergy Advances* Editorial Office E-mail: nanoenergyadv@mdpi.com www.mdpi.com/journal/nanoenergyadv

MDPI St. Alban-Anlage 66 4052 Basel Switzerland Tel: +41 61 683 77 34

www.mdpi.com

ISBN 978-3-0365-5020-6