Symposium NM09—Nanogenerators and Piezotronics

Prof. Wang will first introduce the fundamental science, engineering approach and technological applications of nanogenerator as a sustainable, self-sufficient power source for micro-/nanosystems by harvesting energy from our body and living environment. Prof. Wang will then introduce the fundamentals of piezotronics and piezo-phototronics and to give an updated progress about their applications in energy science, electronics and sensors.

Piezotronics concerns the devices fabricated using the piezopotential as a “gate” voltage to tube/control charge carrier transport at a contact or junction. The piezo-phototronic effect is to use the piezopotential to control the carrier generation, separation, transport, and/or recombination for improving the performance of optoelectronic devices, such as photon detector, solar cell, and LED. In this tutorial, Prof. Pan will first give an in-depth introduction of the mechanisms and applications of nanowire-based piezotronics and piezo-phototronics

Prof. Zhang will introduce fundamental piezotronic theory established, piezotronic p-n junction and metal-semiconductor contact models. The class of materials includes wurtzite and two-dimentional piezoelectric semiconductors, such as ZnO, GaN, InN, CdS and monolayer MoS2.

For TENG, a power management circuit is needed to generate a low DC voltage for the load, while implementing a high voltage interface at the transducer’s side, so to maximize the converted power. Prof. Basset will present an overview of the main architectures for power management systems proposed to date for triboelectric nanogenerators, and the various circuits will be compared based on electrical simulations.

Prof. Sihong Wang will first introduce the overall needs and technological challenges for harvesting mechanical energy from human bodies, towards realizing sustainable power sources for human-integrated electronics. Then Prof. Wang will introduce the major approaches in material and device designs for achieving both high power output and multi-aspect biocompatibility, based on the requirements from different body locations.

Prof. Zi will systematically introduce the evaluation standards and standardized evaluation methods for nanogenerators. Starting from cycles of maximized energy output, the figures-of-merit will be proposed. And then, considering the breakdown effect, the maximized energy output should be modified by reducing the breakdown areas. Finally the experimental characterization methods will be introduced and the output energy density will be introduced and compared between different nanogenerators.

As main topics, Prof. Kim will introduce recent global achievements regarding highly robust and efficient TENGs with multifunctional materials, which provides new insights to an innovative energy solution for various wearable electronics, IoT sensors, bio/healthcare applications. In addition, he will share with audiences about a new wireless energy transfer technology based on ultrasound-driven TENG in a body.

Prof. Yong Qin will first elaborate on the fundamental science of flexotronic effect in centrosymmetric semiconductors, which serve as the basis for understanding and utilizing the interfacial polarization engineering in the new area of flexotronics. Then he will provide an overview of the development of flexotronics, and discuss the relationship between the piezotronics and the flexotronics. Finally, he gave a perspective about their further development and potential applications in emerging fields of enhanced solar technology, active electronics/optoelectronics, hybrid-spintronics etc.

Contact electrification (CE) (or usually called “triboelectrification”) effect, the most fundamental effect for electricity, has been known for over 2600 years since ancient Greek time, but its scientific mechanism remains unclear. The study of triboelectrification is recently revived due to the invention of the triboelectric nanogenerators (TENGs) by using the coupling of triboelectrification and electrostatic induction effects. In this talk, we first present the physics mechanism of triboelectrification for general materials. Our model is extended to liquid-solid contact electrification, reviving the classical understanding about the formation of electric double layers. Secondly, the fundamental theory of the TENGs is explored based on a group of reformulated Maxwell equations. In the Maxwell’s displacement current proposed in 1861, the term dE/dt gives the birth of electromagnetic wave, which is the foundation of wireless communication, radar and later the information technology. Our study indicates that, owing to the presence of surface polarization charges present on the surfaces of the dielectric media in TENG, an additional term dP_s/dt that is due to non-electric field induced polarization should be added in the Maxwell’s displacement current, which is the output electric current of the TENG. Therefore, our TENGs are the applications of Maxwell’s displacement current in energy and sensors.

[1] Z.L. Wang and A.C. Wang “On the origin of contact electrification“ (Review), Materials Today, 30 (2019) 34-51; https://doi.org/10.1016/j.mattod.2019.05.016
[2] S. Lin, L. Xu, A.C. Wang, and Z.L. Wang* “Quantifying electron-transfer and ion-transfer in liquid-solid contact electrification and the formation mechanism of electric double-layer”, Nature Communication, 11 (2020) 399; https://doi.org/10.1038/s41467-019-14278-9.
[2] Z.L. Wang, “On the first principle theory of nanaogenerators from Maxwell’s equations“, Nano Energy, Nano Energy, 68 (2020) 104272; https://doi.org/10.1016/j.nanoen.2019.104272

This seminar introduces three recent progresses of self-powered flexible electronic systems beyond thermal limits. The first part will introduce self-powered systems for IoT sensors and flexible energy source. Flexible nanogenerator converts external bio-mechanical movement into electrical energy for self-powered IoT and biomedical devices such as pacemaker and transportation. In addition, flexible piezoelectric materials detects the minute vibration of membrane or human skin that expands the application of self-powered acoustic sensor and healthcare monitor. The second part will introduce laser material interaction for flexible applications. Laser technology of ultra-short pulse duration becomes important for future flexible electronics since high temperature process can be adopted on plastic substrates, which is essential for high performance electronics. Exciting results of flexible laser material interaction will be explored from both material and device perspectives including nanomaterial synthesis, inorganic laser liftoff and plasmonic material reaction.
The third part will discuss flexible large scale integration (f-LSI) for flexible CPU and high density memory. Flexible LSI is an essential part of future electronics for data processing, storage, and radio frequency (RF) communication. To fabricate f-LSI, we integrated 0.18 CMOS process of single crystal silicon nano-transistors with flexible electronics. Simultaneous roll transfer and interconnection of flexible NAND Flash memory was achieved using anisotropic conductive film (ACF). Finally, we introduce the highly efficient and long-term stable flexible vertical micro LED (f-VLED) for full color displays, wearable and biomedical applications. Using optogenetic mouse models, f-VLED stimulated motor neurons deep below layer III from the brain surface and induced mouse behavior changes. These f-VLED can be also used as tools of skin research and phototherapy.

References of Keon’s corresponding authors since 2014
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Contact electrification (CE) is a well-known phenomenon in physics, in which opposite charges are created at surfaces due to continuous contact and separation processes occurring between two materials, followed by the generation of electric potentials between them. To date, CE has been exploited in a vast range of ways to affect our lives and industry, it has been used in fields such as photocopying, laser printing, and electrostatic separation. Recently, during the COVID-19 pandemic, many kinds of face masks have included filters with electrostatic charges that were created using the CE, these masks have been employed as public and personal health measures against the spread of coronavirus. Here, we will present the strategies for the sustainable output power generation of the triboelectric nanogenerator (TENG) based on the CE in terms of materials aspect by synthesizing a new dielectric, a C₆₀-containing block polyimide (PI–b–C₆₀). Two non-contact mode applications, a keyless electronic door lock system and a speed sensor with a tone wheel for a car, will be demonstrated. A new facile approach to increasing the output voltage of a thermoelectric generator that does not involve material modification will be also reported.

Triboelectric nanogenerator (TENG) is one of the most important technology to solve the problem of distributed energy supply of wireless sensor networks. However, the low charge density constrains TENG output performance and application field. Poor contact with the tribo-material interface, air breakdown effect and insufficient charge generation capability are the main factors affecting TENG charge density. In this talk, I will introduce the strategies for maximizing output charge density of triboelectric nanogenerator in our group, including the material modification, structure design and energy management, specifically, establishing criteria to quantitatively evaluate the contact status and inhibition of the interface air breakdown effect for contact/separation TENG, and the enhancement of charge generation capacity by shielding layer and device structural design of the sliding TENG.
References
1. Yike Liu, Wenlin Liu, Zhao Wang, Wencong He, Qian Tang, Yi Xi, Xue Wang, Hengyu Guo and Chenguo Hu, Quantifying contact status and air breakdown model of charge excitation TENG for maximum charge density, Nature Communications, 2020, 11:1599
2. Wencong He, Wenlin Liu, Jie Chen, Zhao Wang, Yike Liu, Xianjie Pu, Qian tang, Hengyu Guo, Chenguo Hu, Boosting output performance of sliding mode triboelectric nanogenerator by screening layer enabled charge space accumulation effect, Nature Communications, 2020 11:4277
3. Wenlin Liu, Zhao Wang, Gao Wang, Qixuan Zeng, Wencong He, Liyu Liu, Xue Wang, Yi Xi, Hengyu Guo, Chenguo Hu and Zhong Lin Wang, Switched-capacitor-convertors based on fractal designed configurations for double-function output power management of triboelectric nanogenerator, Nature Communications, (2020) 11:1883
4. Wenlin Liu, Zhao Wang, Gao Wang, Guanlin Liu, Jie Chen, Xianjie Pu, Yi Xi, Xue Wang, Hengyu Guo, Chenguo Hu, Zhong Lin Wang, Integrated charge excitation triboelectric nanogenerator, Nature Communications, 2019, 10:1426

Through years of development, the triboelectric nanogenerator (TENG) has been demonstrated as a burgeoning efficient energy harvester. Plenty of efforts have been devoted to further improving the electric output performance through material/surface optimization, ion implantation or the external electric circuit. However, all these methods cannot break through the fundamental limitation brought by the inevitable electrical breakdown effect, and thus the output energy density is restricted. Here we proposed a method for enhancing the threshold output energy density of TENGs by suppressing the breakdown effects in the high-pressure gas environment. With that, the output energy density of the contact-separation mode TENG can be increased by over 25 times in 10 atm than that in the atmosphere, and that of the freestanding sliding TENG can also achieve over 5 times increase in 6 atm. Our research demonstrated the excellent suppression effect of the electric breakdown brought by the high-pressure gas environment, which may provide a practical and effective technological route to promote the output performance of TENGs.

Nanoscale piezoelectric transducers can outperform conventional piezoelectric devices for energy harvesting (nanogenerators), sensing (e.g. piezotronics), implantable systems, wireless piezoelectric nanotransducers, and more. In fact, the properties of transducers greatly depend on dimensions due to both classic scaling laws and, at nanoscale, quantum effects. Consistently, a systematic analysis allowed to identify up to 16 possibly significant advantage of nanoscale piezoelectricity [1]. For instance, nanoscale materials can withstand larger deformations (which translate into higher piezopotentials, more robust and flexible devices and extended measurement ranges), have higher piezoelectric coefficients, offer superior mechanical force-to-displacement sensitivities and operate at higher speed. Moreover, several materials which are not piezoelectric in their bulk form due to the opposite orientations of adjacent atomic layers, are piezoelectric as single layer [1,2]. Besides, some working mechanisms are only possible at nanoscale [1,2]. Additionally, nanoscale results in more degrees of freedom for structures [3], types of mechanical input, positions of contacts, dimensionalities, and shapes [3,4]. Downscaling also uniquely offers the opportunity to fabricate piezoelectric transducers which can be used in vivo for a variety of tasks [5,6].
Despite such an impressive potential, practical applications can be complicated by many difficulties, with special reference to synthesis, characterization, modeling and device implementation. In this talk, also taking advantage of our last results, I will discuss some of the most important open challenges and possible strategies to address them, namely rational design of synthesis procedures [7], real time monitoring [8], dynamic synthesis procedures [8], 3D geometrical characterizations of quasi-1D single crystal nanostructures from conventional SEM images [9] and recent results for both modeling and synthesis.


References
[1] C. Falconi, Piezoelectric nanotransducers, Nano Energy. 59 (2019) 730–744. https://doi.org/10.1016/j.nanoen.2019.03.027.
[2] R. Hinchet, U. Khan, C. Falconi, S.W. Kim, Piezoelectric properties in two-dimensional materials: Simulations and experiments, Mater. Today. 21 (2018) 611–630. https://doi.org/10.1016/j.mattod.2018.01.031.
[3] C. Falconi, G. Mantini, A. D’Amico, Z.L. Wang, Studying piezoelectric nanowires and nanowalls for energy harvesting, Sensors Actuators, B Chem. 139 (2009) 511–519. https://doi.org/10.1016/j.snb.2009.02.071.
[4] R. Araneo, G. Lovat, P. Burghignoli, C. Falconi, Piezo-semiconductive quasi-1D nanodevices with or without anti-symmetry., Adv. Mater. 24 (2012) 4719–24. https://doi.org/10.1002/adma.201104588.
[5] C. Falconi, A. D’Amico, Z.L. Wang, Wireless nanosensors and nanoactuators for in-vivo biomedical applications, in: Eurosensors, Göteborg, 2006.
[6] C. Falconi, A. D’Amico, Z.L. Wang, Wireless Joule nanoheaters, Sensors Actuators, B Chem. 127 (2007) 54–62. https://doi.org/10.1016/j.snb.2007.07.002.
[7] G. Arrabito, V. Errico, Z. Zhang, W. Han, C. Falconi, Nanotransducers on printed circuit boards by rational design of high-density, long, thin and untapered ZnO nanowires, Nano Energy. 46 (2018) 54–62. https://doi.org/10.1016/j.nanoen.2018.01.029.
[8] A. Orsini, C. Falconi, Real-time monitoring of the solution growth of ZnO nanorods arrays by quartz microbalances and in-situ temperature sensors, Sci. Rep. 4 (2014) 1–7. https://doi.org/10.1038/srep06285.
[9] A. Mencattini, A. Orsini, C. Falconi, 3D Reconstruction of Quasi-1D Single-Crystal Nanostructures, Adv. Mater. 27 (2015) 6271–6276. https://doi.org/10.1002/adma.201503522.

Piezoelectric materials and composites enable a wide range of energy harvesting and sensing applications owning to their intrinsic capability of converting mechanical energy to electrical energy and vice versa. Additive manufacturing (AM), also known as 3D printing, is thriving as a category of effective and robust techniques in manufacturing three-dimensionally (3D) architected piezoelectric composites comprising of inorganic/organic piezoelectric materials, and these architected structures possess the potential of delivering anisotropic piezoelectric responses upon specific structural designs. Nonetheless, the serial nature of the additive building processes results in the inherent speed-accuracy trade-off, which seriously limits the scalability and efficiency of manufacturing functional devices that require precise control of fine features. As a result, the reported progress in this field either limit themselves to macroscale devices due to relatively low printing resolution, e.g, fused deposition modeling (FDM), or require extensive amount of fabrication time, e.g., projection micro- stereolithography (PμSL). On the contrary, micro continuous liquid interface production (μCLIP), a recently developed 3D printing technology, has been shown to create 3D geometries at very high speeds, with good surface finish, and uniform mechanical properties.
Herein, we report rapid 3D printing of architected barium titanate (BTO) composite structures with high resolution by using mCLIP. The BTO nanoparticles were surface functionalized prior to mixing to enable stable dispersion in poly(ethylene glycol) diacrylate (PEGDA). Resins with up to 25 wt% of functionalized-BTO (f-BTO) nanoparticles were prepared and printed continuously to yield a variety of architected structures with unprecedented longitudinal printing speeds of up to ~ 25 μm/s. The architected structures were tested with a customized piezoelectric characterization setup and demonstrate effective piezoelectric constant d₃₃ of 37.9 pC/N, and piezoelectric voltage constant g₃₃ of 0.92 Vm N^-1, which is comparable to other reported work. This work not only enables the rapid 3D printing of architected piezoelectric structures with tailorable properties via compositional and structural modifications, but also intrinsically enlightens its potential capability for eco-friendly and biocompatible applications.

Soft piezoelectric materials are an important group of functional material for state-of-the-art energy harvesting, energy conversion and sensing technologies. Driven by the rapid evolution of internet of things and biomedical engineering, current energy and senor technologies, particularly those interfacing with human beings, are placing tremendous emphasizes on the flexibility, processability, conformality and biocompatibility to the piezoelectric components, in addition to their high performance. While polyvinylidene difluoride (PVDF) has been a well-known and broadly used piezoelectric polymer material over years, their mechanical property, piezoelectric performance and processability still cannot meet the high requirements of wearable devices. Recent years see a surge of research efforts on PVDF-based nanostructures to interface with biological systems for energy or sensing applications. However, unlike ceramic-based piezoelectric materials, to reach desired high piezoelectric property and long-term stability in biological environment still stand as a big challenge in PVDF soft nanomaterial development.
In this work, we report a one-step strategy for fabricating core/shell polyvinylidene difluoride (PVDF)/dopamine (DA) nanofibers (NFs) with a very high β-phase content and self-aligned polarization. The self-assembled core/shell structure is believed essential for the formation and alignment of β-phase PVDF, where strong intermolecular interaction between the -NH₂ groups on DA and -CF₂ groups on PVDF is responsible for aligning the PVDF chains and promoting β-phase nucleation. As-received PVDF/DA NFs exhibit significantly enhanced piezoelectric performance and excellent stability and biocompatibility. An all-fiber-based soft sensor is fabricated and tested on human skin and in vivo in mice. The devices show a high sensitivity and accuracy for detecting weak physiological mechanical stimulation from diaphragm motions and blood pulsation. This sensing capability offers great diagnostic potential for the early assessment and prevention of cardiovascular diseases and respiratory disorders.

Electrostimulation (ES) has long been utilized as a versatile therapeutic strategy for treating human diseases. The nanogenerator technology stands at a unique position for connecting biological ES to body motions by converting biomechanical energy to electric pulses. This process forms a closed-loop ES in both energy flow and function feedback. This talk will first elaborate how the closed-loop ES operates as a close analogue to how the body controls its own functions. The most recent progresses of flexible and complex piezoelectric material innovations in our group will be viewed for implantable nanogenerator development, including 3D printing techniques and degradable piezoelectric biomaterials. These materials exhibited tissue-matching mechanical property as well as excellent electromechanical coupling properties, ideal for in situ electrostimulation generation. In our recent research, we also successfully implemented the closed-loop electrostimulation concept for skin wound healing, hair growth, bone healing and vagus nerve stimulation for obesity control with superb performance. An electrical stimulation bandage was developed that could accelerate skin wounds recovery to within 3 days as compared to 12 days of usual contraction-based healing processes in rodents. Via a similar design, the electrical pulses were applied to stimulate hair growth. In another work, an implanted vagus nerve stimulation system was developed in responsive to the peristalsis of stomach. The electric signals generated by this device stimulates the vagal afferent fibers to reduce food intake and achieve weight control. With the advanced electromechanical material and device design and engineering, the closed-loop electostimulation concept will quickly grow into a very impactful application direction of nanogenerator technology for human wellbeing.

Contact-electrification is a universal effect for all existing materials, but it still lacks a quantitative materials database to systematically understand its scientific mechanisms. Using an established measurement method, this study quantifies the triboelectric charge densities of nearly 30 inorganic nonmetallic materials. From the matrix of their triboelectric charge densities and band structures, it is found that the triboelectric output is strongly related to the work functions of the materials. Our study verifies that contact-electrification is an electronic quantum transition effect under ambient conditions. The basic driving force for contact-electrification is that electrons seek to fill the lowest available states once two materials are forced to reach atomically close distance so that electron transitions are possible through strongly overlapping electron wave functions. We hope that the quantified series could serve as a textbook standard and a fundamental database for scientific research, practical manufacturing, and engineering.

Devices that facilitate nonverbal communication typically require high computational loads or have rigid and bulky form factors unsuitable for use on the face or on other curvilinear body surfaces. Here, we report the design and pilot testing of an integrated system for the decoding of facial strains and for predicting facial kinematics. The system consists of mass-manufacturable, conformable piezoelectric thin films for strain mapping, multiphysics modelling for the analysis of the nonlinear mechanical interactions between the conformable device and the epidermis, and three-dimensional digital image correlation for the reconstruction of soft-tissue surfaces under dynamic deformations and for informing device design and placement. In healthy subjects and in subjects with amyotrophic lateral sclerosis, we show that the piezoelectric thin films, coupled with algorithms for the real-time detection and classification of distinct skin-deformation signatures, enable the reliable decoding of facial movements. The integrated system could be adapted for use in clinical settings as a nonverbal communication technology or for use in the monitoring of neuromuscular conditions.

The triboelectric charges on the human body are generated by the contact electrification of the human skin and other surfaces. The mechanism behind this is that the human skin has a very positive charge affinity due to its surface biological composition. Such a high positive charge affinity leads to electron transfer between the skin and other materials when the physical contact-separation process is happening.

Utilizing such unique properties of the human skin, we have constructed different types of triboelectric nanogenerators (TENGs) including human body based TENGs (H-TENGs) for harvesting energy from body motions with maximum output power density above 30 W/m². Moreover, due to the unique charge generating process of the body, we have developed new types of body motion sensors that can sense the movements of the human body. Such sensors have great applications in healthcare and security areas. Beyond these, one can also create human-machine interactions based on the charges generated on the human body.

Our studies have discovered the fundamentals of the triboelectricity of the human body, as well as future applications in different areas. New types of human-machine interaction prototype are expecting to be developed in the future.

References:

(1) Zhang, R.; Hummelgård, M.; Örtegren, J.; Yang, Y.; Andersson, H.; Balliu, E.; Blomquist, N.; Engholm, M.; Olsen, M.; Wang, Z. L.; Olin, H. Sensing Body Motions Based on Charges Generated on the Body. Nano Energy 2019, 63, 103842. https://doi.org/10.1016/j.nanoen.2019.06.038.
(2) Zhang, R.; Hummelgård, M.; Örtegren, J.; Olsen, M.; Andersson, H.; Olin, H. Interaction of the Human Body with Triboelectric Nanogenerators. Nano Energy 2019, 57 (October 2018), 279–292. https://doi.org/10.1016/j.nanoen.2018.12.059.
(3) Zhang, R.; Hummelgård, M.; Örtegren, J.; Olsen, M.; Andersson, H.; Yang, Y.; Olin, H. Human Body Constituted Triboelectric Nanogenerators as Energy Harvesters, Code Transmitters, and Motion Sensors. ACS Appl. Energy Mater. 2018, 1 (6), 2955–2960. https://doi.org/10.1021/acsaem.8b00667.
(4) Zhang, R.; Örtegren, J.; Hummelgård, M.; Olsen, M.; Andersson, H.; Olin, H. Harvesting Triboelectricity from the Human Body Using Non-Electrode Triboelectric Nanogenerators. Nano Energy 2018, 45, 298–303. https://doi.org/10.1016/j.nanoen.2017.12.053.

Piezoelectric composites based on PZT ceramic rods or fibers have emerged long time ago for ultrasonic applications [1]. With the progress in nanotechnologies, similar composites incorporating piezoelectric nanowires have been manufactured for mechanical energy harvesting, generally called nanogenerators [2]. These devices have attracted increasing research interest, as possible micro-power supplies for wireless and mobile electronics, with the growing importance of the Internet of Things (IoT) [2,3].
Here, we will present an original design as well as a modelling strategy of nanogenerator devices. The flexible nanogenerator is manufactured on a polydimethylsiloxane (PDMS) substrate. It consists in a multilayer device with a Ti/Au bottom electrode, a nucleation layer, a zinc oxyde (ZnO)/parylene C composite layer, a Ti/Al top electrode, and a final PDMS encapsulation [4]. ZnO nanowires are synthesized using a low temperature Chemical Bath Deposition (CBD) process (85°C), compatible with various substrates and transferable to industrial processes.
Some of the key properties of the nanogenerator constitutive elements have been studied and optimized: the flexible substrate [4], the bottom electrode layer and the nucleation layer – which may be the same layer and, indeed, the ZnO crystalline quality. The ZnO nanowires properties have been adjusted through the CBD growth conditions (temperature, duration, reactants) [5,6] and the nucleation layer type and thickness (Au, ZnO or Al-doped ZnO) [7,8].
A dedicated test bench has been designed in order to measure the electrical power or energy delivered to an external circuit. Average electrical power and RMS voltage are measured thanks to a double buffer amplifier [9]. The nanogenerator produces a 0.4 µW/ cm² average power (or 2 µW/ cm² peak power) for a 6N force applied at 5Hz [10]. The force sensitivity of the device is 2 V/N at 5Hz. In order to determine the doping properties of the ZnO nanowires, single nanowire – Field Effect Transistors (FETs) were manufactured and characterized [5]. A 10¹⁸ cm^-3 doping level is estimated for pristine nanowires, decreasing by two orders of magnitude thanks to thermal annealing of the nanowires in air. This intrinsic doping, resulting from native defects, is increasing with HMTA (hexamethylenetetramine) concentration used for CBD growth. The ZnO doping values estimated thanks to single nanowire FETs have been correlated to photoluminescence spectroscopy results [6].
In terms of modelling, an analytical model and a Finite Element (FE) model have been developed depending on the simulation purpose. In particular, the analytical model takes into account the possible wide frequency band of the mechanical excitation and allows the prediction of the harvested power for a mechanical force with an arbitrary time-domain form [11]. The FE model of a single cell – with appropriate symmetry conditions – containing a piezoelectric and semiconducting material embedded into a dielectric polymer matrix, on a given rigid or flexible substrate, provides a better understanding of the electromechanical behavior of the transducer. Based on this model, energetic considerations are discussed, like conversion rate or electromechanical coupling factor.

References
[1] R.E. Newnham et al., Mat. Res. Bull. 13 (1978) 525-536
[2] L. Jin et al., Nano Energy 66 (2019) 104086
[3] A.S. Dahiya et al., Journal of The Electrochemical Society 167 (2020) 037516
[4] A. S. Dahiya et al., Advanced Materials Technologies (2017) 1700249
[5] S. Boubenia et al., Scientific Reports 7 (2017) 15187
[6] A. S. Dahiya et al., Nanoscale Research Letters (2018) 13:249, 9 pp.
[7] T. Slimani Tlemcani et al., Chemosensors 7, 7 (2019)
[8] C. Justeau et al., Materials 12 (2019) 2511
[9] K. Nadaud et al., Appl. Phys. Lett. 112 (2018) 063901
[10] S. Boubenia, PhD thesis, Université de Tours, 4^th July 2019
[11] K. Nadaud et al., Mechanical Systems and Signal Processing 133 (2019) 106278

Due to their large number of properties, such as switchable polarization, piezoelectricity or optical absorption, ferroelectrics are particularly promising in order to develop the next generation of electronic microdevices. Among these ferroelectrics, lead zirconium titanate (PZT) has been widely investigated for its high electromechanical properties and more recently for its light-induced deformation¹, commonly named photostriction, combining simultaneously bulk photovoltaic and converse piezoelectric effects². Although this phenomenon was discovered in multiple ferroelectrics materials these last decades³, the photostriction is not fully understood and thus requires further studies in order to promote the future electronic microdevices.
In this work, the interplay between the electric polarization and the photoinduced strain is investigated in PZT epitaxial-based cantilevers. For this purpose, the resonance frequencies of the cantilevers and their deflection were studied under UV illumination, through electrical measurements and optical profilometry respectively. The recent results have shown that the sign of both the photoinduced shift of resonance frequency and photoinduced deflection strongly depend on the polarization state in the ferroelectric thin film.

¹ S. Matzen et al., « Tuning Ultrafast Photoinduced Strain in Ferroelectric-Based Devices », Adv. Electron. Mater., p. 1800709, mai 2019, doi: 10.1002/aelm.201800709.
² C. Paillard, B. Xu, B. Dkhil, G. Geneste, et L. Bellaiche, « Photostriction in Ferroelectrics from Density Functional Theory », Physical Review Letters, vol. 116, n^o 24, juin 2016, doi: 10.1103/PhysRevLett.116.247401.
³ B. Kundys, « Photostrictive materials », Applied Physics Reviews, vol. 2, n^o 1, p. 011301, mars 2015, doi: 10.1063/1.4905505.

Understanding the fundamental mechanism of contact electrification has been the Holy Grail for physicists for centuries. The direct-current generation in semiconductor-based dynamic contacts is a newly discovered triboelectric phenomenon, where multi-scale interaction and multi-physics coupling of contact interfaces under non-equilibrium condition are involved. At microscopic level, mechanical modulation is exerted at local single asperities due to surface roughness. Lattice distortion of a dynamic contact could be extremely large considering a high local pressure at GPa level at the single asperity. As a result, one would expect an ultrahigh ‘local’ temperature as high as 1000 K given rise by the rigorously perturbed surface atoms. In an effort to elucidate the physical picture of the energy conversion process, we investigate the long-existing problem from different scales and different angles: At microscopic level, the contribution of flexoelectric effect due to strain gradient on the interfacial electric field; At atomic level, the contribution of surface dipole formation due to perturbation of surface to the interfacial electric field; At quantum level, the contribution of non-adiabatic electron-phonon interaction on the electronic excitation. It is proposed that those factors should be considered comprehensively, in addition to the simple charge transfer theory based on thermodynamic fermi-level alignment, in order to understand the fundamental mechanism of contact electrification.

Personalized real-time health monitoring in non-clinical environments can significantly improve the life quality by regularly evaluating health status, immediate warning of abnormality, and remote diagnosis. The continuous detection of various vital signals such as respiration, perspiration, and temperature have been realized by wearable sensors. Heart rate is another critical parameter which is closely related to cardiovascular health. Cardiovascular disease is one of the leading causes of death globally, which costs more than $219 billion each year in healthcare service, medicine, and loss of productivity. The precise continuous monitoring of pluse waveform is critical for assessing overall cardiovascular health and the state of the autonomic nervous system (ANS) responsible for regulating cardiac activity.
The emerging frontiers in real-time cardiovascular health monitoring demand that the corresponding sensors be biocompatible, wearable, and self-sustainable. The capability of sensor systems to efficiently scavenge the stray, weak environmental energies through sustainable pathways would enable self-powered bioelectronic systems. Triboelectric nanogenerators (TENG) can effectively transform the otherwise wasted environmental, mechanical energy into electrical power. Recent advances in TENGs witness a significant boost in the output performance. However, obstacles hindering the development of efficient triboelectric devices based on biocompatible materials continue to prevail. Being naturally degradable, biocompatible, low-cost, and lacking in cytotoxicity, chitosan and polyvinyl alcohol (PVA) are two materials with great potential being functional constituents in biomedical devices.
Here, we present for the first time the holistic engineering of chitosan and PVA with tunable mechanical properties, biodegradable rates, and electrical outputs for wearable TENGs, through revealing and understanding the interactions between the constituent materials and the structure-property-performance relations.^{[1, 2]} The feasibility of laser processing of biopolymers was explored for the first time to improve the TENG performances. Leveraging the high mechanical deformability and biocompatibility of the constituent materials, the optimized triboelectric devices built with PVA composite films and chitosan composite films exhibit stable and robust triboelectricity outputs. Such wearable devices are capable of detecting the imperceptible degree of skin deformation induced by human pulse and capture the cardiovascular information encoded in the pulse signals with high fidelity for self-powered cardiovascular health monitoring. The gained fundamental understanding and demonstrated capabilities are expected to enable the rational design and holistic engineering of novel materials for more capable biocompatible triboelectric devices that can continuously monitor vital physiological signals for self-powered health diagnostic and therapeutic systems.
[1] R. Wang, S. Gao, Z. Yang, Y. Li, W. Chen, B. Wu and W. Wu, Adv. Mater. 2018, 30, 1706267.
[2] R. Wang, L. Mu, Y. Bao, H. Lin, T. Ji, Y. Shi, J. Zhu and W. Wu, Adv. Mater. 2020, 32, 2002878.

In this communication, we show an innovative approach to the production of piezoelectric nanogenerators by combining polycrystalline and texturized ZnO layers fabricated at room temperature by plasma enhanced chemical vapor deposition with flexible small-molecule organic nanowires (ONWs) acting as 1D and 3D soft-template. We address the one-wire / one-device concept by developing a core-multishell piezoelectric nanogenerator including gold and silver shells acting as inner nanoscopic electrodes.^[1] Besides, this nanoarchitecture is integrated in high density in a contact-separation mode triboelectric nanogenerator. We aim to demonstrate the advantages of the application of plasma assisted deposition and plasma nanoengineering to optimize the output power of the hybrid nanogenerator at two levels, namely, the nanoengineering of the piezoelectric nanowires and the plasma treatment and functionalization of the triboelectric surfaces. On the one hand, we discuss the role of the piezoelectric shell microstructure, texture, and hierarchical formation of nanotrees. On the other hand, the plasma assisted fluorinated functionalization of PDMS is evaluated as a sustainable approach to increase the electronegativity and hydrophobicity of the negative friction layer. For the optimized vertical separation-contact TENG on ITO/PET, the measured output on manual pressing stimulation reached values higher than 75 V of open-circuit voltage and 0.5 μA of short-circuit current. A setup for the generation of reproducible mechanical stimuli either in frequency or amplitude was developed and applied, which allowed us to obtain a characteristic frequency for the maximal output of the device. Finally, the wetting properties of TENGs surfaces have been tuned by controlling the surface micro and nanoroughness and superhydrophobic hierarchical structures ranging from Petal to Lotus mechanisms. The liquid-solid electrification and its application for fabricating a single electrode TENG have been exploited to harvest energy from microdroplets' impacts and sliding.

High-current density dynamic Schottky junctions hold great promise for ambient energy harvesting, which overcome the transient and limited current output as faced by traditional piezoelectric/triboelectric nanogenerators. However, self-powered applications for wearable/implantable electronics call for excellent flexibility for the device, whereas the materials of dynamic Schottky junction being used hitherto are inorganic bulk materials with poor flexibility. In addition, the low open-circuit voltage and high-cost scaling up have hampered its further implement for practical applications in self-powered electronic devices. Here, we design and optimize the scalable DC generators with high power output as well as excellent flexibility. The flexible DC power generator exhibits a robust mechanical stability and durability under ambient temperature. The design of flexibility in such DC energy generator is inherited from the long chain polymer concept. The Schottky DC power generation nature provides such flexible generator with high current density performance without rectification. The demonstration of scalable and flexible of high current density DC energy generator opens up energy conversion applications with remarkable power output performance (approaching mA level of short-circuit current, and 1 volt open-circuit voltage for single unit) enabled by low frequency movement, high flexibility, and excellent durability performances.

Triboelectric energy harvesting from ambient mechanical sources relies on motion-generated surface charge transfer between materials with different electron affinities. In order to achieve highly efficient energy harvesting performance, choosing materials with a high surface charge density is crucial, and odd-numbered polyamides (Nylons), such as Nylon-11, are particularly promising due to their strong electron-donating characteristics and the possibility to achieve dipolar alignment leading to high surface potential. The use of Nylon-11 as a material for triboelectric energy harvesting has been rather limited due to the extreme processing conditions required for film fabrication, and the high-voltage poling process required for dipole alignment. However, several methods to achieve “self-poled” Nylon-11 nanowires via facile nanoconfinement techniques have been demonstrated recently, leading to highly efficient Nylon-11 nanowire-based triboelectric nanogenerators. In this talk, I will discuss the most recent advances in the fabrication of Nylon-11 nanowires, with a focus on how nanoconfinement-based fabrication methods can be used to control phase and crystallinity. These growth methods lead to self-poled nanowires without the requirement for subsequent electrical poling, facilitating their integration into triboelectric energy harvesting devices. Strategies to fabricate Nylon-11 nanowires for applications in triboelectric devices can be extended to other polymeric families as well, giving rise to improvements in energy harvesting performance.

[1] "Unprecedented dipole alignment in α-phase nylon-11 nanowires for high-performance energy-harvesting applications" Y. S. Choi, S. K. Kim, M. Smith, F. Williams, M. E. Vickers, J. A. Elliott and S. Kar-Narayan*, Science Advances 6, eaay5065 (2020), DOI: 10.1126/sciadv.aay5065

Adding a switch at the output of a TENG or in its conditioning circuit largely promotes the energy harvesting efficiency. However, the switch has to be self-activated with a minimum consumption, so electronic switches are not suitable. We propose to apply MEMS technologies to develop a self-sustained, high-voltage and well-controlled narrow-hysteresis switch for high-efficiency energy transfer.
Our plasma switch is made of bulk silicon. It is part of a 2-stage conditioning system with a first stage including a ~35cm² flexible TENG and a Bennet doubler circuit, plus a second stage including the MEMS switch and a buck converter. The switch includes a movable anode connected to the Bennet output, a fixed cathode connected to the buck input, and a ground reference. At start, the switch is OFF and the voltage progressively increases across the rectifier capacitance C_buf, while an electrostatic force pulls the anode close to the cathode. According to Paschen’s Law, if the plasma breakdown voltage is higher than the physical-contact voltage, a contact occurs between the electrodes and a full-hysteresis ON-OFF loop appears because the contact current is high enough to release all the energy from C_buf. If the breakdown voltage is lower than the physical-contact voltage, micro-plasma occurs before contact, leading to a narrow-hysteresis ON-OFF loop because the low plasma current cannot fully release the energy stored in C_buf due to the end of the plasma.
We used a 2-stage circuits with a Bennet doubler and a MEMS switch having an initial gap of 6µm and 9µm to charge a larger capacitor C_store, or using only a full-wave rectifier. Defining the charging efficiency as the energy per cycle in C_store at 4.5V (the regulator’s operation voltage) divided by the charging time to this voltage, we can conclude that using the 2-stage conditioning circuit with a 6μm-gap MEMS switch improves the energy transfer efficiency by 145 times compared to only a full-wave rectifier. With the 6µm-switch, contact occurs with a full hysteresis, while with the 9µm-switch a µ-plasma is obtained with a very narrow hysteresis. Despite its full-hysteresis, the 6-µm switch has a better energy conversion per mechanical cycle without any load because of its much higher ON voltage. However, in a practical implementation with a commercial regulator and a 660kW load, the circuit with the 6µm-switch has many glitches at the regulated output, on the contrary to the one with the 9µm-switch. This is explained by the longer time for recharging C_buf induced by the full-hysteresis that drastically decreases the mean value of the voltage across C_buff Therefore, the switch can be further improved by designing a 6µm-switch with plasma discharge and no contact.

One of the essential components for practical realization of the smart wearables, is an efficient and standalone power scavenging system. Photovoltaics and wireless power transfer are widely used for such purposes. However, these techniques depend on the presence of an external energy source which poses certain limitations on their practical deployment. Alternatively, conversion of mechanical motion to electrical energy can be considered as a practical and standalone method to generate power for wearable electronics systems such as smart contact lenses. Due to its high output power density, triboelectric power generators are an attractive choice for such purposes. Here, we have investigated a slide triboelectric power generation of liquid-solid movements with respect to each other. Our results confirm the feasibility of supplying power for biomedical electronics just by natural human muscle motions. In this talk we present a triboelectric generator driven by eye blinks suitable for powering up smart contact lenses. A human’s average eye blinking speed is approximately 200 mm/s[1]. Furthermore, an adult blinks about 12 times per minute[2].
One of the main shortcomings of the common solid-solid triboelectric power generators is the materials’ surface properties. Micro and nanoscale surface roughness, together with surface destruction by the friction effect, deteriorate the performance of the solid-solid triboelectric devices. However, in the present study, we have used the liquid as one side of our slide electrification system. Having liquid as one side of the mechanical to electrical power generators, not only benefits from touching all possible surface area of the solid’s surface but also offers a higher lifetime to the triboelectric power generation devices. The solid plate of our device is made up of a grating aluminum (Al) with a surface area and spacing of 1×5 mm2 and 5 mm, respectively. 300 nm of silicon dioxide (SiO2) was sputtered, followed by coating 800 nm of polytetrafluoroethylene (PTFE). SiO2 prevents any leakage current from the slide electrification process. In addition, thin layer of PTFE is used to make the solid’s surface hydrophobic and prevent the wetting of the surface by the motion of the liquid. The liquid side is composed of a simple Al electrode and different types of liquids. Results confirm that the motion of the liquid with respect to the solid plate generates an alternative voltage which is due to the capacitor behavior. Furthermore, the liquid’s ion concentration and the movement speed determine the output generated voltage of the liquid-solid triboelectric system.
A noise-free, battery operated digital oscilloscope (Yeapock ADS1013D) with the input impedance of 1 MΩ was used for the output voltage measurements. In addition, an operational amplifier (ST Microelectronics LM358D) was configured as a unity-gain voltage follower which buffered our device to detect the open-circuit voltage of the triboelectric power generators, properly. We have used different NaCl concentrations (0, 0.01, 0.1, and 1M) as the liquid part of our triboelectric power generator. Higher NaCl concentration results in higher output voltage. At the movement speed of 30 mm/s, deionized (DI) water without NaCl shows no output voltage
while the same value is outstandingly rising to 3.7 V by adding 1 M NaCl to the DI water. Using the commercially available moisturizing eye drop as the liquid part of the slide electrification device at the movement speed of 30 mm/s results in an output voltage of 3.7 V and 4.7V at of 200 mm/s. The calculated effective power density was ~ 0.4 Wm-2.
References
[1] K.-A. Kwon et al., “High-speed camera characterization of voluntary eye blinking kinematics,” J. R. Soc. Interface, vol. 10, no. 85, p. 20130227, Aug. 2013, doi: 10.1098/rsif.2013.0227.
[2] Physiology of the Eye. Elsevier, 1992.

There is nothing more personal than healthcare. Health care must move from its current reactive and disease-centric system to a personalized, predictive, preventative and participatory model with a focus on disease prevention and health promotion. As the world marches into the era of Internet of Things (IoT) and 5G wireless, technology renovation enables industry to offer a more individually tailored approach to healthcare with more successful health outcomes, higher quality and lower costs. However, empowering the utility of IoT enabled technology in personalized health care is still significantly challenged by the shortage of cost-effective and wearable biomedical devices to continuously provide real-time, patient-generated health data. Textiles have been concomitant and playing a vital role in the long history of human civilization. In this talk, I will introduce our current research on smart textiles for biomedical monitoring and personalized diagnosis, textile for therapy, and textile power generation as an energy solution for the future wearable medical devices.

Flexible and wearable devices with the capabilities of both detecting and generating mechanical stimulations are critical for applications in human-machine interfaces, such as augmented reality (AR) and virtual reality (VR). Herein, a flexible patch based on a sandwiched piezoelectret structure is demonstrated to have a high equivalent piezoelectric coefficient of d₃₃ to selectively perform either the actuating or sensing function. As an actuator, mechanical vibrations with peak output force of more than hundreds of mN have been produced, similar to those from the vibration mode of a modern cell phone and can be easily sensed by human skin. As a sensor, both high sensitivity of 1.84 Pa for sensing resolution and excellent stability of less than 1% variations in thousands of cycles have been achieved. The design principle together with the sensing and driving characteristics can be further developed and extended to other soft matters and flexible devices.

Functional polymers provide unique advantages in flexible electronics to improve their electrical performance, flexibility and functions. An integration of flexible energy harvesting and storage components enables a self–powered system that can simultaneously harvest solar/mechanical energy and store electricity for soft robotics, the internet of things and portable electronics. The introduction of functional polymers into a self-powered system pave a new direction for the multifunctional applications in energy harvesting, storage and mechanosensing.

1. R. Liu; M Takakuwa, A Li, et al. An efficient ultra–flexible photo–charging system integrating organic photovoltaics and supercapacitors. Adv. Energy Mater. 2020, 10, 2000523
2. R. Liu, X. Kuang, J. Deng, et al. Shape memory polymers for body motion energy harvesting and self powered mechanosensing. Adv. Mater. 2018, 30, 1705195.
3. R. Liu, J. Wang, T. Sun, et al. Silicon nanowire/polymer hybrid solar cell-supercapacitor: a self-charging power unit with a total efficiency of 10.5%. Nano Lett. 2017, 17, 4240.
4. R. Liu, Y. Liu, H. Zou, et al. Integrated solar capacitors for energy conversion and storage. Nano Res. 2017, 10, 1545.

In-situ converting mechanical energy to electricity is a feasible solution to satisfy the increasing power demand of wearable electronics and Internet of Things (IoTs). Triboelectric nanogenerator (TENG) is considered as a potential solution via building self-powered systems. With its pulsed alternating-current (AC) output characteristics, a conventional TENG, based on triboelectrification effect and electrostatic induction, always needs rectification and energy storage units to obtain a constant direct-current (DC) output to drive electronic devices. Here we report a next-generation TENG, which realizes constant current (crest factor ~ 1) output for the first time by coupling triboelectrification effect and electrostatic breakdown, which has similar essence of lightning. Meanwhile, its output charge density can be improved by nanostructure design, working temperature and atmosphere control. Until now, its charge density is reach ~5.4 mC m^-2, which is over 2-fold the state-of-art of TENGs. The novel direct-current triboelectric nanogenerator (DC-TENG) is demonstrated to power electronics directly without any rectifier or energy storage units. Our findings not only promote the miniaturization trend of the self-powered system used in wearable electronics and IoTs, but also provide a paradigm shifting technique to harvest mechanical energy efficiently.

Tribotronics has attracted great attentions as a new research field about the control and tuning of semiconductor transport by triboelectricity. Here, the tribotronics is firstly reviewed for active mechanosensation and human-machine interfacing. As the fundamental component, contact electrification field-effect transistor is analyzed, in which the triboelectric potential could be used to control the electrical transport in semiconductors. On the basis, several tribotronic functional devices have been developed including tribotronic smart skin, tactile sensing array and tuning diode, which has demonstrated triboelectricity-controlled electronics and established the active mechanosensation for external environment. In addition, the triboelectric power management strategy is also reviewed, in which the triboelectricity can be managed by electronics in reverse action. With the implantation of triboelectric power management module, the harvested triboelectricity by various kinds of human kinetic and environmental mechanical energy could be effectively managed as a power source for self-powered microsystems. By the research prospects for interactions between triboelectricity and semiconductor, tribotronics is expected for significant impacts and potential applications in MEMS/NEMS, flexible electronics, robotics, wireless sensor network, and Internet of Things.

The voltage output of TENG without a rectifier and a capacitor intrinsically displays in sharp AC spikes of several hundred volts. However, numerous functional devices demand continuous DC voltage from the triboelectric power source. Two components are needed for TENG generating a DC voltage (DC-TENG): a rectifier, and a capacitor. Many researches on TENG have employed the conventional silicon diodes to obtain unipolar peaks. Utilizing typical silicon diodes and commercially available aluminum oxide capacitors has been a common way of converting AC voltage to constant voltage, but the integrated triboelectric system based on the hard materials restricts flexibility and patterning. Direct application of the energy harvester as wearable device necessitates a standalone DC-TENG system with flexibility as well as printability. This talk presents the conversion of the spike voltage to continuous DC voltage with the integrated triboelectric system by printing all components of TENG, rectifier, and capacitor on the flexible substrate. And also it demonstrates the self-powered electrochromic display on a single flexible substrate and show the change in color due to the charge accumulation in the ion gel capacitor unit. It holds potential to realize completely wireless and wearable self-power in the future.

Awareness of diverse tactile experiences has been a challenge toward greater human-machine interaction, which requires its full comprehension for both static and dynamic stimuli. Recent advances in triboelectricity-based touch sensors take great advantages in regard of cost, simplicity of design, and use of a wider range of materials. Due to their reliance on the level of contact electrification at the interface of materials, they encounter major difficulties to measure the extent of deformation of materials under a given mechanical force. Here, we describe an ion-doped gelatin hydrogel-based touch sensor, identifying not only contact with an object but also deformation under a certain level of force. Switchable ionic polarization plays an essential role to allow different sensing mechanisms when the touch sensor is contacted and deformed. We successfully demonstrate that this sensor provides dynamic deformation-responsive signals that can be used to control the motion of a miniature car, thereby broadening the potential applications in a human-machine communication system.

Currently, technological progress has pushed increasingly miniaturized devices and systems, stimulating a search for nanoscale piezoelectric materials. Taking the mechanical property with large elastic strain limits, excellent flexibility and the observed piezoelectricity into account, it is expected to develop diverse devices based on the 2D layered nanosheets and their heterostructures, giving rise to ultrathin device technologies towards piezoelectric nanogenerators, actuators, piezo-phototronics, active human-machine smart sensors and self-powered stretchable/wearable devices. Noted that the out-of-plane and in-plane piezoelectricity found in mono- and multilayered α-In₂Se₃ nanosheets offers an opportunity to enable a variety of piezoelectric devices. In my group, we design several kinds of In₂Se₃ layered nanosheet based heterostructures. We have demonstrated the high tunability of In₂Se₃ Raman spectra and PL characteristics under compressive biaxial strain induced by the piezoelectric substrate. The controllable biaxial strain is experimentally imposed on α-In₂Se₃ nanosheets by an electromechanical device. By precisely in situ modulating bias voltage on the substrate, the biaxial strain can be generated and delivered to In₂Se₃ nanosheets, resulting in the prominent shift in Raman and PL spectra. The vibrational behavior of In₂Se₃ is examined based on the Grüneisen parameter. On the other hand, we have demonstrated superior out-of-plane piezoelectric coefficient d₃₃ for In₂Se₃/MoS₂ vdW heterostructure. By combining the experimental studies and density functional theory calculations, the enhanced piezoelectric properties of In₂Se₃/MoS₂ are ascribed to a larger interfacial dipole moment result from the type II band alignment. The high-precision piezoelectric actuators in 2D atomic scale are conceived based on the out-of-plane piezoelectricity of In₂Se₃ based heterostructure. Lastly, we present enhanced performance in the photodetectors based on In₂Se₃ van der Waals heterostructure according to the piezo-phototronic effect. Consequently, our design and strategies enable us not only to achieve enhanced piezoelectric response in the In₂Se₃ nanosheets, but also to offer the possibility for realizing various high-performance devices. The works are supported by the grants from RGC GRF (PolyU153025/19P), CRF (C7036-17W) and NSFC (No. 51972279).

Air-transfer pathogens can cause severe epidemics (e.g., COVID-19), which shows a substantial public health threat. Inactivation of the pathogenic microbes in the air is an essential process, whereas the feasibility of existing air disinfection technologies has encountered obstacles including only achieving physical separation but no inactivation, noticeable pressure drops, and energy intensiveness. Here we report a rapid disinfection method for inactivating air-transfer bacteria and viruses using the nanowire-enhanced localized electric field to damage microbes' outer structures. This air disinfection system is driven by a triboelectric nanogenerator that effectively converts mechanical vibration to electricity and achieves self-powered. Assisted by a rational design for accelerated microbes' accelerated charging and trapping, this self-powered air disinfection system promotes microbial transport. It achieves high performance: >99.99% microbial inactivation within 0.025s in a fast airflow (2 m/s) while only causing low pressure drops (<24 Pa). This rapid, self-powered air disinfection method may fill the urgent need for the air-transmitted microbial inactivation to protect public health.

Development of biocompatible, biodegradable polymer with tunable properties is essential for multifunctioning implantable electronic devices (IEDs). To address this issue, we propose nature-derived polysaccharide, Kappa Carrageenan-Agar composite (kC-Agar composite) based fully biodegradable triboelectric nanogenerator (TENG). We found that triboelectric property of kC-Agar composite can be largely enhanced because of electron-donating oxygen-rich functional groups and abundant presence of Ca²⁺ cations via ionic crosslinking. Through the evaluation of surface potential and triboelectric output, we demonstrate 80 wt% kC-Agar composite is the most highly triboelectric positive as well as surface potential is largely increased by at most 58 % compared to kC and Agar. In addition, to reveal the potential of kC-Agar composite for fully biodegradable triboelectric and encapsulation materials, we demonstrate tunable output performance and lifetime of fully biodegradable triboelectric nanogenerator (BDTENG) based on kC-Agar composite by simply varying the concentration. We believe that our findings would provide multiple options for researchers and engineers in the field of IEDs and biodegradable materials.

The high-output triboelectric nanogenerator (TENG) is indispensable for its practical applications toward industrial products. However, the electricity loss in simple parallel connection among all units and the typically high crest factor output seriously hamper the practical applications of TENG. Here, a rectified TENG is reported in parallel structure to solve the problem of electricity loss in simple parallel connection. The rotational contact-separation structure with phase difference between rectified TENGs addresses high crest factor output and extends service life of rotational TENG simultaneously. The current crest factor is dramatically decreased to 1.31 in multiple rectifier multiple TENG in parallel (MRM-TENG), while that of TENG in simple parallel is higher than 6. A theory based on simulation results was also proposed to reduce the high crest factor and avoid energy loss of traditional TENG in parallel, and its feasibility was confirmed by the experiment. Meanwhile, the current output can retain up to ~93% of its initial performance after 7 200 000 rotations under 2.00 r s⁻¹ of 1000 h. Furthermore, the equivalent current can be in linear growth with low crest factor by making MRM-TENG in parallel for distributed energy supply without electricity loss. This work may provide a new strategy for TENG in parallel to achieve a low crest factor output and long-term cycling stability power generation in distributed energy harvesting for large-scale power application.

Quantum-mechanical models for contact electrification are presented by virtue of tunneling and light-assisted electron transfer between dissimilar solids. Firstly, a description of electronic states in solids (either metal/semiconductor/dielectric) is presented using the one-band effective-mass approach. Then, based on continuity of the wavefunction and its derivative (multiplied by the inverse effective mass), computation of the transmission probability through a barrier potential of vacuum (air) is carried out and weighted by the Fermi functions of the solids to the left and right of the vacuum barrier and the electron density of states. The model captures details of the Fermi levels of the two solids in contact, their temperatures, and the vacuum (air) layer thickness. We investigate the strength of the electron transfer as a function of the effective mass, Fermi levels, temperatures, and the vacuum barrier thickness using the Landauer-Boettiker equation in a one-dimensional formulation. Secondly, we take into account the Coulomb repulsion effect that is gradually established as electrons transfer from one material to the other. This is done by solving for the electric field as a function of the temporal build up of surface charges (positive on the material from which electrons are emitted and equally negative on the other material). The resulting electric potential is a linear function of distance in the vacuum region and the envelope of the electron wavefunction becomes a linear combination of Airy functions. Due to the linearly increasing barrier potential, the electron transfer rate is increasingly reduced as the surface charge build up takes place. This leads to a saturation effect such that the Fermi levels do not become equal as time goes. On the other hand, if the electron transfer is small at all times, the two Fermi levels eventually become equal. In the second part of the talk, light-assisted electron transfer models (spontaneous emission, stimulated emission or absorption) are described using Einstein rate equations and the Fermi Golden Rule. This is first done for a system of coupled atoms separated by a barrier potential and later generalized to arbitrary solids described by Fermi levels.
References
[1] M. Willatzen and Z. L. Wang, “Theory of contact electrification: Optical transitions in two-level systems”, Nano Energy 52, 517 (2018).
[2] M. Willatzen and Z. L. Wang, “Quantum-mechanical model for optical transitions between solids,” Nano Energy 61, 311 (2019).

Development of mechanical energy harvesting devices that are compliant to curvilinear surfaces or mountable onto human body or skin are attractive for human-machine interface and wearable applications. Fabric based or textile based wearable triboelectric nanogenerators (TENGs) have attracted attention since it is conformable and feasible for users to generate electricity from simple biomechanical motions. We develop a conformable textile nanogenerator utilizing black phosphorus as charge trapping layer. The conformable textile nanogenerator was found to deliver considerably high output with a small force (~5 N) and low frequency (~4 Hz), which can be used to power small electronic gadgets. Fabric based TENGs with self-cleaning and antifouling properties are important attributes. Through the preparation of hydrophobic cellulose oleoyl ester nanoparticles, a low-cost and nontoxic coating material can be realized to harvest mechanical energy from water flow. The all-fabric-based dual-mode device harvests both the electrostatic energy and mechanical energy of water, achieving the maximum instantaneous output power density of 0.30 W m^-2. Recently, with the aim of attaining stretchability, breathability and waterproofness simultaneously for wearable applications, we fabricated mechanical interlocked stretchable nanofibers using electrospinning and electrospraying, delivering breathable TENG for tactile interactive sensing and energy harvesting. Designing thermoplastic elastomer that can be 3D printable attains the function of stretchable triboelectric active layer and simultaneously a stretchable current collector. In addition, we have synthesized self-healable elastomers based on porous supramolecular polyurethane which exhibited gas-solid interacted triboelectric behaviour, with high stretchability and adhesion properties.

Water covers about 70% of the earth’s surface and contains tremendous renewable and clean energy. Despite success in the harvesting hydrodynamic energy based on heavy-weight and bulky electromagnetic generators, a great deal of water energies stored in the low-frequency flow of water, such as in the form of raindrop, river/ocean wave, tide, remain largely untapped. In recent years, numerous promising techniques that allow for the efficient harvesting of water/liquid energy have emerged [1-3].

In this talk, I will discuss our recent progress in taking advantage of the fusion of slippery surfaces and transistor-inspired architecture on efficient water energy [4]. On the research line of liquid/liquid-interface based energy harvesting, we developed a novel slippery lubricant-impregnated porous surface (SLIPS) based TENG, termed as SLIPS-TENG, that exhibits distinctive advantages including optical transparency, self-cleaning, impact tolerance, flexibility, and enhanced power generation stability even under harsh environments [5]. On the research line of solid/liquid-interface based energy harvesting, we report an original droplet-based energy generator (DEG) with field-effect transistor (FET)-like architecture that fundamentally overcomes the physical limitation inherent in the traditional approaches which are imposed by the undesirable interfacial effect and achieves the highest energy conversion efficiency [6]. Such a unique design allows for the reversible and efficient transfer of charges between the source and drain, resulting in the enhancement of power density by several orders of magnitude over its counterparts.

References:
1. Scruggs, J. & Jacob, P. Harvesting ocean wave energy. Science, 323, 1176-1178 (2009)
2. Logan, B. E. & Elimelech, M. Membrane-based processes for sustainable power generation using water. Nature, 488, 313-319 (2012).
3. Lin, Z.-H., Cheng, G., Lee, S., Pradel, K. C. & Wang, Z. L. Harvesting water drop energy by a sequential contact-electrification and electrostatic-induction process. Adv. Mater. 26, 4690-4696 (2014).
4. Wanghuai Xu, Zuankai Wang. Fusion of slippery interfaces and transistor-inspired architecture for water kinetic energy harvesting. Joule, https://www.sciencedirect.com/science/article/abs/pii/S2542435120304438
5. Xu, W., Zhou, X., Hao, C., Zheng, H., Yan, X., Liu, Y., Yang, Z., Leung, M., Zeng, X.C., Xu, R., Wang, Z. SLIPS-TENG: Robust triboelectric nanogenerator with optical and charge transparency using slippery interface. Nat. Sci. Review, 6, 540 (2019).
6. Xu, W., Zheng, H., Liu, Y., Zhou, X., Zhang, C., Song, Y., Deng, X., Leung, M., Yang, Z., Xu, R., Wang, Z., Zeng, X.C., Wang, Z. A droplet-based electricity generator with high instantaneous power density. Nature, 578, 392(2020).

Over the years, self-powered devices (SPDs) have been investigated to reduce the dependency of various electronic components on externally connected power sources. These components not only range from biomedical sensors but also basic building blocks of modern electronics-thin film transistors. SPDs are capable of not only powering and storing the energy but also provide the required driving potential to the sensors, which run at minimal input energy. In general, mechanically powered SPDs work on the principle of piezoelectricity; as human motion-powered devices can be considered as the mechanical power source of energy, these self-powered devices can be produced using the same principle. With this motivation, we have fabricated a piezoelectric nanogenerator (PNG) using poly-vinylidene fluoride (PVDF) as an active material spin-coated on a polyethylene terephthalate (PET) coated aluminum substrate. The adopted approach takes care of the device flexibility along with the extrication of mechanical energy via several degrees of motion owing to the involvement of polymeric components. Piezoelectricity in PVDF matrix is due to an electroactive all-trans β phase (re-orientation of H-F dipoles across C-C chain along one direction). To create a self-polarized PVDF film, we have made use of the quenching process, which has not been much explored in fabricating PNGs. The quenching based polarization solely depends on nucleation and growth concept, specifically nucleation of β crystallites under lower temperature. The employed organic solvents during the fabrication process contributed to the appearance of the β phase, which is confirmed by X-ray diffraction and Fourier transform infrared spectroscopy. These films were then incorporated into a PNG device that had a single active layer providing a thin device that is self-polarized and paves the way towards a small area, robust energy harvester. A two-step approach fabrication of the PVDF device showed exceptional performance, resulting in an open-circuit peak-to-peak voltage of 19.2 V upon human hand pressing and short-circuit current >0.7 μA. The average d₃₃ value for the quenched PVDF film is observed to be −15.16 ± 5.81 pm/V. Further, charging of a capacitor and powering the liquid crystal displays (LCDs) are realized through the nanogenerator, thus validating mechanical energy conversion, storage, and powering applications.

The advances of wearable electronics and the Internet of things has triggered tremendous demands for wearable power sources as well as sensors, with multifunctionalities, such as transparency and flexibility/stretchability. Since 2012, triboelectric nanogenerators (TENG) have demonstrated themselves as promising power sources as well as self-powered sensors for various applications. Scientists are devoted to enhancing the output performance of TENG through materials exploration, structural design, and circuit management. From a material point of view, developing tribo-materials with a strong affinity to gain/lose electrons, good mechanical, dielectric, surface properties, and other functionalities is one of the key factors to maximize the output power of TENG.
Herein, we explore a poly(vinyl alcohol)/polyethyleneimine composite, featuring high transparency, ionic conductivity, flexibility, good mechanical property, and plenty of functional groups. These features enable the polymer composite capable of serving as both a tribo-material and a current collector for TENG applications. Through embedding conductive nanofillers with surface capping ligands, the dielectric constant of the composite can be fine-tuned, where the higher power output of the resultant TENG can be achieved. In addition, the ionic conductivity of the composite can be enhanced by tuning the weight ratio of polyethyleneimine in the composite, facilitating its charge collection as a current collector. In this regard, a metal-electrode-free transparent wearable power source and a fully integrated self-powered triboelectric sensor array for self-powered tactile sensing are developed. Furthermore, the interfacial interactions between the functional groups of the composite and the carbon dots enable the composite with enhanced transmittance, resulting in a dual-functional layer for both solar energy and raindrop energy harvesting.

References:
1. L. Wang, W. A. Daoud. Highly flexible and transparent polyionic-skin triboelectric nanogenerator for biomechanical motion harvesting. Advanced Energy Materials, 2019, 9, 1803183.
2. L. Wang, X. Yang, W. A. Daoud. High power-output mechanical energy harvester based on flexible and transparent Au nanoparticle-embedded polymer matrix. Nano Energy, 2019, 55, 433-440.
3. L. Wang, Y. Liu, Q. Liu, Y. Zhu, H. Wang, Z. Xie, X. Yu, Y. Zi. A metal-electrode-free, fully-integrated, soft triboelectric sensor array for self-powered tactile sensing. Microsystems & Nanoengineering, 2020, 6, 1-9.
4. L. Wang, Y. Wang, H. Wang, G. Xu, A. Doring, W. A. Daoud, J. Xu, A. L. Rogach, Y. Xi, Y. Zi. Carbon dot-based composite films for simultaneously harvesting raindrop energy and boosting solar energy conversion efficiency in hybrid cells. ACS Nano, 2020, 14, 10359-10369.

The in-depth understanding on the electrification behavior of dielectric materials is a prerequisite to improve the surface triboelectric charge density for triboelectric nanogenerators (TENGs). Dielectric property is favorable for triboelectric materials due to its high charge sustaining capability and dielectrics are commonly utilized for TENG devices. However, the inherent large impedance of dielectric materials undoubtedly causes limited current and power density outputs, alternatively, semiconductors with higher charge mobility can facilitate the charge transfer thus enhancing the electrical output performances.

Halide perovskites, as one of the most promising semiconductors, have gained tremendous attentions in the past decade given its fascinating optical and electronic properties in solar cells, light emitting diodes and electrochemical sensors. Herein, triboelectric charging affinities and polarities of all-inorganic cesium lead tri-bromine (CsPbBr₃) and ions doped (alkaline earth / alkali metal ions) perovskites are systematically investigated by assembling into vertical contact-separation TENGs. An enriched triboelectric series supplemented with inorganic perovskites and quantified triboelectric charging polarities is delivered through analyzing their dielectric properties, electrical output performances, figure-of-merits (FOM), work function, bond energy and electron binding energy. Moreover, the photoinduced charges play significant role in enhancing the triboelectrification charges and altering charging polarity of perovskite based TENGs under illumination. This work not only supplements inorganic perovskites to triboelectric series and also provides new insights of inorganic perovskite in the foundation of tribo-photovoltaic theory and hybrid devices.

As a promising energy converter, the requirements for miniaturization and high-accuracy of triboelectric nanogenerator (TENG) always keep urgent. In this presentation, a micro triboelectric ultrasonic device will be introduced by integrating TENG and micro-electro-mechanical systems (MEMS) technology. To date, it sets a world record of smallest TENG with a 50 µm-sized diaphragm, and enables to bring the working frequency to Mega Hertz. This dramatically improves the miniaturization and chip integration of triboelectric nanogenerator. With 63 kPa@1 MHz ultrasound input, the micro triboelectric ultrasonic device can generate the voltage signal of 16.8 mV and 12.7 mV through oil and sound-attenuation medium, respectively. It also achieved the signal-to-ratio of 20.54 dB and exhibited the practical potential for signal communication by modulating the incident ultrasound. Finally, the detailed optimization approaches have also been proposed to further improve the output power of the micro triboelectric ultrasonic device.

Reference:
C. Chen, Z. Wen, J. Shi, X. Jian, P. Li, J.T.W. Yeow, X. Sun, Micro triboelectric ultrasonic device for acoustic energy transfer and signal communication, Nature Communications, 11(2020) 4143.

Majority of current IMDs are powered by conventional primary or secondary batteries that contribute up to 90% weight and volume of the entire device. [1.2] While replacement of or recharging the batteries requires substantial surgical or technical efforts, introducing additional suffering and complexity to the patients, other batteries potential issues such as overheat and leakage of toxic electrolyte further prohibit the advancement and miniaturization of IMDs. Therefore, increasing efforts are now being focused on the innovation of designated IMD power sources. Implantable nanogenerators (i-NGs) have been designed to convert biomechanical energy into electricity. [3,4] In spite of their numerous merits, the outputs of state-of-the-art i-NGs are always in a form of largely discrete pulses. Although their theoretical output power could be sufficient for IMDs, battery component is still needed in the i-NG design to produce a steady and useable direct current (DC) output. Moreover, most i-NGs are non-stretchable with incompatible mechanical properties compared to soft biological tissues, which further challenges their practical applications.
In this work, we reported an ultra-soft stretchable i-NG system that could function as a battery-free DC micro-power supply. The i-NG consisted of ultrafine micro-scale interdigital electrodes (IDEs) and multi tribo-active layers with a small working area (approximately 2 cm²), packaged with biocompatible silicone elastomer. While the micro IDEs support the output of high frequency electricity (1 μA at 70 Hz) driven by slow mechanical stimulation, the silicone elastomer and an embedded cavity design enable i-NGs with extremely low Young’s Modulus (46 kPa), exactly matching the mechanical property range of most soft biological tissues. By implanted inside the abdominal cavity of Sprague Dawley (SD) adult rats, the i-NG could convert slow diaphragm movement during normal breath into stable high-frequency electrical spikes, which were readily transmitted into a continuous ~2.2 V DC output on a LED load after being integrated with a basic electrical circuit (rectifier and capacitor) for a relatively long period of time.
This electric output could continuously power the LED without any observable power decay, successfully demonstrating a constant operation of small electronics DC power free of the battery component.This solely biomechanical-energy driven DC micro power supply offers a very promising solution for the development of self-powered IMDs in the near future.
References:
[1] A. B. Amar, A. B. Kouki and H. Cao, Sensors, 2015, 15, 28889-28914.
[2] D. C. Bock, A. C. Marschilok, K. J. Takeuchi and E. S. Takeuchi, Electrochimica Acta, 2012, 84, 155-164.
[3] J. Li and X. Wang, APL Materials, 2017, 5, 073801.
[4] M. Parvez Mahmud, N. Huda, S.H. Farjana, M. Asadnia, C. Lang, Advanced Energy Materials, 2018, 8, 1701210.

Hair loss is classified as a dermatological disorder due to growth factors deficiency and/or hair cycle disorder, which is a common disease for both men and women throughout the world and the number of hair loss cases in 2011 has exceeded 30% of the global population.[1,2] Major current treatments for hair loss including topical treatments, oral medicine and hair transplant to re-establish healthy hair growth cycles, where topical treatments are the most common choice by patients due to the non-invasive nature and high absorbability. Minoxidil (MNX) and oral finasteride are the two FDA-approved drugs to treat hair loss, but they may still induce severe side effects. Hair transplant is invasive and associated with high cost, discomfortness and time commitment.[3] Recent study showed electric stimulation can induce a non-invasive biological effect named electrotrichogenesis (ETG), which could enhance the influx of calcium ions into the dermal papilla cells via voltage-gated transmembrane ion channels, facilitate ATP synthesis in mitochondria, activate protein kinases, and stimulate protein synthesis and cell division.[4,5] As a result, the ETG effect is believed to be able to regulate secretion of multiple hair growth factors, promote hair follicles (HFs) proliferation, prolong the anagen stage, and ultimately promote hair regeneration. However, the electrical stimulation is typically complicated for implementation and not quite suitable for everyday treatment.
Here, a universal motion-activated and wearable electric stimulation device (m-ESD) that can effectively promote hair regeneration via random body motions was designed.[6] Significantly facilitated hair regeneration results were obtained from Sprague-Dawley (SD) rats and nude mice. On shaved SD rats, compared to the other conventional pharmacological MNX and vitamin D₃ treatments, the hair growth rate in m-ESD region is 0.73 mm per day, and the final hair shaft length under m-ESD was 1.8 times and 2.2 times longer than those treated by MNS and vitamin D₃, respectively. Meanwhile, the HF density from m-ESD stimulating was 141% of the MNX and vitamin D₃ regions. We also showed that m-ESD can promote HFs proliferation and improve the secretion of vascular endothelial growth factor and keratinocyte growth factor, and thereby alleviate hair keratin disorder, increase the number of HFs and promote hair regeneration on genetically defective nude mice. In general, our work provided an effective hair regeneration strategy in the context of non-pharmacological self-powered wearable electronic device, and possessed a great value in reshape modern pharmacological physical approaches in biomedical treatments. It is expected to be able to quickly evolve into a practical and facile solution to address the hair loss problem suffered by billions of people over the world.


References:
[1] Lei, M., Chuong, C. M. Aging, alopecia, and stem cells. Science 351 (2016) 559.
[2] Matsumura, H., Mohri, Y., Binh, N. T., Morinaga, H., Fukuda, M., Ito, M., Kurata, S., Hoeijmakers, J., Nishimura, E. K. Hair follicle aging is driven by transepidermal elimination of stem cells via COL17A1 proteolysis. Science 351 (2016) aad4395.
[3] Shapiro, J. Hair Loss in Women. Engl. J. Med. 357 (2007) 1620-1630.
[4] Benjamin, B., Ziginskas, D., Harman, J., Meakin, T. Pulsed electrostatic fields (ETG) to reduce hair loss in women undergoing chemotherapy for breast carcinoma: a pilot study. Psychooncology 11 (2010) 244-248.
[5] Yao, G., Kang, L., Li, J., Long, Y., Wei, H., Ferreira, CA., et al., Effective weight control via an implanted self-powered vagus nerve stimulation device, Nat Commun 9 (2018) 5349.
[6] Yao, G., Jiang, D., Li, J., Kang, L., Chen, S., Long, Y., et al, Self-activated electrical stimulation for effective hair regeneration via a wearable omnidirectional pulse generator, ACS Nano 13 (2019) 12345-56.

Flexible bioelectronics, integrated into a variety of accessories, seamlessly fitting to the skin, or even implanted in the living body, have been extensively researched and have made great progress in biomedical fields such as electronic skin, intelligent monitoring and human-machine interface.[1,2] In particular, a significant interest of flexible bioelectronics lies in monitoring physiological signals, revealing timely information on the state of our body and further providing actionable feedback for disease treatment to sustain a healthy lifestyle.[3,4] In addition, non-pharmacological neuromodulation can manipulate influencing neurophysiological signals or body functions by stimulating through the neural networks to achieve therapeutic purpose, which is a non-destructive and reversible therapeutic strategy.[5,6] However, the device power source still faces the limitation of large size, rigidity and short working life.[7] If wearable electronic devices can be attached to a living body to collect energy generated by low-frequency motion and directly drive a flexible smart biomedical device, the nonnegligible problem of the size, rigidity and working life can be solved.
Here, we present a novel symbiotic non-pharmacological therapy system through low-invasive electrical intervention methods, including a wearable hair regeneration device, an implantable weight loss device and a biodegradable tissue repair device, emphasizing their role in biomedical applications in vivo and problems in practical applications. The work of weight loss provides an efficient and low-invasive strategy, and the weight of the device-implanted rat group was reduced by ~40% compared with the control group. A closed-loop feedback regulation system through food intake, gastric motion, electrical neuromodulation and brain control was established. Meanwhile, in the experiments of hair regeneration and tissue repair, the non-pharmacological electrical stimulation can regulate secretion of multiple growth factors, promote cell proliferation, and ultimately achieve the efficient repair effect. In general, due to the non-pharmacological and low-invasive nature, the widespread application of symbiotic bioelectronics will effectively treat more diseases or symptoms.


References:
[1] J. Y. Sun, C. Keplinger, G.M. Whitesides, Z. Suo, Ionic skin, Adv Mater, 26 (2014), 7608-7614.
[2] T. Someya, Z. Bao, G.G. Malliaras, The rise of plastic bioelectronics, Nature, 540 (2016) 379.
[3] P.J. Soh, G.A.E. Vandenbosch, M. Mercuri, D.M.M. Schreurs, Wearable wireless health monitoring: current developments, challenges, and future trends, IEEE Microw Mag, 16 (2015) 55-70.
[4] M.M. Rodgers, V.M. Pai, R.S. Conroy, Recent advances in wearable sensors for health monitoring, IEEE Sens J, 15 (2015) 3119-3126
[5] Y. Long, H. Wei, J. Li, G. Yao, B. Yu, D. Ni et.al., Effective wound healing enabled by discrete alternative electric fields from wearable nanogenerators, ACS Nano, 12 (2018) 12533-12540.
[6] Yao, G., Jiang, D., Li, J., Kang, L., Chen, S., Long, Y., et al, Self-activated electrical stimulation for effective hair regeneration via a wearable omnidirectional pulse generator, ACS Nano 13 (2019) 12345-56.
[7] T.R. Ray, J. Choi, A.J. Bandodkar, S. Krishnan, P. Gutruf, L. Tian, R. Ghaffari, J.A. Rogers, Bio-Integrated Wearable Systems: A Comprehensive Review, Chem Rev, 119 (2019) 5461-5533.

Energy-autonomous smart sensor systems have been the subject of intense research and innovation activities for applications in multiple and diverse sectors, from wearable wireless sensor patches in sport, wellness and healthcare to battery-assisted distributed sensor nodes for the Internet of Things [1]. Selecting and combining energy harvesting from multiple sources, including from mechanical displacements, thermal gradients and electromagnetic radiations, from radiofrequencies to visible and infrared light, provides a unique opportunity to extend the battery life throughout the entire lifetime of the device [2]. This is particularly important in applications where physical access to the system once in operation is not convenient, such as in distributed environmental sensors for air quality monitoring in metropolitan or rural areas, and indoor [3], as well as in implantable medical devices, such as those operating low voltage (<1V) biosensors in contact with organs and tissues in order to avoid water electrolysis and faradaic reactions [4].
Both fields are critically affected by the appropriate choice of materials in terms of environmental sustainability at the end of life, such as the battery component materials. In recent work we have demonstrated the possibility to operate the front-end and digital electronics via printable thin-film transistors with a carbon nanotube-based channel and hybrid nanodielectrics operating in the deep-subthreshold ambipolar regime, achieving ultralow voltage (<0.5V) and record-low power consumption of 10^-15-10^-12W/µm per gate [5].
Hence our research focuses on combining smart sensor technologies with energy harvesting and ultra-low power electronics to achieve long-lifetime and battery-less biosensor systems and sustainable IoT nodes for “place and forget” applications.
Different types and forms of piezoelectric materials such as thin films, bulk or nanowires have been subject of intense research as a route towards mechanical energy harvesting and triboelectric nanogenerators with both environmental sustainability and biocompatibility. Piezoelectricity has been theoretically predicted first [6] and then reported in low dimensional materials, such as hexagonal boron nitride (hBN), transition metal dichalcogenides (TMDs), including MoS₂, MoSe₂, WS₂, WSe₂, MoTe₂ WTe₂, and graphene engineered by atom adsorption (with e.g. Li, K, F, H) or by incorporating non-centrosymmetric in-plane defects [7], with coefficients of piezoelectric strain (d₁₁) and piezoelectric stress (e₁₁) of the 2H-TMD monolayers comparable to, sometimes even better than, those of the conventional bulk piezoelectric materials, such as AlN and GaN. Additionally, it has been discovered that ultrathin MoS₂ with odd-layers could produce oscillating piezoelectric voltage and current outputs with highest values of the piezoelectric coefficient recorded in the case of a monolayer of MoS₂, whereas no piezoelectric output was observed for the MoS₂ with even-layers [8]. More recent works have reported enhanced values of the piezoelectric effect induced in butterfly-shaped monolayers of MoS₂ with grain boundaries (GB-MoS₂) achieving current densities 50% higher than those measured in MoS2 monolayers without grain boundaries [9] paving the way towards practical energy harvesting applications for self-powered printed electronic devices.

Triboelectric energy generators (TENGs) are characterized by their high electric field and relatively high output power. They usually generate high AC open-circuit voltage around hundreds of volts peak-to-peak, with relatively low short-circuit current around tens of micro ampere (µA) peak-to-peak. However, most applications require DC voltages around a few volts, with DC current of at least a few hundreds of µA, and a conditioning circuit for the TENG is needed to rectify the high AC voltage. Moreover, for a better kinetic-to-electrical energy conversion, TENGs need to maximize their bias voltage, which can be improved with dedicated conditioning circuits (CC). CC can be separated in two categories: Stable Charge Pumps (SCP) and Unstable Charge Pumps (UCP). SCPs reach constant saturation voltage after a few mechanical cycles, whereas the UCPs can generate an exponential increase of the output voltage until the components’ limitation, even with a basic TENG. Although the unstable charge pumps can show biasing advantages over the stable charge pumps, UCP must have a minimum TENG’s capacitance variation for an adequate operation, characterized by the max-to-min ratio η. This constraint may differ from one UCP architecture to another.
In this presentation, we will compare several stable charge pumps (half/full-wave bridge rectifiers and Cockroft Walton multipliers) with different architectures of unstable charge pumps such as Bennet’s doubler and its variations from Lefeuvre and Karami. A series of simulations are conducted for all the conditioning circuits under the same parameters for the TENG and the same simulation conditions. The transducer used in simulations has a surface area S=9 cm², and a surface charge density σ = -50µC/m².
It is not simple to make a valid comparison between the UCPs and SCPs. SCPs reach a saturation voltage so the optimal power and the energy conversion is achieved during a specific mechanical cycle. On the other hand, UCPs’ rectified voltage is always increasing thus the energy, and there is no specific optimal power. Due to such discrepancy between the two, the comparison will be done by choosing a certain mechanical cycle at a given maximal voltage and calculating its QV-cycle area. We show that there is a tradeoff between the time needed to reach the high energy conversion state and the amount of the converted energy. SCPs are more convenient if we need to reach a significant energy conversion rate very quickly. On the contrary, if we want much higher energy conversion, UCPs need to be chosen, but it requires more time, thus average power cannot be that good, unless a 2-stage power management system is used.

Kinetic energy harvesters (KEHs) are typically not considered as stand-alone devices having very high voltage and low current at the output. Given that the direct converted energy cannot be sufficient for powering a system, and at minimum need to be rectified, conditioning circuits are necessary. Traditionally, stable charge pumps such as half- full-wave rectifier, or unstable charge pumps such as Bennet’s doubler are used. It has been previously shown that unstable charge pumps can be significantly more efficient due to no limitations or saturation points in charge multiplication. Keeping the high energy conversion in mind, Bennet’s charge doubler has been studied in detail and applied to different architectures and types of electrostatic KEHs, such as triboelectret nanogenerators (TENGs). Several variations of this circuit have also shown the expected exponential increase with various optimum for specific TENG configurations.
By adding a third electrode to a classical 2-electrode TENG, we can obtain a device with three terminals and two variable capacitors having a 180° phase-shift. Converted energy of such device is higher than of a similar TENG with one variable capacitor, but specific conditioning is needed. In this work, we demonstrate that replacing the fixed capacitor in Bennet’s charge doubler by the 2^nd variable capacitor of the 3-electrode TENG leads to higher converted energy. While this new combination gives promising results, its behavior and details about each phase that occurs during one conversion cycle have not been fully analyzed yet.
To obtain the most information about the circuit’s behavior during the energy conversion process, the best is to observe the change of voltage and charge over the two TENG’s variable capacitors. Taking each of them and examining their behavior through variation of the two mentioned parameters gives the converted energy in each mechanical cycle. This study discussed critical points occurring during one conversion cycle that are a combination of mechanical and electrical variations occurring at the same time. Constraints are set for the TENG, particularly the physical parameters defining η = C_max/C_min ratio conditioning the start of the circuit. Furthermore, we broke down and analytically expressed each significant moment within one mechanical cycle of double variable TENG. Each crucial point is defined by relevant parameters such as local capacitance, voltage and charge over the capacitor, leading to the full definition of such a system.
The analytical study of double variable TENGs is corroborated with simulation models with Bennet’s charge doubler circuit operating in steady state, or in other words, when exponential output is guaranteed. Theoretical study and simulation results have been completed by experimental test through practical implementation of a gap-closing double variable TENG and the charge doubler circuit, leading to the conclusion that, indeed, realistic systems convert expected higher amounts of energy.
Even though this study has been conducted on double variable gap-closing TENG, it can be universally applied to any three-terminal KEH with a phase shift of 180° between its output signals, ensuring steady exponential output.

The last decade highlighted Triboelectric Nanogenerators (TENGs) as a very promising technology for the conversion of mechanical energy into reliable electrical power sources. This technique takes advantage of the triboelectrification effect. Nowadays, lithium batteries supply the biggest part of Internet of Things (IoT) nodes since their energy storage density is significant. However, considering the limits in terms of maintenance cost, resources consumption and environmental dissemination, it is highly desirable to develop alternative self-powering solutions. In the context of the fourth industry era, the wind energy extraction from ventilation systems or areas with permanent airflows can supply enough power for the operation of numerous sensors deployed in the facilities. Ambient energy harvesting is then to be considered as an attractive solution to provide sustainable power sources.

Our initial investigation of an Insulator-Metal-Insulator (IMI) composite thin-film reported an attractive way to obtain compact devices with high power density. The presented work highlighted the great potential of this multilayer structure in a high-performance Wind Actuated Triboelectric Nanogenerator (WATENG). For this study, the collecting wind tunnel is improved. This solution converts wind power to lateral oscillations in a symmetric triboelectric structure using mobile flexible electrodes. This innovative architecture optimizes the triboelectric charges accumulation into metal islands to boost the output power of the WATENG. Different customized stacks and materials are investigated to overcome the localized charge density saturation into the triboelectric harvesting layers.

This study demonstrates the great potential of an IMI structure coupled to a pioneering WATENG architecture in the same device, to greatly enhance the potential airflow-powered connected objects.

Health is a very precious commodity, as we experience every day and especially in these times shaken by a pandemic. Continuous recording of health status through diagnostic techniques that combine high wearing comfort with high reliability, accuracy and cost-effectiveness is therefore very desirable. In this context, we present an ultraflexible solution for the power supply of wireless medical patches for monitoring of heart rate and blood pressure. The ultraflexibility is due to the very low overall thickness (only 2.5 µm in total) of the ferroelectric polymer transducers, organic rectifiers and capacitor components manufactured on a substrate only 1 µm thick, and it guarantees high sensitivity (15 nC N^-1), superior component reliability (over 6000 cyclic loading) and a very high peak power density (1 mW cm^-3 at 2 Hz excitation). Our health patch solution not only includes this radically reduced layer thickness, but also new material combinations (ferroelectric polymers on parylene), novel device concepts (organic thin-film transistor-based diodes) and new integration principles (multi-stack transducers on 3D curved surfaces).<sup>1

1</sup>Petritz et al. submitted; preprint DOI: 10.21203/rs.3.rs-86704/v1;

Triboelectric nanogenerator (TENG) has been widely used as self-powered wearable sensor, and progresses on novel material for high output TENG have attracted tremendous attention. However, simultaneous consideration on materials system design and manufacture process receives limited attention. Manufacture and encapsulation techniques feasible for large scale fabrication compile with functional materials systems need to be developed. Spray coating is an attractive method due to its potential for large scale deposition of ink-based materials. Here, we demonstrate fully sprayed TENG using ink-based functional materials. Several functional inks compatible with spray coating are formulated to fabricate flexible TENG for human pulse monitoring and human-machine interaction. All functional materials are solution processable and can be sprayed layer-by-layer to form the triboelectric sensor. Such flexible sensors could also be sprayed and integrated on arbitrary surfaces for ubiquitous electronics and sensors.

The capability of sensor systems to efficiently scavenge their operational power from stray, weak environmental energies through sustainable pathways could enable viable schemes for self-powered health diagnostics and therapeutics. Triboelectric nanogenerators (TENG) can effectively transform the otherwise wasted environmental, mechanical energy into electrical power. However, obstacles hindering the development of efficient triboelectric devices based on biocompatible materials continue to prevail. I will discuss our recent progress in the design and engineering of bio-derived materials for biocompatible, wearable triboelectric devices.

Such wearable devices are conformable to human skins and can sustainably perform non-invasive functions, e.g., gesture recognition, by harvesting the operation power from the human body. These devices can also detect the imperceptible skin deformation induced by the human pulse and capture the cardiovascular information encoded in the pulse signals with high fidelity. The gained fundamental understanding and demonstrated capabilities enable the rational design and holistic engineering of novel materials for more capable biocompatible triboelectric devices that can continuously monitor vital physiological signals for self-powered health diagnostics and therapeutics.

Strong research efforts have been dedicated towards the miniaturization of energy conversion devices to support the development of self-powered sensors and relevant devices. The harvesting of ambient energy, predominantly vibrational motion, has been demonstrated to convert into the electrical energy by employing various mechanisms, such as electrostatic, electromagnetic, and piezoelectric. These devices face a number of unachievable targets related to the proper output power demanding by specific applications, durability to withstand long life time and flexibility to conform to requirements of wearable electronics. Triboelectric nanogenerators have emerged as an alternative technology to confront with the above challenges. The first TENG devices have been envisaged and fabricated by Wang and co-workers [1]. The working principle is based upon the coupling between electrostatic induction and triboelectric effect. In the current work, we apply a novel method which entails the simultaneous growth and deposition of graphene on any substrate with the use of laser-assisted decomposition of organic compounds. The method allows the deposition of hybrid nanomaterials with a single laser processing step. Employing this method, a single-electrode TENG device architecture was implemented by the direct laser-assisted forward transfer of graphene and graphene/SiO_x hybrids on PDMS substrate acting as one of the tribo-materials. A fairly low sheet resistance of the graphene film and its ability to withstand several cycles of operation are key factors towards replacing the precious metals deposited on the back side of the contact material. The electrical features and the output power of the flexible single-electrode TENG devices were evaluated against various external loads. A remarkable finding of this work is that the current external resistance value providing maximum output of the TENG, is much lower than values reported in the literature. Given the high impedance of other TENGs, this is a significant advantage of the current device, considering that the reduction of the internal impedance is highly desirable in a number of applications requiring low impedance loads for many practical applications.

[1] Fan, F. R., Tian, Z. Q. & Wang, Z. L. Flexible triboelectric generator. Nano Energy 1, 328–334 (2012).

The operation of wearable and portable electronics requires only few tens of microwatts of power supply. These new low power devices, such as the one largely developed for the internet of Things technologies (IoT), have fostered the development of low power energy sources which harvest the environmental green energy. In particular, the fast-growing demand for new sustainable energy, and the ubiquitous and abundant environmental mechanical energy, has raised the interest in energy harvesters able to convert such energy into electrical power. Triboelectric nanogenerators (TENGs) can provide such green power supply. Recent developments in TENGs technology show that they can be used in different applications ranging from wearable self-powered sensors to wind and sea wave energy harvesting (blue energy). Despite the wide number of TENGs developed so far, an in depth understanding of their working mechanism operating at the sub-microscale is still missing. In this work, we highlight the fundamental role played by the interface between the triboelectric material and the electrode collector in contributing to the TENG’s power generation. In particular, we will show the fundamental role played by the electrodes work function and capacitance in boosting triboelectric nanogenerators power outputs. We will describe the guidelines for the combination of electrode and triboelectric materials to optimize the mechanical energy conversion efficiency in TENGs. The outcomes of our studies provide essential insights for the proper design of new TENGs to the aim of further enhancing their performances.

Flexible, wearable, and human-compatible electronics are helping shape a future of integrated nanotechnologies. As such, piezoelectric and semiconducting materials have drawn intense interest by many research groups for the excellent potential to advance the field of wearable devices. Moreover, two-dimensional (2D) nanomaterials, valued for their enhanced mechanical, electronic, and structural properties, have moved to the forefront of compliant electronic devices. Therefore, a thorough understanding of the interfacial charge distribution is required when utilizing flexible piezoelectric semiconductors, such as zinc-oxide (ZnO). We present a method for monitoring the piezoelectric response of nanometer-thick 2D ZNO nanosheets (NS) grown via Ionic Layer Epitaxy (ILE). The direct piezoelectric effect, and the coupling of electronic dipole polarization to lattice strain within ZnO-NSs, is monitored in situ via piezo force microscopy (PFM). Using PFM, we illustrate the effects of modulating strain in piezoelectric nanomaterials and its role in the polarization charge distribution across the surface of the 2D ZnO-NS. Our results illustrate the critical need for understanding the interdependent roles of static piezoelectric charges, semiconducting properties, and strain, for future flexible devices.

The electronic paper (E-paper) display features flexibility, sunlight visibility and low power consumption, which makes it ideal for Internet of Things (IoT) applications where maintenance of bulky power modules is better to be eliminated. In electrophoretic E-papers, the most successfully commercialized E-paper technology, there are generally different color particles, and these particles are usually surface-modified and charged by charge control agents (CCAs) for good stability in nonpolar solvent. The color of a pixel is updated by changing the distribution of charged pigment particles of different colors between two electrodes with an external electric field. With the specific driving waveforms, different distributions of pigment particles are formed to provide various colors according to Grassmann's laws, which indicates the possibility of full-color E-paper.
Currently, smart sensor networks of the emerging Internet of Things (IoT) have to be self-powered by the energy in-situ harvested from the working environment, so as to avoid the impossible maintenance of numerous smart devices. This trend bring E-papers new opportunities, especially if a low-power E-paper can be integrated with an energy-generating device for self-powered displays.
Therefore, we report a unique self-powered E-paper (SPEP), where information inputs and energy supplies are all converted from mechanical motions by a sliding-mode triboelectric nanogenerator (TENG). The operation of an electrophoretic E-paper is firstly investigated, identifying the current density as a determinative parameter for driving pigment particles motion and color change. Electrical and optical responses of the E-paper driven by a sliding-mode TENG are then found to be consistent with that under a current source mode. All-in-one monochromic and chromatic SPEPs integrated with a flexible transparent TENG are finally demonstrated. The sliding-driven mechanism of SPEP allows potential handwriting function, is free of extra power supply and promises certainly a wide range of future applications.

Experimental studies on the physical properties of two-dimensional (2D) materials have grown exponentially since 2D materials offer unique advantages for use in such next-generation devices. Various semiconducting 2D materials have been studied, including transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten diselenide (WSe₂), which are likely to bring breakthroughs in future electronic and optoelectronic devices. The physical properties of 2D MoS₂ nanosheets have been actively explored particularly as a result of their possible integration in both nano/microelectromechanical devices and energy harvesting devices. Monolayer MoS₂ has a direct band gap and high mobility and has been used to successfully fabricate field-effect transistors so it has emerged as an interesting complement to graphene in various semiconducting applications. In additions it is expected that piezoelectric properties of 2D layered materials are very useful to realize highly sensitive, high impact resistive, self-powered sensor. However, studies on the suspended model of 2D MoS₂ have been done mainly on studies on the measurement method or photosensitivity, and studies using the piezoelectric characteristics of suspended MoS₂ have not been actively done. Here we demonstrate a new way to use 2D MoS₂ for highly sensitive 2D piezoelectric acoustic sensor. The acoustic sensor fabricated by using suspended structure of MoS₂ that generate piezoelectric output potential due to vibration transmitted by sound. We have achieved high sensitivity and high impact resistance by using single layer or several layers of MoS₂, and by using piezoelectric properties of MoS₂, we have produced an acoustic sensor that does not require standby power. This study demonstrates a new way of studying 2D MoS₂ research and acoustic sensors by developing a high-sensitivity and high-durability acoustic sensor using suspended MoS₂.

Despite many attempts to harness a strain-driven control of ferroelectric properties in thin films, there remains a technical hurdle to understand the delicate relationship between crystal structure, spontaneous polarization (P_s), and the electronic band gap (ε_gap) modulation under the anisotropic strain. To further our understanding of how these complex anisotropic structure-property relationships may be rationalized by their local atomic arrangements in ferroelectric materials, we perform first-principles density-functional (perturbation) theory calculations to understand how applied epitaxial strain may influence their structural, thermodynamic, electronic, and (anisotropic) polarization properties in polar KNbO₃ polymorphs – a potential contender for Pb-free piezoelectric applications. Specifically, we examine the displacement of the center metal cation (niobium, Nb) as a key descriptor for its polar behavior and find that it exhibits an anisotropic nonlinear relationship between εgap and the commonly-used isotropic scalar quadratic elongation (<λ>). To overcome the inadequacy of the simple <λ>, we propound a revised definition of this key structural descriptor – the new bond elongation index (λ_i), that contains vectorial structural information. Using λ_i, we successfully rationalize the linear dependency of direction-dependent P_s on λ_i for strained KNbO₃ polymorphs with a R² value of 0.998. We further demonstrate and verify that this newfound linear relationship can be used to predict direction-dependent P_s in BaTiO₃ polymorphs without further expensive calculations under strain, and thus allows for the fast and consistent determination of direction-dependent P_s in strained perovskite polymorphs in the course of high-throughput polar materials design.

Piezoelectric nanogenerator (PENG) is a kind of mechanical energy harvester, which utilizes the dipole moment in piezoelectric material to convert mechanical energy to electrical energy. Among all piezoelectric materials, PVDF-based piezoelectric polymer is one of the best materials for PENG due to its high piezoelectricity, light weight, simple fabrication, and high toughness and flexibility comparing with the ceramics. However, since the elasticity of PVDF-based polymers is low, the piezoelectricity in the 31 and 32 directions are underutilized. In this work, auxetic structure is 3D printed onto the drop casted piezoelectric film, of which the synclastic effect enables the bending mode energy harvesting of piezoelectric films by slightly stretching the film in 1 and 2 directions during bending, activating the 31 and 32 direction piezoelectric effect, at the same time the tensile strain of the film can be controlled to avoid over-stretching the device. The structure design and strain distribution are evaluated by finite element analysis. The piezoelectric composites made of P(VDF-TrFE) copolymer and surface modified barium titanate nanoparticles effectively improves the piezoelectricity. With the auxetic structure, the PENG can generate voltage over six times of that without the auxetic structure. Under slight bending, it generates 350 nW/cm² power density. The PENG is also demonstrated as a sensor to detect the motion at various positions on human body.

With the rapid development of 5G network and internet of things (IoT), there are incredibly increasing sensors and corresponding applications emerging to benefit our daily lives. However, the power supplies for the countless sensors become an issue to suppress the IoT further expansion. In this work, we present a strategy which applies triboelectric nanogenerator (TENG) as self-powered sensor in traffic control system. Flexible and reliable polyvinylidene fluoride (PVDF) embedded with multi-wall carbon nanotubes (MWCNTs) fiber-based composite membrane is synthesized by electrospinning to act as triboelectric layers. Vertical contact-separation mode is adopted to design the fiber-based triboelectric sensor toward detecting the incoming vehicles signals on roads. Through systematically investigating the impacts of MWCNT concentration and controlling the electrospinning parameters, the electrical output performances of the fiber-based triboelectric sensors are optimized and an ultra-sensitivity can be achieved for detecting the vehicles signals. Based on the sensors’ electrical charges outputs and the advanced cloud IoT services, a traffic control system including traffic flow management, offending vehicle such as overlapping and overspeed capture, and plate number recognition is developed to facilitate the potential application scenarios of the fiber-based triboelectric sensors. This engineered self-powered triboelectric sensors not only open a new potential area for TENG in traffic monitoring, but also significantly popularized the application of TENG in IoT industry.

Air pollution has become a major environmental concern due to the large amount of air pollutants emitted from human activities such as traffic, industry, and power plants. Particulate matter (PM) serious impacts the living environment in terms of air quality, visibility, climate effects and ecosystems. Haze is mainly formed by PM2.5 (PM with diameter of 2.5um or less), which results in serious health issues such as cardiovascular and respiratory diseases and even lung cancer, increasing the morbidity and mortality. Based on the coupling of the triboelectric effect and electrostatic induction effect, the typical characteristic of triboelectric nanogenerator (TENG) is its high voltage output of hundreds to thousand volts, which is promising for capturing the PMs using electrostatic effect without external power supply. Herein, a honeycomb ceramic based triboelectric PM vehicle exhaust filter (HTPMF) filled with fluorinated ethylene propylene (FEP) particles is introduced to efficiently capture the PMs from vehicle exhaust. Finite element analysis software (COMSOL) is applied to simulate the dynamic behaviors of the triboelectrification process of the PMs in the HTPMF. An open-circuit voltage of 63V can be achieved in one single hole of the HTPMF. Through well controlling the number of FEP particles, the achieved filtration efficiency for PM1.0, PM2.5, PM4.0, PM10 are all over 90% with the highest filtration efficiency of 93.49% for PM2.5. This honeycomb triboelectric PM filter presents promising potential in the application for industrial dust emission and vehicle emission removal.

Water and energy, as two fundamental components of our 21st-century life, cannot be considered separately. Solving this water-energy nexus becomes a promising strategy to relieve the global energy and water crisis facing humanity today. In this regard, the diversiform water-related energies, such as tidal energy, wave kinetic energy, and thermal differential energy, are being explored to generate electricity. Recently, interfacial energy harvesting, relying on the interfacial charges and relative motion between two different materials, is drawing significant attention, as it provides new pathways for collecting the low-frequency water energy. In spite of the extensive progress, the state of the art low-frequency water energy harvesting devices are generally limited by low energy density, poor durability, and hence it remains challenging for multiscale practical implementations.
In this talk, I will focus on high-performance water kinetic energy harvesting by designing novel slippery materials and novel electrode architecture. First, I will report an original droplet-based energy generator (DEG) with transistor-inspired architecture that fundamentally overcomes the physical limitation inherent in the traditional approaches, which are imposed by the undesirable interfacial effect, and achieves the highest energy conversion efficiency and power density. Second, inspired by hierarchical structures on the peristome of the Nepenthes pitcher, we will also report a novel slippery lubricant-impregnated porous surface (SLIPS)-based TENG, termed as SLIPS-TENG, that exhibits distinctive advantages including optical transparency, self-cleaning, impact tolerance, flexibility, and enhanced power generation stability even under harsh environments.
In the future, we envision fusing the advantages inherent in slippery interfaces and transistor-inspired architecture will provide a paradigm shift in the design of efficient energy harvesting devices, and trigger a new wave in the research of the water energy harvesting, including basic fundamental understanding, material design, as well as the multiscale practical applications.

Negative air ions (NAIs) have been widely harnessed in recent technologies for air pollutant removal and their beneficial effects on human health, including allergy relief and neurotransmitter modulation. Herein, we report a corona-type, mechanically stimulated triboelectric NAI generator. Using the high output voltage from a triboelectric nanogenerator, air molecules can be locally ionized from carbon fibre electrodes through various movements, with the electron–ion transformation efficiency reaching up to 97%. Using a palm-sized device, 1 × 1013 NAIs (theoretically 1 × 105 ions cm−3 in 100 m3 space) are produced in one sliding motion, and particulate matter (PM 2.5) can be rapidly reduced from 999 to 0 μg m−3 in 80 s (in a 5,086 cm3 glass chamber) under an operation frequency of 0.25 Hz. This triboelectric NAI generator is simple, safe and effective, providing an appealing alternative, sustainable avenue to improving health and contributing to a cleaner environment.

Emerging deformable energy harvesting systems gain a significant interest as power sources for wearable and implantable electronics because they acquire and convert readily available mechanical, ambient light, biochemical, and thermal energies from the human body and surroundings into electrical energy. Among deformable energy harvesting systems, piezoelectric nanogenerators (PNGs) are the most promising because they harvest the mechanical energy provided by the evermoving and deforming human body to empower wearable electronics. Although PZT is one of the most extensively exploited ceramic piezoelectric materials because of their high piezoelectric coefficient, high Curie temperature, and low cost, the inherent toxicity and rigidity of Pb-based PZT restrict its application to wearable electronics. Hence, tremendous efforts have been made to develop Pb-free perovskite-structured ceramic alternatives to solve the safety issues of PZTs, while the hybrid composite of Pb-free perovskite-structured ceramics with organic polymers ensures the deformability of f-PNGs as the power sources of wearable electronics. However, the excellent mechanical energy harvesting performance of flexible PNGs made of perovskite ceramic nanoparticles (PCNPs) embedded in piezoelectric polymer NFs is challenging to achieve because of the poor adhesion between the ceramic and polymer materials. Because of the weak interaction of the ceramics and polymers together with the very high surface-to-volume ratio of the nanoparticles, the PCNPs easily form agglomerates within the polymer NFs. In this work, we investigated the mechanical and piezoelectric properties of Pb-free NKNS-BZ-BKZ PCNPs uniformly embedded in c-NFs for f-PNGs. A significant reduction in the agglomerate size of the PCNPs and their enhanced interfacial adhesion with the P(VDF-TrFE) matrix were achieved through a tetradecylphosphonic acid (TDPA) surface treatment of the PCNPs. The surfaces of the NKNS-BZ-BKZ PCNPs were modified with TDPA to enhance the interfacial interaction of the PCNPs with the P(VDF-TrFE) matrices. We used the electrospinning method for fabricating the c-NFs of NKNS-BZ-BKZ PCNPs and P(VDF-TrFE) piezoelectric polymers. Particle size distribution analysis, transmission electron microscopy (TEM), and in situ tensile testing in a scanning electron microscope chamber were used to evaluate the effect of the TDPA surface treatment of the PCNPs on the mechanical properties of the c-NFs. The piezoelectric properties of the c-NFs were characterized through Kelvin probe force microscopy (KPFM) and PNG cell performance evaluation. The uniform distribution of the embedded PCNPs made the web of c-NFs highly negative relative surface potential and a high effective modulus of approximately 170 MPa. Consequently, the f-PNG with the TDPA surface-treated PCNPs maintained an open-circuit voltage, a high maximum generated power, and an output voltage of approximately 12.2 V, 33.2 nW, and 1.25 V, respectively, despite being bent for 10,000 cycles at a bending radius (r) of 15 mm.

Tactile electronic skin (e-skin) aims to mimic the functions of skin, which could have profound implications for areas including medical care, prosthetics and robotics. However, it faces a great challenge to simultaneously detect and distinguish between multiple stimuli by a single device. Here, a tactile e-skin that can simultaneously detect and distinguish between temperature and pressure in real time is developed, based on a single-electrode-mode triboelectric nanogenerator (TENG) with a specially prepared thermoresistive electrode combining BiTO and rGO. The TENG based tactile e-skin outputs voltage in response to pressure and its electrode resistance changes in response to temperature (thermal sensitivity β_25/100: 1024 K, temperature coefficient of resistance: -1.15 %/K at 25 °C, range: 25~100 °C). The pressure sensing performance maintains stable with varying temperatures and the temperature sensing performance is undisturbed under pressure. The tactile e-skin also demonstrates excellent sensing reproducibility and durability. When the e-skin is mounted on the human body, it can monitor and distinguish between temperature and pressure with the two signals imposing no interference on each other. This work provides new routes for wearable sensing and sheds new light on the development of electronic skin.

Multifunctional materials have received increasing attention in recent years. Owing to their inherent biological nature as well as piezoelectric properties, piezoelectric biomaterials are considered as promising candidates for application in fields ranging from electrochemical energy storage to biological systems. Rational material design is important to enhance their piezoelectric and biological activity. Herein, recent advancements to piezoelectric biomaterials like peptide-based micro/nanostructures are provided. Synthetic methods, morphological features, and piezoelectric performance of piezoelectric biomaterials are presented. The effect of growth direction, phase and structure of piezoelectric biomaterials on their piezoelectric activity are discussed. The applications of piezoelectric biomaterials in the field of nanogenerators are provided at the end.

Piezoelectric polymers hold physical properties, such as high mechanical flexibility and smaller acoustical impedance, that ceramics do not. Nevertheless, owing to the lack of dipoles, most non-crystalline polymers are hard to generate a high piezoelectric response. Here, we developed a class of hybrid electret films with high piezoelectric performance via electrostatic self-assembly. Their piezoelectric effect stems from an out-plane electric-dipole structure in the inner dielectric polymer constructed by two electret films. The piezoelectret exhibits direct and converse piezoelectric effect (d₃₃ of 77 pC/N, g₃₃ of 4 V m/N), pyroelectric effect, and high mechanical flexibility. Additionally, we also developed different hybrid piezoelectric films and applied them to wearable electronics, such as ultrathin strain sensors and acoustic transducers, based on the ultrathin characteristic (~25 µm thickness), providing a different class of polymer piezoelectric film.

The practical use of biomechanical energy is demonstrated with flexible single crystal PMN-PZT energy harvesters. A flexible PMN-PZT piezoelectric energy harvester is used to demonstrate powered, wireless data transmission activated by biomechanical energy in vivo derived from the movement of the heart in a large animal model. In the contraction and relaxation of the pig's heart, the single crystal flexible energy harvester produced an open circuit voltage of 17.8V and short circuit currents 1.74μA, 4.45 and 17.4 times higher. To demonstrate the biocompatibility of the piezoelectric energy harvesting PMN-PZT flexible energy harvester, various cell viability tests and histological methods were conducted, but no signs of biological damage and inflammatory reactions were shown. Finally, an automated driving system was used to collect the pig's heart energy and transmit communication data wirelessly. It is also visually proven to repeat the light bulb on and off at a distance of about 5 meters without using another power source. This successful automated driving data transfer demonstrates potential applications in implantable biomedical devices that use direct biomechanical energy.

In recent years, triboelectric nanogenerators (TENGs) have been the center of attention for research due to its wide range of applications as a microsystem component. This environmentally friendly energy framework can be implemented in advanced healthcare monitoring as a wearable electronic for sensory applications. Unfortunately, the production cost of TENGs are high and lead to loss of accessibility and production. Our research focuses on a triboelectric universal health monitoring device (TUHMD) that is biocompatible, highly sensible, self powered andeconomically advantageous simple structured, multifunctional, and wearable. The fabrication of the device shows the combination of cellulose paper and polydimethylsiloxane (PDMS) / polytetrafluoroethylene (PTFE) copolymer electrodes. The addition of this gadget into a wearable hardware provides real-time data of physiological movements and vibrations. The TUHMD shows promise as it can potentially be used for personalized medicine, rehabilitation, and remote monitoring of patients. The TUHMD reported a response as a triboelectric nanogenerator with a range of 12 V with negligible charge accumulation, along with a maximum capacitive performance of 11 F. The smart device demonstrated the capability of maintaining full health care monitoring applications.


Keywords
Nanogenerator, Triboelectric nanogenerator, Wearable electronics, Motion Sensor, Health care Monitoring, Remote monitoring

The seamless and adaptive interactions between functional devices and their environment are critical for advancing emerging technologies. The state-of-the-art technologies, however, require a complex integration of heterogeneous components to interface the environmental mechanical stimulus, which is ubiquitous and abundant in the above applications. Here we report a convenient layer-by-layer spray printing method to fabricate Triboelectric Nanogenerator (TENG) sensing unit for tactile sensing. In addition to the general optimization of fabrication process, we also carry out learning-based quantitative study between TENG signals and external factors to extract more in-depth relationship. This research is expected to help construct a smarter sensing system with potential application in biomedical monitoring, consumer electronics, and intelligent robotics.

In this communication, we present the advantages of the application of the remote plasma assisted vacuum deposition method for the development of nanocomposite layers of organic functional molecules as adamantane and Spiro-OMETAD and polymers as PVDF. The synthesis is carried out at room temperature and under a low power plasma activation in an ERC-MW reactor to avoid energetic species or UV radiation of the plasma to reach the substrate surface. This methodology has been previously applied for the fabrication of photonics films based on organic dyes and small functional molecules working as optical sensors, optical filters, tunable photoluminescence emitters and lasing gain media. We present for the first time the exploitation of this approach to develop PENGs and UV photodetectors in combination with texturized ZnO thin films fabricated by plasma enhanced chemical vapour deposition.

Triboelectric nanogenerator (TENG), since its first invention in 2012, has been proved as a promising energy harvesting technology and widely adopted in energy generation, self-powered sensing, actuation, and self-sustained systems, etc. In recent decades, advances in the Human Machine Interface (HMI) have improved from tactile sensors, such as touchpads and joysticks, to now include the accurate detection of dexterous body movements in more diversified and sophisticated devices. In this talk, we discuss a few triboelectric based human-machine interfaces with unique design of electrode patterns aiming for the gaming control. In addition, artificial intelligence (AI) as an effective data analytics tool has been integrated with various triboelectric sensors to achieve intelligent monitoring and recognition system. The AI-enabled triboelectric glove-based HMIs are investigated for gaming and VR/AR applications. On the other hand, the AI-enabled soft robotic manipulator is demonstrated with object recognition capability. Lastly, a smart floor monitoring system is reported with the merits of low cost, high scalability, and self-powered sensing capability, based on the integration of triboelectric sensors and deep learning-based data analytics.

High resolution imaging is a key technology for physics, biology, medicine and material development. Most high resolution imaging relies on cost, large volume and complex equipment. It is desired to have integrated high resolution imaging for invasive internal diagnosis and low cost drug discovery. Here we report our recent results on integrated high resolution imaging based on micro-/nanomaterials: chip-based super-resolution imaging [1-4] and multimode fiber based high resolution endoscope [5,6].

A majority of developed super-resolution techniques rely on nonlinear response to achieve nanoscale resolutions, while their low throughput could limit their practical applications. To achieve fast imaging with a large field-of-view (FOV), linear super-resolution microscopy via synthesis aperture approach is an alternative way that can be used. However, current linear super-resolution imaging is limited in resolution because a missing spatial-frequency band occurs when trying to use a shift magnitude surpassing the cutoff frequency of the detection system beyond a factor of two. In the presentation, we will show our recent results in wide-field far-field super-resolution imaging based on tunable frequency shifting. The missing of spatial-frequency can be effectively solved by developing a spatial-frequency-shift actively tuning approach through wave vector manipulation and operation of optical modes propagating along multiple azimuthal directions on a waveguide chip to interfere.
Multimode fiber, as a potential minimally invasive endoscope, possesses larger mode capacity, higher efficiency of light collection, and much lower cost than fiber bundle. The main challenge in multimode fiber imaging is the complex light propagation process inside multimode fibers: light suffers from modal scrambling caused by environmental fluctuation, meaning that an image injected from one end of a fiber will become scrambled as very little fluctuation of environments. In this presentation, we propose some potential methods to increase the contrast-to-noise ratio (CNR) and stability of multimode fiber imaging. Wavelength modulation is introduced to suppress the background without increasing the number of controllable modes. Bending effect was investigated carefully and used to improve the imaging stability. We combine exhaustive bending and Convolutional Neural Networks to find the bending status precisely and compensate it fast. The results will improve the flexibility of multimode fiber imaging system, paving the way for deep and volumetric tissue endoscope imaging based on multimode fiber.
Acknowledgement
The authors are grateful to the supports from the National Natural Science Foundation of China (No. 51672245, 61822510, 61735017, 62020106002), the Zhejiang Provincial Natural Science of China (No. R17F050003), the Zhejiang University Education Foundation Global Partnership Fund.

Flexible intelligent sensor-stimulation integrated devices can dynamically capture the physiological signals of human tissues and organs, sensing real-time health information, and provide timely and accurate stimulation or treatment for injuries and diseases, which plays a key role in the next generation of intelligent healthcare systems.[1] However, almost all of the wearable electronic devices rely on some form of power supply and battery, and still face the problems of large volume and short working life. If the wearable electronic devices can be attached to the organism to collect the energy generated by low-frequency motion and directly applied to the flexible intelligent sensor-excitation device, it can get rid of the traditional battery and realize the infinite power supply of symbiosis in a sense.[2-4] Piezoelectric materials are the core part of piezoelectric electronic devices and the decisive component to realize the conversion of mechanical energy to electrical energy. Therefore, the research and development of high energy density piezoelectric materials is of great significance for the practical application of miniaturization and high sensing excitation performance flexible electronic devices. However, under the condition of large deformation and extensibility, rigid piezoelectric components are seriously limited due to the relaxation of human-machine interface coupling, low electromechanical coupling conversion/ wearing comfort and brittleness, resulting in short working life. It is not appropriate to directly use hard materials as functional components of wearable devices.
Here, a flexible wireless passive piezoelectric polymer polyvinyldiene fluoride (PVDF) film stimulation device is designed for effective wound healing and repair. Firstly, compared with the traditional inorganic piezoelectric materials, organic piezoelectric polymer PVDF has the advantages of excellent stability, flexibility and low density.[5] Secondly, we designed the flexible PVDF film into grids, at the same time, the pyramid microstructure is introduced. The introduction of the microstructure plays an important role in improving the comprehensive performance of the flexible pressure sensor. Compared with the flexible device that is not combined with the pyramid structure, the piezoelectric output of this device has been increased from the original (3.43 V) to the current (4.61 V), and it can continue to work stably for at least 60 minutes. This provides the possibility for effective wound treatment. In summary, flexible piezoelectric film devices can not only prepare flexible wearable devices for disease treatment, but also the passive devices can get rid of the traditional batteries, which makes the devices further develop towards lightweight, miniaturization and integration. Therefore, the application of piezoelectric passive devices in wound treatment is an important research direction in the field of wearable devices.
References:
[1] Gao, Y., Yu, L., Yeo, J., Lim, C. Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability. Adv. Mater. 32 (2019) 1902133.
[2] Chu, B., Burnett, W., Chung, J., Bao, Z. Bring on the body NET, Nature, 549 (2017) 328.
[3] Wang, X., Dong, L., Zhang, H., Yu, R., Pan, C., Wang, Z. Recent Progress in Electronic Skin, Advanced Science. 2 (2015) 1500169.
[4] Yao, G., Kang, L., Li, J., Long, Y., Wei, H., Ferreira, C., Jeffery, J., Lin, Y., Cai, W., Wang, X. Effective weight control via an implanted self-powered vagus nerve stimulation device. Nat Commun. 9 (2018) 5349.
[5] Sang, M., Wang, S., Liu, S., Liu, M., Bai L., Jiang, W., Xuan, S., Gong, X. A Hydrophobic, Self-Powered, Electromagnetic Shielding PVDF-Based Wearable Device for Human Body Monitoring and Protection. ACS Appl. Mater. Interfaces. 11 (2019) 47340.

With the development of smart city which has been considered as a new lifestyle in the future, the Internet of Things (IoT) technology becomes more and more mature and important. According to a recent report there will be 20 billion devices connected by 2020. Nowadays, with a great progress in communication and embedded technology, emerging technologies such as the big data transfer and microprocessor distributed computing are being rapidly developed. Therefore, research on sensors requires sensing systems to be accurate, stable and small. However, in traditional sensing systems, the power supply circuits are usually complex and may cause environmental issues. In this case, self-powered devices which could work without external power source provide a more sustainable solution to meet the power requirement of such a big sensor network. Triboelectric nanogenerators (TENGs) which work based on the triboelectric effect and electrostatic induction have a list of great advantages, such as high sensitivity to detect motions, significant output voltage, flexibility, light weight, low cost, environmentally friendliness. With that, TENG plays an important role in detecting movement, direction, force, speed, and acceleration signal. However, the extremely-high-voltage output from TENG makes it difficult to directly power most electronics. By using the output electrical signals from TENGs to trigger the traditional electronic control devices, the extremely-high-voltage output from TENGs could break down these electronic devices. Furthermore, the TENG’s output current is too low to meet the power requirement of some switches. Alternatively, TENG’s output signal could be transferred to other kinds of signals which are easily detected and processed, such as transferring to optical signal to realize free-space optical communication. The switching devices which can directly transfer the high-voltage TENG output signal to other types of signals that can be easily processed are highly preferred. On the other hand, the delay characteristic plays an important role in fields of control and signal transmission. It brings convenience and energy-saving features to daily life, for example the delay light control system and the delay air conditioner control system. It can maintain safe operation of systems, such as the alternating current (AC) motor control and the time-limit current quick-break protection. It is also used in controlling the time delay of signals to improve the system’s bandwidth, a.k.a. delay line. A wide kinds of delay characteristic switches have been designed with different mechanical structures or electrical circuits. However, current devices are usually complex and need external power source. It is demanded to achieve delay characteristic switching function in a simple but self-powered way, enabling smart systems. With the principle of dielectric polarization and electrostatic force, an electrostatic force based self-powered switch (EFS) which can work as an electrostatic force optical switch (EFOS) or a controlled electrostatic force electrical switch (CEFES) has been developed. It could be used to detect slight motions through TENG, or work as a high-threshold-voltage (over 1000 V) switch with a high sensitivity and delay characteristic. Upon applying a voltage on the EFOS, the nitrile butadiene rubber (NBR) film in EFOS will be attracted by the electrostatic force to block the light beam with a certain delay time, converting high-voltage electrical signal to optical signal. CEFES could conduct a controlled circuit when the control voltage reaches the threshold value with the delay characteristic. The self-powered EFS is achieved by using a freestanding sliding mode TENG (FS-TENG) as the power and controlling unit to realize the function of delay-characteristic self-powered sensing, which can be applied in various fields such as coding communication, human-machine interface, architecture auto-monitor, etc.

Water is ubiquitously existing on the earth. Natural water sources including rainfall, river water, and ocean waves always contain a certain amount of mechanical energy. When water contacts a specific solid surface, it will either generate a static surface tribo-charge or lead to electrostatic induction. In some circumstances, these both occur. By properly using the static charges generated by water/solid contact and the electrostatic induction process, the mechanical energy contained in the water movements can be converted into electrical energy. In this presentation, I will first introduce the mechanism of electricity generated from water/solid contact and then demonstrate some related application cases.

Deformable and wearable soft devices have attracted great interest because they cannot only extend the scope of smart systems but also provide compliant user experience. Operating those devices inevitably need power sources. However, traditional batteries suffer from heavy weight, bulky volume, and limited capacity and lifetime, hindering the progress and practical uses of those emerging devices. Toward the soft future, it is necessary to explore new energy technology. In this talk, deformable, mechanically-durable, green, and even self-healable energy-collecting triboelectric nanogenerators will be demonstrated for not only serving as new energy providers but also self-powered sensing uses. First, super-stretchable triboelectric nanogenerators will be presented for generating electricity by contacting with other materials regardless of various extreme deformation required from uses, such as extreme stretching, multiple twists and folds. Particularly, even experiencing severe tearing damages, the device can retain its functionality to act as a power source for other components. Such technology can also be introduced into fabric materials for wearable energy and fabric-based self-powered sensing uses. Then, we will discuss the use in self-powered and deformable electronic skins that can actively sense proximity, contact, and pressure to external stimuli via self-generating electricity. Further, the perfect integration of the tribo-skins and soft actuators enables soft robots to perform various actively sensing and interactive tasks including actively perceiving their muscle motions and working states, checking baby diaper conditions, and even detecting subtle human physiological signals. The self-generating signals can even drive optoelectronic devices for visual communication and be processed for diverse sophisticated uses. Then, a self-healing, highly-transparent, and super-stretchable triboelectric nanogenerator with energy-extracting and activity-sensing abilities will be demonstrated in the use of self-powered electronic skin, human-interactive interfaces, and so forth. Last, a triboelectric nanogenerator based on recycled coffee waste is presented for future low-cost and green energy devices. These works may open the crucial doors for the tremendous potentials of wearable/stretchable/deformable electronics, and energy devices.

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Recently. the demand for implantable medical devices has steadily increased as the number of the adult patients is on the rise with westernized diet and aging population. Implantable medical devices normally use batteries, which have a limitation in the lifetime and a need for replacement surgery at the end of life. To overcome these limitations of the batteries, various studies have been conducted and a paper using triboelectric nanogenerator which is capable of transmitting vibration energy using ultrasonic waves from the outside of the body, has been published. The implantable energy harvesting devices for medical devices need to be packaged with titanium plate when they are implanted in the body to avoid unintended immune response or inflammatory reaction from toxic material. However, there is a major disadvantage that large amount of energy is reflected and lost from the titanium surface because titanium has large acoustic impedance compared to the human body tissues. Therefore, we produced an ultrasound implantable triboelectric nanogenerator with bio-implantable and modulus controllable HEMA-based hydrogel which does not require titanium packaging. The voltage and current generated in-vivo by ultrasonic energy transmission reached 3.42 volts and 130 microamps under rat’s tissue which can charge the 100 μF capacitor at the rate of 23.9 μC/s. This HEMA-based US TENG shows higher output than the titanium packaged US TENG device and it generates sufficient energy level to charge the batteries of implantable medical devices.

A pressure sensor is one of the essential electronic devices for human-machine interfaces that enables monitoring from low-level pressure such as vibration and acoustic signals to moderate-level pressure such as physical body signals, including heart rate and blood pressure. Capacitive, resistive, piezoelectric, and triboelectric pressure sensors are commonly used as an electrical signal modifier or transducer responding to mechanical stimuli. Among those sensors, piezoelectric and triboelectric based pressure sensors have been established as self-powered active sensor as they do not consume electrical energy during sensing. Recently, triboelectric-based active sensors are exponentially used, having many advantages such as abundant materials candidate, fast response, and broad dynamic sensing range with linearity. Furthermore, many studies employed hierarchical nano/microstructure in the friction surface of the triboelectric pressure sensor since the micropattern structure improves deformability and contact area variation and enhances the frication surface and charge density. However, the effect of deformation and contact area variation is not significant in a small pressure range (<1kPa), which results in incorrect information in the low-level pressure signal. Herein, a highly sensitive and amplified electrical signal upon low-level pressure input can be achieved by using the convolution of electrostatic induction in a micropatterned triboelectric nanogenerator. By defining electrostatic induction in individual micro-ridge of the microstructure, distance variation between the two separate films can be continuously encoded. The microstructure was fabricated by mold casting of PVDF-TrFE using a commercial 3D printer (Formlabs), and electrodes were prepared by spray coating of AgNW. Face centered cubic (FCC) structure induced-hemisphere micro-ridge patterns were fabricated with diameter varied from 100 to 1000 micro-meter—of which scale pattern cannot be achieved in etched Si mold, and FCC interlocked structure of two separate films enabled spacer-free paired triboelectric system. To identify the electrostatic induction in microstructure, electrodes were embedded at a different distance from the friction surface, and PVDF-TrFE/AgNW/PVDF-TrFE layered structures were formed. Here, spray coating of AgNW on the micro-structured PVDF-TrFE films enabled the conformal distribution of electrode along every micro-ridge morphology, which was confirmed by FE-SEM images and EDS mapping of the respective film layers. As the electrode distance from the friction surface decreased, the open-circuit voltage and short circuit current are exponentially increased. Here, to exclude the capacitance increase effect due to the thinner dielectric layer in a closer distance electrode system, flat samples are also prepared alike the micropattern PVDF-TrFE/AgNW/PVDF-TrFE layered structure. Compared to the linear increase of electrical output when varying the distance between friction surface and electrode in the flat samples, an exponential increase of output signal in micropatterned samples with varying electrode distance can be explained by the convolution of electrostatic induction term from every near micro-ridge. The electric field applied on every surface of the micro-ridge can participate in the triboelectric charge generation and, finally, sensitively respond to every distance variation from individual micro-ridge patterns. This enhancement is typically helpful for detecting pressure levels under 1kPa.

Maximizing resource recycling and finding renewable energy sources are important for a sustainable environment considering the development and consumption of electronic products with the advancement of the internet of things and artificial intelligence technologies. Herein, an economic and environment-friendly triboelectric nanogenerator derived from the discarded coffee ground waste (CG-TENG) is developed to serve as a light-weight and shape-adaptive energy source and self-powered sensitive sensor (CG-TENG sensor). The coffee ground, embedded into the silicone rubber elastomer, is first used as the metal-free electrode feedstock to minimize waste generation. Based on the triboelectrification and electrostatic induction, the shape-adaptive CG-TENG, as wearable power, is capable of harvesting surrounding energy from human motions and extreme deformations, exhibiting excellent stretchability and mechanical durability. The generated electricity can be stored in the conductive coffee ground-derived capacitors to drive portable electronics, which is beneficial for building completely green power systems using coffee waste. Furthermore, the self-powered CG-TENG sensor with optimized structure endows its ultra-high sensitivity for sensing human physiological signals, monitoring motions, emulating gestures, as well as for developing smart tactile epidermal controller and intelligent vending coaster, paving the way for building large-scale energy-efficient artificial sensors and eco-environmental wearable electronics towards humanoid robotics and human-machine interfaces.

Human-machine interface (HMI) has become an indispensable part of our lives. From the original keyboards to the virtual reality today, the goal of HMI is to make life more convenient and fascinating. In certain scenarios, for example, when a user’s two hands are fully occupied, or the disabled expect to achieve necessary self-care, a hands-free operation via eye movement or voice is most definitely the approach of choice for human-machine interfacing. Based on the unique advantages of high sensitivity, low cost, light weight, applicability of structure design, prominent stability, robustness, etc. of triboelectric nanogenerator (TENG), we design a mechnosensational TENG (msTENG) and a triboelectric auditory sensor (TAS) for controlling the equipment via eye-blink and interacting with the robotic via voice, respectively. While in the scenarios of manufacturing industry, deep-sea exploration, telesurgery and disaster salvation etc., gesture control can help bringing about an intuitive interface instead of operation based on amount of learning and training. From this point, we design a joint motion triboelectric quantization sensor (jmTQS) for constructing a robotic hand synchronous control system. Then, keeping on investigating into intuitive interface and coming down to the universal experience in daily life, some traditional interactive modes such as mouse and keyboards, still have a strong vitality and practical value. Thus, we develop a 3D triboelectric touch pad (3D-TTP) for an effective and efficient touch-screen graphical user interface. These studies cover the control modes of eye-movement, voice, gesture, and touch of HMI nowadays. While in the feedback phase of HMI, TENG as a highly sensitive sensor transform mechanic movements into electric signals can also do benefits as in visual, auditory, and tactile modes. Take the TAS study above for example, this resonant frequency-tunable auditory sensor is also a hearing aid for people with impaired hearing in the feedback stage between robotics and them. Further, the 3D-TTP can be another kind of electronic skin of the robotics. With above successful attempts, we may foresee more potentials of TENG-based sensors in the control-feedback loop of HMIs.