A magnetized microneedle-array based flexible triboelectric-electromagnetic hybrid generator for human motion monitoring

Journal Pre-proof A Magnetized Microneedle-Array Based Flexible Triboelectric-Electromagnetic Hybrid Generator for Human Motion Monitoring Yuanyuan Li, Zhipeng Chen, Guizhou Zheng, Wenhao Zhong, Liyuan Jiang, Yawen Yang, Lelun Jiang, Yun Chen, Ching-Ping Wong PII:

S2211-2855(19)31130-9

DOI:

https://doi.org/10.1016/j.nanoen.2019.104415

Reference:

NANOEN 104415

To appear in:

Nano Energy

Received Date: 12 October 2019 Revised Date:

8 December 2019

Accepted Date: 18 December 2019

Please cite this article as: Y. Li, Z. Chen, G. Zheng, W. Zhong, L. Jiang, Y. Yang, L. Jiang, Y. Chen, C.-P. Wong, A Magnetized Microneedle-Array Based Flexible Triboelectric-Electromagnetic Hybrid Generator for Human Motion Monitoring, Nano Energy, https://doi.org/10.1016/j.nanoen.2019.104415. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

A Magnetized Microneedle-Array Based Flexible Triboelectric-Electromagnetic Hybrid Generator for Human Motion Monitoring Yuanyuan Li

#, a

, Zhipeng Chen#, a, Guizhou Zheng a, Wenhao Zhong a, Liyuan Jiang a, Yawen Yang a,

Lelun Jiang*, a, Yun Chen b, and Ching-Ping Wong b a

Guangdong Provincial Key Laboratory of Sensor Technology and Biomedical Instrument, School of

Biomedical Engineering, Sun Yat-Sen University, Guangzhou 510006, PR China b

Faculty of Engineering, The Chinese University of Hong Kong, Shatin, Hong Kong

* Corresponding author, Tel: +86 20-39332153, E-mail: [email protected] #

These authors contributed equally to this work.

Abstract: Most triboelectric-electromagnetic hybrid generators (TEHGs) are not suitable for monitoring human motions as wearable self-powered sensors owing to the rigid, complicated and bulky structures. In this work, we developed a novel magnetized microneedle-array (MA) based flexible TEHG for monitoring human motions. The magnetic-field-induced spray self-assembly method with electro-magnetizing technique was firstly proposed to cost-effectively fabricate magnetized microneedles for TEHG. The magnetized microneedles in TEHG simultaneously serve as the frictional layer of triboelectric generator (TEG) and bendable magnetic poles of electromagnetic generator (EMG). TEG utilized the closed bending-friction-restoring behavior of microneedles to generate triboelectricity while EMG used the rotation of magnetized microneedles to produce the induced electromotive force. TEG could output the open-circuit voltage (VOC) of 10 V and the EMG could generate the short-circuit current (ISC) of 80 µA under the 30 N and 1 Hz compressing-releasing operations. We demonstrated that the MA-based TEHG, integrated in insole and attached on elbow as the self-powered sensors, could exactly detect triggering frequency of human motions. Considering the unique advantages, including flexibility, small dimension, light weight, and easy scale up, further applications of TEHG in wearable electronics, self-powered sensors, and healthcare monitoring system are promising. Keywords:

Triboelectric

generator;

Electromagnetic

generator;

Hybrid

generator;

Magnetic-field-induced spray self-assembly; Microneedle array 1 Introduction Wearable electronics have attracted wide attention for the last decade owing to expanding our capabilities to interact better with the environment and objects [1-5]. Human body produces considerable biomechanical energy during daily activities, which can be used for wearable self-powered sensors [6-8]. Four main mechanisms have been exploited to scavenge biomechanical energy from human activities, including the electrostatic [9-11], piezoelectric [12-16], electromagnetic [17-19], and triboelectric mechanisms [20-30], among which the triboelectric generator (TEG) and electromagnetic generator (EMG) are the two most efficient approaches [31]. Therefore, hybridization of TEG and EMG into a singular triboelectric-electromagnetic hybrid generator (TEHG) to realize the complement of their individual advantages is a fascinating idea [32-35]. TEHG has a broadband range of frequency operation as well as high sensitivity to small excitation amplitudes [36]. TEHG can also simultaneously output relatively high power density, high open-circuit voltage (VOC) of TEG, and high short-circuit current (ISC) of EMG [37]. Therefore, the TEHG possesses many advantages in

self-powered sensing. Currently, plenty of TEHGs with various structures have been developed for scavenging ambient mechanical energy, such as the floating oscillator-embedded TEHG [38], rotating-disk-based TEHG [39, 40], rotating-sleeve structured TEHG [41], waterwheel-like rolling TEHG [32, 34], flexible tube-based TEHG [42], ferrofluid-based TEHG [43], ferromagnetic nanoparticle-embedded TEHG [44], and so on. However, most TEHGs are too cumbersome (>1 kg) or complicated to be conveniently integrated on human body [45]. Zhang et al. [45] developed a small-scale contact-separation TEHG for scavenging biomechanical energy to power wearable electronics by human walking. Among the reported TEHGs, it might be the smallest in volume (50×50×25 mm3) and the lightest in weight (approximately 60 g). However, this TEHG is still slightly bulky and rigid that is not very suitable for wearable electronics. Thus, developing novel TEHGs for wearable electronics with the characteristics of flexible, easy to use, cost-effectiveness, and light-weight remains challenging. Herein, a novel magnetized microneedle-array (MA) based TEHG was developed for human motion monitoring. The MA-based flexible TEHG has a small size of 24×24×3.2 mm3 and a light weight of only 2.8 g. The TEHG operates based on both electromagnetism and triboelectrification of magnetized microneedles. A Roll-to-Roll manufacturing process including magnetic-field-induced spray self-assembly, thermal curing, and electro-magnetization was firstly developed for cost-effective mass production of magnetized microneedles for TEHG. In the following work, the mechanism and process of magnetic-field-induced spray self-assembly of microneedles were investigated. The working mechanism and operation performance of magnetized MA-based TEHG were analyzed. Finally, the MA-based TEHG was integrated in insole and attached on elbow as the self-powered sensors for human motion monitoring. 2 Experimental 2.1 Preparation of curable magnetoliquid The iron nanoparticles (average diameter: 500 nm, Guangzhou Metal Metallurgy Company, China) and NdFeB microparticles without magnetization (average diameter: 10 µm, Magnequench, China) were mixed at a weight ratio of 3:4 as the magnetic particles. Polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning, USA) with approximately 10 % curing agent was uniformly mixed with toluene (Guangzhou Chemical Reagent Factory, China) in a weight ratio of 11:13 as the prepolymer carrier. Subsequently, approximately 7 g magnetic particles were added in 10 mL carrier with sufficient stirring to prepare a homogeneous curable magnetoliquid. 2.2 Fabrication of magnetized MA The fabrication process of magnetized MA consists of the magnetic-field-induced spray self-assembly, thermal curing, and electro-magnetization. (1) Magnetic-field-induced spray self-assembly of liquid MA: the aerosol of prepared curable magnetoliquid was sprayed from an airbrush at a speed of approximately 0.2 m/s onto a PDMS substrate (thickness: 0.5 mm) under an external magnetic field of approximately 300 mT (Figure 1a). The sprayed magnetoliquid droplets were spontaneously assembled and gradually formed into microneedles. The spray process, lasting for 15 seconds, was recorded by an industrial digital camera (CM2000, KUYNICE, China). (2) Thermal curing of liquid MA: the formed liquid microneedles were heated and solidified at 80 °C for 2 hours

under the external magnetic field. The morphology of microneedles was observed by a scanning electron microscope (SEM, JSM-6380LA, JEOL, Japan). (3) Electro-magnetization of solid MA: the solidified microneedles were magnetized using an electro-magnetizing device (MA-2030, Shenzhen JiuJu Company, China). 2.3 Assembly of TEHG TEHG consists of a copper coil sheet, two oppositely magnetized MAs, four aluminum (Al) electrodes and two polyethylene (PE) films (Figure 2a). The sealed copper coil sheet (thickness: 0.6 mm, inner diameter: 1.6 mm, out diameter: 20 mm, thread pitch: 0.2 mm, and internal resistance: 1.7 Ω) was customized. Two magnetized MAs (diameter: 20 mm) were symmetrically assembled on two sides of coil sheet. Two lead wires were connected to the two endpoints of copper coil to obtain electrical output of EMG part. Four Al foils (size: 20×20 mm2 and thickness: 20 µm) were employed as the electrodes for TEG part. Four lead wires were connected to the Al electrodes to get electrical output of TEG. The TEHG was heat-sealed using two PE films (size: 24×24 mm2 and thickness: 160 µm, Runwen Company, China) with the dry air left in the TEHG. 2.4 Performance evaluation of TEHG As the compressions (force: 10, 20, 30, 40, and 50 N, frequency: 0.5, 1.0, 1.5, and 2.0 Hz) were applied on TEHG, the output open-circuit voltage (VOC) and short-circuit current (ISC) were measured using an electrometer (6514, Keithley, USA) and an electrochemical workstation (CHI660E, Chenhua Company, China), respectively. The mechanical performance of TEHG was measured using a universal material testing machine (LLOYD, LR10Kplus, UK). Approximately 50 N compression force was applied on the TEHG at a frequency of 1 Hz for 3000 cycles. The TEHG was assembled in an insole. A 65-kg volunteer was asked to walk (speed: 2.2 km/h) and jog (speed: 3 km/h) on a treadmill wearing the shoe, and the VOC of TEG and ISC of EMG were simultaneously recorded. A TEHG was attached on inner elbow, the volunteer was asked to bend the limb approximately to 90° with a frequency of 0.35 Hz, and the VOC of TEG and ISC of EMG were also simultaneously recorded. 2.5 Finite Element Analysis (FEA) FEA was

employed

to

analyze

the

formation

mechanism

of

microneedles

using

magnetic-field-induced spray self-assembly method. The geometry of microneedles of five states (t≈ 0.1 s, 1 s, 5 s, 10 s, and 15 s) were obtained from optical images and their FEA models were built under 300 mT external magnetic field using COMSOL Multiphysics (Version 5.3, COMSOL Inc., Sweden) (Figure S1a). The magnetic field distributions at the five states were numerically calculated using the equivalent magnetic charge method (Figure S1b-c). The spray process of magnetoliquid micro-droplets towards the microneedles was also simulated by COMSOL Multiphysics. The multi-physics model consisting of magnetoliquid microneedles and airflow was built (Figure S2). The initial spray velocity of micro-droplets was set as 0.2 m/s and the external magnetic field was 300 mT. The magnetic field distribution was firstly calculated using equivalent magnetic charge method. The airflow was subsequently calculated using a stationary and incompressible laminar flow model. The Particle Tracing Modular was employed to analyze the flow path of the micro-droplets during the spray process based on the calculated FEA results of magnetic field distribution and airflow. The trajectory of magnetic micro-droplets under different initial spray

velocity and magnetic field intensity were also analyzed, as shown in Figure S3. The electrical potential distribution of one TEG and magnetic field distribution of EMG part were also calculated using COMSOL Multiphysics (Figure S4). The MA models with rotation angles of 0°, 10°, 20°, 30°, 40°, 50°, and 60° were constructed. The height and mid-diameter of microneedles were 1.2 mm and 0.2 mm, respectively. The relative permittivity of magnetized MA and air were set as 2.75 and 1.0, respectively. The relative permeability of magnetized MA and air were set as 1.42 and 1.0, respectively. The residual magnetization of the magnetized MA was set as 66 kA·m-1 based on the measured hysteresis loop (Figure S7). The surface charge density of MA was assigned to -50 nC/m2 [46]. The electric field distribution of one TEG was analyzed using the “Electrostatics” module. The magnetic field distribution of the EMG was analyzed using the “Magnetic Fields, No Currents” module. The detailed descriptions of above FEA processes can be found in the Supporting Information. 3 Results and discussion 3.1 Fabrication of magnetized MA The formation process of microneedles using the magnetic-field-induced spray self-assembly method was optically observed and analyzed (Figure 1 and Video S1). The microneedles were spontaneously self-assembled and gradually grew up on the PDMS substrate with continuous spray of the magnetoliquid aerosol under the external magnetic field, as shown in Figure 1a&b. The tiny droplets of magnetoliquid aerosol randomly deposited and many aggregations formed on the substrate at the initial state (t ≈ 0.1 s). With continuous spray of magnetoliquid aerosol, the magnetic aggregations grew up along the magnetic field direction and spontaneously formed into ordered microneedle structures (t ≈ 1 s). The microneedle structures continuously attracted the tiny magnetic particles to the tips and rapidly grew up in several seconds (t ≈ 5 s). Meanwhile, some adjacent growing microneedles aggregated with each other and became larger microneedles (t ≈ 10 s). The growing microneedles always kept in rough conical shape with extremely sharp tip during the spray self-assembly process. Gradually, the growth speed of microneedles slowed down (t ≈ 15 s). So, we artificially set the spraying time as 15 s to ensure the fabrication efficiency. The trajectory of the magnetic micro-droplets is greatly affected by the initial spraying velocity and magnetic field intensity (Figure S3). The optimized parameters for fabrication of liquid MA are 0.2 m/s initial spraying velocity and 300 mT magnetic field intensity. The average height and mid-diameter of microneedles increased while the growth speed and density synchronously decreased during spray self-assembly process, as shown in Figure 1c. The spray self-assembly process of microneedles is similar to the growth of grass sprout in nature. The particles in atomized micro-droplets of magnetoliquid aerosol sprayed from airbrush were magnetized by the external magnetic field, inducing the strong anisotropy of dipolar magnetic forces. The additional magnetic force on the magnetized particles promoted the formation of magnetically aligned chains of magnetic dipoles. The micro-droplets were successively attracted by the chains in head-to-tail arrangement, forming larger chainlike aggregations along the magnetic field direction. The magnetic-field-induced spray self-assembly was driven by free-energy gradients to reach a minimum free-energy state where in the ordered microneedle structures appeared. The magnetic field distributions of a liquid microneedle at five states (t≈ 0.1 s, 1 s, 5 s, 10 s, and 15 s) under external

magnetic field were analyzed and shown in Figure 1b and Figure S1. The magnetic field intensity distributed in the liquid microneedle was extremely high, especially at the microneedle tip. The magnetoliquid microneedles were always self-assembled in the conical structures owing to the dynamic balance of magnetic force, surface tension, and gravity in the microneedles [47, 48]. The magnetic field gradient in the air around the microneedle tip was the greatest. The spray self-assembly process of micro-droplets from magnetoliquid aerosol towards the microneedles is shown in Figure 1d and Video S2. The paramagnetic micro-droplets flew towards the formed microneedles at an initial speed, were obviously accelerated near the microneedle tips, and most of micro-droplets were deposited on the microneedle tips owing to the highest magnetic field gradient around tips. Higher magnetic field gradient means larger magnetic force. The dynamic coalescence and rearrangement of microneedles always occurred and larger microneedles continuously formed during spray self-assembly process owing to the sufficient energy feed of continuously deposited micro-droplets [49]. With continuous growth up and coalescence of microneedles, the magnetic field gradient around the microneedle tips decreased (Figure 1b), lowering the attractive force and growth speed of microneedles. The magnetic particles deposited in the liquid microneedles attracted with each other, forming the chains and nets in the carrier [50, 51]. The chains and nets serving as a structural support in the carrier could stop the collapse of microneedles [48]. The shape of self-assembled magnetoliquid microneedles could be maintained under the external magnetic field. Meanwhile, the field-aligned microneedles were solidified by thermal curing, producing a magnetic particles/PDMS composite MA on the substrate. Subsequently, the solidified MA was exposed to a strong external magnetic field generated by an electro-magnetizing device (Figure S5a). The NdFeB microparticles in the solidified MA was magnetized (Figure S5b). Therefore, the solid MA exhibited magnetism. Vibrating sample magnetometer (VSM) was employed to measure the hysteresis loop of the magnetized MA (Figure S6). The magnetized MA exhibited high coercivity and residual magnetic moment. The residual magnetization and relative permeability were 66 kA·m-1 and of 1.42, respectively. Therefore, the microneedles could serve as the flexible permanent magnets [52-55]. Moreover, the mass production of magnetized MA at low cost may be achieved by integration of the magnetic-field-induced spray self-assembly, thermal curing, and electro-magnetization techniques in a Roll-to-Roll manufacturing mode, as shown in Figure S7.

Figure 1 (a) The fabrication illustration of microneedles by the magnetic-field-induced spray self-assembly method. (b) The magnetic-field-induced spray self-assembly process and the magnetic field distribution in the liquid microneedle at five states (t≈ 0.1 s, 1 s, 5 s, 10 s, and 15 s). (c) The average height, mid-diameter, and density of magnetoliquid microneedles during the spray process. (d) The simulated spray self-assembly process of micro-droplets from magnetoliquid aerosol towards the microneedles under the external magnetic field of 300 mT and initial spray velocity of 0.2 m/s. 3.2. Design of TEHG 3.2.1. Structure and characterization of TEHG TEHG has a symmetrically multilayered structure, as schematically illustrated in Figure 2a. It mainly consists of a copper coil sheet, two magnetized microneedles, four Al electrodes and two PE films. Figure 2b1 presents the photograph of TEHG with a small size of 24×24×3.2 mm and a weight of only 2.8 g. It might be the smallest and lightest one among the reported TEHGs. The TEHG was heat-sealed using PE films at an atmospheric pressure and thus perfectly isolated from the external environment. Some dry air was trapped in the TEHG , facilitating the contact and separation behaviors during power generation process [47]. The magnetized MA is the key component of TEHG and its morphology is shown in Figure 2b2 and Figure S8. The microneedles are orderly distributed with a density of 10.1 mm-2. The average height, tip diameter, and mid-diameter of conical microneedles are 1.2 mm, 20 µm and 200 µm, respectively. According to previous report [56], the larger microneedles could provide more separating distance for the contact interfaces, thus producing a better triboelectric property. Therefore, we fabricated a high-aspect-ratio microneedle array with a length of approximately 1.2 mm under a spraying time of 15 s. The microneedle surface is extremely rough owing to the existence of magnetic particles on the surface (Figure S8), which could increase the specific surface area and enhance the electricity generation ability of MA-based TEG [56, 57]. The magnetized microneedles are served as the magnetic poles of EMG and the frictional layers of TEG. The magnetized MA exhibits high elasticity and good anti-fatigue performance (Figure S9), guaranteeing compressibility and flexibility of TEHG for the energy harvesting. The flexible copper coil sheet was fixed on the center of TEHG to induce and collect the electricity of EMG (Figure 2b3).

Figure 2 (a) Illustration of magnetized MA-based TEHG structure. (b1) Photograph of MA-based TEHG. (b2) SEM image of magnetized microneedles. (b3) Photograph of flexible copper coil sheet. (c) Schematic diagrams of the working principle of TEHG for one compressing-releasing operation. 3.2.2. Working principle of TEHG The working mechanism of TEHG is schematically illustrated in Figure 2c. TEHG includes two functional components: TEG part and EMG part. Two symmetrical TEGs are electrically connected in parallel. A TEG consists of a MA and two Al electrodes. The working principle of TEG is based on the coupling of triboelectrification and electrostatic induction [58]. The MA-based TEG utilizes the closed bending-friction-restoring behavior of microneedles for the mechanical energy harvesting. As the TEHG is compressed, the microneedles are gradually bent and rub with the Al electrode, increasing the frictional contact area and resulting in a charge transfer from Al electrode to the surface of microneedles owing to the triboelectrification effect. According to the triboelectric series [59], the microneedle tips and Al electrode are negatively and positively charged, respectively. When the compression is withdrawn, the bended microneedles restore and the frictional layers separate from each other due to the elasticity of the MA and trapped dry air. The electric potential difference between the two charged surfaces drives the electrons to flow through the external circuit. Subsequently, when the compression is applied again, the distance between two electrodes decreases and the electrons flow along the reversed direction. Therefore, the cycled compressing-releasing operations of MA-based TEHG generate the alternating currents in the external circuit. The electricity generation of EMG is based on Faraday's law of electromagnetic induction [60]. EMG is mainly composed of two oppositely magnetized MAs and a copper coil sheet. The magnetized microneedles of MA could be regarded as the miniature magnetic poles, generating a uniform magnetic field through the coil. As TEHG is compressed, the microneedles are gradually bended, resulting in a magnetic field rotation (Figure 2c1) and decline of the equivalent magnetic flux through the coil. According to the Lenz's Law, a positive induced current is produced in the coil to offset the decrease of the magnetic flux. As the compression is stopped, the magnetic flux in the coil keeps unchanged and so the induced current in the coil becomes zero (Figure 2c2). When the compression is withdrawn, the magnetic field direction of MAs reversely rotates with the restoring microneedles (Figure 2c3), leading to an increase of magnetic flux through the coil. Thus, a negative induced current is produced in the

coil to compensate the increase of magnetic flux. When the bended microneedles are completely restored (Figure 2c4), the magnetic flux crossing the coil keeps constant so that no current flows in the coil. Therefore, the closed bending-restoring behaviors of EMG produce the alternating currents flowing in the coil. According to Faraday’s law of electromagnetic induction, the alternating induced electromotive force of EMG (VEMG) proportional to the change rate of magnetic flux through the coil can be calculated by [46]:
 =

 

=

(    ) 

= 

( ∙ ) 

= −  sin !

 

(1)

where Φ is the total magnetic flux through the coil, Bn is the magnetic field through the coil, B0 is the initial magnetic flux density through the coil, Scoil is the equivalent area of the copper coil in TEHG (≈ 10.3 cm2 in this work), and θ is the rotation angle of magnetized microneedles. The short circuit current of EMG (Isc) generated in the coil could be expressed as: # =

$%&' ( 

=

)   *  ( 

+, +-

(2)

where Rcoil is the internal resistance of the coil. Eq. (1) and Eq. (2) indicate that the VEMG and Isc are mainly determined by the initial magnetic flux density and the change rate of rotation angle of the magnetized microneedles. Above all, continuous alternating electrical signals of the TEG and EMG can be simultaneously and severally obtained since the repeated compressing-releasing operations are carried on the magnetized MA-based TEHG. 3.3 FEA for electrical output performance of TEHG To further elucidate the working principle and evaluate the electricity generation performance of TEHG, we carried out FEA to calculate the electrical potential distribution of a TEG and the magnetic field distribution of EMG part, as shown in Figure 3. It could be observed that the magnetized microneedles in the TEHG were bended and rotated during the compression process (Figure 3a1). 1 Hz compressing-releasing operations on TEHG were employed for the simulation (Figure3b1). As the microneedles are bended to 0°, 30° and 60°, the electrical potential distribution of a TEG in open circuit condition was analyzed, as shown in Figure 3a2. The electrical potential distribution varies with the bending angle of the microneedles. The average electric potentials on two Al electrodes were calculated, as shown in Figure 3b2. The gap voltage decreases with the bending angle of microneedles and the highest gap voltage of 11.42 V occurs at bending angle of 60°. The calculated magnetic field distribution of EMG part is shown in Figure 3a3-4. Each magnetized microneedle of MA was assumed as a permanent magnet. The magnetic field around the coil rotated with the bending angle of magnetized microneedles, as highlighted by the arrows in Figure 3a3. As a result, the magnetic flux projection through the coil decreases with the bending angle (Figure 3a4). The mean magnetic flux density through the coil decreases from 7.2 mT to 3.6 mT since the microneedles are bent from 0° to 60° (Figure 3b3). The magnetic flux density is declined by 3.6 mT during the compression process. According to the Faraday's law, the electromotive force induced in a coil is determined by the time-rate change of magnetic flux through the coil. The short-circuit current (Isc) was calculated, as exhibited in Figure 3b4. The Isc periodically fluctuates with the compressing-releasing operations and the peak value is approximately 60 µA.

Figure 3 (a1) The bended microneedles of TEHG during the compression process. (a2) The electric potential distribution of a TEG analyzed by COMSOL. (a3-a4) The front and top view of magnetic field distribution of EMG part analyzed by COMSOL. (b1) 1 Hz compressing-releasing operations on TEHG were employed for the FEA. (b2) The calculated electric potentials on the two Al electrodes of a TEG. (b3) The mean magnetic flux density crossing the copper coil calculated by COMSOL. (b4) The short-circuit current Isc of EMG part calculated by COMSOL. 3.4 Electrical output performance of TEHG Figure 4 presents the electrical output performance of the MA-based TEHG. A 30 N periodic compression force with a frequency of 1 Hz was applied on the TEHG. Both the open circuit voltage (Voc) and short circuit current (Isc) of TEG part and EMG part were correspondingly recorded, as shown in Figure 4a-d. The Voc and Isc of TEG and EMG fluctuate with the compressing-releasing operations on TEHG. It indicates that the MA-based TEHG can well trace the external loading actions. The peak values of Voc and Isc generated by TEG part could reach 10 V and 3 nA, while the peak values of Voc and Isc of EMG were 80 µV and 80 µA, respectively. The TEG usually outputs higher voltage and the EMG part provides higher current [37]. It indicates that the internal resistance of TEG is much higher than EMG [61]. The experimental Voc of TEG and Isc of EMG are consistent with the FEA results. The charging capability of MA-based TEHG on different capacitors was investigated (Figure S10). The voltages of charged capacitors increase with the input time. The voltage of 1 µF capacitor could be rapidly charged to 1 V in 140 s. To investigate the effect of external load resistance on the output performance of TEHG, the

voltage, current and power density of TEG and EMG under different load resistances were tested, as shown in Figure 4e-h. The output voltages of both TEG part and EMG part increase with the load resistances, while their currents exhibit a reversed tendency. Accordingly, the power density of MA-based TEHG first rises at low resistance region and then declines at high resistance region, presenting the maximum values of 16.19 µW/m2 and 9.09 µW/m2 at the load resistances of approximately 106 Ω and 1.5 Ω for TEG part and EMG part, respectively. To examine the influence of external compression force and frequency on the output performance, the Voc and Isc of TEG and EMG under different compression forces (10 N, 20 N, 30 N, 40 N, and 50 N) and frequencies (0.5 Hz, 1 Hz, 1.5 Hz, and 2 Hz) were measured, as shown in Figure 4i-l. For TEG part, the Voc increases with the compression force and the maximum Voc can reach approximately 15 V at 50 N, as shown in Figure 4i. The Voc of TEG is proportional to its triboelectric surface charge density [62]. The contact surface area between magnetized MA and Al electrode increases with the compression force, inducing more triboelectric charges and so enhancing the voltage output. It indirectly indicates that the Voc of TEG may reflect the compression force on TEHG. However, the Voc of TEG is basically constant as the compression frequency increases from 0.5 Hz to 2 Hz, as shown in Figure 4j, which is accordance with the previous report owing to a constant loading force [63]. For EMG part, the Isc varies little with the compression force owing to a constant loading velocity, as shown in Figure 4k. The Isc of EMG increases with the compression frequency and the maximum Isc is 120 µA at a frequency of 2 Hz, as exhibited in Figure 4l. According to the Faraday’s law of electromagnetic induction, the inductive current increases with the change rate of magnetic flux. Therefore, TEHG could simultaneously measure the loading force using TEG part and the velocity using EMG part.

Figure 4 (a,b) The Voc and Isc of TEG. (c,d) The Voc and Isc of EMG. (e-h) The dependence of output voltage, current and power density on the different load resistances of TEG (e,f) and EMG (g,h). (i-l) The dependence of the compression force and frequency on the Voc of TEG (i,j) and the Isc of EMG (k,l). 3.5 Practical applications The MA-based TEHG could serve as a self-powered sensor for human motion monitoring. The

stability and durability of TEHG were firstly investigated. Figure 5a-b correspondingly present the electrical output and mechanical performance for 3000 cycles of compressing-releasing operation. The Voc of TEG and Isc of EMG vary little with the repeated compressions (Figure 5a), demonstrating that the TEHG owns good stability of electrical output performance. After 3000 repeated compressing-releasing operations, the compressed MA-based TEHG could completely restore (Figure 5b), and the magnetized microneedles still kept perfectly intact without breakage (Figure 5c), exhibiting good elasticity and excellent stability. Therefore, the MA-based TEHG has good mechanical performance for the electricity generation. The MA-based TEHG was integrated in the insole as a pedometer to harvest the biomechanical energy from the walking and jogging, as shown in Figure 5d&g and Video S3. The Voc of TEG part and Isc of EMG part collected from walking and jogging are shown in Figure 5e-f and Figure 5h-i, respectively. The collected Voc and Isc fluctuate periodically with the walking/jogging strides. It demonstrates that the MA-based TEHG can accurately measure the human steps. The average peak-to-peak Voc of TEG part collected from jogging (approximately 6 V) is larger than that from walking (2.8 V) owing to the higher step compression force on the TEHG during jogging. The average peak-to-peak Isc of EMG part from jogging (approximately 5.3 µA) are slightly larger than that from walking (approximately 4.5 µA) due to the higher step frequency on the TEHG during jogging. The MA-based TEHG is thin and bendable, which could be attached on the inner elbow as an arm motion sensor, as shown in Video S3. Figure 5k-l respectively present the Voc of TEG part and the Isc of EMG part under 0.35 Hz arm rotation. The fluctuations of Voc and Isc follow with the arm rotation, demonstrating that the TEHG can also exactly detect the arm rotation. Therefore, the MA-based TENG is a promising wearable electronics.

Figure 5 (a) The electrical performance and (b) mechanical performance of TEHG for 3000 cycles of compressing-releasing operations. (c) The SEM images of the magnetized microneedles after 3000 cycles of compressing-releasing operations. (d) The photograph of walking test using a TEHG integrated in the shoe. (e) The recorded Voc of TEG and (f) Isc of EMG during the walking test. (g) The photograph of jogging using a TEHG integrated in the shoe. (h) The recorded Voc of TEG and (i) Isc of EMG during the jogging test. (j) The TEHG was attached on inner elbow. (k) The recorded Voc of TEG

and (l) Isc of EMG during the arm rotation. 4 Conclusions In this work, a novel magnetized MA-based flexible TEHG, as a self-powered sensor, was developed for human motion monitoring. The fabricated MA-based flexible TEHG has a small size of 24×24×3.2 mm3 and a light weight of only 2.8 g. A magnetic-field-induced spray self-assembly method was firstly employed to effectively fabricate the microneedles. With the continuous spray of the magnetoliquid aerosol, the orderly magnetoliquid microneedles were spontaneously self-assembled and gradually grew up on a substrate under an assistance of external magnetic field. A Roll-to-Roll manufacturing mode, including magnetic-field-induced spray self-assembly, thermal curing, and electro-magnetization techniques, was proposed in mass production of magnetized MA for TEHG. The magnetized MA-based flexible TEHG consisted of TEG part and EMG part. TEG part utilized the closed bending-friction-restoring behavior of microneedles to harvest the mechanical energy. EMG part used the bending of magnetic MA to change the magnetic flux through the coil for the generation of induced electromotive force. The peak Voc of TEG part was 10 V and the peak Isc of EMG was 80 µA under 30 N and 1 Hz compressing-releasing operations. The Voc of TEG part increased with the compression force and Isc of EMG part increased with the compression frequency. The maximum power densities were 16.19 µW/m2 and 9.09 µW/m2 at the load resistances of approximately 106 Ω and 1.5 Ω for the TEG part and EMG part, respectively. The MA-based TEHG, assembled in an insole as a wearable pedometer, can exactly trace the walking/jogging strides. The TEHG, attached on the inner elbow as a bendable sensor, can also correctly monitor the arm rotations. However, the output performance of MA-based TEHG is relatively poor, which may not meet the requirement of energy harvesting. In the future work, we will focus on the optimization of structure and materials of MA-based TEHG to enhance its output performance. Acknowledgements This research is financially supported by the National Natural Science Foundation of China (Project No. 51575543 and No. 51975597), and the General Program of Shenzhen Innovation Funding (Project No. JCYJ20170818164246179). Conflicts of interest The authors declare no conflict of interest. References [1] M. Ha, J. Park, Y. Lee, H. Ko, Triboelectric Generators and Sensors for Self-Powered Wearable Electronics, Acs Nano 9(4) (2015) 3421-3427. [2] C.J. Lee, A.Y. Choi, C. Choi, H.J. Sim, S.J. Kim, Y.T. Kim, Triboelectric generator for wearable devices fabricated using a casting method, Rsc Adv 6(12) (2016) 10094-10098. [3] B.N. Chandrashekar, B. Deng, A.S. Smitha, Y. Chen, C. Tan, H. Zhang, H. Peng, Z. Liu, Roll-to-Roll Green Transfer of CVD Graphene onto Plastic for a Transparent and Flexible Triboelectric Nanogenerator, Adv Mater 27(35) (2015) 5210-6. [4] J.N. Deng, X. Kuang, R.Y. Liu, W.B. Ding, A.C. Wang, Y.C. Lai, K. Dong, Z. Wen, Y.X. Wang, L.L. Wang, H.J. Qi, T. Zhang, Z.L. Wang, Vitrimer Elastomer-Based Jigsaw Puzzle-Like Healable Triboelectric Nanogenerator for Self-Powered Wearable Electronics, Adv Mater 30(14) (2018).

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Highlights Magnetized microneedle-array based triboelectric-electromagnetic hybrid generator (TEHG) was developed Magnetic-field-induced spray self-assembly method was proposed to fabricate microneedle-array Flexible TEHG could exactly detect triggering frequency of human motions

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: