Triboelectric micromotors actuated by ultralow frequency mechanical stimuli

A high-speed micromotor is usually actuated by a power source with high voltage and frequency. Here we report a triboelectric micromotor by coupling a micromotor and a triboelectric nanogenerator, in which the micromotor can be actuated by ultralow-frequency mechanical stimuli. The performances of the triboelectric micromotor are exhibited at various structural parameters of the micromotor, as well as at different mechanical stimuli of the triboelectric nanogenerator. With a sliding range of 50 mm at 0.1 Hz, the micromotor can start to rotate and reach over 1000 r min−1 at 0.8 Hz. The maximum operation efficiency of the triboelectric micromotor can reach 41%. Additionally, the micromotor is demonstrated in two scanning systems for information recognition. This work has realized a high-speed micromotor actuated by ultralow frequency mechanical stimuli without an external power supply, which has extended the application of triboelectric nanogenerator in micro/nano electromechanical systems, intelligent robots and autonomous driving.

scanner, in which the multilayered mechanical sliding generated by hand motion at an ultralow frequency of about 0.8 Hz has been converted into electricity and used to drive the micromotor for producing the scanning ray. When the scanning line falls on the ISBN, the barcode reader is activated and recognizes the codes (Supplementary Video 2). Moreover, a TM-based moving obstacle detector is presented in Fig. 4e. The rotational sliding TENG with tire rolling could be used for actuating the micromotor and scanning the moving obstacle. Even though the tire rotates at an ultralow speed of 11.3 r/min, the high-speed scanning could also be realized without external power supply (Supplementary Video 3), which has great application prospects in vehicle-mounted laser radar and automatic drive.
Rev 13. Page 7, line 41-42; Page 8, line 1-2: Furthermore, the TM was demonstrated in two scanning systems for information recognition by the ultralow-frequency energy in human and ambient, in which the book ISBN can be recognized by slow hand motion and the moving obstacle can be detected by low-speed tire rolling, respectively.
Rev 14. Page 8, line 23-26: Finally, a shaft with diameter of 0.7 mm is inserted into a cylindrical hole with diameter of 1.2 mm in the center of the rotor, and assembled with the vertical electrodes. The manufacturing process is shown in Supplementary  Fig. 1.

Respond to the reviewers:
Reviewer #1: This paper draft describes the development of a high-speed micromotor that couples a micromotor and triboelectric nanogenerator. The type of system described is novel to the reviewer's knowledge. Hence, the content should influence the thinking in this field. The system described by the authors sounds exciting to the reviewer. Respond: Thank you very much for your positive comments on our work. We will make persistent efforts.

Reviewer #1:
The correlation of the COMSOL model with the actual behavior of the device could be more developed. Though the results from the model are described in detail, the relationship between the model predictions and the actual device are not in a meaningful way. Respond: Thank you very much for your suggestions. We have added more simulation and calculation on the rotating characteristics of TM, which is very helpful for the behavior predictions of actual devices. All the simulation results in this work are summarized as below.
Firstly, we have simulated the driving torque of rotor with a tilt angle of 20 degrees in Fig. 1e, indicating that the micromotor tends to turn clockwise rather than anticlockwise, in virtue of the larger torque in this direction. As well, the driving torque curves with different tilt angles almost coincide in Supplementary Fig. 3a, which indicates that the tilt angle can only determine the rotational direction but not the driving torque of the micromotor.
Secondly, the simulations on the resistant moment of rotor are elaborated in Supplementary Note 1 and Supplementary Fig. 4, including air resistant moment and friction moment. The air resistant moment is very small and can be ignored, while the friction moment is dominant and goes up with the increasing rotation rate and blade number, in virtue of the increasing rotational centripetal force and friction force on the shaft. Therefore, the total resistant moment is determined by the both rotation rate and blade number. Moreover, the driving torque increases with the blade number for more transferred charges, which is simulated in Supplementary Fig. 3c. For the balanced state that the driving and resistant moments are equal, the rotation rate can be calculated with different blade number in Supplementary Fig. 5b. It is demonstrated that the optimal blade number is 7, which has exactly predicted the measured variation trend in Fig. 2a.
Thirdly, the driving torque grows up with the increase of included angle till 180 degrees, which is simulated in Supplementary Fig. 3b. As the resistant moment is not determined by the included angle, the larger included angle can give rise to the larger rotation rate, which is shown in Supplementary Fig. 5a and consistent with the measured variation tendency shown in Fig. 2b.
Finally, we have investigated the impact of charge quantity on the rotation rate in Supplementary Fig. 5c and 5d, which show that the micromotor rotates faster with more charges. With the increase of either sliding range or frequency, the TENG can produce more charges in one cycle. Therefore, the rotation rate rises with the increasing sliding range and frequency, which are consistent with the measured variation trends in Fig. 2d and 2e.
To sum up, the simulation results have predicted the actual device behavior and the experimental results have verified the structure model, which are beneficial to the device design, optimization and practical application. The relevant revisions are made in Rev 4-8, 18-20, and 28.

Reviewer #1:
The description of the method used for the device fabrication could be more detailed. Respond: Thank you very much for your reminder. We have added the description of the device fabrication method and the detailed process is shown in Supplementary Fig.  1. The 3D modeling of rotor and vertical board are first designed by using the Solidworks software. Accordingly, the 3D printer deposits the polylactic acid (PLA) layer to layer until the desired 3D object is created. After that, we coat copper foils on the rotor and vertical boards, and fix the carbon fibers on the vertical boards. Finally, a shaft with diameter of 0.7 mm is inserted into a cylindrical hole with diameter of 1. Reviewer #1: Other forms of regenerative or low power consumption devices have been reported in the literature, though these devices are not identical to the one described in this paper. The paper quality may be enhanced if this device were compared to one or two other devices of similar application in terms of power density and efficiency. Respond: Thank you very much for your suggestion. Supplementary Table 1 has summarized several low power consumption rotary micromotors, such as ultrasonic, optical and microstreaming types. Compared with these micromotors, we can find that the TM has both higher efficiency and rotate speed. It has also higher power density than the optical and microstreaming micromotors. Although the ultrasonic micromotors powered by AC power source have higher power density than the TM, the speeds are low and the driving frequencies are usually up to KHz or MHZ. While the TM can be actuated by the mechanical stimuli with an ultralow frequency as low as 0.1 Hz, instead of the external high voltage power supply. Therefore, the TM has provided a novel method for a high-speed scanner actuated in ultralow frequency, which are much different and superior to previous micromotors. The relevant revisions are made in Rev 27 and 30-31.  Figure 3.d and 3.e, respectively. Why is there overlap in the plots in Figure 3.d and is there a relationship between the voltage hysteresis and the sliding range that can be described mathematically based on the ranges studied? Respond: Thanks for your valuable suggestions. The overlap in Fig. 3d is caused by the inappropriate range in Y-axis. We have corrected it. The voltage and charge versus sliding range are both linear. For the U-Q curves versus sliding range in Fig. 3e, we have developed theoretical analysis and simulation as shown in Supplementary Note 2 and Supplementary Fig.  10. Based on the working mechanism of TM, we can find out that two electric characteristics in the rotating process. One is that the charges can be accumulated on the vertical electrodes, while the other is that the current can be generated by charges transferring from one electrode to the other. Therefore, we assume that the micromotor can be approximately equivalent to a capacitor and a resistor in parallel, and the equivalent circuit model of the TM is depicted in Supplementary Fig. 10a. With the theoretical derivation and numerical calculation, the simulated U-Q curves versus sliding range are shown in Supplementary Fig. 10b. It is demonstrated that the output electric energy from the TENG in one cycle, described as the encircled area of U-Q curve, increases at larger sliding range, which are consistent with the experimental results in Fig. 3e Reviewer #2: The author reported a high-speed triboelectric motor driven by sliding mode TENG. The author analyzed the effect of various parameters on the TM. The initial analysis of TEM by the author is very interesting but the application part (identity recognition) is too vague and impractical. I would like to suggest a major revision. Respond: Thank you very much for your time and positive comments on the manuscript. This work has realized a high-speed micromotor actuated by ultralow frequency mechanical stimuli without an external power supply, which has extended the application prospects of TENG as a high-voltage source. We have made major revisions in the application part and wish the demonstrations could validate the practicality of TM and improve the manuscript.

Reviewer #2:
The author is advised to provide the output current at different frequencies.
Respond: We thank for your valuable advice. The relevant data have been updated in Supplementary Fig. 8. As shown, with the increase of frequency and sliding range, respectively, the measured output current peak value of the TENG with micromotor increase as well. The current peak value has approximate linear relationship with the frequency and sliding range, respectively. It is because that the sliding speed will be increased and thus the current by increasing either sliding frequency or range. Furthermore, we have measured the short-circuit current peak values of the TENG with different frequencies and sliding ranges, which are included in Supplementary  Fig. 9. The relevant revisions are made in Rev 22-23.