Elsevier

Nano Energy

Volume 90, Part A, December 2021, 106499
Nano Energy

Full paper
Moisture induced electricity for self-powered microrobots

https://doi.org/10.1016/j.nanoen.2021.106499Get rights and content

Highlights

  • The moisture-based energy-harvesting device generates an output voltage of ~1.4 V and an output current of ~43 mA.

  • It is prospective that a moisture-based energy harvesting device can be incorporated into microrobots.

  • The harvesting energy can be used to power a self-powered robot, with an average speed of 4.01 cm/s.

Abstract

Sustainable operation of microrobots mandatorily needs a continuous supply of energy, which is usually provided by a battery. However, with the miniaturization of the microrobot, the reduction of weight, and the limited lifetime of battery, self-powering of microrobot is a key challenge. Inspired by the crawling of cockroaches, we present an untethered insect-scale robot driven by moisture induced electric power. A moisture-based energy harvesting device has been exploited and embedded in the microrobot, which can capture and store atmospheric water under various environmental conditions through a hygroscopic gel and generate electricity based on redox reaction. The device produces an output voltage of ~1.4 V and an output current of ~43 mA. A combination of moisture-electricity powered vertical vibration and the asymmetric structural design of the microrobot enables its forward locomotion at an average speed of ~4.01 cm/s. Our work could facilitate multifunctionality in future self-powered microrobots and mesoscale devices.

Graphical Abstract

Microrobots often lack an energy generation unit and thus need an external power source to provide energy for movement. This work reports an untethered insect-scale robot driven by moisture induced electric power, which can capture and store atmospheric water under various environmental conditions through a hygroscopic gel and generate electricity based on redox reaction, achieving forward locomotion at an average speed of ~ 4.01 cm/s.

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Introduction

Microrobots can perform tasks, such as confined space exploration and actuation with multiple degrees of freedom, which are difficult to achieve with conventional robots [1], [2], [3], [4], [5], [6], [7]. However, providing sufficient power for sustainable operation of microrobots is challenging. Lithium-ion batteries (LIBs) are the main power source for conventional robots [8], [9]. However, the LIBs have limited structure and specific energies, and will create an extra load and some increased volume for the microrobots, restricting the multifunctionality for future microrobot development [10], [11].

There is growing interest in insect-scale (micro-scale) robots that do not need LIBs and have effective and complex mobility, such as robotic flies [12], strider-like robots [13], [14] and soft robots [15], [16], [17], [18]. These robots, however, usually need an external stimulus, such as light, heat, humidity, or magnetism, as an actuation source to realize sufficient mobility, leading to instability under environmental effects [19], [20], [21], [22], [23]. To address these issues, our group developed a floating robotic insect that can harvest energy from water [24]. However, the demand for a water source restricts its activity. Thus, harvesting energy from ambient humidity is an expectant route that can provide portable and clean energy for microrobots [25], [26], [27], [28], [29], [30], because moisture can be freely obtained anywhere on earth.

Some pioneering studies have explored various advanced materials and techniques for enhancing the capability of atmospheric water capture [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41] and moisture-based electricity generation [42], [43], [44], [45], [46]. For example, polypyrrole chloride/poly N-isopropylacrylamide moisture absorbing gel exhibits a daily water harvest of 19.2 L/kg of the corresponding xerogel at a relative humidity (RH) of 60% [47]. A hybrid photocatalysis system that comprises a hygroscopic hydrogel and cupric oxide and barium titanate nanoparticles can split 36.5 mg of absorbed atmospheric water to produce a photocurrent of 224.3 µA/cm2 [48]. Protein nanowires (NWs) were employed to produce a voltage of ~0.5 V and a current of ~110 nA under a load of 2 MΩ from ambient humidity [49]. Bulk graphene oxide was used to exploit a hygroelectric generator unit to produce electricity from air with an output voltage of ~1.5 V. These studies stimulated the construction of self-propulsion systems [50]. However, existing atmospheric water capturing and moisture-based energy harvesting technologies have not yet demonstrated powerful ability to drive soft actuators and electric motors. New strategies for insect-scale robots should be developed along with the introduction of moisture-based energy harvesting devices.

Here, we present a cockroach crawling-inspired microrobot powered by moisture induced electric power. The intrinsic structural design enables the integration of multiple functions into a microrobot: moisture capture, energy harvesting, and actuation. The robot contains a moisture-based generator that can acquire water from ambient air through a hygroscopic gel. The absorbed water is used for electricity generation to drive a micromotor via redox reactions. The micromotor transmits mechanical vibration to the asymmetric folding body of the robot, allowing the robot to rapidly move forward. The microrobot exhibits a power density of approximating 8.5 mW/cm2 and an average forward movement speed of 4.01 cm/s.

Section snippets

Preparation of hygroscopic gel

NFC was individualized from poplar wood powder and its preparation was referred to the previous work [56]. 20 g of 1 wt% NFC aqueous suspension was added into a 150-mL beaker. Then, 40 g of 5 wt% LiCl aqueous solution was slowly added to the NFC suspension along the beaker wall. The beaker was kept at room temperature for 12 h, which led to the generation of an NFC/LiCl composite hydrogel. Excess LiCl solution in the beaker was sucked out and the hydrogel was put into a freezer for 12 h.

Results and discussion

A prototype 3.2 cm × 1 cm robot with folding body design was assembled. Its motion is driven by transforming moisture-electricity powered mechanical vibration to the asymmetric folding body of the robot (Fig. 1a and c). We used a rigid polyethylene terephthalate (PET) plate (thickness: 0.25 mm) as the crossbeam and legs of the robot, and flexible PET film (thickness: 0.1 mm) as the hinge springs. To make the robot body asymmetric, the highest point of the foreleg is lower than the highest point

Conclusion

In summary, we demonstrated an untethered self-powered microrobot that can move without an external power source. The coupling of hygroscopic gel and oxygen reduction reaction enabled the microrobot to harvest energy from environment humidity. The moisture-based energy harvesting device is embedded into the robot body as a power unit. The integration of moisture-electricity powered vertical vibration and asymmetric structural design resulting in the forward movement of the microrobot. This work

CRediT authorship contribution statement

W. C., Z. L.W. and Y. Y. supervised the research and conceived the idea. Y.W., and M.D. fabricated the composite materials. Y.W., M.D., H.W., L.X., and T.Z. carried out the device fabrication and the performance measurement. Y.W., M.D., W.C., Z.L.W., and Y.Y. analyzed the data and co-wrote the manuscript. All authors read and revised the manuscript.

Declaration of Competing Interest

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.

Acknowledgments

This work was supported by the National Key R&D Program of China (No. 2016YFA0202701), the National Natural Science Foundation of China (No. 52072041, 31922056), the University of Chinese Academy of Sciences (No. Y8540XX2D2) and Qingdao National Laboratory for Marine Science and Technology (No. 2017ASKJ01).

Yang Wang is currently a postdoc fellow in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. He received his master's degree in Microelectronics Science and technology from Harbin University of Science and Technology, and Ph.D. degree in Electronics Science and Technology from Harbin Institute of Technology, China. His research currently focuses on self-powered sensors and bionic robot.

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    Yang Wang is currently a postdoc fellow in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. He received his master's degree in Microelectronics Science and technology from Harbin University of Science and Technology, and Ph.D. degree in Electronics Science and Technology from Harbin Institute of Technology, China. His research currently focuses on self-powered sensors and bionic robot.

    Ming Dai is a graduate student in the research group of Prof. Wenshuai Chen at Northeast Forestry University, China. She obtained her B.S. degree in Light Chemical Engineering from Northeast Forestry University in 2017. Her current research interests focus on the fabrication of functional nanostructured gels using wood cellulose nanofibers for air dehumidification.

    Heting Wu is currently a postdoc fellow in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. She received her master's degree in University of Chinese Academy of Sciences. Her research currently focuses on electrochemical cells, triboelectric nanogenerators and there biomedical applications.

    Lin Xu is a postgraduate of the School of Physics and Engineering Technology of Guangxi University. He is a currently visiting student at Beijing Institute of Nanoenergy and Nanosystems (BINN), Chinese Academy of Sciences. His main research is focused on organic composite materials and implantable robot.

    Tongtong Zhang is a Ph.D. student in the research group of Professor Ya Yang at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences (CAS). She received her bachelor's degree in Chemical Engineering and Technology from Guangxi University in 2018. Her current interests focus on self-powered sensors.

    Wenshuai Chen is a professor in the College of Material Science and Engineering at Northeast Forestry University. He received his B.S. degree (2008) and Ph.D. degree (2013) in wood science and technology from Northeast Forestry University. His research interests include wood physics, bionanocomposites, nanocellulose, aerogels, and development of wood-based nanomaterials for energy and environmental sciences.

    Prof. Zhong Lin Wang received his Ph.D. from Arizona State University in physics. He now is the Hightower Chair in Materials Science and Engineering, Regents’ Professor, Engineering Distinguished Professor and Director, Center for Nanostructure Characterization, at Georgia Tech. Dr. Wang has made original and innovative contributions to the synthesis, discovery, characterization and understanding of fundamental physical properties of oxide nanobelts and nanowires, as well as applications of nanowires in energy sciences, electronics, optoelectronics and biological science. His discovery and breakthroughs in developing nanogenerators established the principle and technological road map for harvesting mechanical energy from environment and biological systems for powering a personal electronics. His research on self-powered nanosystems has inspired the worldwide effort in academia and industry for studying energy for micro-nano-systems, which is now a distinct disciplinary in energy research and future sensor networks. He coined and pioneered the field of piezotronics and piezophototronics by introducing piezoelectric potential gated charge transport process in fabricating new electronic and optoelectronic devices. Details can be found at: http://www.nanoscience.gatech.edu.

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    Y. Wang and M. Dai contributed equally to this work.

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