Self-powered ionic sensors overcoming the limitation of ionic conductors as wearable sensing devices
Graphical abstract
Introduction
Wearable sensors have received increasing attention for real-time human motion monitoring, owing to their ability to provide useful insights into the performance and health of individuals [1]. Recently, the wearable sensor devices based on stretchable conductive materials have experienced an explosive growth [[2], [3], [4], [5], [6], [7], [8]]. Compared with electronic conductor-based sensors (E-sensors) that suffer from the degradation of percolating conductive networks [[9], [10], [11], [12], [13], [14]], ionic hydrogels could be more promising materials for wearable sensors due to their high stretchability and stable conductivity [[15], [16], [17], [18], [19], [20], [21]]. Recently, various stretchable ionic hydrogels have been developed for the detection of full-range human motions and they are generally powered by alternating current owing to the difference of signal carriers between the ionic hydrogels and electronic circuits [[19], [20], [21], [22], [23], [24], [25]]. However, as the commonly used portable power source for wearable devices, the ionic batteries actuate the external devices through direct current (DC). The direct connection of the batteries and ionic hydrogels causes continuous electrolytic reaction on the metal electrodes, thus leading to an unstable chemical composition of the ionic hydrogel-based sensors (I-sensors), as well as a lack of long-term reliability of the sensor devices [26]. Currently, there is an urgent need for developing a broadly applicable strategy toward coupling the stretchable ionic conductors and portable power source into one sensor device for practical applications of wearable I-sensors.
As is well-known, the current transport insides a battery relies on ions, during which the insertion/extraction of ions on the two electrodes can ensure constant ionic concentration within electrolyte [27,28]. Hence we put forward the design scheme of directly using the ionic hydrogel sensors as the electrolytes of the batteries. This strategy is expected not only to maintain a stable chemical composition within the ionic sensors during the current transport by virtue of the insertion/extraction of ions on two electrodes but also to endow the sensors with battery's (i.e., self-powering) function for powering the sensors themselves and other wearable devices. Meanwhile, the integration of sensing and self-powering functions in one sensor could be beneficial for sensor miniaturization to broaden the practical applications of wearable sensor devices [[29], [30], [31], [32], [33]]. Although there also exist other types of self-powered sensors (i.e., triboelectric or piezoelectric sensors) that can harvest mechanical energy from human motions, they generally lack the stretchability for large detection strain, the stability in response to temperature or humidity changes, and the ability for measuring static pressures [[30], [31], [32], [33], [34]].
Because the ionic hydrogels are the aqueous electrolytes, the aqueous rechargeable battery systems are required to demonstrate the concept. Among others, the aqueous rechargeable Zn-based batteries have drawn extensive interest in wearable energy-storage devices because of the high abundance, low electrochemical potential, and high theoretical specific capacity of Zn metal [35,36]. In this work, the Zn-Ag battery system was used in view of thier high power density, high safety, and low self-discharge rate [35,37]. The self-powered ionic hydrogel-based sensors (SPI-sensors) were prepared by simply replacing the commonly used metal electrodes of I-sensors with Zn and Ag electrodes (Fig. 1). As predicted, the resulting SPI-sensors showed ultrahigh long-term reliability and high sensitivity. Besides the inherent ultrahigh detection strain of 2000%, the SPI-sensors can also recognize extremely small strain as low as 0.01%, which is superior to the previously reported I-sensors with low sensitivity to the subtle strain [[15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]]. Notably, in contrast to the high ion concentration within hydrogels required for high-performance batteries, a relative low ion concentration is required for sufficient sensitivity of SPI-sensors [[38], [39], [40], [41]], indicating the different functions of two devices give rise to different requirement for ionic hydrogels despite the similar assembly methods of them. This strategy can be readily extended to other ionic hydrogel-battery systems, overcoming the limitation of ionic conductors on wearable sensing devices.
Section snippets
Materials
Acrylamide (AM, >98%), N,N′-methylenebisacrylamide (BIS, ≥98%), and N,N,N′,N′-tetramethylethylenediamine (TEMED, 99%) were purchased from Aladdin Reagents Co. (Shanghai, China). Ammonium peroxodisulfate (APS, 99.99%) and KOH (95%) were supplied by Sinopharm Chemical Reagent, China. The Zn foil (≥99.99%) with a thickness of 0.05 mm and Ag foil (99.9999%) with a thickness of 0.05 mm were purchased from Tianjin Zhongtianpengbo Material Co., Ltd. The reagents were used without further purification.
Hydrogel preparation
Results and discussion
The ionic hydrogels were prepared by directly immersing the PAM hydrogels into KOH solutions with different concentrations for 30 min, followed by drying at 50 °C for ~15 min. The SPI-sensor was integrated by connecting the two sides of ionic hydrogel to Zn electrode and Ag electrode, respectively (Fig. 1). Here, the SPI-sensor containing different concentrations of KOH were defined as SPIm-sensor, where m represents the concentration of KOH solution used in the immersion pretreatment process.
Conclusion
In summary, this work presents a universal and convenient strategy for the design and fabrication of self-powered and high-performance I-sensors. The SPIm-sensors with excellent stability and ultrahigh sensitivity can be prepared by simply replacing the commonly used metal electrodes for I-sensors with battery electrodes (i.e., Zn-Ag electrodes in this work) to maintain a constant ion concentration within the sensors. By modulating the ion concentration to reduce the effect of electrode
Author contributions
DZ and HQ contributed equally to this article, and they carried out most of the material preparation, properties characterizations, data analysis, and prepared the first draft of the article. WF and KZ were in charge of the overall experimental set-up and performance. WF and KS cowrote the article. KS and YX conceived the experiment, oversaw the experiments, and finalized the article. All authors have contributed to the final draft of the article.
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.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51573080, 51873094), the Key Research and Development Project of Shandong Province (2016GGX102005), the China Postdoctoral Science Foundation (2017M622132), Technology Development Project of Shinan District of Qingdao (2018-4-007-ZH) and Program for Taishan Scholar of Shandong Province.
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