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1. Introduction
In the forthcoming sensor network era or so-called the Internet- of-Things (IoT) society, a plenty of electronics including sensors, radio-frequency components, and microbatteries are going to be consumed. To suppress the cost of frequent exchange of battery modules and to extend the longevity of the products over a few years, efforts are paid to develop a new technology to retrieve electrical power from the environments including vibrations,[1–3] radio waves,[4] and lights.[5] Even battery-less sensors[6–9] are under study to solve the issues associated with the power source.
However, a problem remains unsolved; how are we going to recover such numerous sensors after use without leaving impact to the environment? As a matter of fact, it is almost impossible to collect tiny electronics once they are deployed and diffused in the environment, and hence they are alternatively expected to decay in time. Rogers and co-workers[10–15] and other research groups[16–18] proposed an idea of transient electronics for eco-friendly products that are made of, for instance, silicon, silicon oxide, and magnesium that are encapsulated within polylactic-co-glycolic acid (PLGA), which can dissolve in water after use.[14] Other study has reported a supercapacitor based on the transient electronics.[15] Most of them are made of water-based electrolyte, and therefore a drawback is that their electrical proper- ties change in time as the water gradually evaporates. In addition to this, when used in batteries, the energy density is limited by the small potential window that is commonly seen in the water-based electrolyte.
To overcome such problems, ionic liquids (ILs)[19–23] are newly used in this work to compose a water-dissolvable electrolyte. IL is composed of two kinds of molecules with positive and negative charges respectively called cation and anion, and they show various electrochemical properties. Upon voltage application, for instance, accumulated charge layers are formed in the vicinity of electrodes, and they exhibit extremely high capacitance upwards of a few μF cm−2 within a certain voltage range so-called the potential windows that is inherent to the IL species. ILs are nonvolatile at room tempera- ture due to the small vapor pressure associated with the strong ionic bonds. Some ILs are known to form a gel when suspended in eco-friendly polymers, including poly(vinyl alcohol) (PVA), poly(ethylene oxide) (PEO), and poly(ethylene glycol) (PEG).[24–27] Owing to such characteristics, the use of ILs is not limited to the electrolyte of high capacity batteries but also applicable to functional devices such as photovoltaics,[28–32] energy stor- ages,[33–43] and tactile sensors.[44–50] Nonetheless, some ILs have toxicity or nonbiodegradability that hinder their disposable use.
Ionic gels developed in this work use one of IL, tris(2-hydroxy- ethyl) methylammonium ethylsulfate ([MTEOA]+[MeOSO3]−), and (PVA) as schematically shown in Figure 1. Both materials show environmental safety and biodegradability, so do the devel- oped ionic gels. Electrical capacitance of as large as 13 μF cm−2 is experimentally confirmed at 1 Hz with the IL concentration of 60 wt% in the gel. The ionic gels were also found to be stable in dry condition, and dissolved in deionized water (DIW) in 16 h.
出典 A water Dissolvable Electrolyte for Eco-friendly electronics, Small 2018,
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The selectivity of sweat sensors is crucial, because various electrolytes and metabolites in sweat can influence the accuracy of the sensor readings. Extended Data Fig. 7a–d shows that the presence of non- target electrolytes and metabolites causes negligible interference to the response of each sensor. When all five sensors are integrated in the FISA, simultaneous system-level measurements maintain excellent selectivity upon varying concentrations of each analyte (Fig. 2f and Extended Data Fig. 7e–h). Although temperature has a minimal effect on the potentiometric sensors, it greatly influences the performance of the enzymatic sensors. Figure 2g shows that the responses of glucose and lactate sensors increase rapidly upon elevation of the solution temperature from 22 °C to 40 °C, reflecting the effect of increased enzyme activities26. System integration allows for the implementation of real-time compensation to calibrate the sensor readings on the basis of temperature variations. Figure 2h illustrates that with the increase of temperature, the uncompensated sensor readouts can lead to substantial overestimation of the actual concentration of the given glucose and lactate solutions; however, the temperature compensation allows for accurate and consistent readings.
出典 Fully integrated wearable sensor arrays for multiplexed in situ perspiration analysis, Nature 2016, June, Vol. 529.
