**Acknowledgement**

*Wearable Devices - The Big Wave of Innovation*

surface. Small constant current (0.2 mA cm<sup>−</sup><sup>2</sup>

detect the glucose level from the extracted sample.

**4. Conclusions and future prospects**

continuous sweat analysis.

through wearable technology.

**Figure 5B**, a current (0.2 mA cm<sup>−</sup><sup>2</sup>

on-demand sweat generation as the device can be placed to a local skin target. There are two main approaches to extract sweat: (1) iontophoresis with pilocarpine drug and (2) reversed iontophoresis without the drug. These are attractive routes for

First, pilocarpine iontophoresis can be used to stimulate the sweat. In principle, a small electrical current is applied to enable the pilocarpine administration across the epidermis as illustrated in **Figure 5A**. For example, the tattoo-based enzymatic alcohol sensor consists of a pair of electrodes located in contact with the skin

material containing pilocarpine at the anode (positive) iontophoretic side [18]. The applied electrical force will push the pilocarpine drug, which possesses a large positive charge, to eventually enter into the skin. Such transdermal drug delivery of pilocarpine can induce the local sweat, sufficient for the subsequent electrochemical detection. In addition, interstitial fluid (ISF) located under the skin can be extracted. Without this iontophoretic strategy, it is challenging to access ISF

Second, the reversed iontophoresis without pilocarpine drug can be used to extract relevant analytes, such as glucose [17]. For instance, as presented in

reverse iontophoresis process, glucose is pulled out at the negative iontophoretic compartment. Even though glucose holds no charge, the inherent permiselective characteristic of the skin prefers to transport positive species, allowing such glucose extraction. Applying electric field on mobile electric charge can cause Coulombic force, leading to a net convective flow in the skin from the anode to cathode direction. Accordingly, dissolved analytes (e.g., glucose) are also moved toward the cathode where they can be extracted and monitored. Therefore, the glucose amperometric working electrode, adjacent to the cathodic iontophoretic side, can

This chapter has reviewed some examples of new trends of skin-worn enzymebased electrochemical systems, focusing on biosensors, BFC, and self-powered sensors. The existing systems provide significant advances toward the painless and point-of-care applications and personalized electrochemical biodevices, which was not possible without such new biodevices. However, researchers still face many challenges, such as electrochemistry, electrical wiring of enzymes, enzyme behaviors, the fabrication of stretchable electrodes, O2 fluctuations in biofluids, interferences, and difficulty in sweat extraction. Moreover, the workability and reliability of biodevices can be limited due to the limited fluctuating and volume of biofluids. In order to avoid frequent recalibrations, the stability of biodevices or self-calibration systems are also important. Precise electrochemical functions for on-skin applications are still very challenging. Therefore, it is required careful attention to address all challenges in order to advance such wearable technologies. Although main skin-worn BFCs have been driven by glucose and lactate fuels, it is interesting to explore new opportunities, such as from alcohol-based BFCs, where the bioanode can be functionalized with alcohol dehydrogenases. Future efforts may be made to expand the spectrum of current concepts. New integrated devices can be achieved by designing multifunctional sensors that can provide informative series of personalized data. This will require the incorporation of big-data analysis and Internet of things (IoT) to build up integrated networks and personalized baselines of each wearer. Big data collected from networks and individuals can

) was applied through the cryogel

) is applied to extract glucose in ISF. During the

**44**

The author would like to acknowledge Hassler Bueno for proof reading.
