**1. Introduction**

Since 1962 when the first Clark's biosensor was introduced [1], enzymatic electrochemical devices have attracted increasing attention, recently being regarded as a powerful tool for the development of emerging wearable bioelectronics [2]. Integrating enzymes with electrochemical transduction units is one of the most popular and well-built bioelectronic systems due to outstanding selectivity and natural behaviors of enzymes [2–4]. Employing enzymes, as a catalytic system, in order to substitute nonselective metal catalysts, is interesting. Because of inherent behaviors of enzymes, enzyme-based bioelectronics offers favorable operations

under mild physiological conditions of pH and temperature, unlike nonenzymatic approaches [5, 6]. In addition, enzymes will usually catalyze only one particular reaction. Therefore, such enzyme specificity enables bioelectronics to operate selectively even in complex solutions, including biofluids. Recently, there is an increasing interest in transforming traditional enzymatic bioelectronics into modern wearable platforms. Wearable enzyme electronics expands appealing spectra of a variety of applicable fields, ranging from personalized healthcare, fitness, to the environment. These applications comprise of noninvasive diagnosis of biomarkers in biofluids, such as sweat, and the monitoring of the surrounding of the wearer. Besides, electron collectors can be functionalized with enzymes to develop BFCs for energy and self-powered applications. These biodevices employ enzymes to obtain electrocatalytic oxidations of biofuels, such as glucose and lactate. This aims to achieve next-generation energy autonomy for the whole wearable system. In addition to energy-harvesting purposes, BFCs can also act as self-powered electrochemical sensors. Three main applications of enzyme-based

#### **Figure 1.**

*(A) Skin-worn enzyme-based electrochemical devices. The soft electrode platform is functionalized with enzymes, allowing various applications, including (B) biosensors, (C) energy-harvesting biofuel cells, and (D) self-powered biosensors.*

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*Wearable Skin-Worn Enzyme-Based Electrochemical Devices: Biosensing, Energy Harvesting…*

electrodes, including biosensors, biofuel cells (BFCs), and self-powered sensors, along with their relevant aspects, will be discussed (**Figure 1**). An enzymatic biosensor employs an enzyme, immobilized on an electrochemical transducer, to recognize and react with the target, generating a readable electrical signal (**Figure 1B**). A BFC energy harvester can convert chemical energy into electricity and power wearable devices (**Figure 1C**) [7]. A BFC can also be designed to act as a self-powered sensor by displaying power signals proportional to the target con-

Skin-worn enzyme-based electrochemical devices are among the most significant wearables because the skin offers the largest organ interface and unique opportunities to be accessed noninvasively [10–13]. The large epidermal area also provides sweat, which contains a variety of biomarker-rich information, such as levels of glucose, lactate, hormone, urea, pH, and electrolytes. Advantageously, skin-worn electrochemical devices can be attached directly close to the location of sweat generation, enabling the fast access for monitoring or energy harvesting before the unwanted biodegradation. In addition to physical parameters obtained from existing skin-worn biodevices (such as temperature and heartbeat), chemical data is also crucial to step further to understand comprehensive insights of individual [14]. The history of sweat content analysis began many decades ago with the development of cystic fibrosis diagnosis [15]. Establishing new "lab-onskin" electrochemical devices enables noninvasive detection of such biometrics, essential for health monitoring and early disease diagnosis. In addition, such wearable electrochemical tools are also helpful for drug testing and chemical threat screening, such as in sports [12] and in the surrounding environment [16]. Importantly, for emerging energy technologies, sweat also contains relevant biofuels, such as glucose and lactate; this is useful to BFCs as energy-harvesting and self-powered devices, which exemplify new exciting wearable autonomous

Although researchers are battling to create new enzymatic bioelectronics, there

is a continuing need for further development. Revolutionizing traditional electrodes toward wearable bioelectronics needs careful engineering to address several key challenges associated with electrochemistry, the integration of biocatalysts, mechanical stability, environment effects (e.g., O2 fluctuations), and sweat extraction. Therefore, the bulk of this chapter will focus on examples of progress in skinworn enzymatic electrochemical devices. Key working principles and opportunities of biosensors and BFCs will be described. In addition, perspectives emphasizing on main challenges will be discussed. The outlooks of emerging wearable electro-

Wearable enzymatic electrochemical biosensors utilize enzymes, which are functionalized in spatial contact with electrochemical transduction units. In principle, biosensors consist of electrodes and enzyme receptors, allowing the specific binding capabilities and catalytic activity to target analytes. Interfacing enzymes with electrodes will be discussed further in Section 3.3. It should be remarked that the key consideration to fabricate a successful biosensor for nonspecialist wearers is choosing highly specific biocatalysts. Enzymatic biosensors can also function continuously because enzymes are not consumed in reactions, offering an advan-

*DOI: http://dx.doi.org/10.5772/intechopen.85459*

centration (**Figure 1D**) [8, 9].

bioelectronic systems.

chemical technologies will also be concluded.

**2.1 Enzyme-based biosensors**

tage for wearable sensors.

**2. Skin-worn enzyme-based electrochemical devices**

## *Wearable Skin-Worn Enzyme-Based Electrochemical Devices: Biosensing, Energy Harvesting… DOI: http://dx.doi.org/10.5772/intechopen.85459*

electrodes, including biosensors, biofuel cells (BFCs), and self-powered sensors, along with their relevant aspects, will be discussed (**Figure 1**). An enzymatic biosensor employs an enzyme, immobilized on an electrochemical transducer, to recognize and react with the target, generating a readable electrical signal (**Figure 1B**). A BFC energy harvester can convert chemical energy into electricity and power wearable devices (**Figure 1C**) [7]. A BFC can also be designed to act as a self-powered sensor by displaying power signals proportional to the target concentration (**Figure 1D**) [8, 9].

Skin-worn enzyme-based electrochemical devices are among the most significant wearables because the skin offers the largest organ interface and unique opportunities to be accessed noninvasively [10–13]. The large epidermal area also provides sweat, which contains a variety of biomarker-rich information, such as levels of glucose, lactate, hormone, urea, pH, and electrolytes. Advantageously, skin-worn electrochemical devices can be attached directly close to the location of sweat generation, enabling the fast access for monitoring or energy harvesting before the unwanted biodegradation. In addition to physical parameters obtained from existing skin-worn biodevices (such as temperature and heartbeat), chemical data is also crucial to step further to understand comprehensive insights of individual [14]. The history of sweat content analysis began many decades ago with the development of cystic fibrosis diagnosis [15]. Establishing new "lab-onskin" electrochemical devices enables noninvasive detection of such biometrics, essential for health monitoring and early disease diagnosis. In addition, such wearable electrochemical tools are also helpful for drug testing and chemical threat screening, such as in sports [12] and in the surrounding environment [16]. Importantly, for emerging energy technologies, sweat also contains relevant biofuels, such as glucose and lactate; this is useful to BFCs as energy-harvesting and self-powered devices, which exemplify new exciting wearable autonomous bioelectronic systems.

Although researchers are battling to create new enzymatic bioelectronics, there is a continuing need for further development. Revolutionizing traditional electrodes toward wearable bioelectronics needs careful engineering to address several key challenges associated with electrochemistry, the integration of biocatalysts, mechanical stability, environment effects (e.g., O2 fluctuations), and sweat extraction. Therefore, the bulk of this chapter will focus on examples of progress in skinworn enzymatic electrochemical devices. Key working principles and opportunities of biosensors and BFCs will be described. In addition, perspectives emphasizing on main challenges will be discussed. The outlooks of emerging wearable electrochemical technologies will also be concluded.
