**2.1 Enzyme-based biosensors**

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 advantage for wearable sensors.

*Wearable Devices - The Big Wave of Innovation*

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

**32**

**Figure 1.**

*(D) self-powered biosensors.*

*(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* 

Enzymatic biosensors are based on numerous mechanisms. The popular mechanism relies on the conversion of the analyte as an enzymatic substrate into a product, enabling the detection by using electrochemical transducer. Another way is to monitor the analyte (e.g., a toxic compound) that acts as an enzyme inhibitor. In addition, the enzyme can be used as a labeling transducer for bioaffinity recognition. Besides, a reverse approach can be designed to detect the enzyme level. In this case, the enzyme acts as an analyte, while the substrate is immobilized on the electrode surface. When the enzyme reaches the electrode sensor, it will generate the signal, corresponding to the concentration level of the enzyme target.

In recent decades, enzymatic biosensors have been proven to be modern wearables to monitor numerous analytes, such as glucose, lactate, alcohol, and organophosphate nerve agents. Among several enzymes, oxidoreductase and hydrolase, such as glucose oxidase (GOx), lactate oxidase (LOx), alcohol oxidase (AOx), and organophosphorus hydrolase, are predominant for wearable biosensing applications. A temporary tattoo with the integration of transdermal enzymatic glucose biosensor has been introduced since glucose is a key biomarker for diabetes mellitus, which still affects hundreds of millions of patients globally (**Figure 2A**) [17]. The iontophoretic ISF extraction system was coupled with the amperometric

#### **Figure 2.**

*Skin-worn enzyme-based electrochemical biosensors. (A) Transdermal tattoo-based glucose sensors, coupled with reversed iontophoresis [17]. Adapted with permission from ref [17]. Copyright 2015 American Chemical Society. (B) Tattoo-based alcohol biosensors, coupled with pilocarpine iontophoresis and wireless electronics [18]. Adapted with permission from ref [18]. Copyright 2016 American Chemical Society. (C) Biosensors integrated with a microfluidic patch for sweat collection and analysis [20]. Adapted with permission from ref [20]. Copyright 2017 American Chemical Society. (D) Microneedle-based β-lactam sensors [22]. Adapted with permission from ref [22]. Copyright 2019 American Chemical Society. (E) Integrated glucose/lactate enzymatic biosensors with electrolyte and temperature sensors. (F) Integrated sweat monitoring biosensing and transdermal drug delivery system.*

**35**

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

detection to extract the sample containing glucose. The glucose biosensor, located near the negative iontophoretic electrode, relied on GOx immobilization on the Prussian blue (PB)-carbon electrode; this PB facilitates the electroreduction of H2O2 product, generated by the GOx reaction. The amperometric reduction of H2O2 could be detected at a potential of −0.1 V versus Ag/AgCl. The iontophoresis strategy will be discussed in Section 3.5. Additionally, the tattoo-based alcohol sensor was also invented (**Figure 2B**). The AOx-/PB-based sensor was designed to be close to the positive iontophoretic electrode to determine ethanol in sweat induced by transdermal delivery of the pilocarpine drug [18]. Moreover, recent efforts have been made to combine these two concepts, including glucose and alcohol sensors, on a single tattoo [19]. This holds a possibility for multianalyte

Skin-worn microfluidic devices can enable the continuous flow of renewed sweat over operational periods. This addresses the challenge of mixing and carryover between new and old sweat. **Figure 2C** shows an example of sweat collection microfluidic devices, coupled with glucose and lactate biosensors [20]. This offers

Moreover, reading several parameters can complete a clear picture of individual health. A fully integrated sensor array for sweat analysis was demonstrated (**Figure 2E**) [23]. These integrated sensors can monitor information of glucose, lactate, electrolytes (e.g., sodium and potassium ions), and temperature. The temperature sensor is also helpful to standardize the biosensing amperometric response. Furthermore, in order to apply the biosensor glucose device for health management, a transdermal closedloop drug delivery integrated with a sweat-based glucose electrochemical sensor was demonstrated (**Figure 2F**) [24]. The sense-treat concept aimed to give feedback of transdermal administration of type 2 diabetes drugs in response to the glucose level. This idea represents a possible opportunity to overcome insulin overtreatment, helping

BFCs are energy-conversion devices that utilize biocatalysts to convert chemi-

cal energy into electricity. For wearable electronics, the need to anatomically power sources has attracted many research groups to develop a BFC, as a "green" energy-harvesting alternative, in order to extract energy from metabolites present in biofluids, such as perspiration. Since glucose, lactate, and oxygen are present in physiological fluids, in general, a majority of wearable enzymatic BFCs rely on (1) the generation of electrons from glucose or lactate biofuels and (2) the electron reduction by oxidants (such as oxygen). **Figure 1C** shows a typical example of a glucose/O2 BFC. In principle, a glucose BFC uses GOx, functionalized on the bioanode, to catalyze the glucose oxidation reaction to generate electrons. After this oxidation process, these harvested electrons are driven through an external circuit to the biocathode compartment where such electrons are accepted by oxidant molecule (commonly O2) and, eventually, generate complete electrical work. In addition to

wearable effective continuous sweat sampling and flow electroanalysis. Furthermore, minimally invasive microneedles for continuous glucose monitoring have been demonstrated. For example, a GOx/tetrathiafulvalene microneedlebased amperometric sensor (~1.2 mm needle height) could be used for in vivo studies [21]. The data were also validated with the finger-prick technique, indicating a promising alternative for on-skin analysis. In addition, a minimallyinvasive microneedle-based potentiometric sensor for tracking *β*-lactam antibiotic concentrations in vivo and real time was demonstrated **Figure 2C** [22]. This example represents a possibility to tailor individual therapy with the optimal

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

sweat analysis.

efficacy.

patients to maintain their homeostasis.

**2.2 Enzyme-based electrochemical power sources**

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

detection to extract the sample containing glucose. The glucose biosensor, located near the negative iontophoretic electrode, relied on GOx immobilization on the Prussian blue (PB)-carbon electrode; this PB facilitates the electroreduction of H2O2 product, generated by the GOx reaction. The amperometric reduction of H2O2 could be detected at a potential of −0.1 V versus Ag/AgCl. The iontophoresis strategy will be discussed in Section 3.5. Additionally, the tattoo-based alcohol sensor was also invented (**Figure 2B**). The AOx-/PB-based sensor was designed to be close to the positive iontophoretic electrode to determine ethanol in sweat induced by transdermal delivery of the pilocarpine drug [18]. Moreover, recent efforts have been made to combine these two concepts, including glucose and alcohol sensors, on a single tattoo [19]. This holds a possibility for multianalyte sweat analysis.

Skin-worn microfluidic devices can enable the continuous flow of renewed sweat over operational periods. This addresses the challenge of mixing and carryover between new and old sweat. **Figure 2C** shows an example of sweat collection microfluidic devices, coupled with glucose and lactate biosensors [20]. This offers wearable effective continuous sweat sampling and flow electroanalysis.

Furthermore, minimally invasive microneedles for continuous glucose monitoring have been demonstrated. For example, a GOx/tetrathiafulvalene microneedlebased amperometric sensor (~1.2 mm needle height) could be used for in vivo studies [21]. The data were also validated with the finger-prick technique, indicating a promising alternative for on-skin analysis. In addition, a minimallyinvasive microneedle-based potentiometric sensor for tracking *β*-lactam antibiotic concentrations in vivo and real time was demonstrated **Figure 2C** [22]. This example represents a possibility to tailor individual therapy with the optimal efficacy.

Moreover, reading several parameters can complete a clear picture of individual health. A fully integrated sensor array for sweat analysis was demonstrated (**Figure 2E**) [23]. These integrated sensors can monitor information of glucose, lactate, electrolytes (e.g., sodium and potassium ions), and temperature. The temperature sensor is also helpful to standardize the biosensing amperometric response. Furthermore, in order to apply the biosensor glucose device for health management, a transdermal closedloop drug delivery integrated with a sweat-based glucose electrochemical sensor was demonstrated (**Figure 2F**) [24]. The sense-treat concept aimed to give feedback of transdermal administration of type 2 diabetes drugs in response to the glucose level. This idea represents a possible opportunity to overcome insulin overtreatment, helping patients to maintain their homeostasis.
