An EFC is capable to overcome this issue. Studies performed so far showed that there is still lack of stability and mechanical flexibility for wearable devices. In wearable EFCs, glucose and lactate are widely utilized as fuel source, enzymes which are specific to these fuels are used as anode biocatalysts. On cathode side, noble metals provide proper current densities. However, they are costly, poisoner and offer low OCP. In contrast, enzyme-immobilized cathodes are cheaper than the noble metals, offer high OCP and generate little amount of byproducts due to enzyme specificity [39].
A wearable EFC printed directly onto textile materials was reported to generate energy from human sweat. Textile-based EFC, utilized physiologically human sweat lactate as fuel, generated power density up to 100 μW cm−2 at 0.34 V in vitro experiments. It was integrated into a headband and a wristlet to demonstrate that the EFC was producing sufficiently electrical energy. The lactate generated from sweat of human subjects was converted into electrical energy by EFC and a LED and a digital clock were powered by the EFC and operated with the aid of on-board DC/DC converter. In this study a headband was prepared with four parallel EFCs, a subject who weared this headband was performed a stationary bike exercise for perspiring. The LED placed in the headband was flashed with sweat for seven times shortly. Four parallel configurated textile EFCs were placed into a wristlet, and powered a digital watch which remained on for 50 s [40]. Another carbon nanotube-decorated stretchable EFC was made of laminating a bioanode textile, a hydrogel sheet containing fructose as fuel, and a gas-diffusion biocathode textile. The currents of EFC textiles were reported to be consistent for 50% stretching in 30 cycles due to the fractional breaking of the carbon nanotube network at juncture of textile material. With the stretched, twisted and wrapped forms of the textile-based EFC, approximately 0.2 mW cm2 power was generated with a 1.2 kΩ load [41]. A wearable textile-based hybrid super capacitor - biofuel cell, printed on both sides of the fabric and used lactate as fuel, was designed to scevenge the biochemical energy with the user’s sweat and store it in the süper capacitor module for later use. Super capacitor energy storage module was based on MnO2/carbon nanotube. Lactate BFC and super capacitor were integrated across the counterpart of a stretchable fabric, collecting biochemical energy from sweat and stored the generated energy in an integrated stretchable supercapacitor [42]. A flexible enzyme/carbon nanotube composite fiber that uses glucose as biofuel was designed with a series of connections by bonding enzyme fibers with batik based ionic isolation for power generation on a textile fabric. Electrodes were prepared by immobilizing glucose dehydrogenase on anode and bilirubin oxidase on cathode to carbon fibers coated multi walled carbon nanotube. Using optimized electrodes, the highest density of power reached 216 μW cm−2 at the applied voltage of 0.36 V, even when the structure was deformed to like shape of S. In order to increase the output voltage, four BFC series connected between batik-based ionic isolation in the fabric were connected fiber electrodes. The OCV was increased to 1.9 V for quad BFCs and 0.51 V for a BFC. The study showed the lightening of a LED connected in series to four BFCs [38]. Miyake et al. reported that laminated EFC stack was made up of carbon fiber fabrics decorated with carbon nanotubes. Designed the single set of anode/gel/cathode layers were reported to be 1.1 mm of thick, 5 mm × 5 mm in size and used fructose as a fuel. An OCV of 2.09 V which was a 2.8-fold bigger value than a single set cell (0.74 V), was produced by the laminated triple-layer stack EFC. A maximum power of 0.64 mW at 1.21 V was produced by the laminated EFC and its power was able to light the LEDs. The designed EFC was shown in Figure 2.4a (Figures 2.4b and c in the article) [43].
A wearable photocatalytic fuel cell utilized biowaste sources such as lactic acid, ethanol, methanol, urea, etc. was designed. The system generated electrical power by the decomposition of the biowastes and using light irradiation. When the cell was fabricated into a sweatband, it produced a maximum of 4.0 mW cm−2 g−1 power from human sweat. In this system that uses sweat as fuel, the photoelectrochemical performance of the system was investigated using 1 W 365 nm LED and the photograph of designed sweatband was presented in Figure 2.4b [44]. In another study, a flexible and wearable thermoelectric Bi2Te3-based nanogenerator was prepared. The proposed nanogenerator was produced with the Cu conductor and operated as a thermopile with an end-to-end 126 thermoelectric legs connection. Some metal alloys such as Bi, Te, etc. were used to thermoelectric materials by dopping, and thanks to the nickel coating on the surface, the thermoelectric properties deterioration were observed as a result of diffusion of copper atoms or ions. The designed nanogenerator shown in Figure 2.4c was applied to human skin, an OCV of 86.2 mV was generated with 5 μW power output. The authors noted that the nanogenerator using body temperature may be used to power autonomous micro devices in the future [45].
Figure 2.4 (a) Photograph of the biofuel cell sheet and LEDs connected with the triple layer cell, (b) optical images of photocatalytic fuel cell based sweat band in operation, (c) photograph of a watch powered by the wearable thermoelectric nanogenerator and demonstration of harvesting thermal energy from human skin ((a) is adapted from Ref. [43], with permission; Elsevier, (b) is adapted from Ref. [44], with permission; John Wiley and Sons, (c) is adapted from Ref. [45], with permission; Copyright American Chemical Society, 2019).
A highly conductive and catalytic buckypaper electrodes with a structurally stretchable substrate to harvest energy from perspiration was designed. In practice, structural tensile and material intrinsic stretchability was achieved by combining the “island bridge” architecture with the stretchable ink formula to ensure wearable devices withstand rigorous movements and deformation during human exercise. The electrodes of the device were divided into “islands”, which were tightly connected to the substrate, together with serpentine-shaped “bridges” that could loosen under stress. When external strain was applied, the stress was distributed to flexible “bridges” around the “islands” to ensure electrical resistance stability [46]. Electrical energy was generated by epidermal EFC based on temporary transfer tattoos which were designed in the shape of “UC” acronym for “University of California” (bioanode; “U” and biocathode; “C”). The designed screen-printed transfer-tattoo electrodes were reported to be compatible with nonplanarity of the epidermis and resistant to mechanical deformations. The power density obtained from sweating of human subjects with varying levels of fitness was calculated between 5 and 70 μW cm−2 (lactate as fuel). Since the power generation from epidermal EFC depended on the levels of sweating, the power produced was determined to be unstable as expect [47].
A contact lens EFC composed of cured on buckpaper electrodes a silicone elastomer was fabricated. The buckypaper anode and cathode were consisted of lactate dehydrogenase and bilirubin oxidase, respectively. Contact lens EFC experiments were performed in a synthetic tear solution at 35 °C. The OCV, the maximum current and power density were calculated to be 0.413 ± 0.06 V, 61.3 ± 2.9 μA cm−2 and 8.01 ± 1.4 μW cm−2, respectively. In additon, current output of anode side was reported to be unstable in the first 4 hours and then stabilized for the next 13 h [48]. An EFC used lactate as fuel was prepared as a power source for wearable microelectronic devices by modifying anode with Osmium polymer and lactate oxidase, and cathode with bilirubin oxidase. The electrodes were placed between two commercial contact lenses to avoid direct contact with the eye. The designed EFC was shown in Figure 2.5. The system was operated in artificial tear solutions containing lactate, and it generated a power density of 1.7 ± 0.1 μW cm−2 and an open-circuit voltage of 380 ± 28 mV [49].