# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 3 Applications of EBFCs
### Wearable EBFCs
The quaternary ammonium cation-modified Perfluorosulfonic acid film was used to protect and immobilize the BOD as the cathode. The cathode and anode were immobilized on the surface of silicone rubber elastomer and combined with contact lenses to assemble EBFCs that could generate energy from lactic acid in tears [82]. The cell had an open-circuit voltage of 0.413 \(\pm\) 0.06 V and a maximum output power of 8.01 \(\pm\) 1.4 uW cm\({}^{-2}\) and could provide micro wearable devices such as contact lenses with electrical power. However, PMG leaching from the anode easily degraded the performance of the cell and the problem of poor biocompatibility still needs to be solved [82]. These are several examples of wearable biofuel cells that use different approaches to generate electrical power from human sweat or tears. [PERSON] and [PERSON] et al. fabricated printed electrodes on textiles, using tetrathiofulvlene-7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ) as the electron transfer mediator for LOx to obtain the anode and platinum black as the cathode, producing a power density of 100 \(\upmu\)W cm\({}^{-2}\) at 0.34 V [83]. The battery pack was mounted into a textile headband and wristband, and a stable voltage and current were obtained from the sweating human body with the help of a DC/DC converter to directly power LEDs and digital watches [83]. [PERSON] et al. also developed a high-power biofuel cell bracelet using lactic acid production capacity from human sweat [84]. This battery used LOx/Os-based complexes/CNTs/carbon fibers as the anode for lactic acid oxidation and BOD/CNTs/carbon fibers as the cathode for oxygen reduction and is woven into a hydrophilic support fabric for storing sweat. The cell reached a maximum open-circuit voltage of 0.6 V and obtained a maximum power of 74 \(\upmu\)W at 0.39 V in artificial sweat containing 20 mm of lactic acid. The performance remained above 80% after 12 h of operation. Six cells were connected in series and bundled and wrapped around the wrist, and the open-circuit voltage reached 2.0 V, successfully powering an electronic watch [84]. In another work, they coated carbon fibers with MWCNTs, and integrated a GDH bioanode mediated by MG and a BOD bioachode for oxygen reduction on a cotton textile fabric to make a battery with an open-circuit voltage of 0.51 V [85]. The power density generated from 0.1 mM of glucose (simulating the glucose concentration in human sweat) at 0.24 V was 48 \(\upmu\)W cm\({}^{-2}\). When used in a hydrogel containing 200 mM of glucose, a power density of 216 \(\upmu\)W was generated at 0.36 V. They also transformed the cell into an S-shape with no significant loss of performance. In addition, they bundled four cells on clothing to generate 1.9 V and successfully lit LEDs on clothing [85]. [PERSON] and [PERSON] et al. immobilized NQ-mediated LOx as anode and BOD as cathode on a BP paper substrate [86]. The two bioelectrodes were integrated onto a stretchable screen-printed circuit substrate. The battery open-circuit voltage of 0.74 V and the maximum power density of 520 \(\upmu\)W cm\({}^{-2}\) were obtained with in vitro-simulated sweat containing 0.5 M lactic acid [86]. The battery is immobilized in the human arm and can generate a maximum power of 450 \(\upmu\)W during exercise. [PERSON] and [PERSON] et al. developed a microfluidic wearable battery consisting of a skin-interfaced microfluidic module and a cell module based on an ethanol/oxygen biofuel cell [29]. Using 3D-NHCAs as carriers of AOx and BOD to form electrodes, the battery system was placed on different skin regions of the forearm, the back of the neck, and the forehead of the drinker. The battery exhibited a maximum output of 1.01 \(\upmu\)W cm\({}^{-2}\) for the battery system worn on the forearm and a minimum output of 0.65 \(\upmu\)W cm\({}^{-2}\) on the forehead [29]. They also considered and studied some practical conditions related to human drinking and realized the collection of sweat bioenergy in a controlled environment.