# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 3 Applications of EBFCs
### Implantable EBFCs
The activity of human dermal fibroblasts (Human Dermal Fibroblasts-adult, HDF) is a key biological factor affecting post-transplant tissue or organ wound healing. [PERSON] et al. performed a basal cell culture using GDH as an anodic enzyme and BOD as a cathodic enzyme to study the cytocompatibility of EBFCs using HDF as a model cell [69]. The cellular inflammatory response of HDF was low, while EBFCs showed extreme biotoxicity to HDF after replacing GDH with Gox. The power density of EBFCs prepared using GDH ranged from 15.26 to 38.33 nW cm\({}^{-2}\), indicating that GDH is more suitable for implantable EBFCs than GOx and is a promising implantable power source for biomedical applications [69]. [PERSON] et al. were able to generate electrical energy from human tears for the first time [70]. One-hundred \(\upmu\)m diameter gold wires were covered with 17 nm Au nanoparticles to form a three-dimensional nanostructured microelectrode. Anodic enzyme electrode and cathodic enzyme electrode were prepared with CDH (Cellobiose Dehydrogenase) and BOD, respectively. Tested in the tear environment of a healthy human body, the battery had an open-circuit voltage of 0.57 V, and the maximum power density at 0.5 V is 1 \(\upmu\)W cm\({}^{-2}\). After 20 h of continuous operation at 0.51 V, the output current of the cell was still 68% of the initial current with no significant change in the open-circuit voltage. The electrodes were stably stored in buffer at room temperature [70]. These findings suggest that tears could potentially serve as a sustainable and renewable source of energy for various implantable devices. After [PERSON] and [PERSON] et al. developed an EBFC implanted in rats [61]. [PERSON] and [PERSON] et al. implanted a wireless-controlled EBFC in a rabbit [71]. The EBFC functioned well in vivo for a period of 2 months, although at the end of the implantation, the power output diminished, most likely due to an inflammatory process [61; 71]. After developed the implantable EBFC in a living snail [62]. [PERSON] et al. prepared an EBFC based on BP electrodes with GOx and BOD [72]. The resulting biofuel cell was implanted in a living gray garden slug which produced electrical power in the range of 2-10 \(\upmu\)W and it was successfully used to power one microelectronic device. The whole system was able to be operated autonomously by extracting electrical energy from the glucose-containing hemolymph (blood substituting biofluid) in the slug and readout data wirelessly [72]. [PERSON] et al. implanted carbon nanotube-based EBFC in the abdominal cavity of mice [73]. The carbon nanotube electrodes were coated with Nafion and twisted into a micro-sized, two-ply, one-body system with improved mechanical properties for convenient implantation, providing high power density of 0.3 mW cm\({}^{-2}\). The two-ply EBFC system exhibited good biocompatibility in vivo [73]. [PERSON] et al. developed an EBFC using carbon felt electrodes loaded with immobilized Gox and BOD via MWCNTs as bioanode and biocathode, respectively [74]. The EBFC was combined with a brain stimulator and implanted in a pigeon. The power output from the EBFC reached 0.12 mW in vitro and 0.08 mW in vivo using only the natural glucose and oxygen in the pigeon's body [74]. Although implantable EBFCs have been comprehensively studied, in order to achieve sustainable operation, the following issues should be addressed: (1) the enzyme should be simply and firmly anchored to the electrode and no substances with poor biocompatibility should be arbitrarily introduced; (2) it should be ensured that oxygen does not interfere with the catalytic oxidation of the anode biofuel; and (3) the bioelectrode should be able to operate at low concentrations of biofuel and oxygen, especially in the case of slow diffusion and high viscosity of biological tissues, such as in human blood.