# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 3 Applications of EBFCs
### Implantable EBFCs
The open-circuit voltage of the cell was 0.275 V with a maximum output power of 6.5 \(\upmu\)W, and the open-circuit voltage only dropped to 0.25 V after 40 days of operation, indicating good operational stability [61]. To ensure the biocompatibility and minimal impact on living organisms, implantable EBFCs need to be carefully designed. [PERSON] et al. used GDH as the anode enzyme and Lac as the cathode enzyme [62]. The enzymes were immobilized onto the BP electrode by cross-linking. The resulting cell was implanted into a living snail, obtaining an open-circuit voltage of 0.53 V and a maximum output power of 30 \(\upmu\)W cm\({}^{-2}\). The battery was still functional after two weeks of operation in the snail under the condition that the snail survived normal feeding, demonstrating that the metabolically regenerated glucose can continuously produce electricity for the battery implanted in the living body. They also implanted the same EBFCs into clams and lobsters [63; 64]. The EBFCs implanted in clams obtained an open-circuit voltage of 0.3 V\(\sim\)0.4 V with a maximum power density of 40 \(\upmu\)W cm\({}^{-2}\). The two lobster \"batteries\" implanted with EBFCs were connected in series to obtain an open-circuit voltage of 1.2 V, which successfully enabled the operation of an electronic meter. Connecting five lobster \"batteries\" in series also enabled the activation of a medical pacemaker requiring an operating voltage of 2.8 V and a power consumption of 90 \(\upmu\)W [63; 64]. In the development of implantable biofuel cells (EBFCs), biocompatibility and minimal impact on the living organism are crucial considerations. [PERSON] et al. grafted alglucosidase (Trehalase, Tre) and GOx dual enzyme anodes and BOD cathodes onto a polymer backbone with Os complexes and implanted them through an abdominal incision into female _Blaberus discoidalis_ (insect name) [65]. The resulting cell had a maximum power density of about 55 \(\upmu\)W cm\({}^{-2}\) at 0.2 V and only a 5% decrease after 2.5 h of operation. [PERSON] et al. also made EBFCs using Tre and GOx dual enzyme anodes and BOD cathodes. The electrodes were protected with dialysis membranes and implanted into cockroaches [66]. The resulting cell obtained a maximum power density of 6.07 \(\upmu\)W cm\({}^{-2}\) using alginose in the cockroaches. In addition, using air diffusion cathodes, the maximum power density could reach 10.5 \(\upmu\)W cm\({}^{-2}\). They also connected five cockroach EBFCs in series to successfully operate a piezoelectric speaker [66]. This result suggests that insect EBFCs are a promising biobattery that can be used as a power source for environmental monitoring micro-tools. Using KB (Ketjen black, conductive carbon black) as the base electrode [34]. [PERSON] et al. developed a needle bioelectrode using FDH and GDH as dual enzyme anodes and BOD as cathode, which produced a power density of 115 \(\upmu\)W cm\({}^{-2}\) at 0.34 V after implantation into grapes [67]. The needle electrode was inserted into a rabbit ear vein vessel and the open-circuit voltage of the battery could reach 0.81 V [67]. Using CP (carbon paper) as the base electrode, [PERSON] et al. integrated hydrophillically treated with GOx and BOD on carbon fibers to form a bioanode and a biocathode onto a metal needle to form a needle cell [68]. The resulting cell obtained 10 mA cm\({}^{-2}\) in simulated artificial glucose containing 5 mm of glucose. When the tip of the anode needle was inserted into the natural samples of grapes, kiwi and apples, the power generated by the assembled battery from the glucose was 55 \(\upmu\)W, 44 \(\upmu\)W and 33 \(\upmu\)W, respectively. In addition, the needle cell was inserted into the heart of a mouse and produced 16.3 \(\upmu\)W at 0.29 V. Additionally, they coated the anode to be implanted into the organism with an antifouling polymer and sealed the cathode with medical tape to minimize the evaporation of water without affecting the oxygen permeability, thus improving the battery life towards wearable electronics [68].