# Research Progresses and Application of Biofuel Cells Based on Immobilized Enzymes
## 3 Applications of EBFCs  
### Implantable EBFCs  
Many implantable devices have been developed for medical monitoring so that vital life parameters can be acquired to alert medical professionals or device bearers for appropriate care. Implantable EBFCs can help power those implantable devices as well as the devices used outside the bodies. Flexible and biocompatible electrode materials are often required as enzyme carriers in EBFCs for implantable biomedical devices. With this in mind, the carbon materials, such as buckypaper (BP), carbon paper (CP), graphene paper (GP) and carbon cloth (CC), are often selected for EBFCs due to their good mechanical maneuverability and portability, good electrical conductivity and good biocompatibility. For example, [PERSON] et al. employed two-dimensional graphene paper (2D-GP) as the enzyme carrier for fabricating EBFCs toward wearable and implantable devices [53]. EBFCs can convert chemical energy from molecules in living organisms into electrical energy. Glucose is the main energy supplying substance in living organisms and is widely distributed in living organisms, such as blood, tear fluid and sweat. Thanks to innovative advances in the design of nanostructured materials, implantable EBFCs based on molecular energy supply in living organisms have been developed rapidly. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Anode** & **Cathode** & \begin{tabular}{c} **Open-Circuit** \\ **Voltage** \\ \end{tabular} & \begin{tabular}{c} **Maximum Current** \\ **Density** \\ \end{tabular} & \
\begin{tabular}{c} **Maximum Power** \\ **Density** \\ \end{tabular} &
\begin{tabular}{c} **Reference** \\ **Density** \\ \end{tabular} \\ \hline GOx/QN-rGO@CNT/Ni foam & Lac/QN-rGO@CNT/Ni foam & 0.89 V & 2.25 mA cm\({}^{-2}\) & 900 \(\mu\)W cm\({}^{-2}\) & [36] \\ \hline GOx/Au NPs/CNT/PET & MnO2@CNT & 0.58 V & 0.733 mA cm\({}^{-2}\) & 220 \(\mu\)W cm\({}^{-2}\) & [51] \\ \hline GOx/rGO/FP & Lac/rGO/FP & 0.04 V & 0.0007 mA cm\({}^{-2}\) & 0.004 \(\mu\)W cm\({}^{-2}\) & [52] \\ \hline GDH/MB/2D-GP & BOD/2D-GP & 0.665 V & 0.016 mA cm\({}^{-2}\) & 4.03 \(\mu\)W cm\({}^{-2}\) & [53] \\ \hline FMe\({}_{2}\)-LPEI/LOx/PTFE & BOD/CNT/PTFE & 0.55 V & 0.14 mA cm\({}^{-2}\) & 20 \(\mu\)W cm\({}^{-2}\) & [54] \\ \hline GDH/CNT@CNF & Lac/CNT@CNF & 0.434 V & 0.176 mA cm\({}^{-2}\) & 27 \(\mu\)W cm\({}^{-2}\) & [55] \\ \hline GDH/PANI/BP & Lac/PANI/BP & 0.71 V & 0.53 mA cm\({}^{-2}\) & 204 \(\mu\)W cm\({}^{-2}\) & [56] \\ \hline GDH/PLQ/BP & BOD/CNT/BP & 0.5 V & 5.38 mA cm\({}^{-2}\) & 650 \(\mu\)W cm\({}^{-2}\) & [57] \\ \hline GDH/BP & BOD/BP & 0.59 V & 2.62 mA cm\({}^{-2}\) & 1070 \(\mu\)W cm\({}^{-2}\) & [58] \\ \hline GDH/BP & BOD/BP & 0.62 V & 0.141 mA cm\({}^{-2}\) & 470 \(\mu\)W cm\({}^{-2}\) & [59] \\ \hline \hline \end{tabular}  
* PET (polyethylene terephthalate); FP (filter paper); MB (methylene blue); 2D-GP (2D-graphene oxide paper); FcMe\({}_{2}\)-LPEI (linear polyethyleneimine-modified dimethylferrocene); PTFE (polytetrafluoroethylene); CNF (cellulose nanofiber); PLQ (1,10-phenanthroline-5,6-dione).  
\end{table} 
Table 2: A part of paper-based and other flexible materials applied in EBFCs *. [PERSON] and [PERSON] implanted their biofuel cell into a grape [60]. It was the first diaphragmsel single-chamber biofuel cell operating in an organism to oxidize glucose with GOx and reduce oxygen with BOD, yielding an open-circuit cell voltage of 0.8 V and producing a maximum power density of 240 \(\upmu\)W cm\({}^{-2}\) at 0.52 V [60]. [PERSON] and [PERSON] et al. implanted the first glucose biofuel cell based on an independent enzyme electrode in rats [61]. The cell used insoluble ubiquinone (UQ) as an electron transfer mediator bound to GOx for the anode. Polyphenol oxidase (PPO) was used for the cathode with hydroquinone (HQ) as an electron transfer mediator. It was implanted into the retroperitoneal space of rats.