# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 4 Research Progress in Energy Harvesting from Enzyme-Based Biofuel Cells (EBFCs)
### Principles of Electricity Generation from EBFCs  
EBFCs are bio-electrochemical cells that can extract energy from glucose and alcohol based organic substances [257]. In a classic EBFC configuration, glucose oxidase or glucose dehydrogenase (GDH) are immobilized at the bio-anode for glucose oxidation while oxygen is reduced at the biocathode using immobilized lactose or bilirubin oxidase and generating power [33]. Although their principle of operation is similar to MFCs, both anode and cathode of such bio-fuel cells are prepared by embedding pure enzymes [257]. Thus, at a lower level, EBFC electrodes are inherently different from MFC electrodes since they require the presence of specific enzymes without the presence of the microorganisms that generate them. However, from a top view, EBFCs are equivalent to a classic two-electrode MFC configuration since they perform oxidation of glucose at the anode and oxygen reduction at the cathode to generate electrical power [32] and remain connected via an external load resistance. A conceptual implementation of an EBFC is shown in Figure 10.  
These enzyme-immobilized electrodes are an indispensable part of EBFCs where synthetically produced enzyme catalysts are assembled on electrode surfaces [259]. The development of enzyme immobilization techniques has remained as an active research area for the last 20 years [25]. Common enzyme immobilization techniques include enzyme covalent binding or \"wiring\", sandwiching between a protective polymer layer and the electrode surface and entrapment inside the polymeric membrane coating of the electrode [260]. This technology offers an opportunity to miniaturize the cell structure since the configuration does not require separation of fuel and oxidant [259]. A pictorial summary of various immobilization techniques has been given in Figure 11.  
To obtain a summary of the progresses in the enzyme based biofuel cells over the last 30 years, the readers are referred to the works of [PERSON] et al. [261]. According to the authors, the biggest challenge in this field is the low stability and electrochemical performances of EBFCs. [PERSON] et al. [33] predicted that enzyme immobilization of EBFC electrodes can be greatly improved with novel nano carbon materials. For more details on EBFC-targeted carbon nano materials and functionalization, the readers are referred to their work [33]. [PERSON] et al. [262] summarized the most commonly used enzymes for EBFC anodes, wearable EBFC solutions, EBFC-based biosensors and provided a comparison of implantable EBFCs in terms of their output power. They hypothesize that these developments of cost-effective and safe EBFCs can power up key biomarker monitoring and help saving millions of lives. [PERSON] et al. [263] have focused on the interfacing technologies between EBFCs. They hypothesize that these cells, operating either internally or externally on a human body, could pave the way of bionic human-machine hybrid and this scope can extend to cyborg animals as well and can greatly contribute to environmental monitoring, homeland security and military applications. [PERSON] et al. [264] have summarized the key bottlenecks of wearable EBFC technology. They focus on the fact that the sustainable development of wearable EBFCs must be able to dynamically adjust with uncontrolled body changes that occur due to regular movements. Figure 11: Various enzyme immobilization techniques for enzyme-based biofuel cells (EBFCs). Figure 10: A futuristic concept of a biofuel cell for implementation within a blood vessel with enzyme immobilized electrodes, redrawn from the schematic shown in [258]. Carbon nanomaterials are extensively used in fabrication of enzyme-immobilized electrodes for EBFCs owing to the thin diameter, a feature that makes electrodes accessible to the enzyme active sites [33]. For example, [PERSON] et al.