# Towards Bio-Hybrid Energy Harvesting in the Real-World:
## 4 Research Progress in Energy Harvesting from Enzyme-Based Biofuel Cells (EBFCs)
### Classification of EBFCs  
The motivation for developing EBFCs was backed by the demand of electrical power requirements for monitoring [270] and maintaining [258] physiological parameters in living macro-organisms. So far, successful operations of EBFCs have been employed in plants [270; 271; 272; 273], insects [9], mollusks [8], lobsters [34] and mammals [35]. We can classify EBFCs into four major groups: in vitro, plant-powered, animal-powered and human-powered. Animal-powered EBFCs can be further classified into externally and internally implantable sub-groups. Nearly all presently available human-powered EBFCs fall under the wearable sub-group. This classification is depicted in Figure 13.  
Figure 12: Common reasons for implanted EBFC failures. Studies on in vitro EBFCs, i.e., EBFCS which have been tested outside any living organism and in a lab environment, have been widely reported. One such experiment was presented by [PERSON] et al. [274] where bucky paper electrodes were used. The anode was modified with PQQ-dependent glucose dehydrogenase, which is a quinoprotein enzyme with PQQ cofactor for facilitating glucose oxidation [275], and the cathode used lacase. The cell was initially tested in human serum solution and then on exposed rat cremaster tissue. [PERSON] et al. [276] informed about an EBFC with a bienzymatic trehalose glucose oxidase trehalose anode and a bilirubin oxidase dioxygen cathode. [PERSON] et al. [273] reported their study on a needle-type biofuel cell and tested it on artificial blood glucose and from the blood glucose in a mouse heart. Initially and until now in the primary phase, the EBFCs have been tested in plant mediums. [PERSON] et al. [270] utilized cactus leaf and a light source to implement a biosensor for continuous monitoring of O\({}_{2}\) and glucose generation rate. [PERSON] et al. [271] reported their enzyme-based bioreactor for orange pulp and the generated power was used to power a wireless data transmission system. A similar experiment using orange pulp for powering a wireless transmitter was reported by [PERSON] et al. [272]. The authors worked on an EBFC in which electrodes were modified with inorganic nanoparticles deposited and with carbon black. [PERSON] et al. [273] reported a needle type biofuel cell whose performance was tested in various fruits and 55, 44, and 33 \(\mu\)W of power output was obtained from specimens of grape, kiwifruit, and apple, respectively. There is an interesting bio-hybrid EBFC implanted animal group that includes experiments where the EBFC (main cell) is placed outside the energy-harvesting organism. It is inspired by the observation that placement of a complete cell inside the receiving living organism becomes difficult for small-sized organisms like snails [8]. A similar configuration has also been used in live American lobsters [34]. A cyborg insect, _Blaberus discoidalis_, was reported by [PERSON] et al. [9] where the insect hemolymph was used for the substrate for a trehalose/oxygen biofuel cell. As a continuation, a similar system was implemented in a moth for powering up a wireless sensor for environmental monitoring [12]. There are also examples of internally implanted EBFCs in animals. [PERSON] et al. [277] reported the first in vivo implantation employed in the retroperitoneal space of moving rats. [PERSON] et al. [32] reported a similar configuration in rats, capable of producing an average open-circuit voltage of 0.57 V. [PERSON] et al. [278] reported a cell implanted in the gastrointestinal tract of pigs that could perform in vivo temperature sensing and wireless communication. [PERSON] et al. [35] reported implantation of another abdominal implant in a rabbit that operated for 2 months. A new group of EBFCs are now being investigated which can be implanted externally. For example, [PERSON] et al. reported power generation from a transdermal extract of pig skin [279]. Figure 13: Classifying EBFCs according to their placement. Scopes of utilization for such external or wearable EBFCs for human use are also being explored.