Biofuel Cells. Группа авторов

human body contains or releases many forms of energy, such as chemical and physical energy which provide 100 W of power [18, 21]. These energy can be converted to electrical energy via BFCs. In the first attempt in this direction, an enzyme-based BFC was operated in a living system. Mano et al. implanted an EFC in a grape grain including abundant glucose content of >30 mM glucose at pH 5.4. The authors reported that the power density was found to be 2.4 μW mm⁻² when the cathode fiber was near the skin of the grape. The EFC system was preserved 78% of initial power output at the first day with continuous operation [22]. The EFC studies were performed, which were implanted into different living plant, such as a cactus [23] and Carpobrotus acinaciformis [24–26]. Oxygen and glucose produced by photosynthesis were converted into renewable and sustainable electrical energy by sunlight. The next major improvement was animal experiments. For this purpose, first glucose-based EFC was implanted with surgical operation into a retroperitoneal space of freely moving rat in 2010. It was reported that a stable energy production of 2 μW was obtained in the extra-cellular fluid and no signs of inflammatory reaction were observed for the implant during 3 months [27]. In another study, a trehalose based EFC was implanted in a live cockroach [28]. Another important progress was the implantation of glucose-based EFC in a snail. The thin sheet EFC buckypaper electrodes made from an aggregate of carbon nanotubes or carbon nanotube are very popular recently due to its superior properties. They were placed into the hemolymph between the body wall and internal organs. In snail experiments conducted with these electrodes, open circuit potential (OCV), maximum current and generated power was reported as 0.53 V, 42.5 μA and 7.45 μW, respectively. That thanks to the ability of buckpaper electrodes, the EFC operation was reproducible even after a 2-week period and it was not affected by enzyme inactivation or biofouling in the living medium. In addition, reversible decay in the power generation had been interpreted as inadequate glucose regeneration in the buckypaper electrode surface because of slow diffusion in the hemolymph [29]. The biocatalytic electrodes implanted in a snail were presented in Figure 2.2. The same research group placed an EFC into the two living lobsters which were connected in a series, and achieved to activate a digital watch by the living battery. The highest OCV obtained from the implanted EFC system was reported as 1.2 V by the research group. In the same paper, the fluidic EFC including five cells, and designed to imitate circulation of human blood were used for activate pacemaker. The implanted EFC produced enough electrical energy for the pacamaker at least 5 h with this innovative attempt, but long-term stability was not studied for this prototype [30]. In another implanted EFC system which glucose was used as fuel, three electrified clams were connected in serial and parallel. While an OCV of 0.8 V, short-circuit current of 25 μA and power of 5.2 μW were calculated in the serial connection, OCV of 0.36 V, short-circuit current of 300 μA and power of 37 μW were reported in parallel connection. This study was showed the activating possibility of mini or micro devices using the energy generated in vivo medium [31].

Figure 2.2 Photograph of a snail with implanted biocatalytic electrodes (Adapted from Ref. [29], with permission; Copyright American Chemical Society, 2012).

In the future, with the idea of being able to implantable EFCs to humans and evaluate blood sugar for mini/micro devices, EFCs have been tested with more advanced living creatures such as mammals. A miniature EFC system (skin-worn biofuel cell) for power production from ear of rabbit was made of a needle anode and a gas diffusion cathode for using oxygen in the air. The needle electrode substrate was coated with a biocompatible polymer (2-methacryloyloxyethyl phosphorylcholine) defined as antibiofouling agent to prevent blood clotting on the electrode surface. The needle electrode was designed for easy access to blood sugar and insterted in a blood vessel in the ear of rabbit for glucose oxidation. The polymer coated needle electrode was reported to be effective in stabilizing the output power. The needle anode without polymer was lost almost 40% of its power. The power generation was reported to be 0.42 μW at a cell potential of 0.56 V [32]. EFCs (mediator-, cofactor-, and membrane-less) designed with 3D nanostructured micro-scale gold electrodes were operated in cerebrospinal fluid and a rat brain. The implantation photograph of micro bioelectrodes into the rat cortex was shown in Figure 2.3a. The maximum power density was reported to be 2 μW cm⁻² in vivo and 7 μW cm⁻² in vitro at a cell potantial of 0.4 V by using in vivo glucose [23]. Another glucose based EFC was implanted into a rat’s abdominal cavity and produced average OCV of 0.57 V. In this study, it was reported that the power output (38.7 μW) produced by EFC could be sufficient to operate a digital thermometer or a light-emitting diode (LED). Even after 110 days of implantation, inflammation or rejection was not detected in the mammalian body except surrounded by adipose tissue [33]. Miniaturized EFC placed in the left jugular vein of the Wistar rat using a catheter, was tested with glucose in rat blood under physiological medium. A photograph of the catheter implanted into the jugular vein of rat was given in Figure 2.3b. 0.125 V OCV and 95 μW cm⁻² power density were calculated at cell potential of 0.08 V from the EFC with 24 h operation [34].

Figure 2.3 (a) Photograph of the implantation of microbioelectrodes into the rat brain, (b) a photograph of the catheter implanted into the jugular vein of rat and an optical microscope image of the flexible carbon fiber microelectrodes ((a) is adapted from Ref. [23], with permission; Springer Nature, (b) is adapted from Ref. [34], with permission; Royal Society of Chemistry).

There have been few reports on implantable MFCs. A continuous flow single-chamber MFC without membrane was developed to supply power in human transverse colon. This device utilized intestinal contents and microbial community in the colon, and generated a power density of 11.73 mW/m² at an external load of 100 Ω. The MFC operated stably, but pH and ORP values dropped significantly. The authors stated that further works focused on the performance tests, microbial distrubition and the effect on human body should be investigated [35]. Another MFC was implanted in human large intestine and placed in transverse colon. The device consisted of two Plexiglas chambers, and each chamber had a rectangular dimensions of 10 × 25 × 10 cm. The anode and the cathode was made up with activated carbon fiber and carbon paper including Pt catalyst, respectively. The MFC generated electricity stably for 200 h after two months of operation, with a maximum power density of 73.3 mW m⁻² [36]. It is obvious that the reported MFC devices are large in size, the long-term effects on the human body and how electrical connection is provided are uncertain [18]. Luckily, EFCs can be miniaturized, while an MFC can not be miniaturized sufficiently for implantation. This is the major reason fort he development of implantable MFC technology to slow down.

      2.1.2 Wearable BFCs

      By considering the major drawbacks of implantable BFCs, in recent years there is a tendency for developing wearable devices which are miniaturized and integrated easily to human body without surgery. In addition, the difficulties in implantable EFCs applications have led researchers to wearable fuel cell technology and, consequently, to investigate physiological fluids as alternative to blood, for example urine, tears, sweat, saliva and transdermal fluid. These fluids are easily available and do not require blood draw or implantations for testing [37]. It can be thought as conventional batteries can be used for wearable electronics, however they are unsuitable for this application since they are rigid and toxic [38]. Although, EFCs are commonly used, MFCs have also been developed for wearable electronics. One of the drawback of an MFC is that cytotoxicity