# Mini-Review: Recent Technologies of Electrode and System in the Enzymatic Biofuel Cell (EBFC)
## 4 System in EBFC  
Various EBFC systems have been designed for the suitability of EBFC applications. Making the EBFC system is essential for the application involved. For example, to detect blood glucose, the EBFC system developed should be easy to use and dispose of [77]. Most EBFC systems exhibit decreased power and limited lifetime because solvent evaporation in the hydrostatic electrolyte and biofuel depletion require paper exchange replacement and electrolyte refilling [77]. To reduce external components such as pumps, [PERSON] et al. [17] have produced self-pumping EBFCs, as shown in Figure 7. Most fuel cell systems try to minimize the use of external components because it increases the weight of the fuel cell. In addition, the EBFC system that uses a self-pumping technique is suitable for miniaturization applications, where it is ideal for use in portable, wearable, and implantable devices [23]. To produce the self-pumping EBFC system, the flow field produced is driven by the capillary effect, as shown in Figure 7, to supply the fuel, increase efficient mass transfer [75], and reduce fabrication cost and volume of microfluidic biofuel cell [78]. [PERSON] et al. [63] further investigate the factor of electrode microstructure, electrochemical property, and enzyme loading using a self-pumping EBFC that can retain 89.2% on the stability test for 240 h. [PERSON] et al. [51] also developed a self-pumping EBFC but using the Y-shaped flow structured cell. Meanwhile, [PERSON] et al. [73] created a Y-shaped flow structured cell using the printed circuit board (PCB)/double-sided pressure-sensitive adhesive (PSA) method to fabricate the cell. The authors also stated that this method is more accessible than using the thermal- or plasma-coating method and photolithography. This Y-shaped microchannel provides an advantage in reducing the depletion boundary layer and cross-diffusional mixing between fuel and oxidant [79]. Figure 7: The design of flow channel plates for (**a**) typical EBFC (with pump) and (**b**) self-pumping EBFC (without pump); (**c**) schematic diagram of top and bottom end plates in the self-pumping EBFC stack and (**d**) photograph of self-pumping EBFC assembly [17]. Copyright 2019 Elsevier. Several researchers have developed microfluidic EBFCs [73; 80; 81; 82] without using the proton exchange membrane (PEM). Although this system successfully eliminates the use of expensive PEM, problems such as poor mass transport, mixing between fuel and oxidant, high internal resistance, insufficient fuel utilization, and interactive interference still occur [83]. Poor mass transport occurs when reactant/glucose slowly reaches the enzyme for the catalytic to happen. The system of microfluidic EBFC with cross-diffusional mixing was developed by [PERSON] et al. [79]. This study examined the differences in flow rates and microchannel heights that lead to the optimal output of power density and current density at 153 \(\upmu\)W/cm\({}^{2}\) and 450 \(\upmu\)A/cm\({}^{2}\) at a flow rate of 25 mL/h and microchannel height of 450 \(\upmu\)m, respectively. [PERSON] et al. [83] introduced the nanocarrier in a membrane-less EBFC metal-organic framework [[Fe(CN)\({}_{6}\)]3\({}^{-}\) to reduce the internal resistance in EBFC and enhance the power output 700 times. The ability of the enzyme to have high stability and selectivity is essential in EBFC. Therefore, the need to analyze the product resulting in the reaction at the EBFC electrode is needed. [PERSON] et al. [84] produced membrane-less EBFCs and used nuclear magnetic resonance spectroscopy to determine the production of two main products: D-arabinose and formic acid. The authors found that the production of this product was influenced by the enzyme selection on the cathode side of EBFC. Needle-type EBFCs have also been developed to detect glucose by poking the needle on the skin. Two types of microneedles that have been used are polymeric needles and metallic hollow needles. [PERSON] et al.