Abstract
Enzymatic biofuel cells (EBFCs) are lightweight and eco-friendly power sources that employ enzymes as catalysts and sugar as fuel for redox reactions at ambient temperatures. Due to their lightness and biocompatibility, EBFCs recently attract a great deal of attention and have been applied to a wide range of medical, implantable, and wearable devices [1-5]. We previously developed an iontophoresis skin patch with a built-in EBFC for enhancing drug delivery through skin by ionic current generated by the EBFC. The electrodes of the EBFC in the skin patch were made by multi-step fabrication. Carbon fabric was immersed in the dispersion of carbon nanotubes (CNTs) to make electrodes, followed by immobilization of enzymes onto them. Fructose dehydrogenase (FDH) was immobilized on an anode, and bilirubin oxidase (BOD) on a cathode to form a complete EBFC. In this study, we developed ink of CNTs and enzyme that can be printed onto nonwoven fabric that works as a flexible substrate. This material simplifies the fabrication process of CNT/enzyme electrodes, and enables patterning of the electrode with an arbitrary shape.
To prepare the CNT/enzyme ink, CNT and additives of different roles were first mixed in aqueous solution and dispersed by sonication: CNTs as a highly-conductive scaffold for enzyme immobilization, polyvinylimidazole (PVI) as a binder, trehalose as a stabilizer of enzymes during freeze-drying, and 1-pyrenebutyric acid N-hydroxysuccinimide ester (PBSE) for non-covalent immobilization of enzymes to CNTs. Next, FDH as an enzyme catalyst for an anode was added to the mixture above to complete the CNT/enzyme ink. The CNT/enzyme ink was cast on nonwoven fabric in 1 cm2 size to form an anode. Finally, the cast electrode was freeze-dried for 6 hours. After freeze-drying, the fabricated FDH anode had highly porous structures as observed by scanning electron microscopy, and it could be preserved for longer time after freeze-drying because of trehalose.
The performance of the printed FDH anode and the effects of the additives were evaluated. The FDH anode was immersed in a stirred buffer solution containing 200 mM fructose and measured by typical cyclic voltammetry (CV) at 10 mV/s. The FDH anode generated current density of 2 mA cm-2 with fructose as fuel. Though the printed ink was originally aqueous dispersion, the printed shape was kept intact after rehydration, while without PVI the printed electrode readily dissolved in buffer solution. CV of the anode in the presence and absence of PBSE indicated that PBSE was essential for high power density.
[1] Enzymatic Fuel Cells from Fundamental to Applications (Eds: H. R. L uckarift, P. Atanassov, G. R. Johnson), Wiley, New Jersey, 2014
[2] W. Gellett, et al., Electroanalysis., 22, 727, (2010)
[3] G. Valdés-Ramírez, et al., Electrochem. Commun., 47, 58, (2014)
[4] S. Cosnier, et al., Electrochem. Commun., 38, 19, (2014)
[5] Y. Ogawa, et al., Adv. Healthc. Mater., 4, 506, (2015)
Figure 1