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Zhao et al. Soft Sci 2024;4:18 https://dx.doi.org/10.20517/ss.2024.04 Page 17 of 32

The enzyme-electrode interface is crucial for sweat-based BFCs. Electronic mediators enhance the ability of
LOx to oxidize lactate by aiding electron transfer. Choice of mediators influences anode potential,
determining the onset potential of oxidation. 1-methoxy-5-methylphenazinium methyl sulfate (1-methoxy
[121]
PMS) , tetrathiafulvalene (TTF) [120,122-124] , naphthoquinone [125-130] , and ferrocene derivatives [131-133] are
commonly used as the electronic mediators. Yu et al. developed an electronic skin powered by sweat lactate
BFCs that utilizes the TTF as the redox mediator, demonstrating an open-circuit potential (OCP) of
approximately 0.6 V and achieving the highest power output of around 3.5 mW·cm in 40 mM lactate
-2
solutions. The device exhibited exceptional stability, maintaining its performance over a continuous 60-
hour operation . Yang et al. constructed a lactate BFC using a 1,4-naphthoquinone/MWCNT-modified
[124]
-2
bio-anode, which can deliver a power density of 62.2 ± 2.4 μW·cm under bending/torsion conditions
[Figure 8B] . The leaching of enzymes influences the output performance and stability of BFCs. Adding
[130]
biopolymers (such as chitosan) and polymers (such as Nafion) to the anode proves to be highly effective in
safeguarding the enzymes and preventing leaching [122,127,129] . The cathode facilitates the completion of the
biofuel reaction through the oxygen reduction reaction (ORR). Platinum (Pt) and related alloys are
commonly employed as catalysts to enhance the ORR in sweat [35,134] . Biocatalysts such as bilirubin oxidase
(BOD) are also excellent options for facilitating the ORR [135,136] . The performance of the BOD reaction can be
enhanced by molecules such as protoporphyrin IX, which can selectively orient the enzyme towards the T1
center of BOD, facilitating direct electron transfer with CNTs .
[127]
In addition to lactate-based sweat BFCs, glucose and ethanol are two promising metabolites found in sweat
that can be utilized as fuel sources. The working mechanisms are given in the following equations.

As for glucose, the anode and cathode reactions are respectively denoted as [Figure 8C] [137]

Yin et al. developed a cotton textile-based BFC using bioanode and biocathode fibers for glucose oxidation
and oxygen reduction. The cell achieved power densities of 48 μW·cm at 0.24 V and 216 μW·cm at 0.36 V
-2
-2
with varying glucose concentrations. The flexible design maintained performance even when deformed, and
[138]
a series connection of four cells illuminated an light-emitting diode (LED) at 1.9 V [Figure 8D] .
As for ethanol, the reactions are written as [Figure 8E] [139]

Sun et al. created a flexible BFC that captured energy from individuals after alcohol consumption, using
ethanol as a biofuel. The BFC incorporated 3D coralloid nitrogen-doped hierarchical micromesoporous
carbons aerogels (3D-NHCAs), alcohol oxidase (AOx), and terephthalaldehyde (TPA) on one screen-
printed electrode (SPE) anode, and 3D-NHCAs, BOD, and TPA on another SPE electrode as biocathode.
The extensive surface area of the electrode enhanced electrocatalytic performance, resulting in an output of
1.01 μW·cm during exercise [Figure 8E] . These advancements expand the range of biofuels derived from
-2
[139]