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Zhao et al. Soft Sci 2024;4:18 https://dx.doi.org/10.20517/ss.2024.04 Page 15 of 32

Apt2. With secure skin adherence, this flexible sensor enables monitoring of psychoactive drug intake
[108]
during sweating, combining precise aptamers and electrochemistry for fast and accurate identification .
Caffeine, a widely consumed drug found in coffee, tea, and various commercial products, can result in
health issues such as coronary syndromes, high blood pressure, and depression when taken excessively. In
official athletic competitions, it is considered an ergogenic drug, and its presence in urine is tested before
tournaments. The triple-electrode array [Figure 7G] utilizes a carbon electrode modified with carbon
nanotubes (CNTs)/Nafion films as the working electrode, along with a carbon counter electrode and an Ag/
AgCl reference electrode. Caffeine detection via differential pulse voltammetry (DPV) results in a distinct
peak around 1.4 V, providing a quantitative measure of caffeine concentration in sweat. This device
effectively monitors sweat caffeine levels, correlating with dosage and revealing physiological patterns
[18]
associated with caffeine intake . A wearable sweatband is also being proposed as an effective solution for
monitoring nicotine levels in sweat, allowing for a quantitative assessment of an individual’s exposure to
smoking. The working electrode of the sweatband is constructed using gold nanodendrites and is
functionalized with cytochrome P450 2B6 (CYP2B6), an enzyme that can oxidize nicotine. This oxidation
process converts sweat nicotine into a nicotine iminium ion, resulting in a measurable current. By analyzing
the amperometric responses of the sensor, it becomes possible to quantify nicotine concentrations in sweat.
[97]
-1
The presented nicotine sensors demonstrate a sensitivity of 4.3 nA·mM and a linear range of 0-30 μM .
Carbamazepine (CBZ) is a commonly prescribed neurological therapeutic drug used to treat a range of
conditions, including psychomotor seizures, trigeminal neuralgia, clonic seizures, partial seizures, and other
related disorders. Veeralingam et al. have developed a cost-effective and flexible amperometric sensing
platform for detecting its levels in human sweat. This innovative platform employs NiSe nanoclusters as the
2
working electrode. By utilizing their large surface area, these nanoclusters exhibit an impressive sensitivity
of 65.65 μA·mM , covering a wide concentration range from 50 nM to 10 μM . In conclusion, sweat
[109]
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analysis has become increasingly important for drug detection and monitoring, and the promising results
from recent studies emphasize its potential for application in point-of-care medical diagnostics.

SWEAT-BASED ENERGY HARVESTERS
Advancements in flexible electronics enable detailed physiological data collection. Traditional batteries pose
challenges with their bulkiness, inflexibility, and hazardous components. Recently, significant progress has
been made in powering flexible electronics with innovative methods. One promising approach is utilizing
near field communication (NFC) for efficient energy transfer in a sweat sensing system, enabling the
wireless power transmission from an external source to the device and eliminating the need for physical
connections or bulky batteries [115-117] . Another exciting avenue is leveraging energy from the sweat itself to
power flexible electronics. Sweat contains various chemical components, including electrolytes and
metabolites, that can be harnessed to generate electrical energy [28,29,41] . What is more, it is a naturally
[118]
occurring bodily fluid constantly produced by our sweat glands, readily available and easily obtained .
Additionally, utilizing sweat as an energy source is biocompatible and environmentally friendly [66,113,119] . By
converting the chemical energy present in sweat into electricity, these devices enable self-sustainability and
reduce the dependence on traditional power sources. Here, we summarize the recent developments in BFCs,
SABs, and SACs, which can be seamlessly integrated with sweat sampling and sensing modules, enhancing
functionality while reducing risks associated with conventional batteries.

Sweat-based biofuel cells
Sweat-based BFCs are unique electrochemical devices that utilize biofluids as both electrolytes and fuel
sources. They utilize bioelectrocatalytic reactions to generate electricity by harnessing the energy from
redox-active metabolites found in sweat. Lactate, abundant in sweat at concentrations of 2-30 mM, is
particularly important in energy generation . Typically, these BFCs use LOx as the anode, while the
[26]
[120]
cathode incorporates oxygen reduction catalysts [Figure 8A] . The anode and cathode reactions can be