Advancement
of sensing systems, soft robotics, and point-of-care
testing requires the development of highly efficient, scalable, and
cost-effective physical sensors with competitive and attractive features
such as high sensitivity, reliability, and preferably reversible sensing
behaviors. This study reports a highly sensitive and reliable piezoresistive
strain sensor fabricated by one-step carbonization of the MoS2-coated polyimide film to obtain MoS2-decorated
laser-induced graphene. The resulting three-dimensional porous graphene
nanoflakes decorated with MoS2 exhibit stable electrical
properties yielding a reliable output for longer strain/release cycles.
The sensor demonstrates high sensitivity (i.e., gauge factor, GF ≈1242),
is hysteresis-free (∼2.75%), and has a wide working range (up
to 37.5%), ultralow detection limit (0.025%), fast relaxation time
(∼0.17 s), and a highly stable and reproducible response over
multiple test cycles (>12 000) with excellent switching
response.
Owing to the outstanding performances of the sensor, it is possible
to successfully detect various subtle movements ranging from phonation,
eye-blinking, and wrist pulse to large human-motion-induced deformations.
Wearable lactate
sensors for sweat analysis are highly appealing
for both the sports and healthcare fields. Electrochemical biosensing
is the approach most widely used for lactate determination, and this
technology generally demonstrates a linear range of response far below
the expected lactate levels in sweat together with a high influence
of pH and temperature. In this work, we present a novel analytical
strategy based on the restriction of the lactate flux that reaches
the enzyme lactate oxidase, which is immobilized in the biosensor
core. This is accomplished by means of an outer plasticized polymeric
layer containing the quaternary salt tetradodecylammonium tetrakis(4-chlorophenyl)
borate (traditionally known as ETH500). Also, this layer prevents
the enzyme from being in direct contact with the sample, and hence,
any influence with the pH and temperature is dramatically reduced.
An expanded limit of detection in the millimolar range (from 1 to
50 mM) is demonstrated with this new biosensor, in addition to an
acceptable response time; appropriate repeatability, reproducibility,
and reversibility (variations lower than 5% for the sensitivity);
good resiliency; excellent selectivity; low drift; negligible influence
of the flow rate; and extraordinary correlation (Pearson coefficient
of 0.97) with a standardized method for lactate detection such as
ion chromatography (through analysis of 22 sweat samples collected
from 6 different subjects performing cycling or running). The developed
lactate biosensor is suitable for on-body sweat lactate monitoring
via a microfluidic epidermal patch additionally containing pH and
temperature sensors. This applicability was demonstrated in three
different body locations (forehead, thigh, and back) in a total of
five on-body tests while cycling, achieving appropriate performance
and validation. Moreover, the epidermal patch for lactate sensing
is convenient for the analysis of sweat stimulated by iontophoresis
in the subjects’ arm, which is of great potential toward healthcare
applications.
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