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Page 16 of 44 Jung et al. Soft Sci 2024;4:15 https://dx.doi.org/10.20517/ss.2024.02

notable resilience to substantial mechanical deformations, including stretching and twisting [Figure 4D]. In
the glucose and lactate biofuel cells, employing a setup featuring a sole enzyme and absence of a membrane,
-2
remarkable maximum power densities of 160 and 250 μW·cm were observed, alongside open-circuit
potential of 0.44 and 0.46 V, respectively. The textile-based biofuel cells maintained their structural integrity
even after enduring repeated severe mechanical deformations, illustrating a consistent and stable power
output through 100 cycles of 100% stretching. This self-powered biosensing system exhibited minimal
background signals and sustained operational stability for 50 min under physiological conditions,
highlighting its potential for the self-driving logic development in Biocomputing systems and simplified
design of mechanically compliant intelligent wearable electronics.

Wearable sweat sensors have shown promise for on-site measurements, offering potential for preventive
healthcare and prompt diagnosis. However, comprehensive studies are crucial to establish diagnostic
effectiveness and constraints. Overcoming the challenge of extensive population studies requires high-
throughput fabrication of uniform, dependable sweat sensors with minimal preparatory requirements,
enabling simultaneous regional measurements in multisubject and longitudinal trials.

Nyein et al. introduced a high-throughput wearable microfluidic patch for sweat analysis during exercise
and iontophoretic sweat conditions . The patch, using advanced printing and cutting techniques, allows
[214]
uniform production and reveals correlations between sweat secretion rate, sodium levels, and hydration
status. In iontophoretic sweat, individual-specific correlations between sodium and potassium levels are
identified. The study explores the connection between iontophoretic sweat glucose and blood glucose for
healthy and diabetic subjects, emphasizing personalized correlations over universal thresholds for diabetes
diagnostics [Figure 4E]. Continuous monitoring of glucose dynamics in the iontophoretic sweat of both
healthy individuals and those with diabetes reveals varied patterns. In trials involving healthy subjects, the
sweat glucose levels decline from 80 to 72 μM in the initial trial, corresponding to a drop in blood glucose
from 85 to 79 mg/dL. However, in the subsequent trial, despite a higher average sweat glucose level, it
increases from 112 to 122 μM, while blood glucose decreases by 16 mg/dL. Notably, the decrease in sweat
rate occurs earlier in the first trial. Conversely, in the case of the diabetic subject, the sweat rate remains
consistent, and the sweat glucose levels mirror the trend observed in blood glucose, rising in the first trial
and declining in the second.

The research findings suggest that the lack of a consistent association between sweat and blood glucose
levels among different individuals presents a hurdle in establishing standardized sweat thresholds for the
diabetes diagnosis or treatment. Unraveling the potential for personalized connections between sweat and
blood glucose may require extensive longitudinal investigations into the dynamics of sweat glucose.
Additionally, future studies should delve into sweat glucose variation patterns among subjects, intra-
individual regional disparities, and the influence of exercise and iontophoresis-induced sweat rate and
stimulation duration on glucose levels in sweat.

Tear
Tears offer a promising avenue for non-invasive glucose monitoring due to their easy accessibility,
continuous flow, and correlation with blood glucose levels. This has increased interest in developing
wearable tear glucose sensors that utilize optical and electrochemical detection methods [215-219] .
Electrochemical detection, in particular, stands out for its rapid response, high sensitivity, and accuracy.
Additionally, considering the delicate nature of the eye, these wearable tear-based sensors must prioritize
safety and user comfort by being nontoxic, flexible, and miniaturized. Based on these requirements, several
trials were conducted to develop electrochemical biosensors based on tear analysis. As shown in Figure 5A,