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Page 20 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

Tseng et al. reported a wireless saliva sensor consisting of a silk film and a poly(N-isopropylacrylamide)
[109]
(PNIPAM) responsive hydrogel . This wearable saliva sensor was attached to tooth enamel, enabling the
determination of the kind of food being eaten by monitoring changes of the resonant frequency
[Figure 9H]. The PNIPAM hydrogel was responsive to a wide range of fluid properties, including alcohol
content, salinity, sugars, pH, and temperature. The fabricated sensor distinguished the materials by
comparing RF change under DI water, artificial saliva, 50% alcohol, methanol, and high salinity saliva
[Figure 9I]. Compared to DI water, artificial saliva revealed a lower amplitude (and slightly lower resonant
frequency) owing to its higher ionic strength, while alcohol caused an increase of the resonant frequency
due to a lower net permittivity. Four kinds of foods were eaten by the subject, showing different resonance
frequencies [Figure 9J]. The reported wearable saliva sensor opened the opportunity to provide immediate
treatment to patients who have ingested hazardous food via its real-time pH sensing characteristics.

Among the five senses, visual information is the most powerful and memorable due to its high storability of
[110]
~78% . Therefore, numerous researchers have paid attention to the visually vital sign info-communication
device with wearable patches. Hong et al. demonstrated a temperature sensor based on FET (TSFET) with
[111]
visual communication via leuco dye . The TSFET utilized a suspended gate using PNIPAM, which
changed the drain current as airgap decreased with increasing temperature, as shown in the upper right
image of Figure 9K. Leuco, with an easy coating process and low cost, displayed various colors according to
its structural change, resulting in the device sensitivity of 6.5%/°C in the temperature range between 25 and
45 °C. Furthermore, the thermochromic leuco dye showed a transmittance peak shift to the long wavelength
under due to increased temperature [Figure 9L]. This suggested temperature sensor was an innovative
approach demonstrating that the electrical signals can be replaced with visual signals.

Recently, strain sensors with high sensitivity, stability and robustness have been reported; detecting specific
movements, such as finger, wrist and fine vibrations of skin, remained challenging. Chun et al. presented a
skin-conformable wireless mechanical sensor to profile various hand and specific body movements using
advanced acousto-mechanic (ADAM) sensors . The ADAM sensor was placed on the dorsum of the hand
[112]
and captured acousto-mechanic signals from ambient noises. Figure 9M compares the obtained scratching
movements by moving the arm and fingers. The comparison was conducted with two modes of scratching:
one by the arm and the other by only the fingers. The blue line represented the time series of the raw data,
and the red line indicated the final binary classification results, where 0 and 1 correspond to non-scratch
and scratch, respectively. To apply the ADAM system to practical applications, an algorithm-based study
was performed, and the ADAM system showed an accuracy of 99.0% against visual observation. This work
highlighted the potential wide-ranging applications from assessing the efficacy of drugs for conditions
causing itchiness to monitoring disease severity and treatment response.

Biomedical therapeutic devices
In recent decades, many researchers have developed e-skin-based treatment methods to address the medical
inconveniences such as invasive treatments and in-hospital care. Innovative e-skin technologies using opto-
devices, electric fields (EFs), and microneedles have attracted attention due to their non-invasiveness,
convenience, and low side effects. Figure 10 explains biomedical treatment devices integrated into the e-skin
systems. Since alopecia is a common disease with a wide age range of onset, treatment methods have been
developed such as minoxidil (MNX), finasteride, and surgical hair transplants, but these approaches have
low efficiency and side effects [113-115] . Lee et al. reported a wearable photostimulator for hair growth
applications through flexible red vertical µLEDs (f-VLEDs) with high output power (~30 mW/mm ) and
2
low V bias (~2.8 V) . Figure 10A compares the hair growth rate of rat groups between f-VLED and MNX
[116]
f
treatment. After 20 days of treatment, the hair length of the photostimulated mouse was 183.2 μm,
approximately 76.8% longer than that of the MNX group (103.6 μm). Figure 10B compares stained tissues