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Page 22 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

SID was decomposed via electrochemical reaction; and (4) the bubbles increased the internal pressure,
inducing the SID drug release [Figure 10C]. Figure 10D exhibits the drug releasing results by utilizing the
implanted SID in pig skin tissue. The EEG and epileptic seizure signals were measured by the algorithm,
and the model drug (Evans blue) was successfully released through the measured signals.

Since secondary infection from existing wounds can lead to various problems such as high fever, edema,
and convulsion, several ointment-type drugs have been developed. However, these drugs must be applied
multiple times according to the severity of the wound. To solve this issue, a wound treatment patch based
[118]
on EF stimulation has been developed by Wang et al. . A flexible wound healing electrical patch (ePatch)
applying the EF to the wound region was realized using conductive hydrogel electrodes [Figure 10E]. As
shown in Figure 10F, the wound healing was assisted by cell migration and proliferation, induced by the EF.
Figure 10G presents the comparative images of the fibroblast alignment with and without the EF. When the
EF was not applied, the fibroblast migration and alignment were not observed. In contrast, when the EF was
applied, both migration and alignment were evident.

In addition to treating diseases, phototherapeutic devices have recently gained popularity as skincare tools.
However, most optical skincare devices for home-care use have been made with bulky LEDs, providing
non-uniform light irradiation and beauty care effects. Lee et al. accomplished a surface-lighting micro-LED
[119]
(SµLED)-based wearable patch for uniform melanogenesis inhibition [Figure 10H] . The SµLEDs with
630 nm of wavelength were adequate to prevent melanogenesis on the skin and were monolithically
5
fabricated on a plastic substrate. Moreover, the SµLED exhibited outstanding mechanical stability under 10
bending cycles at a 5 mm bending radius and endured a harsh environment within 100 days under 85 °C/
85% relative humidity. According to the melanogenesis inhibition tests, the SµLED-based patch efficiently
reduced the melanin cells without any skin photodamage [Figure 10 (i) and (j)]. These results confirmed
that wearable patches with multifunctionality by integrating various electrical components could non-
invasively treat various diseases and be applied in the beauty care field.

CONCLUSION AND OUTLOOK
This review summarized the latest and most sophisticated stages of e-skins and flexible sensing/treating
technology for personalized wearable biomedical healthcare. Various materials, including conductive
polymers, nanomaterials, and inorganic semiconductors, have been utilized to enhance the electrical and
mechanical properties of the e-skin. Flexible e-skin components with strain-pressure sensors, TFTs,
optoelectrical sensors, and device substrates allowed the integration of multifunctional e-skin systems for
monitoring human health vitals such as heart rate, ion concentration, IOP, and body motions. Independent
and wireless electrical systems could be achieved by adopting TENGs that replace the external power
supply, adding biocompatibility using biodegradable materials. Furthermore, wearable therapeutic
apparatus provided precise treatment with alopecia, seizures, skin wounds, and melanogenesis inhibition.
Despite advancements in e-skin technology, some challenges remain as follows: The reported sensors are
unreliable in harsh conditions of high temperature, humidity, and heavy mechanical impact. While most
studies have concentrated on enhancing the sensing capabilities of devices, their reliability in these
conditions has yet to be confirmed. Furthermore, since the developed devices are directly attached onto the
organ, the durability of the e-skin system has to be guaranteed under periodic and high strain conditions
during human movement. To achieve multifunctionality, although several sensor devices need to be
integrated into a single system, the high-density integration of devices generates excessive heat, leading to
organ damage, device degeneration, and fluctuations in sensitivity. Because signal noise is considered as a
critical and inevitable issue in every sensor system, the packaging technique with electromagnetic shielding
has been actively investigated to minimize inaccurate signal analyses during long-term usage. Despite these