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Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09 Page 11 of 27

Despite novel demonstrations, PLEDs and OLEDs have faced challenges, including low efficiency, short
[55]
lifetimes, and instability in heat/humidity conditions . Micro LEDs (µLEDs) offer significant advantages,
such as high luminous efficacy, long lifespan, and robust endurance, under high temperatures and humid
conditions. Lee et al. demonstrated a wirelessly powered wearable μLED (WμLED) array with high stability,
[56]
enabled by transferring it onto a fabric coat [Figure 5D] . The 30 × 30 WμLED array was operated by
receiving radio frequency (RF) electrical power, transmitted from a power supply to the wearable antenna.
Figure 5E represents that the WμLED array had significant mechanical durability without device breakdown
in the fatigue test with 100,000 bending motions with a 2.5 mm bending radius. The forward voltage (V)
f
and irradiance (E) of the device increased by 0.76 V and decreased by only 1.33mW·mm , respectively. In
-2
addition, the WμLED showed extreme stability even when exposed to high temperature/humidity
(85 °C/85% relative humidity) detergent solutions and artificial sunlight. These results pave the road for
fully functional outdoor environments with stable optoelectrical component-based sensors. Regarding the μ
LED size, the WμLED had negligible degradation in electrical properties [Figure 5E].

For achieving wireless sensing systems, light-based communication, including light fidelity (Li-Fi), was
realized by flexible µLEDs with high bandwidth and high transmission rates. In Figure 5F, Hu et al.
provided gallium nitride (GaN)-based µLEDs enhanced in light efficiency by adding quantum dots (QDs)
and TiO nanoparticles . Figure 5G is the I-V curve of the flexible µLEDs with a bending radius of 3 mm,
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2
indicating a slight change in V regardless of the flatness of the device. The QD-coated µLED panel
f
demonstrated a high data transmission rate of 1.9 Gbps, which also fully operated on a flexible substrate.
According to these results, flexible µLEDs among diverse optoelectronic devices are suitable for various
wearable sensing applications due to their superior stability, high electrical performance, and application
expandability.

Human-device interfaces
Although numerous wearable systems with various optical or electrical devices have been developed, they
have all shown integration with bulky conventional chips onto a wearable substrate . In this case, the
[58]
device delamination from the attached body occurs from the body movement-induced mechanical stresses,
[59]
caused by the Young’s modulus difference between the bulk chip and the wearable substrate .
Furthermore, when conventional flexible devices are utilized for long-term wearable sensing applications,
they can lead to critical dermal problems such as skin irritation, itchiness, and inflammation due to the
[60]
accumulation of skin by-products (e.g., dead cells, sebum, and sweat) . In Figure 6, we introduce various
human-sensing device interfaces designed to minimize skin-related issues. Kim et al. developed a
multifunctional, chip-less, and wireless e-skin system based on a surface acoustic wave (SAW) sensor with a
freestanding single-crystalline piezoelectric thin film of GaN . Figure 6A compares the flexibility of the
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demonstrated SAW e-skin system with conventional chip-based ones. Since the SAW e-skin utilized the
nanoscale GaN thin-film with an ultrathin elastomer, the device showed excellent stretchability and
suitability for skin-adhesive e-skin applications with living organisms. Figure 6B presents wireless pulse
measurements using the SAW e-skin strain sensors on the human wrist, highlighting their high strain
sensitivity and extremely low minimum detectable strain of 0.048%. The wearable SAW sensor could
monitor not only skin-applied strain but also Na ions in perspired sweat by coating the device with ion-
+
selective membranes [Figure 6C]. The membrane-penetrated Na ions attached to the SAW sensor surface
+
changed the resonant frequency of the acoustic wave. Therefore, the SAW device compares the changed
frequency value, quantifying the Na concentration which is a biomarker for organ failures and diseases
+
related to psychological conditions [Figure 6D].