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

Electronic skin and healthcare applications
Organ-attached e-skin systems are strategically applied to monitor significant bio-signals for personal
healthcare [Figure 1]. These systems collect bio-signals from the equipped sensors and then convert them
into personal medical data. This data is wirelessly transmitted to various medical services including real-
time health monitoring, internet network-based telemedicine for fast disease diagnosis, and emergency
medical alerts for acute diseases [21,22] . For example, when the cardiovascular patients are in an acute heart
attack, the following order is carried out in a series: (i) the e-skin systems, which have been continuously
monitoring the heart rate, transmit the recorded information thus far to a nearby medical institution; (ii)
Medical team checks the biosignals of heart rate, saturation of partial pressure oxygen (SpO ) level and
2
blood pressure and (iii) sends the medical alert to caregivers and emergency services. This process is not
only limited to cardiovascular patients but is also critical to other patients requiring real-time vital sign
monitoring.

Figure 2 provides a concise overview of the essential factors for realizing multifunctional and flexible
sensing devices, including e-skin components, integrated sensing devices, bio-signal sensing applications,
and their medical applications. Since the components of skin-attached e-skins should be soft enough to
withstand mechanical stress from the physical movement of the human body, the fabrication processes for
flexible and stretchable devices have been actively developed. To apply the device to the e-skin as a
component, flexible photodetectors (PDs) have been realized with polymer composite materials, organic
materials, and p-n junction semiconductors . Micro-scale light-emitting diodes (µLEDs) have been
[23]
[24]
manufactured to earn flexibility for integration with wearable substrates . Nevertheless, these miniaturized
unit devices can only detect a single signal in a limited small surface region with low sensing reliability. To
overcome this issue, the integrated circuit array has great merit in multifunctional devices with large-scale
[25]
sensing, efficient power consumption, and reduced manufacturing costs .
For reliable bio-signal sensing, although commercial ‘smart watches’ have been commercialized with
sensing features (e.g., PPG, stress score, and nighttime sleep patterns), the intrinsic gap between these
devices and human skin can result in inaccurate vital levels or limited accessibility [26,27] . Skin-adhesive
devices, which can be conformally attached onto the human skin surface, have recently risen in popularity
for providing more precise bio-signal sensing data, as they can send more health vital signals than
[28]
conventional devices . In particular, the sensing accuracy of e-skin has been enhanced due to
advancements in materials engineering and fabrication processes, enabling the application of e-skin systems
with several sensors and actuators in the medical field. For example, these systems regulate therapeutic
actuation in real-time such as administering medication, providing heat transfer treatments, delivering
electrical stimulation, and drug delivery systems (DDSs), all accomplished by signal information from
sensors adhered to the skin [29-31] . Recently, the closed-loop systems with the microneedle-based chemical
sensors and drug delivery devices have been designed for accurate drug dose control with immediate
feedback. Microneedle arrays enabled the detection of the ion concentration in the interstitial fluid and the
control of the DDS for the glucose level monitoring and sustenance of diabetic patients [32,33] . Although these
systems were originally designed for diabetes management, they have been applied to overcome several
other illness models, such as seizures, stroke, and aneurysms, in recent years.

UNIT SENSING COMPONENTS
Piezoelectric and capacitance-based mechanical sensors
Strain sensors are considered as a crucial requisite for skin-attachable devices because the skin, the largest