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Page 2 of 27 Kim et al. Soft Sci 2024;4:24 https://dx.doi.org/10.20517/ss.2024.09

fields. These devices offer a new interaction paradigm and hold the potential to revolutionize healthcare/
[1]
treatment by continuously monitoring and analyzing human vital signs in real time . By directly attaching
the device onto the skin, it accurately measures diverse vital signs such as heart rate, blood pressure, blood
sugar, electromyography, and body temperature, enabling early diagnosis, prevention, and treatment of
diseases. It can also effectively manage chronic disease by continuously monitoring personal health status
and providing personalized healthcare solutions . For example, it can support appropriate insulin
[2,3]
administration by real-time monitoring blood sugar levels in diabetic patients and detect and treat
arrhythmias by analyzing electrocardiograms in patients with heart disease. These techniques have been
very useful in understanding diseases and the various mechanisms behind them. However, patients
suffering from chronic diseases and potentially fatal symptoms still face limited accessibility to medical
treatment and monitoring. Those with chronic diseases require real-time monitoring of bio-signals
throughout their lifetime, such as pulse, blood pressure, and glucose levels. Additionally, new diseases
continue to emerge over the years, increasing the number of patients. The complexity of patient diagnoses
increases the demand for medical personnel and infrastructure. These comprehensive demands on medical
services and personal healthcare for patients and potential patients have exponentially increased in the last
several decades .
[4]

In particular, the COVID-19 pandemic led to the rise of wearable sensors for individual consuming
applications for the long-term health monitoring platforms and early-stage response systems for medical
diagnostics . Recently, several research groups have made efforts to simultaneously monitor multiple bio-
[5]
signals with attachments to the human body for the accurate diagnosis of diseases, which are expressed by
various and complex symptoms . Therefore, it is important to develop wearable sensors with the selectivity
[6]
and high sensitivity required for each physical factor (e.g., mass, displacement, temperature, and voltage
difference) .
[7]
In terms of materials, organic materials have the crucial advantage of inexpensive/simple fabrication
[8]
processes, increasing the mass-production possibility of the realized sensor devices . Despite their complex
fabrication procedures and high-temperature requirements, inorganic sensing materials (e.g., compound
semiconductors, metal oxides, and nanomaterials) have acquired significant attention due to their higher
sensitivity, lower power consumption, and longer lifetimes compared to organic sensing materials [9,10] .
Therefore, studies have been actively conducted to implement sensors with improved performances by
solving the shortcomings of each material or complementing the existing limitations in a hybrid form [11,12] .

Numerous researchers have proposed innovative device structures, such as large-scale arrays, three-
dimensional (3D) stacking, and heterogeneous integration, to overcome the physical and structural
limitations of conventional devices . Thanks to the dedicated efforts of researchers and the progressions in
[13]
state-of-the-art wearable sensors, biomedical research has significantly advanced to achieve remarkable
accuracy in detecting various organ bio-signals, such as skin strain, ions in sweat, and
photoplethysmography (PPG) .
[14]

Electronic skin (e-skin) is defined as an artificial skin system with sensory capabilities, such as sense of
strain, pressure, and temperature, to target complete imitation of human skin sensation. For example, in
order to mimic the sense of touch, the e-skin needs to consist of highly sensitive flexible electronic sensors
with superior resolution for achieving realistic recognition. By integrating the sensors with a signal
transmitting module, the e-skin enables the transfer of its electrical signal to machinery, skin-attachable
wearable systems, and man-machine interaction devices.