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Page 14 of 20 Lu et al. Soft Sci 2024;4:36 https://dx.doi.org/10.20517/ss.2024.29

various areas, including diagnostics, personalized medicine, treatment strategies, and rehabilitation
techniques. Here, examples include voice code recognition based on ferroelectric nanogenerator (FENG)
[29]
[41]
microphone [Figure 4A] , dermatologic malignancy evaluation using CMS system [Figure 4B] , arterial
[20]
stiffness measurement via a conformal ultrasonic device [Figure 4C] , tumor volume regression sensing by
flexible autonomous sensors (FAST) [Figure 4D] , and physiological monitoring for hospital neonatal care
[76]
by a wireless limb unit [Figure 4E] .
[77]
The application of wearable and portable devices in our daily lives necessitates continuous innovation in
[78]
human-computer interaction technology . These advancements aim to make devices more intuitive,
responsive, and seamlessly integrated into everyday routines. However, the push for such innovation
inevitably brings challenges, especially concerning personal privacy and information security. As shown in
Figure 4A, given the flexibility and wearability of FENG, the FENG device can be installed in a wearable
device or personal computer. The high sensitivity of the FENG-based microphone allows speech
recognition, which provides multiple layers of security for the authentication system . When an
[41]
unauthorized user makes an access request, the generated voltage reflects the time-domain acoustic
information. FENG devices analyze the voiceprint information and accurately reveal the user’s identity,
preventing unauthorized users from accessing the owner’s personal computer, even in the case of an
unauthorized user obtaining the correct speech code. Moreover, these advantages can contribute to the
development of new-style loudspeakers and the improvement of active noise cancellation technology in
various domains, making audio systems more efficient, compact, and capable of delivering high-quality
sound .
[70]
Stiffness variations in tissues can be indicative of underlying health conditions, such as abnormalities or
[13]
disease . Therefore, mapping these stiffness differences can provide valuable insights for medical
diagnostics and monitoring. Here, by utilizing reversible soft contact, the CMS [Figure 4A and B] consisting
of ultrathin, stretchable networks of mechanical actuators and sensors can assess the stiffness of different
regions on the skin. Figure 4B presents the image of a piezoelectric device mounted on the forearm for
mapping the lesion regions of dermatologic malignancy and the spatial mapping of modulus . In vivo
[29]
modulus measurement results of the forearm indicate that the modulus in the pathological region is higher
than that in the healthy region. Mapping the mechanical properties of the skin can provide valuable
information for the diagnosis and monitoring of early skin cancer.

Monitoring the central BP (CBP) waveform representing the pressure exerted by the blood on the walls of
the central arteries provides valuable insights into the hemodynamic status of an individual and can help in
the early detection of cardiovascular abnormalities . Figure 4C on the left shows a conformal ultrasound
[79]
device attached to the central vessel pulsation in deep tissues, including the CA (subcutaneous about
25 mm) and internal jugular veins (int. JV) and external jugular veins (ext. JV) for high-precision
[20]
measurement of the behavior of the central vessel pulsation . The CA is a major blood vessel located in the
neck, responding by supplying oxygenated blood to the head and neck region, including the brain. The int.
JV drains deoxygenated blood from the brain, face, and neck and carries it back to the right atrium of the
heart. Another example is the simultaneous measurement of ECG and BP of the brachial artery (BA), as
shown on the right in Figure 4C. PWV is used to assess arterial stiffness with a value of 5.4 m·s , consistent
-1
with commercial devices. The conformal and stretchable ultrasound device can be used to collect a series of
essential characteristics of central blood vessels, such as their structure, blood flow dynamics, and any
abnormalities or pathologies present.