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Page 2 of 20 Lu et al. Soft Sci 2024;4:36 https://dx.doi.org/10.20517/ss.2024.29

are evident in the monitoring of deep tissue signals and changes in tissue characteristics. The results suggest that
wearable biomechanical sensing systems hold substantial promise for applications in healthcare and research.

Keywords: Wearable devices, passive electrodes, active recording, deep-tissue sensing

INTRODUCTION
Employment of digital health and wearable technologies potentially revolutionizes diagnostic and
[1-4]
therapeutic approaches, offering robust support to the healthcare ecosystem . Of particular interest with
epidermal electronics are their mechanical properties (geometries, flexibility/stretchability and unique form
factors), allowing conformal contact with skin and living tissues, thereby quantifying the biological tissue
mechanics in a higher signal-to-noise ratio . Unique emphasis on the measurement of elastic moduli
[5-9]
forms the foundation for healthcare appraisals to assess physiological health status regarding intraocular
[13]
[12]
pressure [10,11] , dermatological pathologies , and cardiovascular status . Furthermore, wearable devices
designed for monitoring physiological health continuously demonstrate accuracy and reliability equivalent
to high-end wired systems utilized in intensive care units, while maintaining cost-effectiveness, adaptability
and portability across various environments, including hospitals, work environments, and home
environments.

Conventional methods to determine tissue moduli consider the non-linear stress-strain relationship based
on the applied force (such as torsion, compression, suction, and indentation) and the dynamic nature of the
tissue [4,14-16] . Normal tissues have varying hardness when assessed on the same strain and temporal scales.
The brain is incredibly soft, having an elastic modulus of around 100 Pa; the liver, while also very soft, is
slightly harder at 400-600 Pa. Medical practice dictates that tissue hardness changes in response to illness.
Several investigations show that fibrotic lungs harden during fibrosis, with elastic modulus values ranging
[17]
from around 2 kPa in normal tissue to about 17 kPa in fibrotic tissue . We discovered that the shear
modulus of normal liver in vitro is less than 1 kPa, whereas the shear modulus of fibrotic liver ranges from 3
to 22 kPa [2,17] . The emphasis on measuring the elastic modulus of tissues aids in the evaluation of various
pathological and physiological conditions [2,17] . Typically, these methods involve the utilization of external
equipment in an invasive manner; however, when applied in vivo, substantial variations play a crucial role
[18]
in influencing the outcomes under different experimental conditions . Simultaneously, the emergence of
novel testing methods, such as magnetic resonance elastography (MRE) and ultrasound elastography ,
[19]
[18]
has come to fruition. Ultrasound elastography, which combines both backscattered and through-
transmission data, shows a comprehensive method to estimate soft tissue elasticity associated with the bulk
modulus as well as shear and Young’s moduli to reach deeper tissues (approximately 10 cm depth) [18,20] . The
most recently used dynamic elasticity imaging techniques exploit MRE for non-invasively mapping the
[21]
viscoelastic properties of soft biological tissues in the extensive diagnostic applications of magnetic
resonance imaging (MRI). MRE offers parameter distributions with high spatial resolution ranging from
[21]
millimeters to sub-millimeters to picture various mechanical functions and structural characteristics of
pathological changes in tissues . However, these high-priced instruments demand careful configuration
[19]
and specialized hospital/lab professionals, thus impeding their swift, direct application for household
diagnostic monitoring or continuous tracking.

Recently developed wearable system categories include ultra-thin, flexible actuator/sensor arrays, which are
becoming increasingly interesting as conceptually different types of methods for measuring soft tissue
biomechanical signals with unique form factors [22-26] . Key points primarily involve precise measurements at
the microscale and in wearable formats, including dimensions of micro-devices, mechanical formats, and
adhesion strength to biological surfaces . It explores possibilities to enhance spatial and temporal
[27]