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Page 12 of 20 Lu et al. Soft Sci 2024;4:36 https://dx.doi.org/10.20517/ss.2024.29

techniques with various measurement scales, from superficial surfaces to tissues and to organs. For example,
optical illumination, mechanical vibration detectors, and strain gauges can be selected when measuring
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dermis, fat, and muscle tissues at the depths of 10 -10 , while piezoelectric actuators are suitable for
superficial surfaces such as stratum corneum and epidermis. By utilizing appropriate measurement
modalities and devices, the mechanical properties of biological tissues can be accurately characterized,
leading to better understanding and potentially improved medical diagnosis and treatment.

The conformal modulus sensor (CMS) is a type of sensor used to measure the elastic modulus or stiffness of
[20]
the targeted tissue surface because it can conform or adapt to various surfaces or objects being tested . The
sensor typically consists of active elements, such as strain gauges or piezoelectric materials, combined with
the metal serpentine configurations providing electrical connectivity, resulting in a stretchable mechanical
structure with a low modulus when held by a thin elastomer . A representative example is shown in
[29]
Figure 3B and C. The device provides a convenient and non-invasive method for coupling to the surface of
the skin or other biological tissues using van der Waals forces, allowing for repeated use without sacrificing
measurement accuracy [Figure 3B]. The conformal sensing attached to the artificial skin surface comprises
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seven actuators with sizes of 200 × 1,000 µm and six sensors with sizes of 100 × 500 µm (piezoelectric film,
500 nm thick) placed between two layers of serpentine configurations of metal traces (bottom: Ti/Pt, 20/300
nm; top: Cr/Au, 10/200 nm), with an encapsulation layer of PI [Figure 3C]. This conformal and
piezoelectric device makes it possible to examine the viscoelasticity of biological tissues near the surface of
the subject’s epidermis and to map the mechanical features of the skin or other organs in space, which has a
broad application prospect in therapeutic diagnosis and therapy of diseases [29,69] . Analyzing the data obtained
from CMS applied to ex vivo skin samples provides valuable insights into the efficacy and mechanisms of
action of moisturizing agents, as well as the relationship between hydration and skin properties. The
directional and spatial mapping suggests that these devices can not only assess stiffness values but also
provide information about the distribution of stiffness across different regions. This mapping capability
allows for a detailed understanding of the mechanical properties of the skin in various areas, which can be
important for the diagnosis and monitoring of early skin cancer .
[29]
The development and application of miniaturized electromagnetic (MEM) devices with vibratory actuators
and soft strain-sensing sheets represent a promising advancement in the field of deep tissue
characterization. They offer non-invasive, dynamic measurements of Young’s modulus in skin and other
soft tissues at depths ranging from approximately 1 to 8 mm, providing valuable insights for medical
diagnostics, research, and treatment optimization. The electromechanical modulus (EMM) sensor described
in Figure 3D is designed to capture the mechanical behavior of underlying soft tissue by applying periodic
forces and measuring the resulting displacements . It consists of two main components: a vibratory
[73]
actuator and a strain-gauge sheet. The vibratory actuator located on the top layer (top coil, 200 μm) is
responsible for generating a periodic Lorenz force with a fixed frequency applied to the tissue. The strain-
gauge sheet in the form of serpentine metal traces (modulus sensor, ~2 µm) is used to measure the
amplitude of displacement caused by applied forces. The total thickness of the device is ~2.5 mm and the
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contacting area is ~2 cm . Near-surface muscle tissue activity affects the modulus change, which can be
measured with this EMM sensor [Figure 3E]. When the device is installed on the forearm, the forearm
muscles contract to generate force to lift the dumbbell. The modulus values obtained through continuous
recording of Vs changes over time suggest that the muscle tissue exhibits varying degrees of stiffness or
elasticity during repeated iterations of movements. The modulus values of 205 and 320 kPa correspond to a
relaxed state and a tensed state of muscle activity, respectively. Furthermore, the use of device arrays
provides a powerful tool for obtaining detailed spatial and depth-wise information about the moduli of
biological tissues. This information is invaluable for monitoring and diagnosing a range of health
conditions.