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Tu et al. Soft Sci 2023;3:25 https://dx.doi.org/10.20517/ss.2023.15 Page 7 of 15

selectivity when sensing various sweat biomarkers. Through the integration of multiple sensors, e-skins can
thus afford the ability to interact with the environment and obtain vital health evaluation indices from
human bodies in the meantime.

While massive works have been reported to detect multimodal physical signals simultaneously from skins,
one of the challenges is the signal interference between sensing components. The electrical output signals of
the flexible electronic device may show motion artifacts due to deformations such as stretching,
compressing, and bending . In the meantime, some multimodal sensing systems include physical signals
[65]
and thus require decoupling methods to differentiate deformation modes. Self-decoupling materials and
novel structural designs are adopted to differentiate multicomplex signals for accurate and reliable
measurement.

Self-decoupling materials can intrinsically suppress signal interference through novel sensing mechanisms.
Ionic-based materials are suitable for self-decoupling sensing systems with frequency-dependent ion
relaxation dynamics [53,67] . For example, You et al. proposed a new artificial receptor that can differentiate
thermal and mechanical information without signal interference . The bulk resistance (R) and capacitance
[53]
−1
(C) show different behaviors under different frequencies. The charge relaxation frequency (τ ) does not
change with stretching [Figure 3A, ii]. Meanwhile, the normalization of capacitance at the measured
temperature can remove the effect of temperature. Thus, the systems can provide complete temperature and
force sensing through a self-decoupling ionic conductor. Further, the receptor provides real-time force
directions and strain profiles in various tactile motions. In addition, magnetic mechanisms can also be used
for force self-decoupling. The force directions can be differentiated by detecting the change of magnetic flux
densities. Yan et al. introduce a soft tactile sensor that possesses self-decoupling and super-resolution
capabilities by utilizing a sinusoidally magnetized flexible film . In detail, the embedded Hall sensor
[68]
located at the middle layer can sense deformation, whether it is from the normal or shear direction
[Figure 3B, ii]. The normal force and shear force can be decoupled by calculating two different parameters,
which are the magnetic field rotation angle and the translational movement of the magnetic field.
Subsequently, the sensor converts this deformation into electric signals through the use of a printed circuit
board (PCB). Different mechanisms of the same materials are also combined to differentiate multimodal
singles. Ferroelectric materials can be candidates for multimodal systems with their triboelectric and
[69]
pyroelectric effects. Shin et al. developed a self-powered multimodal sensor based on . Based on an
interlocked ferroelectric copolymer microstructure, this sensor enables simultaneous detection of
mechanical and thermal stimuli without a spacer in a single device, overcoming the drawbacks of
conventional sensors. The temperature and pressure are detected through the pyroelectric and triboelectric
mechanisms, respectively. The response and relaxation times of the triboelectric and pyroelectric effects are
different, as shown in the output signals [Figure 3C, ii]. Herein, this multimodal tactile sensor can
intrinsically decouple pressure and temperature information by analyzing the multiple signals based on the
response and relaxation times. The above-mentioned self-decoupling mechanisms can be integrated to
further develop the design of multimodal sensing systems. For example, Zhang et al. proposed a multilayer
structure that includes an ionic hydrogel film, a wrinkle-patterned polydimethylsiloxane (PDMS) film, and
a carbon nanotube (CNT)/PDMS electrode with self-decoupled pressure, strain, and temperature sensing
[70]
abilities . The temperature was decoupled through an ionic hydrogel with an aligned polymer chain
structure, which processed an ultrahigh temperature sensitivity in a wide range from 0 °C to 50 °C. In the
meantime, it shows surprisingly low strain sensitivity and intrinsic pressure-insensitive properties. The
mechanochromic core-shell magnetic nanoparticles with a photonic crystal structure were fast responsive to
external strain via interactive color switching. Further, a triboelectric structure comprising a wrinkle-
patterned PDMS friction layer with gradient modulus and a CNT-based elastic electrode detected voltage