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Page 6 of 38 Zhu et al. Soft Sci 2024;4:17 https://dx.doi.org/10.20517/ss.2024.05

Baek et al. demonstrated a maskless method for fabricating flexible pressure sensors through high-precision
3D printing and thin-film coatings of conductive polymers, which can be used to create very precise
[51]
microstructures . The team demonstrated diverse pressure sensors with complex multi-scale dome/spike +
step-like structures, showing extremely high process controllability [Figure 2D]. The prefabricated pressure
sensors presented in this work exhibit very sensitive responsiveness (up to 185 kPa ) and ultra-fast response
-1
times (≈ 36 μs), which could be utilized in smart mechanical devices.

(2) Capacitive pressure sensors operate by converting external pressure into a capacitive signal. The
capacitance of a medium is known to be defined as:

where ε is the dielectric constant, A is the area of the device, and d is the distance between two conductive
plates, and thus the pressure-induced changes in d and A can be used to measure the applied pressure [34,52] .
Piezo-capacitive pressure sensors typically have the advantage of high sensitivity and no inherent
temperature sensitivity compared to resistive pressure sensors [53-55] .

It should be noted that using microstructural design to improve device performance is not exclusive to
piezoresistive conductive composite pressure sensors [56-58] . Tee et al. prepared a pyramid-structured
capacitive pressure sensor using PDMS material and investigated the compressibility of the PDMS
microstructures and the effect of spatial arrangement on the mechanical sensitivity of the microstructured
film . It was used as a parallel plate capacitor structure in pressure sensors. The sensors showed potential
[59]
for applications such as blood pulse monitoring and force-sensing touch panels.

Niu et al. present a facile technique based on a simple melt infiltration process and a commercial cone-
shaped nanoporous anodic aluminum oxide (AAO) transfer method for large-scale and low-cost
preparation of interlocking asymmetric nanocones on poly(vinylidenefluoride-co-trifluoroethylene)
[P(VDF-TrFE)] films [Figure 3A] [60-62] . The developed capacitive tactile sensors have excellent
[60]
performance, including a sensitivity of 6.583 kPa in the low-pressure region (0-100 Pa), a detection limit as
-1
low as ≈ 3 Pa, response/recovery times as low as 48/36 ms, and excellent stability and reproducibility (10,000
cycles).

(3) Triboelectric pressure sensors utilize the triboelectric nanogenerator (TENG) triboelectric effect and the
principle of electrostatic induction to detect pressure changes. In normal operation, this sensor usually
contains two or more material layers inside; when subjected to pressure, these layers contact each other and
move in the mechanical stress of spontaneous charge separation to generate electrostatic charge. The
external pressure stimulus can produce different characteristics of the electrical signal, the contact area, the
degree of contact tightness and relative sliding distance, and other factors will affect the characteristics of
the generated charge; therefore, by detecting the electrical signal changes, triboelectric pressure sensors can
indirectly measure the size of the applied pressure on the sensor. These sensors are self-powered and are
suitable for the development of self-powered e-skins [63-66] .

It has been reported that designing microstructures on the dielectric surface to provide a larger contact area
and more active sites to transfer electrostatic charges during contact electrification can enhance the
triboelectric effect. Yao et al. developed a self-powered TENG e-skin sensor mimicking a conical array of
interlocking microstructures on the surface of a plant [Figure 3B], an interlocking microstructure that