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Page 16 of 22 Wu et al. Soft Sci 2024;4:29 https://dx.doi.org/10.20517/ss.2024.21

pixels fabricated an eight-digital flexible ECD, where the electrochromic layer, electrolyte, and electrode
were in the same pixel arrangement. Each flexible ECD subpixel’s positive and negative electrodes were
connected with copper foil so that they were controlled individually showing different numbers
[84]
[Figure 10H] . Besides, a novel multicolor fiber-based ECD has been fabricated by independently woven
1,1’-Diethyl-4,4’-bipyridilium dibromide (EtVio)-, 1-heptyl-4-(4-pyridyl)pyridinium bromide (HVio)- and
1,1’-bis(p-cyanophenyl)-4,4’-bipyridilium dichloride (p-CVio)-based electrochromic fibers into nylon yarns
forming patterns. This display as a smart wearable application has shown its promising potential for
[47]
adaptive camouflage in response to changes in the surrounding environment . In conclusion, the above
design strategies of device structures provide an adequate research foundation for high-performance flexible
and stretchable ECDs. Due to their easier processibility, flexible and stretchable pixelated ECDs are
primarily developed with point-to-point modes. This approach arises from significant limitations related to
pixel size and number. To improve the current situation, the processing difficulty of active/passive matrix-
based ECDs with flexible substrates should be addressed from three aspects: (1) The flexible interconnect
should be firmly prepared to accommodate the bending/stretching of the substrate without losing electrical
conductivity; (2) A thinner patterned electrolyte with precise alignment of the patterned flexible electrode is
expected to address the signal crosswalk issue; (3) Develop reliable, high-performance transistors on a
flexible substrate to integrate well with electrochromic pixels. Addressing these limitations requires ongoing
research and development in materials science, device engineering, and fabrication technologies to enhance
the performance, durability, and scalability of flexible and stretchable ECDs.

Featured by the point-to-point driving mode, these flexible ECDs, which exhibit dynamic information, are
promising to be integrated with various sensors to visualize corresponding signals through color changes of
+
[28]
resultant electrochromic pixels. These signals, including the temperature , strain [28,51] , Na and Vitamin C
concentration [49,65] , pressure , and energy storage , have been demonstrated in current flexible ECDs. For
[120]
[27]
instance, the flexible ECD pixels function as energy storage devices integrated with the temperature and
strain sensors, which power the sensors and detect the bio-signals of skin temperature and wrist bending
[28]
angle with the help of a microcontroller [Figure 11A] . Besides, Yin et al. have successfully integrated
various chemical sensors with individual flexible electrochromic pixels, such as the PH sensor, Na and
+
[65]
Vitamin C sensors [Figure 11B] . The corresponding working voltage of the chemical sensors obtained by
different chemical substance concentrations was converted into a logic output. After programming by the
microcontroller, the obtained voltage value of the electrochromic pixelated display is associated with the
display content, visualizing the corresponding chemical signals. Moreover, the pressure of the pen tip can be
detected using the novel vertically integrated device with the PANI-based pixelated ECD module on top and
PVDF-based patterned piezo electrical pressure sensor module on bottom. Specifically, the piezoelectric
signal collected by the PVDF-based pressure sensor was amplified by an amplifier circuit and converted into
a direct current (DC) signal by a bridge rectifier, which was then applied to the pressed PANI pixels for
[120]
electrochemical reaction and showed corresponding color change [Figure 11C] . Similarly, the resistance
change upon stretching in the strain sensor can trigger the “on/off” of the stretchable ECD pixel, showing
[51]
the corresponding display response [Figure 11D] . Moreover, a voltage level can be directly visualized
when the battery is connected with the multicolor electrochromic pixels showing the corresponding color of
relevant voltage [Figure 11E] . The above interactive visual displays have greatly provided new avenues in
[27]
future flexible IoT systems and intelligent robot applications.

CONCLUSIONS AND PROSPECTS
In this review, the latest research on flexible and stretchable ECDs has been summarized. We have provided
an overview of the strategies for fabricating high-performance flexible and stretchable ECDs from three
aspects, including flexible and stretchable electrodes, active layers, and electrolytes. For these strategies, the