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Page 2 of 22 Wu et al. Soft Sci 2024;4:29 https://dx.doi.org/10.20517/ss.2024.21

significant role in IoT, providing bendable and conformable display solutions to create versatile and
[1,2]
adaptive interfaces that seamlessly integrate with interconnected devices . So far, these technologies have
[3]
been widely studied and can be divided into emissive and reflective (non-emissive) displays . The flexible
[5]
[4]
emissive displays, including liquid crystal (LCDs) , light-emitting diode (LED) , organic LED (OLED) ,
[6]
and quantum-dot LED (QLED) displays , have been extensively developed even with some commercial
[7]
products as they have advantages of high resolution and vibrant colors. However, these displays strongly
rely on internal backlight sources, suffer from limited lifespan issues and environmental concerns, and are
susceptible to image retention with high voltage input and power consumption. In this case, the reflective
(non-emissive) flexible displays, including electrophoretic (EPDs) , electrowetting (EWDs) , photonic
[8,9]
[10]
[11]
crystal (PCDs) , and electrochromic displays (ECDs) , utilize the ambient light with lower voltage input
[12]
and energy consumption would be a more advanced choice. They have shown superior eye-friendly
properties, high suitability for outdoor use, and lower energy consumption . Among them, the flexible
[13]
ECDs provide unique advantages compared to other reflective flexible displays. EWDs are known for fast
response and good readability but face challenges with dielectric breakdown and high power consumption.
ECDs are more energy-efficient than EWDs, which benefit from their inherent bistable nature allowing
them to maintain the display content without continuous power input . Compared to EPDs, ECDs also
[14]
offer faster response speed and better durability . PCDs provide high reflectivity and vibrant color but
[15]
struggle with complex and costly preparation processes, while ECDs can be fabricated with facile and cost-
effective methods. These advantages make ECDs stand out in functionalities and receive extensive attention,
and successfully expand their applicability into wearable electronics, innovative packaging, flexible signage,
and e-readers [16,17] . Generally, the flexible ECD is constructed by the flexible electrode, active layers
(electrochromic layer and ion storage layer), and ionic conducting electrolyte with a vertical or lateral
structure . The underlying mechanism of the ECD is based on the redox reaction of electrochromic (EC)
[18]
materials driven by external electricity input. Then, ECDs can present corresponding optical changes
(absorbance/transmittance/reflectance) to display content and convey information . To have a
[15]
systematical understanding of the flexible and stretchable ECDs, we have summarized the strategies for
constructing these displays based on different device layers. Materials selection or modification and device
structure design or distribution have been categorically introduced in this part. Besides, we have elaborated
on recent advances in these flexible and stretchable ECDs from the perspectives of designing electrode
patterns, active layer patterns, electrolyte patterns, and ECD pixels. Moreover, the versatile visual displays
on the interactive system consisting of ECD pixel and sensor modules have also been summarized
according to various signal types.

STRATEGIES FOR FABRICATING FLEXIBLE AND STRETCHABLE ELECTROCHROMIC
DISPLAYS
When constructing flexible and stretchable ECDs, it is important to select the materials and consider the
design and fabrication strategies for three aspects: the electrode, active layer, and electrolyte.

Flexible and stretchable electrode
The strategies aimed at developing flexible and stretchable electrodes for ECDs predominantly centered on
the materials selection and integration of conductive layer and substrate to achieve desirable chemical,
thermal, mechanical, and electrical characteristics . The prevalent choice for flexible electrodes is the
[17]
commercially available indium tin oxide (ITO) deposited on polyethylene terephthalate (PET) or
polyethylene naphthalate (PEN) substrates [19-22] . However, the ITO exhibits a brittleness trait, constraining
its chemical and mechanical performances . In this case, researchers have explored various alternative
[23]
conductive materials onto conventional flexible PET substrates as flexible electrodes in ECDs, including
poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [24,25] , silver nanowires (AgNWs) [26,27] ,