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Page 2 of 15 Zhong et al. Soft Sci. 2025, 5, 3 https://dx.doi.org/10.20517/ss.2024.52

INTRODUCTION
Soft robots are characterized by their compliance, deformability, and adaptability, allowing them to adapt to
complex environments and interact safely with humans and delicate objects. Notably, these soft robots have
found diverse applications in delicate object interactions , bionic manipulation , assisting in human
[1,2]
[3]
[5]
[4]
rehabilitation and surgical procedures , etc. Correspondingly, soft sensors assume a vital role in
facilitating perception for these robots, contributing significantly to their ability to sense and respond to
their surroundings. Specifically, soft bending sensors play a key role in detecting attitude changes, providing
[6,7]
essential posture and position information . Achieving accurate self-shape perception and external
environment perception necessitates the seamless integration of soft robots and soft bending sensors ,
[8,9]
thereby ushering groundbreaking advancements that enable soft robots to adeptly execute precise and
delicate tasks [10-12] .

Soft bending sensors can be categorized into two distinct types: non-stretchable bending sensors and
stretchable bending sensors. The former is typically fabricated using inextensible substrate materials such as
[14]
polyimide (PI) and polyethylene terephthalate (PET) , which induce significant residual stress during
[13]
adaptation to bending surfaces. On the other hand, the latter employs deformable substrates such as soft
silicone [15,16] , leading to reduced residual stress. Both types of bending sensors utilize smart materials as their
sensing units, including liquid metal [17-19] , soft sensitive polymers [20-22] , and piezoelectric material [23-25] . These
smart materials respond to the strain on the sensor surface and generate variable electrical properties, such
as resistance, conductance, electric charge, and capacitance. Notably, when the same joint is bent, the larger
the angle, the greater the change in the electrical signal. However, it is essential to acknowledge that, at a
constant degree of bending, the joint with a large radius experiences more substantial deformation to the
sensor. This phenomenon necessitates repeated sensor calibration whenever it is mounted on different
[26]
joints before use , posing a common challenge for both commercially available and research-stage soft
bending sensors. Furthermore, a notable limitation of most soft bending sensors based on smart materials is
their inability to distinguish bending directions [27-29] . This arises from the generation of identical electronic
signal responses when the sensor is bent in positive and negative directions. Additionally, the smart
materials are sensitive to both pressures and bending, leading to unexpected pressure interference with the
bending measurement.

An optical waveguide is a structure that confines and directs light propagation along a predetermined path
using total internal reflection, consisting of a core with a higher refractive index than the surrounding
cladding or substrate. The soft optical waveguide exhibits remarkable characteristics, including high anti-
electromagnetic and excellent extensibility , making it well-suited for detecting bending signals and
[30]
[31]
facilitating widespread applications in soft robots and wearable devices [32-39] . In this research, we present a
dual-colored layer structured (DCLS) bending sensor based on the soft optical waveguide [Figure 1A,
middle]. Red and blue lights are regulated via the dual-colored layers (red and blue). By capitalizing on the
light attenuation effect of dual-colored layers, the red and blue output light intensity can be adaptively
regulated during the bending process. Consequently, the bending angle can be accurately determined
through a light intensity difference algorithm, even for joints of varying sizes. Compared to conventional
bending sensors, the DCLS sensor can be directly applied to detect joints of various sizes without requiring
calibration, making it, for example, highly suitable for measuring a diverse range of joints in the human
body [Figure 1A, left]. Moreover, the variations in the light intensity difference of the DCLS sensor show its
capability to measure bi-directional bending angles without recalibration [Figure 1A, right]. Finally, the
DCLS sensors are applied in the soft sorting robots for processing agricultural products [Figure 1B, left],
fish-inspired robots for exploring the underwater world [Figure 1B, middle], and rehabilitation exoskeleton
robots for helping the disabled [Figure 1B, right] to validate their performance and capabilities.