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Page 4 of 39 Yang et al. Soft Sci. 2025, 5, 46 https://dx.doi.org/10.20517/ss.2025.44

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Figure 1. System diagram of flexible optical waveguides for human health assistance . Copyright 2020, Elsevier; Copyright 2023,
Wiley; Copyright 2020, Wiley; Copyright 2024, American Chemical Society; Copyright 2020, American Chemical Society; Copyright
2022, American Chemical Society; Copyright 2023, Opto-Electronic Journals Group; Copyright 2024, American Association for the
Advancement of Science.

While traditional rigid materials are unable to adapt to human tissue deformation due to their hard and
brittle properties, flexible materials can achieve a Young’s modulus that matches that of soft tissue due to
their molecular connectivity properties. This mechanical adaptability effectively solves the stress shielding
problem of implantable devices and avoids the occurrence of inflammatory reactions. Among the flexible
materials, those that can be used for photoconductive and biological applications still need to possess
several necessary properties . First, it should exhibit a high transparency to minimize energy loss and
[60]
improve the propagation efficiency. Second, the refractive index (RI) should satisfy the requirements of the
total internal reflection (TIR) mechanism. Third, the mechanical properties of the materials, such as high
tensile strength, high breaking stress, and high Young’s modulus, are crucial for achieving flexible fiber
sensor preparation. In addition, implantable flexible optical waveguide materials must also be nontoxic and
highly biocompatible to protect the host tissues. Some representative flexible optical waveguide preparation
materials and related properties are summarized in Table 1. In this section, some of the properties of these
materials, including hydrogels, elastomers, and biodegradable polymers, are described and discussed in
detail.

Hydrogels
Hydrogels are promising materials for biomedical applications because of their biocompatibility and ability
to incorporate functional groups for sensing. As a cross-linked network of hydrophilic polymers, a hydrogel
[83]
contains a large amount of water and exhibits excellent biocompatibility . Its 3-dimensional (3D) polymer
network [Figure 2A] is connected by physical entanglement or chemical bonding [Figure 2B and C ], and
[56]
each of the chains is not fixed but can be relatively freely rotated and bent, which makes a hydrogel highly
flexible as an optical waveguide and able to undergo elastic deformation within a certain range . The
[84]
optical and mechanical properties of hydrogels are affected by their polymer content, molecular weight, and