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Yang et al. Soft Sci. 2025, 5, 46 https://dx.doi.org/10.20517/ss.2025.44 Page 7 of 39

hydrogels by infiltrating functional molecules into the hydrogels to produce fluorescence, amplified
spontaneous emission, whispering gallery mode (WGM) lasers, and photothermal devices, giving PEG
hydrogel optical fibers rich functionality.

PEGDA
PEGDA is a PEG derivative containing acrylate groups, with two acrylate groups in the main chain of PEG,
as shown in Figure 2F. These acrylate groups can be rapidly cured into a hydrogel by a photoinitiated free
radical polymerization reaction to form a cross-linked network when irradiated with ultraviolet (UV) or
[95]
visible light . This structure can enhance the optical transparency, toughness, and stress-cracking
resistance of the material . In addition, factors such as the amount of photoinitiator, the amount of
[96]
monomer, and the light intensity significantly affect the polymerization rate and gelation behavior of
hydrogels [97-99] . Specifically, the polymerization rate of the hydrogel polymerization precursors linearly
increases with the square root of the photoinitiator dosage, monomer dosage, and light intensity, whereas
the gel transition time exponentially decreases with these factors. The tunable mechanical and optical
properties and flexible chemical modification possibilities make PEGDA ideal for use as an optical
[100]
waveguide material .

In addition, the light-curing characteristics of PEGDA allow precise control of the curing process of the
material, which is particularly important for the fabrication of optical waveguide materials, as it ensures that
the precise geometries and optical properties of the optical waveguide are suitable for precise transmission
of optical signals in vivo . This precise control is achieved through a photolithography process in which a
[101]
photosensitive material is used to form the desired optical waveguide pattern after exposure to UV light,
thereby reducing the optical loss and improving the transmission efficiency of the optical waveguide.

PAM
PAM, a hydrogel material commonly used in the preparation of optical waveguides, is a linear, water-
[102]
soluble polymer with many amide groups in its molecular chain . This structure endows PAM with
excellent hydrophilicity and biocompatibility, enabling it to exhibit nontoxicity and good histocompatibility
in biomedical applications [85,103] . Compared with PEG and PEGDA, PAM has greater mechanical strength
and a longer service life in biomedical applications. This is mainly attributed to its linear polymer structure,
which endows PAM with good film-forming properties and mechanical strength, enabling it to exhibit
excellent light transmission properties among optical waveguide materials. This also makes PAM promising
for tissue engineering applications. For example, nanohydroxyapatite (nHAP) particles can be introduced
into a PAM hydrogel system to prepare nHAP/PAM nanocomposite hydrogels, which can be used to make
scaffolds for bone tissue repair .
[104]
More possibilities for the application of PAM in optical waveguide technology are enabled through
chemical modification, polymerization with other materials, and the introduction of a cladding-core
[68]
structure . For example, through one-step polymerization and cross-linking of alginate and PAM
precursors, alginate–PAM hydrogel optical fibers with a low modulus [the Young’s modulus of the fiber
slowly increases from 48 to 90 kPa with increasing acrylamide (AAm) concentration] and high stretchability
(120%-140% of the original length under the condition of little change in the electrical conductivity) can be
prepared . The feasibility of the use of these fibers for chronic optogenetic brain conditioning was
[105]
[105]
demonstrated in free-moving animals, as shown in Figure 2G .