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Page 6 of 39 Yang et al. Soft Sci. 2025, 5, 46 https://dx.doi.org/10.20517/ss.2025.44

Figure 2. Characterization of hydrogel and features of flexible optical waveguides prepared from it. (A) Schematic of the three-
dimensional structure of hydrogels; (B) Physical cross-linking of hydrogels including (a) hydrogen bonding interactions, (b) electrostatic
interactions, and (c) hydrophobic interactions; (C) Chemical cross-linking of hydrogels including (a) chemically reactive polymerization,
(b) free-radical polymerization, and (c) photopolymerization [56] . Copyright 2024, American Association for the Advancement of
Science; (D) SEM images of hydrogels with PEGDA and PEGDA/ALG as examples. (a) SEM images of PEGDA hydrogels. (b-d) SEM
images of PEGDA/ALG hydrogels with gradually increasing alginate concentration. It can be seen that the hydrogel pore size decreases
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gradually with the increase of alginate concentration . Copyright 2023, Elsevier; (E) Case of flexible optical waveguide prepared with
PEG. (a) Preparation process of PEG hydrogel. The UV photo cross-linking was first performed through a mold, followed by the passage
of dichloromethane to remove the core, and finally, the alginate shell was generated by dip-coating on the surface of the core. (b)
Physical drawing of the prepared 1 m-long optical fiber. (c) Light conduction of an optical waveguide sandwiched between two pieces of
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thin pig tissue . Copyright 2015, Wiley; (F) Chemical formula of PEGDA; (G) Uniform luminescence after fiber-coupled blue light
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based on PAM . Copyright 2018, Wiley. SEM: Scanning electron microscope; PEGDA: poly (ethylene glycol) diacrylate; ALG:
alginates; PEG: polyethylene glycol; UV: ultraviolet; PAM: polyacrylamide.
w/v PEG material as the core and a 1%-2% w/v alginate hydrogel as the cladding [see Figure 2E(b) for the
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actual picture] . As shown in Figure 2E(c), the optical loss of this fiber in pork tissue is about 0.42 dB/cm
(λ = 402 nm) with a 1/e attenuation distance of over 10 cm. At the same time, they exploited the properties of