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

Table 1. Summary of typical materials used in the fabrication of flexible optical waveguides and their relevant properties
Material RI Optical loss Flexibility metrics Structural metrics Other properties Ref.
Hydrogel PEG 1.35-1.47 0.17-25 dB/cm Max elongation: 300%-2,000% Young’s modulus: 1-44 kPa Nontoxic [61-64]
PEGDA 1.33-1.48 / Elongation at break: 20%-80% Elastic modulus: 30 kPa - 85 MPa Tensile strength: 1.5-4.0 MPa (highly cross-linked); [65-67]
36 kPa -20 MPa (composite)
Nontoxic
PAM 1.46-1.50 1-11 dB/cm Max elongation: 13%-74% Young’s modulus: 20-27 MPa / [68]
Elastomer PDMS 1.41-1.47 0.5 dB/cm Max elongation: 95%-140% Young’s modulus: 0.57-3.7 MPa / [69-74]
Eco-flex 1.40 68.6%-78% Elongation at break: 900% Shear modulus: 1.3-35 kPa Tensile strength: 200 psi [75-77]
Shore hardness: 00-30
Solaris / 80% Max elongation: 290% Young’s modulus: 1.08 MPa Shore A hardness: 15 [75,78]
Shear modulus: 0.6-175 kPa Tensile strength: 180 psi
Biodegradable polymers PLA 1.46-1.47 1.5 dB/cm Max elongation: 3%-100% Young’s modulus: 2.7-7 GPa Biodegradation speed: 1 week to 4 months [79-81]
Nonelastic
PU 1.46 2 dB/cm Max elongation: 10% Young’s modulus: 0.3 MPa / [82]

RI: Refractive index; PEG: polyethylene glycol; PEGDA: poly (ethylene glycol) diacrylate; PAM: polyacrylamide; PDMS: polydimethylsiloxane; PLA: polylactide; PU: polyurethane.

cross-linking density, which can be adjusted to closely match the optical and mechanical properties of soft tissues for a wide range of biomedical
[86]
applications . In addition, as shown in Figure 2D , the pore size of hydrogels can be controlled by changing the relevant properties of the polymer
[85]
precursor. This feature makes hydrogels attractive materials for preparing functional biophotonic waveguides that provide drug delivery and controlled drug
release [87,88] . In this section, we will focus on three hydrogel materials, polyethylene glycol (PEG), poly (ethylene glycol) diacrylate (PEGDA), and
polyacrylamide (PAM), which are commonly used to prepare flexible optical waveguides for human health.

PEG
PEG hydrogels are usually prepared by cross-linking PEGDA in an aqueous solution and have antifouling properties or anti-protein-adsorption properties
similar to those of PEGDA, as well as excellent biocompatibility, optical transparency, no immunoreactivity, etc. . The mechanical and optical properties of
[85]
[89]
PEG hydrogels can be adjusted by adjusting their molecular weight and water content . In addition, owing to its excellent biocompatibility, optical
[91]
[90]
transparency, and lack of immunoreactivity, PEG has promising potential for applications in the fields of biosensing , tissue engineering , surface coating of
[92]
nanoparticles , and so on.
One of the major limitations of photoconductive structures in biomedical applications is that the effective light transmission distance is smaller than the organ
scale. The organ-scale distance in the human body is more than 10 cm, whereas the 1/e attenuation range of waveguides is no more than a few centimeters .
[93]
Choi et al. addressed this limitation by preparing a core-cladding hydrogel fiber with an overall step RI by the process depicted in Figure 2E(a) using 80%-90%