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Page 8 of 39 Yang et al. Soft Sci. 2025, 5, 46 https://dx.doi.org/10.20517/ss.2025.44

Elastomers
Elastomers are characterized by high flexibility and an elastic recovery ability, making them reliable
materials for the fabrication and modification of flexible optical waveguides. The source of these
outstanding capabilities is some of their structural features at the micromolecular level, such as polymer
chains and cross-linking points. The soft segments in the polymer chain produce large deformations, giving
the material a high degree of flexibility. A high proportion of soft segments in the molecular chain leads to a
lower Young’s modulus of elastomers. In contrast, cross-linking points act as interchain junctions,
restricting molecular movement and providing elastic recovery of the network structure. According to
different types of cross-linking, elastomers can be divided into thermoset elastomers (TSEs) and
thermoplastic elastomers (TPEs), which differ in the fabrication and application areas of different flexible
optical waveguides . Together, these materials significantly expand the design and application potential of
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flexible optical waveguides for human health assistance.

TSEs
The intermolecular structure of a TSE is formed by chemical cross-linking, forming a 3D network structure
with irreversible cross-linking bonds. Therefore, a TSE has a high degree of mechanical stability due to its
resistance to temperature and fatigue. Moreover, considering the different optical properties and
biocompatibilities, the TSEs most commonly used in flexible optical waveguides are polydimethylsiloxane
(PDMS) and the Eco-flex series.

Characterized by low cost, high elasticity, thermal stability, and high transmittance in the near-ultraviolet to
near-infrared regions, PDMS is an important material for preparing flexible optical waveguides . PDMS,
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with an RI of approximately 1.41 and a transmittance of nearly 95% in the visible spectrum, is widely used in
waveguides requiring high optical clarity. The Sylgard® 184 silicone is one of the most commonly utilized
commercial formulations of PDMS. The properties of PDMS, including its mechanical strength , optical
[108]
[109]
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clarity , and gas permeability , can be significantly influenced by varying the ratio of monomers to
curing agent. Compared to Sylgard® 184, Solaris has lower Shore A Hardness (15), indicating that it has
better elasticity and can provide sufficiently sensitive bending feedback to pressure changes. Additionally,
Solaris has a clear color, which will contribute to obtaining sufficiently strong signals; the fabrication
process of Solaris is presented in Figure 3A . Another commonly used type of TSE is Eco-flex, which
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shares many of the excellent properties of PDMS. Nevertheless, Eco-flex is softer and more malleable
because of the specially designed silicone groups in its chemistry. Owing to its excellent skin safety, Eco-flex
has become a popular choice for manufacturing prosthetics and artificial skin. Notably, after curing, Eco-
flex rubbers are translucent rather than optically transparent, which partially limits their use as waveguide
materials. Figure 3B shows that a liquid bladder sensor encapsulated with the Eco-flex material for human
wrist pulse wave monitoring demonstrates excellent flexibility . Figure 3C indicates that PDMS can be
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used as an overpack for flexible skin-friendly sensors .
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TSEs typically need to consist of blended and cross-linked two-component compositions. They are prone to
bubble formation during optical waveguide preparation and molding, which affects their optical properties.
In addition, optical waveguides prepared from these materials with commonly used preparation methods
are limited in length and are thicker .
[56]

TPEs
The intermolecular structure of TPEs is formed by physical cross-linking through phase separation,
resulting in a two-phase morphology that is reversible under thermal processing. This characteristic enables
TPEs to exhibit recyclability and reprocessability, making them suitable for sustainable manufacturing.