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Page 14 of 31 Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36

Another approach to controlling deformation in homogeneous elastomers is by spatially patterning the
crosslink density. Differences in crosslink density within the elastomer lead to changes in modulus, and by
patterning these variations, the deformation behavior of the film can be controlled even under the same
external tensile conditions [Figure 9B]. In elastomers, crosslinking occurs by supplying energy, such as heat
or light, to form crosslink sites and create a 3D polymer network, depending on the types of monomers and
crosslinkers. The crosslink density can be adjusted not only by the amount of external energy supplied but
also by the content of crosslinkers within the material [101-106] . Methods for spatially patterning crosslink
density include using patterned masks to selectively distribute thermal or light energy across the elastomer
film or directly irradiating with patterned energy sources [101-104] . Notably, photo-crosslinking enables precise
and complex pattern formation, allowing modulus patterning not only in-plane but also in the out-of-plane
[105]
direction by adjusting penetration conditions according to wavelength within the elastomer matrix . This
crosslink density patterning approach makes it possible not only to regulate the overall mechanical behavior
of a homogeneous elastomer substrate but also to enhance deformation resistance or flexibility in specific
regions. Nauman et al. produced a film from a thermally cross linkable elastomer material containing
photo-responsive color-changing dyes, patterning the crosslink density by generating spatial temperature
variations using a gradient grayscale patterned film and an infrared light source . When an external
[102]
actuation light source was applied to the fabricated film, variations in photochromic dynamics appeared
according to the crosslink density in different regions, and this technology was subsequently applied to
artificial iris devices for contact lenses [Figure 11A]. Park et al. achieved a remarkable enhancement in the
modulus of specific regions within a photo-cross linkable elastomer by up to 38,000% by selectively
irradiating the material with patterned light [Figure 11B]. This innovative selective patterning technique
[103]
allowed them to precisely create distinct soft and hard regions within the film, facilitating the development
of high-performance pressure sensors. These sensors are capable of maintaining exceptional sensitivity
while effectively reducing the adverse effects of deformation, thus offering robust performance across
various applications. Kang et al. developed a bio-based thermoplastic vulcanizate (TPV) by blending
polyester elastomer (BPE) with polylactide (PLA), followed by a dynamic crosslinking process involving
high temperatures and strong shear forces [Figure 11C]. They adjusted the ratio of BPE to PLA to control
[104]
the overall modulus of the TPV elastomer film. By optimizing the blend ratio and crosslinking process, they
achieved optimal tensile strength and elongation at break for the TPV under external tensile conditions.
This composite material exhibits high reprocessability and maintains its mechanical performance under
repeated deformation, providing stable deformation behavior over an extended period.

Structural assembly of heterogeneous materials
The structural assembly of heterogeneous materials involves integrating pre-designed structures into an
elastomer matrix to achieve desired deformations [Figure 12]. Unlike merely patterning homogeneous
materials, this approach leverages the deformation control effects of assembled structures to more
effectively manage both macroscopic and localized deformations, enabling in-plane and 3D deformation
modes that broaden potential applications [107-116] . Heterogeneous material structures for controlling substrate
deformation are manufactured through processes such as mold forming [109,111] and laser cutting [107,108] , with
recent widespread adoption of 3D printing-based additive manufacturing processes further enhancing
[116]
design flexibility by enabling the realization of complex structures.

The incorporation of 1D structures involves embedding linear elements such as fibers, rods, and ribbons
into elastomers, where these components are predominantly aligned in a single direction to enhance axial
deformation characteristics and control mechanical properties along that axis [109-112] [Figure 12A]. For
instance, integrating frame-like structures or fibers into an elastomer substrate allows these elements to
serve as mechanical supports that effectively control deformation when the substrate is stretched in a
specific direction. Nauman et al. employed elastomer materials with high modulus to create a 1D line