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Page 16 of 31 Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36

[Figure 13A]. By coating the substrate with metal nanoparticles and allowing the distance and arrangement
of these particles to vary with substrate tensile direction, the surface plasmonic effects produced a range of
visible light wavelengths. This enabled the substrate to be used in plasmonic optical sensors capable of
directional color tuning across a broad spectrum. Matsuda et al. conducted studies using a heterogeneous
silicone substrate composed of PDMS and Ecoflex, employing a molding process to regulate spatial
[111]
deformation distribution and overall mechanical deformation [Figure 13B]. They fabricated dot-shaped
PDMS pillars with high modulus and positioned them within the Ecoflex matrix to create rigid regions that
suppress excessive deformation in pressure-sensitive areas. Additionally, they integrated porous silicone for
pressure sensing and conductive silicone for strain sensing, enabling precise and independent
measurements. This device maintains stable performance even under strains up to 50%, providing an
effective solution for advanced sensing applications. Paik et al. developed a composite substrate capable of
programmed anisotropic deformation by embedding 1D Cr patterns within an elastomer matrix and
[112]
adjusting the width and spacing of these patterns [Figure 13C]. The Cr line patterns were pre-fabricated
on a silicon wafer using electron-beam lithography, followed by overcoating with a PDMS elastomer,
curing, and subsequent detachment as a single layer. This composite substrate functions as a variable
photomask, enabling diverse pattern light transmission depending on the designed deformation range, with
adjustable deformation range and resolution based on the arrangement and width of the metal lines.

The 2D structure assembles frame elements capable of 2D deformation within the elastomer matrix,
allowing for control of in-plane deformation through bi-directional or multi-directional expansion of the
structure [Figure 12B]. Various types of planar-expanding structures, such as re-entrant, chiral, and
Kagome grids, can be utilized in the form of frames, which are primarily fabricated using 3D printing
[113]
methods [Figure 14A]. Lee et al. developed a 2D re-entrant structure with a negative Poisson’s ratio
based on a honeycomb design and integrated it into an elastomer matrix . By leveraging the competitive
[114]
interaction between the expansion of the structure and the contraction of the matrix, they created a
composite stretchable substrate with a negative Poisson’s ratio, which was applied to a strain sensor, and
this application improved the gauge factor by approximately 3.2 times compared to sensors without such
structures [Figure 14B]. Additionally, Ha et al. conducted research on controlling the performance of
[115]
resistive strain sensors by embedding a hard film with various integrated 2D patterns into the matrix
[Figure 14C]. The strain sensor was designed to concentrate surface deformation in specific areas,
optimizing its performance to achieve both high sensitivity and stretchability while demonstrating long-
term reliability.

Recently, innovative approaches have been exploring the integration of 3D structures into elastomer
substrates to enable multi-dimensional deformation [117,118] . By employing intricate 3D designs, these methods
aim to address the limitations of traditional planar electronic devices and investigate the potential of next-
generation electronics with dynamic and adaptable 3D shapes. Such research goes beyond simple structural
enhancements, seeking to fundamentally transform the interaction between substrates and devices in
advanced applications, and drive significant technological advancements.

Rigid islands on elastomers for localized strain control
Previously, we concentrated on controlling the macroscopic deformation behavior of substrates by spatially
adjusting the modulus patterns of elastomers. To expand the range of device applications, managing
localized strain in areas where devices are positioned is essential; for this purpose, the rigid island pattern is
commonly used. A rigid island involves placing island-shaped patterns with a higher modulus than the
elastomer onto the substrate, thereby minimizing the deformation transmitted to the device under tensile
strain [119-121] . Thus, positioning rigid devices on a stretchable substrate, with independent local deformation
control, effectively mitigates damage or performance degradation in external deformation environments,