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Lee et al. Soft Sci 2024;4:38 https://dx.doi.org/10.20517/ss.2024.36 Page 11 of 31

MODULUS-PATTERNED ELASTOMERIC SUBSTRATES
In the previous section, we explored the introduction of various structures into plastic substrates to impart
stretchability and examined the effects of different structural design approaches on deformation. These
structured plastic films possess mechanical and optical properties suitable for display device applications,
yet a challenge remains in balancing the substrate’s stretchability with the effective area needed for device
[77]
formation . Additionally, achieving stretchability often requires adding compensatory structures
perpendicular to the surface and expanding void spaces, which may compromise reliability in applications
requiring complex shape deformation, thereby limiting their practical application range. Recent
advancements in low-temperature and printing processes have enabled the formation of devices at relatively
low temperatures, leading to increased exploration of using inherently stretchable elastomer materials as
substrates [78-81] . Elastomers can undergo tensile deformation without the need for compensatory structures,
as the chains within the matrix extend, and the elastomer matrix, which forms a random network, allows for
tensile deformation in all directions . Researchers have recently been exploring strain engineering
[79]
techniques that spatially pattern the modulus of elastomers to address unwanted deformations in traditional
elastomer substrates (e.g., unnecessary contraction due to the Poisson’s effect) or to achieve multi-
directional dynamic tensile deformation instead of unidirectional static stretching [82,83] . This section
examines methods for controlling the deformation behavior of elastomer substrates by spatially patterning
the modulus, including the regulation of molecular alignment and crosslink density in homogeneous
elastomers or the incorporation of heterogeneous structures.

Direct-patterning on homogeneous elastomer substrates
To control the dynamic behavior of homogeneous elastomer films, one effective method is to pattern the
modulus of the film by spatially reconfiguring the polymer chain network. This approach involves aligning
the molecules that constitute the elastomer network in specific directions or selectively controlling the
crosslink density of the network at different locations [84,85] [Figure 9]. Key methods for controlling molecular
alignment in elastomer networks include introducing spatially oriented rubbing patterns on the mother
substrate surface where the film is fabricated, guiding molecules - especially those with anisotropic
structures - to align in the desired direction . Additionally, techniques such as applying mechanical
[86]
stretching to the substrate to rearrange molecules through shear forces [87,88] and using directional
electromagnetic fields to control molecular orientation can also be utilized . By physically aligning
[89]
polymer chains in a specific direction, differences in modulus arise between the aligned and non-aligned
directions, and designing and patterning this alignment allows for various deformations to be induced even
under the same external tensile input [Figure 9A]. Among various elastomer materials, these molecular
alignment techniques are commonly used in block copolymer (BCP) elastomers [90-93] with chemical
incompatibility between molecules or liquid crystal elastomers (LCEs) [94-100] that have anisotropic rigid
domains.

BCPs are polymer chains made up of two or more monomers with unique properties, and due to the
chemical incompatibility between these monomers, phase separation occurs in response to external stimuli,
resulting in alignment . This alignment generally leads to modulus anisotropy along specific axes and is
[90]
primarily achieved through mechanical or chemical methods. Ye et al. investigated the effects of
unidirectional alignment of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) thin films on their
macroscopic deformation and mechanical properties . By utilizing the cold zone annealing-soft shear
[91]
(CZA-SS) technique, they successfully aligned PDMS domains within the PS matrix [Figure 10A]. This
alignment resulted in a significant increase of up to 31% in the elastic modulus along the alignment
direction, causing notable deformation anisotropy in the film’s mechanical behavior. Park et al. employed a
solvent annealing method using a chamber with solvent and a heater to induce the self-assembly of BCPs,
[92]
leading to the successful reorganization and alignment of domains within the film [Figure 10B]. This