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D. George and M. J. Madou
PDMS sheets to make two-dimensional shapes was challenging too. Moreover, the
PDMS sheets had the same material property throughout their planform. Despite
these shortcomings, the capillary origami method gave a new facade to one of the
lithography limitations and opened a new door to three-dimensional fabrication. High
elastic modulus may pose a limitation to this approach when other materials are used.
A thinner sheet can be used to overcome the constraints posed by the high elastic
modulus values of the materials. For instance, a silicon nitride with a thickness of
100 nm can be bent by the surface tension of water. Another approach to bend a
high elastic modulus material is by softening it temporarily, followed by subjecting
the material to capillary bending. Polymer sheets, for example, can be softened by
heating them above its glass transition temperature.
Magnetic force is exploited for the reversible bending of the elastomers. The
fabrication of sheets that can be bent using an external magnet is achieved either using
photolithography or using direct writing techniques. The orientation of magnetic
particles determines the final folding shape of the sheet. Differently oriented magnetic
particles on different faces are initially achieved by orienting the particles inside a
curable silicone material using an external magnetic field followed by locking them
in place by curing the silicone. A precision of 100 µm is possible with photocurable
silicone materials, as shown by Xu et al. [161]. A sinusoidally arranged magnetic
particles inside the sheet induce uniform bending on the sheet in a magnetic field, as
shown in Fig. 14.
Releasable multilayered 2D precursors are buckled to form complex threedimensional shapes. These shapes are fabricated using SOI wafers and sacrificial
layers. A bonding location is designed, and multiple layers are transferred using
polyvinyl alcohol onto a stretched elastomer. When the strain of the elastomer is
released, the transferred structure buckles (Fig. 14). Numerous configurations may
be formed if the strain releasing sequence is controlled.
3.3.2 From Bending to Folding Using External Field Approach
Capillary-based folding of two-dimensional sheets made of rigid panels and flexible hinges can result in folded three-dimensional shape rather than a bent shape
(Fig. 15). The shape obtained represents the configuration corresponding to the
energy-optimized state, as explained earlier. (Therefore, the angle to which the sheet
folds can be estimated by minimizing both bending and surface energies by assuming
that the volume of the droplet placed on the structure remains constant during the
folding process [153]). In a negative photoresist, the exposure energy density in a
location determines the local degree of crosslinking. The higher the crosslinking
density, the stiffer the sheet becomes [36, 43, 44]. Therefore, an origami precursor
sheet with stiff faces and compliant folds can be made easily by controlling the exposure. Polymer softens when they are heated to a temperature close to or beyond its
glass transition temperature. The softening temperature of a polymer is a function of
the degree of crosslinking. As a result, when heated, the precursors soften more at
the fold region, leading to folding of the shape upon capillary actuation [43].
D. George and M. J. Madou
PDMS sheets to make two-dimensional shapes was challenging too. Moreover, the
PDMS sheets had the same material property throughout their planform. Despite
these shortcomings, the capillary origami method gave a new facade to one of the
lithography limitations and opened a new door to three-dimensional fabrication. High
elastic modulus may pose a limitation to this approach when other materials are used.
A thinner sheet can be used to overcome the constraints posed by the high elastic
modulus values of the materials. For instance, a silicon nitride with a thickness of
100 nm can be bent by the surface tension of water. Another approach to bend a
high elastic modulus material is by softening it temporarily, followed by subjecting
the material to capillary bending. Polymer sheets, for example, can be softened by
heating them above its glass transition temperature.
Magnetic force is exploited for the reversible bending of the elastomers. The
fabrication of sheets that can be bent using an external magnet is achieved either using
photolithography or using direct writing techniques. The orientation of magnetic
particles determines the final folding shape of the sheet. Differently oriented magnetic
particles on different faces are initially achieved by orienting the particles inside a
curable silicone material using an external magnetic field followed by locking them
in place by curing the silicone. A precision of 100 µm is possible with photocurable
silicone materials, as shown by Xu et al. [161]. A sinusoidally arranged magnetic
particles inside the sheet induce uniform bending on the sheet in a magnetic field, as
shown in Fig. 14.
Releasable multilayered 2D precursors are buckled to form complex threedimensional shapes. These shapes are fabricated using SOI wafers and sacrificial
layers. A bonding location is designed, and multiple layers are transferred using
polyvinyl alcohol onto a stretched elastomer. When the strain of the elastomer is
released, the transferred structure buckles (Fig. 14). Numerous configurations may
be formed if the strain releasing sequence is controlled.
3.3.2 From Bending to Folding Using External Field Approach
Capillary-based folding of two-dimensional sheets made of rigid panels and flexible hinges can result in folded three-dimensional shape rather than a bent shape
(Fig. 15). The shape obtained represents the configuration corresponding to the
energy-optimized state, as explained earlier. (Therefore, the angle to which the sheet
folds can be estimated by minimizing both bending and surface energies by assuming
that the volume of the droplet placed on the structure remains constant during the
folding process [153]). In a negative photoresist, the exposure energy density in a
location determines the local degree of crosslinking. The higher the crosslinking
density, the stiffer the sheet becomes [36, 43, 44]. Therefore, an origami precursor
sheet with stiff faces and compliant folds can be made easily by controlling the exposure. Polymer softens when they are heated to a temperature close to or beyond its
glass transition temperature. The softening temperature of a polymer is a function of
the degree of crosslinking. As a result, when heated, the precursors soften more at
the fold region, leading to folding of the shape upon capillary actuation [43].