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D. George and M. J. Madou
dinks contract considerably upon heating because of strain release, and as a result,
folding of the sheet happens. Instead of colored inks on the hinges, a graphene ink
may be deposited to bend Shrinky dink material to actuate the folding with microwave
[26]. When exposed to microwave, since graphene is not transparent to microwave,
the sheet is locally heated. Like the previous case, this local heating results in bending.
However, the orientation of the fold with respect to the wave affects the folding of
the material, limiting the versatility of the technique. In nanoscale, a focused ion
beam (FIB) is used for the fabrication of folded shapes. FIB, when bombarded
with the nanofilm, induces tensile or compressive stress on the film depending on
the operating parameters. Gallium ion from FIB when colliding with the film can
remove atoms from the gold film. The vacancies formed via this bombardment lead
to grain coalescence and as a result, residual tensile stress on the top layer of the
film, as illustrated in Fig. 12. This leads to the bending of the thin film [133].
Heating-based folding can be induced in an LCE by locally heating the sheet
beyond its transition temperature with IR irradiation (Fig. 11). The side that is heated
experiences a shrinkage due to the molecular rearrangement, leading to the bending
of the LCE sheet. While folding in LCE is achieved through IR illumination of
an isotropic material, an SMP-based folding requires local preprogramming of an
SMP. A direct writing technique may be employed to fabricate folds with SMP and
the faces with an inactive material. By training this SMP-made fold region, folded
origami can be obtained (Fig. 11).
3.3 External Field Approach
Here we discuss a new approach where bending of a sheet is carried out without
changing its properties across the cross-section, contrasting previously discussed
cases where the material properties of the sheets are tuned to induce bending. Various
forces, including capillary, magnetic, and external compressive forces, can be the
cause of bending in an external field approach. Since the external field approach
involves different forces, it is essential to discuss them briefly before delving into
the fabrication aspects of it.
Capillary effects scale favorably for the micron-scale actuation. A comparison of
the body forces (ρgl
3 ) with the surface tension forces (γ l), where l is the length scale,
γ is the surface tension, ρ is the density, and g is the gravitational constant, reveals
that the surface tension effects dominate in a smaller length scale. Therefore, surface
tension can act as an actuation mechanism for micro-origami fabrications. Nature
takes advantage of this fact, as observed in the case of the drinking mechanisms
of both phalaropes and hummingbirds. For a phalarope, this heightened effect of
surface tension at small scales is a way to accomplish, a gravity-defying intake of
prey-laden water toward its mouth. A Hummingbird’s tongue exploits the surface
tension to close lamellae on its tongue to entrap honey at each of its dips into honeyfilled flowers. Here, the tongue closes because of the interaction between the soft
D. George and M. J. Madou
dinks contract considerably upon heating because of strain release, and as a result,
folding of the sheet happens. Instead of colored inks on the hinges, a graphene ink
may be deposited to bend Shrinky dink material to actuate the folding with microwave
[26]. When exposed to microwave, since graphene is not transparent to microwave,
the sheet is locally heated. Like the previous case, this local heating results in bending.
However, the orientation of the fold with respect to the wave affects the folding of
the material, limiting the versatility of the technique. In nanoscale, a focused ion
beam (FIB) is used for the fabrication of folded shapes. FIB, when bombarded
with the nanofilm, induces tensile or compressive stress on the film depending on
the operating parameters. Gallium ion from FIB when colliding with the film can
remove atoms from the gold film. The vacancies formed via this bombardment lead
to grain coalescence and as a result, residual tensile stress on the top layer of the
film, as illustrated in Fig. 12. This leads to the bending of the thin film [133].
Heating-based folding can be induced in an LCE by locally heating the sheet
beyond its transition temperature with IR irradiation (Fig. 11). The side that is heated
experiences a shrinkage due to the molecular rearrangement, leading to the bending
of the LCE sheet. While folding in LCE is achieved through IR illumination of
an isotropic material, an SMP-based folding requires local preprogramming of an
SMP. A direct writing technique may be employed to fabricate folds with SMP and
the faces with an inactive material. By training this SMP-made fold region, folded
origami can be obtained (Fig. 11).
3.3 External Field Approach
Here we discuss a new approach where bending of a sheet is carried out without
changing its properties across the cross-section, contrasting previously discussed
cases where the material properties of the sheets are tuned to induce bending. Various
forces, including capillary, magnetic, and external compressive forces, can be the
cause of bending in an external field approach. Since the external field approach
involves different forces, it is essential to discuss them briefly before delving into
the fabrication aspects of it.
Capillary effects scale favorably for the micron-scale actuation. A comparison of
the body forces (ρgl
3 ) with the surface tension forces (γ l), where l is the length scale,
γ is the surface tension, ρ is the density, and g is the gravitational constant, reveals
that the surface tension effects dominate in a smaller length scale. Therefore, surface
tension can act as an actuation mechanism for micro-origami fabrications. Nature
takes advantage of this fact, as observed in the case of the drinking mechanisms
of both phalaropes and hummingbirds. For a phalarope, this heightened effect of
surface tension at small scales is a way to accomplish, a gravity-defying intake of
prey-laden water toward its mouth. A Hummingbird’s tongue exploits the surface
tension to close lamellae on its tongue to entrap honey at each of its dips into honeyfilled flowers. Here, the tongue closes because of the interaction between the soft