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D. George and M. J. Madou
where field induces a bending moment without interfering with the material properties). Therefore, a property gradient that is permanently embedded in the material is
preferred to a method that purely relies on an external field to fold origami. A permanent property gradient across the thickness of a sheet is fulfilled during its synthesis
[75, 120]. Fabrication of photopolymer sheets with varying crosslinking density is
one of such synthesis methods. Photopolymers crosslinks when exposed to light. The
crosslinking occurs first in the region that is closer to the light source. For explaining
the process, this region is referred as the first layer, even though the crosslinking is
not really a layer-by-layer process. The polymer undergoes shrinkage as a result of
the crosslinking. As far as the first layer is concerned, the polymer is free to shrink
in every direction. However, once a solidified layer is formed, the contraction of
the subsequent layers would be constrained by the first layer. This constraint results
in residual stress inside the material. The residual stress this region experiences is
analogous to a stretched band. If this residual tension is released, the sheet will bend
as shown in Fig. 10. This photopolymer-based execution of the gradient approach,
frontal photopolymerization, can create complex shapes having millimeter length
scale [173]. A similar type of bending can be achieved using direct writing techniques too. Like the frontal photopolymerization-driven bending, the residual strain
build-up during the writing of the second layer can lead to the bending of the film.
The degree of crosslinking across the thickness of a photopolymer reduces with the
distance from the light source. This crosslinking density gradient directly correlated
to the porosity density inside its polymer matrix. Specifically, a less cross-linked SU8
sheet possesses a relatively more porosity compared to a more cross-linked sheet.
Porosity in a material allows for the absorption of various solvents by the substance.
The more porosity the material has, the more solvent it absorbs. Consider a thin
sheet with a crosslinking density gradient varying from a high level of crosslinking
on the top to a low level of crosslinking on the bottom. If one fabricates such flat
sheets with absorbed liquid (e.g., developer solution) inside them, the bending of the
faces occurs toward the bottom surface when the solvent is removed. The cause of the
bending is attributed to the shrinkage resulting from the removal of the large quantity
of the solvent from the highly porous bottom surface of the sheet [63]. Please keep in
mind that the reported fabrication based on this technique showed only a reversible
bending of the SU8 film since nothing prevents the folded structures from going back
to the original shape when placed inside the same solvent again. We will discuss a
strategy to make permanent folding later in this chapter.
In the case of an SMP with a preprogrammed bending trained on it, the sheet
bends to the “memorized” shape upon heating. Unlike in the case of bimorph, where
the linear deformation of the SMP bends a bilayer sheet consisting of the polymer
and another material, bending, in this case, is not assisted by a second material.
3.2.2 From Bending to Folding Using Material Gradient Approach
Localized modification of material properties is easy to achieve with photopolymers by selectively illuminating the polymer sheet. In one of the methods, selective
D. George and M. J. Madou
where field induces a bending moment without interfering with the material properties). Therefore, a property gradient that is permanently embedded in the material is
preferred to a method that purely relies on an external field to fold origami. A permanent property gradient across the thickness of a sheet is fulfilled during its synthesis
[75, 120]. Fabrication of photopolymer sheets with varying crosslinking density is
one of such synthesis methods. Photopolymers crosslinks when exposed to light. The
crosslinking occurs first in the region that is closer to the light source. For explaining
the process, this region is referred as the first layer, even though the crosslinking is
not really a layer-by-layer process. The polymer undergoes shrinkage as a result of
the crosslinking. As far as the first layer is concerned, the polymer is free to shrink
in every direction. However, once a solidified layer is formed, the contraction of
the subsequent layers would be constrained by the first layer. This constraint results
in residual stress inside the material. The residual stress this region experiences is
analogous to a stretched band. If this residual tension is released, the sheet will bend
as shown in Fig. 10. This photopolymer-based execution of the gradient approach,
frontal photopolymerization, can create complex shapes having millimeter length
scale [173]. A similar type of bending can be achieved using direct writing techniques too. Like the frontal photopolymerization-driven bending, the residual strain
build-up during the writing of the second layer can lead to the bending of the film.
The degree of crosslinking across the thickness of a photopolymer reduces with the
distance from the light source. This crosslinking density gradient directly correlated
to the porosity density inside its polymer matrix. Specifically, a less cross-linked SU8
sheet possesses a relatively more porosity compared to a more cross-linked sheet.
Porosity in a material allows for the absorption of various solvents by the substance.
The more porosity the material has, the more solvent it absorbs. Consider a thin
sheet with a crosslinking density gradient varying from a high level of crosslinking
on the top to a low level of crosslinking on the bottom. If one fabricates such flat
sheets with absorbed liquid (e.g., developer solution) inside them, the bending of the
faces occurs toward the bottom surface when the solvent is removed. The cause of the
bending is attributed to the shrinkage resulting from the removal of the large quantity
of the solvent from the highly porous bottom surface of the sheet [63]. Please keep in
mind that the reported fabrication based on this technique showed only a reversible
bending of the SU8 film since nothing prevents the folded structures from going back
to the original shape when placed inside the same solvent again. We will discuss a
strategy to make permanent folding later in this chapter.
In the case of an SMP with a preprogrammed bending trained on it, the sheet
bends to the “memorized” shape upon heating. Unlike in the case of bimorph, where
the linear deformation of the SMP bends a bilayer sheet consisting of the polymer
and another material, bending, in this case, is not assisted by a second material.
3.2.2 From Bending to Folding Using Material Gradient Approach
Localized modification of material properties is easy to achieve with photopolymers by selectively illuminating the polymer sheet. In one of the methods, selective