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D. George and M. J. Madou
intersections of those tubes. This localized bending enables folding without any
additional patterning [136] (Table 2).
As far as the execution of the bimorph-based folding strategies is concerned, most
of the examples discussed so far were based on lithography. For patterning some of the
materials, we may have to rely on other techniques such as direct writing. Let us revisit
the direct writing-based additive manufacturing method that was mentioned earlier
in this chapter. It is a process of making complex three-dimensional shapes by layerby-layer writing. In this technique, the volume of the target shape determines the time
required for the fabrication, irrespective of whether a structure is hollow or solid.
Consequently, fabrication time for a two-dimensional patterned origami precursor
sheet is an order of magnitude less than the time required for the fabrication of corresponding three-dimensional shape. Therefore, folding of two-dimensional sheets is
preferred over the direct writing of the corresponding three-dimensional shapes, as
far as the throughput of fabrication is concerned. A further improvement could be
made by introducing 4D printing, where the morphing of the shape with respect to
time is the fourth degree of freedom (Fig. 9). This kind of morphing can reduce the
production time and enables an extra degree of freedom, giving rise to free-form
manufacturing. This kind of manufacturing is generally realized by incorporating
active materials such as LCE and SMP in additive manufacturing (multi-material
printing), followed by actuation [42].
Incorporation of the LCE into direct writing technologies was made possible
with a relatively less viscous LCE ink. Low viscosity simplified the writing process
and helped introducing the required degree of molecular alignment in the resulting
precursor pattern. Shear-thinning during direct writing causes the needed alignment
of liquid crystals. The folding action can be initiated through Joule heating of an
embedded wire inside a bimorph containing LCE [168].
Multi-materials polymer 3D printers that are compatible with SMPs can fabricate
foldable multi-material precursors. SMP is printed only on one side of the fold region
to introduce a strain difference favorable for folding [41]. At a temperature higher
than the transition temperature, the printed sheets are stretched and brought to a
temperature lower than the transition temperature. When cooled, the SMP retains
its original shape, but the elastomer to which it is attached does not. This difference
in strain results in the bending of the sheet. When this bent shape is heated again,
SMP goes back to the original shape leading to the flattening of the sheet. Printing
at a temperature above the material’s transition temperature can simplify the overall
process even further as shown by Van Manen et al. [154]. During the printing, the ink
is extruded. This stretching is memorized by the polymer and leads to the morphing
when reheated (Fig. 9).
3.2 Material Gradient Approach
Fabrication of bimorphs requires either multilayer photolithography or a multimaterial writing, increasing the complexity of the process. Moreover, the strain
D. George and M. J. Madou
intersections of those tubes. This localized bending enables folding without any
additional patterning [136] (Table 2).
As far as the execution of the bimorph-based folding strategies is concerned, most
of the examples discussed so far were based on lithography. For patterning some of the
materials, we may have to rely on other techniques such as direct writing. Let us revisit
the direct writing-based additive manufacturing method that was mentioned earlier
in this chapter. It is a process of making complex three-dimensional shapes by layerby-layer writing. In this technique, the volume of the target shape determines the time
required for the fabrication, irrespective of whether a structure is hollow or solid.
Consequently, fabrication time for a two-dimensional patterned origami precursor
sheet is an order of magnitude less than the time required for the fabrication of corresponding three-dimensional shape. Therefore, folding of two-dimensional sheets is
preferred over the direct writing of the corresponding three-dimensional shapes, as
far as the throughput of fabrication is concerned. A further improvement could be
made by introducing 4D printing, where the morphing of the shape with respect to
time is the fourth degree of freedom (Fig. 9). This kind of morphing can reduce the
production time and enables an extra degree of freedom, giving rise to free-form
manufacturing. This kind of manufacturing is generally realized by incorporating
active materials such as LCE and SMP in additive manufacturing (multi-material
printing), followed by actuation [42].
Incorporation of the LCE into direct writing technologies was made possible
with a relatively less viscous LCE ink. Low viscosity simplified the writing process
and helped introducing the required degree of molecular alignment in the resulting
precursor pattern. Shear-thinning during direct writing causes the needed alignment
of liquid crystals. The folding action can be initiated through Joule heating of an
embedded wire inside a bimorph containing LCE [168].
Multi-materials polymer 3D printers that are compatible with SMPs can fabricate
foldable multi-material precursors. SMP is printed only on one side of the fold region
to introduce a strain difference favorable for folding [41]. At a temperature higher
than the transition temperature, the printed sheets are stretched and brought to a
temperature lower than the transition temperature. When cooled, the SMP retains
its original shape, but the elastomer to which it is attached does not. This difference
in strain results in the bending of the sheet. When this bent shape is heated again,
SMP goes back to the original shape leading to the flattening of the sheet. Printing
at a temperature above the material’s transition temperature can simplify the overall
process even further as shown by Van Manen et al. [154]. During the printing, the ink
is extruded. This stretching is memorized by the polymer and leads to the morphing
when reheated (Fig. 9).
3.2 Material Gradient Approach
Fabrication of bimorphs requires either multilayer photolithography or a multimaterial writing, increasing the complexity of the process. Moreover, the strain