Structural Colors - Engineered Biomimicry - page 284

mechanism but also the structure of the ridges
on the scales. They implemented a hybrid
focused-ion-beam chemical vapor deposition
(FIB-CVD) technique. When illuminated by
white light and observed under a microscope,
the artificial structure shimmers with blue to
violet color, which, by means of a photonic multi
channel spectral analyzer system, was proven to
be similar to the reflection from real Morpho
scales. Although indicative of technoscientific
accomplishment, the hybrid technique does not
appear to be industrially scalable in the foresee-
able future [89] .

but also the surface hydrophobicity of butterfly
wings is exhibited by ALD replicas [107] .
The conformal evaporated film by rotation
(CEFR) method, classified as a physical vapor
deposition method, has also been used for
bioreplication, as discussed in Chapter 15. The
method employs thermal evaporation of a solid
material in a low-pressure chamber to create a
collimated vapor flux that is directed toward the
biotemplate, which itself is rotated in a complex
fashion so that the incoming vapor flux con-
denses and solidifies as a conformal coating of
the biotemplate [48] . Details of a butterfly wing
with and without the coating of a chalcogenide
glass are shown in Figure 11.21 , clearly demon-
strating the high fidelity delivered by the CEFR
method. Removal of the wing from the coating
has been achieved by chemical etching to leave
behind a positive replica [92] .
Nanocasting methods have also been used to
make replicas of butterfly wings. In the sol-gel
method, replication begins by filling the void
regions of a butterfly wing with a sol, drying it,
and then either etching or calcining away the
actual wing [46, 108-110] . This process forms a
negative replica of the dried sol. Sonication may
assist in the infilling step [111] . Instead of the
sol-gel method, chemical vapor deposition may
be used to infiltrate the void regions [112] .

11.5.3 Bioreplication

Perhaps the most direct way to reproduce the
iridescent colors of butterfly wings is by copy-
ing the wing morphology itself with nanoscale
fidelity. Several bioreplication techniques [89]
are currently under development, as discussed
elsewhere in this topic.
Low-temperature atomic layer deposition
(ALD) of alumina was used for the replication of
the Morpho butterfly wing [45] . After the wing itself
had been removed by prolonged high-temperature
annealing, the negative replica left behind exhib-
ited optical response characteristics quite similar to
that of original wing. Not only the optical response

FIGURE 11.21 SEMs of (left) a butterfly wing and (right) the same wing coated with chalcogenide glass using the CEFR

method [48] .