Biomedical Engineering Reference
In-Depth Information
7.4.3
Biomimetics of Structural Color
Color production in the animal kingdom normally takes advantage of either
pigmentation or structural coloration. Color produced from pigmentation is also
called chemical color, which comes from the selective absorption of the incident
light and the reflection from the remaining spectra of the light by pigments.
Structural color, also called physical color, is caused by complicated interaction
between light and microstructures with featured sizes comparable to the visible
wavelengths [
132
]. In this case, fundamental optical processes such as reflection,
refraction, interference, diffraction, and scattering are the basic physical origination
of structural color. Comparing with the colors arising from pigments, structural
colors possess many interesting features since it is produced structurally. Generally,
structural color is of high brightness and saturation and is hence sometimes called
metallic color. It may display iridescence [
133
], i.e., a color change with perspective.
The variations of structures or contrast of refractive indices may alter structural
color, which can be achieved by applying pressure or infiltrating liquids in air voids.
In contrast to pigmentary coloration, structural color never fade away provided that
the corresponding photonic structure retains unchanged.
In recent years, structural color and relevant photonic structure have been
subjected to extensive studies because of their scientific and practical importance
[
132
-
139
]. The study of structural color may render tremendous important informa-
tion related from evolution, biological functions, structural formation, to strategies
of light steering. On the other hand, structural color may have potential applications
in a variety of industries including photonics, display, painting, and textile. Natural
photonic structures and the ingenious ways of light steering may have been a great
source of inspiration in our design and fabrication of new optical materials and
devices for future technological applications.
A wide variety of structural colors can be found in nature, among which the most
famous examples are vivid colors of some breeds of butterfly wings and peacock
feathers [
140
-
144
]. Metallic reflection from the elytra of beetles and iridescent
stripes in some kinds of fish are also typical natural structural colors [
145
-
147
]. As
illustrated in Fig.
7.10
a, the 2D photonic crystals in the cortex of different colored
barbules are responsible for the coloration of peacock feather [
140
,
144
], while the
multilayer structures of wing scales produce the structural coloration of some breeds
of butterflies [
148
].
7.4.3.1
Applications of Colloidal Crystals in Mimicking
of Structural Color
During the last two decades, much effort has been devoted to mimicking natural
structural color, although to obtain the dedicated structures with the structural color
as seen in animal kingdom (Fig.
7.15
a) remains to be a big challenge. One way
to acquire structural color was to adopt the nature structural color materials as
