Biomedical Engineering Reference
In-Depth Information
Structural colors
are produced by optical phe-
nomena, such as scattering of light from ordered
microstructures, thin films, and even irregular
arrays of scatterers, the key being the morphol-
ogy of the substance. Striking are the luster of
pearls and the iridescence of precious opals,
both materials commonly used in jewelry and
adored because of the
play of colors
that they
display. Most fascinating are the brilliant colors
of some rocks and shells, butterflies, beetles,
fish, and birds, which arise from the texture or
microstructure of their surfaces.
Structural colors
are characterized by an
intensely brilliant sheen that is almost metallic.
As opposed to pigments, which scatter light dif-
fusely, structural colors usually exhibit direc-
tional effects, i.e., they vary either with the
viewing angle or with the change from reflected
to transmitted light. For example, the colors of
a peacock feather undergo a complete cycle of
change with the viewing angle of reflected light.
In addition, when the feather is held up to the
light, the vivid signature colors are replaced by
a dull brown or black tint.
Structural color
is usually destroyed by injury
to the surface or by immersion in a neutral
medium whose refractive index is substantially
different from that of air. For example, the
colors of a peacock feather disappear when the
feather is thoroughly wetted with water or oil.
In contrast to pigmental colors, structural colors
exhibit high stability to acids, alkali, and light
and do not fade with time. Change in tempera-
ture should not significantly affect structural
color unless the thermal strains are very high.
According to Mason
[6]
, a structural color is not
affected by chemical treatment of the structure
creating the color unless the chemical reagents
swell, shrink, or destroy the structure. Cer-
tainly, pressure, distortion, swelling, or shrink-
ing will alter the hue; however, a structural
color cannot be bleached away, and no pig-
ments can be extracted by solvents or revealed
by chemical tests. Structural colors are gener-
ally iridescent and all constituents of the
incident white light can be found in the scat-
tered light. The presence of neighboring pig-
ments may, however, change the hue of a
structural color.
Structural colors arise from different types of
textured surfaces. For example, the wings of
the mango moth (
Bombotelia jocosatrix
) are cov-
ered with a multilayered thin film. The color
changes from green to yellow to orange and
purple, simply depending on the layer thick-
nesses and the viewing angle
[7]
. In some spe-
cies of moths and butterflies, color arises from
a diffraction grating formed by parallel ridges
[8]
. Both types of structures may be combined
in the wings of some species, and some pig-
ment may also contribute color
[9, 10]
. In addi-
tion to the blue iridescence of the
Morpho
genus,
the equally spectacular iridescence of beetles
and birds, illustrated in
Figure 11.2
, has
attracted scientific attention for more than a
century.
Electron microscopy has greatly assisted in the
elucidation of structural color in biological struc-
tures
[11, 12]
. It has revealed very complicated
architectures, often involving a uniformly repeat-
ing structure, which interact with light to produce
color. Therefore, structural color is very difficult
to reproduce with high fidelity using artificially
constructed structures, even when sophisticated
manufacturing techniques of nanotechnology are
employed. See Chapters 14-16 on solution-based
techniques, vapor-deposition techniques, and
atomic layer deposition.
Very importantly, during the last two
decades many researchers have been inspired
by structural colors in nature to develop new
and safe alternatives to the conventional pig-
ments in order to reduce the use of hazardous
and volatile chemicals. Applications have been
sought in many industries such as cosmetics,
textiles, and automotive paints. Our goal here is
to guide the reader to a few of these applica-
tions, but let us also provide some insights into
engineered biomimicry for coloration before
proceeding to the applications.
