Biomedical Engineering Reference
In-Depth Information
white luster shows but the interference color does
not; whereas both the pearly luster and the inter-
ference color does not appear for the
Infinite Color
TM
even when it is applied on white background.
In addition to cosmetics, the technology of
Infinite Color
TM
is used to color artificial leather,
construction materials, and automobile bodies.
A recent review paper suggests replacing of the
central mica core with a void that may result in
an increased reflectance, especially in the short-
wavelength range
[94]
.
species in that genus, the scales of
M. sulkowskyi
have a higher ridge density with slightly slender
shapes and a more regular lamellar structure,
and, most important they contain only a neg-
ligible amount of a pigment that absorbs light
and decreases reflectance
[26]
. The reflectance of
the scale changes with either thermal expansion
[136]
or due to the entrapment of a vapor in the
void regions of the scale
[137]
.
When a wing of
M. sulkowskyi
is illuminated
with midwave infrared (3-8
μ
m) light, a part of
the incident energy is absorbed in the wing. This
causes thermal expansion of the wing, resulting
in increased spacing between the ridges, expan-
sion of the tree-like lamellar structure, and a
thermally induced reduction in the refractive
index of chitin (of which the wing is made);
accordingly, spectral signatures of the wing
shift. Doping the scales with single-walled car-
bon nanotubes enhanced the sensitivity and the
dynamic response to the infrared exposure, and
temperature changes as small as 0.02
o
C were
detected within 25 ms
[136]
, which is very prom-
ising for high-resolution microbolometry.
The scales trap air between the chitin structures.
When that air is replaced by a vapor, the overall
reflectance spectrum of a
M. sulkowskyi
wing
changes. Very diverse reflectance spectra are pro-
duced with different vapors, as has been noted
with water, methanol, ethanol, and dicholoroeth-
ylene
[137]
. Replicas of these wings could therefore
be used as optical sensors. Both applications--
thermal imaging and optical sensing--of butterfly
wings would require
standardization
before becom-
ing industrially viable.
11.6.5 Nacreous Pigments in Imaging
Elements
A remarkable application of nacreous pigments
is in making imaging elements
[135]
. Such an
imaging element is shaped like either a platelet
or a needle. A layer of an oxide of titanium, alu-
minum, or barium is first deposited as a highly
reflecting layer on a substrate, then that layer is
coated with a layer comprising white pigment
(from the group consisting of titanium oxide,
zinc oxide, zinc sulfide, barium sulfate, calcium
carbonate, talc, and clay), and finally a layer
comprising a nacreous pigment and a polymer
is deposited. The nacreous pigments contain
flakes of metal-oxide-coated mica, feldspar, sili-
cates, and quartz. The difference between the
refractive indices of the second and the third
layers should exceed 0.2. The thickness of the
layer containing the nacreous pigment must be
between 2 and 10 times the longest linear dimen-
sion of the nacreous flakes.
11.6.6 Sensor Applications Inspired by
the
Morpho Sulkowskyi
Butterly
Inspiration for the next two applications came
from highly iridescent wings of the butter-
ly
Morpho sulkowskyi.
The reason this species
was chosen from the
Morpho
genus is due to
the peculiarities of the morphology of its wing
scales; see Section
11.4.3
. Compared to the other
11
.7 CONCLUDING REMAR
KS
Nature is not as simple as claimed by scientific
reductionism. Complex structures have evolved
over very long periods of time to perform one or
many functions. There may be several routes to
the display of the same functionality. Dyes and
pigments occur widely in nature to produce color
