Biomedical Engineering Reference
In-Depth Information
Fig. 8.27
(
a
) Cross-sectional SEM image of the wall region of a spine in the sea mouse
Aphrodita
sp. (
b
)and(
c
) Optical images of a spine showing the color change when the orientation in the
horizontal plane varies (by 90
ı
) with respect to the direction of the light source. Scale bar: (
a
)8
m (Reproduced from [
45
])
along this direction. As a result, strong reflection for frequency within this partial
bandgap is expected, leading to iridescent structural coloration. Iridescence can be
understood by the fact that partial photonic bandgaps are direction dependent.
8.5.4.1
2D Photonic Crystal
The spines of the sea mouse
Aphrodita
sp. (Polychaeta: Aphroditidae) display
spectacular iridescence. A spine appears normally dull red in color. However, if
illuminated perpendicularly to its axis, different colors can be seen as stripes running
parallel to the axis, as shown in Fig.
8.27
. SEM characterizations revealed that the
spines consist of a wall and a hollow center [
120
]. The wall region is composed
of hollow cylinders, made of pure
˛
-chitin, along the spine axis. The hollow
cylinders are close-packed hexagonally, forming a 2D photonic-crystal structure.
The cylinders have thicker walls near the spine edge, but after a few layers the wall
thickness slightly decreases to a constant value. The spacing of adjacent layers of
cylinders is about 0.51
m and remains constant throughout the wall cross-section.
From the calculated photonic band structure of the 2D photonic crystal, there exists
a partial photonic bandgap along the
M
direction, responsible for the observed
structural coloration and iridescence.
