Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 220

Fig. 8.9 Cross section of a
bioinspired compound lens
with fractal design

z

x

1 st stage

0 th stage

a b

glass

FOV FOV

T < 32°C

oil

T = 32°C

T < 32°C

hydrogel
ring

water

aperture slip

glass

Fig. 8.10 ( a ) Bioinspired lens with adjustable focus. ( b ) Overlapping field of views of two such

lenses.

the superiority of nature's design: for an incident wavelength of 600 nm, the zeroth-
stage design of silicon dioxide bioinspired compound lenses has a higher light
coupling efficiency than antireflection layers, especially for large angles, the first
stage structure improves the performance of direct-illuminated lenses but not that
of diffused-illuminated lenses, while second-order structures do not further improve
the performances. For silicon compound lenses, the zeroth-stage design outperforms
antireflection-coated structures only for diffused light illumination, the first-order
stage is more performant than the zero-order stage, whereas the second-order stage
does not improve significantly the light coupling efficiency. Note that only the first
stage is encountered in nature.
Not only have the compound eyes of insects inspired innovative devices, but
the human eye is also worth copying. What makes human eye special is its ability
to focus on different distances by modifying the shape of its lens through the
action of ciliary muscles. On the contrary, man-made optical lenses can adjust their
focal distance by a refraction index change induced by the electro-optic effect,
for example, or by varying their position. A lens that can autonomously adjust
its focal length by changing its shape in response to external stimuli has been
demonstrated in Dong et al. ( 2006 ). It consists of a cylinder-shaped liquid droplet
container surrounded by a hydrogel ring and integrated into a microfluidic system
(see Fig. 8.10 a). The optical lens is the meniscus between water and oil, which is
pinned at the top edge of the aperture slip due to the different properties of its top
surface and sidewall and bottom surface: hydrophobic and hydrophilic, respectively.
If fabricated from a temperature-sensitive NIPAAm hydrogel, the hydrogel ring
expands when the temperature decreases below 32 ı C due to water absorption in
its network interstitials, such that the available container volume for the water
droplet decreases, the pressure difference across the water-oil interface changes
and the meniscus curvature grows. On the contrary, at temperatures above 32 ı C,
the meniscus first retreats and then changes sign. Thus, the focal length of the
microlens changes: it is divergent for temperatures below 33 ı C, attaining a focal