Biomedical Engineering Reference
In-Depth Information
Practically, the applications require colloidal crystals to be either fixed or replicated
by other more robust materials [
158
-
163
]. Alternatively, one can replicate the col-
loidal crystal structure using a durable material, creating an inverse opal structure.
In this regard, colloidal crystals serve as the templates, with the voids infiltrated by
a material that solidifies in place without disrupting the order of the crystals. The
original colloidal particles are subsequently removed, leaving behind an inverted
structure of the colloidal crystals. The method has by now been used to make
highly ordered porous materials from a wide variety of precursors and templates. By
adjusting the dimension of the pores, as well as the refractive index of the material,
PBG of the inverse opal structure can be tuned accordingly.
Another important property that can be seen in nature is tunable structural
color, such as the color change that can be seen on the surface of a damselfish
[
164
]. In order to mimic such color change, several methods have been developed
to create tunable structural color films. Responsive photonic crystals (RPCs) are
materials with photonic band-gap properties that can be tuned by external stimuli.
To create such materials, a stimulus-response mechanism needs to be coupled with
the photonic crystal structure [
165
]. There are generally two approaches to introduce
such responsive materials. In the first case, the responsive materials are directly
prepared in the form of building blocks that can be used for constructing photonic
crystals. A typical example is the 1D Bragg stacks formed by self-assembly of block
copolymers that contain segments that can expand when exposed to certain solvents.
In the second case, the periodic structures are defined first, and then the responsive
materials are filled into the interstitial space to form a composite material that is
optically tunable and mechanically stable [
166
-
171
]. If the responsive material
cannot provide enough mechanical strength, an inverse opal structure will be formed
by first infiltrating an inert material into the interstitial space of the periodic structure
to form a robust framework and then removing the original periodic template
through calcination or chemical etching. Finally, the responsive materials are filled
into the porous structure of the inverse opals. In the second case, the responsibility
of nature structural color was mimicked by fabricating various RPCs (thermal,
chemical, optical) based on colloidal crystals and templating.
To create structural colors on fabrics is extremely important, practically, but
remains as a challenging task. As color fading caused by leaching or oxida-
tion/bleaching is a key issue in fabric care, producing with vivid and durable
structural colors on fabrics by fabricating the 3D colloidal crystals onto the surface
of fabrics will revolutionize textile and fashion industries [
158
]. If the physical
structure of photonic crystals on the fabrics is strong enough, the colors will last
forever. Recently, a combined surface treatment technology [
158
] allows us to
create opal and/or inverse opal structures on silk fabrics (cf Fig.
7.15
). In producing
structural colors on silk fabrics, polystyrene spheres with different diameters were
assembled on the surface of silk fabrics. Silk fibroin was dispersed on the colloidal
crystals as binding materials (Fig.
7.15
b). Thus, silk fabrics with different reflection
peaks ranging from ultraviolet to near-infrared can be obtained. It follows that the
colors (the reflected wave lengths) can be created by tuning the lattice constant of
