Fabrication
of interpenetrated hydrogel network BCP PC. (A) Schematic illustration of the
fabrication processes of interpenetrated hydrogel network block copolymer
photonic crystal (IHN BCP PC). (B) Surface morphology of a PS-b-QP2VP film
swollen by ethanol, followed by being dried. Credit: Science Advances, doi:
10.1126/sciadv.abb5769
A novel
three-dimensional (3-D) touchless interactive display can change color based on
the distance of the user's finger from the screen by detecting subtle shifts in
ambient relative humidity, according to a new study. The technology may find
future applications in wearable electronics and electronic skins (e-skins) that
artificially mimic human skin's ability to sense pressure, temperature, and
humidity. While scientists have already developed a variety of interactive
touch displays, most of these involve variations in the intensity of light
emission or chromic reflection in response to a stimulus rather than changes in
color, which can provide more striking and distinct visual feedback.
To develop
a touchless interactive display based on changes in structural color, Han Sol
Kang and colleagues in materials science, nano engineering and chemical
engineering in the Republic of Korea and the U.S., designed a new display using
chemically cross-linked, interpenetrated hydrogel network layers within
photonic crystals that respond to changes in water vapor when a finger is moved
from 1 to 15 millimeters from the surface. The process could shift the
configuration of its surface structures to produce blue, green and orange
colors. The researchers then demonstrated the possibility of easily
transferring the photonic crystal-based film from one substrate to another by
swapping it from a silicon surface to a printed one-dollar bill. By combining
ionic liquid dopants (which alter a semiconductor's electrical properties) as
printing inks, the researchers note applications of the technology for
printable and rewritable displays.
User-interactive
displays (UIDs) facilitate the visualization of invisible information that can
be sensed such as touch, smell and sound, with potential applications in
wearable and patchable electronics suited for a futuristic hyperconnected
society. The tremendous demand for electronic skin that can artificially mimic
human skin to sense temperature, pressure and humidity has led to the
development of a variety of human-interactive touch displays. A touch platform
is in demand to visualize a stimulus without touch on 3-D interactive touchless
displays. Kang et al. envision a stimuli-sensitive, low-power, reflective-mode,
visible-range structural color (SC) of a photonic crystal (PC) to satisfy the
engineering requirements of a user-interactive 3-D touchless display. The scientists
developed a printable 3-D touchless interactive display using a hygroscopic
ionic liquid ink with facile structural color variation relative to humidity.
As proof of concept, they showed 3-D position-sensing of water vapor emanating
from a human finger (humidity) for touchless display from finger to film, with
emerging applications in wearable electronics.
Developing
an interpenetrated hydrogel network block copolymer photonic crystal (IHN BCP
PC)
The team
used self-assembled 1-D block copolymer (BCP) photonic crystals (PC) whose
layered periodic microstructure developed spontaneously upon film formation.
They then developed chemically cross-linked interpenetrated hydrogel network
(IHN) layers in a BCP PC microdomain. Kang et al. controlled the amount of
interpenetrated hydrogel network in the construct using UV irradiation to
control its structural color (SC) across the full visible range. Using
photographs of the engineered interpenetrated hydrogel network block copolymer
photonic crystals (IHN BCP PCs), they showed the irradiation-dependent
variation of SC. The polymer film was pseudoelastic (the material recovered
completely after unloading large strains) with excellent mechanical robustness,
flexibility and without sticky, gel-like viscoelasticity on the upper surface
to make it suitable for solid-state sensing.
Characterizing
the solid-state IHN BCP PCs
Kang et
al. extensively characterized the solid-state construct using grazing incident
small angle X-ray scattering (GISAXS) and transmission electron microscopy
(TEM). The results showed the development of highly ordered 1-D photonic crystal
structures and their calculated in-plane lamellae were consistent with
finite-difference time-domain (FDTD) simulations. For cross-sectional
transmission electron microscopy, they used cross-sectioned samples of the
mechanically robust film via focused ion beam milling and noted the different
layers of the material lamellae.
The TEM
images of BCP films showed screw dislocations (defects in crystals) distributed
across the sample surface to facilitate the transport of liquid and oligomeric
agents into the BCP films. The BCP film allowed water molecules to diffuse
through screw dislocations to facilitate humidity based touchless sensing. The
team obtained additional mechanical properties including the effective modulus
of the IHN BCP PCs using nanoindentation. The pseudoelastic material had an
effective elastic modulus approximating 5.3 GPa—as expected and similar to
those observed for conventional glassy polymers.
Obtaining
full color display and developing a user-interactive 3-D touchless screen
To obtain
a full color display, Kang et al. used an inkjet printer for direct deposition
of an ink known as L-ethyl-3-methylimidazolium
bis-(trifluoromethylsulfonyl)-imide, abbreviated EMIMTFSI, on an IHN BCP PC
film. The color of the film depended on the amount of EMIMTFSI deposited in a
given region. The inkjet printer only required a single ink for deposition on
the IHN BCP PC film, which markedly differed from a commercial inkjet printer
with red, green, and blue dye inks. Kang et al. produced a given colored image
by first programming the appropriate color information into a black/gray/white
contrast. As proof of concept, they converted a U.S. dollar bill to a black and
white contrast using software, and reconstructed the full color structural
color image using EMIMTFSI inkjet printing on an IHN BCP PC film.
For
further applications of the IHN BCP PC display, Kang et al. used another
hygroscopic ionic liquid named bis(trifluoromethylsulfonyl)amine lithium salt
(abbreviated LiTFSI). Upon diffusion of this ionic liquid into the material,
the structural color of the photonic crystal became sensitive to environmental
humidity. The LiTFSI allowed association with water molecules for structural
color variations to occur across the visible range as a function of humidity.
The absorbed water could be diffused out in a reversible process. The setup
allowed the human finger with natural humidity approximating 90 percent to be
an excellent source to modulate the structural color of the display film, which
the team confirmed experimentally. The 3-D touchless sensing display worked
successfully under multiple sensing events with different finger-to-photonic
crystal distances. Increased capacitance due to water uptake approximated a
response time of 20 seconds and the reversible change in structural color
lasted 55 time cycles.
In this way, Han Sol Kang and colleagues demonstrated a user-interactive 3-D touchless sensing display based on block copolymer photonic crystals with interconnected hydrogel networks (abbreviated IHN BCP PCs). The engineering technique allowed for mechanically soft and robust full-visible-range structural colors on a film with an effective modulus. The team combined the film with various ionic liquid printing inks to create printable and rewritable displays for 3-D touchless sensing through varying capacitance and structural color changes, to demonstrate a new approach for solid-state sensors and 3-D touchless displays.