29 research outputs found
Fabrication of Well-Defined Mushroom-Shaped Structures for Biomimetic Dry Adhesive by Conventional Photolithography and Molding
Biomimetic dry adhesives have many attractive features, such as reversible and repeatable adhesion against various surfaces. This paper presents a method for the simple fabrication of biomimetic dry adhesives composed of a mushroom-shaped structure, which is based on conventional photolithography and molding. Firstly a masked and a maskless exposure are performed on the top and bottom of a photoresist, respectively, that generates microholes with an undercut after development. This structured photoresist is then used for molding, leading to mushroom-shaped structural features after sacrificing the photoresist. Because of the convenience of photolithography, the proposed method has the potential to fabricate various dry adhesives cost-efficiently
Adhesion Circle: A New Approach To Better Characterize Directional Gecko-Inspired Dry Adhesives
The number of different
designs of directional gecko-inspired adhesives has proliferated over
the past 15 years, but some basic characterization tools are still
nonstandardized, which can make direct comparisons of different adhesives
in the literature difficult. By far the most common type of test for
directional adhesives, the load-drag-pull (LDP) test is useful but
can miss substantial information on the exact behavior of gecko-inspired
adhesives in a variety of loading conditions. Other test techniques,
including angled approaches and pull-offs, have been employed by a
few groups but they are not as widely adopted; peel tests can be employed
but require a larger amount of adhesive material to use in the test,
which is not always practical given some current manufacturing constraints.
Very few tests have looked at the effect of off-main axis loads on
the performance of directional adhesives, however, and this quality
of performance may be very important in applications where direct
control over displacements or angle of pull-off in pitch and yaw of
the peeling interface may not be practical or possible. To address
this overlooked area of characterization, we introduce a new test
concept for anisotropic adhesives, the adhesion circle, and also compare
how the radial normal adhesion performance is altered depending on
whether the pull-off comes after a displacement drag or when pulled
at a constant angle from vertical after a preload. Testing directional
adhesive designs made with different geometries shows that unexpected
behaviors at pull-off angles not in the direction of the strongâweak
axis can sometimes be seen. The complete adhesion circle tests should
help better design directional adhesives for scaled up performance,
and can be completed with relatively simple hardware that is typically
used in most current directional adhesive tests
Electrically Templated Dewetting of a UV-Curable Prepolymer Film for the Fabrication of a Concave Microlens Array with Well-Defined Curvature
This
paper presents an economic method, based on electrically templated
dewetting of a UV-curable prepolymer, for fabricating a concave microlens
array (MLA) of high quality and high density. In our strategy, a voltage
is applied to an electrode pair consisting of a conductive substrate
coated with a UV-curable prepolymer film and a microhole-arrayed silicon
template, sandwiching an air gap, to dewet the prepolymer film into
a curved airâliquid interface. At or beyond a critical voltage,
the curved prepolymer
can be pulled quickly into contact with the protrusive underside of
the silicon template. Contact of the prepolymer with the template
can be detected by monitoring the leaky current in the polymer, followed
by a UV curing of the prepolymer. Finally, by separating the mold
from the solidified polymer, a concave MLA is obtained. The curvature
of the MLA can be well-defined simply by changing the air gap between
the mold and prepolymer film.
Besides, the dewetting strategy results in a much smaller adhesion
area between the mold and solidified polymer structures, which allows
for easy separation of the mold from the MLA in a large-area operation
Electrically Modulated Microtransfer Molding for Fabrication of Micropillar Arrays with Spatially Varying Heights
The ability to generate a large area micropillar array
with spatially
varying heights allows for exploring numerous new interesting applications
in biotechnology, surface engineering, microfluidics, and so forth.
This Letter presents a clever and straightforward method, called electrically
modulated microtransfer molding (EM3), for generating such unique
microstructures from a silicon mold arrayed with microholes. The key
to the process is an application of electrically tunable wettability
caused by a spatially modulated voltage, which electrohydrodynamically
drives a photocurable and dielectric prepolymer to fill the microholes
to a depth depending on the voltage amplitude. Using EM3, micropillar
arrays with stepwise or continuously varying heights are successfully
fabricated, with the diameter scalable to 1.5 ÎŒm and with the
maximum height being equal to the depth of the high-aspect-ratio (more
than 10:1) microholes
UV-Catalytic Preparation of Polypyrrole Nanoparticles Induced by H<sub>2</sub>O<sub>2</sub>
As
a green oxidant, H<sub>2</sub>O<sub>2</sub> can be used to induce
the polymerization of pyrrole. This approach avoids the issue of metal
residue in the polymer caused by metal oxidants, whereas the reaction
efficiency is low and the corresponding reaction mechanism not clear.
In this study, uniform polypyrrole (PPy) nanoparticles were prepared
using H<sub>2</sub>O<sub>2</sub> as an oxidant under UV irradiation
in the presence of polyvinylpyrrolidone (PVP). The morphology characterization
indicated that the spherical PPy nanoparticles were capped by a PVP
shell. Through the investigation of reaction process, it was found
that the photolysis of H<sub>2</sub>O<sub>2</sub> led to the formation
of hydroxyl radicals, which then initiated the oxidative polymerization
of pyrrole. The coalescence of small PPy particles formed nanoparticles
which were stabilized by PVP. The effects of several reaction conditions
on the polymerization rate and the size distribution of nanoparticles
were investigated in detail, including radiation intensity (0â30
W), temperature (0â50 °C), and the concentrations of PVP
(5â20 g/L), H<sub>2</sub>O<sub>2</sub> (0.06â0.6 M),
H<sub>2</sub>SO<sub>4</sub> (0â0.<i>2</i>2 M), and
the monomer pyrrole (0.03â0.2 M), respectively. UV-catalytic
preparation of PPy nanoparticles induced by H<sub>2</sub>O<sub>2</sub> is an effective and environmentally friendly approach, which could
be expected to be extended to other conductive polymers
Polydopamine-Coated Main-Chain Liquid Crystal Elastomer as Optically Driven Artificial Muscle
Optically
driven active materials have received much attention because their
deformation and motion can be controlled remotely, instantly, and
precisely in a contactless way. In this study, we investigated an
optically actuated elastomer with rapid response: polydopamine (PDA)-coated
liquid crystal elastomer (LCE). Because of the photothermal effect
of PDA coating and thermal responsiveness of LCE, the elastomer film
contracted significantly with near-infrared (NIR) irradiation. With
a fixed strain, light-induced actuating stress in the film could be
as large as 1.5 MPa, significantly higher than the maximum stress
generated by most mammalian skeletal muscle (0.35 MPa). The PDA-coated
LCE films could also bend or roll up by surface scanning of an NIR
laser. The response time of the film to light exposure could be as
short as 1/10 of a second, comparable to or even faster than that
of mammalian skeletal muscle. Using the PDA-coated LCE film, we designed
and fabricated a prototype of robotic swimmer that was able to swim
near the waterâair interface by performing âswimming
strokesâ through reversible bending and unbending motions induced
and controlled by an NIR laser. The results presented in this study
clearly demonstrated that PDA-coated LCE is a promising optically
driven artificial muscle, which may have great potential for applications
of soft robotics and optomechanical coupling devices
Polydopamine-Coated Main-Chain Liquid Crystal Elastomer as Optically Driven Artificial Muscle
Optically
driven active materials have received much attention because their
deformation and motion can be controlled remotely, instantly, and
precisely in a contactless way. In this study, we investigated an
optically actuated elastomer with rapid response: polydopamine (PDA)-coated
liquid crystal elastomer (LCE). Because of the photothermal effect
of PDA coating and thermal responsiveness of LCE, the elastomer film
contracted significantly with near-infrared (NIR) irradiation. With
a fixed strain, light-induced actuating stress in the film could be
as large as 1.5 MPa, significantly higher than the maximum stress
generated by most mammalian skeletal muscle (0.35 MPa). The PDA-coated
LCE films could also bend or roll up by surface scanning of an NIR
laser. The response time of the film to light exposure could be as
short as 1/10 of a second, comparable to or even faster than that
of mammalian skeletal muscle. Using the PDA-coated LCE film, we designed
and fabricated a prototype of robotic swimmer that was able to swim
near the waterâair interface by performing âswimming
strokesâ through reversible bending and unbending motions induced
and controlled by an NIR laser. The results presented in this study
clearly demonstrated that PDA-coated LCE is a promising optically
driven artificial muscle, which may have great potential for applications
of soft robotics and optomechanical coupling devices
Polydopamine-Coated Main-Chain Liquid Crystal Elastomer as Optically Driven Artificial Muscle
Optically
driven active materials have received much attention because their
deformation and motion can be controlled remotely, instantly, and
precisely in a contactless way. In this study, we investigated an
optically actuated elastomer with rapid response: polydopamine (PDA)-coated
liquid crystal elastomer (LCE). Because of the photothermal effect
of PDA coating and thermal responsiveness of LCE, the elastomer film
contracted significantly with near-infrared (NIR) irradiation. With
a fixed strain, light-induced actuating stress in the film could be
as large as 1.5 MPa, significantly higher than the maximum stress
generated by most mammalian skeletal muscle (0.35 MPa). The PDA-coated
LCE films could also bend or roll up by surface scanning of an NIR
laser. The response time of the film to light exposure could be as
short as 1/10 of a second, comparable to or even faster than that
of mammalian skeletal muscle. Using the PDA-coated LCE film, we designed
and fabricated a prototype of robotic swimmer that was able to swim
near the waterâair interface by performing âswimming
strokesâ through reversible bending and unbending motions induced
and controlled by an NIR laser. The results presented in this study
clearly demonstrated that PDA-coated LCE is a promising optically
driven artificial muscle, which may have great potential for applications
of soft robotics and optomechanical coupling devices
Spray-Coated CsPbBr<sub>3</sub> Quantum Dot Films for Perovskite Photodiodes
Large-area
film deposition and high material utilization ratio
are the crucial factors for large-scale application of perovskite
optoelectronics. Recently, all-inorganic halide perovskite CsPbBr<sub>3</sub> has attracted great attention because of its high phase stability,
thermal stability, and photostability. However, most reported perovskite
devices were fabricated by spin-coating, suffering from a low material
utilization ratio of 1% and a small coverage area. Here, we developed
a spray-coating technique to fabricate a CsPbBr<sub>3</sub> quantum
dot (QD) film photodiode which had a high material utilization ratio
of 32% and a deposition rate of 9 nm/s. The film growth process was
studied, and substrate temperature and spray time were two key factors
for the deposition of uniform and crack-free QD films. The spray-coated
photodiode was demonstrated to be more suitable for working in the
photodetector mode because a low dark current density of 4 Ă
10<sup>â4</sup> mA cm<sup>â2</sup> resulting from an
extremely low recombination current contributed to a high detectivity
of 1 Ă 10<sup>14</sup> Jones. A high responsivity of 3 A W<sup>â1</sup> was obtained at â0.7 V under 365 nm illumination,
resulting from a low charge-transfer resistance and a high charge
recombination resistance. We believe that the spray deposition technique
will benefit the fabrication of perovskite QD film optoelectronics
on a large scale
Polydopamine-Coated Main-Chain Liquid Crystal Elastomer as Optically Driven Artificial Muscle
Optically
driven active materials have received much attention because their
deformation and motion can be controlled remotely, instantly, and
precisely in a contactless way. In this study, we investigated an
optically actuated elastomer with rapid response: polydopamine (PDA)-coated
liquid crystal elastomer (LCE). Because of the photothermal effect
of PDA coating and thermal responsiveness of LCE, the elastomer film
contracted significantly with near-infrared (NIR) irradiation. With
a fixed strain, light-induced actuating stress in the film could be
as large as 1.5 MPa, significantly higher than the maximum stress
generated by most mammalian skeletal muscle (0.35 MPa). The PDA-coated
LCE films could also bend or roll up by surface scanning of an NIR
laser. The response time of the film to light exposure could be as
short as 1/10 of a second, comparable to or even faster than that
of mammalian skeletal muscle. Using the PDA-coated LCE film, we designed
and fabricated a prototype of robotic swimmer that was able to swim
near the waterâair interface by performing âswimming
strokesâ through reversible bending and unbending motions induced
and controlled by an NIR laser. The results presented in this study
clearly demonstrated that PDA-coated LCE is a promising optically
driven artificial muscle, which may have great potential for applications
of soft robotics and optomechanical coupling devices