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Basically made up of polymers in the form of hydrogels with liquid crystalline properties, featuring with reversible stimuli responsive properties.
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CHEMISTRY OF MOLECULAR ACTUATORS
1.0 INTRODUCTION
Soft Actuator
(Viscoelastic state) Hard Actuator
(Glassy state)
Handling soft living
tissues Handling metals
2.0 CLASSIFICATION OF POLYMERIC ACTUATORS
There are FOUR main classes of polymeric actuators based on different actuation mechanisms:~
Molecular Actuator
Materials and devices that are able to change their
shape in response to changes in environmental
conditions and thus perform mechanical work on the
nano-, micro-, and macroscales.
Metals
Metal oxides Polymers
Bimetal
strings
Hydrogels
1
Elastic
relaxation a)
Change of
order b)
Change in
volume c)
Surface
tension d)
CHEMISTRY OF MOLECULAR ACTUATORS
a) Elastic Relaxation
Still can’t get a clear picture? Please refer to Figure 2.2.
Figure 2.2 shows the permanent shape transferred to temporary shape by programming (heating)
process. Heating above the temperature of switching transition Ttrans for the sample results in the
recovery of permanent shape.
Recovery Programming
Mechanism in shape memory polymers.
Consists of two shapes:
Permanent (chemical cross-
linking or cooling below melting
pont where no shape changes).
Temporary (application of
stimulus leads to shape
deformation).
Advantages
Able to act in wet and dry
environments.
Transition temperature can be
tuned.
Biocompatible and biodegradable
E.g. polyethylene, polynorbonene,
styrene-based, acrylate-based, epoxy-
based and thioene-based polymer.
Figure 2.1 shows the transition from
temporary shape (spiral) to permanent
shape (rod) for a shape-memory network
synthesized from poly(ɛ-caprolactone)
dimethylacrylate and butylacrylate.
Switching temperature of this polymer is
46 oC. Recovery process take 35 s after
heating to 70 oC. Figure 2.1
2
Permanent Permanent Temporary
CHEMISTRY OF MOLECULAR ACTUATORS
b) Change of Order
3
Mechanism in liquid crystalline polymers.
Shrinks and stretches anisotropically along
the director orientation
A change in temperature or chemical
environment triggers the change of
order for the polymers.
Advantages
Able to act in wet and dry
environments (solvent-free).
Reversibility of actuation
Figure 2.3 shows the scheme for liquid
crystalline actuator.
Polymer backbones experience an
anisotropic environment
Leads to extended chain confor-
mation
For phase transition to isotropic phase, the
polymer regains its coiled conformation,
giving rise to a macroscopic shape change.
An intelligent approach to control direction
and degree of orientation of mesogenic
groups is by photoisomerization.
E.g. azobenzene chromophore can
repeatedly bent along any chosen
direction using polarized light
Figure 2.3
Phase transition
CHEMISTRY OF MOLECULAR ACTUATORS
c) Change in Volume
3D polymer networks imbibed
with aqueous solutions.
Divided into homogeneous and
inhomogeneous hydrogels.
Perform actuation when heat,
light, pH are being tuned.
Degree of swelling according to
Flory Rehner Theory (depends
on cross-linking density, inter-
actions between polymer chains
and solvents as well as mixing
entropy).
Advantages
Considerable volume
change.
Can be easily fabricated
using photolithography
and molding.
Biocompatible and bio-
degradable.
Limitation:
Requires an aqueous
environment or humid
air to act.
Consists of:
Two cross-linked polymers
with different melting points.
One cross-linked polymers with
a broad melting range.
Illustrate similar actuation as shape
memory polymers.
Reversible actuation occurred:
Sample heated to above melting
point of the poymer with lower
melting point but below the
melting point of polymer with
higher melting point.
Reversible volume changes
upon melting/crystallization of
one of the polymers.
Advantages
Provide alternative to the
development of shape memory
polymers.
High reversibility.
Opportunity:
Miniaturization of reversible
shape memory actuators.
Mechanism in:
i. Wet hydrogel actuators and
ii. Dry actuators based on thermal expansion
and shrinking.
Wet Hydrogel Actuators
Dry Actuators Based on Thermal
Expansion and Shrinking.
4
CHEMISTRY OF MOLECULAR ACTUATORS
d) Surface Tension
Mechanism in surface-tension-driven polymers.
In macroscale, surface tension is negligible. Size
of the actuator decreases to microscale and
nanoscale, surface tension become stronger,
which will contribute in producing movement.
E.g. non-spherical fusible particles. The particles
become softer and adopt equilibrium spherical
shape upon melting.
E.g. highly viscous polymer such as poly-
caprolactone (PCL). Due to high viscosity, the
particle can’t change their shape in reasonable
period of time. Stimuli applied leads to quick
relaxation of shape to spherical one.
Disadvantages
Irreversibility of actuation
High environment-dependent.
Figure 2.4 shows the video capture sequence (A to
D over 15 s) showing a 1-mm-sized, six-windowed
polymeric container self folding at 60 oC .
Figure 2.4
A
B
C
D
5
CHEMISTRY OF MOLECULAR ACTUATORS
3.0 Application of Actuators
Applications Description References
Sensors Designed using hydrogels (broad range of stimuli such as
temperature, pH, and specific ions and chemicals).
AFM cantilevers coated on one side with a stimuli-
responsive hydrogel and are able to bend depending on the
swelling state of hydrogel.
Bending detected by the change in the reflection of laser
beam from the surface of the cantilever.
Advantage: any AFM device can be used for sensing.
Bashir, R. et
al. (2002).
Appl. Phys.
Lett. 81,
3091-3093.
Hilt, J. Z. et
al. (2003).
Bio-med.
Micro-
devices, 5,
177-184.
Imaging devices Hydrogels used to design lenses with tunable focal
lengths. An example of imaging devices based on poly(N-
isopropylacrylamide)-based hydrogel as shown in Figure
3.1.
Dong, Li. et
al. (2006).
Nature,
442, 551.
Figure 3.1 shows the geometry of liquid meniscus
determined by pressure changes of water-oil
interface, resulted from expanding or shrinking of
hydrogel ring upon exposure to appropriate
stimulus.
6
CHEMISTRY OF MOLECULAR ACTUATORS
An example of artificial skin with tunable topography
device, which contains 4225 thermoresponsive hydrogel
actuators within an area of 37.7 mm × 37.7 mm as shown
in Figure 3.2.
Swelling and shrinking properties of hydrogels are being
utilized to carry out the mechanical motions.
Ritcher, A.
et al.
(2009). Adv.
Mater., 21,
979
-983.
Figure 3.2 shows the geometry of liquid meniscus
determined by pressure changes of water-oil
interface, resulted from expanding or shrinking of
hydrogel ring upon exposure to appropriate
stimulus.
7
CHEMISTRY OF MOLECULAR ACTUATORS
Control of
liquid flow Hydrogel pieces used to act as smart valve to control liquid
flow in microfluidic devices as shown in Figure 3.3.
The valve consists of a bistrip formed by pH-sensitive and
pH-indifferenr hyfrogels. Back pressure closes the
leaflets, thereby restricting backflow, whereas forward
pressure opens the leaflets and allow fluid to pass.
Another potential materials are electroactive polymers
such as polypyrolle, which is able to swell in one redox
state and does not swell in another state.
Yu, Q. et al.
(2001).
Appl. Phys.
Lett., 78,
2589-2591.
Arndt, K. F.
et al. (2000)
Polym. Adv.
Technol.,
11, 496-
505.
Infinite Bio-
medical
Technologie
s,
http://www.
i-
biomed.com
/
Figure 3.3 shows smart valve for the control of
liquid flow based on poly(2-hydroxyethyl
methacrylate)-based hydrogel.
8
CHEMISTRY OF MOLECULAR ACTUATORS
Walkers and
swimmers Applied cyclical stimuli leads to walking or swimming.
Example for “walking”:
In 2015, Yang, C. et al. demonstrated a hydrogel
walker using poly(2-acrylamido-2-methylpropane
sulfonic acid-co-acrylamide).
The hydrogel walkers allow controllable and
reversible banding/stretching behaviors via
repeated “on/off” electro-triggering cycles.
The liquid-crystalline polymers that are able to
bend in one direction or another can be used for
movement, can be used in developing electro-
controlled soft systems in the soft robotic field for
remote manipulation and transportation.
Yang, C. et
al. (2015).
Sci. Rep. 5,
13622-
13631.
Figure 3.4 shows the walking behaviors of hydrogel
walkers loaded with different weight of cargo,
which are: a) 25 m0, b) 50 m0, c) 75 m0, d) 100 m0
and e) 125 m0 in which m0 is the weight of the
walker in dried state.
a) b) c) d) e)
9
CHEMISTRY OF MOLECULAR ACTUATORS
Example for “swimming”: Performs when the shape changes cyclically.
In 2008, Lee, S. H. et al. used the inhomogeneous
deformation of hydrogels and pH-sensitive
hydrogel actuators mimic shape and swimming
motion of octopus as shown in Figure 3.5.
Initial angles of tentacles #1 and #2 are each in
clockwise direction relative to the “Ref Line” while
tentacles #3 and #4 are bent in counter-clockwise
direction.
During receding phase, all four tentacles were slowly
bent upward, ready to propel;
During propelling phase, the aquabot rapidly moved
upward by the propulsion of all four tentacles.
Such aquabots produces directional motion in
response to changes in electrochemical potential can
be used to sense and destroy certain microbes.
Lee, S. H. et
al. (2008).
Small, 4,
2148-2153.
Figure 3.5 shows locomotion of octopus aquabot under an
electric field (scale bar = 1 mm).
10
CHEMISTRY OF MOLECULAR ACTUATORS
Smart textiles Design of smart clothes by incorporating individual fibers
with a shape memory effect.
Shrinking of such fibers leads to the folding and shrinking
of a piece of textile.
E.g. a shirt with long sleeves could be programmed so that
the sleeves shorten as the temperature increases.
The fabric can be rolled up, pleated, creased and
returned to original shape by using a hair dryer.
Baurley, S.
(2004).
Pers.
Ubiquit.
Comput., 8,
274-281.
Switchable
surfaces In 2012, Aizenberg, M. et al. reported an arrays of poly(N-
isopropylacrylamide) (PNIPAM) pillars were fabricated
with a catalyst as a cap in a thermoresponsive hydrogel for
each pillar as shown in Figure 3.6.
When temperature reached lower critical solution
temperature (LCST), the rods bent into a region of liquid.
This caused the pillars elongate once more and
continue the cycles.
The system used to catalyze the decomposition of
hydrogen peroxide. The color arise from pH
indicator bromophenol blue shows presence of
oxygen
Aizenberg,
M. et al.
(2012).
Nature,
487, 214-
218.
Figure 3.6 shows the self-regulating oscillating surfaces
based on poly(acrylamide-co-acrylic acid) hydrogel with
PNIPAM pillars topping with catalyst caps.
11
CHEMISTRY OF MOLECULAR ACTUATORS
Paper Reviewed: A Light-Driven Supramolecular Nanowire Actuator
Author Information : 1) Junho Lee. (Pohang University, Korea)
2) Seungwhan Oh (Hanyang University, Korea)
3) Jaeyeon Pyo (Pohang University, Korea)
4) Jong-Man Kim (Hanyang University, Korea)
5) Jung Ho Je (Pohang University, Korea)
Journal : Nanoscale (Impact factor: 7.394).
Volume, Page Number : 7, 6457-6461
First Publication Online : 16 March 2015
Abstract:
A single photomechanical supramolecular nanowire actuator with an azobenzene-containing
1,3,5-tricarboxamide derivative is developed by employing a direct writing method. Single
nanowires display photoinduced reversible bending and the bending behavior follows first-order
kinetics associated with azobenzene photoisomerization. A wireless photomechanical nanowire
tweezers that remotely manipulates a single micro-particle is also demonstrated.
Background of Research
By using the principle of actuation, polymers and supramolecules that can be fabricated into gels
are being employed for such purpose. Incorporating with the sustainable energy source especially
light as well as to get rid of invasive wires and electrodes, authors have clearly emphasized the
advantages of photomechanical actuators against conventional electrostatic actuators.
Clearly, photomechanical actuators can be finely-tuned via the adjustment of light in terms of
wavelength, direction and desired intensity of input light. Meanwhile, photochromic crystals,
azobenzene derivatives and supramolecular hydrogels have been developed in recent decades.
However, it is a big great challenge because most of the suitable molecular candidates have yet to
be explored and developed as actuating components in nanodevices.
10
12
CHEMISTRY OF MOLECULAR ACTUATORS
Besides that, this paper main objective focused on the practical usage of photoinduced 3D motion
comprised of bending, twisting and rotation using NANO-wire actuators, previous report shows
that only MICRON-sized actuators been synthesized recrystallization, inkjet printing and melt-
and-draw method due to lack of dimensional control. Besides that, previous study only able to
illustrate one- and two-dimensional actuating motion, showing the method to synthesize
photomechanical nanowire actuators with omnidirectional actuating in nanoscale and give great
performance has yet to be developed.
The authors have report in this paper on a novel photomechanical nanowire actuator featured with
three-dimensional (3D) actuation in nanoscale, using tris(4-((E)-phenyldiazenyl)phenyl)-benzene-
1,3,5-tricarboxamide (Azo-1) have been carried out.
13
tris(4-((E)-phenyldiazenyl)phenyl)-benzene-1,3,5-tricarboxamide
CHEMISTRY OF MOLECULAR ACTUATORS
Instead of the previous method, authors decided to use bottom-up meniscus-guided solidification
method. This method depends on restricted solidification of an organic solution in nanoscale
meniscus. This method can be easily carried out at room temperature using micropipette. Besides
that, the Azo-1 nanowires have smooth surfaces in high aspect ratio and diameters in range of 200-
1000 nm that are pulling speed governed.
Figure 1 shows the illustration of growing a vertical nanowire by meniscus-guided method.
The fabricated Azo-1 nanowires adopt a monoclinic crystal structure as shown in Figure 2, due to
the high degree of supramolecular interactions between Azo-1 molecules.
Figure 2 shows the X-ray Powder Diffraction (XRD) patterns of solution-crystallized sample,
meniscus-guided microwires and freeze-dried sample of Azo-1.
14
CHEMISTRY OF MOLECULAR ACTUATORS
Experimental Set Up
Figure 3 shows the experimental set-up for fabrication of Azo-1 nanowires and to determine the
photoinduced actuation.
Results and Discussion
The observation was carried out on the bending and unbending of a single Azo-1 nanowire (d ~
500 nm, l (length) ~ 20 µm). Respective UV (λ ~ 365 nm, 1.5 mW cm-2; t0 to t1) and visible light
(λ ~ 455 nm, 4.0 mW cm-2; t1 to t2) irradiation was used. Reversible 3D bending motion was
demonstrated as shown in Figure 4 and Figure 5.
Figure 4 shows the schematic representation of 3D bending motion by Azo-1 nanowire.
15
CHEMISTRY OF MOLECULAR ACTUATORS
Figure 5 shows the top-view optical microscope images for the 3D bending motion of an Azo-1
nanowire (500 nm (d) x 45 µm (l)). Scale bar, 20 µm.
Significantly, the single nanowire reversibly bent and unbent towards the direction of UV and
visible irradiation, and this phenomenon was monitored by repeating alternative UV and visible
light irradiation. After several cycles, the nanowire was characterized using FESEM, and it shows
no fragmentation or “crack” within the structure of the nanowire as shown in Figure 6 (a) and (b)
due to high surface-to-volume ratio provides sufficient strain relief during the transition.
Figure 6 (a) shows a plot of the tip displacement as function of time showing the reversible
photoinduced bending and unbending process of Azo-1 nanowire by controlling the UV (purple)
and visible (green) light irradiation whereas (b) shows a FESEM image of bent Azo-1 nanowire
after UV irradiation where there is no sign of “crack” or fragmentation.
(a) (b)
16
CHEMISTRY OF MOLECULAR ACTUATORS
This bending and unbending phenomenon can be explained by the photoisomerization of
azobenzene-containing supramolecules. UV irradiation promotes formation of cis-azobenzene,
which followed by decrease in the overall length of the ca. 9.0 Å (trans-form) to ca. 5.5 Å (cis-
form) caused by trans-to-cis photoisomerization. Consequently, the supramolecules will undergo
contraction when irradiated with UV light as shown in Figure 7.
Figure 7 (a) shows a schematic trans-to-cis photoisomerization of Azo-1 nanowire while (b)
shows photoinduced bending of Azo-1 nanowire upon UV irradiation
Furthermore, the contraction of Azo-1 nanowire at surface region (< 100 nm) upon UV irradiation
was determined using real-time grazing incidence X-ray diffraction (GIXD) method (refer to
Appendix 1), indicating that length contraction of the azo π-bonds are responsible for the
photoinduced bending. In addition, Figure 8 also shows that the bending motion of Azo-1
nanowire is directly linked to the geometrical changes of azobenzene with the increased exposure
time.
(a) (b)
17
CHEMISTRY OF MOLECULAR ACTUATORS
Figure 8 (a) shows real-time bending of Azo-1 nanowire (scale bar, 10 µm) within 25 s while (b)
show the plots of bending strains as a function of exposure time for various UV intensities
(Coefficients of determination (R2) are 0.987 (2.0 mW cm-2), 0.993 (1.6 mW cm-2), 0.995 (1.2 mW
cm-2) and 0.992 (0.8 mW cm-2)).
As a result, authors claimed that the irradiation using low intensity (0.8 mW cm-2) of UV light for
a few seconds is sufficient to induce bending of a nanowires. The authors also ensure the system
uses light intensity level that meets the accepted safety standard (ACGIH, 1.0 mW cm-2 for period
lasting <1000 s), which can be used as a potential biomedical devices.
Other than light irradiation, the Azo-1 nanowire shows a remarkable results of thermal stability
with only little variations up to one month as shown in Figure 9. Authors explained this
phenomenon was due to strong intermolecular interactions between the cis-form, thermal cis-to-
trans isomerization occurred only in exceptionally low rate.
Figure 9 shows little variation happened up to one month where the bending strains measured as
function of a given temperature (25, 60 and 90 oC) after UV light irradiation (1.6 mW cm-2, 10 s).
18
CHEMISTRY OF MOLECULAR ACTUATORS
Lastly, the Azo-1 nanowire were used to investigate its photomechanical properties as wireless
nano-tweezers remotely. As shown in Figure 10, this testing was carried out easily with a simple
set up by fixing two nanowires (red one is Azo-1 nanowire, green one is a polystyrene (PS)
nanowire) at the tip of micropipette using the meniscus-guided method.
Figure 10 shows an optical microscope images on how the resulting tweezers from Azo-1 and a
PS nanowire (500 nm (d) x 12.5 µm (l)) successfully grips a PS microparticle (d ~ 4 µm) on a
silica substrate upon irradiation with UV light (1.5 mW cm-2 for 20 s): 1 (contact) 2 (gripping)
3 (detachment). Scale bar, 20 µm.
The successful actuation properties shown by Azo-1 nanowire with low UV dose required for
activation shows that it can be a suitable candidate as a novel photomechanical tweezers to be
applied in biomechanical systems.
19
CHEMISTRY OF MOLECULAR ACTUATORS
Conclusions
In conclusion, authors have successfully develop a new method to solidify the Azo-1
supramolecules into nanowire. Besides that, Azo-1 nanowire shows omnidirectional bending and
unbending with large actuating displacement (<1.7 µm) in presence of UV and visible light
alternatively. In addition, high thermal stability able prolongs the photoresponses of Azo-1
nanowire actuator. And lastly, authors have successfully demonstrated a new photomechanical
tweezers that can manipulate a single micro-particles remotely.
Opportunity
These type of supramolecules containing photochromic molecules can be developed as novel
nanorobotics devices for manipulation of micro- or even nano-objects.
Reviews
This paper have mainly focused on the characterization and application of the Azo-1 molecules,
and the results for characterization are well-explained. For the fabrication part, the manipulation
of bottom-up pulling speed significantly affect the diameter of the nanowire, which remains a great
challenge to ensure each nanowire is with the same diameter and length. Resistivity towards attack
of biological antibody and environmental pH instability can be tested for application in biomedical
science and engineering, as well as the mechanical strength of the nanowire. Overall, this paper
gives a clear picture on how the contraction works at molecular level as well as physical level in
the mechanism of actuation.
20
CHEMISTRY OF MOLECULAR ACTUATORS
APPENDIX ONE
(a) Experiemntal set-up of real-time grazing incidence X-ray diffraction (GIXD) for an Azo-1 microwire (2 μm in
diameter), line-patterned on Si(100) substrate (plane view). By this total X-ray reflection condition (θ (= 0.1°) < θc (=
0.11°)), the penetration depth of the incident X-rays could be adjusted as ~ 0.1 m from the Azo-1 microwire surface.
(b) A two-dimensional GIXD pattern of the Azo-1 microwire before UV irradiation. By the GIXD geometry, we
successfully identified two Azo-1 (12 ̅ 0 12) and (12 0 10) domains from the microwire surface region with q1 = 2.0432
Å-1 and q2 = 2.2871 Å-1, respectively. (c) The Bragg peaks of the (12 ̅0 12) domain (q1 = 2.0432 Å-1, domain size = 40
nm, measured from the full width at half maximum (FWHM)), measured in real-time during UV irradiation. We find
that the peak position gradually increases from q = 2.0432 Å-1 before UV irradiation to q = 2.0439 Å-1 under 10 min
UV irradiation, and further to q = 2.0445 Å-1 under 20 min UV irradiation. This result immediately indicates that the
interplanar spacing of the Azo-1 domain at the surface region gradually contracts with UV irradiation. (d) The Bragg
peaks of the (12 0 10) domain (q2 = 2.2871 Å-1, domain size = 80 nm) also showed similar contraction behavior under
UV irradiation. From these GIXD results, the Azo-1 crystal at the surface region proved to be really contracts by trans
to cis conversion under UV irradiation
21