Journal Meeting on Molecular Actuator

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)


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


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


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


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


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


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


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

http://www.i-biomed.com/





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


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).

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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.

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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

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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.

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tris(4-((E)-phenyldiazenyl)phenyl)-benzene-1,3,5-tricarboxamide


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.

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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.

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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)

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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)

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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).

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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.

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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.

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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

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Documents

Journal Meeting on Molecular Actuator