Controlled Placement of Microparticles at the WaterâLiquid Crystal

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Applications of Polymer, Composite, and Coating Materials

Controlled placement of microparticles at the water-liquid crystal elastomer interface Greta Babakhanova, Hao Yu, Irakli Chaganava, Qihuo Wei, Paul Shiller, and Oleg D. Lavrentovich ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 30, 2019

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ACS Applied Materials & Interfaces

Controlled placement of microparticles at the waterliquid crystal elastomer interface Greta Babakhanova,1,2 Hao Yu, 1,2 Irakli Chaganava, 3,4 Qi-Huo Wei, 1,2,5 Paul Shiller,6 Oleg D. Lavrentovich1,2,5,* 1Advanced 2Chemical 3Institute

Materials and Liquid Crystal Institute, Kent State University, Kent, 44242 USA Physics Interdisciplinary Program, Kent State University, Kent, 44242 USA

of Cybernetics of Georgian Technical University, Tbilisi, Georgia

4Georgian

State Teaching University of Physical Education and Sport, Tbilisi, Georgia

5Department 6Civil

of Physics, Kent State University, Kent, 44242 USA

Engineering-Timken Engineered Surface Laboratory, The University of Akron, Akron,

USA

KEYWORDS: liquid crystal elastomer photoresponsive coatings, disclinations, self-assembly

ABSTRACT

Controlled placement of microparticles is of prime importance in production of microscale superstructures. In this work we demonstrate the remote control of microparticle placement using a photo-activated surface profile of a liquid crystal elastomer (LCE) coating. We employ light-

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responsive LCEs with pre-imposed patterns of molecular orientation (director) in the plane of coating. Upon UV illumination, these in-plane director distortions translate into deterministic topographic change of the LCE coating. Microparticles placed at the interface between the LCE coating and water, guided by gravity, gather at the bottom of photoinduced troughs. The effect is reversible: when the substrates are irradiated with a visible light, the coatings become flat and the microparticle arrays disorganize again. The proposed non-contact manipulation of particles by a photo-activated LCEs may be useful in development of drug delivery or tissue engineering applications.

1. INTRODUCTION Placing

microparticles

in

predetermined

locations

is

a

challenging

problem

of

microtechnology.1-4 Many current approaches are based on heterogeneous patterning of substrates.5 One of the issues is the strength of binding forces and reversibility of particle placement. In this work, we propose an approach that offers reversible light-controlled predesigned placement of microparticles. The technique is based on photoresponsive liquid crystal elastomer (LCE) coatings with predesigned molecular orientation. Ultraviolet (UV)

   365 nm 

photoactivation of the LCE translates in-plane patterns of molecular orientation into variable surface topography.6-14 When such a photoactivated LCE elastomer coating with spatially varying profile serves as a substrate in contact with a water dispersion of microparticles, the particles are guided by gravity towards preprogrammed locations with the lowest potential energy. When UV irradiation is terminated, the LCE recovers its flat profile and the positions of particles are randomized. We consider different geometries of LCE patterning and discuss the advantages of reversibility of the proposed technique.


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2. MATERIALS AND METHODS 2.1. Preparation of the liquid crystal cells. Indium tin oxide coated glass substrates were cleaned in an ultrasonic bath and subsequently rinsed with isopropyl alcohol. The glass substrates were then dried in an oven at 90 ˚C to evaporate the solvent. It is very important for the surface of the glass substrate to be free from organic residuals for better spreading of the polyimide layer which was used to achieve planar anchoring. Thus, the substrates are treated with a UV-ozone for 5 minutes to remove organic residual and increase the hydrophilicity of the glass for better spreading of polyimide layer.15 The uniform planar alignment layer was achieved by spin coating polyimide layer PI2555 (HD Microsystems) onto a cleaned glass and baked in an oven at 275 ˚C for one hour, after which the substrates were rubbed ten times with a velvet cloth in a unidirectional fashion. The photosensitive alignment layer was deposited by spin coating a solution of the photosensitive azo-dye SD1 in N,N-dimethylfomamide (DMF) at 0.5 wt% concentration onto the glass and the substrates were baked in a hot oven at 95 ˚C for 30 min. The SD1 coated glass substrates were used to pre-pattern a spatially-varying liquid crystal director field using plasmonic metamask (PMM) composed of 200x100 nm nanoaperture arrays in Al film.16-17 The PMM was exposed to an unpolarized metal halide light source with emission spectrum   300  750 nm (EXFO X-Cite series 120). After passing through each nanoaperture in the PPM, the outgoing beam of light becomes linearly polarized. The locally polarized light incident onto the SD1 coating, causes the azo dye molecules to reorient their long axes perpendicularly to the polarization of light.18 Azo dyes such as SD1, Figure 1f, absorb UV light strongly when their transition dipole is parallel to the polarization of light, experiencing trans-tocis-isomerization of the azobenzene moieties.18 During isomerization and relaxation back to the trans-state, orientation of the molecule changes. A sequence of trans-to-cis and cis-to-trans


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transformations eventually brings the molecules to the state with the transition dipole perpendicular to the local polarization of light, making further changes in orientation highly unlikely.19-22 The orientational pattern of nanoapertures in the PMM produces the desired pattern of azodye orientation that is followed by the liquid crystal molecules that are in direct contact with the azodye molecules at the bounding plates.16 Liquid crystal mixture (Figure 1) was injected into the assembled sandwich-type cells (Figure 2a) via capillary action to create disclination free liquid crystal elastomers (LCEs), while the assembly in Figure 2b,c was used to create LCEs with periodic disclinations. The cell spacing was controlled using Micropearl microspacers with diameter (D) of  8 μm (Sekisui Chemical Co. Ltd.) pre-mixed with NOA71 UV glue.

Figure 1. Chemical structures of liquid crystal monomers (a) RM82, (b) RM23, (c) RM105, (d) azo-dye, A3MA; (e) photoinitiator Irgacure 819; (f) azobenzene dye SD1.


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Figure 2. Schematic representation of assembled cells consisting of a) an identical linear “Vstripes” at the top and bottom glass boundaries, b) linear photopatterned “V-stripes” and unidirectional planar substrates, c) circular “C-stripes” and unidirectional planar glass substrates. Scale bar 40  m . 2.2. Preparation of the light responsive LCE coatings: The composition of the photosensitive mixture presented in Figure 1 included monomers RM23 (24.8 wt%), RM105 (46.9 wt%) and RM82 (23.9 wt%) which were homogeneously mixed with a photoinitiator Irgacure 819 (0.9 wt%) (Merck) and azo dye A3MA (3.5 wt%) (Synchom) in Dichloromethane solvent (Sigma Aldrich). The monomeric mixture was injected in the cell in an isotropic phase (T  80 C) using capillary action, after which the system was brought to 25 ˚C using Linkam PE 94 temperature controller and the LTS 150 hot stage with precision of 0.01 C (Linkam Scientific Instruments, Ltd.). The aligned monomeric mixture was photopolymerized using metal halide unpolarized light source (EXFO X-Cite series 120) with intensity of 18.3 mW/cm2 for 10 minutes at T  25 C . Additionally, a 400 nm high-pass filter was used to avoid exposure to the UV light to prevent premature isomerization of the photoresponsive A3MA molecules.23 After curing, one substrate was removed leaving a light responsive LCE coating on the remaining substrate. Namely, a force


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was applied using a razor blade which was positioned in between two glass plates in one corner (far away from the photopatterned area). Careful separation of the two plates leaves an LCE coating, and especially the photopatterned area, intact.

3. RESULTS AND DISCUSSION We use a special class of liquid crystal (LC) molecules with polymerizable acrylate groups (Figure 1a-c). When using such reactive LC mesogens, the LC molecular order can be fixed via UV photopolymerization.24-27 Anisotropic properties such as orientational order parameter are coupled to the rubber-like elasticity of the LCE coatings. Such coupling presents an opportunity to control the elastomer deformations in predesigned locations.26,

28-29

In particular, in-plane

patterns of molecular orientation specified by the so-called director, nˆ , control the local thickness of the LCE coatings activated by temperature.7, 17 In this work, we employ photoactivation of LCE coatings. The LCE coatings were created using reactive mesogens, Figure 1a-c, mixed with a photoresponsive azobenzene monomer A3MA (Figure 1d), and a photoinitiator Irgacure 819, Figure 1e. The director patterning of the LCE was achieved by the technique of PMM photoalignment, as described in Section 2.1.16 The mixture of monomers was confined between two glass plates with a predesigned surface alignment pattern (Figure 2). We used three different patterns of the director at the bounding plates. The first is a uniform director aligned along a single direction in the xy -plane of the cell, which we call the y-axis, nˆ  (nx , n y , nz )   0, 1, 0  . The second represents a one-dimensional periodic system of stripes with V-shaped director field, written as

nˆ  ( cos  y / p , sin  y / p, 0) ,

(1)


6

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where p is half the period. The third represents circular stripes with the director resembling a letter “C” and written in cylindrical coordinates as

nˆ  (nr , n , nz )  (cos  r / p, sin  r / p, 0) ,

(2)

where r is the distance to the center of the pattern. After the monomeric mixture was filled in the space between two glass plates, the surface anchoring forces control the bulk director of the monomer mixture. The bulk director structure depends on the type of bounding plates used. If both plates are predesigned as V-stripes in phase with each other, then the bulk director forms a periodic linear nonsingular (LN) splay-bend structure, Figure 2a. When the cell is comprised of one plate treated uniformly and the second plate with V-stripes, then a linear singular (LS) system forms, with linear singular disclinations of strength ½ running along the x-axis in the bulk of the sample. The disclinations form along the lines where the director orientations at two plates are perpendicular to each other and separate the right- and left-handed twist domains of the director.30-32 In a similar fashion, a combination of circular stripes and a uniform plate shown in Figure.2c, produces a system of concentric singular disclinations, which we call a circular singular (CS) pattern. After the cells were filled with the monomeric mixtures and adopted director fields specified by the boundary conditions, the material was polymerized using a metal halide lamp with

  300  750 nm emission spectrum to form a flat  8 μm thick LCE slab. During photopolymerization of the monomers, a high pass filter was placed after the light source, to avoid the premature photoisomerization of A3MA photoresponsive molecules.23 At this stage, the LCE elastomer slab is of a uniform thickness, defined by the gap thickness between the two plates. Finally, one of the bounding plates was removed to produce an LCE coating. The bottom surface


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of LCE coating is affixed to the glass substrate. A water dispersion of microparticles is deposited onto the free top surface of the coating. The LCE coating remains almost flat until it is irradiated with UV light at   365 nm , Figure 3f. Irradiation results in the trans-cis isomerization of A3MA azo dye, which reduces the scalar order parameter of the LCE and triggers activation forces that lead to expansion/shrinkage of the coating, as described in Ref.17. Previously, we demonstrated that upon decreasing the orientational order, splay deformation of the director causes the coating to develop valleys, and bend develops protrusions.17 The activation force, f , that links the director field gradients to the topographical changes of the surface profile,17

f    nˆ divnˆ  nˆ  curlnˆ  , where

(3)

 is an activation parameter, increases as the director gradients become stronger. The

activation forces cause transport of the material, which produces surface deformations of the coating that are determined by the prescribed director pattern (Figure 4a, 5a, 6a). As the experiments reveal, these surface topography changes are sufficiently strong to control placement of microparticles, Figures 3-6.


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Figure 3. LS system. a) 3D visualization of an actuated LCE with LS pattern showing alternating linear hills and valleys via Zygo optical profilometer, b) light microscopy image showing the random distribution of microparticles at the initial deposition on an LCE surface; scale bar 100  m , c) histogram showing the 80  m periodicity of microparticle separation after UV light

illumination (the image in panel b was used to generate the histogram), d) light microscopy image showing linear chains of assembled microparticles at the disclination sites; scale bar 100  m , e)


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2D image of a dry, initial LCE surface before the UV photoirradiation visualized via digital holographic microscope (DHM); scale bar 50  m , f) height profiles of line VV’ shown in panel (e) before and after the UV light exposure obtained via DHM.

Figure 4. CS system. a) 3D visualization of an actuated LCE showing circular surface topography via DHM, b) light microscopy image showing the microparticles assembling at the disclination sites after UV irradiation, c) histograms showing periodic 80  m separation of microparticles upon UV light irradiation (the image in panel b was used to generate the histogram). The number


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of particles increases with distance, R, since the radius and length of each trough increase with the distance. Scale bar 50  m .

Figure 5. NS system. a) 3D visualization of an actuated LCE showing linear, periodic hills and valleys via DHM, b) linear V-striped prepatterned LCE showing microparticles forming chain-like


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assemblies in the splay regions of the LCE coating where valleys are formed upon UV irradiation, c) histogram showing spatial distribution of the microparticles upon UV light irradiation exhibiting the 80  m periodicity (the image in panel b was used to generate the histogram). Scale bar 100  m .

Figure 6. CS system. a) initial deposition of microparticles dispersed in water onto a flat LCE surface showing random distribution, b) microparticles aggregated at the disclination sites where the valleys were formed upon UV illumination for 3 min, c) blue light illumination for 3 min yielded random distribution of microparticles, thus, showing the reversibility of the system, regions. Scale bars 50  m .


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Figure 7. CS system. a) 2D visualization of an LCE coating during UV light irradiation showing three disclinations via DHM; scale bar 50  m ; regions b) #1 and c) #2 indicated in panel (a) show the rate of an LCE surface deformation while irradiating it with UV light.

Deposition of spherical resin microparticles (Sekisui Chemical Co. Ltd.) of a radius R  5 μm dispersed in water onto a flat LCE coating yields a random distribution (Figure 3b, 6a). Water prevents sticking of the microspheres to the dry LCE coating. Upon UV irradiation at   365 nm, locations with y  80, 160, 240 μm (Figure 2a) at which the director deformations are predominantly of the splay type, develop valleys (Figure 3a, 4a, 5a).17 As a result, the microparticles move towards the valleys, driven by gravity (Figure 3d, 4b, 5b, 6b).


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Figure 7 illustrates the rate of surface deformations as a function of UV light photoirradiation time. Under UV light irradiation, the dynamic changes of the surface profile start to saturate after about ~10 s. The microparticles organize at the valleys after about 50-60 s since the start of irradiation. The assembly/disassembly remains reversible after at least 10 cycles of UV/blue light irradiation, Figure 8. The temperature of an LCE coating after irradiating the sample with UV light for 20 minutes measured using a thermocouple remained constant at 25 ˚C, within 1 ˚C. A small temperature increase is not expected to alter the photo-induced profiles, as we did an experiment in which the sample was heated from 25 ˚C to 30 ˚C and the surface profile remained unchanged, with profile variations on the order of 1-2 nm or less. The microparticles form chain-like assemblies, spatially separated by 80 μm period, matching the period of the inscribed photopattern (Figure 3c,d, 4b,c, 5b,c, 6b). Note here that the final distribution of the microparticles is affected by their random initial distribution in water. Additionally, the light scattering due to surface roughness, microparticles and water might cause slight uneven distribution of the light intensity that may be responsible for slight variations of surface topographies, visible, for example, in Figure 5a.


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Figure 8. Bright field microscopy observations of the photoinduced assembly/disassembly of microparticles dispersed in water showing the reversibility and reproducibility of the CS system. Scale bars 50  m . The disclinations located in splay regions of LS and CS coatings are clearly visible under light microscope (Figure 3b,d, 4b, 6, 8). Upon UV illumination at   365 nm , these regions develop periodic valleys (Figure 3a, 4a) that collect microparticles (Figure 3d, 4b, 6b). The histograms in Figures 3c and 4c show 80  m spatial separation of the microparticles matching the periodicity of the photopatterned director field. The disclinations should yield sharper tilts of the LCE coatings, as suggested by Equation 3, since the director gradients near disclinations are stronger than in smoothly deformed director. The experiments confirm this expectation: the amplitude of the protrusions in Figure 3a is : 20% greater than in Figure 5a. Thus, the disclinations provide a deeper potential minimum for the assemblies of microparticles. Microparticle assembly at the troughs is guided by gravity. In our system, the component of the gravitational force acting on microparticles, Fg    particleVparticle   waterVwater  g sin , is in the range of (2  6)  1014 N , where  particle  1190 kg m -3 and Vparticle  4 R 3 / 3 are the density and the volume of the microsphere, respectively,  water  997 kg m -3 is the density of water at 25 ˚C,

g  9.8 m s-2 is the gravitational acceleration,  LN  0.0035 rad and  CS  0.01 rad are the slope angles of the LN and CS coatings, respectively, measured from the surface topography as   h / d , where h  130 nm is the average amplitude of deformations of the coating’s profile and

d  10  40  μm is the distance between the minima and maxima of the profile. The driving force Fg overcomes the friction, f    particleVparticle   waterVwater  r g cos force, where r is the


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coefficient of rolling friction. A typical value of rolling friction is r  105 .33 Since   r , the microparticles migrate towards the troughs. The difference in potential energy of the microspheres between the elevated regions and valleys, calculated as U g  Vparticle (  particle   water ) gh

 1.3  10-19J , is significantly higher than the thermal energy, kBT  4.11021 J , thus providing a robust trapping at the troughs. Obviously, as the particles become smaller, the trapping becomes weaker; for radii smaller than about ~1.5 μm, the Brownian motion could even detach the particles from the substrate. Applicability of the method to control positions of larger particles is limited by the period of the pattern. Another factor that might limit the approach is sticking of the particles to the substrate. Note here that the initial state of the coating shows nanoscale profile variations, of amplitude about hnano  10 nm and an angle of about 104 rad , Figure 3f. Although these tilts are still larger than r , they do not cause any visible repositioning of the microparticles, apparently because of a very small potential energy, U g hnano / h ~ 1020 J and finite adhesion of the particles to the substrate. The discussion above considers only the gravitational force, rolling friction and thermal fluctuations. A more realistic model should also account for substrate-particle anchoring or absence of thereof, hydrophobic and hydrophilic forces, etc. These interactions could be potentially used for further control of particles assembly; the corresponding studies are planned for the future work. Note here also that similar manipulation of microparticles through a dynamic surface of LCEs can be achieved by means other than photoirradiation, such as temperature 17 or electric field.34 The light-controlled mechanism demonstrated in our work has an advantage of remote application and also of potentially local character, as different regions of the substrate can


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be irradiated with different light beams. Optical control is also often less harmful to living matter as compared to heat and electric field.

4. CONCLUSION To conclude, we demonstrated that light-responsive liquid crystal elastomer coatings can be used for a controlled placement of microparticles. Contact-free UV irradiation creates a non-flat surface topography that is preprogrammed through in-plane molecular alignment of the liquid crystal during the preparation of coatings. Splay deformations of molecular alignment result in depressions of the topography which collect the microparticles thanks to gravity. Assemblies of microparticles are reversible, as the elastomer coatings restore their initial profile under visible light illumination. We demonstrated both linear and circular assemblies of the particles driven by linear defects-disclinations and by disclination-free coatings. In both cases, the microparticles migrate to regions of the lowest potential energy. LCE coatings of the LS/Cs type containing disclinations with stronger gradients of the molecular alignment produce stronger topography variations and valleys that are almost three times steeper than those produced in the disclinationfree LN coatings. Thus, they may serve as better traps. More complex director field patterns could be used for desired particle assembly patterns. For example, periodic lattice of point defects in molecular alignment presented in Ref. 17 can be used to create disk- and aster-like clusters of microparticles. Different periodicities of the director patterns presented in this work may also allow one to trap particles of different sizes, since stronger gradients due to smaller periodicities yield steeper valleys and thus may attract particles with smaller diameter. It would be of interest to explore whether the proposed non-contact photocontrol of surface topography can be expanded


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to biological applications to control cell growth, migration, tissue formation, and in designing targeted/site-specific drug delivery systems.11, 35-37

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The work was funded by the Office of Sciences, DOE, grant DE-SC0019105. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors thank Dr. Albert P.H.J. Schenning and Dr. Dirk J. Broer for providing the liquid crystal monomers; Dr. Frank Liu and Lyncée Tec for providing the DHM for characterization of the LCE coatings; Dr. Barbara Tury for her help with Zygo profilometer, Dr. Jagat Budhathoki and Taras Turiv for helpful discussions. ABBREVIATIONS


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LCE, liquid crystal elastomer; UV, ultraviolet; LC, liquid crystal; LN, linear nonsingular; LS, linear singular; CS, circular singular, PMM, plasmonic metamask. REFERENCES 1. Whitesides, G. M.; Grzybowski, B., Self-Assembly at All Scales. Science 2002, 295 (5564), 2418-2421. 2. Pishnyak, O. P.; Tang, S.; Kelly, J. R.; Shiyanovskii, S. V.; Lavrentovich, O. D., Levitation, Lift, and Bidirectional Motion of Colloidal Particles in an Electrically Driven Nematic Liquid Crystal. Phys Rev Lett 2007, 99 (12), 127802. 3. Yoon, D. K.; Choi, M. C.; Kim, Y. H.; Kim, M. W.; Lavrentovich, O. D.; Jung, H.-T., Internal Structure Visualization and Lithographic Use of Periodic Toroidal Holes in Liquid Crystals. Nat Mater 2007, 6 (11), 866. 4. Peng, C.; Turiv, T.; Zhang, R.; Guo, Y.; Shiyanovskii, S. V.; Wei, Q. H.; de Pablo, J.; Lavrentovich, O. D., Controlling Placement of Nonspherical (Boomerang) Colloids in Nematic Cells with Photopatterned Director. J Phys Condens Matter 2017, 29 (1), 014005. 5. Peng, C.; Turiv, T.; Guo, Y.; Shiyanovskii, S. V.; Wei, Q. H.; Lavrentovich, O. D., Control of Colloidal Placement by Modulated Molecular Orientation in Nematic Cells. Sci Adv 2016, 2 (9), e1600932. 6. Liu, D. Q.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J., Photo-Switchable Surface Topologies in Chiral Nematic Coatings. Angew Chem Int Edit 2012, 51 (4), 892-896. 7. Liu, D. Q.; Broer, D. J., Liquid Crystal Polymer Networks: Switchable Surface Topographies. Liq Cryst Rev 2013, 1 (1), 20-28. 8. Liu, D. Q.; Broer, D. J., Light Controlled Friction at a Liquid Crystal Polymer Coating with Switchable Patterning. Soft Matter 2014, 10 (40), 7952-7958. 9. Liu, D. Q.; Broer, D. J., Self-Assembled Dynamic 3D Fingerprints in Liquid-Crystal Coatings Towards Controllable Friction and Adhesion. Angew Chem Int Edit 2014, 53 (18), 4542-4546. 10. Liu, D. Q.; Broer, D. J., New Insights into Photoactivated Volume Generation Boost Surface Morphing in Liquid Crystal Coatings. Nat Commun 2015, 6, 8334. 11. Kocer, G.; ter Schiphorst, J.; Hendrikx, M.; Kassa, H. G.; Leclere, P.; Schenning, A. P. H. J.; Jonkheijm, P., Light-Responsive Hierarchically Structured Liquid Crystal Polymer Networks for Harnessing Cell Adhesion and Migration. Advanced Materials 2017, 29 (27), 1606407. 12. Hendrikx, M.; Schenning, A. P. H. J.; Broer, D. J., Patterned Oscillating Topographical Changes in Photoresponsive Polymer Coatings. Soft Matter 2017, 13 (24), 4321-4327. 13. Hendrikx, M.; Schenning, A. P. H. J.; Debije, M. G.; Broer, D. J., Light-Triggered Formation of Surface Topographies in Azo Polymers. Crystals 2017, 7 (8), 231. 14. Hendrikx, M.; Liu, D.; Schenning, A. P. H. J.; Broer, D. J. In Oscillatory Dynamic Surface Structures in Patterned Liquid Crystal Network Coatings, SPIE Organic Photonics and Electronics, SPIE: 2018; p 10. 15. Delplanque, A.; Henry, E.; Lautru, J.; Leh, H.; Buckle, M.; Nogues, C., UV/Ozone Surface Treatment Increases Hydrophilicity and Enhances Functionality of SU-8 Photoresist Polymer. Applied Surface Science 2014, 314, 280-285.


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37. Sanjay, S. T.; Dou, M. W.; Fu, G. L.; Xu, F.; Li, X. J., Controlled Drug Delivery Using Microdevices. Curr Pharm Biotechno 2016, 17 (9), 772-787.

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