Replica Molding of High-Aspect-Ratio Polymeric Nanopillar Arrays with High Fidelity

Zhang, Ying; Lo, Chi-Wei; Taylor, and J. Ashley; Yang, Shu

doi:10.1021/la061372+

Cited by 214 publications

(214 citation statements)

References 34 publications

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“…Anisotropic assemblies can also be obtained by the evaporation frontdirected method. This top-down/bottom-up hybrid strategy for preparing hierarchical structures features simplicity, scalability, and high flexibility in comparison with other state-of-the-art approaches such as photolithography (39), electron-beam lithography (20,40), and template replicating (4,16). Moreover, the assembled cells can be used as automatic microgrippers for selective trapping and controllable releasing of microobjects, suggesting numerous potential applications in the fields of chemistry, biomedicine, and microfluidic engineering.…”

Section: Selective Trapping and Releasing Of Microobjectsmentioning

confidence: 99%

“…When one aims to create slender structures, the dominant capillary force drives them to collapse, cluster, or be completely destroyed. Some efforts such as adopting a supercritical-point dryer (3,4) have been made to eliminate these unwanted behaviors when fabricating high-aspect-ratio structures. However, from another point of view, the capillary-driven bottom-up self-assembly can be exploited as a valuable tool to construct complex architectures.…”

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confidence: 99%

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Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly

Lao

Cumming

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Capillary force is often regarded as detrimental because it may cause undesired distortion or even destruction to micro/nanostructures during a fabrication process, and thus many efforts have been made to eliminate its negative effects. From a different perspective, capillary force can be artfully used to construct specific complex architectures. Here, we propose a laser printing capillaryassisted self-assembly strategy for fabricating regular periodic structures. Microscale pillars are first produced by localized femtosecond laser polymerization and are subsequently assembled into periodic hierarchical architectures with the assistance of controlled capillary forces in an evaporating liquid. Spatial arrangements, pillar heights, and evaporation processes are readily tuned to achieve designable ordered assemblies with various geometries. Reversibility of the assembly is also revealed by breaking the balance between the intermolecular force and the elastic standing force. We further demonstrate the functionality of the hierarchical structures as a nontrivial tool for the selective trapping and releasing of microparticles, opening up a potential for the development of in situ transportation systems for microobjects.laser printing | capillary force | self-assembly | hierarchical structures | microobject trapping P eople learn from daily life experience that individual filaments can coalesce, as with wet hairs or paintbrushes pulling out of paint. Analogs of this elastocapillary coalescence exist and are often amplified in micro/nanoelectromechanical system manufacturing processes because the capillary force tends to dominate over the standing force that needs to be overcome for coalescence when the scale is reduced (1, 2). When one aims to create slender structures, the dominant capillary force drives them to collapse, cluster, or be completely destroyed. Some efforts such as adopting a supercritical-point dryer (3, 4) have been made to eliminate these unwanted behaviors when fabricating high-aspect-ratio structures. However, from another point of view, the capillary-driven bottom-up self-assembly can be exploited as a valuable tool to construct complex architectures. Compared with other driving forces for self-assembly such as magnetism (5), electrostatic force (6), and gravity (7), capillary force self-assembly features the advantage of simplicity, low cost, scalability, and tunability.The desire for scalable and cost-effective self-assembly schemes comes from observations of the natural world, where self-assembled filamentous structures with mechanical compliance at the microscopic and mesoscopic scale are ubiquitous and include, to name a few, actin bundles (8), gecko feet hairs (9), and Salvinia leaf (10). These structures give rise to diverse functions ranging from mechanical strengthening (11) and directional adhesion (12, 13) to superhydrophobicity (14) and structural colors (15), thus inspiring researchers to devote their efforts to mimic these multifunctional structures for decades. Many top-down fabricat...

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Section: Selective Trapping and Releasing Of Microobjectsmentioning

confidence: 99%

mentioning

confidence: 99%

Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly

Lao

Cumming

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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“…All of the patterned substrates were fabricated by replica molding from commercially available epoxy (D.E.R. 354; Dow Chemical) on glass slides, using poly(dimethylsiloxane) (PDMS) molds, following the procedure reported in the literature (50). Preparation of substrates with desired LC anchoring.…”

Section: -Nitro 14-benzenedicarboxylic Acid 14-bis[4-[(2-oxiranymentioning

confidence: 99%

Direct mapping of local director field of nematic liquid crystals at the nanoscale

Xia

Serra

Kamien

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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Liquid crystals (LCs), owing to their anisotropy in molecular ordering, are of wide interest in both the display industry and soft matter as a route to more sophisticated optical objects, to direct phase separation, and to facilitate colloidal assemblies. However, it remains challenging to directly probe the molecular-scale organization of nonglassy nematic LC molecules without altering the LC directors. We design and synthesize a new type of nematic liquid crystal monomer (LCM) system with strong dipole-dipole interactions, resulting in a stable nematic phase and strong homeotropic anchoring on silica surfaces. Upon photopolymerization, the director field can be faithfully "locked," allowing for direct visualization of the LC director field and defect structures by scanning electron microscopy (SEM) in real space with 100-nm resolution. Using this technique, we study the nematic textures in more complex LC/colloidal systems and calculate the extrapolation length of the LCM.nematic liquid crystal polymers | dipole-dipole interactions | topological defects | photo-cross-linking | nanoscale imaging

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“…However, only rod arrays with diameters larger than ∼500 nm (aspect ratios <50) have survived the drying process without complete collapse, as demonstrated in the scanning electron micrographs seen in Figure 2. Previous work 15,16 has shown that the collapse of similar structures with low Young's moduli and high aspect ratios is due to surface adhesive forces between neighboring structures (lateral collapse) or between the rod structures and the substrate (ground collapse). The aspect ratio at which structures become susceptible to collapse depends on the work of adhesion of the material, which in turn is a function of the medium surrounding the structures.…”

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confidence: 99%

Magnetically Actuated Nanorod Arrays as Biomimetic Cilia

et al. 2007

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We present a procedure for producing high-aspect-ratio cantilevered micro-and nanorod arrays of a PDMS−ferrofluid composite material. The rods have been produced with diameters ranging from 200 nm to 1 µm and aspect ratios as high as 125. We demonstrate actuation of these superparamagnetic rod arrays with an externally applied magnetic field from a permanent magnet and compare this actuation with a theoretical energy-minimization model. The structures produced by these methods may be useful in microfluidics, photonic, and sensing applications.High-aspect-ratio nanostructures have attracted increasing attention in the nanotechnology community due to their potential applications as sensors 1-3 and actuators 4-7 and the effect of their presence on the surface properties of a material such as adhesion 8-11 and wetting. 12-14 We are interested in producing high-aspect-ratio nanostructures to serve as biomimetic cilia for the purpose of studying the mechanics of nanoscale fluid flow in a ciliated system. To this end, we have produced soft polymeric, actuable nanostructures of the size of biological cilia (∼10 µm in length by ∼200 nm diameter.) High-aspect-ratio polymer rods have been produced with materials with elastic moduli on the order of 100 MPa, 12-13 but these are unsuitable as actuating mechanisms due to their stiffness. Softer materials, such as poly(dimethyl siloxane) (PDMS, E ∼ 2 MPa), have been reported to fail at large aspect ratios due to lateral or ground collapse. 15,16 In addition, in many cases, rodlike microstructures are fabricated via a photolithographic master 15,16 or anodized aluminum oxide (AAO) membrane. 17 However, conventional photolithographic molds involve lengthy or specialized processing to produce large arrays of upright high-aspectratio structures, and AAO membranes impose severe limits on the diameter and spacing of the pores. Furthermore, with soft materials, photolithographic lift-off procedures may lead to structure collapse. Particle track-etched membranes have successfully been used as a template for a variety of materials [18][19][20] and are able to produce high-aspect-ratio structures with variable spacing and diameter. We use polycarbonate track-etched (PCTE) membranes as templates, allowing us to freely select the length and diameter of the rods and the density of the rod array by choosing an appropriate membrane.The high-aspect-ratio and low elastic modulus of our PDMS rods lend them a flexibility that makes them ideally suited to serve as actuators. To this end, we have produced micro-and nanorod arrays using a composite material of PDMS and iron oxide nanoparticles, which results in flexible superparamagnetic rods that may be actuated by applied external magnetic fields. Other groups have devised highaspect-ratio magnetically actuated microstructures via linkedbead chains, 3,7 and Singh, et al. have succeeded in tethering these structures to a substrate. 6 Our templated structures do not necessarily require a liquid medium, have the advantage of being scalable b...

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Replica Molding of High-Aspect-Ratio Polymeric Nanopillar Arrays with High Fidelity

Cited by 214 publications

References 34 publications

Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly

Laser printing hierarchical structures with the aid of controlled capillary-driven self-assembly

Direct mapping of local director field of nematic liquid crystals at the nanoscale

Magnetically Actuated Nanorod Arrays as Biomimetic Cilia

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