Mechanocapillary Forming of Filamentary Materials.

The hierarchical structure and organization of filaments within natural materials determine their collective chemical and physical functionalities. Synthetic nanoscale filaments such as carbon nanotubes (CNTs) are known for their outstanding properties including high stiffness and strength at low density, and high electrical conductivity and current carrying capacity. Ordered assemblies of densely packed CNTs are therefore expected to enable the synthesis of new materials having outstanding multifunctional performance. However, current methods of CNT synthesis have inadequate control of quality, density and order. In pursuit of these needs, a new technique called capillary forming is used to manipulate vertically aligned (VA-) CNTs, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation; followed by evaporation-induced shrinkage. Three-dimensional geometric transformations result from the heterogeneous strain distribution within the microstructures during the vapor-liquid-solid interface shrinkage. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using this technique. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 104 S/m respectively. These values are at least hundred-fold higher than as-grown CNT properties, and exceed the properties of typical microfabrication polymers. Responsive CNT-hydrogel composites are prototyped by combining isotropic moisture-induced swelling of the hydrogel with the anisotropic stiffness of CNTs to induce reversible self-directed shape changes of up to 30% stroke. Centimeter scale sheets are fabricated by mechanical rolling and capillary assisted joining of CNTs. The mechanical stiffness, strength and electrical conductivity of CNT sheets are comparable to those of continuous CNT microstructures; and can be tuned by engineering the morphology of the CNT joints. Finally, the applicability of mechanocapillary forming to other nanoscale filaments is demonstrated using silicon nanowires synthesized by metal assisted chemical etching. Further work using the methods developed in this dissertation could enable applications such as directional liquid transport, adhesives, and biosensors; toward an end goal of creating multifunctional surfaces having arbitrary structural, interfacial, and optical responsiveness.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/1/stawfick_1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/91466/2/stawfick_2.pd

Advances in patterned polymer nanoestructures in-situ polymerization and polymer infiltration in AAO templates

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, Departamento de Química-Física, leída el 19-04-2017Las plantillas porosas de alúmina (AAO) son sistemas ordenados, formados por una matriz de poros cilíndricos uniformemente dimensionados. Estas plantillas han sido ampliamente empleadas en nuestro grupo para la obtención de nanoestructuras poliméricas como nanorods, nanofibras y nanotubos mediante el proceso infiltración de polímeros (nanomoldeo). Asimismo, numerosos trabajos han demostrado que las propiedades del polímero en confinamiento dentro de los nanoporos cambian con respecto a las propiedades del mismo polímero en masa. Sin embargo, cuando se trata de infiltrar polímeros termoestables o el proceso de infiltración debe llevarse a cabo a alta temperatura y/o durante un tiempo relativamente largo, de horas a días, el polímero se puede degradar y es conveniente buscar otro método. Esta tesis doctoral plantea la polimerización in situ de un monómero dentro de los poros de la plantilla AAO (nanoreactor) como método alternativo para producir nanoestructuras poliméricas. Una vez polimerizado el polímero puede extraerse para el estudio de sus propiedades y para su uso en distintas aplicaciones. Esta tesis está basada en el empleo de plantillas AAO y comprende dos aspectos importantes, su empleo como nanoreactores y nanomoldes. Los objetivos de esta tesis son: el estudio de las reacciones de polimerización de diferentes monómeros en las nanocavidades y la preparación de nuevas nanoestructuras poliméricas nunca reportadas en la literatura, mediante la infiltración de polímeros en AAO. El primer objetivo además plantea la modelización de la polimerización en confinamiento estudiando las diferencias respecto al bulk. Para alcanzar estos objetivos se prepararon plantillas de alúmina mediante el proceso de doble anodización. El empleo de distintas condiciones de anodización (temperatura, naturaleza y concentración del electrolito, voltaje y tiempo) permitió la elaboración de plantillas de distintos tamaños de diámetro y longitud de poro, entre 15-400 nanómetros y 0.7-100 micras, respectivamente...Anodic aluminium oxide templates (AAO) are ordered systems, formed by a matrix of uniformly sized cylindrical pores. These templates have been widely used in our group to obtain polymer nanostructures such as nanorods, nanofibers and nanotubes through the process of polymer infiltration (nanomolding). Also, numerous studies have shown that the properties of the polymer in confinement inside the nanopores change acoording to the properties of the same polymer in bulk. However, when we need to infiltrate thermosetting polymers or the infiltration process must be carried out at high temperature and / or for a relatively long time, from hours to days, the polymer may be degraded and it is required to use other method. This doctoral thesis proposes the in situ polymerization of a monomer within the pores of the AAO template (nanoreactor) as an alternative method to produce polymer nanostructures. Once it polymerized, the polymer can be extracted for the study of its properties and can be the use in different applications. This thesis is based on the use of AAO templates and comprises two important aspects, their use as nanoreactors and nanomolds. The objectives of this thesis are: the study of the reactions of polymerization of different monomers in the nanocavities and the preparation of new polymer nanostructures never reported in the literature, by the polymer infiltration process in AAO. The first objective also proposes the modelling of the polymerization in confinement by studying the differences with respect to the bulk. To achieve these objectives, alumina templates were prepared by the double anodization process. The use of different anodizing conditions (temperature, nature and concentration of electrolyte, voltage and time) allowed the development of templates of different sizes of pore diameter and pore length, between 15-400 nanometers and 0.7-100 microns, respectively...Depto. de Química FísicaFac. de Ciencias QuímicasTRUEunpu

Fabrication of high aspect ratio silicon nanostructure arrays by metal-catalyzed etching

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2010.Includes bibliographical references (p. 167-178).The goal of this research was to explore and understand the mechanisms involved in the fabrication of silicon nanostructures using metal-assisted etching. We developed a method utilizing metal-assisted etching in conjunction with block copolymer lithography to create ordered and densely-packed arrays of high-aspect-ratio single-crystal silicon nanowires with uniform crystallographic orientations. Nanowires with sub-20 nm diameters were created as either continuous carpets or as carpets within trenches. Wires with aspect ratios up to 220 with much reduced capillary-induced clustering were achieved through post-etching critical point drying. The size distribution of the diameters was narrow and closely followed the size distribution of the block copolymer. Fabrication of wires in topographic features demonstrated the ability to accurately control wire placement. The flexibility of this method will facilitate the use of such wire arrays in micro- and nano-systems in which high device densities and/or high surface areas are desired. In addition, we report a systematic study of metal-catalyzed etching of (100), (110), and (111) silicon substrates using gold catalysts with varying geometrical characteristics. It is shown that for isolated catalyst nanoparticles and metal meshes with small hole spacings, etching proceeded preferentially in the direction. However, etching was confined in the direction vertical to the substrate surface when a catalyst mesh with large hole spacings was used. This result was used to demonstrate the use of metal-assisted etching to create arrays of vertically-aligned polycrystalline and amorphous silicon nanowires etched from deposited silicon thin films using catalyst meshes with relatively large hole spacings. The ability to pattern wires from polycrystalline and amorphous silicon thin films opens the possibility of making silicon nanowire-array-based devices on a much wider range of substrates. Finally, we demonstrated the fabrication of a silicon-nanopillar-based nanocapacitor array using metal-assisted etching and electrodeposition. The capacitance density was increased significantly as a result of an increased electrode area made possible by the catalytic etching approach. We also showed that the measured capacitance densities closely follow the expected trend as a function of pillar height and array period. The capacitance densities can be further enhanced by increasing the array density and wire length with the incorporation of known self-assembly-based patterning techniques such as block copolymer lithography.by Shih-wei Chang.Ph.D

From high-content to super-resolution investigation of cell behaviour on nanostructured surfaces

The environment in which cells find themselves is a complex, three dimensional one which provides a variety of inputs and cues capable of controlling and guiding cell behaviour. These environmental signals take a fertilised egg through development to become an adult human being made up of trillions of cells. As such, the power of environmental cues to provide context and guidance to cell behaviour cannot be understated. Without attempting to directly mimic the in vivo environment, it has been shown that micro- and nanostructured surfaces can influence cell behaviour when we try and engineer biology in vitro. Identification and optimisation of powerful topographies is, however, tedious, and so this thesis provides techniques to expedite the discovery of new and potent surfaces to drive cell behaviour. A new fabrication technique has been developed which allows for the fabrication of gradients of feature height at both the micro- and nanoscale. This involves the use of plasma polymer gradients as novel etch masks alongside existing lithographic techniques. After fabrication and mass replication by injection moulding, use of these surfaces as platforms for the high-content screening of cell response is demonstrated. These can be considered high-content due to both the range of surface structures on a single sample, and also the microscopy techniques used to investigate cell response. Distinct cell types were found to respond differently to topographical cues, exhibiting varying degrees of alignment, proliferation, and organisation in both mono- and co-culture systems. A new cell culture device has also been developed and patented which ensures that screening experiments begin with an accurate and repeatable distribution of cells across the high content array. The impact of uneven cell seeding on studies involving stem cell differentiation was also investigated – showing the importance of improved control. Finally, the interaction of cells with such nanostructured surfaces is investigated using new super-resolution microscopy techniques. New methods are presented for the correlation of multiple nanoscale imaging techniques to view cell-nanostructure interactions with unprecedented resolution. This reveals insights into the way in which the cellular substructure is being modulated by underlying nanotopography. Indeed, it paints a picture which is remarkably different to the structure observed under a standard widefield microscope over the past 10 years

3D integration of micro- and nanostructures into bio-analytical devices

This study aims to develop a process which allows 3D integration of micro and nanostructures in microchannels. A fabrication process was established for the large area integration of hierarchical micro and nanostructures in microchannels. This novel process, which is called 3D molding, takes advantage of an intermediate thin flexible stamp such as PDMS from soft lithography and a hard mold such as brass from hot embossing process. However, the use of a thin intermediate polydimethylsiloxane (PDMS) stamp inevitably causes dimensional changes in the 3D molded channel, with respect to those in the brass mold protrusion and the intermediate PDMS stamp structures. We have investigated the deformation behavior of the 3D molded poly(methyl methacrylate) (PMMA) substrate and the intermediate PDMS stamp in 3D molding through both experimentation and numerical simulation. It was found that for high aspect ratio brass mold protrusion, the maximum strain of the intermediate layer occurs in the bottom center of the 3D channels. However, with decreasing the aspect ratio of brass mold protrusion the highest elongation occurs at the bottom corners of the channel causing less elongation of the intermediate PDMS stamp and imprinted structures on the bottom surface of the 3D channel. A modified 3D molding process which is called 3D nanomolding is developed which allows nanopatterning the surface of small microfeatures. Using 3D nanomolding process and solvent assisted bonding microdevices with no side, one side, three sides and four sides patterned were fabricated. To characterize 3D flow patterns induced by the surface structures on microdevices, confocal microscopy was used as dyed water and undyed water injected from separate inlets of micromixer were mixed along the microchannel at flow rates of 10 and 40 μL/min. The standard deviation of the normalized intensity measured in the confocal image of the cross section of the channel was used for quantifying the degree of mixing and evaluating the mixing performance of all four different microdevices. Experimental and simulation results show that by patterning the surface of the micromixer, flow patterns can be manipulated, which can improve mixing through stretching and folding of fluid elements and therefore increasing the interfacial area between fluids and cutting down the diffusion length. The effect of increasing velocity on increasing standard deviation (decreasing mixing) was also found to be less for the micromixers whose surfaces are patterned compared to the plain channel

Bio-oriented Micro- and Nano- Structures Based on Stimuli-responsive Polymers

Nowadays, the ability to pattern surfaces on the micro- and nano- scale is the basis for a wide range of research fields. Over last few decades, a lot of processing technologies offer the possibility to fabricate complex 2D and 3D polymeric designs which are mostly static in nature since they cannot be physically and chemically modified once fabricated. The aim of the present thesis is to overcome such a limitation, exploiting stimuli-responsive materials (Chapter I). We allow to engineer polymeric architectures adding interesting functionalities, by providing an active manipulation of pre-structured systems, which could be helpfully in a wide variety of applications, such as biosensing and cell conditioning. In the first part of the present dissertation (Chapter II), a thermos-sensitive material is employed. We investigate the thermo-responsive behavior of Poly(N-isopropylacrylamide) (pNIPAAm)-based crosslinkable hydrogel as active binding matrix in optical biosensors. In this study, we propose an extension of surface plasmon resonance (SPR) and optical waveguide mode (OWS) spectroscopy, for in situ observation of nano-patterned hydrogel film that are allowed to swell and collapse by varying the external temperature of the aqueous environment. Weak refractive index contrast of hydrogel structures arranged in periodic pattern, is generally associated with intrinsically low diffraction efficiency. In order to enhance the intensity of diffracted light, the surface is probed by resonantly excited optical waveguide modes, taking advantage of the fact that the hydrogel can serve as optical waveguide (HOW) enabling the excitation of additional modes besides surface plasmons. Thus, we provide a hydrogel optical waveguide-enhanced diffraction measurements, taking advantage of strong electromagnetic field intensity enhancements that amplifies the weak diffracted light intensity. The main part of the thesis is focused in the study of azopolymer-containing materials, a specific class of light-responsive materials. Upon photon absorption, azobenzene undergo reversible trans-cis photoisomerization, which induces a substantial geometrical change of its molecular structure, that can be translated into larger-scale movements of the material below the glass transition temperature (Tg) of the polymer. In Chapter III, by exploiting the light-induced mass migration phenomenon, we demonstrate that an azopolymeric film patterned by soft imprinting technique, can be anisotropically deformed and consequent restored in its initial shape via single irradiation just by controlling the polarization state of the incident laser beam. We also propose that the light-driven morphological manipulation can induce anisotropic wettability changes. Lastly, a polarization driven birefringence effect on flat and structured surfaces is discussed. Chapter IV focuses in the design of novel azopolymeric systems, where the optical response is provided by azobenzene molecules, which doped two different host materials. The photo-responsive behavior and potential applications of azo compounds incorporated into either a soft elastomeric and in rigid matrix is discussed. Azo-embedded poly(dimethylsiloxane) (PDMS) is studied as tunable optical lens and an azo-doped photocurable commercial polymeric resin is developed to study the photo-mechanical transduction of a 3D suspended membrane fabricated by two photon lithography technique. In Chapter V, we propose a light-deformable azopolymeric micro-pillars patterned substrate as a biocompatible and “smart” platform for dynamic material-cell observation in 2D environment, modified by a holographic optical conditioning. The aim is to observe by time-lapse acquisitions, how an in situ deformation of a pre-patterned structure can influence cell functions and fate. Finally, in Chapter VI, general remarks of the present work are discussed, and directions for future perspective are summarized

Functional surface microstructures inspired by nature : From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

Functional surface microstructures inspired by nature – From adhesion and wetting principles to sustainable new devices

In the course of evolution nature has arrived at startling materials solutions to ensure survival. Investigations into biological surfaces, ranging from plants, insects and geckos to aquatic animals, have inspired the design of intricate surface patterns to create useful functionalities. This paper reviews the fundamental interaction mechanisms of such micropatterns with liquids, solids, and soft matter such as skin for control of wetting, self-cleaning, anti-fouling, adhesion, skin adherence, and sensing. Compared to conventional chemical strategies, the paradigm of micropatterning enables solutions with superior resource efficiency and sustainability. Associated applications range from water management and robotics to future health monitoring devices. We finally provide an overview of the relevant patterning methods as an appendix

3D self-folding tissue engineering scaffold origami

In the field of tissue engineering complex 3D architecture has become increasingly relevant in the pursuit of precisely engineered control over living tissue. It is needed to recreate the heterogeneous and complex arrangements of cells seen in nature, and to be able to influence their proliferation, differentiation and fate. A method for the 3D structuring of cells is therefore desired and is something standard lithographic methods cannot provide - the precision engineered 3D cellular niche. This work transfers traditional 2D lithographic techniques used in MEMS (E-beam lithography, photolithography, soft lithography and nanoimprint lithography) to the construction of 3D as well as complex hierarchical structures compatible with cell culture. To address this, hydrogel bilayers act as biocompatible, flexible and environmentally responsive hinges to fold the 2D structure into a 3D conformation. To achieve this, a rapid method of producing nanopatterns with the potential for large area patterning was developed. These were fluorinated ethylene propylene (FEP) and polydimethylsiloxane (PDMS) replica stamps with 2D and 2.5D hierarchical patterns. They were capable of bending and conforming to uneven and curved surfaces. These were used in a novel combinational lithography approach to construct complex hierarchical structures by photolithography through photomasks with nanopatterned transparent FEP inlays to create unfolded 3D cellular niches by a 2D method. Several different hydrogels were synthesised and patterned by photolithography to be used as bilayer hinges. Actuation mechanisms included thermoresponsive N-isopropylacrylamide (NIPAAm), and anionic acrylic acid (AA) monomers. Successful bilayers were formed using acrylate based photochemistry with poly(ethylene glycol) dimethacrylate (PEGDMA) and pH responsive polyacrylic acid (PAA) in a novel sacrificial layer functionalisation method. These structures would bend and roll due to differential swelling in neutral pH and when acting as a hinge would result in self-folding of photolithographically defined 2D structures into 3D containers. To test the compatibility of this method of manufacture with cell culture hESCs were trialled on the container materials, and showed excellent adhesion on the SU8 structures. More ambitiously to see if they could in the future be used for the directed differentiation of stem-cells, hESCs were cultured on nanopatterned injection moulded polymer substrates with varying nanofeature type. It was found that hESEs had improved adhesion on vitronectin coated nanotopographies even at extremely low vitronectin concentrations, and showed an increased 3D colony structure leading to the enhanced expression of certain lineage markers. It was found that hESC attachment could be mediated by feature height and substrate elasticity. This work has demonstrated as a proof-of-principle, a rapid and simple method of producing nanopatterned 3D self-folding containers, compatible with cell culture which could in the future serve as 3D self-folding nanopatterned cellular niches for tissue engineering