3,478 research outputs found

    Extreme mechanical resilience of self-assembled nanolabyrinthine materials

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    Low-density materials with tailorable properties have attracted attention for decades, yet stiff materials that can resiliently tolerate extreme forces and deformation while being manufactured at large scales have remained a rare find. Designs inspired by nature, such as hierarchical composites and atomic lattice-mimicking architectures, have achieved optimal combinations of mechanical properties but suffer from limited mechanical tunability, limited long-term stability, and low-throughput volumes that stem from limitations in additive manufacturing techniques. Based on natural self-assembly of polymeric emulsions via spinodal decomposition, here we demonstrate a concept for the scalable fabrication of nonperiodic, shell-based ceramic materials with ultralow densities, possessing features on the order of tens of nanometers and sample volumes on the order of cubic centimeters. Guided by simulations of separation processes, we numerically show that the curvature of self-assembled shells can produce close to optimal stiffness scaling with density, and we experimentally demonstrate that a carefully chosen combination of topology, geometry, and base material results in superior mechanical resilience in the architected product. Our approach provides a pathway to harnessing self-assembly methods in the design and scalable fabrication of beyond-periodic and nonbeam-based nano-architected materials with simultaneous directional tunability, high stiffness, and unsurpassed recoverability with marginal deterioration

    Polymer Pen Printing: A Tool for Studying 2D Enzymatic Lithography and Printing 3D Carbon Features

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    Polymer Pen Lithography (PPL) is a promising molecular printing approach which combines the advantages of both microcontact printing (low cost, high-throughput) and the dip pen lithography (DPN) (arbitrary writing, high-resolution) into one cohesive lithography method to create 2 dimensional (2-D) patterns with micro/nano-features on different substrates. The goal of this dissertation is to design and develop a new tool based upon PPL, which is not limited to forming 2D parallel patterns, but can also create 3D complex microstructures, finding applications in both biotechnology and Micro-Electro-Mechanical systems (MEMS) technology. This novel approach is named Polymer Pen Printing. Different from PPL using traditional dry-ink printing methods, an inking step is added to each printing repetition in the polymer pen printing process. Thus a wide range of ink materials with diverse viscosities can be transferred to substrates to create functional 2D and 3D microstructures. The polymer pen printing apparatus used in this thesis has been accomplished and introduced in Chapter 2. As a preliminary attempt, the single polymer pen printing approach was developed by simply attaching a solid polydimethylsiloxane (PDMS) pen tip to a multi-axis robot for small microarray fabrication. Compared to the single pen printing method, multi-pen printing can create large arrays of features. Therefore, an improved apparatus for polymer pen printing with high-throughput was discussed and built. Silicon molds, which consist of hundreds of uniform pyramidal openings, were photolithographically defined and etched using hydrofluoric acid (HF) followed by potassium hydroxide solution; after surface-modification with fluorosilane, these silicon molds were used to cast arrays of PDMS pyramidal pen tip. The cast PDMS pen array was mounted to a hollow holder with a 45° mirror inside. Therefore, each PDMS pen can be observed and monitored from the microscope on the side. To achieve prints less than 1 micron across, a Z axis stage with nanometer resolution was incorporated; and to control the compression of PDMS pen tips, a force gauge was also incorporated to detect 1 mg of applied force from the tips. The printing process for the multi-pen system is almost the same as single pen system. PDMS pens are coated with ink solution before each printing cycle by dipping into an inkwell and then brought into contact with the substrate surface. Thus multiple patterns, one from each tip, are created in parallel simultaneously. Furthermore, with control of the printing force, feature sizes could be controlled over the range submicron to tens of microns. Three ink candidates have been printed by polymer pen printing approach to fabricate 2D&3D microstructures. The first ink material is Barium Strontium Titanate (BST) nanocrystallites dispersed in a furfuryl alcohol (FA), which was printed by the single PDMS pen with 100 μm tip diameter (Chapter 3). After printing, samples were heated to crosslink FA monomers, forming a stable polymeric matrix with embedded BST nanocrystallites. Without shear-thinning properties, BST/FA ink cannot be used to build 3D posts, but it has the capability to create circular patterns with different thickness by the single or multi-tier deposition method. It was found that the thickness of film increased linearly with the number of deposits without changing the diameter significantly. This encouraging result could enable the formation of microcapacitors with multi-tiered structure. Moreover, the study of printing parameters, including printing height and ink pick-up position, shows that changes to the pen positions in the ink reservoir or substrate have essentially no impact on deposit thickness or diameter. Beyond that, the effect of surface chemistry of PDMS pen and silicon wafer have also been studied. The plasma treated hydrophilic PDMS pen can pen transfer more BST/FA than untreated one; and the larger diameters with smaller thickness were obtained on a hydrophilic silicon wafer. The second ink candidate is a dilute aqueous solution of enzyme Candia antartica lipase B (CALB), which is known to catalyze the decomposition of poly (ε-caprolactone) (PCL) films. By bringing enzymes into contact with pre-defined regions of a surface, a polymer film can be selectively degraded to form patterned features that are requited for applications in biotechnology and electronics. This so-called enzymatic lithography is an environmentally friendly process as it does not require any actinic radiation or synthetic chemicals to develop required features. But the need to restrict the mobility of the enzyme in order to maintain control of feature sizes poses a significant challenge. In Chapter 4, after writing 2D enzyme patterns onto a spin-cast PCL film by single pen printing, samples with CALB were incubated at 37 ℃ and 95% relative humidity (RH) for up to 7 days to develop features. The CALB selectively degraded the PCL film during incubation, forming openings through the film. The size of these features (10 to 50 μm diameter) is well suited for use as biocompatible micro-reactors. Previous study of patterning CALB by single polymer pen printing technique resulted in slow etch rates, low throughput and poor image quality. In Chapter 5, I present an improved enzymatic lithography approach, still based on enzyme CALB and PCL system, which can resolve fine-scale features (\u3c 1 μm across) in thick (0.1 - 2.0 μm) polymer films after 5 minutes to 2 hours of incubation at 37 ℃ and 87% RH. Immobilization of the enzyme on the polymer surface was monitored using fluorescence microscopy by labeling CALB with FITC. The crystallite size in the PCL films was systematically varied; small crystallites resulted in significantly faster etch rates (20 nm/min) and the ability to resolve smaller features (as fine as 1 μm). The effect of printing conditions and RH during incubation is also presented. Patterns formed in the PCL film were transferred to an underlying copper foil demonstrating a Green approach to the fabrication of printed circuit boards. In parallel, the third ink material is a mixture of 25 wt% graphite dispersed in a high viscosity phenolic resin n-methyl-2-pyrrolidone (NMP) solution, which can be converted into carbon/carbon composites after a pyrolysis process. The 3D polymeric posts were created by depositing multilayers of thixotropic phenolic ink on a silicon substrate by single polymer pen printing method with a 10 μm radius PDMS pen tip (Chapter 6). After pyrolysis at 1000 ℃ in a nitrogen (N2) atmosphere, the polymeric features were converted to the glassy carbon/graphite features with a high aspect ratio (\u3e2). These features may be used as microelectrodes. Last, arrays of needle-shaped glassy carbon have been developed by a drawing approach using multi-pen printing technique followed by simple pyrolysis process (Chapter 7). To build polymeric needles with ultra-high aspect ratio, the polymeric ink was prepared by dissolving phenolic resin in the high boiling point (204 ℃) solvent NMP without fillers to achieve good printability and suitable viscosity. By slowly lifting up the print head from substrate, liquid needle structures were formed and then solidified on silicon substrates or gold electrodes due to the solvent evaporation. In addition, suspended resin fibers connected to two electrodes have also been fabricated by precisely controlling the movement of the PDMS pen. After pyrolysis, these resin features were converted to glassy carbon and the 3D structures remained. The electrical characterization results showed that glassy carbon made by this method had relatively low resistivity (2.5 x 10-5 Ωm). Therefore the glassy carbon based microneedles are well-suited to be electrodes for electrochemical sensors for biological applications

    Microscale Strategies for Generating Cell-Encapsulating Hydrogels

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    Hydrogels in which cells are encapsulated are of great potential interest for tissue engineering applications. These gels provide a structure inside which cells can spread and proliferate. Such structures benefit from controlled microarchitectures that can affect the behavior of the enclosed cells. Microfabrication-based techniques are emerging as powerful approaches to generate such cell-encapsulating hydrogel structures. In this paper we introduce common hydrogels and their crosslinking methods and review the latest microscale approaches for generation of cell containing gel particles. We specifically focus on microfluidics-based methods and on techniques such as micromolding and electrospinning.National Science Foundation (U.S.) (DMR0847287)National Institutes of Health (U.S.) (EB008392)National Institutes of Health (U.S.) (DE019024)National Institutes of Health (U.S.) (HL099073)National Institutes of Health (U.S.) (AR057837)National Institutes of Health (U.S.) (HL092836)United States. Army Research Office (contract W911NF-07-D-0004)United States. Army Research Office (Institute for Soldier Nanotechnology)United States. Army. Corps of EngineersInnovative Med Tech (Postdoctoral fellowship

    Freeform terahertz structures fabricated by multi-photon lithography and metal coating

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    Direct-write multi-photon laser lithography (MPL) combines highest resolution on the nanoscale with essentially unlimited 3D design freedom. Over the previous years, the groundbreaking potential of this technique has been demonstrated in various application fields, including micromechanics, material sciences, microfluidics, life sciences as well as photonics, where in-situ printed optical coupling elements offer new perspectives for package-level system integration. However, millimeter-wave (mmW) and terahertz (THz) devices could not yet leverage the unique strengths of MPL, even though the underlying devices and structures could also greatly benefit from 3D freeform microfabrication. One of the key challenges in this context is the fact that functional mmW and THz structures require materials with high electrical conductivity and low dielectric losses, which are not amenable to structuring by multi-photon polymerization. In this work, we introduce and experimentally demonstrate a novel approach that allows to leverage MPL for fabricating high-performance mmW and THz structures with hitherto unachieved functionalities. Our concept exploits in-situ printed polymer templates that are selectively coated through highly directive metal deposition techniques in combination with precisely aligned 3D-printed shadowing structures. The resulting metal-coated freeform structures offer high surface quality in combination with low dielectric losses and conductivities comparable to bulk material values, while lending themselves to fabrication on planar mmW/THz circuits. We experimentally show the viability of our concept by demonstrating a series of functional THz structures such as THz interconnects, probe tips, and suspended antennas. We believe that our approach offers disruptive potential in the field of mmW and THz technology and may unlock an entirely new realm of laser-based 3D manufacturing

    Creating Collagen 3D Microenvironment by Developing Pneumatic Actuated Soft Micromold (PASMO) for Biological Application

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    Small volume, high surface area and protective fibulas scaffold of collagen modular microenvironment improves cell viability and proliferation. Therefore, the ability to produce collagen modular microenvironment accurately and reliably is of most importance to the advancement of tissue engineering. Currently, no such fabrication technique exists due to the inherent fragility of collagen. Herein, we report the very first platform that addresses such challenges. Pneumatic actuated soft micro mold (PASMO) composes asymmetric structure, which performs different mechanical properties. PASMO device is classified as particles’ template (top layer), air channel layer (middle layer) and resistance(bottom layer). The major deformation of PASMO is assigned to particles’ template layer because the mechanical property of particles’ template is less than resistance layer. Therefore, the deformation would form on particles’ template to expand and extract microparticles from PASMO device after increasing inner pressure. Soft micro mold with pneumatic extraction actuator not only can produce arbitrary shapes of collagen microstructures precisely but also can encapsulate cells inside without causing damage during the extraction process. MDA-MB-231-GFP encapsulated in collagen microcubes can fully stretch and survive well. Moreover, MDA-MB-231-GFP in collagen microcubes can sense the treatment of Paclitaxel and the size of microcubes changed as following the different concentration of Paclitaxel due to the inhibition of cellar division. Furthermore, creating cancer microenvironment can efficiently localize cancer cells at the specific location so that it minimizes the varieties of experiments. Another application is for cell therapy, like beta cells encapsulation for creating artificial islet. Artificial islets are micro-disk can secrete insulin based on the stimulation of glucose from the surrounding. Blood vessels can successfully form around the implanted artificial islets. This formation of the blood vessel for subcutaneous transplantation not only can regulate the level of glucose in blood but also can simplify surgery to avoid the risk. For multiple locations of implanted artificial islets clusters, blood vessels can also form connections between these two groups

    Creating Collagen 3D Microenvironment by Developing Pneumatic Actuated Soft Micromold (PASMO) for Biological Application

    Get PDF
    Small volume, high surface area and protective fibulas scaffold of collagen modular microenvironment improves cell viability and proliferation. Therefore, the ability to produce collagen modular microenvironment accurately and reliably is of most importance to the advancement of tissue engineering. Currently, no such fabrication technique exists due to the inherent fragility of collagen. Herein, we report the very first platform that addresses such challenges. Pneumatic actuated soft micro mold (PASMO) composes asymmetric structure, which performs different mechanical properties. PASMO device is classified as particles’ template (top layer), air channel layer (middle layer) and resistance(bottom layer). The major deformation of PASMO is assigned to particles’ template layer because the mechanical property of particles’ template is less than resistance layer. Therefore, the deformation would form on particles’ template to expand and extract microparticles from PASMO device after increasing inner pressure. Soft micro mold with pneumatic extraction actuator not only can produce arbitrary shapes of collagen microstructures precisely but also can encapsulate cells inside without causing damage during the extraction process. MDA-MB-231-GFP encapsulated in collagen microcubes can fully stretch and survive well. Moreover, MDA-MB-231-GFP in collagen microcubes can sense the treatment of Paclitaxel and the size of microcubes changed as following the different concentration of Paclitaxel due to the inhibition of cellar division. Furthermore, creating cancer microenvironment can efficiently localize cancer cells at the specific location so that it minimizes the varieties of experiments. Another application is for cell therapy, like beta cells encapsulation for creating artificial islet. Artificial islets are micro-disk can secrete insulin based on the stimulation of glucose from the surrounding. Blood vessels can successfully form around the implanted artificial islets. This formation of the blood vessel for subcutaneous transplantation not only can regulate the level of glucose in blood but also can simplify surgery to avoid the risk. For multiple locations of implanted artificial islets clusters, blood vessels can also form connections between these two groups

    Photophysical And Photochemical Factors Affecting Multi-photon Direct Laser Writing Using The Cross-linkable Epoxide Su-8

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    For the past decade, the epoxy based photoresist SU-8 has been used commercially and in the lab for fabricating micro- and nano-structures. Investigators have studied how processing parameters such as pre- and post-exposure bake temperatures affect the resolution and quality of SU-8 structures patterned using ultraviolet or x-ray lithography. Despite the advances in understanding the phenomena, not all of them have been explored, especially those that are specific to multi-photon direct laser writing (mpDLW). Unlike conventional exposure techniques, mpDLW is an inherently three-dimensional (3D) process that is activated by nonlinear absorption of light. This dissertation reports how several key processing parameters affect mpDLW using SU-8 including pre-exposure bake duration, focal depth, incident laser power, focal-point scan speed, and excitation wavelength. An examination of solvent content of films at various stages in the mpDLW by 1H-NMR shows that even moderate solvent content (over 1 wt-%) affects film viscosity and photoacid diffusion lengths, and can greatly affect the overall fidelity of small features. A study of micro-fabricated feature size versus writing depth in the material shows that even slight refractive index mismatch between SU-8 and the medium between it and the focusing objective introduces spherical aberration that distorts the focus, causing feature size to decrease or even increase in size with writing depth, depending on the average exposure power used. Proper adjustment of the average exposure power was demonstrated as a means to fabricate more uniform features with writing depth. Third, when varying the power and scan speed, it was observed that the feature-size scales with these two parameters in a manner that is consistent with a three-photon absorption mechanism at an excitation wavelength of 800 nm. When an iii excitation wavelength of 725 nm is used, the feature-size scaling becomes consistent with that of two photon absorption. This shows that the photoinitiators in the SU-8 can be activated by either two- or three-photon absorption over this wavelength range. Using an irradiance of ~2 TW cm-2 and elongated femtosecond pulses resulted in an observed fourth order power dependence. This observation is in agreement with the literature and suggests that the effective absorptive nonlinearity is also sensitive to pulse duration. These findings will be useful for creating accurate models of the process of mpDLW in SU-8. These models could be used to optimize the processing parameters and develop new processing methods and materials for high-resolution fabrication of robust 3D microstructures. Some of the findings were used to develop a method for fabricating functional microlenses on the tip of optical fibers. This approach opens a new route to functional integrated photonic devices

    Mechanical BioMEMS Technologies for Advanced Label-free Sensing of Biomolecular Species in Microfluidic Channels

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    The aim of this PhD project is to investigate alternative sensing methodologies that can possibly improve the sensing performances of lab–on–chip (LOC) designed for biochemical applications. Suspended microchannel resonator (SMR) for bio–mechanical sensing applications have become very popular as detection of weigths of chemicals integrated in LOC. They exploit laser doppler vibrometry (LDV) for dynamic mode detection. In this thesis two different SMRs designs have been investigated, involving either technological challenges – the use of polymers as material and processing techniques based on laser micromachining – and different sensing phenomena – the use of the parametric resonance rather than the standard harmonic resonance response. The flexibility of two–photon direct laser writing is exploited to optimize a highly– versatile fabrication strategy based on a shell–writing procedure with the aim to reduce fabrication time of big inlet/outlet sections compatible with most microfluidic systems for LOCs. With respect to standard microfabrication techniques, requiring several technological steps to obtain suspended hollow structures, this method allows to fabricate complex SMR sensors in only one fabrication step, by virtue of its intrinsically three– dimensional nature. A SMR fixed-fixed beam has been fabricated and characterized by LDV. A different sensing mechanism based on the parametric resonance instead of the harmonic resonance has been investigated to develop a novel platform for the characterization of biomolecules in free–flow with unique specificity, sensitivity, and speed: to this purpose a PDMS based device was realized by laser machining, a rapid prototype fabrication technique; beside to it, a commercial fused silica capillary tubing was also employed in the realization of a prototype for this sensing mechanism, and both solutions were tested through LDV
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