SYNTHETIC GECKO INSPIRED DRY ADHESIVE THROUGH TWO- PHOTON POLYMERIZATION FOR SPACE APPLICATIONS

This work aims to develop an advanced and cost-effective fabrication process to produce a simplified gecko-inspired microstructure with two-photon polymerization and polymer molding, aimed to improve the adhesive properties of microstructures. Such adhesive microstructures can be implemented for multi-purpose adhesive grasping devices, which have recently gained significant interest in the space exploration sector. Previous gecko-inspired microstructures were reviewed, and the new gecko-inspired microstructures have been developed with the adaptation of additive manufacturing methods for facile fabrication. The examined microstructures in this thesis were the tilted mushroom-shaped and wedge-shaped designs, which could both maximize adhesion by shearing the micropillars toward the tilted direction when preload force is applied. The improved microstructure fabrication process could produce micropillars in the height of 270 μm with soft polymer without defects. However, the miniaturized micropillars in the height of 40 μm, frabricated with the same process, had broken tips and missing structures. The effects of the scale, height, and shape of the micropillars in controllable dry adhesion were investigated through the experiments. The adhesion of the microstructures with artificial gecko setae in the height of 270 μm was 2 times higher than the microstructures with 40 μm of height. Meanwhile, the microstructures that consisted of long and short artificial gecko setae had inferior adhesive performance to the microstructures having uniform long setae on all tested surfaces. Meanwhile, the result showed no direct correlation between the surface roughness of the attached surface and the adhesive performance of the microstructures. The wedge-shaped design was determined to have higher adhesion than the tilted mushroom-shaped design due to lower structural resistance on bending and higher effective contact area

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

Development of microstructures for application on a controllable bioinspired adhesive mechanism for gripping system in pick-n-place task

Dissertação de mestrado integrado em Engenharia MecânicaA natureza oferece uma variedade de ideias para uma adesão transitória e reversível a diferentes substratos. Até agora, os soft dry adhesives (SDAs) bioinspirados mais estudados são superfícies com matrizes de micropilares. A inspiração veio de espécies terrestres, como as osgas, cujas almofadas dos pés são cobertas por intrincadas fibrilhas que permitem uma forte adesão (que se deve a forças intermoleculares) bem como uma fácil libertação. Um dos objetivos atuais dos esforços da investigação é transferir a solução da natureza para estruturas artificiais que possam um dia encontrar aplicações tecnológicas. Este trabalho visa replicar os comportamentos de agarra e libertação das osgas, utilizando como base as suas estruturas fibrilares pegajosas. Para este objetivo, serão utilizadas várias técnicas de fabrico e ensaios experimentais para determinar o melhor protocolo para a criação de microestruturas. Foram estudados micropilares cilíndricos lisos e micropilares com forma de cogumelo, tendo sido escolhidos estes últimos dado que aderem melhor aos substratos lisos em comparação com pilares cilíndricos. O presente trabalho começou por delinear o estado de arte, no qual se investiga o desenvolvimento de SDAs bioinspirados e as suas qualidades adesivas. Além disso, também foram abordados os fundamentos de adesão fibrilar. Prosseguiu-se para o desenvolvimento de amostras em polidimetilsiloxano (PDMS) com o objetivo de caracterizar este material num equipamento de ensaio universal (UTM). Neste trabalho foram examinadas várias técnicas de microfabricação. Tendo em consideração a dimensão das micropartículas utilizadas, foram produzidas microestruturas utilizando uma metodologia de baixo custo. Foram utilizados e testados vários tipos de métodos para fabricar os moldes, ou seja, para produzir pilares cilíndricos lisos foi utilizada fresagem e para produzir micropilares em forma de cogumelo foi utilizada impressão 3D. Devido à sua forma, os micropilares em forma de cogumelo requerem uma dupla moldagem, com um molde intermediário constituído por um material altamente flexível. Finalmente, com o auxílio do UTM para realizar testes de aderência, foi avaliada a eficiência das microestruturas.Nature offers a variety of ideas for transient and reversible adhesion to different substrates. Geckos and insects use hairy structures whose adhesion is due to intermolecular forces. So far, the most widely studied SDAs are surfaces with arrays of micropillars. The inspiration came from terrestrial species including lizards and geckos whose toe pads are covered by intricate fibrils that enable strong attachment as well as easy release. The current goal of research and development efforts is to transfer nature's solution into artificial structures that might someday be applied in different technologies. Hence, this work aims to replicate the grasping and releasing behaviors of geckos using their fibrillar sticky structures as a basis. In order to achieve this goal, different kinds of designs, fabrications, and testing will be used to determine the best protocol for creating microstructures. Smooth cylindrical and mushroom-shaped micropillars were studied. The latter were chosen because they adhere to smooth substrates better than cylindrical micropillars. This work began by outlining the state of the art on the development of soft dry adhesives (SDAs) with natural inspiration and an examination of their dry adhesive properties. Additionally, the fundamentals of fibrillar adhesion were also covered. The work then proceeded to the development of polydimethylsiloxane (PDMS) specimens with the goal of characterizing this material in a universal testing machine (UTM). A number of microfabrication techniques were examined. Based on the size of the employed microparticles, microstructures were produced applying a low-cost method. Different methods were employed depending on the shape of the molds, i. e., to produce cylindrical flat pillars, it was used a CNC milling machine whereas to produce mushroom shaped micropillars, it was used 3D printing. Due to their design, the micropillars with the mushroom shape required double molding with an intermediary mold made of a highly flexible material. Finally, using the UTM to perform adhesion tests, the efficiency of the microstructure was evaluated

Development of self-cleaning polymeric surfaces using polymer processing systems for application to high-voltage insulators

Herein, polymer processing systems are used to fabricate superhydrophobic high-temperature vulcanized (HTV) silicone rubber surfaces by direct replication. HTV silicone rubber is one of the main polymeric housing materials used in high-voltage insulators. The selected polymer processing techniques are compression molding and injection molding.The direct replication approach requires that a template or insert having the desired surface patterns be replicated onto a target polymer surface via a polymer processing. The appropriate micro-nanostructures, required for achieving ultra-water-repellency, were created on the insert materials (an aluminum alloy) using a wet-chemical etching method. As a flawless demolding is essential to acquire desirable replication quality, an antistiction coating was applied to the insert surfaces prior to the molding process to ensure the thorough removal of the silicone rubber during the demolding. The resulting silicone rubber surfaces possessed micro-nanostructures producing a water contact angle (WCA) of >160° and a contact angle hysteresis (CAH) of <3°. The surface roughness of the aluminum inserts was optimized at HCl concentrations of 15 wt.%. The self-cleaning properties of the produced ultra-water-repellent silicone rubber surfaces were rigorously investigated to ensure a self-cleaning surface at real outdoor imitated conditions. The presence of air pockets in between the surface asperities produced the Cassie-Baxter regime. The consistency of these air pockets is crucial for attaining the self-cleaning properties. A series of tests, including droplet impact, water-jet impact, trapped air layer, and severe droplet contact tests were conducted to confirm the stability of the Cassie-Baxter regime. A comprehensive series of self-cleaning experiments involving both suspended and non-suspended contaminants, e.g., kaolin, carbon black, and silica as well as contaminant-applying methods, e.g., dropwise, spraying, wet or dry contamination were performed. Self-cleaning tests were organized from less severe, i.e., non-suspended contamination tests, to severe, i.e., the wet suspended contamination test, to most severe, i.e., the dry suspended contamination test. Due to their ultra-low CAH, the produced surfaces demonstrated favorable self-cleaning properties against the various types of contaminants and the different means of contaminant application. The produced surfaces retained their water repellency following the application of the contaminants and successful cleaning of the surfaces, thereby verifying the self-cleaning performance and resistance of the fabricated superhydrophobic silicone rubber surfaces. The anti-icing properties (delayed ice formation) and de-icing properties (reduced ice adhesion strength) of the produced surfaces were evaluated. Two types of icing (atmospheric glaze and bulk ice) were considered to accumulate ice on the surfaces. The well-known ice adhesion measurement techniques, i.e., the centrifuge adhesion and push-off tests were employed to provide quantitative comparisons of the ice adhesion strength of the produced surfaces. The produced surfaces significantly delayed ice formation and reduced the ice adhesion strength. To rigorously assess the durability of the produced surfaces, a comprehensive series of experiments that covered a wide range of real-life conditions were carried out. In some cases, where the water repellency was lost, the silicone rubber surfaces demonstrated a satisfactory recovery of their anti-wetting properties. Given the importance of replication quality in the direct replication of micro-nanostructures and the role of micro-nanostructures in the formation of superhydrophobic and icephobic surfaces, the effect of processing parameters on the superhydrophobicity, icephobicity, and replication quality in the compression molding of silicone rubber surfaces were evaluated. Curing time, mold temperature, molding pressure, and part thickness were assessed via response surface methodology to determine the optimal processing parameters. Molding pressure and part thickness were revealed as two main influencing parameters in the superhydrophobic properties. The crosslink density of the fabricated silicone rubber samples, however, was found to be significantly affected by curing time and mold temperature. Replication quality was determined for various molding pressures and part thicknesses. There was an optimal molding pressure value at each part thickness level to obtain the best replication quality. Surfaces having the highest replication quality showed the longest freezing delay reflecting their potential use as anti-icing surfaces. Although all superhydrophobic surfaces offered potential icephobic properties, identifying the influential parameters controlling ice adhesion was more complicated. As this PhD project is part of an industrial-academic collaboration, the results obtained in the laboratory experiments were used for implementation in the industry (K-Line Insulators Limited). This step includes the use of aluminum and stainless-steel inserts. Using the injection molding system available at K-Line Insulators Ltd., silicone rubber insulators having superhydrophobic properties were produced successfully. The industrial partner provided facilities to modify its mold to produce superhydrophobic insulators in an industrial scale. Dans cette thèse, les systèmes de transformation des polymères sont utilisés pour fabriquer des surfaces superhydrophobes de caoutchouc de silicone vulcanisé à haute température (HTV) à partir d’une réplication directe. Le HTV est l’un des principaux matériaux polymères utilisés dans la fabrication des isolateurs à haute tension. Les systèmes considérés sont des procédés de moulage par compression et de moulage par injection. L'approche de réplication directe nécessite un modèle ou un insert ayant les structures de surface souhaitée à répliquer sur la surface du polymère. Les micronanostructures appropriées pour obtenir la non-mouillabilité de la surface ont été créées sur les matériaux d'insert (alliage d'aluminium) en utilisant un procédé de gravure chimique. Comme un démoulage sans défaut est essentiel pour obtenir la qualité de réplication souhaitable, un revêtement antiadhésif est appliqué sur les surfaces de l'insert avant le processus de moulage afin d’assurer l'élimination complète du caoutchouc de silicone lors du démoulage. Les surfaces de caoutchouc de silicone développées possédaient des micronanostructures produisant un angle de contact eau (WCA) de > 160 ° et une hystérésis angle de contact (CAH) de < 3 °. La rugosité optimale de surface des inserts en aluminium est obtenue à une concentration massique de HCl de 15%. Les propriétés autonettoyantes des surfaces produites ont été rigoureusement étudiées pour assurer que ces propriétés autonettoyantes demeuraient efficaces dans des conditions extérieures réelles. La présence de poches d'air entre les aspérités de surface est responsable de la formation du régime de Cassie-Baxter. La consistance de ces poches d’air est cruciale pour obtenir des propriétés autonettoyantes. Par conséquent, une série d’essais ont été effectués pour confirmer la stabilité du régime Cassie-Baxter. Ensuite, une série complète d'expériences de propriétés autonettoyantes a été réalisée en impliquant des contaminants en suspension et non suspendus (non dispersés) utilisant divers matériaux (par exemple, le kaolin, le noir de carbone, la silice, etc.) et des méthodes d'application de contaminants (par exemple, goutte à goutte, pulvérisation, contaminants humides ou secs) ont été effectuées. Les tests d’autonettoyage ont été organisés, du test le moins sévère, c’est-à-dire de la contamination non suspendue (non dispersée), au test plus sévère, c’est-à-dire de la contamination en suspension humide, et se terminant par le test le plus sévère, à savoir la contamination en suspension sèche. En raison du CAH ultra-bas, les surfaces produites ont montré des propriétés autonettoyantes favorables contre les différents types de contaminants et de différents moyens d'application de contaminants. Les surfaces produites ont conservé leurs propriétés répulsives après l'application des contaminants et après le nettoyage des surfaces, permettant ainsi de vérifier les performances d'autonettoyage et la résistance des surfaces en silicone superhydrophobe fabriquées. Les propriétés anti-givrantes (la formation retardée de la glace) et les propriétés glaciophobes (la force d'adhérence réduite de la glace) des surfaces produites ont été évaluées. Les surfaces produites sont exposées à la formation de deux types de givrage. Les techniques bien connues de mesure de l'adhésion sur la glace, à savoir le test d'adhérence par centrifugation et le test de poussée, ont été utilisées pour obtenir une comparaison précise des résultats. Les surfaces superhydrophobes produites ont considérablement retardé la formation de glace et réduit la force d'adhérence de la glace. Afin d’évaluer de manière rigoureuse les propriétés de durabilité, une série complète d’expériences a été réalisée sur les surfaces. Les expériences de durabilité ont été menées pour couvrir un large éventail d'applications réelles. En ce qui concerne la capacité attractive du caoutchouc de silicone dans la récupération des propriétés anti-mouillantes, la perte de la propriété de répulsion de l’eau a été régénérée jusqu’à un niveau satisfaisant dans certains cas. Compte tenu de l’importance de la qualité de la réplication dans la réplication directe des micronanostructures d’une part, et d’autre part du rôle des micronanostructures dans la formation de surfaces superhydrophobes et glaciophobes, les effets des paramètres de moulage par compression des surfaces en caoutchouc de silicone sur la superhydrophobicité, la glaciophobicité et la qualité de la réplication ont été évaluées. Le temps de durcissement, la température de moulage, la pression de moulage et l'épaisseur de la pièce ont été choisis comme paramètres de traitement à évaluer. La méthodologie de surface de réponse a été utilisée pour déterminer les paramètres de traitement optimaux. Bénéficiant des résultats, la pression et l'épaisseur ont été révélées comme les deux paramètres d'influence principaux des propriétés superhydrophobes. La densité de réticulation des échantillons de caoutchouc de silicone fabriqués s'est toutefois révélée être significativement affectée par le temps et la température. Les valeurs de qualité de réplication ont été déterminées en fonction de diverses pressions et épaisseurs. Il y avait une valeur de pression optimale à chaque niveau d'épaisseur pour obtenir la meilleure qualité de réplication. Il a également été observé que les surfaces présentant la meilleure qualité de réplication affichaient le plus long retard de gel de la gouttelette d’eau, ce qui représentait leur potentiel élevé d'utilisation en tant que surfaces antigivrantes. Bien que toutes les surfaces superhydrophobes aient présenté des propriétés potentiellement glaciophobes, il a été constaté que le scénario d’adhérence sur la glace était plus compliqué en termes de paramètres influents. Ce projet de doctorat fait partie d'une collaboration industrielle-académique. Les résultats obtenus en laboratoire ont été utilisés pour la mise en œuvre dans l'industrie (K-Line Insulators Limited). À cette étape, des inserts en aluminium et en acier inoxydable ont été utilisés. En utilisant le système de moulage par injection disponible chez K-Line Insulators Ltd., des isolateurs en caoutchouc de silicone ayant des propriétés superhydrophobes ont été produits avec succès. Par conséquent, le partenaire industriel fournit des installations pour modifier son moule afin de produire des isolateurs superhydrophobes à l'échelle industrielle

Micro- and Nanostructured Microfluidic Devices for Localized Protein Immobilization and Other Biomedical Applications

A new immobilization method for the localized adsorption of proteins on thermoplastic surfaces is introduced. Artificial three-phase interfaces were realized by surface structuring to control the wetting behavior which lead to a preferred adsorption in these modified areas. Additionally, different fabrication methods were analyzed to determine mass fabrication capabilities. These fabrication methods also allowed the production of fully structured microchannels to tune the fluids behavior within

Micro- and Nanostructured Microfluidic Devices for Localized Protein Immobilization and Other Biomedical Applications

A new immobilization method for the localized adsorption of proteins on thermoplastic surfaces is introduced. Artificial three-phase interfaces were realized by surface structuring to control the wetting behavior which lead to a preferred adsorption in these modified areas. Additionally, different fabrication methods were analyzed to determine mass fabrication capabilities. These fabrication methods also allowed the production of fully structured microchannels to tune the fluids behavior within

Development of an optimal magneto-rheological elastomer for application on a controllable bioinspired adhesive mechanism

Dissertação de mestrado integrado em Engenharia MecânicaMagneto-rheological elastomers (MREs) are considered to be a type of smart material, as their rheological and mechanical properties can be changed when an external magnetic field is applied, as well as their shape or dimensions, which is known as magnetostriction. This unique feature makes this kind of elastomer a suitable candidate for the application on a controllable bioinspired adhesive mechanism, such as those that attempt to mimic the attachment pads of insects or geckos. MREs are composed of a polymeric matrix with embedded magnetic particles, along with other additives. In this work, magneto-rheological elastomers were fabricated, using PDMS Sylgard 184 as a matrix and three types of magnetic particles were tested: iron oxide, recycled iron and carbonyl iron particles. Each type of particle was characterized morphologically having been found that the carbonyl iron particles have a more homogenous size distribution, while the iron oxide and recycled iron particles have more tendency to form agglomerates of distinct sizes. By performing a magnetic characterization, it was found that carbonyl iron particles have exhibited a higher saturation magnetization and magnetic remanence. The effect of particle concentration on the mechanical properties of the MREs was investigated, through the performance of tensile and compressive tests for MREs with different mass concentrations of particles (20%, 40%, 60%, 80%). The results have shown that the Young’s modulus increase as the concentration of particles rises. A preliminary model for the magnetic force was developed and experimental measurements of this force were performed for the same MRE samples. It was found that the magnetic force is dependent and increases with particle concentration, as well as the magnetic field. The property change capacity was also investigated with compressive tests performed on the MRE samples with an indenter under an external magnetic field and without a magnetic field. It was expected to obtain a higher Young’s modulus for the tests performed under a magnetic field, but the opposite happened. Although the Young’s modulus of the samples was different under and without magnetic field, it cannot be concluded that this was due to the magnetic field, as other external factors could have influenced the obtained results. An MRE-based adhesive prototype was also developed, incorporating the MRE mixture into a micropillar mold, with adhesion tests being performed.Os elastómeros magneto-reológicos (MREs) são um tipo de material inteligente, uma vez que as suas propriedades reológicas e mecânicas, assim como as suas dimensões e forma, alteram na presença de um campo magnético externo. Este aspeto único torna este tipo de elastómeros como candidatos adequados para a aplicação num mecanismo de adesão controlável, de inspiração biológica. Os MREs são constituídos por uma matriz polimérica, na qual estão embebidas partículas magnéticas, assim como outros aditivos. Nesta dissertação, recorreu-se ao fabrico de MREs cuja matriz consistia em PDMS Sylgard 184, tendo sido testadas três tipos de partículas magnéticas: ferro carbonílico, óxido de ferro e ferro reciclado. Cada tipo de partícula foi caracterizado morfologicamente, tendo sido observado que as partículas de ferro carbonílico apresentavam uma distribuição de tamanho mais homogénea, enquanto que as partículas de óxido de ferro e de ferro reciclado mostraram tendência para formar aglomerados de tamanhos distintos. Através de uma caracterização magnética, concluiu-se que as partículas de ferro carbonílico exibiam maior saturação e remanência magnética. O efeito da concentração das partículas nas propriedades mecânicas dos MREs foi investigado, através da realização de testes de tensão e compressão, para MREs produzidos com diferentes concentrações, em massa, de 20%, 40%, 60%, 80% de partículas. Os resultados mostraram que o módulo de Young aumenta com o aumento da concentração de partículas. Um modelo da força magnética foi desenvolvido e testes experimentais de medição dessa força foram realizados. Os resultados demonstraram a dependência da força magnética em relação à concentração de partículas, assim como ao fluxo magnético. A capacidade de alteração das propriedades mecânicas com a aplicação de um campo magnético foi testada para as amostras de MREs, através da realização de um teste compressivo com e sem campo magnético. Era esperado que o módulo de Young aumentasse com a aplicação do campo magnético, tendo acontecido o oposto. Apesar de o módulo de Young registado ter sido diferente para as situações com e sem campo magnético, não pode ser concluído que essa diferença foi provocada pelo campo magnético, uma vez que outros fatores externos podem ter afetado os resultados. Por último, foi fabricado um protótipo de um adesivo, através da implementação de um MRE

NANOCHANNEL-ASSISTED ACTIVE CONTROL OF MASS TRANSPORT IN POLYDIMETHYLSILOXANE-BASED MICRO- /NANOFLUIDIC SYSTEMS

Department of Mechanical EngineeringNanofluidic devices have been extensively studied due to a fascinating nature of their small size which facilitates biosensing, bio-chemical separations, seawater desalination, nanofluidic transistors, protein, and preconcentration for a Lab-on-a-Chip (LOC). Such applications could be achieved by a control of electrokinetic transport in a nanochannel produced by sophisticated nanofabrication technique. However, it has been a challenge from a fabrication to the control of electrokinetic phenomena in nanochannel because of the cost, time, incompatibility, and addressability issues. Therefore, an innovative method is required to achieve simple fabrication and versatile operations of micro/nanofluidic device with limited resources. This dissertation proposes a new method for nanochannel-assisted active manipulation of mass transport by switching physicochemical environment. In the early chapters of this dissertation, unconventional fabrication methods for hybrid-scale micro-/nanofluidic devices is described by using both crack-photolithography and polydimethylsiloxane (PDMS) based soft lithography. The late chapters introduce the mechanism of the mass transport in micro/nanofluidic device using solutes gradient and humidity for manipulation of colloidal motion and molecule valves, respectively. These studies can be introduced as follows. First, crack-photolithography is employed to facilitate large-scale reproducible channel fabrication through a single molding process and thus enable the fabrication of hybrid-scale micro-/nanofluidic devices at a wafer level with advantages seen in the throughput, cost-effectiveness, reliability, and reproducibility. In addition, modified soft lithography process is developed to fabricate stable nanochannel which is free from the collapse and the crumbling. Second, crack-assisted nanochannel is introduced to manipulate physicochemical environment of neighboring microchamber. Diffusion-controlled ion transport produces solutes gradient inducing spontaneous electric field which affects the motion of colloidal particles. Since the single nanochannel allows the production of concentration gradient in a long-term and stable manner, least source is required to maintain the spontaneous electric field without any external power source, which is appropriate for a portable and self-containable LOC. As a practical application, integrated micro/nanofluidic device facilitates concentration, on-demand extraction, and separation of the colloidal particles. Third, gas permeable PDMS nanochannel with high hydraulic resistance is employed to develop humidity-based gating nanochannel. The rate of mass transport can be manipulated by humidity due to the evaporation of water and the adsorption of solutes to the wall of channel. To demonstrate functionality of humidity for liquid gating or capacitor of molecules, the effect of humidity on mass transport was investigated. This new concept of manipulation of nanofluidic transport made it possible to successfully perform individual mass transport control in a nanochannel array, which is difficult with conventional technique using electricity. It further facilitated on-demand addressable bio/chemical assay using humidity-based molecule valves and pumps. The role of nanochannel as a passage for mass transfer is essential to allow stable and precise control of transport of ions and molecules in the microchannel. It provides wide range of applications using a diffusion-based control of microfluidic environment to induce not only solute gradient for production of electric field but also liquid gating for a valve at molecular level. Thus, achievements of this dissertation contribute to raise the insight about nanochannel-assisted system for simple and precise control of mass transport in hybrid-scale micro-/nanofluidic devices, which is facilitated by the help of the cracking-assisted micro-/nanofabrication technologies.clos

Dynamic photopolymerization produces complex microstructures on hydrogels in a moldless approach to generate a 3D intestinal tissue model

Epithelial tissues contain three-dimensional (3D) complex microtopographies that are essential for proper performance. These microstructures provide cells with the physicochemical cues needed to guide their self-organization into functional tissue structures. However, most in vitro models do not implement these 3D architectural features. The main problem is the availability of simple fabrication techniques that can reproduce the complex geometries found in native tissues on the soft polymeric materials required as cell culture substrates. In this study reaction-diffusion mediated photolithography is used to fabricate 3D microstructures with complex geometries on poly(ethylene glycol)-based hydrogels in a single step and moldless approach. By controlling fabrication parameters such as the oxygen diffusion/depletion timescales, the distance to the light source and the exposure dose, the dimensions and geometry of the microstructures can be well-defined. In addition, copolymerization of poly(ethylene glycol) with acrylic acid improves control of the dynamic reaction-diffusion processes that govern the free-radical polymerization of highly-diluted polymeric solutions. Moreover, acrylic acid allows adjusting the density of cell adhesive ligands while preserving the mechanical properties of the hydrogels. The method proposed is a simple, single-step, and cost-effective strategy for producing models of intestinal epithelium that can be easily integrated into standard cell culture platfor

Replica molding-based nanopatterning of tribocharge on elastomer with application to electrohydrodynamic nanolithography

Replica molding often induces tribocharge on elastomers. To date, this phenomenon has been studied only on untextured elastomer surfaces even though replica molding is an effective method for their nanotexturing. Here we show that on elastomer surfaces nanotextured through replica molding the induced tribocharge also becomes patterned at nanoscale in close correlation with the nanotexture. By applying Kelvin probe microscopy, electrohydrodynamic lithography, and electrostatic analysis to our model nanostructure, poly(dimethylsiloxane) nanocup arrays replicated from a polycarbonate nanocone array, we reveal that the induced tribocharge is highly localized within the nanocup, especially around its rim. Through finite element analysis, we also find that the rim sustains the strongest friction during the demolding process. From these findings, we identify the demolding-induced friction as the main factor governing the tribocharge’s nanoscale distribution pattern. By incorporating the resulting annular tribocharge into electrohydrodynamic lithography, we also accomplish facile realization of nanovolcanos with 10 nm-scale craters