Poly(N-isopropyl-acrylamide) (PNIPAM) is a thermoresponsive polymer with a wide range of biological applications, including drug delivery, biosensing, and tissue engineering. The tunability of the structural and mechanical properties of PNIPAM makes it particularly at- tractive in emulating cell environments and dynamic cytoskeletal deformations. This thesis discusses PNIPAM's properties and applications in different forms i.e., solution, brushes, hydrogels, and surface patterned hydrogels, with specific focus on lithographically patterned substrates coated with PNIPAM films. The scaffolds are investigated for structural and me- chanical responses to thermally driven changes in the PNIPAM hydration states using atomic force microscopy (AFM). AFM measurements on our lithographically patterned substrates show that the substrate pattern and coating method enable the fabrication of scaffolds with different topographic and mechanical properties across a wide thermal range. Importantly, these scaffolds exhibit variations in both lateral topography and Young's modulus, rendering them well suited for investigations of differential mechanical stresses experienced by cells and cell membranes. / Master of Science / Poly(N-isopropyl-acrylamide) (PNIPAM) is a polymer which can change its water absorption depending on the temperature of its aqueous environment. It transitions from a swollen state at room temperature to a collapsed state at around 32 °C. These thermally tunable properties make PNIPAM an attractive candidate in a variery of applications, including biomedical and biophysical applications. In this thesis, PNIPAM is coated on lithographically patterned substrates to emulate the cellular cytoskeleton. Atomic force microscopy (AFM) measurements are performed to measure the topography and mechanical properties of the fabricated scaffolds. The results show that the coating method and the features of the used substrate allow the fabrication of different surface topographies with biologically relevant mechanics.

Surfactants, polymers, and their mixtures are widely used in commercial formulations of paints, water-based adhesives, detergents, food, and other products. This thesis describes measurements of the forces acting on colloidal particles in surfactant and polymer solutions. The change in force on addition of surfactants and polymers is usually caused by adsorption to an interface. In this thesis, I also describe the effect of surface charge density, surface crystallinity, surface heterogeneity, and preadsorbed polymer on surfactant adsorption. A new method for the stabilization of colloidal particles is introduced via the synthesis and adsorption of unnatural proteins. Unnatural proteins can be synthesized using the natural "machinery" of a bacterial cell with almost any primary sequence, and provide an environmentally friendly route to colloidal stabilization. As a model system, we study the stabilization of alumina, because alumina has a high Hamaker constant and is therefore difficult to stabilize. An unnatural protein with the sequence, thioredoxin-Pro39Glu10 is used. The Glu10 is anionic (pH > 3) and is designed to adsorb to positively charged alumina (pH<9). The thioredoxin-Pro39 is hydrophilic so it should remain in solution, thereby providing a steric barrier to the approach of two particles in a range of salt and pH conditions. Ellipsometry experiments show that thioredoxin-Pro39Glu10 adsorbs to alumina. Force measurements with the Atomic Force Microscopy (AFM) colloid probe technique show that adsorption of the unnatural protein leads to repulsive forces that decay exponentially with the separation between the surfaces, and are independent of salt concentration. The loss of a salt-dependent force shows that adsorption of the unnatural protein has effectively neutralized the charge on the alumina. Thus, I have shown that an unnatural protein can be used to control the stability of a colloidal system. In general, the same hydrophilic block can probably be added to a variety of anchoring blocks to stabilize different colloidal particles. Electrostatic forces are frequently responsible for the stabilization of colloidal particles. The decay length of these forces is dictated by the electrolyte concentration. The relationship between the decay length and the concentration is well understood for fully dissociated 1:1 electrolytes. Here, I examine the decay-length in solutions where the ions associate strongly. The forces are measured between silica surfaces in aqueous carboxylic acid and surfactant solutions. The decay lengths of the electrostatic double-layer force in both these solutions are well described by the usual expression for decay length when the concentration of ions is obtained from an activity measurement. The effect of the surface properties of the solid substrate on surfactant adsorption is also described in this thesis. The adsorption characteristics of a charged surfactant onto fixed charged surfaces as a function of surface charge density is reported. This is the first time that a method has been introduced for making a series of known fixed charged surfaces. Investigating surfactant adsorption to these surfaces has improved our understanding of the role of charge density in surfactant adsorption and desorption. The desired surface charge density is achieved by the use of gold-thiol self-assembled monolayers (SAMs) of different Ï -groups ("OH and "N+(CH3)3). The mole fraction of "N+(CH3)3 on the mixed SAM dictates the surface charge density. The charge on "N+(CH3)3 is fixed and does not self-regulate. The adsorption of sodium dodecyl sulfate (SDS) to the interface between these model surfaces and aqueous solutions of SDS is investigated. Atomic Force Microscopy (AFM) of the adsorbed surfactant reveals no surface micelles above the critical micelle concentration, cmc, over a wide variety of "N+(CH3)3 densities. This shows that the lateral mobility of ions other than surfactant at the interface is important for the formation of surface micelles of ionic surfactants. Adsorption isotherms of SDS (with no added salt) measured by Surface Plasmon Resonance (SPR) show a plateau region in which the surface excess of SDS is equal to the known fixed surface charge. This demonstrates that the adsorption is electrostatically driven. There is no critical surface charge density at which adsorption rises rapidly. Thus there appears to be no 'hemimicelle concentration'. My work suggests that the formation of hemimicelles depends on the lateral mobility of the surface ions. Desorption experiments starting above the cmc show rapid desorption of SDS into water until the surface excess is equal to the surface charge density. The rapid desorption is followed by a much slower desorption. The elucidation of this fast-slow desorption pattern based on charge density is made possible by the preparation of a set of constant charge surfaces. / Ph. D.

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, and the height increased by 6-12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10-550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of un-aggregated AgNP dissolution. / Master of Science

According to the American Cancer Society, Cancer is the second most common cause of death in the United States, only exceeded by heart disease. Over the past decade, deciphering the complex structure of individual cells and understanding the symptoms of cancer disease has been a highly emphasized research area. The exact cause of Cancer and the genetic heterogeneity that determines the severity of the disease and its response to treatment has been a great challenge. Researchers from the engineering discipline have increasingly made use of recent technological innovations, namely the Atomic Force Microscope (AFM), to better understand cell physics and provide a means for cell biomechanical profiling. The presented work's research objective is to establish a fundamental framework for the development of novel biosensors for cell separation and disease diagnosis. By using AFM nanoindentation, several studies were conducted to identify key distinctions in the trends of cell viscoelasticity between healthy, nontumorigenic cells and their malignant, highly tumorigenic counterparts. The possibility of identifying useful 'biomarkers' was also investigated. Due to the lack of an available human ovarian cell line, experiments were done on a recently developed mouse ovarian surface epithelial (MOSE) cell line, which resembles to human cell characteristics and represents early, intermediate, and late stages of the ovarian cancer. Material properties were extracted via Hertz model contact theory. The experimental results illustrate that the elasticity of late stage MOSE cells were 50% less than that of the early stage. Cell viscosity also decreased by 65% from early to late stage, indicating that the increase in cell deformability directly correlates with increasing levels of malignancy. Various cancer treatment and component-specific drugs were used to identify the causes for the changes in cell biomechanical behavior, depicting that the decrease in the concentration levels of cell structural components, predominantly the actin filament framework, is directly associated with the changes in cell biomechanical property. The investigation of MOSE cells being subject to multiple mechanical loads illustrated that healthy cells react to shear forces by stiffening up to 25% of their original state. On the other hand, cancerous cells are void of such response and at times show signs of decreasing rigidity. Finally, deformation studies on MOSE cancer stem cells have shown that these cells carry a unique elasticity profile among other cell stage phenotypes that could allow for their detection. The results herein carry great potential into contributing to cell separation methods and analysis, furthering the understanding of cell mechanism dynamics. While prior literature emphasizes on the elastic modulus of cells, the study of cell viscosity and other key material properties holds a critical place in the realistic modeling of these complex microstructures. A comprehensive study of individual cells holds a great amount of promise in the development of effective clinical research in the fight against cancer. / Master of Science

京都大学 / 新制・課程博士 / 博士(工学) / 甲第25287号 / 工博第5246号 / 新制||工||1999(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 杉村 博之, 教授 邑瀬 邦明, 准教授 小林 圭 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM

Metal-based nanoparticles (MNPs) are becoming increasingly common in commercial products. Release of these materials into the environment raises concerns about the potential risks they pose to aquatic life. Predicting these risks requires an understanding of MNPs' chemical transformations. In this study, arrays of immobilized MNPs fabricated by nanosphere lithography (NSL) were used to investigate environmental transformations of MNPs. Specifically, sulfidation of silver nanoparticles (Ag NPs) and dissolution of copper-based nanoparticles (Cu NPs) were investigated. Atomic force microscopy (AFM) and transmission electron microscopy were the primary analytical techniques for these investigations. Because the MNPs were immobilized on a solid surface, the samples were field deployable, environmentally relevant metal concentrations were maintained, and the confounding influence of MNP aggregation was eliminated. Ag NP samples were deployed in a full-scale wastewater treatment plant. Sulfidation occurred almost exclusively in anaerobic zones of the WWTP, where the initial sulfidation rate was 11-14 nm of Ag converted to Ag2S per day. Conversion to Ag2S was complete within 7-10 d. Dissolution rates of Cu-based NPs were measured in situ over a range of pH by flow-cell AFM. Based on the measured rates, CuO/Cu(OH)2 NPs dissolve completely within a matter of hours at any pH, metallic Cu NPs persist for a few hours to days, and CuxS NPs do not dissolve significantly over the time scales studied. Field deployment of samples in a freshwater stream confirmed these conclusions for a natural aquatic system. This research demonstrates that environmental transformations of MNPs will be a key factor in determining the ultimate form and concentration of NPs that aquatic organisms will be exposed to. / Ph. D.

During morphogenesis, epithelial tissues undergo dramatic changes in shape, transitioning from flat sheets to three-dimensional folded structures. This remarkable transformation relies on dynamic changes in mechanical tension at both their apical and basal surfaces. While it is well-established that the generation of mechanical tension at the apical side is driven by the actomyosin network, research on this process has often overlooked the generation of mechanical tension at the basal surface. Moreover, the mechanical response to stress, encompassing both elastic (spring-like) and viscous (fluid-like) properties, is important for epithelial transformations, yet this mechanical response is poorly understood for the basal cell surface. In this thesis, we investigated how basal tension is influenced by the basement membrane - an extracellular matrix layer which has been widely regarded as a passive scaffold for cells. We probed the material mechanical response of the basement membrane and directly measured and analyzed basal tension in the wing imaginal disc epithelium of Drosophila. To study the mechanical response, I used long-term confocal imaging and fluorescence recovery after photobleaching (FRAP) to analyze the turnover and mobility of Collagen IV, a component of the basement membrane. The low Collagen IV mobility and turnover (≈ 40 hours) suggest a solid-like behavior of the basement membrane at the time scale of hours. Moreover, Atomic Force Microscopy (AFM) force-indentation curves reveal low hysteresis and an elastic solid-like response. To measure basal mechanical tension, I probed the basement membrane with an AFM. Interpreting the results of AFM shallow indentations on the basal side of explanted wing discs as indenting into a fixed, elastic, stretched thin film, I investigated in control conditions and after molecular perturbations basal mechanical tension. Mechanical tension was ≈ 0.4 mN/m. The removal of collagen IV by collagenase significantly reduced basal tension while increasing basal cell surface area. In addition, inhibition of actomyosin activity through different reagents reduces basal tension while decreasing basal cell surface area. These results indicate that basal tension depends on both the ECM and actomyosin activity. They also indicate that the basement membrane is under expansile stress. Finally, to further investigate the mechanisms underlying the generation of stretch in the basement membrane, I analyzed the influence of hydrostatic pressure and actomyosin contractility along the lateral cell surfaces. These mechanisms exert mechanical forces that increase basal cell area, inducing a stretch in the basement membrane. Mild hypo- or hyperosmotic shocks resulted in increased or decreased basal cell area and basal tension, respectively. Moreover, optogenetic activation of actomyosin at lateral cell surfaces resulted in an increase in both basal cell area and basal tension. In summary, our research quantifies basal tension and unveils that the basement membrane is an elastic material (at time scale of hours). Furthermore, our data suggest that the basement membrane is under elastic stretch generated by hydrostatic pressure and actomyosin contractility. Thus, rather than being a passive scaffold for cells, the elastic properties of the basement membrane contribute to basal tension and thereby the shaping of cells and tissues.

In the first part of this study, frictional and normal forces in aqueous solutions were measured between a glass particle and oligopeptide films grafted from a glass plate. Homopeptide molecules consisting of 11 monomers of different amino acids were each "grafted from" an oxidized silicon wafer using microwave-assisted solid phase peptide synthesis. Oligopeptides increased the magnitude of friction compared to a bare hydrophilic silicon wafer. Friction was a strong function of the nature of the monomer unit and was lower for hydrophilic films. There was a strong adhesion and therefore friction between surfaces of opposite charges. Changes in adhesion and friction depended on the hydrophobicity and electrostatic forces: hydrophobic films and oppositely charged films produced high friction, whereas hydrophilic and like-charges produced low friction. Friction was lower in phosphate buffered saline than in pure water due to the screening of the double layer attraction for oppositely charged surfaces and additional lubrication by hydrated salt ions. We also investigated antimicrobial action of poly (allyl amine) (PA) when covalently bonded to glass. Glass surfaces were prepared by a two-step procedure where the glass was first functionalized with epoxide groups using 3-glycidoxypropyltrimethoxy silane (GOPTS) and then exposed to PA to bind via reaction of a fraction of its amine groups. Antibacterial properties of these coatings were evaluated by spraying aqueous suspensions of bacteria on the functionalized glass slides, incubating them under agar, and counting the number of surviving cell colonies. The PA film displayed strong anti-microbial activity against both Gram-positive and Gram-negative bacteria. Films that were prepared by allowing the PA to self assemble onto the solid via electrostatic interactions were ineffective antimicrobials. Such films had an insufficient positive charge and did not extend far from the solid. Thus we found that antimicrobial activity was correlated with a combination of the ability of the polymer chain to extend into solution and a positive surface potential. / Ph. D.

A number of microcantilever systems of fundamental importance are explored using theoretical and numerical methods to quantify and provide physical insights into the dynamics of experimentally accessible systems that include a variety of configurations and viscous fluids. It is first shown that the correlated dynamics of both a laterally and vertically offset cantilever pair can be accurately predicted by numerical simulations. This is verified by comparing the correlated dynamics yielded by numerical simulations with experimental measurement. It is also demonstrated that in order to obtain these accurate predictions, geometric details of the cantilever must be included in the numerical simulation to directly reflect the experimental cantilever. A microrheology technique that utilizes the fluctuation-dissipation theorem is proposed. It is shown that by including the frequency dependence of the fluid damping, improvements in accuracy of the predictions of the rheological properties of the surrounding fluid are observed over current techniques. The amplitude spectrum of a 2-D cantilever in a power-law fluid is studied. The resulting amplitude spectrum yielded a curve similar to an overdamped system. It is observed that the amplitude and noise spectrum yield the same qualitative response for a 2-D cantilever in a shear thinning, power-law fluid. The correlated dynamics of a tethered vertically offset cantilever pair is investigated. It is shown that for a range of stiffness ratios, which is the ratio of the spring constant of the tethering relative to the cantilever spring constant, the change in the correlated dynamics of a Hookean spring tethered cantilever pair can be seen in the presence of fluid coupling. The dynamics of a spring-mass tethered, vertically offset cantilever pair is qualitatively studied by simplifying the model to an array of springs and masses. The resulting study found that the correlated dynamics of the displacement of mass of the tethered object yielded newly observed features and characteristics. It is shown that the curve shape of the cross-correlation of the displacement of the mass of the tethered object is similar to that of the auto-correlation of the displacement of the mass representing a step forced cantilever. The cross-correlation of the displacement of the mass of the tethered object, however, is found to be significantly more dependent on the stiffness ratio than the auto-correlation of the displacement of the mass representing a cantilever for t > 0. At t = 0, it is observed that the mass of the tethered object yields the same finite value for the cross-correlation for all studied values of the stiffness ratio. This characteristic is a result of the symmetry of the studied spring-mass system. / Ph. D.

This dissertation focused on the potential use of highly charged nanoparticles to stabilize dispersions of weakly charged microparticles. The experimental components of the project centered on a model colloidal system containing silica microparticles at the isoelectric point where the suspensions are unstable and prone to flocculation. The stability of the silica suspensions was studied in the presence of highly charged nanoparticles. Initial experiments used polystyrene latex with either sulfate or amidine surface groups. Effective zeta potentials were measured with nanoparticle concentrations ranging from 0.001% to 0.5% vol. Adsorption levels were determined through direct SEM imaging of the silica microparticles, showing that the nanoparticles directly adsorbed to the microparticles (amidine more than sulfate), producing relatively large effective zeta potentials. However, stability experiments showed that the latex nanoparticles did not stabilize the silica but merely provided a reduction in overall flocculation rate. It was concluded that the zeta potential was an insufficient predictor of stability as there was still sufficient patchiness on the surface to allow for the silica surfaces to aggregate. Experiments using zirconia and alumina nanoparticles did achieve effective stabilization; both types stabilized the silica suspensions for longer than the observation period of approximately 15 hours. Stability was observed at concentrations of 10^-4% to 1.0% (zirconia) and 10^-2% vol. (alumina). These particles adsorbed directly to the microparticles (confirmed via SEM) and produced increasing effective zeta potentials with increasing nanoparticle concentrations. The adsorption resulted in significant electrostatic repulsion that was determined to be effectively irreversible using colloidal probe AFM. The improved stabilizing ability was attributed to the increased van der Waals attraction between the oxide nanoparticles (compared to polystyrene). Finally, an unexpected result of the CP-AFM force measurements showed that the repulsive forces between a nanoparticle-coated particle and plate lacked the normal dependence on the radius of the probe as predicted by the Derjaguin approximation. The forces observed in nanoparticle suspensions were virtually identical for 5 µm and 30 µm probes. Based on calculations of the shear rate in the gap, it was theorized that this phenomenon may have resulted from the shearing of adsorbed particles from the surfaces, which leads to similar interaction geometries for the two probe sizes. / Ph. D.