Jonathan T. Pham

Macroscopic nanoparticle ribbons and fabrics.

Hierarchical structures that extend over several orders of magnitude in dimensions offer extreme multifunctionality by providing different properties and performance over multiple length scales. Nature offers stunning examples of such materials (e.g., collagen assembly into fi brils, fi bers, and extracellular matrices and tissues). [ 1 , 2 ] Numerous synthetic materials have complex hierarchies, from the nanometer to micrometer length scales, including dendrimers, [ 3 ] block-copolymer micelles, [ 4 ] and nanoparticle assemblies. [ 5–7 ] However, all of these examples are limited in their ability to organize into larger structures, reaching to the human scale. Fabrics represent an intricate example of macroscale hierarchical materials, comprising individual micrometer-scale fi bers (inset of Figure 1 a) arranged into continuous structures fabricated easily on extremely large scales (kilometers or greater). Such materials possess properties provided by the fi ber structure itself, as well as from ensemble capabilities that balance strength and density, present an environmental barrier, and allow draping [ 8 ] over three-dimensional objects (Figure 1a). Beyond conventional fabrics, relatively few examples of hierarchical materials, built from nanoscale objects, have been developed with the ability to tune function and response across a signifi cant size range. The ability to prepare materials that resemble fabrics by “stitching” nanoparticles into hierarchical structures (Figure 1 b) that extend to macroscopic length scales, while maintaining the inherent desirable properties of the nanoparticles, would establish a broad and novel platform for advanced technologies. Using CdSe quantum dots (QDs), we combine unique assembly processes and tailored organic ligands to create ribbons that can be a single nanoparticle thick (measuring vertically from the substrate), as thin as 200 nm in width ( x – y plane), and as long as 10 cm (inset of Figure 1 c, and Supporting Information, Figure S1) or more. While these structural dimensions naturally afford the ribbons with a great deal of fl exibility (Figure 1 c), combining similarly sized organic or polymer connecting ligands with rigid NPs offers a new balance of mechanical properties that mimic those of single polymer molecules but on a much larger scale. Ribbons are virtually inextensible along their axis, controlled by the strength of covalent bonds, yet bend with ease due to the rotational freedom of their interparticle

Highly conductive ribbons prepared by stick-slip assembly of organosoluble gold nanoparticles.

Precisely positioning and assembling nanoparticles (NPs) into hierarchical nanostructures is opening opportunities in a wide variety of applications. Many techniques employed to produce hierarchical micrometer and nanoscale structures are limited by complex fabrication of templates and difficulties with scalability. Here we describe the fabrication and characterization of conductive nanoparticle ribbons prepared from surfactant-free organosoluble gold nanoparticles (Au NPs). We used a flow-coating technique in a controlled, stick-slip assembly to regulate the deposition of Au NPs into densely packed, multilayered structures. This affords centimeter-scale long, high-resolution Au NP ribbons with precise periodic spacing in a rapid manner, up to 2 orders-of-magnitude finer and faster than previously reported methods. These Au NP ribbons exhibit linear ohmic response, with conductivity that varies by changing the binding headgroup of the ligands. Controlling NP percolation during sintering (e.g., by adding polymer to retard rapid NP coalescence) enables the formation of highly conductive ribbons, similar to thermally sintered conductive adhesives. Hierarchical, conductive Au NP ribbons represent a promising platform to enable opportunities in sensing, optoelectronics, and electromechanical devices.

Highly Stretchable Nanoparticle Helices Through Geometric Asymmetry and Surface Forces

Geometric asymmetry and surface forces are used directly the shape transformation of two-dimensional nanoparticle (NP)-based ribbons into three-dimensional helices. The balance between elasticity and surface tension dictates the helical radius dimension. NP helical ribbons have exceptional mechanical properties, displaying high stretchability, helical shape recovery after extension, and low-strain stiffness values similar to biological helices.

Direct Patterning of Engineered Ionic Gold Nanoparticles via Nanoimprint Lithography

Gold nanoparticles are engineered for direct imprinting of stable structures. This imprinting strategy provides access to new device architectures, as demonstrated through the fabrication of a prototype photoswitchable device.

Duration of urination does not change with body size

Significance Animals eject fluids for waste elimination, communication, and defense from predators. These diverse systems all rely on the fundamental principles of fluid mechanics, which we use to predict urination duration across a wide range of mammals. In this study, we report a mathematical model that clarifies misconceptions in urology and unifies the results from 41 independent urological and anatomical studies. The theoretical framework presented may be extended to study fluid ejection from animals, a universal phenomenon that has received little attention. Many urological studies rely on models of animals, such as rats and pigs, but their relation to the human urinary system is poorly understood. Here, we elucidate the hydrodynamics of urination across five orders of magnitude in body mass. Using high-speed videography and flow-rate measurement obtained at Zoo Atlanta, we discover that all mammals above 3 kg in weight empty their bladders over nearly constant duration of 21 ± 13 s. This feat is possible, because larger animals have longer urethras and thus, higher gravitational force and higher flow speed. Smaller mammals are challenged during urination by high viscous and capillary forces that limit their urine to single drops. Our findings reveal that the urethra is a flow-enhancing device, enabling the urinary system to be scaled up by a factor of 3,600 in volume without compromising its function. This study may help to diagnose urinary problems in animals as well as inspire the design of scalable hydrodynamic systems based on those in nature.

Large deformation and adhesive contact studies of axisymmetric membranes.

A model membrane contact system consisting of an acrylic copolymer membrane and a PDMS substrate was utilized to evaluate a recently developed nonlinear large-deformation adhesive contact analysis. Direct measurements of the local membrane apex strain during noncontact inflation indicated that the neo-Hookean model provides an accurate measure of membrane strain and supports its use as the strain energy function for the analysis. Two membrane contact geometries, exhibiting significantly different strain distributions during withdrawal, were investigated. The first examines the wet contact of an air pressurized membrane. The second looks at the dry contact of a fluid deformed membrane in which a stepper motor controls membrane-substrate separation. A time-dependent modulus emerges from the analysis, with principal tensions obtained from a comparison of predicted and experimental membrane profiles. The applicability of this numerical analysis for determining membrane tension, however, is limited by wrinkling instabilities and viscoelasticity. For this reason, a conceptually simpler method, based on the direct measurement of the membrane tension and contact angle, was also utilized. The traditional peel energy defined with this direct measurement accurately described the membrane/substrate adhesive interactions, giving well-defined peel energies that were independent of the detailed strain state of the membrane.

Stick–Slip Friction of PDMS Surfaces for Bioinspired Adhesives

Friction plays an important role in the adhesion of many climbing organisms, such as the gecko. During the shearing between two surfaces, periodic stick-slip behavior is often observed and may be critical to the adhesion of gecko setae and gecko-inspired adhesives. Here, we investigate the influence of short oligomers and pendent chains on the stick-slip friction of polydimethylsiloxane (PDMS), a commonly used material for bioinspired adhesives. Three different stick-slip patterns were observed on these surfaces (flat or microstructured) depending on the presence or absence of oligomers and their ability to diffuse out of the material. After washing samples to remove any untethered oligomeric chains, or after oxygen plasma treatment to convert the surface to a thin layer of silica, we decouple the contributions of stiffness, oligomers, and pendant chains to the stick-slip behavior. The stick phase is mainly controlled by the stiffness while the amount of untethered oligomers and pendant chains available at the contact interface defines the slip phase. A large amount of oligomers and pendant chains resulted in a large slip time, dominating the period of stick-slip motion.

When the going gets rough - studying the effect of surface roughness on the adhesive abilities of tree frogs

Tree frogs need to adhere to surfaces of various roughnesses in their natural habitats; these include bark, leaves and rocks. Rough surfaces can alter the effectiveness of their toe pads, due to factors such as a change of real contact area and abrasion of the pad epithelium. Here, we tested the effect of surface roughness on the attachment abilities of the tree frog Litoria caerulea. This was done by testing shear and adhesive forces on artificial surfaces with controlled roughness, both on single toe pads and whole animal scales. It was shown that frogs can stick 2–3 times better on small scale roughnesses (3–6 µm asperities), producing higher adhesive and frictional forces, but relatively poorly on the larger scale roughnesses tested (58.5–562.5 µm asperities). Our experiments suggested that, on such surfaces, the pads secrete insufficient fluid to fill the space under the pad, leaving air pockets that would significantly reduce the Laplace pressure component of capillarity. Therefore, we measured how well the adhesive toe pad would conform to spherical asperities of known sizes using interference reflection microscopy. Based on experiments where the conformation of the pad to individual asperities was examined microscopically, our calculations indicate that the pad epithelium has a low elastic modulus, making it highly deformable.

Stretching of assembled nanoparticle helical springs.

Hybrid materials that possess high inorganic fractions of nanoscale particles can be advantageous for a wide range of functions, from optoelectronic or electronic devices to drug delivery. However, many current nanoparticle (NP) based materials lack the necessary combination of simple fabrication and robust mechanical properties that span across length scales greater than tens of microns. We have developed a facile, evaporative assembly method called flow coating to create NP based ribbons that can subsequently form helical structures. Here we analytically examine the stretching properties of these helical ribbons which are nanometers thick, microns wide, and arbitrarily long. We find that the force-extension behavior is well described by the elastic and surface energies, which can be used as a guideline for their design. In addition, we show that the properties may be tuned by changing the ribbon dimensions or material composition to yield a different stiffness. These macroscale mechanical properties, along with properties inherent to the nanometer length scale of the particles can provide tunable multifunctionality for a number of applications.

Guiding cell migration with microscale stiffness patterns and undulated surfaces.

UNLABELLED By placing stiff structures under soft materials, prior studies have demonstrated that cells sense and prefer to position themselves over the stiff structures. However, an understanding of how cells migrate on such surfaces has not been established. Many studies have also shown that cells readily align to surface topography. Here we investigate the influence of these two aspects in directing cell migration on surfaces with 5 and 10μm line stiffness patterns (a cellular to subcellular length scale). A simple approach to create flat, stiffness-patterned surfaces by suspending a thin, low modulus polydimethylsiloxane (PDMS) film over a high modulus PDMS structure is presented, as well as a route to add undulations. We confirm that cells are able to sense through the thin film by observation of focal adhesions being positioned on stiff regions. We examine migration by introducing migration efficiency, a quantitative parameter to determine how strongly cells migrate in a certain direction. We found that cells have a preference to align and migrate along stiffness patterns while the addition of undulations boosts this effect, significantly increasing migration efficiency in either case. Interestingly, we found speed to play little role in the migration efficiency and to be mainly influenced by the top layer modulus. Our results demonstrate that both stiffness patterns and surface undulations are important considerations when investigating the interactions of cells with biomaterial surfaces. STATEMENT OF SIGNIFICANCE Two common physical considerations for cell-surface interactions include patterned stiffness and patterned topography. However, their relative influences on cell migration behavior have not been established, particularly on cellular to subcellular scale patterns. For stiffness patterning, it has been recently shown that cells tend to position themselves over a stiff structure that is placed under a thin soft layer. By quantifying the directional migration efficiency on such surfaces with and without undulations, we show that migration can be manipulated by flat stiffness patterns, although surface undulations also play a strong role. Our results offer insight on the effect of cellular scale stiffness and topographical patterns on cell migration, which is critical for the development of fundamental cell studies and engineered implants.