Direct visualization of cooperative adsorption of a string-like molecule onto a solid.

Natural systems, composite materials, and thin-film devices adsorb macromolecules in different phases onto their surfaces. In general, polymer chains form interfacial layers where their aggregation states and thermal molecular motions differ from the bulk. Here, we visualize well-defined double-stranded DNAs (dsDNAs) using atomic force microscopy and molecular dynamics simulations to clarify the adsorption mechanism of polymer chains onto solid surfaces. Initially, short and long dsDNAs are individually and cooperatively adsorbed, respectively. Cooperative adsorption involves intertwining of multiple chains. The dependence of adsorption on the chain affects the formation of the interfacial layer, realizing different mechanical properties of DNA/filler bulk composites. These findings will contribute to the development of light and durable polymer composites and films for various industrial, biomedical, and environmental applications.

Soft robotic constrictor for in vitro modeling of dynamic tissue compression.

Here we present a microengineered soft-robotic in vitro platform developed by integrating a pneumatically regulated novel elastomeric actuator with primary culture of human cells. This system is capable of generating dynamic bending motion akin to the constriction of tubular organs that can exert controlled compressive forces on cultured living cells. Using this platform, we demonstrate cyclic compression of primary human endothelial cells, fibroblasts, and smooth muscle cells to show physiological changes in their morphology due to applied forces. Moreover, we present mechanically actuatable organotypic models to examine the effects of compressive forces on three-dimensional multicellular constructs designed to emulate complex tissues such as solid tumors and vascular networks. Our work provides a preliminary demonstration of how soft-robotics technology can be leveraged for in vitro modeling of complex physiological tissue microenvironment, and may enable the development of new research tools for mechanobiology and related areas.

Direct observation of morphological transition for an adsorbed single polymer chain.

A better understanding of the structure of polymers at solid interfaces is crucial for designing various polymer nano-composite materials from structural materials to nanomaterials for use in industry. To this end, the first step is to obtain information on how synthetic polymer chains adsorb onto a solid surface. We closely followed the trajectory of a single polymer chain on the surface as a function of temperature using atomic force microscopy. Combining the results with a full-atomistic molecular dynamics simulation revealed that the chain became more rigid on the way to reaching a pseudo-equilibrium state, accompanied by a change in its local conformation from mainly loops to trains. This information will be useful for regulating the physical properties of polymers at the interface.

Mechanical Stabilization of Deoxyribonucleic Acid Solid Films Based on Hydrated Ionic Liquid.

Solid films of deoxyribonucleic acid (DNA) containing a hydrated ionic liquid, choline dihydrogen phosphate (CDP), were prepared by a solvent-casting method. Thermal properties, aggregation structure, thermal molecular motion, and tensile properties of CDP-containing DNA films were examined by thermogravimetry (TG), wide-angle X-ray diffraction (WAXD) measurement, dynamic mechanical analysis (DMA), and tensile tests, respectively. The water retentivity of the films at room temperature was much improved with CDP. The packing density of DNA helical chains clearly depended on the amount of CDP in the film. A small amount of CDP contributed to the suppression of the BI → BII conformational transition and the cooperative motion of the DNA duplex in the film. The tensile properties of the film drastically changed in the presence of CDP. When the amount of hydrated CDP in the film increased, the mechanical response of the film changed from glassy-like to rubbery-like via a semicrystalline-like state. The above results make it clear that CDP plays two major roles as a water absorber and plasticizer in the DNA film. Thus, it can be concluded that the use of an ionic liquid as an additive significantly increases the possibility of using a DNA solid film as a structural material.

Protein Resistance Driven by Polymer Nanoarchitecture.

We report that the nanometer-scale architecture of polymer chains plays a crucial role in its protein resistant property over surface chemistry. Protein-repellent (noncharged), few nanometer thick polymer layers were designed with homopolymer chains physisorbed on solids. We evaluated the antifouling property of the hydrophilic or hydrophobic adsorbed homopolymer chains against bovine serum albumin in water. Molecular dynamics simulations along with sum frequency generation spectroscopy data revealed the self-organized nanoarchitecture of the adsorbed chains composed of inner nematic-like ordered segments and outer brush-like segments across homopolymer systems with different interactions among a polymer, substrate, and interfacial water. We propose that this structure acts as a dual barrier against protein adsorption.

Structure-induced switching of interpolymer adhesion at a solid-polymer melt interface.

Here we report a link between the interfacial structure and adhesive property of homopolymer chains physically adsorbed (i.e., via physisorption) onto solids. Polyethylene oxide (PEO) was used as a model and two different chain conformations of the adsorbed polymer were created on silicon substrates via the well-established Guiselin's approach: "flattened chains" which lie flat on the solid and are densely packed, and "loosely adsorbed polymer chains" which form bridges jointing up nearby empty sites on the solid surface and cover the flattened chains. We investigated the adhesion properties of the two different adsorbed chains using a custom-built adhesion testing device. Bilayers of a thick PEO overlayer on top of the flattened chains or loosely adsorbed chains were subjected to the adhesion test. The results revealed that the flattened chains do not show any adhesion even with the chemically identical free polymer on top, while the loosely adsorbed chains exhibit adhesion. Neutron reflectivity experiments corroborated that the difference in the interfacial adhesion is not attributed to the interfacial brodening at the free polymer-adsorbed polymer interface. Instead, coarse-grained molecular dynamics simulation results suggest that the tail parts of the loosely adsorbed chains act as "connector molecules", bridging the free chains and substrate surface and improving the interfacial adhesion. These findings not only shed light on the structure-property relationship at the interface, but also provide a novel approach for developing sticking/anti-sticking technologies through precise control of the interfacial polymer nanostructures.

Transparent deoxyribonucleic acid substrate with high mechanical strength for flexible and biocompatible organic resistive memory devices.

Biocompatible deoxyribonucleic acid (DNA), with high mechanical strength, was employed as the substrate for a Ag nanowire (Ag NW) pattern and then used to fabricate flexible resistor-type memory devices. The memory exhibited typical write-once-read-many (WORM)-type memory features with a high ON/OFF ratio (104), long-term retention ability (104 s) and excellent mechanical endurance.