Su Ryon Shin

pH-Dependent Structures of an i-Motif DNA in Solution

We have investigated for the first time the structure of i-motif DNA in solution at various pH conditions by using synchrotron small-angle X-ray scattering technique. To facilitate direct structural comparison between solution structures of i-motif DNA at various pH values, we created atomic coordinates of i-motif DNA from a fully folded to unfolded atomic model. Under mild acidic conditions, the conformations for i-motif DNA appeared to be similar to that of the partially unfolded i-motif atomic model in overall shape, rather than the fully folded i-motif atomic model. Collectively, our observations indicate that i-motif DNA molecule is structurally dynamic over a wide pH range, adopting multiple conformations ranging from the folded i-motif structure to a random coil conformation. As the i-motif structure has been used as an important component in nanomachines, we can therefore believe that the structural evidence presented herein will promote the development of future DNA-based molecular-actuator devices.

Hydrogel-Assisted Polyaniline Microfiber as Controllable Electrochemical Actuatable Supercapacitor

Flexible, controllable, and stable electrochemical supercapacitors serving as actuators at low operating voltage combining the advantages of the high power of the dielectric capacitors and the high specific energy of rechargeable batteries are important in artificial muscle technology, hybrid electric vehicles, and in short-term power sources for mobile electronic devices [Baughman, Science, 300, 268 (2003); Winter and Brodd, Chem. Rev. (Washington, D.C.), 104, 4245 (2004); Ebron, et al., Science, 311, 1580 (2006)]. High capacitance, a surprising 99% inner charge contribution and actuation in the hydrogel-assisted actuatable electrochemical supercapacitor (HAES) microfiber fabricated through wet spinning of a chitosan solution, followed by the in situ chemical polymerization of aniline was made possible through the perfect utilization of the large surface area provided by the nanostructured polyaniline grown inside as well as on the surface of the fiber. The HAES electrodes with an actuation strain of 0.33% showed 703 F/g specific capacitance in 1 M methane sulfonic acid, and more than 3000 cycles durability. The change in impedance as well as capacitance was achieved by the controlled strain as a function of applied stress, which can establish a direct relationship between the actuation strain and specific capacitance of electrochemical supercapacitors.

Electromechanical properties of hydrogels based on chitosan and poly(hydroxyethyl methacrylate) in NaCl solution

A semi-interpenetrating polymer network (semi-IPN) hydrogel, composed of chitosan and poly(hydroxyethyl methacrylate) (PHEMA), exhibited electrically sensitive behavior. The swelling behavior of the chitosan/PHEMA semi-IPN hydrogel was studied by immersion of the gel in aqueous NaCl solutions at various concentrations. The stimulus response of the chitosan/PHEMA semi-IPN hydrogel in electric fields was also investigated. When swollen, the semi-IPN was placed between a pair of electrodes, and showed bending behavior upon the application of an electric field. The electro-responsive behavior of the present semi-IPN was also affected by the electrolyte concentration of the external solution, and also showed various degrees of increased bending behavior depending on the electrical stimulus.

DNA Hydrogel Fiber with Self‐Entanglement Prepared by Using an Ionic Liquid

DNA hydrogels have a wide range of biomedical applications in tissue engineering and drug-delivery systems. There are two ways to create hydrogel structures: one is enzymecatalyzed assembly of synthetic DNA and the other is by crosslinking natural DNA chemically. For natural DNA, formaldehyde and metal compounds such as arsenic, chromate, and nickel are widely used as crosslinkers. However, these modified DNA hydrogels are unsafe to apply in biological systems because the crosslinkers have potentially adverse side effects, with some being carcinogens. Besides this, these DNA hydrogels are difficult to form into hydrogel fibers by using conventional spinning methods in the absence of chemical crosslinking. In solution, DNA resembles modular proteins such as titin, silk, and polysaccharides. The very flexible linear DNA strands and their noncovalent assemblies can form compacted interwound supercoils in bulk aqueous solution with cationic salts. Alternatively, they can roll into soluble clusters of toroids. In concentrated DNA solutions in poor solvents, rodlike multiple-chain bundling occurs and, simultaneously, single or multiple loops form knots with themselves or with adjacent loops through a nucleation-growth pathway. Thus, these condensates are seen primarily as intertwined aggregates of toroids. We were inspired by the spinning processes used by insects (for example, silkworms and spiders) to develop spinning conditions to create the desired DNA hydrogel fibers. It is known that the last process to occur in insect spinning is the formation of a dragline in air. It can be considered that the air effectively removes water through evaporation to produce dense, dried fibers. To replicate this process in wet spinning, we need to ensure that the coagulation solvent does not fill the space created in the spinning droplet by the exiting water. In addition, the diffusion rates of the coagulation solvent and water must be controlled to prevent the formation of a dense skin on the fiber, which could trap water and create a porous structure. If the coagulation solvent also contains crosslinking cations, then the concentrated DNA solution can form hydrogel fibers with intertwined toroidal entanglements. We have found that room-temperature hydrophilic ionic liquids (RTILs) can produce suitable conditions. The feasibility of using RTILs was suggested by our previous work, which showed that 100% of RTILs will absorb water even when bound to a polymer network. Moreover, some RTILs can create low-pH conditions when in contact with water, and such acidic conditions have been used to promote coagulation in the wet spinning of DNA fibers. The cations present in RTILs condense the DNA as a matter of course. In this work, we prepared a DNA hydrogel fiber in a single step by injecting aqueous DNA solution into a coagulation bath of an RTIL.

Tough Supersoft Sponge Fibers with Tunable Stiffness from a DNA Self-Assembly Technique

Tough and soft: Highly porous, spongelike materials self-assemble by calcium ion condensation of DNA-wrapped carbon nanotubes (SWNTs-DNA; see picture, IL = ionic liquid). The toughness, modulus, and swellability of the electrically conductive sponges can be tuned by controlling the density and strength of interfiber junctions. The sponges have compliances similar to the softest natural tissue, while robust interfiber junctions give high toughness.

Swelling Behavior of Semi‐Interpenetrating Polymer Network Hydrogels Based on Chitosan and Poly(acryl amide)

Semi‐interpenetrating polymer networks (semi‐IPNs) composed of chitosan and polyacrylamide (PAAm) hydrogels have been prepared, and the effect of changing pH, temperature, ionic concentration, and applied electric fields on the swelling of the hydrogels was investigated. The swelling kinetics increased rapidly, reaching equilibrium within 60 min. The semi‐IPN hydrogels exhibited a relatively high swelling ratios of 385%–569% at T=25°C. The swelling ratio increased with decreasing pH below pH=7 due to the dissociation of ionic bonds. The swelling ratio of the semi‐IPN hydrogels was pH, ionic concentration, temperature, and electric field dependent. Differential scanning calorimetry (DSC) was used to determine the volume of free water in the semi‐IPN hydrogels, which was found to increase with increasing PAAm content.

The fabrication of polyaniline/single-walled carbon nanotube fibers containing a highly-oriented filler

Highly uniform composite nanofibers composed of well-oriented single-walled carbon nanotubes (SWCNTs) wrapped in a conducting polymer have been fabricated using electrospinning. Water-soluble polyaniline (WS-PANI) was used as a conducting material to improve the processability during electrospinning. The WS-PANI formed a homogeneous dispersion with the SWCNTs and poly(vinyl alcohol), and good compatibility of the WS-PANI with the SWCNTs was demonstrated by data showing interactions between two components and the wrapping of the SWCNTs by the WS-PANI. Through transmission electron microscopy, atomic force microscopy, and polarized Raman spectroscopy, we confirmed that the WS-PANI plays an important role as a conducting polymer matrix to achieve aligned SWCNTs in composite nanofibers and to form uniform nanofibers.

Effect of C60 Fullerene on the Duplex Formation of i-Motif DNA with Complementary DNA in Solution

The structural effects of fullerene on i-motif DNA were investigated by characterizing the structures of fullerene-free and fullerene-bound i-motif DNA, in the presence of cDNA and in solutions of varying pH, using circular dichroism and synchrotron small-angle X-ray scattering. To facilitate a direct structural comparison between the i-motif and duplex structures in response to pH stimulus, we developed atomic scale structural models for the duplex and i-motif DNA structures, and for the C(60)/i-motif DNA hybrid associated with the cDNA strand, assuming that the DNA strands are present in an ideal right-handed helical conformation. We found that fullerene shifted the pH-induced conformational transition between the i-motif and the duplex structure, possibly due to the hydrophobic interactions between the terminal fullerenes and between the terminal fullerenes and an internal TAA loop in the DNA strand. The hybrid structure showed a dramatic reduction in cyclic hysteresis.

Enhancement of the electromechanical behavior of IPMCs based on chitosan/polyaniline ion exchange membranes fabricated by freeze-drying

The electromechanical behavior of an ionic polymer?metal composite (IPMC) based on a chitosan/polyaniline (CP) interpenetrating polymer network (IPN) is influenced by the internal structure of the CP ion exchange membrane. The freeze-drying method was found to be successful in improving the IPMC electromechanical properties. Scanning electron microscopy (SEM) observations indicated that the ion exchange membranes became more porous after freeze-drying. From swelling ratio and differential scanning calorimetry (DSC) measurements, the freeze-dried membranes exhibited a higher swelling ratio and increased free water content. Electron beam deposition was used to manufacture the IPMC metal electrodes. Experiments on the bending of IPMC samples in a direct current electric field showed that the freeze-dried samples had a faster and larger bending motion than non-freeze-dried samples.

Thermal Characteristics of Polyelectrolyte Complexes Composed of Chitosan and Hyaluronic Acid

Abstract The thermal characterization of the polyelectrolyte complexes (PEC) was investigated by thermogravimetric analysis (TGA) and dielectric analysis (DEA). In the results of the DEA, two relaxation peaks appeared around 50 and 180°C in the sample, CSHA11. The relaxation peak of all samples increased the temperature because of an increase in the number of chitosan segments in the PEC films. The activation energy of the relaxation process was obtained by performing a linear least squares analysis (Arrhenius plot) on the plot of ln(frequency) vs. 1/T max, where T max is the temperature which corresponds to the loss factor peak maximum at various test frequencies using the Arrhenius equation. The thermal decomposition and thermal stability of hyaluronic acid (HA), chitosan, and PEC films were investigated using TGA.