Seolhee Baek

Flexible piezocapacitive sensors based on wrinkled microstructures: toward low-cost fabrication of pressure sensors over large areas

Flexible pressure sensors are a key component of electronic skin (e-skin) for use in future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we demonstrated the development of a highly sensitive, simple, and low-cost capacitive pressure sensor, which acted as a flexible capacitive dielectric, based on a microstructured elastomeric template that could be fabricated over a large area. To achieve this goal, the dielectric template was prepared simply by stretching and releasing a flexible Ecoflex film to produce wrinkled surface microstructures with a feature size on the order of tens of micrometers. The effects of the wrinkled surface microstructure on the sensing performance were systematically investigated by comparing the nonwrinkled film, one-side wrinkled film, and double-side wrinkled film. The response and release times of the double-side wrinkled pressure sensor were improved by 42% and 25% in comparison with the values obtained from the unwrinkled case, respectively. These results showed that the introduction of wrinkled surface microstructures to the elastomeric template efficiently enhanced the pressure sensor performance. We also demonstrated that our sensor could be used to detect a variety of changes in the surroundings, such as variations in the angle of a stimulus, object loading/unloading, or an exhaled breath.

Dense Assembly of Soluble Acene Crystal Ribbons and Its Application to Organic Transistors

The preparation of uniform large-area highly crystalline organic semiconductor single crystals remains a challenge in the field of organic field-effect transistors (OFETs). Crystal densities in the channel regions of OFETs have not yet reached sufficiently high values to provide efficient charge transport, and improving channel crystal densities remains an important research area. Herein we fabricated densely well-aligned single crystal arrays of the 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS_PEN) semiconductor using a straightforward scooping-up (SU) methodology to quickly produce a large-area self-assembled semiconductor crystal layer. The resulting crystalline TIPS_PEN strip arrays obtained using the SU method revealed a packing density that was 2.76 times the value obtained from the dip-coated channel, and the mean interspatial distance between the crystal strips decreased from 21.5 to 7.8 μm. The higher crystal packing density provided efficient charge transport in the FET devices and directly yielded field-effect mobilities as high as 2.16 cm(2)/(V s). These field-effect mobilities were more than three times the values obtained from the OFETs prepared using dip-coated channels. Furthermore, the contact resistance between the source/drain electrodes and the TIPS_PEN crystals decreased by a factor of 2. These contributions represent a significant step forward in improving semiconductor crystal alignment for the fabrication of large-area high-performance organic electronics.

Thermally Crosslinked Biocompatible Hydrophilic Polyvinylpyrrolidone Coatings on Polypropylene with Enhanced Mechanical and Adhesion Properties

We developed a stable hydrophilic biocompatible hydrogel-forming coating for polypropylene (PP)-based disposal medical applications. Although PP has a variety of advantages, including good stability and inertness in medical applications, tissue damage and insertion resistance are observed upon insertion of PP-based devices into the human body due to the high hydrophobicity of the PP surface. These issues limit the utility of PP in medical applications. To address these problems, we sought to develop a stable hydrophilic and biocompatible hydrogel-forming layer using polyvinyl pyrrolidone (PVP) combined with a crosslinked polyethyleneglycolacrylate (PEGDA) matrix. Systematic studies of the blended hydrogel-forming PVP:PEGDA were conducted using a variety of blending ratios between the two polymers. The hydrophilicity and water-affinity of the hydrogel-forming layer improved significantly as the PEGDA-to-PVP blending ratio increased. Importantly, the tensile strain at the break point increased by a factor of more than 7, and the strength of adhesion to the PP surface for the 1:1 PVP:PEGDA (PVP(1):PEGDA(1)) blend ratio was 54 times that of the PVP film, determined using tensile strain–stress and peel tests. The water stability of the PVP(1):PEGDA(1) improved significantly. This approach is potentially useful as a biocompatible hydrophilic polymer coating in a variety of low-priced consumable PP commercial medical applications.

Chemically Robust Superhydrophobic Poly(vinylidene fluoride) Films with Grafting Crosslinkable Fluorinated Silane

A superhydrophobic surface with excellent chemical stability was fabricated using the spraying method, one of the most efficient technologies for producing large-area coatings at low cost. Poly(vinylidene fluoride) (PVDF) was used as a hydrophobic polymer material, and heptadecafluoro-1,1,2,2,-tetra-hydrodecyl)trichlorosilane (FTS), which reacts with moisture during curing, was used to improve the water repellency and durability. Spray coating of PVDF alone yielded PVDF nanostructures described by the Cassie-Baxter model. The water contact angle of a water droplet on this surface, however, was 128°, indicating that the surface was not superhydrophobic. On the other hand, spray-coating a mixed PVDF-FTS solution provided a complex and homogeneous nanostructured surface with excellent water repellency and a contact angle of up to 159°. Immersion of the PVDF-only film for 20 min in N,N-dimethylformamide (DMF), a good solvent for PVDF, led to complete dissolution of the film. By contrast, the PVDF-FTS film maintained its superhydrophobicity with a water contact angle of 151° after 20 min of immersion in DMF, and still exhibited a high contact angle of 142° after 1 h. The PVDF-FTS film developed in the present work should enable the production of large-area superhydrophobic coatings at low cost using a simple spray process. Moreover, the PVDF-FTS film displayed excellent stability against solvents, thus increasing its suitability for robust superhydrophobic applications.

Crisscross-Designed Piezoresistive Strain Sensors with Cracked Microtectonics for Direction-Selective Tensile Perception

Flexible strain sensors are a key component of electronic skin (e-skin), a technology that is currently receiving considerable research attention with a view to future applications ranging from human healthcare monitoring to robotic skins and environmental risk detection. Here, we developed a highly sensitive, simple, and low-cost piezoresistive strain sensor, which acted as a flexible reactive resistor with a cracked microtectonic architecture that could be fabricated over a large area. In particular, our strain sensor recognizes the direction of tensile stimulation through its rational crisscross electrode design, allowing it to overcome some of the shortcomings of traditional flexible strain sensors. Under a given stress, the strain sensor developed here showed a variation in the relative resistance (ΔR/R0) of up to 24-fold depending on the direction of the applied stress. For example, application of a 1% strain changed ΔR/R0 by 0.11 when the strain was applied parallel to the direction of current flow, but by only 0.012 when the strain was applied perpendicular to that direction. Similarly, a 5% strain changed ΔR/R0 by 0.85 and 0.062, and a 20% strain changed ΔR/R0 by 2.37 and 0.098, depending on whether the strain was applied parallel or perpendicular to the current flow, respectively. In addition, ΔR/R0 varied approximately linearly as a function of the strain over the operational range. The results thus show that the proposed sensor is sensitive to the direction in which an external stress is applied. Finally, we demonstrated that our sensor could be used to detect the bending of a human finger.