Elastic Moduli of Organic Electronic Materials by the Buckling Method

Mechanical moduli of common organic electronic materials are measured by the buckling method. The organic layers were prepared on the elastomer polydimethylsiloxane (PDMS) substrate by transfer, direct spin-coating, or thermal evaporation. When a small (∼2%) compressive strain is applied to organic/PDMS film samples, the layer becomes buckled with a characteristic wavelength. Fitting the experimentally measured data of buckling wavelength as a function of layer thickness with a model equation yields the mechanical modulus of the organic layer. The measured values compare well with those from theoretical predictions for materials such as poly(3-hexylthiophene) (P3HT) and its blend with [6,6]-phenyl C61-butyric acid methyl ether (PCBM). The modulus of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is similar to that of pure PSS, which is contrary to the common expectation that the ionic interaction between PEDOT and PSS chains may lead to a modulus value 2−3 times larger than that of the constituent polymers. The similarity is likely due to a very small amount of PEDOT added and the oligomeric nature of PEDOT. Thermally evaporated pentacene film has a modulus value of ∼15 GPa, which is an order of magnitude larger than those of other polymeric materials investigated here, and reveals delamination bukling behavior when the magnitude of compression is relatively large. The residual solvent in polyaniline (PANI) plays the role of plasticizer and leads to a very small modulus. The measured mechanical moduli of common organic electronic materials would be valuable for designing and implementing flexible and/or stretchable organic electronics

Fabrication of Antireflection and Antifogging Polymer Sheet by Partial Photopolymerization and Dry Etching

We present a simple method to fabricate a polymer optical sheet with antireflection and antifogging properties. The method consists of two consecutive steps: photocross-linking of UV-curable polyurethane acrylate (PUA) resin and reactive ion etching (RIE). During photopolymerization, the cured PUA film is divided into two domains of randomly distributed macromers and oligomers due to a relatively short exposure time of 20 s at ambient conditions. Using the macromer domain as an etch-mask, dry etching was subsequently carried out to remove the oligomer domain, leaving behind a nanoturf surface with tunable roughness. UV−vis spectroscopy measurements demonstrate that transmittance of a nanoturf surface is enhanced up to 92.5% as compared to a flat PUA surface (89.5%). In addition, measurements of contact angle (CA) reveal that the etched surface shows superhydrophilicity with a CA as small as 5°. To seek potential applications, I−V characteristics of a thin film organic solar cell were measured under various testing conditions. It is shown that the efficiency can be increased to 2.9% when a nanoturf film with the surface roughness of 34.73 nm is attached to indium tin oxide (ITO) glass. More importantly, the performance is maintained even in the presence of water owing to superhydrophilic nature of the film

Magnetic Nanoparticle-Embedded Hydrogel Sheet with a Groove Pattern for Wound Healing Application

Endothelial progenitor cells (EPCs) can induce a pro-angiogenic response during tissue repair. Recently, EPC transplantations have been widely investigated in wound healing applications. To maximize the healing efficacy by EPCs, a unique scaffold design that allows cell retention and function would be desirable for in situ delivery. Herein, we fabricated an alginate/poly-l-ornithine/gelatin (alginate-PLO-gelatin) hydrogel sheet with a groove pattern for use as a cell delivery platform. In addition, we demonstrate the topographical modification of the hydrogel sheet surface with a groove pattern to modulate cell proliferation, alignment, and elongation. We report that the patterned substrate prompted morphological changes of endothelial cells, increased cell–cell interaction, and resulted in the active secretion of growth factors such as PDGF-BB. Additionally, we incorporated magnetic nanoparticles (MNPs) into the patterned hydrogel sheet for the magnetic field-induced transfer of cell-seeded hydrogel sheets. As a result, enhanced wound healing was observed via efficient transplantation of the EPCs with an MNP-embedded patterned hydrogel sheet (MPS). Finally, enhanced vascularization and dermal wound repair were observed with EPC seeded MPS

Continuous and Scalable Fabrication of Bioinspired Dry Adhesives via a Roll-to-Roll Process with Modulated Ultraviolet-Curable Resin

A simple yet scalable strategy for fabricating dry adhesives with mushroom-shaped micropillars is achieved by a combination of the roll-to-roll process and modulated UV-curable elastic poly­(urethane acrylate) (e-PUA) resin. The e-PUA combines the major benefits of commercial PUA and poly­(dimethylsiloxane) (PDMS). It not only can be cured within a few seconds like commercial PUA but also possesses good mechanical properties comparable to those of PDMS. A roll-type fabrication system equipped with a rollable mold and a UV exposure unit is also developed for the continuous process. By integrating the roll-to-roll process with the e-PUA, dry adhesives with spatulate tips in the form of a thin flexible film can be generated in a highly continuous and scalable manner. The fabricated dry adhesives with mushroom-shaped microstructures exhibit a strong pull-off strength of up to ∼38.7 N cm^–2 on the glass surface as well as high durability without any noticeable degradation. Furthermore, an automated substrate transportation system equipped with the dry adhesives can transport a 300 mm Si wafer over 10 000 repeating cycles with high accuracy