Jaeyoon Park

Material Approaches to Stretchable Strain Sensors

With the recent progress made in wearable electronics, devices now require high flexibility and stretchability up to large strain levels (typically larger than 30 % strain). Wearable strain sensors or deformable strain sensors have been gaining increasing research interest because of the rapid development of electronic skins and robotics and because of their biomedical applications. Conventional brittle strain sensors made of metals and piezoresistors are not applicable for such stretchable sensors. This Review summarizes recent advances in stretchable sensors and focuses on material aspects for high stretchability and sensitivity. It begins with a brief introduction to the Wheatstone bridge circuit of conventional resistive strain sensors. Then, studies on the manipulation of materials are reviewed, including waved structural approaches for making metals and semiconductors stretchable, the use of liquid metals, and conductive filler/elastomer composites by using percolation among the fillers. For capacitive strain sensors, the constant conductivity of the electrode is a key factor in obtaining reliable sensors. Possible approaches to developing capacitive strain sensors are presented. This Review concludes with a discussion on the major challenges and perspectives related to stretchable strain sensors.

Comparison of the efficacy of Burch colposuspension, pubovaginal sling, and tension-free vaginal tape for stress urinary incontinence

Objective: To compare the cure rate and confirm the clinical efficacy of the 3 most frequently performed surgical procedures for stress urinary incontinence (SUI). Methods: Between January 2001 and May 2003, 92 women with SUI were randomly assigned to undergo the Burch colposuspension (n = 33), pubovaginal sling (n = 28), or tension‐free vaginal tape (n = 31) at the Department of Obstetrics and Gynecology, Yonsei Medical Center, Seoul, Korea. Patient characteristics, urodynamic study results, cure rates at 3, 6, and 12 months, and complication rates were compared using the χ2 test. Results: There were no statistically significant differences in the cure rates initially, but after 12 months the cure rate of the pubovaginal sling procedure was found to be significantly higher than those of the tension‐free vaginal tape or Burch colposuspension procedures. Conclusion: The cure rate of the pubovaginal sling procedure was significantly higher after 1 year, but no difference in efficacy was observed between the 2 other procedures. A randomized prospective study of a larger population should be conducted.

Stretchable E‐Skin Apexcardiogram Sensor

A new strategy to measure the apex cardiogram with electronic skin technology is presented. An electronic skin apexcardiogram sensor, which can compensate the conventional electrocardiogram for cardiac diagnosis, is demonstrated through a highly sensitive and stretchable strain sensor with gold-nanoparticle composites.

Surfactant-free CuInSe 2 nanocrystals transformed from In 2 Se 3 nanoparticles and their application for a flexible UV photodetector

In(2)Se(3) nanoparticles were synthesized in an aqueous solution without using any surfactant and then chemically transformed into CuInSe(2) nanocrystals. The transformation was thermodynamically favorable and fast. The 93% production yield in mild reaction conditions allowed mass production of the CuInSe(2) nanocrystals. By the virtue of the surface charges, the CuInSe(2) nanocrystals were well dispersed in polar solvents. The surfactant-free nanocrystals enabled the formation of semiconducting CuInSe(2) films on a flexible polymer substrate without any thermal treatment. We took advantage of this to fabricate a flexible UV photodetector. The current and sensitivity of the devices could be improved by utilizing CuInSe(2) nanocrystals annealed at 160 °C in the reaction batch. On bending test, the detection sensitivity remained the same until the bending radius was reduced down to 4 mm. The dynamic response of the film device was stable and reproducible during light illumination (350 nm).

Interfacing Liquid Metals with Stretchable Metal Conductors

UNLABELLED Highly stretchable conductors are essential components in deformable electronics. Owing to their high stretchability and conductivity, liquid metals have attracted significant attention for use as circuits and interconnections. However, their poor wettability to stretchable metal electrodes prevents the formation of stable electrical connections. This study examined two approaches for creating a stable interface between a liquid metal (EGaIn) and stretchable metal electrodes via: (i) the use of honeycomb-structured stretchable metal electrodes and (ii) the addition of a conducting polymer interlayer. The line width of the honeycomb had a significant influence on the formation of a stable interface. The liquid metal formed a stable film layer on honeycomb metal electrodes, which have line widths of less than 50 μm. Coating PEDOT PSS with a nonionic surfactant lowered the interfacial energy of EGaIn with flat stretchable metal surfaces; hence EGaIn was coated uniformly on the stretchable metal surfaces. Strain sensors were fabricated as a demonstrative example of an application that utilizes the stable interface.

Application of patterned Ag-nanowire networks to transparent thin-film heaters and electrodes for organic light-emitting diodes

We present patterned Ag-nanowire (AgNW) networks for their application to transparent electrodes in flexible devices. Using capillary-force-based soft lithography (CFL), we formed 25- to 30-µm-wide line patterns of AgNWs on flexible polymer substrates. Organic light-emitting diodes (OLEDs) and transparent thin-film heaters (TFHs) were successfully fabricated on the patterned substrates, which verified the potential of AgNW patterns formed by CFL as interconnects in flexible devices.

Suppressing Instability of Liquid Thin Films by a Fibril Network and its Application to Micropatterning without a Residual Layer

This study proposes a method to coat thin films of non-volatile solvents on substrates. A small amount of crystalline polymer dissolved in solvents forms a network of crystalline fibrils during the coating process. The network suppresses dewetting of the solvent liquid and helps the liquid film sustaining on the substrate. This strategy can be used in soft lithography to generate micropatterns of diverse materials without having a residual layer. This process does not request etching for achieving residual layer-free micropatterns, which has been a long challenge in soft lithography. As examples, we demonstrate micropatterns of polymer hydrogels and metal oxides (ZnO, In2O3).

Enhanced stretchability of poly(3-hexylthiophene) thin films by ion gel gate embedding

AbstractWe investigated the stretchability of poly(3-hexylthiophene) (P3HT) thin film and its network structure. We found that stretchability of P3HT thin film is less than 3% strain and can be barely improved by the network structure. But, the ion-gel layer on the P3HT film could improve the electrical stability up to 6%. Based on the results, we fabricated high-performance polymer transistors (1 cm2/Vs and 10E4 on-off ratio) which are reasonably working at 10% tensile strain.