Joung Sook Hong

CNT/PDMS Composite Flexible Dry Electrodesfor Long-Term ECG Monitoring

We fabricated a carbon nanotube (CNT)/ polydimethylsiloxane (PDMS) composite-based dry ECG electrode that can be readily connected to conventional ECG devices, and showed its long-term wearable monitoring capability and robustness to motion and sweat. While the dispersion of CNTs in PDMS is challenging, we optimized the process to disperse untreated CNTs within PDMS by mechanical force only. The electrical and mechanical characteristics of the CNT/PDMS electrode were tested according to the concentration of CNTs and its thickness. The performances of ECG electrodes were evaluated by using 36 types of electrodes which were fabricated with different concentrations of CNTs, and with a differing diameter and thickness. The ECG signals were obtained by using electrodes of diverse sizes to observe the effects of motion and sweat, and the proposed electrode was shown to be robust to both factors. The CNT concentration and diameter of the electrodes were critical parameters in obtaining high-quality ECG signals. The electrode was shown to be biocompatible from the cytotoxicity test. A seven-day continuous wearability test showed that the quality of the ECG signal did not degrade over time, and skin reactions such as itching or erythema were not observed. This electrode could be used for the long-term measurement of other electrical biosignals for ubiquitous health monitoring including EMG, EEG, and ERG.

Nonlinear viscoelasticity of polymer nanocomposites under large amplitude oscillatory shear flow

In this study, the nonlinear response of polymer nanocomposites under large amplitude oscillatory shear (LAOS) flow was investigated. We first investigated polycaprolactone (PCL)/multiwall nanotube (MWNT) composites under LAOS flow using different analyzing methods including Lissajous plot analysis, stress decomposition, and Fourier transform rheology (FT-rheology). The nonlinear parameter Q (≡I3/1/γ02) was obtained from the FT-rheology as a function of strain amplitude, and the zero-strain nonlinearity Q0 (≡limγ0→0Q) was also calculated. We compared the linear and nonlinear viscoelastic properties as we increase MWNT concentration (ϕ). It was found that the zero-strain nonlinearity (Q0) was more sensitive to detect the effect of MWNT concentration than the linear viscoelastic properties. We also investigated the effect of particle shape on nonlinear viscoelastic properties of the polymer composites containing particles of different shape, e.g., PCL/MWNT (one-dimensional thread shape), PCL/organomodified ...

Self-adhesive epidermal carbon nanotube electronics for tether-free long-term continuous recording of biosignals

The long-term, continuous, inconspicuous, and noiseless monitoring of bioelectrical signals is critical to the early diagnosis of disease and monitoring health and wellbeing. However, it is a major challenge to record the bioelectrical signals of patients going about their daily lives because of the difficulties of integrating skin-like conducting materials, the measuring system, and medical technologies in a single platform. In this study, we developed a thin epidermis-like electronics that is capable of repeated self-adhesion onto skin, integration with commercial electronic components through soldering, and conformal contact without serious motion artifacts. Using well-mixed carbon nanotubes and adhesive polydimethylsiloxane, we fabricated an epidermal carbon nanotube electronics which maintains excellent conformal contact even within wrinkles in skin, and can be used to record electrocardiogram signals robustly. The electrode is biocompatible and can even be operated in water, which means patients can live normal lives despite wearing a complicated recording system.

CNT/PDMS-based canal-typed ear electrodes for inconspicuous EEG recording

OBJECTIVE Current electroencephalogram (EEG) monitoring systems typically require cumbersome electrodes that must be pasted on a scalp, making a private recording of an EEG in a public place difficult. We have developed a small, user friendly, biocompatible electrode with a good appearance for inconspicuous EEG monitoring. APPROACH We fabricated carbon nanotube polydimethylsiloxane (CNT/PDMS)-based canal-type ear electrodes (CEE) for EEG recording. These electrodes have an additional function, triggering sound stimulation like earphones and recording EEG simultaneously for auditory brain-computer interface (BCI). The electrode performance was evaluated by a standard EEG measurement paradigm, including the detection of alpha rhythms and measurements of N100 auditory evoked potential (AEP), steady-state visual evoked potential (SSVEP) and auditory steady-state response (ASSR). Furthermore, the bio- and skin-compatibility of CNT/PDMS were tested. MAIN RESULTS All feasibility studies were successfully recorded with the fabricated electrodes, and the biocompatibility of CNT/PDMS was also proved. SIGNIFICANCE These electrodes could be used to monitor EEG clinically, in ubiquitous health care and in brain-computer interfaces.

Shear induced CNT/PDMS conducting thin film for electrode cardiogram (ECG) electrode

A CNT-based electrode was developed for flexible electrocardiogram (ECG) electrodes with a mixture of polydimethylsiloxane (PDMS) and carbon nanotubes (CNT). In this study, the electrical characteristics of the electrode were evaluated as to the CNT concentration, CNT dispersion and fabrication condition. To obtain good electrical performance of film, the good dispersion of CNT and PDMS is one of great challenges, and we employed the modified two-step method for this purpose. The electrical property of the film by the two step method was much more enhanced than that by the conventional one step method. The electrical properties of the film depended on the film thickness which was controllable by spin coating speed and the flow property of the fluid. Based on these experiments, we found that ECG electrodes require less than 1.5wt% CNT for the desired performance. The 1.5wt% CNT/PDMS film showed optimal mechanical and electrical performance which enabled it to be used as an ECG electrode. We measured the ECG signals from the human skin, and the signal quality was comparable to commercial ECG electrodes.

Extension-induced dispersion of multi-walled carbon nanotube in non-Newtonian fluid

In this study, we devised an extension-induced mixer to disperse carbon nanotube (CNT) clumps into individual tubes by imposing extensional stress continuously and periodically. The rheological behavior of the CNT-dispersed suspension was investigated to examine whether the hydrodynamic dragging induces the dispersion of individual CNT from strongly entangled CNT by van der Waals force between adjacent tubes. When CNT clumps were subjected to a continuous extensional flow for a prolonged time, the optical microscopic and cryo-transmission electron microscopy studies showed that the CNT agglomerates were effectively dispersed and some of the individual nanotubes were disentangled from the CNT clump. Shear viscosity of the dispersion increased when the CNT-Boger fluid suspension was subjected to the continuous and periodic extensional flow for a prolonged time, indicating the dispersion of the individual tubes by the extensional flow. Under shear flows, however, aggregated CNT clumps formed larger, weakly b...

Interfacial localization of nanoclay particles in oil-in-water emulsions and its reflection in interfacial moduli

The localization of nanoclay particles dispersed in the oil phase of a model oil-in-water emulsion depends on the wetting property of layered nanoparticles. Investigation at a single droplet interface shows that nanoclay is located at different interfacial regions depending on the hydrophilic property of the nanoclay surface. Hydrophobic nanoclays do not present Pickering phenomena at the interface and hardly form an interfacial layer. Hydrophilic nanoclay particles quickly move to the interface and form a Pickering interface with a high interfacial shear modulus. With surfactant, poor hydrophilic nanoclays can be located at the interface due to improvement of the wetting behavior caused by the surfactants dissolved in the aqueous continuous phase. With ionic molecules changing the wetting behavior of particles, the interfacial localization of nanoclays can be controlled and improve the mechanical property of emulsion.

Self-Adhesive and Capacitive Carbon Nanotube-Based Electrode to Record Electroencephalograph Signals From the Hairy Scalp

We fabricated a carbon nanotube (CNT)/adhesive polydimethylsiloxane (aPDMS) composite-based dry electroencephalograph (EEG) electrode for capacitive measuring of EEG signals. As research related to brain-computer interface applications has advanced, the presence of hairs on a patients scalp has continued to present an obstacle to recorder EEG signals using dry electrodes. The CNT/aPDMS electrode developed here is elastic, highly conductive, self-adhesive, and capable of making conformal contact with and attaching to a hairy scalp. Onto the conductive disk, hundreds of conductive pillars coated with Parylene C insulation layer were fabricated. A CNT/aPDMS layer was attached on the disk to transmit biosignals to the pillar. The top of disk was designed to be solderable, which enables the electrode to connect with a variety of commercial EEG acquisition systems. The mechanical and electrical characteristics of the electrode were tested, and the performances of the electrodes were evaluated by recording EEGs, including alpha rhythms, auditory-evoked potentials, and steady-state visually-evoked potentials. The results revealed that the electrode provided a high signal-to-noise ratio with good tolerance for motion. Almost no leakage current was observed. Although preamplifiers with ultrahigh input impedance have been essential for previous capacitive electrodes, the EEGs were recorded here by directly connecting a commercially available EEG acquisition system to the electrode to yield high-quality signals comparable to those obtained using conventional wet electrodes.

Oil-Free Generation of Small Polymeric Particles Using a Coaxial Microfluidic Channel

In this study, a microfluidic method to generate small polymeric particles ( approximately 10 mum in diameter) via the control of interfacial tension without using oil and in situ photopolymerization immediately after drop generation was introduced. For the reduction in size, the selection of proper sample and sheath liquid to minimize the interfacial tension is extremely important, and 4-HBA (4-hydroxybutyl acrylate) and PVA (poly(vinyl acrylate)) were employed as core and sheath fluid pair because of much smaller surface tension than the case using oil. In addition, PVA is easily washable by aqueous solution, which is a strong advantage when the particle is applied in biomedical fields. The viscosity effect of sheath flow was also examined for further size reduction. The loading and release properties of proteins were evaluated using fluorescently labeled bovine serum albumin for the potential application as drug carrier. The protein was uniformly loaded into particles, and the protein release rate was dependent on the particle size. For utility in the biomedical area, the cyto-compatibility test of 4-HBA was performed by culturing glioma cells on the 4-HBA sheet, and the cells were alive well after 4 days culture. Conclusively, this oil-free particle generation methods facilitates the generation of uniform and small particles in a simple way without an oil-washing process.

Variation of interfacial tension by nanoclay particles in oil-in-water emulsions

To investigate whether nanoclay particles vary with the interfacial tension of oil-in water emulsion, interfacial tension is measured using dynamic drop volume method. From the critical volume of a single droplet at the moment of detachment, interfacial tension is calculated based on the force balance between gravity force and capillary force. The interfacial tension of a droplet is reduced by the addition of nanoclay and the interfacial tension reduction is further varied depending on nanoclay type. The addition of hydrophilic nanoclay particles (5000 ppm) in an oil drop reduces the interfacial tension from 31.4 to 19.6 mN/m by interfacial localization. In the case of organically modified nanoclay (5000 ppm), interfacial tension is only slightly reduced from 31.4 to 26 mN/m. This study shows that nanoclay effectively reduces the interfacial tension of a droplet when nanoclay is located at the interface.