Published papers

The prolonged and continuous monitoring of mechanoacoustic heart signals is essential for the early diagnosis of cardiovascular diseases. These bodily acoustics have low intensity and low frequency, and measuring them continuously for long periods requires ultrasensitive, lightweight, gas-permeable mechanoacoustic sensors. Here, we present an all-nanofiber mechanoacoustic sensor, which exhibits a sensitivity as high as 10,050.6 mV Pa-1 in the lowfrequency region (<500 Hz). The high sensitivity is achieved by the use of durable and ultrathin (2.5 μm) nanofiber electrode layers enabling a large vibration of the sensor during the application of sound waves. The sensor is ultralightweight, and the overall weight is as small as 5 mg or less. The devices are mechanically robust against bending, and show no degradation in performance even after 1,000- cycle bending. Finally, we demonstrate a continuous long-term (10 h) measurement of heart signals with a signal-to-noise ratio as high as 40.9 decibels (dB).

The transepidermal water loss has been widely used as a method for directly evaluating the barrier function of the stratum corneum of the skin. However, transepidermal water loss could not be measured continuously for a long period of time, and there were no reports of continuous monitoring of skin barrier functions. Here, a method is reported to continuously monitor the skin electrical resistance by nanomesh electrodes for a long period of time while maintaining the natural skin condition that does not inhibit water evaporation. Simultaneous measurements of the skin electrical resistance by nanomesh electrodes and transepidermal water loss exhibits a linear fit with a high negative correlation. Furthermore, dynamics of skin physiological functions are successfully visualized by monitoring of the skin electrical resistance by nanomesh electrodes for 30 h in daily life.

High-pressure resistivity studies on ß-[CH3)4N][Pd(dmit)2]2 and [(CH3C2H5)2N]-[Pd(dmit)2]2 have revealed the system to be new molecular superconductors with Tc=6.3 K at 6.5 kbar and Tc=4K at 2.4kbar, respectively. ß-[(CH3)4][Pd(dmit)2]2, though they have the isomorphous to the high-pressure superconductor, [(CH3)4N][Ni(dmit)2]2 has the structure almost dimeric columns of Pd(dmit)2 molecules. The determination of the phase diagram has revealed the existence of a narrow temperature region where the superconducting phase invades into the insulating phase. The crystal of [(CH3)2(C2H5)2N][Pd(dmit)2]2 belongs to triclinic system and has the dimeric columns of Pd(dmit)2. It is of interest that the insulating phase emerges when superconducting state is suppressed at high pressure. © 1993.

The unique molecular conductors with pπ-d mixing band structures (R1,R2-N,N'-dicyanoquinonediimine)2Cu [(R1,R2-DCNQI)2Cu] (R1,R2=CH3,CH3O,Cl,Br) were examined. General features of the phase diagram of the DCNQI-Cu system were depicted. A region that is related to the existence of anomalously heavy-metal electrons has been found. The T2 dependence of the low-temperature resistivity of the alloyed system (DMe1-xMeBrx-DCNQI)2Cu (where Me=CH3) suggests a large enhancement of the electron mass at the critical situation where the system begins to exhibit a characteristic metal instability. The mixed valency of Cu (Cu+1.3) in (DMe-DCNQI)2Cu was confirmed by ir experiments performed on neutral DMe-DCNQI crystals and (DMe-DCNQI)2M (M=Li, Ba, Cu). The same conclusion was also derived from a low-temperature x-ray-diffraction experiment. The gradual temperature dependences of the ir absorption intensities of totally symmetric modes of (DBr-DCNQI)2Cu observed below the metal-insulator transition temperature (TMI) are in contrast with the discontinuous resistivity and susceptibility changes at TMI. This may be attributable to the existence of two driving forces characterizing the M-I transition. One is the sharp charge ordering in Cu sites and the other is the continuous development of charge-density waves on DCNQI stacks. The arrangement of Cu2+ and Cu+ below TMI was determined by an x-ray crystal-structure analysis of the threefold insulating phase of (MeBr-DCNQI)2Cu at 110 K. The nearest-neighbor Cu2+ ions interact with each other via two DCNQI molecules. A plausible spin structure of the antiferromagnetic ground state was proposed. According to this spin-structure model, the magnetic moments of Cu2+ along the crystallographic c axis will be arranged ferromagnetically. © 1993 The American Physical Society.

Evaporated films of lead phthalocyanine (PbPc) show a unique conductivity change after a very slight amount of gases are adsorbed. The change in the conductivity of the films depends on the polarity and the strength of input voltages, and therefore is analogous to the change of synaptic connections in nerve systems. Therefore, a perceptron circuit has been fabricated using PbPc films as synaptic connections. After learning via loading of input signals and teacher signals, this circuit can classify random inputs of eight-dimensional voltage patterns into two categories when the input patterns are linearly separable.

Contact resistance at the organic semiconductor (OSC) - metal electrode interface has been successfully estimated by transmission line method and AFM potentiometry. We have found that contact resistance of source edge was drastically reduced by surface treatment of UV/ozone or self-assembled monolayers of thiol before OSC fabrication at the first time.

The electrical resistivities (ρ{variant}) of (DMeO-DCNQI)2Cu were measured up to 12.5 kbar and down to 1.5 K. Similar to (DMe-DCNQI)2Cu, (DMeO-DCNQI) 2Cu shows a pressure-induced metal instability. But the critical pressure (Pc = 8 kbar), above which the system exhibits the metal instability, is 102 times larger than that of (DMe-DCNQI)2Cu. Reentrance of the metallic phase was observed at 8-9.5 kbar. Above 10 kbar, the reentrant behavior disappears and the system undergoes a sharp metal-insulator transition. At the "reentrant pressure region", ρ{variant} begins to increase at Tmin with decreasing temperature (T) and reaches its maximum at Tmax. Between Tmin and Tmax, ρ{variant} is linearly dependent on log T indicating the scattering of conduction electrons by the magnetic ions (Cu2+): ρ=ρ0 + c log T. The content of Cu2+ seems to vary with pressure. This unique behavior suggests (DMeO-DCNQI)2Cu to be a new family of the dense Kondo materials. © 1989.

The importance of continuous monitoring of skin hydration in daily life, to aid in the diagnosis of skin diseases, is rising. Electrodes that can be worn directly on the skin are attracting attention as an effective means. However, they should not inhibit natural water evaporation from the skin and should not cause inflammation or irritation even if they are attached to the body for long periods of time. In this study, nanomesh electrodes that have previously been reported to exhibit high biocompatibility are also found to exhibit high water vapor permeability, resulting in properties that prevent skin dampness. Furthermore, the skin impedance measured using nanomesh electrodes is found to correlate with the hydration level of skin measured using existing medical equipment. This study provides a new approach to measure skin hydration in conditions close to bare skin.

Structural characterization of alkylated tetracyanoquinodimethane (TCNQ) Langmuir and Langmuir-Blodgett (LB) films has been carried out. Scanning electron microscope (SEM) and atomic force microscope (AFM)observations reveal numerous platelet microcrystal domains in the alkylated TCNQ Langmuir films transferredonto a quartz substrate. The analysis of height distribution in the AFM data proves that these domains haveperiodic layered structure. The thickness of these layered structures agrees well with the d-values obtained byX-ray diffraction analysis of the alkylated TCNQ LB films. Moreover, the high-resolution AFM image and thed-value of the 2-dodecyl-TCNQ Langmuir film suggest that the structure of the second and the upper layers inthe domains is identical to that of the single crystal. SEM and AFM measurements prove that the LB filmsconsist of numerous platelet microcrystal domains which are superposed upon each other, resembling a mosaicstructure. Therefore, the periodic structures observed in X-ray diffraction measurements of the alkylated TCNQLB films are ascribed to the layered structure in these domains, although they are usually explained as well-definedmultilayered structure. © 1995 The Japan Society of Applied Physics.