Seiichi Takamatsu

Enzymatic sensing with organic electrochemical transistors

Since their development in the 1980s organic electrochemical transistors (OECTs) have attracted a great deal of interest for biosensor applications. Coupled with the current proliferation of organic semiconductor technologies, these devices have the potential to revolutionize healthcare by making point-of-care and home-based medical diagnostics widely available. Unfortunately, their mechanism of operation is poorly understood, and this hinders further development of this important technology. In this paper glucose sensors based on OECTs and the redox enzyme glucose oxidase are investigated. Through appropriate scaling of the transfer characteristics at various glucose concentrations, a universal curve describing device operation is shown to exist. This result elucidates the underlying device physics and establishes a connection between sensor response and analyte concentration. This improved understanding paves the way for rational optimization of enzymatic sensors based on organic electrochemical transistors.

Transparent conductive-polymer strain sensors for touch input sheets of flexible displays

A transparent conductive polymer-based strain-sensor array, designed especially for touch input sheets of flexible displays, was developed. A transparent conductive polymer, namely poly(3, 4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS), was utilized owing to its strength under repeated mechanical bending. PEDOT:PSS strain sensors with a thickness of 130 nm exhibited light transmittance of 92%, which is the same as the transmittance of ITO electrodes widely used in flat panel displays. We demonstrated that the sensor array on a flexible sheet was able to sustain mechanical bending 300 times at a bending radius of 5 mm. The strain sensor shows a gauge factor of 5.2. The touch point on a flexible sheet could be detected from histograms of the outputs of the strain sensors when the sheet was pushed with an input force of 5 N. The touch input could be detected on the flexible sheet with a curved surface (radius of curvature of 20 mm). These results show that the developed transparent conductive polymer-based strain-sensor array is applicable to touch input sheets of mechanically bendable displays.

Flexible fabric keyboard with conductive polymer-coated fibers

We proposed flexible fabric keyboard which consists of conductive polymer of PEDOT:PSS-coated fibers, standard Micro Control Unit (MCU) with capacitance measurement circuits and personal computer (PC). In the sensor structure, PEDOT:PSS and UltraViolet(UV) -curable adhesive-coated fibers were woven as wefts and warps. The conductive polymer and insulating polymer coating technique was developed to form functional material on enough long fibers to weave fabric with. The weaving with automatic looming machine was employed, forming 1.2 m × 3 m sensor fabric. The sensors could detect human touch by measuring capacitance change between human fingers. The values of capacitance change under touch input ranges from 1.0 to 2.0 pF which is able to be detected by conventional capacitance meters that were integrated in MCUs. By using 9 by 3 touch sensors, keyboard was constructed and the demonstration of inputting character was made.

Wearable Keyboard Using Conducting Polymer Electrodes on Textiles

A wearable keyboard is demonstrated in which conducting polymer electrodes on a knitted textile sense tactile input as changes in capacitance. The use of a knitted textile as a substrate endows stretchability and compatibility to large-area formats, paving the way for a new type of wearable human-machine interface.

Direct patterning of organic conductors on knitted textiles for long-term electrocardiography

Wearable sensors are receiving a great deal of attention as they offer the potential to become a key technological tool for healthcare. In order for this potential to come to fruition, new electroactive materials endowing high performance need to be integrated with textiles. Here we present a simple and reliable technique that allows the patterning of conducting polymers on textiles. Electrodes fabricated using this technique showed a low impedance contact with human skin, were able to record high quality electrocardiograms at rest, and determine heart rate even when the wearer was in motion. This work paves the way towards imperceptible electrophysiology sensors for human health monitoring.

Micro-patterning of a conductive polymer and an insulation polymer using the Parylene lift-off method for electrochromic displays

We have developed a process to pattern the conductive polymer of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) film and the insulation polymer of the perfluoro polymer (Cytop) with a Parylene lift-off method for the purpose of fabricating a PEDOT:PSS-based electrochromic display device. Because conventional micro-patterning processes decrease the conductivity of organic electronic polymers due to the destructive solutions of photolithography, PEDOT:PSS and Cytop polymers were patterned by using the dry lift-off film of Parylene. Its patterning resolution of PEDOT:PSS was found to be as low as 20 µm. The insulation layer of Cytop was also patterned on the PEDOT:PSS pattern with the same resolution. This process was able to pattern the 300 µm wide wiring and 1 mm square pixels of PEDOT:PSS and the Cytop to cover the entire area except for the pixels constructed to form an electrochromic display. Finally, the fabricated 4 × 4 pixel device displayed a simple shape, a transverse line.

Unique Activity-Meter with Piezoelectric Poly(vinylidene difluoride) Films and Self Weight of the Sensor Nodes

We have developed piezoelectric switches for application in ultra low-power wireless sensor nodes to monitor the health condition of chickens. Using Pb(Zr0.52,Ti0.48)O3 (PZT) thin films, we have developed S-shaped PZT cantilevers with proof masses. Since the resonant frequency of PZT devices is approximately 24 Hz, we have utilized their superharmonic resonance to detect chicken movements with frequencies as low as 5–15 Hz. By attaching sensor nodes to chickens, we successfully measured the activity of chickens. However, the PZT devices of other sensor nodes broke down. S-shaped PZT devices are adequate for low vibrations, but are beset by the structural problems of fragmentation upon impact. To address these problems, we examine a method of utilizing poly(vinylidene difluoride) (PVDF) films, which are tough and generate high piezoelectric output voltages under a large stress, as piezoelectric switches. We suggest that the self-weight of sensor nodes be used as the mass of the cantilevers of the PVDF films. One end of a PVDF film is fixed to the case of a sensor node, and the other end is attached to the sensor node. Since PVDF films are subjected to force generated by the self-weight of sensor nodes, high output voltages are expected. A result of measuring output voltages, we confirm the output voltages to be approximately the same as those of PZT devices below 15 Hz at 0.5 m/s2 vibration, which is close to chicken movements. Thus, we consider that we have successfully fabricated a tough wireless sensor node for chickens, utilizing the features of PVDF films.

Liquid-phase packaging of a glucose oxidase solution with parylene direct encapsulation and an ultraviolet curing adhesive cover for glucose sensors.

We have developed a package for disposable glucose sensor chips using Parylene encapsulation of a glucose oxidase solution in the liquid phase and a cover structure made of an ultraviolet (UV) curable adhesive. Parylene was directly deposited onto a small volume (1 μL) of glucose oxidase solution through chemical vapor deposition. The cover and reaction chamber were constructed on Parylene film using a UV-curable adhesive and photolithography. The package was processed at room temperature to avoid denaturation of the glucose oxidase. The glucose oxidase solution was encapsulated and unsealed. Glucose sensing was demonstrated using standard amperometric detection at glucose concentrations between 0.1 and 100 mM, which covers the glucose concentration range of diabetic patients. Our proposed Parylene encapsulation and UV-adhesive cover form a liquid phase glucose-oxidase package that has the advantages of room temperature processing and direct liquid encapsulation of a small volume solution without use of conventional solidifying chemicals.

Ultra-Thin Piezoelectric Strain Sensor Array Integrated on a Flexible Printed Circuit Involving Transfer Printing Methods

In this paper, we present a novel sensor array manufacturing process that involves transfer printing methods using a chip mounter with a vacuum collet. Using these methods, one can mount not only ultra-thin microsensors but also microcontroller and amplifier chips required for sensor device fabrication. We successfully transfer-printed a very fragile microelectromechanical systems-based 5-mm-long, 1-mm-wide, 5-μm-thick high-aspect-ratio ultra-thin Pb(Zr, Ti)O3 (PZT) (1.9 μm)/Si (3 μm) strain sensor onto a flexible printedcircuit (FPC) substrate with etched Cu wiring. Then, we connected the sensor to the wiring by printing a conductive paste using a screen printer. Large ferroelectric polarization-voltage hysteresis curves were obtained even after the transfer printing process. Since an output voltage corresponding to the magnitude of the strain from the developed sensor was generated, it was confirmed that ultra-thin sensors could be integrated to the FPC substrate by these transfer and screen printing techniques without damage. The PZT thin films have shown 0.18 mV/με of dynamic strain sensitivity according to the resonant vibration of the stainless plate.

The photo charge of a bacterioRhodopsin electrochemical cells measured by a charge amplifier

We propose that electrochemical cells based on bacterioRhodopsin (bR), a photosensitive protein, generate electric charges under illuminated light and that a suitable signal detection apparatus for this cell type is a charge amplifier. By using charge amplifiers, we observed bR-photocell electric charges that were proportional to the intensity of incident light, which ranged from 0 to 8mW/cm2. Then, the 4 × 4 micro-patterned bR photosensor array was connected to charge amplifiers to form an image sensor system. Static and moving images were detected with the constructed image sensor system.