Search

Tactile sensing, particularly the detection of object slippage, is required for skillful object handling by robotic grippers. The real-time measurement and identification of the dynamic shear forces that result from slippage events are crucial for slip detection and effective object interaction. In this study, a ferroelectric polymer-based printed soft sensor for object slippage detection was developed and fabricated by screen printing. The proposed sensor demonstrated a sensitivity of 8.2 &mu, C&middot, cm&minus, 2 and was responsive to shear forces applied in both the parallel and perpendicular directions. An amplifier circuit, based on a printed organic thin-film transistor, was applied and achieved a high sensitivity of 0.1 cm2/V&middot, s. Therefore, this study experimentally demonstrates the effectiveness of the proposed printable high-sensitivity tactile sensor, which could serve as part of a wearable robotic e-skin. The sensor could facilitate the production of a system to detect and prevent the slippage of objects from robotic grippers.

Facile fabrication and high ambient stability are strongly desired for the practical application of temperautre sensor in real-time wearable healthcare. Herein, a fully printed flexible temperature sensor based on cross-linked poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was developed. By introducing the crosslinker of (3-glycidyloxypropyl)trimethoxysilane (GOPS) and the fluorinated polymer passivation (CYTOP), significant enhancements in humidity stability and temperature sensitivity of PEDOT:PSS based film were achieved. The prepared sensor exhibited excellent stability in environmental humidity ranged from 30% RH to 80% RH, and high sensitivity of −0.77% °C−1 for temperature sensing between 25 °C and 50 °C. Moreover, a wireless temperature sensing platform was obtained by integrating the printed sensor to a printed flexible hybrid circuit, which performed a stable real-time healthcare monitoring.

The demonstration of the complementary integrated circuit using printing processes is indispensable for realizing electronic devices using organic thin film transistors. Although complementary integrated circuits have advantages such as low power consumption and a wide output voltage range, complementary integrated circuits fabricated by the printing method have problems regarding driving voltage and performance. Studies on fabrication processes of electronic circuits for printing technology, including optimization and simplification, are also important research topics. In this study, the fabrication process of the printed complementary integrated circuit was simplified by applying a p-type donor-acceptor (D-A) polymer semiconductor, which is not strongly affected by the electrode work function. An inverter circuit and the ring oscillator circuit were demonstrated using this process. The fabricated ring oscillator array showed excellent performance, with low voltage operation and low performance variation.

The ability to monitor subtle changes in vital and arterial signals using flexible devices attached to the human skin can be valuable for the detection of various health conditions such as cardiovascular disease. Conventional Si device technologies are being utilised in traditional clinical systems; however, its fabrication is not easy owing to the difficulties in adapting to conventional processes. Here, we present the development of a fully printed, wearable, ferroelectric-polymer vital sensor for monitoring the human pulse wave/rate on the skin. This vital sensor is compact, thin, sufficiently flexible, and conforms to the skin while providing high pressure sensitivity, fast response time, superior operational stability, and excellent mechanical fatigue properties. Moreover, the vital sensor is connected to a communication amplifier circuit for monitoring the pulse waves with a wireless sensing system. This sensor system can realise the development of new healthcare devices for wearable sensor applications.

We investigated a mechanism of improving adhesiveness depending on sintering temperature for printed silver (Ag) electrodes prepared from nanoparticle ink on an insulating polymer layer as the under layer. The adhesion strength significantly improved by sintering the Ag electrodes at a temperature above the glass transition temperature ( T g ) of the polymer layer. In the sample with improved adhesiveness, the interfacial fusion between the printed Ag electrodes and polymer layer was observed in a cross sectional scanning electron microscope image. Based on this mechanism, we have successfully demonstrated the improvement in mechanical durability of organic thin-film transistors (TFTs) with printed Ag electrodes. The degradation of the organic TFTs after applying bending stress was significantly suppressed by interfacial fusion.

We report on the mechanical fatigue behavior of printed, organic, thin-film transistors (OTFTs) based on a polymer semiconductor, investigated by repeatedly applying strain to the flexible OTFT devices and assessing their electrical characteristics after 60,000 bending cycles. As part of our investigation, we established that the rates of reduction in source/drain currents in the OTFT device depended on bending directions. Our improved understanding of the mechanical fatigue behavior of the flexible printed OTFT devices provides valuable insights into their employment in practical flexible electronics applications.

The realization of wearable electronic devices with extremely thin and flexible form factors has been a major technological challenge. While substrates typically limit the thickness of thin-film electronic devices, they are usually necessary for their fabrication and functionality. Here we report on ultra-thin organic transistors and integrated circuits using device components whose substrates that have been removed. The fabricated organic circuits with total device thicknesses down to 350 nm have electrical performance levels close to those fabricated on conventional flexible substrates. Moreover, they exhibit excellent mechanical robustness, whereby their static and dynamic electrical characteristics do not change even under 50% compressive strain. Tests using systematically applied compressive strains reveal that these free-standing organic transistors possess anisotropic mechanical stability and a strain model for a multilayer stack can be used to describe the strain in this sort of ultra-thin device. These results show the feasibility of ultimate-thin organic electronic devices using free-standing constructions.

Flexible printed organic electronics have various advantages over conventional inorganic electronics. These organic devices such as sensors and actuators can be fabricated on a plastic thin-film substrate with low-temperature printing processes[1,2]. Especially, physical sensors based on a ferroelectric polymer have a great potential in the development of wearable and disposable health-care devices such as pressure sensors. As the ferroelectric polymer for the sensors, poly(vinylidene difluoride trifluoroethylene) [P(VDF–TrFE)] (Piezotech, 62-010, VDF:TrFE molar ratio of 75:25) is the most promising material because of high ferroelectricity and piezoelectricity[3]). Moreover, the P(VDF–TrFE) is compatible with printing processes because of its solubility in several organic solvents[4,5]. A representative printed physical sensor based on P(VDF–TrFE) was fabricated on a flexible plastic film substrate by screen printing method. PEDOT:PSS (Heraeus, Clevios SV 4 Stab) was used for the top and bottom electrodes. Thicknesses of the P(VDF–TrFE) layer and the electrodes were 2 µm and 1 µm, respectively. The P(VDF–TrFE) layer could generate the voltage of 50 mV by applied pressure of 50 kPa[5]. The generated voltage clearly depended on the thickness of P(VDF–TrFE) layer. Our printed sensor exhibited high sensitivity in the low pressure region, which is indicating the potential application to wearable sensors which monitoring health condition of human. In order to monitor the health condition of a volunteer, the sensor was attached to the skin near the neck using a skin-compatible adhesive patch. We have succeeded in the detection of the pulse rate with 55 pulse per minute. These results indicate that our P(VDF–TrFE) based sensor is significantly promising for wearable health-care applications. The detection of the pulse rate from volunteer was authorized by the Ethics Committee of Yamagata University (authorization code: 29-2). This study was partially supported by the Japan Science and Technology Agency (JST). T. S. would like to thank all member of Research Center for Organic Electronics in Yamagata University, Piezotech and Arkema corporation. [1] K. Fukuda et al., Sci. Rep., 3, 2048 (2013). [2] S. Conti et al., Adv. Mater. Technol., 2, 1600212 (2017). [3] D. Thuau et al., J. Mater. Chem. C, 5, 9963 (2017). [4] G. Knotts et al., Appl. Phys, Lett., 104, 233301 (2014). [5] T. Sekine et al., Sci. Rep., 8, 4442 (2018).

Organic thin-film transistor (OTFT) devices have attracted much attention in viewpoint of printed-electronics that realizes inexpensive, large-area and flexible devices in the near future. Mitsubishi Chemical Corporation (MCC) has investigated organic semiconductor (OSC) for more than a decade, and focuses recently on development of donor/acceptor (DA) type polymer semiconductors[1-4]. Improvement of OTFT device mobility by novel OSC materials is still one of the biggest issues in the development phase. However, their additional features that can make device fabrication process simpler are also an important industrialization standpoint in material selection. In this paper, we present DA type polymer semiconductor that enables uncomplicated device fabrication process without self-assembled monolayer (SAM) modification on printed silver electrode surface. Top-gate, bottom-contact devices were fabricated as follows. Silver source and drain (S/D) electrodes were patterned by ink-jet printing on Parylene underlayer on a glass substrate. After preparation of bank structure by a dispenser system using solution of amorphous fluoropolymer, semiconducting layer (MCC’s OSC; MOP-01) was printed into an area defined by the bank by a dispenser equipment using its dilute mesitylene solution. Parylene polymer dielectric (thickness: 300 nm) was then deposited on the surface under vaccum. Finally silver gate electrode was patterned by ink-jet printing on the polymer dielectric. Channel length and channel width of the devices are 30-40 μm and 1000 μm, respectively. During the fabrication process described above, three kinds of the silver S/D electrode surface were prepared with or without SAM modification. The SAM molecule introduced at the electrode surface is either pentafluorobenzenethiol (PFBT) commonly used for p-type OSC or 4-methylbenzenethiol (4MBT) normally used for n-type OSC. Silver S/D electrodes without SAM modification were also prepared for comparison. Transfer characteristics of the fabricated OTFT were measured by semiconductor parameter analyzer. The OTFTs exhibited a field-effect mobility of 0.4 cm2/Vs with 4MBT SAM, 0.5 cm2/Vs without SAM, and 0.6 cm2/Vs with PFBT SAM in a saturation region (VDS = -10V) as an average with an on/off current ratio of about 106. The mobility difference is quite small regardless of the presence or absence of the SAM and its species. In addition, mobility among several devices was adequately uniform in each case. According to these findings, we were able to fabricate complementary organic inverter circuits successfully with fewer process steps by combining MOP-1 and n-type OSC (TU-3, Future Ink corporation) where only 4MBT was utilized as an electrode surface modifier[5]. The inverter circuit realized low power operation with excellent performance. T.M. would like to thank all the collaborators in Mitsubishi Chemical Center for Advanced Materials (MC-CAM) in University of California, Santa Barbara (UCSB). This work was partially supported by the Japan Science and Technology Agency (JST, the center of Innovation Program). L. Ying et al., J. Am. Chem. Soc., 2011, 133, 18538–18541. H.-R. Tseng et al., Adv. Mater., 2014, 26, 2993–2998. M. Wang et al., Chem. Commun., 2016, 52, 3207-3210. M. Wang et al., J. Am. Chem. Soc., 2017, 139, 17624–17631. Y. Takeda et al., the 65th JSAP Spring Meeting, 2018, 18p-D102-17.

The resolution, thickness and printing speed range of the printed electrode are determined by the printing method. Printing method having a film thickness (50-200 nm) that can be used for organic thin film transistors (OTFTs) include ink-jet printing, flexographic printing, mCP, etc. In general, the reproducible line spacing (channel length) that can be realized by 10 pL ink-jet printing is about 10 micrometer, and special patterning assistance is necessary for further higher resolution. The reverse offset printing method is known as a printing method with resolution approaching that of photolithography, and it is possible to realize line spacing below 1 micrometer[1]. In previous study, we have fabricated OTFTs with short channel length by using reverse offset printing method. In this study, we fabricated complementary inverter circuit[2], ring oscillator, operational amplifier by integrating printed p-type and n-type OTFT. Those integrated circuits including ring oscillators showed good results with short channel and small overlap electrodes. The p-type and n-type OTFT have a bottom-gate, bottom-contact structure and a top-gate, bottom-contact structure, respectively[2]. Crosslinked poly(4-vinylphenol) was used as a base layer for controlling the surface wettability of the glass substrat. The silver nanoparticle inks were prnited by using an offset printer for all electrodes. The silver nanoparticle ink was patterned onto the base layer to form source / drain (S/D) electrodes for the n-type OTFTs. Next, an amorphous fluoropolymer solution was printed using a dispenser system to create hydrophobic bank layer. Before depositing the n-type semiconductor, the substrate was treated with 4-methylbenzenethiol (4-MBT). A n-type semiconductor (TU-3, FIC) ink was printed using dispenser system in the bank region. A parylene (diX-SR) layer was deposited to form a gate dielectric layer of the n-type OTFT. The silver nanoparticle ink was printed under the same conditions as the S/D electrodes to form gate electrodes. After forming the gate electrodes, the parylene was deposited under the same conditions as those for the n-type OTFT dielectric layer to form the gate dielectric layer for the p-type OTFT. Next, the silver nanoparticle ink was printed under the same condition as the S/D electrodes of n-type OTFT for S/D electrodes of the p-type OTFT. The hydrophobic bank layers were printed using the dispenser system to define the channel width (W) of the p-type OTFT under the same conditions as for the n-type semiconductor bank layer. Prior to printing the p-type semiconductor layer, the S/D electrode surfaces were treated with PFBT. Finally, a mesitylene-based solution containing the p-type semiconductor (diF-TES-ADT/PS blend) was printed in the areas defined by bank layer using the inkjet printer. Transfer characteristics of the fabricated OTFT were measured by semiconductor parameter analyzer (4200A-scs, Keithley). Each of the OTFT devices with diF-TES-ADT/PS as the p-type and TU-3 as the n-type exhibited negligible hysteresis with excellent carrier mobilities of 0.05 and 0.03 cm2/Vs, respectively. The fabricated ring oscillator started oscillating at an operating voltage of 1.25 V. The propagation delay time was 51.7 microsecond at 12.5 V, which is high speed operation in the printed type organic circuit. The operational amplifier also operated well, succeeded in amplifying a weak signal (50 mV) by 20 times (1.0 V).[3] This work was partially supported by the Japan Science and Technology Agency (JST, the center of Innovation Program). K. Fukuda et al., Advanced Electronic Materials, 1, 1500145 (2015). Y. Takeda et al., Advanced Electronic Materials, 4, 1700313 (2018). Y. Takeda et al. JSPS Spring meeting 2018, 18p-D102-6.

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has been fixed in the paper.

Thin, ultra-flexible devices that can be manufactured in a process that covers a large area will be essential to realizing low-cost, wearable electronic applications including foldable displays and medical sensors. The printing technology will be instrumental in fabricating these novel electronic devices and circuits; however, attaining fully printed devices on ultra-flexible films in large areas has typically been a challenge. Here we report on fully printed organic thin-film transistor devices and circuits fabricated on 1-μm-thick parylene-C films with high field-effect mobility (1.0 cm(2) V(-1) s(-1)) and fast operating speeds (about 1 ms) at low operating voltages. The devices were extremely light (2 g m(-2)) and exhibited excellent mechanical stability. The devices remained operational even under 50% compressive strain without significant changes in their performance. These results represent significant progress in the fabrication of fully printed organic thin-film transistor devices and circuits for use in unobtrusive electronic applications such as wearable sensors.

Mechanical flexibility and compatibility of printing processes are key advantage that organic electronic devices have over conventional inorganic devices. However, one of the major remaining issues for organic devices is insufficient mechanical durability of printed electrodes. Here we have investigated the mechanical durability of both p-type and n-type organic thin-film transistors (TFTs) with ink-jet printed silver electrodes from silver nanoparticle inks. The modified silver nanoparticle inks enabled the strong adhesion to the underlying polymer layer, and the fabricated organic TFTs exhibited excellent reproducibility in the bending cycle tests. The strong channel length dependence on the strain sensitivity was observed in both p-type and n-type organic TFTs. The organic TFTs with a short-channel exhibited higher sensitivity to the bending strain. These results suggest that the flexible organic TFTs with printed silver electrodes have excellent mechanical durability and are useful for bending and strain sensors.