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In the emerging Internet of Things (IoT) society, there is a significant need for low-cost, high-performance flexible humidity sensors in wearable devices. However, commercially available humidity sensors lack flexibility or require expensive and complex fabrication methods, limiting their application and widespread use. We report a high-performance printed flexible humidity sensor using a cellulose nanofiber/carbon black (CNF/CB) composite. The cellulose nanofiber enables excellent dispersion of carbon black, which facilitates the ink preparation and printing process. At the same time, its hydrophilic and porous nature provides high sensitivity and fast response to humidity. Significant resistance changes of 120% were observed in the sensor at humidity ranging from 30% RH to 90% RH, with a fast response time of 10 s and a recovery time of 6 s. Furthermore, the developed sensor also exhibited high-performance uniformity, response stability, and flexibility. A simple humidity detection device was fabricated and successfully applied to monitor human respiration and noncontact fingertip moisture as a proof-of-concept.

A reverse offset-printed stacked-structure complementary ring oscillator and operational amplifier (OPA) circuits were constructed on a large-area substrate. Uniform and fine electrodes were fabric...

In recent years, softening a ferroelectric polymer for wearable tactile electronics has been challenging because of its high robustness and crystallinity. In this study, a functional piezoelectric material system was fabricated by optimizing the blend ratio of a copolymer of PMMA-PBA, which is an amorphous polymer, and a ferroelectric polymer P(VDF-TrFE). The stretchable film was able to extend it up to maximum 400%. By fabricating a capacitor-type device combined with a conductive polymer, we demonstrated the possibility of applying it to a stretchable sensor.

Flexible strain sensors are essential to realizing novel wearable electronics applications that include dynamic human-motion capture sensing devices. Various technologies have been proposed to fabricate flexible strain sensors for motion capture using low-cost printing methods. A new approach utilizing piezoelectric polymers has recently been proposed whereby ferroelectric polymer layers are formed on plastic substrates and employed in wearable sensor devices. However, in printed strain sensor devices using ferroelectric polymers, the influence of layer formulation on sensor properties has not been clear. We have developed fully-printed flexible strain sensors by optimizing the concentration and viscosity of a ferroelectric polymer solution. It was found that the ferroelectric layers using screen printing can be formed with flat morphologies and good crystallinity using solutions with viscosities of over 3.0 Pa s. Moreover, these printed ferroelectric strain sensors achieved practical performance levels, exhibiting ferroelectric characteristics of 6.0 μC cm-2 and 50 MV m−1. The sensors were employed in wearable bending sensors for human fingers to demonstrate their feasibility in motion capture applications. Despite a wide range of operating speeds, the fabricated sensors effectively measured bending rates.

Flexible printed analogic amplifier circuits are required in wearable sensors to enhance their sensitivity for application in various fields such as healthcare, artificial skins, and soft robotics. Various technologies have been proposed to develop wearable sensors for healthcare. However, the development of piezopolymer-based printed healthcare devices for monitoring human vital signs that simultaneously achieve high sensitivity and low operating voltage remains a challenging task. Here, a highly pressure-sensitive printed sensor with low operating voltage is demonstrated and applied to monitor human pulse wave velocity (PWV). The printed sensor consists of a 2 μm thick pressure detector and an organic analogic amplification circuit that are simultaneously formed on flexible substrates. The printed organic analogic circuit can amplify the generated signal by a gain factor of 10. This configuration makes it possible to combine good pressure sensitivity (∼10 kPa) with a low operating voltage of −3 V. We at...

Herein, two donor–acceptor conjugated polymers based on dithieno[3,2-b:2′,3′-d]thiophene (DTT) and thiophene-flanked diketopyrrolopyrrole (DPP) or furan-flanked DPP, namely PDPPT-DTT and PDPPF-DTT were synthesised, characterised and evaluated for their electrical performance in transistor devices for comparison. The influence of the heteroatoms (thiophene and furan) flanked DPP on their thermal, optical, electronic structure and charge carrier transport properties were investigated. The absorption spectra of polymer PDPPT-DTT are broader and red-shifted (26–30 nm) compared with that of polymer PDPPF-DTT, indicating the resonance energy of thiophene is greater than furan, which may allow for different electron localization and result in the difference of optical properties. In addition, energy levels of polymers were slightly affected by the aromatic remote end-groups (thiophene to furan) in DPP-based molecule. Hole transport properties of copolymers were investigated by fabricating the field-effect transistors in the bottom gate top contact (BGTC) configurations for three different self-assembled monolayers (SAMs)/gate dielectric interfaces and different annealing temperatures of polymeric active layer. The BGTC organic thin film transistor (OFET) devices having PDPPT-DTT and PDPPF-DTT thin film annealed at 200 °C exhibit the hole mobility of 0.18 and 0.20 cm2 V−1 s−1, respectively. The OFET devices with trichloro(octyl)silane SAM, fabricated and characterized in ambient environment (temperature ∼25 °C, humidity ∼50%), were found to retain 90% of their performance up to 1000 h.

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.

There are few reports on resistive strain sensors that exhibit both high sensitivity and a wide working range under stretching. We have newly developed a high-performance strain sensor based on a brittle-stretchable conductive network that consists of both brittle and stretchable conductive layers and is fabricated on a stretchable substrate using the screen-printing method. Adding strain usually generates structural cracks in brittle conductive layers leading to a significant increase in resistance, while a stretchable conductive layer bridges these cracks to maintain the conductive pathways under high-strain conditions. This novel conductive network endows superior electrical-mechanical performance to the strain sensors, which possess high sensitivity (gauge factor > 870) over the entire working range (∼100%). Additionally, the developed sensors showed unique anisotropic bend-sensing characteristics, which could be used to detect the bending directions. This high degree of comprehensive performance results in a strain sensor with the capability for full-range human motion detection and robotic motion sensing.

A novel intrinsically stretchable ABA triblock copolymer can be synthesized where A and B are poly(3-hexylthiophene) (P3HT) and polyisobutylene (PIB) segments, respectively. The deformation of the ...

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.

Flexible and low-power consuming integrated circuits are some of the basic requirements for smart wearable devices. High mobility solution-processed organic field-effect transistors (OFETs) have the potential to make a big impact in printed electronic circuits, but their overall performance is currently limited by unusually high threshold voltages (Vth). Herein, systematic optimization of donor–acceptor conjugated polymer, based on dithienothiophene (DTT) and thiophene-flanked diketopyrrolopyrrole (DPP), namely, PDPPT–DTT, OFETs by application of self-assembled monolayers (SAMs) at the semiconductor–dielectric, and semiconductor–metal interfaces is reported. The results clearly exhibit that simultaneous application of octyltrichlorosilane (OTS) as semiconductor–dielectric interface modifying layer and pentafluorobenzene thiol (PFBT) as semiconductor–metal interface modifying layer results in significantly lower Vth and subthreshold slope values from −14.07 V and 13.26 (V Dec−1) to +1.06 V and 7.11 (V Dec−1), respectively. This tailored approach is also beneficial in enhancing hole mobility values by an order of magnitude from 0.01 to 0.5 cm2 V−1 s−1 along with the possibility of switching from hole accumulation mode (Vth = −3.75 V) to depletion mode (Vth = +1.06 V) through device engineering. Simultaneous interface engineering reveals OFET electronic properties can be fine-tuned for robust circuits and low power electronic applications.

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.

We evaluated the electrochemical behaviors and reliability of printed silver (Ag) electrodes prepared from nanoparticle inks with the use of protective self-assembled monolayers (SAMs) under electronic bias conditions. The printed Ag electrodes were fabricated by inkjet printing on a hydrophobic substrate. The SAMs, which acted as barriers to moisture, were prepared by immersing the substrate in a pentafluorobenzenethiol solution at ambient temperature (25 °C). We investigated the electrochemical migration phenomenon using the water drop method, and the results showed that the formation of dendrites connecting the cathode and the anode, which can affect the electrochemical reliability of an electric device, was suppressed in the presence of the SAMs. The time before short circuit occurred was found to depend on the spacing between the electrodes, i.e., 130 s, when the distance between the electrodes was 200 μm in the presence of an SAM. We demonstrated that Ag electrodes treated using the procedure described in this work suppress the occurrence of electrical short circuits caused by Ag dendrite formation and thus their electrochemical properties are substantially improved.

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.

A concept of three dimensional visual characterization is proposed here for the performance deterioration behavior of flexible electronic devices under bending deformation. Here the loading space is defined in a two dimensional manner with two axes. One is to indicate the severity of bending, starting from completely flat ending with ultimately loaded where the device is folded into half. The other is to indicate the number of loading cycles with specified levels of bending severity. By plotting the performance, whatever it is, in a three dimensional manner over the plane spanned with these two axes, its deterioration behavior is fully visualized quantitatively and entirely over the possible space of bending load. Feasibility of such a characterization is eventually demonstrated with silver nano-particle printed flexible wires. The experimental results are examined in detail from the view point of finding useful information for reliability improvement, i.e. promising contribution of the concept for further development of more robust flexible devices.

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.

Organic thin-film transistors (OTFTs) using high-k polymeric materials have attracted considerable attention. Their fabrication processes based on printing can dramatically reduce material waste and manufacturing costs compared with conventional photolithography. However, the high operating voltage of printed OTFTs remains a significant problem. In this study, we fabricated OTFTs that can operate in low voltages by using a high-k dielectric bilayer and a paraelectric polymers. The bilayer structure provided a much higher dielectric constant (>18). The bilayer dielectric exhibited a residual polarization value of 0.3 μC cm−2 and the measured field-effect mobility of the printed OTFT was 0.91 cm2 V s−1.

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.

Organic integrated circuits based on organic thin-film transistor (TFT) devices are fabricated with solution-based electrodes by using dense inks of silver nanoparticles, which can be sintered at room-temperature. The TFT devices fabricated at a sintering temperature of 30 °C exhibit good electrical characteristics. There is a strong relation between the sintering temperature of silver nanoparticle inks and transistor characteristics. A work function of silver electrodes can be controlled by changing the sintering temperature of silver nanoparticle inks, thereby threshold voltage of fabricated TFT devices are shifted accordingly. Fabricated pseudo-CMOS inverter circuits are successfully operated at low voltage with small hysteresis, and large gains are obtained. These results suggest that printed organic TFT devices fabricated with a low-temperature process enable large-area and low-cost integrated circuits by using these techniques in future applications.

We have fabricated electronically stable organic thin-film transistor (TFT) devices that are fully solution-processed and adopt printed electrodes using silver nanoparticles dispersed in organic solvents, whose sintering temperatures are 100 °C or less. The bottom-contact organic TFT devices showed good electrical characteristics, and exhibited threshold voltage shifts less than 2.0 V after applying a DC bias voltage stress for 10 4 s, which is attributed to relatively low contact resistance. These results demonstrate the feasibility of producing stable organic TFTs that are fully solution-processed at relatively low temperatures, for use in large-area flexible electronics applications.

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.

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