Inkjet-printed Silver Electrodes Research Articles

Fabrication of PDMS based flexible capacitive tactile sensor array with inkjet printed silver electrodes for robotic object grasp control

Abstract Human interactive robots deployed for advanced services such as nursing and elderly assistance systems, operate in close proximity to humans in a home environment, unlike the conventional industrial robots. For the adaptive and safe interaction with humans, it is important to endow the sense of touch to such robots. Tactile sensors are embedded on the robotic body to perceive touch. The existing rigid tactile sensors have limited flexibility and conformability to robot surfaces. This paper presents the development of tactile sensors for robots to detect human touch, biomimicking the flexible nature of human skin. To emulate flexibility, we propose the fabrication of flexible capacitive tactile sensors based on polymers like Polyimide (PI) and Polydimethylsiloxane (PDMS). PI is used as the substrate to develop 3 × 3 array by thermal evaporation. PDMS is used as the dielectric between silver electrodes printed on Polyethylene terephthalate (PET) substrate adopting the state of the art and cost-effective inkjet printing. The sensitivity evaluation of the fabricated sensor was carried out using a customized force measurement setup. The pressure sensitivity of 0.32% /KPa was obtained. Force sensitive resistor (FSR) was integrated with the tactile sensor array to estimate force at contact location and was placed on the custom 3D printed robotic arm, for object grasp control. This novel tactile sensor module (TSM) can potentially offer improved user experience in human-robot interaction (HRI) as in Robotic elderly assistance systems.

Polymer field-effect transistors with inkjet-printed silver electrodes: from device fabrication to circuit simulation

Polymer field-effect transistors with inkjet-printed silver electrodes: from device fabrication to circuit simulation

Hardware implementation of a true random number generator integrating a hexagonal boron nitride memristor with a commercial microcontroller.

The development of the internet-of-things requires cheap, light, small and reliable true random number generator (TRNG) circuits to encrypt the data-generated by objects or humans-before transmitting them. However, all current solutions consume too much power and require a relatively large battery, hindering the integration of TRNG circuits on most objects. Here we fabricated a TRNG circuit by exploiting stable random telegraph noise (RTN) current signals produced by memristors made of two-dimensional (2D) multi-layered hexagonal boron nitride (h-BN) grown by chemical vapor deposition and coupled with inkjet-printed Ag electrodes. When biased at small constant voltages (≤70 mV), the Ag/h-BN/Ag memristors exhibit RTN signals with very low power consumption (∼5.25 nW) and a relatively high current on/off ratio (∼2) for long periods (>1 hour). We constructed TRNG circuits connecting an h-BN memristor to a small, light and cheap commercial microcontroller, producing a highly-stochastic, high-throughput signal (up to 7.8 Mbit s-1) even if the RTN at the input gets interrupted for long times up to 20 s, and if the stochasticity of the RTN signal is reduced. Our study presents the first full hardware implementation of 2D-material-based TRNGs, enabled by the unique stability and figures of merit of the RTN signals in h-BN based memristors.

Controlled poling of a fully printed piezoelectric PVDF-TrFE device as a multifunctional platform with inkjet-printed silver electrodes

A novel all-printed piezoelectric device and its preparation methodology is presented as a multifunctional device platform with controllable polarization of the PVDF-TrFE layer, reversibly switching between piezoelectric and non-piezoelectric states.

Characterization and Comparison of Biodegradable Printed Capacitive Humidity Sensors.

Flexible and biodegradable sensors are advantageous for their versatility in a range of areas from smart packaging to agriculture. In this work, we characterize and compare the performance of interdigitated electrode (IDE) humidity sensors printed on different biodegradable substrates. In these IDE capacitive devices, the substrate acts as the sensing layer. The dielectric constant of the substrate increases as the material absorbs water from the atmosphere. Consequently, the capacitance across the electrodes is a function of environmental relative humidity. Here, the performance of polylactide (PLA), glossy paper, and potato starch as a sensing layer is compared to that of nonbiodegradable polyethylene terephthalate (PET). The capacitance across inkjet-printed silver electrodes is measured in environmental conditions ranging from 15 to 90% relative humidity. The sensitivity, response time, hysteresis, and temperature dependency are compared for the sensors. The relationship between humidity and capacitance across the sensors can be modeled by exponential growth with an R2 value of 0.99, with paper and starch sensors having the highest overall sensitivity. The PET and PLA sensors have response and recovery times under 5 min and limited hysteresis. However, the paper and starch sensors have response and recovery times closer to 20 min, with significant hysteresis around 100%. The PET and starch sensors are temperature independent, while the PLA and paper sensors display thermal drift that increases with temperature.

Flexible and Stretchable Temperature Sensors Fabricated Using Solution-Processable Conductive Polymer Composites.

Accurate monitoring of physiological temperatures is important for the diagnosis and tracking of various medical conditions. This work presents the design, fabrication, and characterization of temperature sensors using conductive polymer composites (CPCs) patterned on both flexible and stretchable substrates through both drop coating and direct ink writing (DIW). These composites were formed using a high melting point biopolymer polyhydroxybutyrate (PHB) as the matrix and the graphenic nanomaterial reduced graphene oxide (rGO) as the nanofiller (from 3 to 12 wt%), resulting in a material that exhibits a temperature-dependent resistivity. At room temperature the composites exhibited electrical percolation behavior. Around the percolation threshold, both the carrier concentration and mobility were found to increase sharply. Sensors were fabricated by drop-coating PHB-rGO composites onto ink-jet printed silver electrodes. The temperature coefficient of resistance was determined to be 0.018 /°C for pressed rGO powders and 0.008 /°C for the 3 wt% samples (the highest responsivity of all composites). Composites were found to have good selectivity to temperature with respect to pressure and moisture. Thermal mapping was demonstrated using 6 × 7 arrays of sensing elements. Stretchable devices with a meandering pattern were fabricated using DIW, demonstrating the potential for these materials in healthcare monitoring devices.

Flexible inkjet-printed dual-gate organic thin film transistors and PMOS inverters: Noise margin control by top gate

Low operating voltage dual-gate p-type organic thin film transistors (OTFTs) and PMOS inverters have been fabricated on 125 μm-thick flexible polyethylene naphthalate (PEN) substrates by solution-processed techniques. Inkjet-printed silver electrodes are used for gate, source, and drain contacts while the active organic semiconducting material is printed by a dispenser system over parylene dielectric. Bottom channel, top channel, and dual-gate OTFTs separately display average charge carrier mobilities of 0.03, 0.11, and 0.37 cm 2 V −1 s −1 , respectively. Top-gate is used to independently tune the threshold voltage for bottom-gate OTFT and vice versa. Two PMOS inverters are fabricated with high gain and low supply voltage operation. Ideal symmetric voltage transfer characteristics with high noise margin are achieved by controlling the threshold voltage of the driver OTFT by top-gate bias for low supply voltage, V DD = −2.5 V. • Flexible low voltage dual-gate OTFTs are fabricated by inkjet printing technique. • Dual-gate OTFTs are characterized in bottom-, top-, and dual-gate mode. • V Th of bottom channel OTFT is linearly control by top gate and vice versa. • DLL and ZVLL, PMOS inverters are compared in terms of gain and noise margin. • Ideal PMOS inverters are achieved by controlling V Th of bottom OTFT by top gate.

Reduced contact resistance of a-IGZO thin film transistors with inkjet-printed silver electrodes

In this study, high performance amorphous In–Ga–Zn–O (a-IGZO) TFTs were successfully fabricated with inkjet-printed silver source-drain electrodes. The results showed that increased channel thickness has an improving trend in the properties of TFTs due to the decreased contact resistance. Compared with sputtered silver TFTs, devices with printed silver electrodes were more sensitive to the thickness of active layer. Furthermore, the devices with optimized active layer showed high performances with a maximum saturation mobility of 8.73 cm2 · V−1 · S−1 and an average saturation mobility of 6.97 cm2 · V−1 · S−1, Ion/Ioff ratio more than 107 and subthreshold swing of 0.28 V/decade, which were comparable with the analogous devices with sputtered electrodes.

Sensitivity enhancement of flexible gas sensors via conversion of inkjet-printed silver electrodes into porous gold counterparts

This work describes a facile, mild and general wet chemical method to change the material and the geometry of inkjet-printed interdigitated electrodes (IDEs) thus drastically enhancing the sensitivity of chemiresistive sensors. A novel layer-by-layer chemical method was developed and used to uniformly deposit semiconducting single-wall carbon nanotube (SWCNT)-based sensing elements on a Kapton® substrate. Flexible chemiresistive sensors were then fabricated by inkjet-printing fine-featured silver IDEs on top of the sensing elements. A mild and facile two-step process was employed to convert the inkjet-printed dense silver IDEs into their highly porous gold counterparts under ambient conditions without losing the IDE-substrate adhesion. A proof-of-concept gas sensor equipped with the resulting porous gold IDEs featured a sensitivity to diethyl ethylphosphonate (DEEP, a simulant of the nerve agent sarin) of at least 5 times higher than a similar sensor equipped with the original dense silver IDEs, which suggested that the electrode material and/or the Schottky contacts between the electrodes and the SWCNTs might have played an important role in the gas sensing process.

Inkjet printed silver electrodes on macroporous paper for a paper-based isoelectric focusing device.

We demonstrate a combined printing process utilizing inkjet printing of silver electrodes and solid-ink technology for printing hydrophobic wax barriers for fabricating paper microfluidic devices with integrated electrodes. Optimized printing parameters are given for achieving conducting silver lines on the top of macroporous chromatography paper down to 250 μm-300 μm resolution. Electrical characterization and wicking experiments demonstrate that the printed silver patterns are simultaneously conductive and porous enough to allow reliable capillary wicking across the electrodes. The combined wax and silver printing method is used for fabrication of paper microfluidic isoelectric focusing devices for separation and concentration of proteins.

Stable fully-printed polymer resistive read-only memory and its operation in mobile readout system

The read-only memory (ROM) is a key component for a wide range of printed electronics applications. The resistive type ROM based on conductive polymer poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT: PSS) would be a promising technology of choice, which can be “manufactured-on-demand” via digital printing for high throughput and material saving. However, the instability issues associated with the conventional PEDOT: PSS and its interface with contact electrodes would be a critical hurdle preventing the technology from practical applications. This work proves that, by removing the hydrophilic acidic groups in conventional PEDOT: PSS, these instability issues can be well addressed. The ROM tags fabricated based on the modified PEDOT: PSS of neutral pH and inkjet printed silver electrodes present extremely stable performance under both aging and electrical stress tests in air ambient. A self-designed memory readout circuit board, communicating with mobile phone through near field communication, is also implemented to demonstrate the feasibility of using the ROM tags in real mobile systems. It is shown that, without any encapsulation, the ROMs can have stable output under high humidity condition (>60% RH), after either being stored in the ambient condition for 30 days or being operated after 20000 reading cycles.

Electrochemical detection of free chlorine at inkjet printed silver electrodes

A low-cost, reliable and sensitive electrochemical method for free chlorine analysis in water using inkjet printed silver electrodes is presented. Free chlorine detection was based on linear sweep voltammetry (LSV) analysis of AgCl/Ag2O films formed over an inkjet printed silver electrode by the spontaneous reaction between silver and free chlorine species (i.e. HClO and ClO−) present in solution. The formation of AgCl/Ag2O films was studied and characterized by high resolution scanning electron microscopy (HR SEM) and X-ray photoelectron spectroscopy (XPS) techniques. LSV characterization demonstrated a quantitative linear relationship between the amount of AgCl/Ag2O formed and the concentration of free chlorine in water within a range from 1 to 100 ppm. After optimization of several parameters (e.g. scan rate, reaction time, starting potential), lowest detectable free chlorine concentration was 0.4 ppm (by standard addition method), while the limit of detection (S/N = 3) was equal to 2 ppm, with a sensitivity of 30 μC/ppm. The validation of the proposed methodology was performed by comparison with the standard N,N-diethylparaphenylenediamine (DPD) method for analyzing swimming pool water samples. Finally, it was demonstrated that reproducible and disposable silver electrodes could be easily prepared by inkjet printing in a large scale and in any required geometry to fit on-line and on-site free chlorine analyses requirements.

A capacitive angular sensor with flexible digitated electrodes

Purpose – This paper aims to present a prototype of a capacitive angular-position sensor which exploits advantages of flexible/printed electronics. The novelty of the sensor is that the capacitor structure is placed at the circumference of the rotor and stator, that it posses two channels (capacitor structures) electrically shifted for p/4 and that the rotor is common for both channels. The electrodes of the sensing capacitor are digitated, providing a triangular transfer function. Design/methodology/approach – This sensor prototype consists of two flexible inkjet-printed silver electrodes forming a cylindrical capacitor structure. One of them is wrapped around the stator and another is wrapped around the rotor part of a simple mechanical platform used to precisely adjust the angular displacement. Findings – The capacitance as a function of angular position was measured using an inductance capacitance impedance (LCZ) Meter, and results are presented for a full-turn measurement range. The experimental results are compared with analytical ones and very good agreement has been achieved. Originality/value – The proposed capacitive sensor structure can be used as an absolute or an incremental encoder with different resolutions, and it can be applied in automotive industry or robotics.

ZnO Nanodisk Based UV Detectors with Printed Electrodes

The fabrication of highly functional materials for practical devices requires a deep understanding of the association between morphological and structural properties and applications. A controlled hydrothermal method to produce single crystal ZnO hexagonal nanodisks, nanorings, and nanoroses using a mixed solution of zinc sulfate (ZnSO4) and hexamethylenetetramine (HMTA) without the need of catalysts, substrates, or templates at low temperature (75 °C) is introduced. Metal-semiconductor-metal (MSM) ultraviolet (UV) detectors were fabricated based on individual and multiple single-crystal zinc oxide (ZnO) hexagonal nanodisks. High quality single crystal individual nanodisk devices were fabricated with inkjet-printed silver electrodes. The detectors fabricated show record photoresponsivity (3300 A/W) and external quantum efficiency (1.2 × 10(4)), which we attribute to the absence of grain boundaries in the single crystal ZnO nanodisk and the polarity of its exposed surface.

Automating fluid delivery in a capillary microfluidic device using low-voltage electrowetting valves

A multilayer capillary polymeric microfluidic device integrated with three normally closed electrowetting valves for timed fluidic delivery was developed. The microfluidic channel consisted two flexible layers of poly (ethylene terephthalate) bonded by a pressure-sensitive adhesive spacer tape. Channels were patterned in the spacer tape using laser ablation. Each valve contained two inkjet-printed silver electrodes in series. Capillary flow within the microchannel was stopped at the second electrode which was modified with a hydrophobic monolayer (valve closed). When a potential was applied across the electrodes, the hydrophobic monolayer became hydrophilic and allowed flow to continue (valve opened). The relationship between the actuation voltage, the actuation time, and the distance between two electrodes was performed using a microfluidic chip containing a single microchannel design. The results showed that a low voltage (4.5 V) was able to open the valve within 1 s when the distance between two electrodes was 1 mm. Increased voltages were needed to open the valves when the distance between two electrodes was increased. Additionally, the actuation time required to open the valve increased when voltage was decreased. A multichannel device was fabricated to demonstrate timed fluid delivery between three solutions. Our electrowetting valve system was fabricated using low-cost materials and techniques, can be actuated by a battery, and can be integrated into portable microfluidic devices suitable for point-of-care analysis in resource-limited settings.

Chip-on-foil devices for DNA analysis based on inkjet-printed silver electrodes

For a rapid on-site diagnosis of pathogens, low-cost chip-based devices are of great interest. Here, we report the successful fabrication of inkjet printed silver electrodes on polymer foils as disposable chips for molecular DNA analytics. In order to manufacture these electrode structures, silver nanoparticle inks were inkjet printed onto planar polypropylene substrates. Due to the low thermal stability of the foils, substrate preserving sintering techniques, including low temperature thermal sintering and low pressure argon plasma sintering, were implemented. Thus, sufficient electrical conductance of the printed structures at processing temperatures ≤100 °C was achieved. To test the applicability of the manufactured chips, specific capture DNA was immobilized within the gaps of the conductive electrode paths and hybridized in the next step with biotin-labeled target DNA. Subsequently, an enzymatically generated silver nanoparticle deposition was induced that bridges the electrode gap. This enabled both conductance measurement and gray value analysis as a fast, simple and robust electrical and optical read-out system. The proof-of-principle experiments successfully demonstrated the applicability of these convenient chip-on-foil devices for nucleic acid based pathogen detection.

Graphene-based Flexible Field Effect Transistor with Inkjet Printed Silver Electrodes

Graphene-based Flexible Field Effect Transistor with Inkjet Printed Silver Electrodes

Strain sensitivity and durability in p-type and n-type organic thin-film transistors with printed silver electrodes

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.

Profile Control of Inkjet Printed Silver Electrodes and Their Application to Organic Transistors

We report on the cross-sectional profile control of printed electrodes fabricated from silver nanoparticle inks with water-based solvents by inkjet printing. Systematically varying the ambient conditions and time for the drying process corresponded to changes in electrode shape. In general, lower humidity levels resulted in concave electrode profiles due to the coffee-ring effect, while higher humidity levels resulted in convex profiles. Printed capacitors with trapezoidal-shaped lower electrodes showed much better electrical breakdown strength than those with concave-shaped lower electrodes. Solution-processed organic thin-film transistors with trapezoidal gate electrodes operated reproducibly and exhibited good electrical characteristics with very low gate-leakage currents. The methods can be utilized in the fabrication of printed electronic devices with stacked layers, such as thin-film capacitors and transistors.

Nanoimprint assisted inkjet printing to fabricate sub-micron channel organic field effect transistors

Solution processed poly(3-hexylthiophene) organic field effect transistors with channel lengths down to 750nm were fabricated by nanoimprint assisted inkjet printing. The nanoimprint lithography was used to define sub-micron channels into a resist because of its high resolution. A silver-containing ink was inkjet-printed onto a pre-patterned resist layer to form a metallic film, which acts as source and drain electrodes after lift-off. This process replaces the expensive vacuum evaporation of gold electrodes. The transistor short channel effect was suppressed successfully by constant field downscaling. However, samples with inkjet-printed silver electrodes have limited current density. They also have lower effective charge mobility due to higher charge injection barrier, as well as the rough metal surface. Gold nanoparticles were added into the silver ink to modify its work function and therefore reduce the contact resistance between electrodes and polymer. This emphasizes the importance of the metal-semiconductor contact especially for short channel organic transistors.