Inkjet Printing of PEDOT:PSS Based Conductive Patterns for 3D Forming Applications

Cleanroom-Free Direct Laser Micropatterning of Polymers for Organic Electrochemical Transistors in Logic Circuits and Glucose Biosensors

Organic electrochemical transistors (OECTs) are promising devices for bioelectronics, such as biosensors. However, current cleanroom-based microfabrication of OECTs hinders fast prototyping and widespread adoption of this technology for low-volume, low-cost applications. To address this limitation, a versatile and scalable approach for ultrafast laser microfabrication of OECTs is herein reported, where a femtosecond laser to pattern insulating polymers (such as parylene C or polyimide) is first used, exposing the underlying metal electrodes serving as transistor terminals (source, drain, or gate). After the first patterning step, conducting polymers, such as poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), or semiconducting polymers, are spin-coated on the device surface. Another femtosecond laser patterning step subsequently defines the active polymer area contributing to the OECT performance by disconnecting the channel and gate from the surrounding spin-coated film. The effective OECT width can be defined with high resolution (down to 2 µm) in less than a second of exposure. Micropatterning the OECT channel area significantly improved the transistor switching performance in the case of PEDOT:PSS-based transistors, speeding up the devices by two orders of magnitude. The utility of this OECT manufacturing approach is demonstrated by fabricating complementary logic (inverters) and glucose biosensors, thereby showing its potential to accelerate OECT research.

Engineering Proteins for PEDOT Dispersions: A New Horizon for Highly Mixed Ionic-Electronic Biocompatible Conducting Materials

Poly (3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrene sulfonate (PSS) is the most used conducting polymer from energy to biomedical applications. Despite its exceptional properties, there is a need for developing new materials that can improve some of its inherent limitations, e.g., biocompatibility. In this context, doping PEDOT is propose with a robust recombinant protein with tunable properties, the consensus tetratricopeptide repeated protein (CTPR). The doping consists of an oxidative polymerization, where the PEDOT chains are stabilized by the negative charges of the CTPR protein. CTPR proteins are evaluated with three different lengths (3, 10, and 20 identical CTPR units) and optimized varied synthetic conditions. These findings revealed higher doping rate and oxidized state of the PEDOT chains when doped with the smallest scaffold (CTPR3). These PEDOT:CTPR hybrids possess ionic and electronic conductivity. Notably, PEDOT:CTPR3 displayed an electronic conductivity of 0.016 S cm-1, higher than any other reported protein-doped PEDOT. This result places PEDOT:CTPR3 at the level of PEDOT-biopolymer hybrids, and brings it closer in performance to PEDOT:PSS gold standard. Furthermore, PEDOT:CTPR3 dispersion is successfully optimized for inkjet printing, preserving its electroactivity properties after printing. This approach opens the door to the use of these novel hybrids for bioelectronics.

Future Thread: Printing Electronics on Fibers

This article introduces a methodology to increase the integration density of functional electronic features on fibers/threads/wires through additive deposition of functional materials via printed electronics. It opens the possibility to create a multifunctional intelligent system on a single fiber/thread/wire while combining the advantages of existing approaches, i.e., the scalability of coating techniques and the microfeatures of semiconductor-based fabrication. By directly printing on threads (of diameters ranging from 90 to 1000 μm), micropatterned electronic devices and multifunctional electronic systems could be formed. Contact and noncontact printing methods were utilized to create various shapes from serpentines and meanders to planar coils and interdigitated electrodes, as well as complex multilayer structures for thermal and light actuators, humidity, and temperature sensors. We demonstrate the practicality of the method by integrating a multifunctional thread into a FFP mask for breath monitoring. Printing technologies provide virtually unrestricted choices for the types of threads, materials, and devices used. They are scalable via roll-to-roll processes and offer a resource-efficient way to democratize electronics across textile products.

Fast Visible-Light 3D Printing of Conductive PEDOT:PSS Hydrogels

Functional inks for light-based 3D printing are actively being searched for being able to exploit all the potentialities of additive manufacturing. Herein, a fast visible-light photopolymerization process is showed of conductive PEDOT:PSS hydrogels. For this purpose, a new Type II photoinitiator system (PIS) based on riboflavin (Rf), triethanolamine (TEA), and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated for the visible light photopolymerization of acrylic monomers. PEDOT:PSS has a dual role by accelerating the photoinitiation process and providing conductivity to the obtained hydrogels. Using this PIS, full monomer conversion is achieved in less than 2 min using visible light. First, the PIS mechanism is studied, proposing that electron transfer between the triplet excited state of the dye (3 Rf*) and the amine (TEA) is catalyzed by PEDOT:PSS. Second, a series of poly(2-hydroxyethyl acrylate)/PEDOT:PSS hydrogels with different compositions are obtained by photopolymerization. The presence of PEDOT:PSS negatively influences the swelling properties of hydrogels, but significantly increases its mechanical modulus and electrical properties. The new PIS is also tested for 3D printing in a commercially available Digital Light Processing (DLP) 3D printer (405 nm wavelength), obtaining high resolution and 500 µm hole size conductive scaffolds.

A micropatterned thermoplasmonic substrate for neuromodulation of in vitro neuronal networks

Understanding how the spatial organization of a neural network affects its activity represents a leading issue in neuroscience. Thanks to their accessibility and easy handling, in vitro studies remain an essential tool to investigate the relationship between the structure and function of a neuronal network. Among all the patterning techniques, ink-jet printing acquired great interest thanks to its direct-write approach, which allows the patterned substrate realization without mold, leading to a considerable saving of both cost and time. However, the inks commonly used give the possibility to control only the structure of a neuronal network, leaving aside the functional aspect. In this work, we synthesize a photosensitive ink combining the rheological and bioadhesive properties of chitosan with the plasmonic properties of gold nanorods, obtaining an ink able to control both the spatial organization of a two-dimensional neuronal network and its activity through photothermal effect. After the ink characterization, we demonstrate that it is possible to print, with high precision, different geometries on a microelectrode array. In this way, it is possible obtaining a patterned device to control the structure of a neuronal network, to record its activity and to modulate it via photothermal effect. Finally, to our knowledge, we report the first evidence of photothermal inhibition of human neurons activity. STATEMENT OF SIGNIFICANCE: Patterned cell cultures remain the most efficient and simple tool for linking structural and functional studies, especially in the neuronal field. Ink-jet printing is the technique with which it is possible to realize patterned structures in the fastest, simple, versatile and low-cost way. However, the inks currently used permit the control only of the neuronal network structure but do not allow the control-modulation of the network activity. In this study, we realize and characterize a photosensitive bioink with which it is possible to drive both the structure and the activity of a neuronal network. Moreover, we report the first evidence of activity inhibition by the photothermal effect on human neurons as far as we know.

Fabrication of Flexible Wiring with Intrinsically Conducting Polymers Using Blue-Laser Microstereolithography

Recently, flexible devices using intrinsically conductive polymers, particularly poly(3,4-ethylenedioxythiophene) (PEDOT), have been extensively investigated. However, most flexible wiring fabrication methods using PEDOT are limited to two-dimensional (2D) fabrication. In this study, we fabricated three-dimensional (3D) wiring using the highly precise 3D printing method of stereolithography. Although several PEDOT fabrication methods using 3D printing systems have been studied, few have simultaneously achieved both high conductivity and precise accuracy. In this study, we review the post-fabrication process, particularly the doping agent. Consequently, we successfully fabricated wiring with a conductivity of 16 S cm^-1. Furthermore, flexible wiring was demonstrated by modeling the fabricated wiring on a polyimide film with surface treatment and creating a three-dimensional fabrication object.

Two-photon polymerization lithography enabling the fabrication of PEDOT:PSS 3D structures for bioelectronic applications

Conductive 3D platforms have gained increasing attention in bioelectronics thanks to the improvement in the cell-chip coupling. PEDOT:PSS is nowadays widely employed in bioelectronic applications thanks to its electrical and mechanical properties. In this work, an innovative fabrication method for the realization of PEDOT:PSS-based conductive micropillars and 3D cage-like structures is presented, combining two-photon lithography and electrodeposition techniques.

Modeling 3D Droplet Movement Using a Drop-on-Demand Inkjet Printhead Model

This article presents a numerical simulation of a printhead model for drop-on-demand (DoD) inkjet printers. A three-dimensional droplet model is provided for the numerical study of inks, ejection parameters, droplet movement, and the analysis of droplet impacts on the surface. This work is devoted to the analysis of different droplet ejection settings during the printing process, when the behavior of the droplet directly affects the accuracy of the printing process itself. A numerical model was also developed to investigate the effect of various settings on droplet stability, including printhead size and nozzle orifice, motion parameters (pulse strength and droplet ejection amplitude) and fluid properties. The results reflect the behavior of the ink droplet over time. The behavior of the drop was tested at different waveform ejection parameters and a mass turnover was observed.

Stretchable printed device for the simultaneous sensing of temperature and strain validated in a mouse wound healing model

Temperature and strain are two vital parameters that play a significant role in wound diagnosis and healing. As periodic temperature measurements with a custom thermometer or strain measurements with conventional metallic gauges became less feasible for the modern competent health monitoring, individual temperature and strain measurement modalities incorporated into wearables and patches were developed. The proposed research in the article shows the development of a single sensor solution which can simultaneously measure both the above mentioned parameters. This work integrates a thermoelectric principle based temperature measurement approach into wearables, ensuring flexibility and bendability properties without affecting its thermo-generated voltage. The modified thermoelectric material helped to achieve stretchability of the sensor, thanks to its superior mechano-transduction properties. Moreover, the stretch-induced resistance changes become an additional marker for strain measurements so that both the parameters can be measured with the same sensor. Due to the independent measurement parameters (open circuit voltage and sensor resistance), the sensing model is greatly attractive for measurements without cross-sensitivity. The highly resilient temperature and strain sensor show excellent linearity, repeatability and good sensitivity. Besides, due to the compatibility of the fabrication scheme to low-temperature processing of the flexible materials and to mass volume production, printed fabrication methodologies were adopted to realize the sensor. This promises low-cost production and a disposable nature (single use) of the sensor patch. For the first time, this innovative temperature-strain dual parameter sensor concept has been tested on mice wounds in vivo. The preliminary experiments on mice wounds offer prospects for developing smart, i.e. sensorized, wound dressings for clinical applications.

Design of Chopsticks-Shaped Heating Resistors for a Thermal Inkjet: Based on TaN Film

Efficient printing frequency is critical for thermal bubble inkjet printing, while the difficulty lies in the structural design and material selection of the heating resistors. In this paper, a TaN film was used as the main material of the heating resistors, and two TaN films were placed in parallel to form the chopsticks-shaped structure. The heating time was divided into two sections, in which 0–0.1 μs was the preheating and 1.2–1.8 μs was the primary heating. At 1.8 μs, the maximum temperature of the Si₃N₄ film could reach about 1100 °C. At the same time, the SiO₂ film was added between the TaN film and Si₃N₄ film as a buffer layer, which effectively avoided the rupture of the Si₃N₄ film due to excessive thermal stress. Inside the inkjet print head, the maximum temperature of the chamber reached about 680 °C at 2.5 μs. Due to the high power of the heating resistors, the working time was greatly reduced and the frequency of the inkjet printing was effectively increased. At the interface between the back of the chip and the cartridge, the SiO₂ film was used to connect to ensure a timely ink supply. Under the condition of 12 V at 40 kHz, the inkjet chip could print efficiently with 10 nozzles at the same time. The inkjet chip proposed in this paper is not limited to only office printing, but also provides a new reference for 3D printing, cell printing, and vegetable and fruit printing.

An Atlas for the Inkjet Printing of Large-Area Tactile Sensors

This review aims to discuss the inkjet printing technique as a fabrication method for the development of large-area tactile sensors. The paper focuses on the manufacturing techniques and various system-level sensor design aspects related to the inkjet manufacturing processes. The goal is to assess how printed electronics simplify the fabrication process of tactile sensors with respect to conventional fabrication methods and how these contribute to overcoming the difficulties arising in the development of tactile sensors for real robot applications. To this aim, a comparative analysis among different inkjet printing technologies and processes is performed, including a quantitative analysis of the design parameters, such as the costs, processing times, sensor layout, and general system-level constraints. The goal of the survey is to provide a complete map of the state of the art of inkjet printing, focusing on the most effective topics for the implementation of large-area tactile sensors and a view of the most relevant open problems that should be addressed to improve the effectiveness of these processes.

Design of H-Shape Chamber in Thermal Bubble Printer

The utilization rate of ink liquid in the chamber is critical for the thermal bubble inkjet head. The difficult problem faced by the thermal bubble inkjet printing is how to maximize the use of ink in the chamber and increase the printing frequency. In this paper, by adding a flow restrictor and two narrow channels into the chamber, the H-shape flow-limiting structure is formed. At 1.8 μs, the speed of bubble expansion reaches the maximum, and after passing through the narrow channel, the maximum reverse flow rate of ink decreased by 25%. When the vapor bubble disappeared, the ink fills the nozzle slowly. At 20 μs, after passing through the narrow channel, the maximum flow rate of the ink increases by 39%. The inkjet printing frequency is 40 kHz, and the volume of the ink droplet is about 13.1 pL. The structure improves the frequency of thermal bubble inkjet printing and can maximize the use of liquid in the chamber, providing a reference for cell printing, 3D printing, bioprinting, and other fields.

Inkjet-Printed Highly Conductive Poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) Electrode for Organic Light-Emitting Diodes

Recently, inkjet printing technology has attracted much attention due to the advantages of drop-on-demand deposition, low-cost and large-area production for organic light-emitting diode (OLED) displays. However, there are still some problems in industrial production and practical application, such as the complexity of ink modulation, high-quality films with homogeneous morphology, and the re-dissolution phenomenon at interfaces. In this work, a printable poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) ink is developed and obtains an adjustable viscosity. Finally, a patterned PEDOT:PSS electrode is fabricated by inkjet printing, and achieves a high conductivity of 1213 S/cm, a transparency of 86.8% and a uniform morphology without coffee-ring effect. Furthermore, the vacuum-evaporated and solution-processed OLEDs are fabricated based on this electrode and demonstrate a current efficiency of 61 cd/A, which is comparable to that of the indium tin oxide counterpart. This work confirms the feasibility of inkjet printing technology to prepare patterned electrodes and expects that it can be used to fabricate highly efficient optoelectronic devices.

Screen Printing Carbon Nanotubes Textiles Antennas for Smart Wearables

Electronic textiles have become a dynamic research field in recent decades, attracting attention to smart wearables to develop and integrate electronic devices onto clothing. Combining traditional screen-printing techniques with novel nanocarbon-based inks offers seamless integration of flexible and conformal antenna patterns onto fabric substrates with a minimum weight penalty and haptic disruption. In this study, two different fabric-based antenna designs called PICA and LOOP were fabricated through a scalable screen-printing process by tuning the conductive ink formulations accompanied by cellulose nanocrystals. The printing process was controlled and monitored by revealing the relationship between the textiles' nature and conducting nano-ink. The fabric prototypes were tested in dynamic environments mimicking complex real-life situations, such as being in proximity to a human body, and being affected by wrinkling, bending, and fabric care such as washing or ironing. Both computational and experimental on-and-off-body antenna gain results acknowledged the potential of tunable material systems complimenting traditional printing techniques for smart sensing technology as a plausible pathway for future wearables.

An Inkjet-Printed PEDOT:PSS-Based Stretchable Conductor for Wearable Health Monitoring Device Applications

A stretchable conductor is one of the key components in soft electronics that allows the seamless integration of electronic devices and sensors on elastic substrates. Its unique advantages of mechanical flexibility and stretchability have enabled a variety of wearable bioelectronic devices that can conformably adapt to curved skin surfaces for long-term health monitoring applications. Here, we report a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based stretchable polymer blend that can be patterned using an inkjet printing process while exhibiting low sheet resistance and accommodating large mechanical deformations. We have systematically studied the effect of various types of polar solvent additives that can help induce phase separation of PEDOT and PSS grains and change the conformation of a PEDOT chain, thereby improving the electrical property of the film by facilitating charge hopping along the percolating PEDOT network. The optimal ink formulation is achieved by adding 5 wt % ethylene glycol into a pristine PEDOT:PSS aqueous solution, which results in a sheet resistance of as low as 58 Ω/□. Elasticity can also be achieved by blending the above solution with the soft polymer poly(ethylene oxide) (PEO). Thin films of PEDOT:PSS/PEO polymer blends patterned by inkjet printing exhibits a low sheet resistance of 84 Ω/□ and can resist up to 50% tensile strain with minimal changes in electrical performance. With its good conductivity and elasticity, we have further demonstrated the use of the polymer blend as stretchable interconnects and stretchable dry electrodes on a thin polydimethylsiloxane (PDMS) substrate for photoplethysmography (PPG) and electrocardiography (ECG) recording applications. This work shows the potential of using a printed stretchable conducting polymer in low-cost wearable sensor patches for smart health applications.

Optimizing Aerosol Jet Printing Process of Platinum Ink for High-Resolution Conductive Microstructures on Ceramic and Polymer Substrates

Printing nano-ink with platinum nanoparticles to generate conductive microstructures for electronics on different types of substrates has gained increasing interest in recent years. To solve the problem of the low conductivity of platinum (Pt) nano-ink, we synthesized chemically pure Pt nanoparticles with sizes of 18.2 ± 9.0 nm by spark discharge method. A low toxic solvent, ethylene glycol with water, was used to ensure the aggregation stability of Pt nanoparticles. Polyvinylpyrrolidone was used as an adhesive additive and binder in the nano-ink. Narrow and conductive Pt lines were generated by aerosol jet printing technology. The resistivity of the Pt lines sintered at 750 °C on alumina substrate was found to exceed the bulk Pt by about 13%. Moreover, the Pt film fabricated on polymer substrates has demonstrated excellent mechanical flexibility in terms of twisting tests.