Machine learning approach to monitor inkjet jetting status based on the piezo self-sensing.

One of the advantages of inkjet printing in digital manufacturing is the ability to use multiple nozzles simultaneously to improve the productivity of the processes. However, the use of multiple nozzles makes inkjet status monitoring more difficult. The jetting nozzles must be carefully selected to ensure the quality of printed products, which is challenging for most inkjet processes that use multi-nozzles. In this article, we improved inkjet print head monitoring based on self-sensing signals by using machine learning algorithms. Specifically, supervised machine learning models were used to classify nozzle jetting conditions. For this purpose, the self-sensing signals were acquired, and the feature information was extracted for training. A vision algorithm was developed to label the nozzle status for classification. The trained models showed that the classification accuracy is higher than 99.6% when self-sensing signals are used for monitoring. We also proposed a so-called hybrid monitoring method using trained machine learning models, which divides the feature space into three regions based on predicted jetting probability: certain jetting, certain non-jetting, and doubt regions. Then, the nozzles with uncertain status in the doubt region can be verified by jet visualization to improve the accuracy and efficiency of the monitoring process.

Fabrication of Highly Sensitive Capacitive Pressure Sensors Using a Bubble-Popping PDMS.

Attempts have been made to introduce microstructures or wrinkles into the elastomer surface to increase the sensitivity of the elastomer. However, the disadvantage of this method is that when a force is applied to the pressure sensor, the contact area with the electrode is changed and the linear response characteristic of the pressure sensor is reduced. The biggest advantage of the capacitive pressure sensor using an elastomer is that it is a characteristic that changes linearly according to the change in pressure, so it is not suitable to introduce microstructures or wrinkles into the elastomer surface. A method of increasing the sensitivity of the capacitive pressure sensor while maintaining the linearity according to the pressure change is proposed. We proposed a bubble-popping PDMS by creating pores inside the elastomer. The sensitivity of the pressure sensor made of the bubble-popping PDMS was approximately 4.6 times better than that of the pressure sensor without pores, and the pressure sensor made of the bubble-popping PDMS showed a high linear response characteristic to the external pressure change. These results show that our pressure sensor can be used to detect applied pressures or contact forces of e-skins.

Multi-Geometry Parameters Optimization of Large-Area Roll-to-Roll Nanoimprint Module Using Grey Relational Analysis and Artificial Neural Network.

Micro- and nanofabrication on polymer substrate is integral to the development of flexible electronic devices, including touch screens, transparent conductive electrodes, organic photovoltaics, batteries, and wearable devices. The demand for flexible and wearable devices has spurred interest in large-area, high-throughput production methods. Roll-to-roll (R2R) nanoimprint lithography (NIL) is a promising technique for producing nano-scale patterns rapidly and continuously. However, bending in a large-scale R2R system can result in non-uniform force distribution during the imprinting process, which reduces pattern quality. This study investigates the effects of R2R imprinting module geometry parameters on force distribution via simulation, using grey relational analysis to identify optimal parameter levels and ANOVA to determine the percentage of each parameter contribution. The study also investigates the length and force ratio on a backup roller used for bending compensation. The simulation results and the artificial neural network (ANN) model enable the prediction of nip pressure and force distribution non-uniformity along the roller, allowing the selection of the optimal roller geometry and force ratio for minimal non-uniformity on a specific R2R system. An experiment was conducted to validate the simulation results and ANN model.

Hybrid Device Fabrication Using Roll-to-Roll Printing for Personal Environmental Monitoring.

Roll-to-roll (R2R) printing methods are well known as additive, cost-effective, and ecologically friendly mass-production methods for processing functional materials and fabricating devices. However, implementing R2R printing to fabricate sophisticated devices is challenging because of the efficiency of material processing, the alignment, and the vulnerability of the polymeric substrate during printing. Therefore, this study proposes the fabrication process of a hybrid device to solve the problems. The device was created so that four layers, composed of polymer insulating layers and conductive circuit layers, are entirely screen-printed layer by layer onto a roll of polyethylene terephthalate (PET) film to produce the circuit. Registration control methods were presented to deal with the PET substrate during printing, and then solid-state components and sensors were assembled and soldered to the printed circuits of the completed devices. In this way, the quality of the devices could be ensured, and the devices could be massively used for specific purposes. Specifically, a hybrid device for personal environmental monitoring was fabricated in this study. The importance of environmental challenges to human welfare and sustainable development is growing. As a result, environmental monitoring is essential to protect public health and serve as a basis for policymaking. In addition to the fabrication of the monitoring devices, a whole monitoring system was also developed to collect and process the data. Here, the monitored data from the fabricated device were personally collected via a mobile phone and uploaded to a cloud server for additional processing. The information could then be utilized for local or global monitoring purposes, moving one step toward creating tools for big data analysis and forecasting. The successful deployment of this system could be a foundation for creating and developing systems for other prospective uses.

Strain Optimization of Tensioned Web through Computational Fluid Dynamics in the Roll-to-Roll Drying Process.

Unpredictable web temperature distributions in the dryer and strain deviations in the cross-machine (CMD) and machine (MD) directions could hamper the manufacture of smooth functional layers on polymer-based webs through the roll-to-roll (R2R) continuous process system. However, research on this topic is limited. In this study, we developed a structural analysis model using the temperature distribution of the web as a boundary condition to analyze the drying mechanism of the dryer used in an R2R system. Based on the results of this model, we then applied structural modifications to the flow channel and hole density of the aluminum plate of the dryer. The model successfully predicted the temperature and strain distributions of the web inside the dryer in the CMD and MD by forming a tension according to the speed difference of the driven rolls at both ends of the span. Our structural improvements significantly reduced the temperature deviation of the moving web inside the dryer by up to 74% and decreased the strain deviation by up to 46%. The findings can help prevent web unevenness during the drying process of the R2R system, which is essential to minimize the formation of defects on functional layers built over polymer-based webs.

AI-assisted reliability assessment for gravure offset printing system.

In printed electronics, flawless printing quality is crucial for electronic device fabrication. While printing defects may reduce the performance or even cause a failure in the electronic device, there is a challenge in quality evaluation using conventional computer vision tools for printing defect recognition. This study proposed the computer vision approach based on artificial intelligence (AI) and deep convolutional neural networks. First, the data set with printed line images was collected and labeled. Second, the overall printing quality classification model was trained and evaluated using the Grad-CAM visualization technique. Third and last, the pretrained object detection model YOLOv3 was fine-tuned for local printing defect detection. Before fine-tuning, ground truth bounding boxes were analyzed, and anchor box sizes were chosen using the k-means clustering algorithm. The overall printing quality and local defect detection AI models were integrated with the roll-based gravure offset system. This AI approach is also expected to complement more accurate printing reliability analysis firmly.

IoT device fabrication using roll-to-roll printing process.

With the development of technology, wireless and IoT devices are increasingly used from daily life to industry, placing demands on rapid and efficient manufacturing processes. This study demonstrates the fabrication of an IoT device using a roll-to-roll printing process, which could shorten the device fabrication time and reduce the cost of mass production. Here, the fabricated IoT device is designed to acquire data through the sensor, process the data, and communicate with end-user devices via Bluetooth communication. For fabrication, a four-layer circuit platform consisting of two conductive layers, an insulating layer including through holes, and a solder resist layer is directly printed using a roll-to-roll screen printing method. After the printing of the circuit platform, an additional layer of solder paste is printed to assemble the electrical components into the device, inspiring the fully roll-to-roll process for device fabrication. Successful IoT device deployment opens the chance to broaden the roll-to-roll fabrication process to other flexible and multilayer electronic applications.

Empirical design of slot-die having shallow reservoir for thin-film printed electronics.

Slot-die coating plays an important role in printed electronics, which are fabricated by stacking thin films and patterns. As electronic devices are being required to have higher performance, the importance of coating uniformity cannot be overestimated in the slot-die coating. The coating uniformity consists of two directions: nozzle direction, which is affected by the interior design of the head, and machine direction, which is majorly related to exterior operating conditions. In this research, the empirical design procedure of a slot-die head is proposed. The internal resistance values of the slot-die were calculated using the experimental parameter obtained through a coating experiment, and the acquired resistances were reflected in the modeling of the interior design of the slot-die head. The hanger type reservoir was adapted to minimize the consumption of ink. After fabricating the slot-die head, coating experiments were carried out using PEDOT:PSS ink. The resulting deviation of the coating thickness was within ±1.7%, thus proving that the proposed design predicted the uniformity of the actual thickness of the coating correctly. The significance of the slot-die design method presented in this paper is that it is based on the experimental equation that can be readily applied to the printed electronics industry.

Roll-to-roll-compatible, flexible, transparent electrodes based on self-nanoembedded Cu nanowires using intense pulsed light irradiation.

Copper nanowire (Cu NW)-based flexible transparent conductive electrodes (FTCEs) have been investigated in detail for use in various applications such as flexible touch screens, organic photovoltaics and organic light-emitting diodes. In this study, hexadecylamine (HDA) adsorbed onto the surface of NWs is changed into polyvinylpyrrolidone (PVP) via a ligand exchange process; the high-molecular-weight PVP enables high dispersion stability. Intense pulsed light (IPL) irradiation is used to remove organic species present on the surface of the NWs and to form direct connections between the NWs rapidly without any atmospheric control. NWs are self-nanoembedded into a plastic substrate after IPL irradiation, which results in a smooth surface, strong NW/substrate adhesion, excellent mechanical flexibility and enhanced oxidation stability. Moreover, Cu NW FTCEs with high uniformities are successfully fabricated on a large area (150 mm × 200 mm) via successive IPL irradiation that is synchronized with the motion of the sample stage. This study demonstrates the possibility of roll-to-roll-based, large-scale production of low-cost, high-performance Cu NW-based FTCEs.

Development of a precision reverse offset printing system.

In printed electronics technology, the overlay accuracy of printed patterns is a very important issue when applying printing technology to the production of electric devices. In order to achieve accurate positioning of the printed patterns, this study proposes a novel precision reverse offset printing system. Furthermore, the study evaluates the effects of synchronization and printing force on position errors of the printed patterns, and presents methods of controlling synchronization and printing force so as to eliminate positional errors caused by the above-mentioned reasons. Finally, the printing position repeatability of 0.40 µm and 0.32 µm (x and y direction, respectively) at a sigma level is obtained over the dimension of 100 mm under repeated printing tests with identical printing conditions.

All-solution-processed PbS quantum dot solar modules.

A rapid increase in power conversion efficiencies in colloidal quantum dot (QD) solar cells has been achieved recently with lead sulphide (PbS) QDs by adapting a heterojunction architecture, which consists of small-area devices associated with a vacuum-deposited buffer layer with metal electrodes. The preparation of QD solar modules by low-cost solution processes is required to further increase the power-to-cost ratio. Herein we demonstrate all-solution-processed flexible PbS QD solar modules with a layer-by-layer architecture comprising polyethylene terephthalate (PET) substrate/indium tin oxide (ITO)/titanium oxide (TiO2)/PbS QD/poly(3-hexylthiophene) (P3HT)/poly(3,4-ethylenedioxythiophene) : poly(styrene sulfonate) (PEDOT : PSS)/Ag, with an active area of up to 30 cm(2), exhibiting a power conversion efficiency (PCE) of 1.3% under AM 1.5 conditions (PCE of 2.2% for a 1 cm(2) unit cell). Our approach affords trade-offs between power and the active area of the photovoltaic devices, which results in a low-cost power source, and which is scalable to larger areas.

Exposure assessment of workers in printed electronics workplace.

Printed electronics uses converging technologies, such as printing, fine mechanics, nanotechnology, electronics and other new technologies. Consequently, printed electronics raises additional health and safety concerns to those experienced in the traditional printing industry. This study investigated two printed electronics workplaces based on a walk-through survey and personal and area sampling. All the printed electronics operations were conducted in a cleanroom. No indication of exposure to excess silver nanoparticles or carbon nanotubes (CNTs) was found. While the organic solvents were lower than current occupational exposure limits, there was a lack of engineering controls, such as local exhaust ventilation, correct enclosure and duct connections. There was also an insufficient quantity of personal protective equipment, and some organic solvents not described in the safety data sheets (SDSs) were detected in the air samples. Plus, the cleaning work, a major emissions operation, was not conducted within a hood, and the cleaning waste was not properly disposed of. Therefore, the present exposure assessment results from two printed electronics workplaces suggest that the printed electronics industry needs to take note of the occupational safety and health risks and hazards already established by the traditional printing industry, along with new risks and hazards originating from converging technologies such as nanotechnology.

Direct and precise measurement of displacement and velocity of flexible web in roll-to-roll manufacturing systems.

Interest in the production of printed electronics using a roll-to-roll system has gradually increased due to its low mass-production costs and compatibility with flexible substrate. To improve the accuracy of roll-to-roll manufacturing systems, the movement of the web needs to be measured precisely in advance. In this paper, a novel measurement method is developed to measure the displacement and velocity of the web precisely and directly. The proposed algorithm is based on the traditional single field encoder principle, and the scale grating has been replaced with a printed grating on the web. Because a printed grating cannot be as accurate as a scale grating in a traditional encoder, there will inevitably be variations in pitch and line-width, and the motion of the web should be measured even though there are variations in pitch and line-width in the printed grating patterns. For this reason, the developed algorithm includes a precise method of estimating the variations in pitch. In addtion, a method of correcting the Lissajous curve is presented for precision phase interpolation to improve measurement accuracy by correcting Lissajous circle to unit circle. The performance of the developed method is evaluated by simulation and experiment. In the experiment, the displacement error was less than 2.5 µm and the velocity error of 1σ was about 0.25%, while the grating scale moved 30 mm.

Roll-printed organic thin-film transistor using patterned poly(dimethylsiloxane) (PDMS) stamp.

The roll-printed gate, source, and drain electrodes of organic thin-film transistors (OTFTs) were fabricated by gravure printing or gravure-offset printing using patterned poly(dimethylsiloxane) (PDMS) stamp with various channel lengths and low-resistance silver (Ag) pastes on flexible 150 x 150 mm2 plastic substrates. Bottom-contact roll-printed OTFTs used polyvinylphenol (PVP) as polymeric dielectric and bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) as organic semiconductor; they were formed by spin coating or ink-jetting. Depending on the choice of roll-printing method, the printed OTFTs obtained had a field-effect mobility of between 0.08 and 0.1 cm2/Vs, an on/off current ratio of between 10(4) and 10(5), and a subthreshold slope of between 1.96 and 2.32 V/decade. The roll-printing using patterned PDMS stamp and soluble processes made it possible to fabricate a printed OTFT with a channel length of between 12 to 74 microm on a plastic substrate; this was not previously possible using traditional printing techniques. The proposed fabrication process was 20 steps shorted than conventional fabrication techniques.