Direct printing methods have been used as manufacturing tools for printed electronics applications due to their cost effectiveness. In this review, the piezo-driven inkjet is discussed in detail since it is a mature technology and suitable for the production printing of printed electronics. In addition, other printing methods are considered for using higher viscosity ink and for producing smaller printed feature size. Various direct printing methods are compared in terms of jet mechanism, printing algorithm, and their applications. In particular high resolution printing methods using high viscosity inks, such as electrohydrodynamic jet, aerosol jet and micro-plotter are reviewed. To understand the recent status of industrial printing applications, display (liquid crystal display and organic light emitting diode) materials and printing issues are discussed. Finally, a brief overview of nano-particle metal based conductive inks is included because these inks have been widely used for printed electronics applications.
The Organic and Printed Electronics Association (OE-A) was founded in December 2004 and is the leading international industry association for organic and printed electronics. OE-A represents the entire value chain of this emerging industry. Our members are world-class global companies and institutions, ranging from R&D institutes, mechanical engineering companies and material suppliers to producers and end-users.
Well over 200 companies from Europe, Asia, North America, South America, Africa and Oceania are working together to promote the establishment of a competitive production infrastructure for organic and printed electronics.
The vision of OE-A is to build a bridge between science, technology and application. OE-A is a working group within VDMA. More than 3100 member companies from the engineering industry make VDMA the largest industry association in Europe.
Korea Flexible Printed Electronics (KFPE) is a representative academic association of Korea in the field of flexible printed electronics. Promoting the communication between academia and industry, KFPE contributes to the development of the related technology and industry. The conferences and the publications of journal may lead the technology to the front edge. The exhibitions are organized to enhance the technology of industry and the preparation for the future business related to flexible printed electronics like, display, solar cell and semiconductor.
Purpose-led Publishing is a coalition of three not-for-profit publishers in the field of physical sciences: AIP Publishing, the American Physical Society and IOP Publishing.
Together, as publishers that will always put purpose above profit, we have defined a set of industry standards that underpin high-quality, ethical scholarly communications.
We are proudly declaring that science is our only shareholder.
Endorsed by
ISSN: 2058-8585
Flexible and Printed Electronics is a multidisciplinary journal devoted to publishing cutting edge research articles on electronics that can be either flexible, plastic, stretchable, conformable or printed.
Research related to electronic materials, manufacturing techniques, components or systems which meets any one (or more) of the above criteria is suitable for publication in the journal.
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Kye-Si Kwon et al 2020 Flex. Print. Electron. 5 043003
Yvan Bonnassieux et al 2021 Flex. Print. Electron. 6 023001
This roadmap includes the perspectives and visions of leading researchers in the key areas of flexible and printable electronics. The covered topics are broadly organized by the device technologies (sections 1–9), fabrication techniques (sections 10–12), and design and modeling approaches (sections 13 and 14) essential to the future development of new applications leveraging flexible electronics (FE). The interdisciplinary nature of this field involves everything from fundamental scientific discoveries to engineering challenges; from design and synthesis of new materials via novel device design to modelling and digital manufacturing of integrated systems. As such, this roadmap aims to serve as a resource on the current status and future challenges in the areas covered by the roadmap and to highlight the breadth and wide-ranging opportunities made available by FE technologies.
William N Hartnett et al 2024 Flex. Print. Electron. 9 025006
Resistors are basic yet essential circuit components that must be fabricated with high precision at low cost if they are to be viable for flexible electronic applications. Inkjet printing is one of many additive fabrication techniques utilized to realize this goal. In this work, a process termed self-aligned capillarity-assisted lithography for electronics (SCALE) was used to fabricate inkjet-printed resistors on flexible substrates. Capillary channels and reservoirs imprinted onto flexible substrates enabled precise control of resistor geometry and straightforward alignment of materials. More than 300 devices were fabricated using poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the resistive material and silver as the electrode material. By varying PEDOT:PSS ink formulation and resistor geometry, resistances spanning from 170 Ω to 3.8 MΩ were achieved. Over 98% of devices were functional and the relative standard deviation in resistance ranged from 3% to 18% depending on resistor length and ink composition. The resistors showed no significant change in resistance after 10 000 cycles of bend testing at 1.6% surface tensile strain. In summary, this work demonstrated a fully roll-to-roll compatible process for inkjet printing resistors with superior properties.
Ethan B Secor 2018 Flex. Print. Electron. 3 035002
Aerosol jet printing (AJP) has emerged as a promising method for microscale digital additive manufacturing using functional nanomaterial inks. While compelling capabilities have been demonstrated in the research community in recent years, the development and refinement of inks and process parameters largely follows empirical observations, with an extensive phase space over which to optimize. While this has led to general qualitative guidelines and ink- and machine-specific correlations, a more fundamental understanding based on principles of aerosol physics and fluid mechanics is lacking. This contrasts with more mature printing technologies, for which foundational physical principles have been rigorously examined. Presented here is a broad framework for describing the AJP process. Simple analytical models are employed to ensure generality and accessibility of the results, while experimental validation using a silver nanoparticle ink supports the physical relevance of the approach. This basic understanding enables a description of process limitations grounded in fundamental principles, as well as guidelines for improved printer design, ink formulation, and print parameter optimization.
C S Buga and J C Viana 2022 Flex. Print. Electron. 7 043001
The emergence of novel materials with flexible and stretchable characteristics, and the use of new processing technologies, have allowed for the development of new connected devices and applications. Using printed electronics, traditional electronic elements are being combined with flexible components and allowing for the development of new smart connected products. As a result, devices that are capable of sensing, actuating, and communicating remotely while being low-cost, lightweight, conformable, and easily customizable are already being developed. Combined with the expansion of the Internet of Things, artificial intelligence, and encryption algorithms, the overall attractiveness of these technologies has prompted new applications to appear in almost every sector. The exponential technological development is currently allowing for the 'smartification' of cities, manufacturing, healthcare, agriculture, logistics, among others. In this review article, the steps towards this transition are approached, starting from the conceptualization of smart connected products and their main markets. The manufacturing technologies are then presented, with focus on printing-based ones, compatible with organic materials. Finally, each one of the printable components is presented and some applications are discussed.
Muhammad Atif Khan et al 2024 Flex. Print. Electron. 9 025008
Flexible heaters (FHs) have applications ranging from defoggers to flexural warmers, food processors, and thermotherapy. Printed FHs are particularly of interest as they offer unique advantages like high resolution, customization, low cost, and ease of fabrication. Here, we report printed FHs on polyethylene terephthalate substrate. The heater design is optimized to operate on a low voltage of five volts and yield high temperatures with a uniform temperature distribution across the surface. The heater has a fast response time of 15 s to reach its maximum temperature and does not show any degradation in performance after three months of operation. The heater maintains its temperature after continuous use for two hours and exhibits a minimum change in temperature upon bending. We have also developed and tested designs for zone heaters and nano heaters, where zone heater is suited for applications requiring heating in specified locations on a surface only. Whereas nano heater has an area of 1 mm2 and can produce high temperatures in this small area. Finally, we developed similar printed heaters on paper and polyimide (PI) substrates as well. Paper-based heater can achieve a temperature of 210 °C and can be used in disposable applications due to its low cost, whereas PI heater can achieve a temperature of 380 °C and is suitable for attaining high temperatures. These results manifest the use of FHs for various practical applications.
L Sokka et al 2024 Flex. Print. Electron. 9 015007
Within the past years, there has been a growing demand for sustainable, cost-efficient on-line sensing of chemical and physical properties and locations of products. Measuring of products' physical properties, such as temperature and humidity, could improve product safety and efficiency of logistic operations. In the future measurement of temperature of food items could also aid in reducing food wastage. The aim of this study was to calculate the life cycle environment impacts of a temperature logger, hereafter called smart label, primarily targeted for the monitoring of the packed food products. According to the results, the largest normalised impacts of the smart label production are resource use (both use of fossil fuels and use of minerals and metals), eutrophication and particulate matter formation. The main materials causing these impacts were the printed electronics inks and adhesives. In addition, energy used in the production, and plastics used as substrates had large impacts on the results. It should be noted that the present calculations have mainly been made on a laboratory scale. The impacts are likely to get smaller on an industrial scale with more efficient production. In the future, the label could potentially bring environmental benefits through product savings when used in products with high environmental load.
Dongping Wang et al 2022 Flex. Print. Electron. 7 023004
Thin-film transistor (TFT) active matrix (AM) arrays have been developed to achieve many applications, including flat panel displays, digital x-rays, digital microfluidics (DMF) and high-throughput biosensors. Here, we focus on a review on TFT array technologies for biological sensing systems, which are regarded as one of the most promising emerging application fields of TFTs. As an important part of the biological sensing system, the DMF chip will be introduced. In particular, development of the TFT-based AM DMF (AM-DMF) chips, which possess the characteristics of higher throughput and higher flexibility of manipulating liquid samples, will be discussed in details. Further, the developed TFT array based biological sensing systems will be summarized and discussed as well. Finally, we present prospects for AM-DMF chips and biosensors, along with a brief conclusion.
Pierre Kateb et al 2023 Flex. Print. Electron. 8 045006
Printable, self-healing, stretchable, and conductive materials have tremendous potential for the fabrication of advanced electronic devices. Poly(3,4-ethylenedioxithiopene) doped with polystyrene sulfonate (PEDOT:PSS) has been the focus of extensive research due to its tunable electrical and mechanical properties. Owing to its solution-processability and self-healing ability, PEDOT:PSS is an excellent candidate for developing printable inks. In this study, we developed printable, stretchable, dry, lightly adhesive, and self-healing materials for biomedical applications. Polyurethane diol (PUD), polyethylene glycol, and sorbitol were investigated as additives for PEDOT:PSS. In this study, we identified an optimal printable mixture obtained by adding PUD to PEDOT:PSS, which improved both the mechanical and electrical properties. PUD/PEDOT:PSS free-standing films with optimized composition showed a conductivity of approximately 30 S cm−1, stretchability of 30%, and Young's modulus of 15 MPa. A low resistance change (<20%) was achieved when the strain was increased to 30%. Excellent electrical stability under cyclic mechanical strain, biocompatibility, and 100% electrical self-healing were also observed. The potential biomedical applications of this mixture were demonstrated by fabricating a printed epidermal electrode on a stretchable silicone substrate. The PUD/PEDOT:PSS electrodes displayed a skin-electrode impedance similar to commercially available ones, and successfully captured physiological signals. This study contributes to the development of improved customization and enhanced mechanical durability of soft electronic materials.
Redwan Ahmad et al 2024 Flex. Print. Electron. 9 025005
We employed the screen-printing method to fabricate terahertz (THz) frequency selective surfaces (FSSs) featuring an inductive metallic checkerboard (i-MCB) pattern based on conductive silver ink onto a flexible polyethylene terephthalate substrate, chosen for its excellent THz transmission properties below 1 THz [Jin et al 2006 J. Korean Phys. Soc.49 513–17]. Analytical studies, along with simulations and experiments, were conducted to investigate the filtering characteristics of the printed FSSs, confirming their functionality as a band-pass filter. Subsequently, we demonstrated the reconfigurability of a two-layer system by vertically stacking two layers. This was achieved by systematically shifting the position of the second layer in the x or y-direction relative to the first layer. Experimental verification revealed a significant variation in normalized transmission, ranging from 94% to 6% at 0.15 THz for type-I:i-MCBs and 90% to 5% at 0.20 THz for type-II:i-MCBs, respectively. This study presents a simple scheme for a reconfigurable screen-printed i-MCB-FSS operating in the THz range. Consequently, our findings demonstrate that screen printing method can effectively be employed for the large-scale production of THz FSSs.
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Cem Odaci et al 2024 Flex. Print. Electron. 9 025010
Layered GaS structures have been attracting increasing research interest due to their highly anisotropic structural, electrical, optical, and mechanical properties. However, the investigation of the performance based on the responsivity, external quantum efficiency, and detectivity of printed GaS based photodetector on a flexible PET substrate with respect to a period of time under the environmental conditions have not been reported so far. This experimental study shows that the printed GaS based photodetector stored in ambient conditions undergoes a change in terms of performance in a few weeks after the fabrication. This work also holds an importance being premier study experimentally investigating the printed III–VI group monochalcogenide based photodetector stored under the environmental conditions and contributing the literature to improve the printed device performance in further applications.
Shangjian Liu et al 2024 Flex. Print. Electron. 9 025009
Direct ink writing (DIW) represents a technical branch of additive manufacturing technology, particularly suitable for prototyping or small-batch printing production of printed electronic components. However, the existing print heads required for near or sub-100 μm line width are quite sophisticated, limiting the accessibility and application of the DIW method. This paper reports the use of the vibrating membrane ejector (VME) as a new option for the print head of DIW. The structure of the VME-based print head was specially designed for this purpose. Finite element modeling and analysis of the VME's vibration characteristics were performed to provide insights into the ejection conditions and behaviors. The factors influencing the size of printed structures were identified and analyzed through the printing of a metal–organic complex silver (Ag) ink. After optimizing several operational parameters to limit the spreading effects and suppress the satellite droplets, the DIW printed line width has reached about 100 μm. The effectiveness of the VME-based print head was further demonstrated through the DIW fabrication of interdigitated electrodes and microstrip transmission lines. This highlights the versatility of the VME-based print head as a practical tool for device prototyping and ink development in the field of printed electronics.
Muhammad Atif Khan et al 2024 Flex. Print. Electron. 9 025008
Flexible heaters (FHs) have applications ranging from defoggers to flexural warmers, food processors, and thermotherapy. Printed FHs are particularly of interest as they offer unique advantages like high resolution, customization, low cost, and ease of fabrication. Here, we report printed FHs on polyethylene terephthalate substrate. The heater design is optimized to operate on a low voltage of five volts and yield high temperatures with a uniform temperature distribution across the surface. The heater has a fast response time of 15 s to reach its maximum temperature and does not show any degradation in performance after three months of operation. The heater maintains its temperature after continuous use for two hours and exhibits a minimum change in temperature upon bending. We have also developed and tested designs for zone heaters and nano heaters, where zone heater is suited for applications requiring heating in specified locations on a surface only. Whereas nano heater has an area of 1 mm2 and can produce high temperatures in this small area. Finally, we developed similar printed heaters on paper and polyimide (PI) substrates as well. Paper-based heater can achieve a temperature of 210 °C and can be used in disposable applications due to its low cost, whereas PI heater can achieve a temperature of 380 °C and is suitable for attaining high temperatures. These results manifest the use of FHs for various practical applications.
Md Abu Mosa et al 2024 Flex. Print. Electron. 9 025007
Aerosol jet printing (AJP) outperforms inkjet printing by significantly reducing printed line width, effectively addressing issues such as bulging and surface irregularities. This technology allows for line widths as narrow as 10–100 μm with high aspect ratios, making it well-suited for radio frequency (RF) applications. Consequently, AJP emerges as a valuable tool for direct printing in RF applications. Among conductive inks, silver nanoparticle (Ag-NP) ink is preferred for its straightforward direct printing process and lower sintering temperature requirements. However, the conductivity of printed Ag NP traces falls markedly below that of bulk silver due to significant porosity, limiting its use in RF applications where a high-quality factor is essential. The quality factor of an inductor, indicative of its efficiency in energy storage and release, inversely correlates with its resistance. Our research combines AJP with selective electroplating to reduce the resistance of printed traces, thereby enhancing the inductor's quality factor for RF applications. We fabricated spiral inductors on alumina substrates using silver NP ink and subsequently applied selective gold electroplating to these traces. This approach led to a significant increase in the inductors' quality factor, improving it by a factor of 3–5 in the RF frequency range of 100–700 MHz.
William N Hartnett et al 2024 Flex. Print. Electron. 9 025006
Resistors are basic yet essential circuit components that must be fabricated with high precision at low cost if they are to be viable for flexible electronic applications. Inkjet printing is one of many additive fabrication techniques utilized to realize this goal. In this work, a process termed self-aligned capillarity-assisted lithography for electronics (SCALE) was used to fabricate inkjet-printed resistors on flexible substrates. Capillary channels and reservoirs imprinted onto flexible substrates enabled precise control of resistor geometry and straightforward alignment of materials. More than 300 devices were fabricated using poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the resistive material and silver as the electrode material. By varying PEDOT:PSS ink formulation and resistor geometry, resistances spanning from 170 Ω to 3.8 MΩ were achieved. Over 98% of devices were functional and the relative standard deviation in resistance ranged from 3% to 18% depending on resistor length and ink composition. The resistors showed no significant change in resistance after 10 000 cycles of bend testing at 1.6% surface tensile strain. In summary, this work demonstrated a fully roll-to-roll compatible process for inkjet printing resistors with superior properties.
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Shimin Liu et al 2024 Flex. Print. Electron. 9 023001
Due to its conformal capability, the flexible pressure sensor has a wide range of applications in wearable devices, health monitoring, human–machine interfaces, and other fields. Sensors designed according to various principles and application scenarios exhibit a variety of good characteristics such as high sensitivity, high transparency, a wide detection limit, and low crosstalk. However, achieving all these exceptional functions within a single sensor is evidently challenging. Therefore, it is prudent to emphasize specific advantageous features depending on the unique usage environments and application scenarios. This paper first describes the classification of flexible pressure sensors based on their working principle, then summarizes the commonly used materials and sensor characteristics, and finally reviews the application characteristics of flexible pressure sensors based on different application fields and scenarios. The bottleneck challenges encountered in the development of flexible pressure sensors are discussed, and the foreseeable development strategy is predicted.
Shunsuke Yamamoto 2024 Flex. Print. Electron. 9 013001
This article summarizes recent developments in organic mixed ion-electron conductors for organic electrochemical transistors (OECTs) in the field of materials science. Materials for the active layers, including crosslinkers and additives, are summarized, with a focus on poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). Particular aspects of organic devices are highlighted, including their flexibility, biocompatibility, and facile fabrication processes.
Hyeon-Soo Lee et al 2023 Flex. Print. Electron. 8 043001
Wearable neuromorphic devices have gained attention because of the growth in the Internet of Things and the increasing demand for health monitoring. They provide meaningful information and interact with the external environment through physiological signal processing and seamless interaction with the human body. The concept of these devices originated from the development of neuromorphic and flexible/stretchable electronics, which offer a solution to the limitation of conventional rigid devices. They have been developed to mimic synaptic functions and flexibility/stretchability of the biological nervous system. In this study, we described the various synaptic properties that should be implemented in synaptic devices and the operating mechanisms that exhibit these properties with respect to two- and three-terminal devices. Further, we specified comprehensive methods of implementing mechanical flexibility and stretchability in neuromorphic electronics through both structure and material engineering. In addition, we explored various wearable applications of these devices, such as wearable sensors for danger detection, auxiliary equipment for people with sensory disabilities, and neuroprosthetic devices. We expect this review to provide an overall understanding of concepts and trends for flexible and stretchable neuromorphic devices, with potential extensions to state-of-the-art applications such as cybernetics and exoskeleton.
Long Yang et al 2023 Flex. Print. Electron. 8 033005
The optimization of electrochemical energy storage devices (EES) for low-temperature conditions is crucial in light of the growing demand for convenient living in such environments. Sluggish ion transport or the freezing of electrolytes at the electrode-electrolyte interface are the primary factors that limit the performance of EES under low temperatures, leading to fading of capacity and instability in device performance. This review provides a comprehensive overview of antifreeze strategies for various electrolytes (including aqueous electrolytes, organic electrolytes, and ionic liquids), and optimization methods for ion transport at the electrolyte-electrode. Additionally, the main challenges and forward-looking views are highlighted on the design and development of low-temperature electrolytes and EES devices.
Liming Xie and Wenming Su 2023 Flex. Print. Electron. 8 033004
Quantum dot light-emitting diodes (QLEDs) have attracted a great deal of interests due to their unique advantages such as tunable spectrum, high color saturation, compatibility with low-cost inkjet-printing (IJP) technology and potential for use in large-area full-color pixelated display. To date, the efficiency and lifetime of red, green, and blue QLEDs have been significantly improved, in which hole-transporting materials (HTMs) play the key role in determining the device performance. In this review, we highlight to summarize the diverse types of HTMs in QLEDs, including small-organic materials, polymers, crosslinkable materials and inorganic p-type semiconductors, and their properties such as charge carrier mobility, thermal stability, and structural configuration are also reviewed. The significant effects of these properties on device performances are discussed, which would help to understand device physics and improve their performances and reliability of QLEDs. In addition, the development of IJP for QLEDs fabrication and the influence factors of IJP on quantum dot film-forming property are also reviewed, in an effort to provide guidance to continue the advancement of QLED displays.
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Nga et al
Recently, there has been an increasing interest in the development of wearable sensors for monitoring vitamin C (ascorbic acid) in sweat. These sensors can help assess personal nutritional status, prevent vitamin imbalances, and determine the effectiveness of certain medical treatments. This study presents the first example of non-enzymatic ascorbic acid sensor based on the catalytic activity of printed AuNPs. The three-electrode electrochemical system sensor was fabricated by printing a working and counter-electrode from an AuNP-based ink and a pseudo-reference electrode from a silver ink, on a flexible Kapton® substrate. SEM examination of the printed gold layer revealed a highly divided material, which facilitates electron transfer and accelerates the oxidation of ascorbic acid. In vitro amperometry demonstrated a proportional increase in current with ascorbic acid concentration ranging from 10 to 390 µM, with a sensitivity of 14 µA.mM-1.cm-2. The low oxidation potential of +0.2 V vs Ag/AgCl effectively avoided most interfering oxidations. These results pave the way toward evaluation on the body of healthy volunteers, by placing the sensor directly on their skin, for example for tracking the changes in ascorbic acid concentration in sweat when eating Vitamin C tablets or Vitamin C-containing food such as orange juice. The very first results demonstrated good real-time performance in testing in such experimental conditions. This has potential applications in healthcare, not only on skin but also on therapeutic bandages applied to chronic wounds.
Oliveira et al
Reverse micelles composed of polystyrene-b-poly(2-vinylpyiridine) have been used to synthesize nanoparticles composed of a wide range of materials, including metals, metal oxides, dielectrics, semiconductors perovskites, and core-shell nanoparticles. In this contribution, we examine the effect of deposition parameters on two-dimensional nanoparticle arrangements from colloidal solutions created using spin coating, dip coating, slot-die coating, and electrospray deposition. Despite the importance of achieving uniform coatings of ordered arrays of colloidal particles, previous studies have not thoroughly addressed this challenge. We show that the adjustability of interparticle distance depends on the deposition technique used and only occurs within the stable defect-free operating window of the deposition parameters. Establishing the specific operating window for each technique for a model system, we propose general guidelines that can be used for ensuring uniform coatings regardless of precursor loading and provide a guide for adjusting the deposition conditions when coating defects occur. We introduces a novel application of ellipsometry to evaluate interparticle spacing in nanoparticle arrays, enhancing our ability to assess film uniformity, allowing for quick and easy tuning of nanoparticle dispersion. Comparisons between spin, dip, and slot-die coating techniques reveal insights into the correlation between interparticle spacing and ordering, highlighting the importance of fitting relationships for various coating samples. This comprehensive comparison and discussion provide a roadmap for future research, outlining current challenges and trends and offering insights into achievable spacings and ordering in coating processes. This information allows the classification of various deposition techniques with respect to their suitability for tailored applications.
Lee et al
Direct-printed quantum dot (QD) patterns utilizing inkjet method are a next-generation alternative to achieve high-resolution quantum dot light-emitting diodes (QLED). However, research on high-resolution pixel patterns is still limited due to issues such as thickness uniformity. In this study, we explored the feasibility of direct printing QLEDs to attain high-resolution pixel-patterned displays. Here, we successfully fabricated arrays of green QLED devices with a resolution of 200 pixels per inch, employing a precise 1 pL fine printer nozzle and optimizing ink formulation to achieve superior surface roughness and uniformity at the desired thickness. The inkjet-printed QD layers demonstrated a surface roughness of 1.9 nm and a flatness ratio of 0.2, comparable to those achieved through conventional spin-coating processes. This investigation offers the potential for extending the methodology to the fabrication of cost-effective, high-resolution full-color QLED displays.
Li et al
Active sensing technology plays an essential role in environment–robot interactions. Inspired by the proximity sensing approach of weakly electric fish, which relies on distributed electroreceptors capable of detecting electric fields, we propose a flexible electronic skin (e-skin) or proximity and pressure detection. Conductive thermoplastic polyurethane and dielectric polyurethane are employed for flexible electrodes and substrates, respectively. An ecoflex-based elastic layer enables decoupling proximity and pressure information from the electric field. The proposed electronic skin can detect objects as far as 160 mm while performing real-time proximity and pressure sensing. Finally, we demonstrate that robots equipped with the e-skin can easily explore their surroundings and perform specific tasks such as recognition, avoidance, and grasping. The developed e-skin, with proximity and pressure-sensing capabilities and a low-cost fabrication process, can have broad application potential in robot active sensing.
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Pedro Quintella Oliveira et al 2024 Flex. Print. Electron.
Reverse micelles composed of polystyrene-b-poly(2-vinylpyiridine) have been used to synthesize nanoparticles composed of a wide range of materials, including metals, metal oxides, dielectrics, semiconductors perovskites, and core-shell nanoparticles. In this contribution, we examine the effect of deposition parameters on two-dimensional nanoparticle arrangements from colloidal solutions created using spin coating, dip coating, slot-die coating, and electrospray deposition. Despite the importance of achieving uniform coatings of ordered arrays of colloidal particles, previous studies have not thoroughly addressed this challenge. We show that the adjustability of interparticle distance depends on the deposition technique used and only occurs within the stable defect-free operating window of the deposition parameters. Establishing the specific operating window for each technique for a model system, we propose general guidelines that can be used for ensuring uniform coatings regardless of precursor loading and provide a guide for adjusting the deposition conditions when coating defects occur. We introduces a novel application of ellipsometry to evaluate interparticle spacing in nanoparticle arrays, enhancing our ability to assess film uniformity, allowing for quick and easy tuning of nanoparticle dispersion. Comparisons between spin, dip, and slot-die coating techniques reveal insights into the correlation between interparticle spacing and ordering, highlighting the importance of fitting relationships for various coating samples. This comprehensive comparison and discussion provide a roadmap for future research, outlining current challenges and trends and offering insights into achievable spacings and ordering in coating processes. This information allows the classification of various deposition techniques with respect to their suitability for tailored applications.
Muhammad Atif Khan et al 2024 Flex. Print. Electron. 9 025008
Flexible heaters (FHs) have applications ranging from defoggers to flexural warmers, food processors, and thermotherapy. Printed FHs are particularly of interest as they offer unique advantages like high resolution, customization, low cost, and ease of fabrication. Here, we report printed FHs on polyethylene terephthalate substrate. The heater design is optimized to operate on a low voltage of five volts and yield high temperatures with a uniform temperature distribution across the surface. The heater has a fast response time of 15 s to reach its maximum temperature and does not show any degradation in performance after three months of operation. The heater maintains its temperature after continuous use for two hours and exhibits a minimum change in temperature upon bending. We have also developed and tested designs for zone heaters and nano heaters, where zone heater is suited for applications requiring heating in specified locations on a surface only. Whereas nano heater has an area of 1 mm2 and can produce high temperatures in this small area. Finally, we developed similar printed heaters on paper and polyimide (PI) substrates as well. Paper-based heater can achieve a temperature of 210 °C and can be used in disposable applications due to its low cost, whereas PI heater can achieve a temperature of 380 °C and is suitable for attaining high temperatures. These results manifest the use of FHs for various practical applications.
William N Hartnett et al 2024 Flex. Print. Electron. 9 025006
Resistors are basic yet essential circuit components that must be fabricated with high precision at low cost if they are to be viable for flexible electronic applications. Inkjet printing is one of many additive fabrication techniques utilized to realize this goal. In this work, a process termed self-aligned capillarity-assisted lithography for electronics (SCALE) was used to fabricate inkjet-printed resistors on flexible substrates. Capillary channels and reservoirs imprinted onto flexible substrates enabled precise control of resistor geometry and straightforward alignment of materials. More than 300 devices were fabricated using poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) as the resistive material and silver as the electrode material. By varying PEDOT:PSS ink formulation and resistor geometry, resistances spanning from 170 Ω to 3.8 MΩ were achieved. Over 98% of devices were functional and the relative standard deviation in resistance ranged from 3% to 18% depending on resistor length and ink composition. The resistors showed no significant change in resistance after 10 000 cycles of bend testing at 1.6% surface tensile strain. In summary, this work demonstrated a fully roll-to-roll compatible process for inkjet printing resistors with superior properties.
Redwan Ahmad et al 2024 Flex. Print. Electron. 9 025005
We employed the screen-printing method to fabricate terahertz (THz) frequency selective surfaces (FSSs) featuring an inductive metallic checkerboard (i-MCB) pattern based on conductive silver ink onto a flexible polyethylene terephthalate substrate, chosen for its excellent THz transmission properties below 1 THz [Jin et al 2006 J. Korean Phys. Soc.49 513–17]. Analytical studies, along with simulations and experiments, were conducted to investigate the filtering characteristics of the printed FSSs, confirming their functionality as a band-pass filter. Subsequently, we demonstrated the reconfigurability of a two-layer system by vertically stacking two layers. This was achieved by systematically shifting the position of the second layer in the x or y-direction relative to the first layer. Experimental verification revealed a significant variation in normalized transmission, ranging from 94% to 6% at 0.15 THz for type-I:i-MCBs and 90% to 5% at 0.20 THz for type-II:i-MCBs, respectively. This study presents a simple scheme for a reconfigurable screen-printed i-MCB-FSS operating in the THz range. Consequently, our findings demonstrate that screen printing method can effectively be employed for the large-scale production of THz FSSs.
Qiushi Li et al 2024 Flex. Print. Electron. 9 025002
The electrical resistance of metal-polymer conductive inks increases as they undergo cyclic loading, posing a major challenge to their reliability as interconnect materials for flexible electronic devices. To characterize an ink's fatigue performance, extensive electro-mechanical testing is usually performed. Phenomenological models that can accurately predict the resistance increase with cyclic loading can save time and be useful in flexible conductor design against fatigue failure. One such model was recently developed for only one composite ink type. The model is based on experiments monitoring resistance under monotonic stretch data and multiple experiments measuring the rate of increase of the resistance under different strain amplitudes and mean strains. The current work examines whether such resistance rate model could be generalized to apply for more types of composite inks. Two composite inks with different binder material, metal flake sizes and shapes, and substrate material were experimentally tested under monotonic and cyclic loading. It was found that the two new inks are also more sensitive to strain amplitude than mean strain. The resistance rate model accurately predicts early/catastrophic failure (<1000 cycles) in all inks and conservatively estimates high fatigue life for low strain amplitudes. A protocol detailing the procedures for applying the resistance model to new inks is outlined.
Aziz Radwan et al 2024 Flex. Print. Electron. 9 025001
This paper presents the development of planar zinc (Zn) resistor–capacitor (RC) filters from a single printed layer that are both printed and treated at room temperature. The fabrication process involves screen printing the resistor, capacitor and interconnects in a single patterned layer on kraft paper substrates using a Zn microparticle ink. In order to form a distinct resistor and capacitor in the patterned structure, reactive inkjet printing (RIJ) was performed to selectively dispense acetic acid on the RC filter pattern to achieve regions with highly contrasting resistance. The required high degree of spatial contrast was achieved using the positional control of the inkjet printer combined with the number of print passes and drop spacing used to dispense the acetic acid droplets. X-ray diffraction and scanning electron microscopy showed the crystal structure and grain size of Zn microparticles remained unchanged with increasing acetic acid exposure while the prominence of cold-welding increased with increasing exposure. Zn-based RC filters sharing a common set of dimensions but with a wide range of corner frequencies were successfully fabricated using this process. For a fixed filter geometry, the corner frequencies could be tuned from ∼7 kHz to ∼1 MHz as the number of print passes used to form the resistor increased in a stepwise manner. To the best of our knowledge, this is the first room temperature printing process to produce side-by-side resistors and conductors from the same printed layer and the first printing process of any type to produce RC filters with such a wide range of corner frequencies.
Milad Ghalamboran et al 2024 Flex. Print. Electron. 9 015011
When fabricating inkjet-printed electronic devices and circuits, inkjet-printed conductive materials require drying and sintering to improve electrical conductivity. Electrical conductivity should be the same irrespective of pattern design, size, location, or density of adjacent patterns. However, we demonstrate that spatial variations in the drying process for inkjet-printed patterns with proximity to others cause resistivity variations. These resistivity variations are studied here experimentally for different circuit patterns and in arrays of inkjet-printed square electrodes. This variation depends not only on the location of each electrode in an array but also on the number of electrodes. This means that for the same drying temperature and duration, the array with a larger number of electrodes exhibits a larger resistivity variation. The sooner an electrode dries, the lower resistivity it achieves. The resistivity variation between an individual electrode and the center electrode in a 7 × 7 electrode array can be a factor of seven. This variation decreases for lower numbers of electrodes to a factor of three for a 3 × 3 array. Furthermore, x-ray photoelectron spectroscopy analyses provide evidence for the residual presence of carbon-based materials within electrodes after the drying process. These results confirm that the location of electrodes within an array significantly influences the amount of residual carbon-based materials, thereby contributing to resistivity variations. Although intense pulsed light sintering can decrease this variation, its optimal parameters depend on the printed designs, and our simulation results show a non-uniform temperature profile over the electrode arrays. Temperature increases more at the center of patterns than the corners, which can be useful in this case to improve resistivity uniformity. In this study, for the first time, we show how different printed shapes and designs can result in non-uniform resistivity after drying and sintering.
Swarup Kumar Subudhi et al 2024 Flex. Print. Electron. 9 015013
In this paper, we develop multifunctional, physically soft, mechanically compliant, and magnetically responsive PDMS films, with embedded Fe3O4 nanoparticles, that show robust magnetic properties over a significant range of mechanical deformation. First, we establish that the magnetic properties, namely the saturation magnetization (Ms), remanent magnetization (Mr), and intrinsic coercivity (Hci) of these PDMS films in highly deformed configurations, i.e. in folded, twisted (with different twist angles), and bent (flexed) configurations, show very little degradation compared to those obtained in undeformed configurations. Next, the films were subjected to repetitive cycles of zero-to-max deformation (R = 0) and the saturation magnetization of the films was shown to not exhibit any significant degree of progressive degradation as a function of cyclic deformation history. These findings confirm the excellent robustness and cyclic durability of magnetic properties shown by these magnetic and compliant PDMS films and point to their suitability for wearable electronics applications.
Mathieu N Tousignant et al 2024 Flex. Print. Electron. 9 015012
With the rise of the internet of things and applications such as smart packaging, the need for low cost, disposable temperature sensors with minimum environmental impact are critical. In this study, we report fully printed capacitive temperature sensors made from bio-degradable dielectric materials. All layers were aerosol jet printed and the areal capacitance was characterized at several temperatures between room temperature (22 ˚C) and 80 °C. Using a bilayer dielectric structure, a layer of poly (vinyl alcohol) (PVA) was encapsulated with polycaprolactone (PCL) through interfacial crosslinking to protect it against humidity. Various concentrations and layer amounts of PVA were investigated, with the most effective capacitors consisting of a single layer of PVA deposited from a 5.0 mg ml−1 solution followed by a layer of the UV-crosslink-able PCL deposited from 2.0 mg ml−1 solution, achieving a 43 ± 6% increase in areal capacitance at 80 °C when compared to room temperature, measured at a frequency of 501 Hz.
Tomoya Koshi et al 2024 Flex. Print. Electron. 9 015009
For conventional flexible printed circuit board widely used in industry, jointing islands of electric components with polyimide-supported copper serpentine interconnects is an effective approach to ensure circuit stretchability. The stretchability of the interconnects varies significantly due to the soft elastomer encapsulating the interconnect, as the encapsulation essentially constrains the lateral buckling of the serpentine structure during stretching. Previous studies have indicated that thin encapsulation with a low Young's modulus is required to maximize stretchability. However, extremely low modulus and thinness lead to the elimination of the encapsulation function, and the design criteria for maximizing stretchability while maintaining adequate modulus and thickness are still unclear. This study investigates the dependence of stretchability on encapsulation stiffness, an index that simultaneously considers modulus and thickness. The interconnects with core–shell and single-elastomer encapsulations, each with a different stiffness, were prepared. The relationships between the elongation to failure of the interconnect and the tensile and bending stiffness of the encapsulation were investigated through experiments and finite element method calculations. The results indicate that the tensile stiffness is a more useful index in encapsulation design than the bending stiffness because the elongation to failure monotonically decreases as the tensile stiffness increases. The results also indicate that the required tensile stiffness to maximize interconnect stretchability, essentially making the interconnect almost freely deformable, ranges from 5 to 34 N m−1 when the interconnects use an 18 μm thick copper and 50 μm thick polyimide.