Flexible and Printed Electronics

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.

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.

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.

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.

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.

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.

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.

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.

High-quality roll-to-roll (R2R) manufacturing for flexible and printed electronics often requires uniform web tension. Nonuniformity in web tension can lead to nonuniform performance of printed electronic devices across the width of the web, and excessive nonuniformity in web tension can lead to web wrinkling. Here we develop and test a noncontact resonance (NCR) method and a gentle contact stiffness mapping (GCSM) method for measuring the average web tension and its linear variation across the width of the web. The NCR method uses the lowest symmetric and anti-symmetric frequencies of a web with a closed-form expression to obtain its linearly varied tension. The closed-form expression includes the significant effects of air loading on web vibrations through accurate hydrodynamic functions. While the GCSM method is based on nonlinear regression of the contact stiffness on multiple locations of the web. Both methods are accurate, reliable, and inexpensive, and are compatible for a wide range of web properties, web path, web tension, measurement configurations, and environmental conditions. We cross-validate the two methods on a stationary test stand and in-line test the NCR method in two spans of a moving commercial R2R system. We measure up to 35.58% cross-span tension variation in that system, and both average tension and its linear variation can vary in different spans of the same R2R system. We expect the results presented in this article can improve quality control of R2R processes for flexible and printed electronics and maximize device yields.

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.

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.

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.

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.

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.

Egg-albumen, a natural polymer, in bilayer combination with ultrathin HfO_x is explored as an active switching layer component in flexible resistive random access memory devices. The fabricated devices have shown excellent switching characteristics with a current on/off ratio of greater than 10⁴, stable retention of both low resistance and high resistance states, reliable multiple cycle switching, and very low switching power (with set power as 0.5 µW and reset power as 3.1 mW). To investigate the electro-mechanical stability, devices were bent with different bending radii and it was found that negligible degradation in device performance was observed until a 5 mm bending radius. Furthermore, a simple mathematical model is used to simulate the devices' characteristics and the values of fitting parameters were extracted with a root mean square error of less than 4.5%. Moreover, a switching variation was introduced by utilizing variations of the physical parameters, and a near practical physics based mathematical device model was demonstrated which can enable the strengthening of simulation capabilities for exploration of unique flexible resistive memory devices and related circuits.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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 (M_s), remanent magnetization (M_r), and intrinsic coercivity (H_ci) 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.

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.

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.

The innovative field of printed sensor with a demand for high accuracy, sensitivity and durability has enabled a wide application area in sensing, healthcare etc. A large-area printed sensor system on a flexible foil substrate emplying p-type organic field-effect transistors (OFETs) is presented. Thereby, the OFET is fabricated through a hybrid manufacturing process, including photolithographically structured source- and drain-electrodes, ink-jet printed organic semiconductor, and spin-coated dielectric. Moreover, a dedicated device model, derived from the variable range hopping model, is developed and integrated together with process related design rules, materials properties and geometric information into a comprehensive process design kit (FH_OPDK). The FH_OPDK is integrated in a commercial electronic design automation tool and is used to design and perform post-layout simulations on logic gates, such as INV, NAND2, and NOR2 as well as circuitry such as ring oscillators and a 4 × 16 digital decoder. Several circuit topologies have been tested and evaluated in a detailed model-hardware correlation analysis. Finally we have optimized logic gates and the decoder in a PMOS only, pseudo CMOS design style. To demonstrate the feasibility of the full sensor system in hardware a 16 × 16 active matrix pressure sensor on a flexible substrate integrated with a 4 × 16 binary decoder was fabricated and tested. We have integrated our flexible hybrid sensor system with a PCB board and a microcontroller to demonstrate the hardware readout platform capable of detecting the weight of objects and visualizing a digital map of applied forces.