This paper introduces sustainable sensor solution of tire pressure measurement sensor (TPMS). Solution utilizes thin and flexible plastic substrate patterned by screen printing. Reference sensor (REF) circuitry patterned by screen printing using silver paste on PET substrate. Sustainable sensor patterned by screen printing using copper paste. The reliability of silver pastes printed samples on PET substrate evaluated first by bending tests. Slightly pre-curved samples enabled bending in both directions inducing either compressive or tensile stress in a sample. By control software, a cyclic stress with approximately 20 mm radius at minimum in free bending addressed to circuitries. The recorded data showed stable performance for circuitries on both PET and Bio-PET substrates in compression and tension. Measurement results for PET and Bio-PET samples in cyclic push tests recorded similarly as with bending tests. Measured resistance of a PET sample remained approximately at the same level over fifty thousand pushes. Bio-PET samples had notably increased electrical performance after ten thousand cycles. This indicated initiation and propagation of minor defects either at measurement wire or printed conductor interface and/or on flex PCB circuitry. Assembly of required SMD components bonded on copper paste substrates using SnBi solder in low-temperature soldering process having peak temperature of 158°C. UV-curable adhesive applied in component post-bonding to improve system reliability. Functional testing of assembled systems performed by placing system into garden pressure bottle together with industrial reference sensor ContiPressureCheck™ by Continental. Pressure inside the garden bottle increased gradually up to four bars in functional testing. Measured data of sustainable sensors gathered through Bluetooth® using specific mobile phone application. Industrial reference sensor data monitored wirelessly through handheld device. Sustainable sensors showed high consistency with the reference sensor. In the final step of testing, sensors integrated on the tires and subsequently installed on a passenger car. In testing the car operated at speeds ranging from 30 to 120 km/h on both city streets and highways. Operation of sensors followed during testing. Samples provided correct temperature and pressure measurement results without any disturbances or breaks.

Objective: This paper describes the full development of a sensor for measuring optical heart rate (OHR) and blood oxygen saturation (SpO2). Methods: A wearable sensor with a new type of skin compatible dispensed lens was designed and manufactured. All critical optical components, light emitting diodes (LEDs) and photodiode (PD) were close to skin and gave maximum light intensity due to minimal loss in the lens structure. Lens and optical components formed a thin monolithic structure. Results: Suppressed crosstalk between LED and PD was achieved by using two types of dispensed material: light blocking and transparent. High signal to noise ratio (SNR) and amplitude in the alternating current (AC) part of the photoplethysmography (PPG) signal were achieved. User comfort was achieved by having a small sensor located on the upper arm. When re-training the algorithm from our first iteration, the multiwavelength PPG sensor showed an SpO2 RMSE of 2.61% with a 7-second average analysis for 25 participants. The average RMSE of heart rate over all 25 participants was 1.6 ± 1.1%. Conclusion: This study demonstrates a sensor with a clinical grade SpO2 measurement and a highly accurate OHR measurement that is also comfortable and easy to wear. Significance: A dispensing method provides a new way of manufacturing sensor elements for wearable sensors with increased performance with reduced crosstalk.

High-resolution reverse-offset printing (ROP) is developed for miniaturization of printed electronics, resulting in a notable decrease in material usage compared to conventional printing processes. Two alternative ROP processes for patterning of metal conductors are available that are comparable in their cost per sample: direct nanoparticle (NP) printing (e.g., Ag and Cu) and patterning of vacuum-deposited metal (Ag, Al, Au, Cu, Ti, etc.) films using ROP printed polymer resist ink and the lift-off (LO) process. In this work, we focus on ROP of Cu NP ink followed by intense pulsed light (IPL) sintering and vacuum-deposited Cu patterned by ROP lift-off (LO). The good large-scale uniformity of the two processes is demonstrated by a grid of 300 individual thickness, sheet resistance, and resistivity measurement points with low variation over the 10 cm × 10 cm printed sample area. Sheet resistances of 0.56 ± 0.03 and 1.23 ± 0.05 Ω/□ are obtained at 113 and 40 nm thickness for Cu NP and Cu LO, respectively. Both processes show <5% thickness variation over a large area. A line-space (L/S) resolution of 2 μm is obtained for ROP patterned vacuum-deposited Cu having very low line edge roughness (LER) (∼60 nm), whereas for direct ROP printed Cu NP ink, the L/S resolution (2-4 μm) is limited by LER (∼900 nm) and influenced by the printed layer thickness. Based on the two fabrication routes, a flexible chip component assembly process is presented. Preliminary bending resistance results indicate that both ROP-based patterning processes yield a robust electrical interconnection between the ultrathin polyimide (PI) 5 mm × 5 mm chip and thermoplastic polyurethane (TPU). ROP shows promise as a scalable and sustainable patterning method for flexible ICs/chips that are assembled on flexible, stretchable, or biodegradable substrates and used, e.g., in wearable, large-scale sensing, and in environmental monitoring.

A key aspect of printed electronics manufacturing is layer-layer registration and potential deformation of the substrate during processing (e.g. printing) which involves thermal treatment under tension or steps without any tension (e.g. surface mount hybrid integration of electronics). The influence of thermal processing on the registration control and material interfaces for printed and hybrid processing was evaluated on a pilot printing line using roll-to-roll screen-printed silver microflakes and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) inks. The registration was not fully stabilized on polyethylene terephthalate (PET) whereas on thermally pre-treated PET (PET_t) it reached +/- 60 mu m accuracy in the machine direction (MD) and +/- 100 mu m in the cross-machine direction (CD) after settling. The print on PET deviated 0.18% in MD and -0.01% in CD, implying that the print dimensions were changing along the MD elongation, whereas the changes on PET_t occurred during the pre-treatment, thus, the print deviated only -0.01% in MD and 0.08% in CD. Silver-substrate interface obtained good adhesion, however partial delamination was detected after thermal post-treatment at zero tension, most likely due to substrate shrinkage. Differences in the thermal response may partially delaminate the silver. Delamination of PEDOT:PSS was not observed, which may be related to its polymeric nature. Silver resistance on PET was 8% higher than on PET_t in MD, due to the elongation of the PET that occurs during print curing. Respectively, the silver resistance on PET was 2% lower in CD, with the dimensions slightly below the nominal layout value. Thermal post-treatment at zero tension reduced the resistance on both substrates, being greater in the MD as well as the substrate shrinkage. The impacts of thermal treatment are particularly important if the hybrid electronics processing is utilizing manufacturing lines with different web tension, such as printing and surface mount technology assembly, since the dimensional changes of the substrate can cause significant reduction in accuracy and reliability.

Advanced manufacturing techniques combined with state-of-the-art materials and conventional surface mounted devices enable producing of stretchable electronic systems with conformable form factor. This paper investigates various temperature sensor implementations on elastic substrates and assess their reliability under mechanical loadings. The primary sensor implementations include printed sensor, glob-topped SMD sensor (thermistor) and glob-topped SMD sensor (thermistor) on a flexible interposer integrated on an elastic substrate. The study features iterative rounds, in which mechanical structures and material combinations change for the fixed design, and their effect with respect to baseline is quantified. The reliability assessment for test vehicles is performed with maximum and cyclic stretching tests including torsion. The conducted mechanical tests and failure analysis pinpoint critical interfaces with cracking under elongation. The quantitative results show unsatisfactory performance of printed sensors under linear elongation (<5%) and about 20% maximum stretchability for baseline SMD based structures. Iterative designs prove increased stretchability up to 30% in linear elongation for thermistor structures by means of a softer glob-top material. This also results in a change in major failure mechanism from rigid-soft interface to a printed conductor. Investigating the novel substrate and conductor materials records significant improvement in maximum stretching tests. The best material combinations withstand over 150% stretching. The study includes also test method development for stretchable electronics. A setup used for linear elongation test upgrades with a rotating base enabling torsional force. The advanced test setup mimics operational environment of elastic electronic systems more realistically and quantifies the difference between linear elongation and multi-dimensional forces.

To improve the potential application opportunities of roll-to-roll flexible printed electronics, improved system integration and area reduction of hybrid systems is possible using two-sided functional carrier foils. These double-sided circuits, and systems, on flexible substrates can have multiple functional layers, such as conductive or semiconductive materials, on both sides. This study aimed to increase the density and complexity of the hybrid systems, while reducing the material and processing costs. The focus was on three main tasks: roll-to-roll (R2R) via laser cutting and filling, R2R etching of two-sided indium tin oxide (ITO)/polyethylene terephthalate (PET), and a double-sided ITO system backplane for organic photovoltaic (OPV) integration. Critical parameters for choice of Ag layer thickness, interconnection pitch and additional via filling need to be considered in the system design phase to improve manufacturing efficiency.

This paper presents a manufacturing flow to integrate surface mounted device (SMDs) on elastic fabric. In this concept, manufacturing processes are based on printing, converting and assembly to integrate rigid SMD electronics with high-performance functionality on a stretchable substrate, and hot laminating this entity on an elastic fabric. The presented manufacturing flow exploits novel material solutions like stretchable inks and adhesive bonding technology to achieve needed electro-mechanical performance at low processing temperatures. The study includes reliability assessment for test samples under mechanical loadings. The test structures comprise 0402'' sized SMD thermistors and printed wiring with multiple material and design configurations integrated on both thermoplastic polyurethane (TPU) substrate and elastic fabric by means of TPU lamination. Test coverage for prepared samples encompasses maximum stretching test, cyclic stretching, and cyclic stretching with rotation, thus enabling extensive reliability assessment for various sample configurations under interest. The results show mainly over 100% stretchability without catastrophic failure for textile integrated SMD samples in maximum stretching tests. In cyclic tests covering 25% elongation with and without +/- 90 degrees rotation, durability of SMD-based structures is limited in comparison to baseline samples without component integration. The conducted tests and electro-mechanical characterization pinpoint the importance of material stack strength and layout design. Narrow TPU cut for integrating SMD electronics on textile has lower effect on mechanical behavior under stretching. This reflects also on electrical performance and reliability.

This study presents an advanced method to manufacture large-area flexible electronic circuits in a continuous roll-to-roll process. Transfer foil method founds on converting processes to cut and laminate thin metal foils to designed patterns onto a target substrate. The processing technique utilizes die-cutting tools, carrier foils and adhesive films to embed functionalities with bulk metal level conductivity directly on various surfaces. Transfer foil method is a chemical-free process and operates at room temperature favoring sustainable manufacturing. Furthermore, integration of rigid electronics on transfer foil patterns is realizable by low temperature soldering as proven in the paper.

The demand for flexible large area optoelectronic devices such as organic light-emitting diodes (OLEDs) and organic photovoltaics (OPVs) is growing. Roll-to-roll (R2R) printing enables cost-efficient industrial production of optoelectronic devices. The performance of electronic devices may significantly suffer from local electrical defects. The dark lock-in infrared thermography (DLIT) method is an effective non-destructive testing (NDT) tool to identify such defects as hot spots. In this study, a DLIT inspection system was applied to visualize the defects of R2R printed silver conductors on flexible plastic substrates. A two-stage automated defect recognition (ADR) methodology was proposed to detect and localize two types of typical electrical defects, which are caused by complete or partial breaks on the printed conductive wires, based on localized segmentation and thresholding methods.

Flexible electronics comprises several manufacturing techniques and material choices for assembling electronic circuits on plastic substrates. A fundamental characteristic in such assemblies is the performance and mechanical stability of bonding between rigid surface-mounted devices and flexible circuitry. This study quantifies bonding strength of 0402' sized chip resistors by a pull-off test measuring the needed perpendicular force to detach a component from a substrate. The test structures include screen-printed conductors and adhesive bonding technology prepared at roll-to-roll pilot manufacturing lines of VTT Technical Research Centre of Finland, as well as commercially available counterparts based on etched wiring and soldering process for bonding. The pull-off testing results indicate higher forces in average for solder-based samples denoting enhanced bonding strength in comparison to printed specimens with adhesive bonding. The observations are founded on higher mechanical integrity of etched wiring and differences in nature between soldering and adhesive bonding methods. However, additive printing and adhesive bonding technology provide sufficient mechanical stability for flexible electronics with low-cost substrates at lower processing temperatures. These manufacturing techniques and proven materials favor sustainability, and enable together with elastic substrates improved skin-contact in future wearable electronic applications.

Electrically conductive adhesives offer a low-temperature alternative for soldering process in electronics assembly. Adhesive bonding technology is especially advisable when integrating components on printed structures on soft substrates with low heat-tolerance. This study investigates mechanical durability of surface mounted chip resistors bonded on a flexible substrate with an isotropic conductive adhesive. The test specimen are manufactured in fully automated roll-to-roll processes by rotary screen printing a conductive circuitry, and subsequently dispensing silver epoxy, pick-and-placing components and heat-curing the adhesive. The adhesive bonding strength is studied with an automated test setup pulling a component off from the substrate. This destructive test method is used to evaluate the mechanical strength when varying adhesive volume at the interconnection areas with different component sizes. The results prove that the adhesive bonding strength for printed and hybrid electronics can be improved by increasing the adhesive volume in bonds when using a silver epoxy. At the same time, spreading of adhesive dots during assembly process limits the applicable adhesive volume to avoid short circuits and to maintain the electrical functionality. The study provides guidelines to optimize dispensing routines to achieve stable electro-mechanical interconnections.

Sustainability in electronics has a growing importance due to, e.g. increasing electronic waste, and global and European sustainability goals. Printing technologies and use of paper as a substrate enable manufacturing of sustainable electronic devices for emerging applications, such as the multi-layer anti-counterfeit label presented in this paper. This device consisted of electrochromic display (ECD) element, NFC (near field communication) tag and circuitry, all fully roll-to-roll (R2R) printed and assembled on plastic-free paper substrate, thus leading to a sustainable and recyclable device. Our setup uses harvested energy from HF field of a smartphone or reader, to switch an electrochromic display after rectification to prove authenticity of a product. Our novelty is in upscaling the manufacturing process to be fully printable and R2R processable in high-throughput conditions simulating industrial environment, i.e. in pilot scale. The printing workflow consisted of 11 R2R printed layers, all done in sufficient quality and registration. The printed antennas showed sheet resistance values of 32.9±1.9 mΩ/sq. The final yield was almost 1500 fully printed devices, and in R2R assembly over 1400 labels were integrated with 96.5% yield. All the assembled tags were readable with mobile phone NFC reader. The optical contrast (ΔE*) measured for the ECDs was over 15 for all the printed displays, a progressive switching time with a colour change visible in less than 5 s. The smart tag is ITO-free, plastic-free, fully printed in R2R and has a good stability over 50 cycles and reversible colour change from light to dark blue.

Capability of high-speed and low-cost manufacturing makes the printing techniques a very promising approach for large-area flexible electronics mass manufacturing. Due to fast and intensive technology development, the lack of knowledge about the reliability and lifetime of printed electronics is obvious, requiring further investigation. Especially, the effect of torsional bending on lifetime is a mostly unexplored field of reliability testing. In this article, a torsional bending test of parallel printed silver conductors (0.3-, 0.5-mm pitch) on polymer substrate (polyethylene terephthalate, 125-mu m thickness) was conducted and analyzed. According to the experimental results, torsional bending causes wear-out type failures in conductors and the length-to-width (LTW) ratio of the sample's substrate was observed to have a significant impact on reliability. If the LTW ratio is smaller than 3, the lifetime of printed conductor seems to collapse and samples lasted for approximately only 17 bending cycles on average. Lifetime was improved by increasing the LTW ratio and samples withstood over hundreds of cycles with LTW ratio of higher than 15. However, the distance of a conductor fromthe edge of the substrate was not observed to have any significant influence on the reliability under torsional bending.

This paper introduces digital wallpaper demonstrator manufacturing based on flexible PET substrate with R2R printed conductive tracks and surface-mounted LEDs assembled on printed substrate on R2R assembly line. Heat management of applied LEDs simulated using Lighting Design Calculator (LDC) and simulated results compared to measurement results. Very good consistency between simulated and characterized results achieved with red Luxeon Color Line 3535L surface-mounted LEDs. In addition, measured junction temperatures of LEDs were about 80°C with 60mA operational driving current when laboratory temperature was 25°C. Such a reasonable low junction temperature for LEDs suggests long operational lifetime for components in wallpaper application.

Stretchable electronics offers a new way for making wearable devices that are unobtrusive and conform to the user's body shape. Electronics manufactured on a stretchable substrate allows integration of electronics into e.g. clothing, disposable patches and wearable accessories while retaining user comfort. This paper presents a LED matrix display manufactured on thermoplastic polyurethane. A 5x17 LED matrix was fabricated with bare die semiconductor LED chips and a reference device with chip scale packaged components. Reliability under stretching is investigated with and without protective measures, namely support adhesive and laminated cover layer. It was found that the bare die samples could withstand no more than 6% strain whereas the packaged components could reach 18%. In a cyclic stretch test the best performance was recorded from a sample laminated with pressure sensitive adhesive, again chip scaled components faring better that the bare dies.

In this case study, the possibilities of hybrid integration of printed and flexible electronics in combination with conventional electronic components to create new types of product concepts is demonstrated. The final result is a personal activity meter demonstrator, which is realized by utilizing various flexible electronics manufacturing and integration techniques. Roll-to-roll printing was used to print the electronic backplane as well as co-planar electrochromic (EC) display. A pick-and-place assembled microcontroller unit and accelerometer, together with passive components, provided the brains for the system. Injection molding was then utilized to create a structural electronics system including an EC display. To validate the feasibility and scalability of the processes used, 100 pieces of the personal activity meter were fabricated. Modeling with continuum computational fluid dynamics and numerical heat transfer, using the high-performance finite volume method, showed that high filling pressure and shear-stress are the key factors causing broken devices. The stability of the devices in harsh environmental conditions as well as in bending seem to be slightly improved in the over molded samples.

Development of reliable glucose sensors for noninvasive monitoring without interruption or limiting users' mobility is highly desirable, especially for diabetes diagnostics, which requires routine/long‐term monitoring. However, their applications are largely limited by the relatively poor stability. Herein, a porous membrane is synthesized for effective enzyme immobilization and it is robustly anchored to the modified nanotextured electrode solid contacts, so as to realize glucose sensors with significantly enhanced sensing stability and mechanical robustness. To the best of our knowledge, this is the first report of utilizing such nanoporous membranes for electrochemical sensor applications, which eliminates enzyme escape and provides a sufficient surface area for molecular/ion diffusion and interactions, thus ensuring the sustainable catalytic activities of the sensors and generating reliable measureable signals during noninvasive monitoring. The as‐assembled nanostructured glucose sensors demonstrate reliable long‐term stable monitoring with a minimal response drift for up to 20 h, which delivers a remarkable enhancement. Moreover, they can be integrated into a microfluidic sensing patch for noninvasive sweat glucose monitoring. The as‐synthesized nanostructured glucose sensors with remarkable stability can inspire developments of various enzymatic biosensors for reliable noninvasive composition analysis and their ultimate applications in predictive clinical diagnostics, personalized health‐care monitoring, and chronic diseases management.

Recent technological advancements in wearable sensors have made it easier to detect sweat components, but our limited understanding of sweat restricts its application. A critical bottleneck for temporal and regional sweat analysis is achieving uniform, high-throughput fabrication of sweat sensor components, including microfluidic chip and sensing electrodes. To overcome this challenge, we introduce microfluidic sensing patches mass fabricated via roll-to-roll (R2R) processes. The patch allows sweat capture within a spiral microfluidic for real-time measurement of sweat parameters including [Na + ], [K + ], [glucose], and sweat rate in exercise and chemically induced sweat. The patch is demonstrated for investigating regional sweat composition, predicting whole-body fluid/electrolyte loss during exercise, uncovering relationships between sweat metrics, and tracking glucose dynamics to explore sweat-to-blood correlations in healthy and diabetic individuals. By enabling a comprehensive sweat analysis, the presented device is a crucial tool for advancing sweat testing beyond the research stage for point-of-care medical and athletic applications.

Stretchable electronics has attracted much interest recently because of its potential applications in the area of wearable electronics and healthcare. Highly elastic polydimethyl siloxane (PDMS) has been for decades a widely used material in prototyping purposes. It enables the realization of a variety of mechanical and optical functions besides being a substrate for other processes or applications. As a substrate, PDMS enables high stretchability and easy integration of other parts made of PDMS. In this work, we demonstrated a high-volume production of stretchable electrical interconnections on PDMS substrates. We used roll-to-roll (R2R) rotary screen printing that has been conventionally applied in high-throughput fabrication of electronics on flexible, but not stretchable, substrates. We demonstrated silver conductivity remains sufficient for signal transmission, for example, in sensor structures under repeated 20% strain over 100 cycles. We also demonstrated R2R compatible PDMS encapsulation of electrical interconnections that increased the strain repetition durability by a factor of 2.

The increasing capabilities of roll-to-roll (R2R) printing processes present challenges for quality control, requiring in-process inspection of large substrates with high resolution at high speed. In this paper, an all-optical difference engine (AODE) sensor has been developed for in-process defect inspection for R2R printed electronics. The AODE sensor achieves high-speed inspection by utilising the principle of coherent optical subtraction to minimise data processing. The capability of the developed sensor is demonstrated using industrial printed electrical circuity samples and the sensor is capable of inspecting areas of 4 mm width with a resolution of the order of several micrometres.