Printed strain sensor based on silver nanowire/silver flake composite on flexible and stretchable TPU substrate

A printed strain gauge based on silver/carbon composite was successfully fabricated and characterized for strain monitoring applications. The silver-carbon (Ag/C) composite ink was prepared by blending 81% wt. of silver with 19% wt. of carbon ink. The strain gauge was fabricated by screen printing Ag/C composite ink on a flexible and stretchable thermoplastic polyurethane (TPU) substrate in a meandering pattern to achieve a desired resistance of ~350 Ω. The capability of the printed strain gauge to detect varying strains ranging from 0% to 5% was investigated. It was observed that the strain gauge had a linear response till 2.5% strain. At 2.5% of tensile strain a relative resistive change of 7.8% and a gauge factor of 3.1 was observed. However, as the strain increased beyond 2.5% the strain gauge had a non-linear response. It was observed that at a tensile strain of 5%, the strain gauge had a maximum relative resistive change of 285.6% resulting in 57.2 gauge factor. The results demonstrate that a screen-printed Ag/C composite ink-based strain gauge with on a TPU substrate can be utilized for strain monitoring. The electromechanical response of the fabricated strain gauge as a function of resistance is investigated and presented in this paper.

The lightweight and bendable features of printed flexible electronics are increasingly attractive. Currently stretchable silver inks are formulated for wide traces, typically larger than 2 mm. To attach ultra-thin silicon chips that have fine pitch onto printed organic substrate, it is necessary to print fine trace width/space that matches the pitch of the chips, which may be less than 200 microns. This paper presents the development and optimization of the screen printing process for printing stretchable silver ink onto stretchable thermoplastic polyurethane (TPU) substrate. A test vehicle was designed including 50 μm/5 mm (line width/line length) to 350 μm/35 mm lines (at 4 biases). The stretchable ink selected was DuPont PE 873 and Dupont's PE 5025 ink (non-stretchable conductive flake silver) was used as a “control” to baseline the printing process. The substrate used was Bemis TPU ST604. The experiment was done on a DEK Horizon 03i printer. A DEK squeegee 200 (Blue) and a DEK 265 flood bar (200 mm) were used. A 2-level factorial design with three replicates was selected to investigate the effect of process parameters on the quality of prints. The quality of the prints is characterized by 1) resistance of traces, 2) sheet resistance, 3) z-axis height, and 4) trace width/spacing. We observed significant noise in the z-axis printed silver ink height measured by profilometry and concluded z-axis height is not a good response variable for characterizing screen printing stretchable silver ink onto TPU substrate, mainly due to high roughness of the TPU substrate. We proposed calculated sheet resistance based on the measured resistance value, trace width, and trace length, which can replace trace height measurements on rough profile substrates. We found that squeegee pressure and emulsion thickness have statistically significant effects on calculated sheet resistance of print traces while print speed does not have statistically significant effects. In our experiment setting levels, the lower the squeegee pressure, the lower the calculated sheet resistance that is achieved. The emulsion with higher emulsion over mesh (EOM) is better than the emulsion with lower EOM since it can achieve lower sheet resistance. After optimizing the screen printing process, we were able to print 100 μm (4 mils) trace width and spacing with high consistency.

Development of a novel wrinkle-structure based SERS substrate for drug detection applications

Highly flexible wristband based on utilization of structural electronics processing technologies and applied for bio-signal monitoring is introduced in this paper. Wristband designed to measure heart rate (HR) and oxygen saturation (SpO2) based on reflective photoplethysmography (PPG) method. Wristband consisted of stretchable thermoplastic polyurethane (TPU) substrate equipped with conductive printed tracks. Three rigid FR4 PCBs contacted on TPU substrate using isotropic conductive adhesive (ICA) and supported mechanically using non-conductive adhesive (NCA). Distances of two sensor PCBs from main PCB were 20 mm and 50 mm to allow measurement of bio-signals from two different locations on wrist or forearm having distance of 115mm in between. Sensor modules were equipped with photonic transceiver component providing capability for PPG sensing. RGB LEDs placed on the main PCB and covered with decorative crystal designed to inform operational status of wristband to the user. Specific aluminum mold tool designed and tooled by mechanical milling for elastomer embedding. Tool equipped with thin pins, which lifted the TPU assembly up from the tool cavity surface to enable embedding of the TPU substrate fully within elastomer. Tool design enabled photonic transceivers optical window surfaces to be open for bio-signals sensing after elastomer embedding. Silicone and polyurethane elastomers applied for TPU assembly embedding. Functionality of processed wristband samples tested by performing HR and SpO2 measurements with volunteer test persons under physical stress. Achieved results verified that both silicone and polyurethane wristbands were functional and capable to monitor HR and SpO2 levels of test persons.

Traditional multifunctional composite structures are produced by embedding parasitic parts, such as foil sensors, optical fibers and bulky connectors. As a result, the mechanical properties of the composites, especially the interlaminar shear strength (ILSS), could be largely undermined. In the present study, we demonstrated an innovative aerosol-jet printing technology for printing electronics inside composite structures without degrading the mechanical properties. Using the maskless fine feature deposition (below 10 μm) characteristics of this printing technology and a pre-cure protocol, strain sensors were successfully printed onto carbon fiber prepregs to enable fabricating composites with intrinsic sensing capabilities. The degree of pre-cure of the carbon fiber prepreg on which strain sensors were printed was demonstrated to be critical. Without pre-curing, the printed strain sensors were unable to remain intact due to the resin flow during curing. The resin flow-induced sensor deformation can be overcome by introducing 10% degree of cure of the prepreg. In this condition, the fabricated composites with printed strain sensors showed almost no mechanical degradation (short beam shearing ILSS) as compared to the control samples. Also, the failure modes examined by optical microscopy showed no difference. The resistance change of the printed strain sensors in the composite structures were measured under a cyclic loading and proved to be a reliable mean strain gauge factor of 2.2 ± 0.06, which is comparable to commercial foil metal strain gauge.

Computer embroidery is one of the modern types of garment decoration. But in our country this industry is insufficiently studied. Instead, today there are entire associations of embroidery companies abroad, periodicals are published, special schools operate, international conferences are held, and Internet conferences on computer embroidery are organized. The article discusses the issues of improving the quality of applying an embroidered element to a textile material in order to increase the competitiveness of garments in the domestic market of goods and services. It was found that during machine embroidery, the most vulnerable point is the border of the “fabric-embroidery” system. If the embroidered pattern along the contours of the edge is characterized as a “straight line”, then the maximum value of the destruction of the samples at the warp occurs with tatami stitches, and weft with tatami stitches and zigzag. When the pattern is embroidered in the form of a circle, the destruction already occurs not only along the perimeter of the “arc line”, but also in the middle. If the embroidered pattern is a rectangle with wavy edges, in contrast to the straight and arc border lines in the system “fabric-embroidery”, the process of destruction occurs within, starting from the upper and then the lower contours. There is also a decrease in rupture characteristics at (S), (Z), and (T) –stitches. When studying the effect of embroidery needles on the physical and mechanical characteristics of textile materials, it was experimentally established that this process should be attributed to the destructive, the degree of which depends on their number, as well as the step and type of stitches. This is evidenced by the increase in the values of the coefficient of air permeability of the samples of materials and the decrease in the breaking indicators in comparison with the initial values. Thus, the research and their analysis shows that the degree of change in rupture characteristics, as a control indicator, primarily depends on the contour of the edge of the pattern, as well as the type of computer embroidery weave, but the greatest influence of these factors occurs when the geometry of the system boundary ” fabric-embroidery “is a straight line, and the smallest – a wavy line that does not contradict the mathematical model, the conclusions of which were used in the design of the embroidered element for children’s clothing (pants).

Computer embroidery is one of the modern types of garment decoration. But in our country this industry is insufficiently studied. Instead, today there are entire associations of embroidery companies abroad, periodicals are published, special schools operate, international conferences are held, and Internet conferences on computer embroidery are organized. The article discusses the issues of improving the quality of applying an embroidered element to a textile material in order to increase the competitiveness of garments in the domestic market of goods and services. It was found that during machine embroidery, the most vulnerable point is the border of the “fabric-embroidery” system. If the embroidered pattern along the contours of the edge is characterized as a “straight line”, then the maximum value of the destruction of the samples at the warp occurs with tatami stitches, and weft with tatami stitches and zigzag. When the pattern is embroidered in the form of a circle, the destruction already occurs not only along the perimeter of the “arc line”, but also in the middle. If the embroidered pattern is a rectangle with wavy edges, in contrast to the straight and arc border lines in the system “fabric-embroidery”, the process of destruction occurs within, starting from the upper and then the lower contours. There is also a decrease in rupture characteristics at (S), (Z), and (T) –stitches. When studying the effect of embroidery needles on the physical and mechanical characteristics of textile materials, it was experimentally established that this process should be attributed to the destructive, the degree of which depends on their number, as well as the step and type of stitches. This is evidenced by the increase in the values of the coefficient of air permeability of the samples of materials and the decrease in the breaking indicators in comparison with the initial values. Thus, the research and their analysis shows that the degree of change in rupture characteristics, as a control indicator, primarily depends on the contour of the edge of the pattern, as well as the type of computer embroidery weave, but the greatest influence of these factors occurs when the geometry of the system boundary ” fabric-embroidery “is a straight line, and the smallest – a wavy line that does not contradict the mathematical model, the conclusions of which were used in the design of the embroidered element for children’s clothing (pants).

A surface enhanced Raman spectroscopy (SERS) substrate was developed for the detection of cocaine. The SERS substrate was fabricated by gravure printing a metallic layer of silver nanoparticle (Ag NP) ink, with average particle size of 150 nm, on a flexible and stretchable thermoplastic polyurethane (TPU) substrate. The feasibility of the printed substrate to enhance the Raman spectra of cocaine was investigated. An enhancement factor (EF) of three, in the intensity of Raman spectrum of cocaine on the printed SERS substrate, was observed when compared to target molecules absorbed on bare TPU substrate. This EF is based on the large electromagnetic fields, that are localized at hot spots, created by interaction of the Ag NPs with light. The SERS based response of the printed substrate thus demonstrated the capability of the printed SERS substrate to be used in drug detection applications.

Screen-printing of functional materials is a well-established process in the printed electronics industry. Recent advancements of flexible electronics demand screen-printing of a wide range of functional materials on flexible and stretchable polymeric substrates. The printability, quality of the printed structures, and their reliability during storage and usage are determined by the key fabrication process parameters such as the material properties, printing parameters, curing conditions, and environmental factors. Therefore, proper analysis of distinctive fabrication processes is crucial for problem-solving and process improvements in order to ensure optimum outcomes. In the present study, a low-cost, disposable, multilayer Sweat Rate Electrode (SRE) was fabricated on a flexible and highly stretchable Thermoplastic Polyurethane (TPU) substrate by screen-printing, using a Silver-filled conductive ink and a water-based Carbon-filled conductive ink. Exposure to low humidity and high temperature adversely affected the fabrication process and the robustness of the SRE. These challenges were overcome by better control of environmental conditions in the printing atmosphere and proper selection of the materials, printing parameters, curing conditions, and fabrication process flow. We demonstrate the fabrication of a defect-free, robust SRE on TPU substrate.

Highly stretchable and soft electronics as emerging technologies exhibit promising physical and electromechanical characteristics. One of the key efforts for expanding these emerging technologies includes developing intrinsically stretchable conductors as stretchable interconnects, which can withstand high tensile strain amplitude and repeated cyclic deformation while maintaining electrical conductivity and mechanical integrity. In this study, a highly conductive fluid phase conductor developed by Liquid Wire Inc. is stencil printed on a thermoplastic polyurethane (TPU) substrate followed by lamination of TPU as an encapsulant layer on top of the trace. The conductor is an alloy composed of a eutectic Gallium-Indium-Tin alloy stabilized by an oxide microstructure and additives. Test coupon includes 4 probe structure including stretch zone and non-stretch zone. Non-stretch zone is formed by assembling woven fabrics on TPU substrate. Electrical resistance is measured via 4-point probe in-situ monitoring of electrical resistance during fatigue cycling. Microstructure and morphology of the conductor are investigated by Confocal Laser Scanning Microscopy and Scanning Electron Microscopy (SEM). Electromechanical properties of the printed conductor are investigated by subjecting the trace to continuous and discrete tensile cyclic loading at strain amplitudes of 30% and 50% and distinct extension rates of 5, 10, 20 and 30 mm/s. The results of electromechanical analysis reveal that the conductor's resistance exhibits a linear response to changes in the length and exhibits no hysteresis. Extension rate and relaxation of TPU substrate does not show significant impact on rate of change in electrical resistance. The conductor shows fatigue free performance during continuous and discrete fatigue cycling at strain amplitude of 30% and 50% for 8000 cycles of fatigue cycling.

Smart surfaces with switchable wettability are widely studied for environmental application. Although a large number of stimulation routes provide broad prospects for the development of smart surfaces, achieving high sensitivity, fast response and recovery, simple operation, security and good stability is still challenging. Herein, a Janus membrane via electrospinning, chemical bath deposition and heat treatment is constructed. By using the hydrophilic ZIF-L nanosheet to functionalize the hydrophobic thermoplastic polyurethane (TPU) substrate, a smart surface utilizes the ZIF-L crack induced by strain in the hydrophilic layer to control surface wettability is obtained. In the range of 0%–100% strain, the wettability of the smart surface presents an obvious change with stretching, and water contact angle of the surface shows a monotonic increase with a maximum tuning range from 47° to 114°. Due to local fusion of the TPU microfibers and good binding between the ZIF-L layer and the TPU substrate after heat treatment, the prepared Janus membrane exhibits consistent and symmetrical hydrophilic–hydrophobic–hydrophilic transition curves in 50 stretching-releasing cycles. Thanks to the porous and asymmetric architecture, the membrane shows good oil–water separation performance, and the separation flux increases with the increase of strain, while the separation efficiency is always higher than 98%. Because of the excellent structural stability, the robust membrane with 100% strain maintains its oil–water separation property for 50 stretching-releasing cycles. This study provides a new perspective for the development of smart material with stimuli responsive surface for oily wastewater purification.

High conductive free-written thermoplastic polyurethane composite fibers utilized as weight-strain sensors

Thermoplastic polyurethanes reinforced with glass fibres

The influence of MWNT composite on the stretchability of conductive nanopaste screen-printed on elastomeric substrate

On‐skin electronics have drawn extensive attention as they revolutionize many aspects of healthcare, motion tracking, rehabilitation, robotics, human–machine interaction, among others. Flexible and stretchable strain sensors represent one of the most explored devices for on‐skin electronics. Many printing techniques have recently emerged showing great promises for manufacturing strain sensors. Herein, it is aimed to provide a timely survey of recent advancements in printed strain sensors for on‐skin electronics. This review starts with an overview of sensing mechanisms for printed strain sensors, followed by a review of various printing techniques employed in fabricating these sensors. The materials, structures, and printing processes of representative strain sensors are discussed in detail for each printing method. Finally, potential applications of printed flexible and stretchable strain sensors are presented focusing on three areas: healthcare, sports performance monitoring, and human–machine interfaces. The review concludes with a discussion of challenges and opportunities for future research.