Polyurethane Substrate Research Articles

Development of stretchable electrodes for wearables using vacuum thermal pressure

Several recent studies have investigated textile-electrodes, as they can be used in various types of wearable devices in many different fields. However, if an electrode is printed directly on top of a textile, the pattern cannot spread or permeate the textile to form an electrode. To solve this problem, this study printed electrodes on stretchable and printable polyurethane substrates. They were then integrated with a textile through a vacuum-thermal-pressure (VTP) process. This process forms an electrode pattern on the textile and complements the recovery of the polyurethane substrates. Various analyses were conducted to assess the properties of these fabricated textile-electrodes compared to those of conventional TPU-electrodes. We compared the mechanical durabilities and conductivities of textile-electrodes vs. conventional TPU-electrodes. The degree of waterproofing was also increased through the encapsulation process, and the characteristics of the encapsulation-material were analyzed through environmental reliability tests and washable-durability tests to examine their suitability for future applications in wearable products.

Highly stretchable transparent bar coated Ag NW/PEDOT:PSS hybrid electrode for wearable and stretchable devices.

In this study, we demonstrated poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) as a composite with Ag nanowire (Ag NW) to enhance the stretchability of the Ag NW network electrode. The composite Ag NW/PEDOT:PSS hybrid ink (AP ink) was prepared at a ratio of 1 : 10, 1 : 20, and 1 : 30, respectively and bar coated on polyurethane substrate. The different ink ratios were studied and optimized with a sheet resistance of 14.93 Ω sq−1. and a transmittance of 88.6% showing a high performance in mechanical stress tests such as bending, folding, rolling, twisting, and stretching. It also showed a conductive bridge effect where the PEDOT:PSS acted as an anchor or support to Ag NW during mechanical strain and PEDOT:PSS also enhanced the electrical conductivity of the Ag NW. Therefore, to prove the real time performance of the electrode as a wearable device, we fabricated transparent electroluminescence devices and thin film heater devices which are highly flexible and demonstrated excellent performance proving that the AP electrode is more suitable candidate for future wearable transparent devices.

Thermal and UV light adaptive polyurethane elastomers for photolithography-transfer printing of flexible circuits.

Flexible polymers are widely used in the fields of wearable devices, soft robots, sensors, and other flexible electronics. Combining high strength and elasticity, electrical conductivity, self-healability, and surface tunable properties in one material becomes a challenge for designing polymeric materials for these applications, especially in flexible electronics. Herein, we propose a "two birds with one stone" strategy to synthesize thermal and UV light adaptive polyurethane elastomers with high-strength, self-healable, surface-modifiable and patternable functions for photolithography-transfer printing flexible circuits. The "stone", dihydroxybenzophenone, plays two roles in the synthesized polyurethanes as both a dynamic covalent bond and a UV-sensitive unit. On one hand, the phenolic group reacts with isocyanate to form a dynamic covalent phenol-carbamate bond, making the polymer self-healable, processable, and surface-embeddable with conductive fillers utilizing dynamic network rearrangement. On the other hand, the benzophenone group acts as a UV-sensitive unit to graft other functional groups to the polymer surface or self-crosslink on the surface under UV irradiation. Based on the dynamic covalent network and UV self-crosslinking properties, self-healable patterned flexible circuits can be obtained by photolithography-transfer printing. The flexible circuits prepared by loading silver nanowires on the dynamically crosslinked polyurethane substrate show little change of electric resistance when stretched up to 125% and can withstand thousands of stretching cycles.

Me-Doped Ti-Me Intermetallic Thin Films Used for Dry Biopotential Electrodes: A Comparative Case Study.

In a new era for digital health, dry electrodes for biopotential measurement enable the monitoring of essential vital functions outside of specialized healthcare centers. In this paper, a new type of nanostructured titanium-based thin film is proposed, revealing improved biopotential sensing performance and overcoming several of the limitations of conventional gel-based electrodes such as reusability, durability, biocompatibility, and comfort. The thin films were deposited on stainless steel (SS) discs and polyurethane (PU) substrates to be used as dry electrodes, for non-invasive monitoring of body surface biopotentials. Four different Ti–Me (Me = Al, Cu, Ag, or Au) metallic binary systems were prepared by magnetron sputtering. The morphology of the resulting Ti–Me systems was found to be dependent on the chemical composition of the films, specifically on the type and amount of Me. The existence of crystalline intermetallic phases or glassy amorphous structures also revealed a strong influence on the morphological features developed by the different systems. The electrodes were tested in an in-vivo study on 20 volunteers during sports activity, allowing study of the application-specific characteristics of the dry electrodes, based on Ti–Me intermetallic thin films, and evaluation of the impact of the electrode–skin impedance on biopotential sensing. The electrode–skin impedance results support the reusability and the high degree of reliability of the Ti–Me dry electrodes. The Ti–Al films revealed the least performance as biopotential electrodes, while the Ti–Au system provided excellent results very close to the Ag/AgCl reference electrodes.

Two Birds with One Stone: Preparation of 4, 4-Diaminodiphenylmethane Functionalized GO@SiO2 with Mechanical Reinforcement and UV Shielding Properties and Its Application in Thermoplastic Polyurethane.

This study prepared 4,4-diaminodiphenylmethane (DDM)-functionalized graphene oxide (GO)@silica dioxide (SiO2) nano-composites through amidation reaction and low-temperature precipitation. The resulting modified GO, that was DDM−GO@SiO2. The study found that DDM−GO@SiO2 showed good dispersion and compatibility with thermoplastic polyurethane (TPU) substrates. Compared with pure TPU, the tensile strength of the TPU composites increased by 41% to 94.6 MPa at only 0.5 wt% DDM−GO@SiO2. In addition, even when a small amount of DDM−GO@SiO2 was added, the UV absorption of TPU composites increased significantly, TPU composites can achieve a UV shielding efficiency of 95.21% in the UV-A region. These results show that this type of material holds great promise for the preparation of functional coatings and film materials with high strength and weather resistance.

Vacuum‐Assisted Layer‐by‐Layer Carbon Nanotube/Ti3C2TX MXene Films for Detecting Human Movements

AbstractThe rapid development of the Internet of Things has promoted the application of wearable devices in human's daily life and industrialization. Strain sensors play an indispensable role as a member of wearable devices. Here, a strain film sensor with multi‐walled carbon nanotubes (MWCNTs) and Ti3C2TX MXene, which is obtained by vacuum‐assisted filtration and integrated onto a thermoplastic polyurethane substrate, is developed. Since the MWCNTs and Ti3C2TX MXene composite sensor has a characteristic ‘mud‐brick’ microstructure. The sensor has the ability to detect various human movements, such as, bending of finger joints, wrists, elbow joints, and shoulder. The sensors show excellent mechanical property and can be folded more than 2000 times without failure. It also exhibits short response time, low detection limit, and wide range of use. In addition, the sensor shows excellent electrical heating performance. The temperature of the composite film can rise to 96.6 °C in a short period of time even under low voltage power supply (25 V). It likewise proves its excellent performance in terms of cyclability and long‐time heating capability. The film has low production cost, easy preparation method, and simple operation. The MWCNT/Ti3C2TX MXene film can be integrated into wearable devices and shows great potential for future applications.

Fabrication of High-Performance Flexible Supercapacitor Electrodes with Poly(3,4-ethylenedioxythiophene) (PEDOT) Grown on Carbon-Deposited Polyurethane Sponge

Composite porous supercapacitor electrodes were prepared by growing poly(3,4-ethylenedioxythiophene) (PEDOT) on graphite nanoplatelet- or graphene nanoplatelet-deposited open-cell polyurethane (PU) sponges via a vapor phase polymerization (VPP) method. The resulting composite supercapacitor electrodes exhibited great capacitive performance, with PEDOT acting as both the conductive binder and the active material. The chemical composition was characterized by Raman spectroscopy and the surface morphology was characterized by scanning electron microscopy (SEM). Cyclic voltammetry (CV), charge-discharge (CD) tests and electrochemical impedance spectroscopy were utilized to study the electrical performance of the composite electrodes produced in symmetrically configured supercapacitor cells. The carbon material deposited on PU substrates and the polymerization temperature of PEDOT affected significantly the PEDOT morphology and the electrical properties of the resulting composite sponges. The highest areal specific capacitance 798.2 mF cm−2 was obtained with the composite sponge fabricated by VPP of PEDOT at 110 °C with graphene nanoplatelet-deposited PU sponge substrate. The capacitance retention of this composite electrode was 101.0% after 10,000 charging–discharging cycles. The high flexibility, high areal specific capacitance, excellent long-term cycling stability and low cost make these composite sponges promising electrode materials for supercapacitors.

3D printing-directed flexible strain sensors of accordion-like architecture to achieve ultrastretchability with the assist of ultrasonic cavitation treatment

A new class of accordion-like cellular architecture with sinusoidal struts is designed to enhance the planar stretchability of cellular solids, aiming to fabricate flexible strain sensors with ultrastretchability. The combination manufacturing process of fused deposition modeling (FDM) 3D printing technique and ultrasonic cavitation-enabled treatment was introduced into the fabrication of flexible strain sensors made of thermoplastic polyurethane (TPU) substrate and carbon nanotubes (CNTs). A negative Poisson’s ratio (NPR) architecture made of TPU was firstly 3D-printed by FDM. The ultrasonic cavitation treatment was then conducted on the soft auxetic structure immersing in CNTs liquid, aiming to embed the CNTs into the surface layer of the flexible TPU substrate with NPR configurations. Instead of 3D printing the TPU matrix composite after hybridization inside the matrix material, the hybrid manufacturing procedure can ensure that the intrinsic excellent mechanical properties of TPU are not embrittled. Besides, the sinusoidal struts in accordion-like cellular architectures offer a design route to extend the material property chart to achieve ultrahigh stretchability in lightweight 3D printable flexible polymers for the applications that require combined stretchability, lightweight, and energy absorption such as soft robotics, stretchable electronics, and wearable protection shields.

In-situ laser sintering for the fabrication of fully 3D printed electronics composed of elastomeric materials

In-situ laser sintering can be used to locally sinter conductive inks in an uninterrupted process, enabling the embedding of printed electronics within 3D printed structures. In this work we apply a laser sintering method to create highly conductive silver features (50–125 cm) on top of and embedded within thermosensitive polymers printed by fused deposition modelling (FDM). We exploit this method to locally sinter silver inks deposited by direct ink writing (DIW) without external processing. Furthermore, we perform this while thermally preserving the 3D printed thermoplastic polyurethane (TPU) substrate which has a low glass transition temperature. By analyzing the effect of the process for both small and large shapes of conductive features, we show the importance of the transmitted power and sintering time. Lastly, we apply the developed method to produce flexible conductors and sensors. These include pressure and bending sensors that could find their way into cost-effective and customized soft electronic devices.

Influence of silver/silver chloride electroless plating on the Shore hardness of polyurethane substrates for dry EEG electrodes

Abstract Dry electrodes enable a shorter preparation time for infant EEG. Since infant skin is more sensitive than adult skin, soft electrodes are required to reduce the mechanical stress for this sensitive skin. Thus, soft electrodes are crucial for eventual repetitive and long-term use like in neonatal intensive care units. A biocompatible polyurethane (PU) can be produced in low hardness resulting in a soft and flexible electrode substrate. Silver/silver chloride (Ag/AgCl) electroless plating provides a conductive, electrochemically stable coating but the process may alter the mechanical properties of the electrode substrate. In this study, we assess the hardness of PU material before and after Ag/AgCl plating. The test sample design for Shore hardness measurement is based on ISO 7619-1:2010. Sample production consists of a 3D print master model, silicone molding, PU casting, and finally electroless plating. UPX 8400-1 (Sika AG, Switzerland) is used for the sample substrates. Test samples are produced with 7 different Shore hardness (range A40-A95) and 14 samples (each hardness: 1 uncoated and 1 coated). The hardness measurements are carried out with a lever-operated test stand Shore hardness tester model with a digital hardness tester (TI-AC with HDA 100-1, KERN &SOHN GmbH, Germany).. It is shown that there is a hardness increase (Shore A) due to Ag/AgCl coating with a grand average of 1.1±0.7 (p<0.05). The largest increase of 2.1±0.2 is seen on the initial lowest Shore hardness sample (Shore hardness: 43.4±0.1). The absolute increase of hardness due to the Ag/AgCl coating decreases with increasing substrate hardness. It is concluded that there is no strong hardness increase of PU substrates due to Ag/AgCl plating. Therefore, the material is suitable as a soft electrode for repetitive and long-term use in infant applications.

Trace width effects on electrical performance of screen-printed silver inks on elastomeric substrates under uniaxial stretch

This work investigates the origins of electrical performance degradation under uniaxial stretching of a silver filled polyurethane ink (DuPont PE 874) screen printed onto a thermoplastic polyurethane substrate. The ink develops surface ruptures at strains of only a few percent yet remains conductive through continued elongation. We identify increasing sensitivity to surface damage beyond 10% applied strain, ɛapp, as the trace width, w, is reduced from 2 to 0.1 mm. This lowers the threshold strain for open circuit failure, from approximately 180% for w = 2 mm down to 25% for w = 0.1 mm. The damage progression remains largely consistent across trace widths: surface cracks coalesce to form longer channels, which grow perpendicular to the direction of elongation. These channels both deepen and widen with increasing ɛapp and some become laterally linked. The evolution of the network of interlinked channels is not width dependent, but a width effect manifests as a result of the channels constituting a larger fraction of specimen width for narrower traces. In addition, the narrower traces exhibit reduced cross sections due to an edge taper—an artifact of the screen printing process—which attenuates ink thickness by as much as 50% for w = 0.1 mm.

Intensifying Solar Interfacial Heat Accumulation for Clean Water Generation Excluding Heavy Metal Ions and Oil Emulsions

Solar‐driven interfacial steam generation has emerged as an innovative technique for seawater desalination due to its high photothermal conversion efficiency and potential industrial applications. Herein, a superior interfacial heat accumulation structure composed of semiconductive in situ polymerization (polypyrrole) of nickel foam (IPNF) is reported. The IPNF photothermal layer is assembled with superhydrophilic polyurethane substrate for synchronous water transport and excellent thermal insulation. The 2D ultrablack mesh induces multiple incident rays within the diffused polymerized surface, which allows omnidirectional solar absorption (88.5 %) and intensifying heat localization (49.5 °C @ 1 sun). The state‐of‐the‐art evaporation performances reveal that the integrated IPNF solar evaporator exhibits an excellent evaporation rate (1.74 kg m−2 h−1) and solar‐to‐vapor conversion efficiency (90% excluding heat losses) under 1 kW m−2 solar intensity. Besides this, the long‐term evaporation experiments show negligible discrepancy under seawater conditions (13.27 kg m−2/8 h under 1 kW m−2) and engrain its functioning potential for multimedia and salt rejection (3.2 g × NaCl/240 min). More importantly, herein, insights into different water states in the polymeric network systems during solar‐driven evaporation are provided. This work shows a significant potential to generate freshwater excluding heavy metals and other oil emulsions for industrial applications.

Sustained Antibacterial Effect of Levofloxacin Drug in a Polymer Matrix by Hybridization With A Layered Double Hydroxide

AbstractThe immobilization of antimicrobial drugs can be used to expand the application of antibacterial properties to consumer products. The purpose of this study was to stabilize an antimicrobial agent, levofloxacin (LVX), for sustained antibacterial activity by immobilizing the drug molecules in a layered double hydroxide (LDH) and embedded in a polyurethane substrate. As-prepared MgAl-LDH was calcined at 400°C and reconstructed with LVX for intercalation. The X-ray diffraction patterns and cross-sectional transmission electron microscopy images showed lattice expansion along the crystallographic c axis upon LVX intercalation, suggesting successful loading of the drug. Fourier-transform infrared spectra revealed that the structure of LVX was well preserved between LDH layers. Elemental analysis indicated that the loading capacity of LVX in the hybrid was 41.7%. Bacterial-colony forming inhibitory assay on Bacillus subtilis exhibited ~100% antibacterial activity of both LVX alone and LVX-LDH hybrid (LL). To determine sustainability of antibacterial activity by the hybrid, either LVX alone or LL hybrid was loaded in the polyurethane (PU) substrate for which antibacterial activity was evaluated before and after immersion in a phosphate-buffered saline for 3 days. The LVX-composited PU showed a dramatic decrease in antibacterial activity, down to 0% after buffer treatment; LL-composited PU still contained antibacterial activity (~34% of colony suppression) after phosphate-buffered saline immersion.

Surface Modification of Polyurethane Membrane with Various Hydrophilic Monomers and N-Halamine: Surface Characterization and Antimicrobial Properties Evaluation.

Reducing microbial infections associated with biomedical devices or articles/furniture noted in a hospital or outpatient clinic remains a great challenge to researchers. Due to its stability and low toxicity, the N-halamine compound has been proposed as a potential antimicrobial agent. It can be incorporated into or blended with the FDA-approved biomaterials. Surface grafting or coating of N-halamine was also reported. Nevertheless, the hydrophobic nature associated with its chemical configuration may affect the microbial interactions with the chlorinated N-halamine-containing substrate. In this study, a polymerizable N-halamine compound was synthesized and grafted onto a polyurethane surface via a surface-initiated atom transfer radical polymerization (SI-ATRP) scheme. Further, using the sequential SI-ATRP reaction method, different hydrophilic monomers, namely poly (ethylene glycol) methacrylate (PEGMA), hydroxyethyl methacrylate (HEMA), and [2-(methacryloyloxy) ethyl] dimethyl-(3-sulfopropyl) ammonium hydroxide (SBMA), were also grafted onto the polyurethane (PU) substrate before the N-halamine grafting reaction to change the surface properties of the N-halamine-modified substrate. It was noted that the chains containing the hydrophilic monomer and the polymerizable N-halamine compound were successfully grafted onto the PU substrate. The degree of chlorination was improved with the introduction of a hydrophilic monomer, except the HEMA. All of these hydrophilic monomer-containing N-halamine-modified PU substrates demonstrated a more than 2 log CFU reduction after microbial incubation. In contrast, the surface modified with N-halamine only exhibited significantly less antimicrobial efficacy instead. This is likely due to the synergistic effects caused by the reduced chlorine content, as well as the reduced surface interactions with the microbes.

Development of chemically grafted multiwall carbon nanotube onto cellulose fiber sheet and polyurethane based resin composite for an active paper

Cellulose fibers (CFs) and carbon nanotubes (CNTs) were successfully developed on polyurethane substrate as a flexible composite paper. With small amount of CNTs, the composite was prepared by a suction filtration method. The existence of CNT additive in cellulose matrix was investigated based on the correlation of mechanical properties, thermal stability, and electrical properties. Although the highly transparent cellulose sheets impregnated with polyurethane were successfully fabricated, the low transmittance was obtained as the increasing of CNT additive. However, the dielectric properties of composite were enhanced with an addition of CNTs in the composite paper. The electrical conductivity was increased from the insulator to 4.91  10-4 Scm-1 at small amount of CNTs of 5 wt%. In addition, the minimal amount of CNTs of 1.5% showed the transmittance of 35%, adequate dielectric constant, and the conductivity of 5.59 × 10-7 Scm-1. The role of CNTs with well distribution presents as a polar cluster of well-defined electrically charge in cellulose composite.

水耕栽培の葉ネギの培地に発生するホシチョウバエに対する温湯処理による防除効果

水耕栽培の葉ネギの培地に発生するホシチョウバエに対する温湯処理による防除効果

Stretchable Inorganic GaN-Nanowire Photosensor with High Photocurrent and Photoresponsivity.

To effectively implement wearable systems, their constituent components should be made stretchable. We successfully fabricated highly efficient stretchable photosensors made of inorganic GaN nanowires (NWs) as light-absorbing media and graphene as a carrier channel on polyurethane substrates using the pre-strain method. When a GaN-NW photosensor was stretched at a strain level of 50%, the photocurrent was measured to be 0.91 mA, corresponding to 87.5% of that (1.04 mA) obtained in the released state, and the photoresponsivity was calculated to be 11.38 A/W. These photosensors showed photocurrent and photoresponsivity levels much higher than those previously reported for any stretchable semiconductor-containing photosensor. To explain the superior performances of the stretchable GaN-NW photosensor, it was approximated as an equivalent circuit with resistances and capacitances, and in this way, we analyzed the behavior of the photogenerated carriers, particularly at the NW-graphene interface. In addition, the buckling phenomenon typically observed in organic-based stretchable devices fabricated using the pre-strain method was not observed in our photosensors. After a 1000-cycle stretching test with a strain level of 50%, the photocurrent and photoresponsivity of the GaN-NW photosensor were measured to be 0.96 mA and 11.96 A/W, respectively, comparable to those measured before the stretching test. To evaluate the potential of our stretchable devices in practical applications, the GaN-NW photosensors were attached to the proximal interphalangeal joint of the index finger and to the back of the wrist. Photocurrents of these photosensors were monitored during movements made about these joints.

Self-healable and recyclable polyurethane-polyaniline hydrogel toward flexible strain sensor

Flexible electronics have been extensively investigated in recent years because of their potential applications in artificial skin, soft robotics, energy storage, healthcare technology, etc. However, since the rigid and brittle nature of the conductive or semi-conductive materials, it's a huge challenge to project stretchable, self-healable and recyclable soft electronics to mimic skin. Herein, the interpenetrating and synergistic dual-network hydrogel of Diels-Alder chemistry-based dynamic polyurethane substrates and polyaniline conductive polymer (PU-DA-1/1-PANI) was prepared and employed as a strain sensor. The PU-DA-1/1-PANI hydrogel exhibited large elongation at break of 500%, soft Young's modulus of 0.3 MPa, and robust tensile strength of 1.1 MPa. Meanwhile, the strain sensors with optimized composition presented an ideal conductivity of 7.9 S m−1, which could monitor the physical motions precisely and repeatedly with a gauge factor (GF) of 2.9. In addition, the mechanical performance and conductivity could be self-healed, which was enabled by the hydrogen bonds, ionic interactions, reversible DA covalent bonds, and rearrangement of polymer chains. Built on the thermally stimuli-responsive nature of the DA covalent cross-linking points, the PU-DA-1/1-PANI composites could be recycled.

Large-Scale Rapid Laser Sintering of Highly Stretchable Electrodes Using a Homogenized Rectangular Laser Beam.

This study explored the feasibility of a fast and uniform large-scale laser sintering method for sintering stretchable electrodes. A homogenized rectangular infrared (IR) laser with a wavelength of 980 nm was used in the sintering process. A highly stretchable composite electrode was fabricated using silver (Ag) microparticles and Ag flakes as the fillers and polyester resin as the binder on the polyurethane substrate. This laser-sintering method showed a sintering time of 1 sec and a very uniform temperature across the surface, resulting in enhancing the conductivity and stretchability of the electrodes. The effects of the laser power on the electrical and electromechanical properties of the electrodes were investigated. Using stretching, bending, and twisting tests, the feasibility of the laser-sintered stretchable electrodes was comprehensively examined. The electrode that was sintered at a laser power of 50 W exhibited superior stretchability at a strain of 210%, high mechanical endurance of 1,000 repeated cycles, and excellent adhesion. The stretchable electrodes showed excellent bendability and twistability in which the electrodes can be bent up to 1 mm and twisted up to 90° without any damage; thus, they are highly applicable as stretchable electrodes for wearable electronics. Additionally, the Ag composites were explored for use in a radio-frequency (RF) stretchable antenna to confirm the application of the laser-sintering method for stretchable and wearable electronic devices. The stretchable dipole antenna showed an excellent radiation efficiency of 95% and a highly stable operation, even when stretched to 90% strain.

Biaxial Inflation Stretch Test for Flexible Electronics

Printed flexible ink manufacturers typically report electrical performance under uniaxial strains. Under observed real‐world conditions, flexible ink systems experience strain in multiple dimensions. This indicates a potential for misrepresentation of the failure modes of these materials in application if strictly characterized using uniaxial methods. A novel test method is proposed in which a device is clamped around its periphery and subjected to an inflation pressure to induce a biaxial strain‐state. The electrical performance of the ink is monitored via in situ resistance measurement. 3D digital image correlation and finite‐element analysis are used to validate the calculated stress–strain state. Comprehensive analytical and numerical models are developed and compared with experimental results. The following findings are reported: 1) a commercial silver‐based stretchable ink screen‐printed onto a thermoplastic polyurethane substrate can withstand upward of 70% uniaxial strain without electrical failure, yet experiences a loss of conductivity at just 12% biaxial strain, 2) the proposed inflation test setup successfully creates a biaxial strain‐state in the substrate–ink system while measuring the system's electrical performance, and 3) models, based on 3D percolation theory and the mechanics of pressurized membranes, accurately describe the electrical and mechanical behavior of the substrate–ink system.