Flexible Thermoplastic Polyurethane Research Articles

One-shot manufacturable soft-robotic pump inspired by embryonic tubular heart.

Soft peristaltic pumps, which use soft ring actuators instead of mechanical pistons or rollers, offer advantages in transporting liquids with non-uniform solids, such as slurry, food, and sewage. Recent advances in 3D printing with flexible thermoplastic polyurethane (TPU) present the potential for single-step fabrication of these pumps, distinguished from handcrafted, multistep traditional silicone casting methods. However, because of the relatively high hardness of TPU, TPU-based soft peristaltic pumps contract insufficiently and thus cannot perform as well as silicone-based ones. Improving the performance is crucial for fully automated, one-step manufactured soft pumps to lead to industrial use. This study aims to enhance TPU-based soft pumps through bioinspired design. Specifically, it proposed a design inspired by embryonic tubular hearts, in contrast to previous studies that mimicked digestive tracts. The new design facilitated long-axis stretching of an elliptical lumen during non-concentric contractile motion, akin to embryonic tubular hearts. The design was optimized for ring actuators and pumps 3D-printed with shore hardness 85 A TPU filament. The ring actuator achieved over 99% lumen closure with the best designs. The soft pumps transported water at flow rates of up to 218 ml min-1and generated a maximum discharge pressure of 355 mm Hg, comparable to the performance of blood pumps used in continuous renal replacement therapy.

Multifunctional 3D-Printed Thermoplastic Polyurethane (TPU)/Multiwalled Carbon Nanotube (MWCNT) Nanocomposites for Thermal Management Applications

In this work, multiwalled carbon nanotubes (MWCNTs) were melt-compounded into a novel thermal energy storage system consisting of a microencapsulated paraffin, with a melting temperature of 6 °C (M6D), dispersed within a flexible thermoplastic polyurethane (TPU) matrix. The resulting materials were then processed via Fused Filament Fabrication (FFF), and their thermo-mechanical properties were comprehensively evaluated. After an optimization of the processing parameters, good adhesion between the polymeric layers was obtained. Field-Emission Scanning Electron Microscopy (FESEM) images of the 3D-printed samples highlighted a uniform distribution of the microcapsules within the polymer matrix, without an evident MWCNT agglomeration. The thermal energy storage/release capability provided by the paraffin microcapsules, evaluated through Differential Scanning Calorimetry (DSC), was slightly lowered by the FFF process but remained at an acceptable level (i.e., >80% with respect to the neat M6D capsules). The novelty of this work lies in the successful integration of MWCNTs and PCMs into a TPU matrix, followed by 3D printing via FFF technology. This approach combines the high thermal conductivity of MWCNTs with the thermal energy storage capabilities of PCMs, creating a multifunctional nanocomposite material with unique thermal management properties.

Multivariate gradient structure design of flexible thermoplastic polyurethane based composite foam for enhanced electromagnetic interference shielding performance

Flexible and efficient electromagnetic interference (EMI) shielding polymer composite foams are rapidly developing, but it is difficult to avoid the deterioration of shielding properties after foaming. To solve the issue, in this work, composite foams with multivariate gradient structure of thermoplastic polyurethane (TPU) mixed with multiwalled carbon nanotubes (MWCNTs) were constructed by combining the soft-hard domains proportional gradient of TPU with filler content gradient. Gradient-structured TPU/MWCNTs foams with impedance matching layer and high reflector layer were obtained and impedance matching layer with large cells significantly reduced the density while high reflector layer with a few cells maintained relatively perfect conductive network, whose shielding efficiency was 38.2 % higher than that of unfoamed sample. Furthermore, TPU/MWCNTs composite foams presented great stability even after 1000 times bending. This special multivariate gradient structure shows potential applications in microelectronic devices and aerospace fields and is critical to the development of high-performance EMI shielding polymer foams.

Additive manufacturing of flexible thermoplastic polyurethane (TPU): enhancing the material elongation through process optimisation

AbstractThermoplastic polyurethane (TPU) is used to produce elastomeric parts with superior wear/abrasion resistance, toughness, shock absorption properties, and flexibility, even at low temperatures. The production of this material through additive manufacturing (AM) techniques has been increasing because of the possibility of tuning the mechanical properties using structural design and build process parameters. Despite the data being limited, TPU produced by AM, mainly based on material extrusion, is much stiffer than the corresponding produced by conventional manufacturing, and, therefore, it shows a limited elongation. This study presents the mechanical characterization of TPU produced by the infrared light powder bed fusion (PBF-IrL) system (HP multi-jet fusion), which has recently been introduced. The properties are compared with TPU produced by open (3ntrA4) and closed (Markforged) material extrusion (MEX) systems. For the open FDM, the effects of the processing conditions are investigated to improve the material elongation and UTS with respect to the data reported in the literature for AM and conventional manufacturing. For this reason, an extensive and comprehensive review has been carried out. Compared to material extrusion, PBF-IrL TPU specimens showed higher Young’s modulus, but poorer A%. Considering the samples produced by MEX and compared to previous results in the literature, the properties obtained in this study are superior both in terms of UTS and A%.

A multimodal sensing network based on synergistically sensitized polyaniline composites strategy for safety monitoring in pesticide spraying

Ensuring the safety of farmers during pesticide spraying is crucial in smart agricultural practices due to the potential risks of pesticide poison and the associated irreversible damages. However, there is a notable deficiency of efficient monitoring strategies for providing timely and cost-effective safety warnings. In this study, we have devised an advanced and real-time sensing network using polyaniline (PANI) composites to address this pressing concern. To enhance sensing performance, diverse quantum dots (QDs) sensitizers and rigid or flexible substrates are strategically integrated with PANI, resulting in sensors tailored to different sensing capabilities. Specifically, the proton conduction is heightened by incorporating carboxyl graphene QDs (GQDs) into PANI, obtaining the superior gas-sensing performance for NH3 detection. Moreover, a strain sensor employing Ti3C2Tx QDs (MQDs) and a flexible thermoplastic polyurethane (TPU) substrate has been developed for real-time monitoring of physical biosignals. Finally, a smart sensing network with real-time display integrated with the above sensors is designed to detection NH3 concentration released from pesticide volatilization and physiological tracking in cases of mild pesticide poisoning. This sensing network can wirelessly interface with a laptop for convenient safety monitoring of farmers engaged in smart agriculture. The proof of concept demonstrated by this sensing network holds promise for enhancing safety monitoring in smart agriculture.

Development of AgNPs-PVP/TPU based flexible strain sensors for structural health monitoring of composite structures

Real-time in-stu structural health monitoring (SHM) of composite structure developed through highly sensitive and flexible smart sensors based on colloidal solution of Silver Nano Particles (AgNPs). Synthesis of AgNPs through chemical reduction method by citrate and then in Haxen phase transferred by using surfactant Cetyltrimethylammonium bromide (CTAB). Colloidal solution of AgNPs-Haxen and Polyvinylpyrrolidone (PVP) - Haxen has been followed by mixed, calcined and deposited that thick paste on flexible Thermoplastic Polyurethane (TPU) film. These sensors become more sensitive than metallic foil made sensors and farther this sensor pasted on polymer composite for monotonic loading like tensile loading. The data extracted from sensors was monitored effectively by data acquisitioning technique using Keithley KUSB-3100. This research based on chemical synthesis of metallic nanoparticles like silver, its functionalization, protection of these nanoparticles from environment by capping agents and by creating inert atmosphere respectively. Then these functionalized capped nanoparticles are embedded on the surface of thermoplastic Urethane (TPU) film to make piezo resistive semi conductive strips which serve as flexible strain gauge/smart sensor. One other technique is also being used to make sensor is that colloidal solution of metallic Nanoparticles sprays over TPU substrate using Connective Self Assembly (CSA) method. The sensor then properly attached to Glass fabric reinforced polymer (GFRP) composite and calibrated using data acquisitioning (DAQ) under different mode of testing.

Online structural health monitoring of polymer composite structure using gold nanoparticles (AuNPs)

A mechanism for online structural health monitoring of composite structure was established by developing highly sensitive strain gauges based on colloidal Gold Nano Particles (AuNPs). They were synthesized using well known Turkevich-Frens method followed by phase transfer in chloroform using surfactant Cetyltrimethylammonium bromide (CTAB). Flexible Strain gauges / smart sensors were formed by mixing colloidal AuNPs-chloroform solution in polystyrene (PS) – chloroform solution and depositing their thick paste on hand-made flexible thermoplastic polyurethane (TPU) film. These sensors were used as strain gauges on composite substrates for tensile testing. The developed AuNP based strain gauges were found to have 10 times higher sensitivity than normal metal foil strain gauges.

Crystallization, thermal properties, rheological properties, mechanical properties, and morphology of thermoplastic polyurethanes/calcium sulfate whiskers composites

The research shows that thermoplastic polyurethane (TPU) which is easy to process has great application potential in practical application. In this study, a calcium sulfate whisker (CSW) with high strength and stability was used as a reinforcing filler to improve the processability of flexible TPU. In this paper, TPU/CSW composites were prepared by the melt blending method, and the effects of modified CSW on the thermal, crystalline, and melt properties, as well as the rheological and mechanical properties of TPU composites were investigated. The results showed that when the mass content of modified CSW was 10%, the maximum values of ΔH c and ΔH m were 222.81°C and 9.24 J/g, respectively, and the thermal properties of the composites were the most stable. When the content of modified CSW is 5%, the maximum impact strength, bending strength, and tensile strength are 50.5 KJ/m2, 38.8 MPa, and 927.7 MPa, respectively, and the mechanical properties of the composites are the best. The excellent stability and high strength of TPU/CSW composites show great promise for various applications in transportation and building materials.

Dynamic Characterization of a Low-Cost Fully and Continuously 3D Printed Capacitive Pressure-Sensing System for Plantar Pressure Measurements.

In orthopedics, the evaluation of footbed pressure distribution maps is a valuable gait analysis technique that aids physicians in diagnosing musculoskeletal and gait disorders. Recently, the use of pressure-sensing insoles to collect pressure distributions has become more popular due to the passive collection of natural gait data during daily activities and the reduction in physical strain experienced by patients. However, current pressure-sensing insoles face the limitations of low customizability and high cost. Previous works have shown the ability to construct customizable pressure-sensing insoles with capacitive sensors using fused-deposition modeling (FDM) 3D printing. This work explores the feasibility of low-cost fully and continuously 3D printed pressure sensors for pressure-sensing insoles using three sensor designs, which use flexible thermoplastic polyurethane (TPU) as the dielectric layer and either conductive TPU or conductive polylactic acid (PLA) for the conductive plates. The sensors are paired with a commercial capacitance-to-voltage converter board to form the sensing system. Dynamic sensor performance is evaluated via sinusoidal compressive tests at frequencies of 1, 3, 5, and 7 Hz, with pressure levels varying from 14.33 to 23.88, 33.43, 52.54, and 71.65 N/cm2 at each frequency. Five sensors of each type are tested. Results show that all sensors display significant hysteresis and nonlinearity. The PLA-TPU sensor with 10% infill is the best-performing sensor with the highest average sensitivity and lowest average hysteresis and linearity errors. The range of average sensitivities, hysteresis, and linearity errors across the entire span of tested pressures and frequencies for the PLA-TPU sensor with 10% infill is 11.61-20.11·10-4 V/(N/cm2), 11.9-31.8%, and 9.0-22.3%, respectively. The significant hysteresis and linearity error are due to the viscoelastic properties of TPU, and some additional nonlinear effects may be due to buckling of the infill walls of the dielectric.

Employing additive manufacturing to create reusable TPU formworks for casting topologically optimized facade panels

Concrete is a building material with excellent structural qualities that can be molded into any shape. However, it is also responsible for approximately 8% of the world's total greenhouse gas emissions. There is a need to reduce material consumption during the design and construction of concrete structures. This research proposes a new manufacturing workflow for the design and fabrication of precast façade panels. Structural topology optimization (TO) is used to design perforated façade panels with optimized material distribution according to the panel's connection to a sub-structure. Structural simulations are conducted to validate the stiffness and strength of the TO panels. Finally, flexible Thermoplastic Polyurethane (TPU) molds are additively manufactured and repeatedly cast. The study results show that TO decreases the panels' volumes within a predefined WWR by anywhere from 23% to 30% of the total volume, while maintaining satisfactory structural performance. Additionally, the study demonstrates the potential of additively manufactured TPU molds for the repeatable casting of concrete while accommodating undercuts in the complex cast geometries. This study is novel as TO has not previously been used for designing façade panels. Furthermore, the use of 3D printed flexible molds made of TPU for repeatable casting of geometries is still in its early stages, with only one publication reporting the investigations at the time of writing this article. This workflow is innovative as a whole as it addresses material saving in the design phase through TO, as well as the creation of reusable 3D printed formworks in the fabrication phase.

Highly fire safe and flexible nanoarchitectures with tunable interface towards excellent electromagnetic interference shielding

To meet the demand for practical applications, it is imperative to develop flexible electromagnetic interference (EMI) shielding materials with fire safety. In this work, highly fire safe and flexible thermoplastic polyurethane (TPU)-based EMI shielding nanocomposites were prepared via melt blending polyphenyl phosphate ester amide (PPPEA) with MXene/carboxylic multiwalled carbon nanotubes hybrid (MC), air-assisted thermocompression and layer-by-layer stacking. Combining PPPEA with MC led to peak of heat release rate, total heat release and total smoke release of TPU nanocomposite containing 5.0 wt% PPPEA and 2.0 wt% MC decreased by 39.0%, 24.5% and 23.8%, respectively. Furthermore, the hierarchical TPU/5PA/2.0MC/carbon fiber cloth (CF) nanocomposite showed a prominent averaged EMI shielding effectiveness (SE) of 35.5 dB and a high normal SE value of 29.6 dB/mm, which were superior to most previously reported results. Catalytic carbonization of PPPEA and multiply reflected and scattered effect of MC together with continuous conductive network of CF were responsible for the improvement of these performances.

Bio-Inspired Deformable Propeller Concept for Smooth Human-UAV Interaction and Efficient Thrust Generation

This letter presents a novel deformable propeller concept, 3D-printed in flexible thermoplastic polyurethane, which stores impact energy in the form of elastic energy for smooth collisions, reducing risk in human-UAV interactions. However, such a design experiences elastic deformations when exposed to aerodynamic and centrifugal loads. The most relevant occur in the 3-5 krpm range. At higher speeds, the centrifugal force is dominant and the propeller becomes quite stiff. This work provides an investigation of the propeller deformation angles (which ultimately alter the aerodynamic profile and limit thrust generation) based on Fluid-Structure Interaction (FSI) simulations. These results are leveraged to introduce two complementary solutions: deformation reduction oriented internal fiber distributions, inspired by the ultra-efficient dragonfly wings (I); anticipatory designs which pre-modify pitch and roll angles based on simulation results to ensure optimal performance at the target rotational velocity (II). This letter offers a multi-configuration analysis, yielding an easy to manufacture propeller with a specific design methodology which results in increased efficiency and reduced impact recovery time. This work presents collision tests with various objects, as well as a proof of concept for its flight capabilities in a conventional quadrotor, including physical interaction with humans. These findings are valuable for the development of collision control strategies for UAVs.

Highly Stretchable, Highly Sensitive, and Antibacterial Electrospun Nanofiber Strain Sensors with Low Detection Limit and Stable CNT/MXene/CNT Sandwich Conductive Layers for Human Motion Detection

With the rise of the concept of “Internet of Everything”, the development of wearable sensing devices is growing rapidly. Among them, crack-based strain sensors have received much attention due to their high sensitivity. However, the application of crack-based strain sensors has been limited by the relatively narrow sensing range. Here, a sandwich structure of a CNT/MXene/CNT sensing layer was realized on the flexible thermoplastic polyurethane substrate prepared by electrospinning, followed by layer-by-layer vacuum filtration. The two stable carbon nanotube (CNT) layers broaden the sensing range, and the strain sensor exhibits excellent sensing range (390%) and stability (3000 tensile tests). The strain sensor also shows excellent sensitivity (GF = 2159.5), low detection limit (0.05%), and fast response time (∼50 ms). The sensor is responsive to different movements when being applied to human motion detection, and the smart wearable device based on the strain sensor shows excellent performance in gesture language recognition. In addition, the antibacterial property of the conductive material inherently facilitates the wider application of the strain sensors.

Strain-adjustable reflectivity of polyurethane nanofiber membrane for thermal management applications

Passive radiative cooling technologies are highly attractive in pursuing sustainable development. However, current cooling materials are often static, which makes it difficult to cope with the varying needs of all-weather thermal comfort management. Herein, a strategy is designed to obtain flexible thermoplastic polyurethane nanofiber (Es-TPU) membranes via electrospinning, realizing reversible in-situ solvent-free switching between radiative cooling and solar heating through changes in its optical reflectivity by stretching. In its radiative cooling state (0% strain), the Es-TPU membrane shows a high and angular-independent reflectance of 95.6% in the 0.25–2.5 μm wavelength range and an infrared emissivity of 93.3% in the atmospheric transparency window (8–13 μm), reaching a temperature drop of 10 °C at midday, with a corresponding cooling power of 118.25 W/m2. The excellent mechanical properties of the Es-TPU membrane allows the continuous adjustment of reflectivity by reversibly stretching it, reaching a reflectivity of 61.1% (ΔR = 34.5%) under an elongation strain of 80%, leading to a net temperature increase of 9.5 °C above ambient of an absorbing substrate and an equivalent power of 220.34 W m−2 in this solar heating mode. The strong haze, hydrophobicity and outstanding aging resistance exhibited by this scalable membrane hold promise for achieving uniform illumination with tunable strength and efficient thermal management in practical applications.

Soft Wireless Headband Bioelectronics and Electrooculography for Persistent Human-Machine Interfaces.

Recent advances in wearable technologies have enabled ways for people to interact with external devices, known as human-machine interfaces (HMIs). Among them, electrooculography (EOG), measured by wearable devices, is used for eye movement-enabled HMI. Most prior studies have utilized conventional gel electrodes for EOG recording. However, the gel is problematic due to skin irritation, while separate bulky electronics cause motion artifacts. Here, we introduce a low-profile, headband-type, soft wearable electronic system with embedded stretchable electrodes, and a flexible wireless circuit to detect EOG signals for persistent HMIs. The headband with dry electrodes is printed with flexible thermoplastic polyurethane. Nanomembrane electrodes are prepared by thin-film deposition and laser cutting techniques. A set of signal processing data from dry electrodes demonstrate successful real-time classification of eye motions, including blink, up, down, left, and right. Our study shows that the convolutional neural network performs exceptionally well compared to other machine learning methods, showing 98.3% accuracy with six classes: the highest performance till date in EOG classification with only four electrodes. Collectively, the real-time demonstration of continuous wireless control of a two-wheeled radio-controlled car captures the potential of the bioelectronic system and the algorithm for targeting various HMI and virtual reality applications.

A unique, flexible, and porous pressure sensor with enhanced sensitivity and durability by synergy of surface microstructure and supercritical fluid foaming

Flexible thermoplastic polyurethane (TPU)/multi-walled carbon nanotube (MWCNT) nanocomposite piezoresistive sensor with microsphere surface and internal porous structure were prepared using a facile glass template method combining with supercritical carbon dioxide (Sc-CO2) foaming process. Effects of surface microstructure and cell structure on the piezoresistive sensing performance of TPU/MWCNT nanocomposite foam had been investigated in detail: the novel hybrid structure significantly improved the sensing performance with a fast response time (≤100 ms), a wide operating range (0 ∼ 80 kPa) and a stability of over 1000 cycles. In addition, the prepared piezoresistive sensor can be used to monitor a wide range of human physiological signals (facial activity, small arm muscle contraction and arterial pulse, etc.) due to its excellent integrated sensing performance. This work provides a route for the design of advanced piezoresistive sensors that show great potential for applications in human motion detection, artificial intelligence devices, and medical detection, etc.

A Novel Soft Glove Utilizing Honeycomb Pneumatic Actuators (HPAs) for Assisting Activities of Daily Living.

Fabric-based pneumatic actuators (FPAs) are extensively employed in the design of lightweight and compliant soft wearable assistive gloves. However, conventional FPAs typically exhibit limited output force, thereby restricting the applications of such gloves. This paper presents the development of a novel honeycomb pneumatic actuator (HPA) constructed using flexible thermoplastic polyurethane (TPU) coating through hot pressing or ultrasonic welding techniques. Compared to the previously utilized double-layer fabric-based pneumatic actuators (DLFPAs), the HPAs yields a remarkable 862% increase in end output force. It can produce a tip force of 13.57 N at a pressure of 150 kPa. The integration of HPAs onto a soft pneumatic glove enables the facilitation of various activities of daily living. A series of trials involving nine patients were conducted to assess the effectiveness of the soft glove. The experimental results indicate that when assisted by the glove, the patients' finger metacarpophalangeal (MCP) and proximal interphalangeal (PIP) joints achieved angles of 87.67 ± 19.27° and 64.2 ± 30.66°, respectively. Additionally, the average fingertip force reached 10.16 ± 4.24 N, the average grip force reached 26.04 ± 15.08 N, and the completion rate of daily functions for the patients increased from 39% to 76%. These outcomes demonstrate that the soft glove effectively aids in finger movements and significantly enhances the patients' daily functioning.

Polyurethane-based nanocomposite film with thermal deicing capability and anti-erosion for wind turbine blades protection in extreme environments

In some high-altitude areas, solid particle erosion and ice accretion on the wind turbine blades may reduce the power generation efficiency.

Flexible TPU inverse opal fabrics for colorimetric detecting of VOCs.

Recently, responsive structure color fibers and fabrics have been designed and prepared for colorimetric detecting of volatile organic compounds (VOCs). Fabric substrates can offer greater flexibility and portability than flat and hard substrates such as glass, silicon wafers, etc. At present, one-dimensional photonic crystal (multilayer films) and three-dimensional dense photonic crystal layers are mainly constructed on fabrics to achieve the response to VOCs. However, the binding force between these structural color coatings and the fabrics was poor, and the dense structures inevitably hindered the diffusion of VOCs. Here, thermoplastic polyurethane (TPU) inverse opal (IOs) fabrics were prepared by sacrificing the SiO2 photonic crystal templates to achieve colorimetric detecting of VOCs. The IOs layer of TPU was cured directly on the fabric surface, TPU infiltrated into the fabric yarns, and bonded the fabrics and IOs layer into a whole, which greatly improved the binding force, and the porous structure and large specific surface area of IOs were conducive to the diffusion of VOCs. The results showed that the TPU IOs fabrics have large reflection peak shifts to DMF, THF, toluene and chloroform vapors, and its concentration has a good linear relationship with the maximum reflection peak value of TPU IOs fabrics. The theoretical detection limits are 1.72, 0.89, 0.78 and 1.64 g m-3, respectively. The response times are 105, 62, 75 and 66 seconds, with good stability. Finally, it was calculated that the discoloration of the TPU IOs fabrics in VOCs was due to the joint-effects of lattice spacing and effective refractive index increase.

3D printed flexible wearable sensors based on triply periodic minimal surface structures for biomonitoring applications

Soft piezoresistive wearable conductors have led to a paradigm shift in the monitoring of human bodily motions. Cellular additively manufactured conductors are promising piezoresistive components as they offer mechanical tunability and provide controllable percolation pathways. In the present study, we engineer high surface-area cellular structures with the triply periodic minimal surface (TPMS)-based architectures to tailor their piezoresistive response for use in wearable devices. A simple and economical fabrication process is proposed, wherein a fused deposition modeling 3D printing technique is utilized to fabricate flexible thermoplastic polyurethane (TPU) cellular structures. Interconnectivity of TPMS designs enables the coating of a continuous graphene layer over the TPU internal surfaces via a facile dip-coating process. The effects of pore shape on piezoresistivity are studied in four different TPMS structures (i.e. Primitive, Diamond, Gyroid, and I-WP). Mechanical properties of sensors are evaluated through experimental procedures and computation methods using finite element analysis of the Mooney–Rivlin hyperelastic model. The piezoresistive performance of sensors exhibits durability under cyclic compression loading. Finally, we conclude that the Primitive structure offers suitable piezoresistive characteristics for sensing of walking, whereas the Diamond structure presents favorable results for respiration monitoring.