A Short Review on Self‐Healing Thermoplastic Polyurethanes

Regulation of mechanical properties of microcapsules and their applications

Microcapsules encapsulating payloads are one of the most promising delivery methods. The mechanical properties of microcapsules often determine their application scenarios. For example, microcapsules with low mechanical strength are more widely used in biomedical applications due to their superior biocompatibility, softness, and deformability. In contrast, microcapsules with high mechanical strength are often mixed into the matrix to enhance the material. Therefore, characterizing and regulating the mechanical properties of microcapsules is essential for their design optimization. This paper first outlines four methods for the mechanical characterization of microcapsules: nanoindentation technology, parallel plate compression technology, microcapillary technology, and deformation in flow. Subsequently, the mechanisms of regulating the mechanical properties of microcapsules and the progress of applying microcapsules with different degrees of softness and hardness in food, textile, and pharmaceutical formulations are discussed. These regulation mechanisms primarily include altering size and morphology, introducing sacrificial bonds, and construction of hybrid shells. Finally, we envision the future applications and research directions for microcapsules with tunable mechanical properties.

Mechanical Property of Thermoplastic Polyurethane Vascular Stents Fabricated by Fused Filament Fabrication

Vascular stents have many applications in treating arterial stenosis and other vascular-related diseases. The ideal vascular stent for clinical application should have radial support and axial bending mechanical properties that meet the requirements of vascular deformation coordination. The materials used for vascular stents implanted in the human body should have corresponding biocompatibility to ensure that the stents do not cause coagulation, hemolysis, and other reactions in the blood. This study fabricated four types of vascular stents, including inner hexagon, arrowhead, quadrilateral, and outer hexagonal, using fused filament fabrication technology and thermoplastic polyurethane (TPU) as materials. By evaluating the effects of edge width and wall thickness on the radial support and axial bending performance, it was found that the inner hexagonal stent exhibited the best radial support and axial bending performance under the same conditions. The design and fabrication of vascular stents based on 3D printing technology have promising application prospects in personalized customized vascular repair therapy.

Alkaline-Responsive, Self-Healable, and Conductive Copolymer Composites with Enhanced Mechanical Properties Tailored for Wearable Tech

Despite significant advancements, current self-healing materials often suffer from a compromise between mechanical robustness and functional performance, particularly in terms of conductivity and responsiveness to environmental stimuli. Addressing this issue, the research introduces a self-healable and conductive copolymer, poly(ionic liquid-co-acrylic acid) (PIL-co-PAA), synthesized through free radical polymerization, and further optimized by incorporating thermoplastic polyurethane (TPU). This combination leverages the unique properties of each component, especially ion-dipole interactions and hydrogen bonds, resulting in a material that exhibits exceptional self-healing abilities and demonstrates enhanced mechanical properties and electrical conductivity. Moreover, the PIL-co-PAA/TPU films showcase alkaline-responsive behavior, a feature that broadens their applicability in dynamic environments. Through systematic characterization, including thermogravimetric analysis, tensile testing, and electrical properties measurements, the mechanisms behind the improved performance and functionality of these films are elucidated. The conductivities and ultimate tensile strength (σuts) of the PIL-co-PAA/TPU films regain 80% under 8 h healing process. To extend the applications for wearable devices, the self-healing properties of commercial cotton fabrics coated with the self-healable PIL-co-PAA are also investigated, demonstrating both self-healing and electrical properties. This study advances the understanding of self-healable conductive polymers and opens new avenues for their application in wearable technology.

Self-healing, stretchable and recyclable polyurethane-PEDOT:PSS conductive blends

Future electronics call for materials with mechanical toughness, flexibility, and stretchability. Moreover, self-healing and recyclability are highly desirable to mitigate the escalating environmental threat of electronic waste (e-waste). Herein, we report a stretchable, self-healing, and recyclable material based on a mixture of the conductive polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS) with a custom-designed polyurethane (PU) and polyethylene glycol (PEG). This material showed excellent elongation at brake (∼350%), high toughness (∼24.6 MJ m-3), moderate electrical conductivity (∼10 S cm-1), and outstanding mechanical and electrical healing efficiencies. In addition, it demonstrated exceptional recyclability with no significant loss in the mechanical and electrical properties after being recycled 20 times. Based on these properties, as a proof of principle for sustainable electronic devices, we demonstrated that electrocardiogram (ECG) electrodes and pressure sensors based on this material could be recycled without significant performance loss. The development of multifunctional electronic materials that are self-healing and fully recyclable is a promising step toward sustainable electronics, offering a potential solution to the e-waste challenge.

Investigating the Impact of Hardness on Dielectric Breakdown Characteristics of Polyurethane

Polymeric materials play a vital role in high-voltage insulation, but their insulating properties can deteriorate over time, leading to insulation failures. The presence of voids resulting from manufacturing defects or external stresses can create a highly divergent field, further contributing to this issue. However, certain polymers, such as polyurethane (PU), possess self-healing properties that enable them to repair these voids and restore a uniform electric field distribution, thereby ensuring the reliability of the insulation. Surprisingly, the potential of PU as an insulating material in high-voltage applications remains unexplored. However, the self-healing capability of PU decreases with an increase in the hardness of the material. Therefore, in this study, the dielectric breakdown properties of PU with different levels of hardness, rated on the Shore scale as 40° (soft), 70° (medium), and 90° (hard), were investigated. The AC and DC dielectric breakdown characteristics of these PU variants and dielectric spectra were examined. Additionally, the study explores the relationship between the dielectric properties and the hardness of the material. Our findings revealed that the dielectric breakdown strength of PU increases as the material's hardness is increased under both AC and DC electric stress. However, this may come at the cost of reduced self-healing capabilities of PU. Therefore, there is a need to balance the hardness of the material with its ability to recover from breakdown events. The findings from this study can be useful for researchers and engineers, as they offer valuable insights into the dielectric properties of PU at various hardness levels.

Shock Hugoniot Calculations of Newly Designed Thermoplastic Elastomers and Comparison with Classical Binder Estane

Our purpose is to design excellent binder candidates used in polymer-bonded explosives (PBX) according to the calculated shock of Hugoniot. Here, we mainly examined the thermoplastic elastomer (TPE) binders commonly used in PBX formulations. Equilibrium molecular dynamics (MD) simulation and mixing rule methods were used to calculate the shock Hugoniot values of 180 newly designed TPEs. We focused on the influence of the polymerization degree, contents, and types of soft and hard segments composed of TPEs on the shock Hugoniot and compared them with the classic PBX binder, Estane. The results show that the hard segment has an effect on the Hugoniot curve, which gradually diminishes as the degree of polymerization increases. The underlying physical mechanism can be attributed to the presence of a large number of hydrogen bonds in hard segment domains. The shock Hugoniot of TPEs also depends on the type of soft segments. The volume compression rate of TPEs decreases with increasing content of hard segments under a given shock. By comparing with Estane, a TPE binder commonly used in PBX, we ultimately chose several new TPEs with the potential to serve as PBX binders in terms of shock performance.

Slippery Porous-Liquid-Infused Porous Surface (SPIPS) with On-Demand Responsive Switching between "Defensive" and "Offensive" Antifouling Modes

Slippery liquid-infused porous surfaces (SLIPS) have received widespread attention in the antifouling field. However, the reduction in antifouling performance caused by lubricant loss limits their application in marine antifouling. Herein, inspired by the skin of a poison dart frog which contains venom glands and mucus, a porous liquid (PL) based on ZIF-8 is prepared as a lubricant and injected into a silicone polyurethane (SPU) matrix to construct a new type of SLIPS for marine antifouling applications: the slippery porous-liquid-infused porous surface (SPIPS). The SPIPS consists of a responsive antifoulant-releasing switch between "defensive" and "offensive" antifouling modes to intelligently enhance the antifouling effect after lubricant loss. The SPIPS can adjust antifouling performance to meet the antifouling requirements under different light conditions. The wastage of antifoulants is reduced, thereby effectively maintaining the durability and service life of SLIPS materials. The SPIPS exhibits efficient lubricant self-replenishment, self-cleaning, anti-protein, anti-bacterial, anti-algal, and self-healing (97.48%) properties. Furthermore, it shows satisfactory 360-day antifouling performance in actual marine fields during boom seasons, demonstrating the longest antifouling lifespan in the field tests of reported SLIPS coatings. Hence, the SPIPS can effectively promote the development of SLIPS for neritic antifouling.

Recent Study Advances in Flexible Sensors Based on Polyimides

With the demand for healthy life and the great advancement of flexible electronics, flexible sensors are playing an irreplaceably important role in healthcare monitoring, wearable devices, clinic treatment, and so on. In particular, the design and application of polyimide (PI)-based sensors are emerging swiftly. However, the tremendous potential of PI in sensors is not deeply understood. This review focuses on recent studies in advanced applications of PI in flexible sensors, including PI nanofibers prepared by electrospinning as flexible substrates, PI aerogels as friction layers in triboelectric nanogenerator (TENG), PI films as sensitive layers based on fiber Bragg grating (FBG) in relative humidity (RH) sensors, photosensitive PI (PSPI) as sacrificial layers, and more. The simple laser-induced graphene (LIG) technique is also introduced in the application of PI graphitization to graphene. Finally, the prospect of PIs in the field of electronics is proposed in the review.

Structural and dynamical properties of thermoplastic polyurethane/fullerene nanocomposites: a molecular dynamics simulations study

In this work, the structural and dynamical properties of thermoplastic polyurethane (TPU)/fullerene (C60) nanocomposites are investigated using atomistic molecular dynamics simulations, focusing on the glass transition, thermal expansion, polymer mobility, polymer-C60 interactions, and diffusion behavior of C60. The results show a slight increase in the glass transition temperature (Tg) with increasing C60 weight fraction (wt%), attributed to hindered polymer dynamics, and a remarkable reduction in the coefficient of thermal expansion above Tg. Results of the mean squared displacement and the time decay of bond-reorientation autocorrelation indicate that the mobility of TPU hard segments is more restricted than that of soft segments, owing to the electrostatic attractions and the π-π stacking between isocyanate groups and C60 molecules. Analysis of TPU-C60 interaction energy reveals that the electrostatic interactions are weakened with an increase in the C60 wt%, while the van der Waals contributions become more significant due to the TPU-C60 interfacial characteristics. Further analysis shows that the translational and rotational diffusion of C60 are both increasingly suppressed with the increase of C60 wt%, indicating a violation of Stokes-Einstein (SE) and Stokes-Einstein-Debye (SED) relations, presumably due to the polymer chain-mediated hydrodynamic interactions arising from chain bridges between neighboring C60 particles. This is highlighted by a stronger decoupling of translational-rotational diffusion and a lower ratio of translational-rotational diffusion coefficient (DT/DR) with increasing C60 wt%. This work elucidates an atomistic understanding of the structure and properties of polymer/C60 nanocomposites.

Polyurethane-Based Nanocomposites for Regenerative Therapies of Cancer Skin Surgery with Low Inflammatory Potential to Healthy Fibroblasts and Keratinocytes In Vitro

Nanocomposites based on thermoplastic polyurethanes (TPUs) filled with halloysite nanotubes (HNTs) were studied for their physicochemical and biological properties. Nanocomposites containing halloysite nanotube filler contents of 1 and 2% (E+1 and E+2), respectively, were obtained by extrusion. The newly formed E+1 and E+2 nanomaterials exhibited better flexibility and similar thermal properties compared to neat polyurethane. The use of atomic force microscopy (AFM) and differential scanning calorimetry (DSC) thermogram analysis showed that the distribution of halloysite nanotubes in the polymer matrix is more evenly dispersed in the E+1 nanomaterial, where the grains in the E+2 nanomaterial have a greater tendency to form agglomerates. Mechanical tests have shown that nanocomposites with the addition of HNT are characterized by a higher stress at break and elongation at break compared to neat TPU. The results of cytotoxicity tests suggest that the nanocomposite materials express lower toxicity to normal HaCaT and NHDF than to cancer Me45 cells. Further studies showed that the tested materials induced the expression of proinflammatory interleukins IL6 and IL8 in normal cells, but their overexpression in the cancer cell line resulted in cytostatic effects and proliferation reduction. Such a conclusion suggests the possible application of tested materials for regenerative therapies in cancer surgeries.

Morphological Electrical and Hardness Characterization of Carbon Nanotube-Reinforced Thermoplastic Polyurethane (TPU) Nanocomposite Plates

The impacts on the morphological, electrical and hardness properties of thermoplastic polyurethane (TPU) plates using multi-walled carbon nanotubes (MWCNTs) as reinforcing fillers have been investigated, using MWCNT loadings between 1 and 7 wt%. Plates of the TPU/MWCNT nanocomposites were fabricated by compression molding from extruded pellets. An X-ray diffraction analysis showed that the incorporation of MWCNTs into the TPU polymer matrix increases the ordered range of the soft and hard segments. SEM images revealed that the fabrication route used here helped to obtain TPU/MWCNT nanocomposites with a uniform dispersion of the nanotubes inside the TPU matrix and promoted the creation of a conductive network that favors the electronic conduction of the composite. The potential of the impedance spectroscopy technique has been used to determine that the TPU/MWCNT plates exhibited two conduction mechanisms, percolation and tunneling conduction of electrons, and their conductivity values increase as the MWCNT loading increases. Finally, although the fabrication route induced a hardness reduction with respect to the pure TPU, the addition of MWCNT increased the Shore A hardness behavior of the TPU plates.

Recent advances in self-healing polyurethane based on dynamic covalent bonds combined with other self-healing methods

To meet more application requirements, improving mechanical properties and self-healing efficiency has become the focus of current research on self-healing PU. The competitive relationship between self-healing ability and mechanical properties cannot be avoided by a single self-healing method. To address this problem, a growing number of studies have combined dynamic covalent bonding with other self-healing methods to construct the PU structure. This review summarizes recent studies on PU materials that combine typical dynamic covalent bonds with other self-healing methods. It mainly includes four parts: hydrogen bonding, metal coordination bonding, nanofillers combined with dynamic covalent bonding and multiple dynamic covalent bond bonding. The advantages and disadvantages of different self-healing methods and their significant role in improving self-healing ability and mechanical properties in PU networks are analyzed. At the same time, the possible challenges and research directions of self-healing PU materials in the future are discussed.

Mechanoluminescent Device: In Situ Renewable Carbazole Derivatives Sandwiched by Self-Healing Disulfide-Containing Polyurethane for Mechanical Signals Detection

Mechanoluminescent (ML) materials can emit visible light by utilizing mechanical energy, which shows unique advantages in visual mechanical sensing, displays, and biomechanical monitoring due to the correlation between force stimulation and luminescence intensity. Most organic ML materials exhibit luminescence intensity attenuation, disappearing completely with force stimulation and failing to recover. Here, organic luminogens (Cz-alkyl₆) can be synthesized by introducing a soft alkyl chain into the carbazole, which exhibits ML emission with self-assembly units. Furthermore, organic luminogens can be generated repeatedly by simply recrystallizing the fracture crystal in situ after a short thermal treatment (70 °C) within 14 s. More importantly, the quantitative correlation between force pressure and ML intensity has been established by a sandwich-type ML device based on a novel carbazole derivative (Cz-alkyl₆). The ML device presents a capacity for detecting mechanical signals up to 13 N according to its ML intensity (≤275 a.u.), exhibiting potential application value in engineering damage detection, anticounterfeiting, and advanced visual mechanical sensing.

Passive Daytime Radiative Cooling by Thermoplastic Polyurethane Wrapping Films with Controlled Hierarchical Porous Structures

Current research has focused on effective solutions to mitigate global warming and the accelerating greenhouse gas emissions. Compared to most cooling methods requiring energy and resources, passive daytime radiative cooling (PDRC) technology offers excellent energy savings as it requires no energy consumption. However, existing PDRC materials encounter unprecedented problems such as complex structures, low flexibility, and performance degradation after stretching. Thus, this study reports a porous structured thermoplastic polyurethane (TPU) film with bimodal pores to produce high-efficiency PDRC with efficient solar scattering using a simple process. The TPU film exhibited an adequately high solar reflectivity of 0.93 and an emissivity of 0.90 in the atmospheric window to achieve an ambient cooling of 5.6 °C at midday under a solar intensity of 800 W m^-2 . Thus, the highly elastic and flexible TPU film was extremely suitable for application on objects with complex shapes. The radiative cooling performance of 3D-printed models covered with these TPU films demonstrated their superior indoor cooling efficiency compared to commercial white paint (8.76 °C). Thus, the proposed design of high-efficiency PDRC materials is applicable in various urban infrastructural objects such as buildings and vehicles.

Aerogel-Functionalized Thermoplastic Polyurethane as Waterproof, Breathable Freestanding Films and Coatings for Passive Daytime Radiative Cooling

Passive daytime radiative cooling (PDRC) is an emerging sustainable technology that can spontaneously radiate heat to outer space through an atmospheric transparency window to achieve self-cooling. PDRC has attracted considerable attention and shows great potential for personal thermal management (PTM). However, PDRC polymers are limited to polyethylene, polyvinylidene fluoride, and their derivatives. In this study, a series of polymer films based on thermoplastic polyurethane (TPU) and their composite films with silica aerogels (aerogel-functionalized TPU (AFTPU)) are prepared using a simple and scalable non-solvent-phase-separation strategy. The TPU and AFTPU films are freestanding, mechanically strong, show high solar reflection up to 94%, and emit strongly in the atmospheric transparency window, thereby achieving subambient cooling of 10.0 and 7.7 °C on a hot summer day for the TPU and AFTPU film (10 wt%), respectively. The AFTPU films can be used as waterproof and moisture permeable coatings for traditional textiles, such as cotton, polyester, and nylon, and the highest temperature drop of 17.6 °C is achieved with respect to pristine nylon fabric, in which both the cooling performance and waterproof properties are highly desirable for the PTM applications. This study opens up a promising route for designing common polymers for highly efficient PDRC.

A Study on Preparation and Property Evaluations of Composites Consisting of TPU/Triclosan Membranes and Tencel®/LMPET Nonwoven Fabrics

This study investigated eco-friendly antibacterial medical protective clothing via the nonwoven process and characteristic evaluations. Firstly, Tencel^® fibers and low melting point polyester (LMPET) fibers (re-sliced and granulated from recycled PET bottles) were mixed at different ratios and then needle punched at diverse needle rolling depths. The influences of manufacturing parameters on the Tencel^®/LMPET nonwoven fabrics were examined in terms of mechanical properties, water vapor transmission rate, and stiffness. Next, Tencel^®/LMPET nonwoven fabrics were combined with thermoplastic polyurethane (TPU)/Triclosan antibacterial membranes that contained different contents of triclosan using melt processing technology. The resulting Tencel^®/LMPET/TPU/Triclosan composites were characterized via different measurements; an optimal bursting strength of 86.86 N, an optimal horizontal tensile strength of 41.90 N, and an optimal stiffness along the MD and CD of 8.60 cm were recorded. Furthermore, the Tencel^®/LMPET/TPU/Triclosan composites exhibited a distinct inhibition zone in the antibacterial measurement, and the hydrostatic pressure met the requirements of the EN 14126:2003 and GB 19082-200 disposable medical protective gear test standards.

Self-Healing Materials for Electronics Applications

Self-healing materials have been attracting the attention of the scientists over the past few decades because of their effectiveness in detecting damage and their autonomic healing response. Self-healing materials are an evolving and intriguing field of study that could lead to a substantial increase in the lifespan of materials, improve the reliability of materials, increase product safety, and lower product replacement costs. Within the past few years, various autonomic and non-autonomic self-healing systems have been developed using various approaches for a variety of applications. The inclusion of appropriate functionalities into these materials by various chemistries has enhanced their repair mechanisms activated by crack formation. This review article summarizes various self-healing techniques that are currently being explored and the associated chemistries that are involved in the preparation of self-healing composite materials. This paper further surveys the electronic applications of self-healing materials in the fields of energy harvesting devices, energy storage devices, and sensors. We expect this article to provide the reader with a far deeper understanding of self-healing materials and their healing mechanisms in various electronics applications.

An unparalleled H-bonding and ion-bonding crosslinked waterborne polyurethane with super toughness and unprecedented fracture energy

Self-healing polyurethane elastomers have been extensively studied; however, developing an eco-friendly self-healable waterborne polyurethane (WPU) with exceptional mechanical properties remains a great challenge. Herein, we report healable, and highly tough WPU elastomers with unprecedented crack tolerance by introducing the concerted interactions of strong multiple H-bonds and ionic bonds in the network. The WPU elastomer demonstrated that the microphase separation structure contributes to an ultrahigh tensile strength (≈58 MPa), super toughness (≈456 MJ m^-3), and unprecedented record fracture energy (≈320 kJ m^-2). Due to the dynamic reconstruction of reversible H-bonds and ionic bonds, the WPU elastomer demonstrates a robust self-healability at 50 °C, allowing complete recovery of mechanical properties. Importantly, the thermoplasticity and reprocessability of WPUs enable direct 3D printing of different objects and electrospinning of tubes, showing great potential for expanding their application scope in soft robots and artificial stents.

Dynamic Covalent Polyurethane Network Materials: Synthesis and Self-Healability

Polyurethane (PU) has not only been widely used in the daily lives, but also extensively explored as an important class of the essential polymers for various applications. In recent years, significant efforts have been made on the development of self-healable PU materials that possess high performance, extended lifetime, great reliability, and recyclability. A promising approach is the incorporation of covalent dynamic bonds into the design of PU covalently crosslinked polymers and thermoplastic elastomers that can dissociate and reform indefinitely in response to external stimuli or autonomously. This review summarizes various strategies to synthesize self-healable, reprocessable, and recyclable PU materials integrated with dynamic (reversible) Diels-Alder cycloadduct, disulfide, diselenide, imine, boronic ester, and hindered urea bond. Furthermore, various approaches utilizing the combination of dynamic covalent chemistries with nanofiller surface chemistries are described for the fabrication of dynamic heterogeneous PU composites.