Self-healing flexible strain sensor fabricated through 3D printing template sacrifice for motion monitoring with enhanced healing and mechanical performance.

With the advancements in flexible materials and information technology, flexible sensors are becoming increasingly pervasive in various aspects of life and production. They hold immense potential for further development in areas such as motion detection, electronic skin, soft robots, and wearable devices. Aminopropyl-terminated polydimethylsiloxane (PDMS) was used as the raw material, while a diisocyanate reagent served as the cross-linking agent for the polymerization reaction, which involved the introduction of ureido groups, containing N-H and C=O bonds, into the long siloxane chain. The dynamic hydrogen bonding between the clusters completes the self-healing of the material. Using 1-[3-(trimethoxysilyl)propyl]urea as a grafting agent, the urea groups are introduced into graphene oxide and carbon nanotubes (CNTs) as conductive fillers. Subsequently, a flexible polymer is used as the substrate to prepare conductive flexible self-healing composites. By controlling the amount of conductive fillers, flexible strain materials with varying sensitivities are obtained. Design the structure of the flexible strain sensor using three-dimensional (3D) modeling software with deposition printing method.

Carbohydrate-Based Reprocessable and Healable Covalent Adaptable Biofoams.

Polymeric foams derived from bio-based resources and capable of self-healing and recycling ability are of great demand to fulfill various applications and address environmental concerns related to accumulation of plastic wastes. In this article, a set of polyester-based covalent adaptable biofoams (CABs) synthesized from carbohydrates and other bio-derived precursors under catalyst free conditions to offer a sustainable alternative to conventional toxic isocyanate-based polyurethane foams is reported. The dynamic ß-keto carboxylate linkages present in these biofoams impart self-healing ability and recyclability to these samples. These CABs display adequate tensile properties especially compressive strength (≤123 MPa) and hysteresis behavior. The CABs swiftly stress relax at 150 °C and are reprocessable under similar temperature conditions. These biofoams have displayed potential for use as attachment on solar photovoltaics to augment the output efficiency. These CABs with limited swellability in polar protic solvents and adequate mechanical resilience are suitable for other commodity applications.

Mechanically Adaptive Polymers Constructed from Dynamic Coordination Equilibria.

Designing materials capable of adapting their mechanical properties in response to external stimuli is the key to preventing failure and extending their service life. However, existing mechanically adaptive polymers are hindered by limitations such as inadequate load-bearing capacity, difficulty in achieving reversible changes, high cost, and a lack of multiple responsiveness. Herein, we address these challenges using dynamic coordination bonds. A new type of mechanically adaptive material with both rate- and temperature-responsiveness was developed. Owing to the stimuli-responsiveness of the coordination equilibria, the prepared polymers, PBMBD-Fe and PBMBD-Co, exhibit mechanically adaptive properties, including temperature-sensitive strength modulation and rate-dependent impact hardening. Benefitting from the dynamic nature of the coordination bonds, the polymers exhibited impressive energy dissipation, damping capacity (loss factors of 1.15 and 2.09 at 1.0 Hz), self-healing, and 3D printing abilities, offering durable and customizable impact resistance and protective performance. The development of impact-resistant materials with comprehensive properties has potential applications in the sustainable and intelligent protection fields.

An Interdisciplinary Tutorial: A Self-Healing Soft Finger with Embedded Sensor.

In the field of soft robotics, knowledge of material science is becoming more and more important. However, many researchers have a background in only one of both domains. To aid the understanding of the other domain, this tutorial describes the complete process from polymer synthesis over fabrication to testing of a soft finger. Enough background is provided during the tutorial such that researchers from both fields can understand and sharpen their knowledge. Self-healing polymers are used in this tutorial, showing that these polymers that were once a specialty, have become accessible for broader use. The use of self-healing polymers allows soft robots to recover from fatal damage, as shown in this tutorial, which increases their lifespan significantly.

An Overview of Self-Healable Polymers and Recent Advances in the Field.

The search for materials with better performance, longer service life, lower environmental impact, and lower overall cost is at the forefront of polymer science and material engineering. This has led to the development of self-healing polymers with a range of healing mechanisms including capsular-based, vascular, and intrinsic self-healing polymers. The development of self-healable systems has been inspired by the healing of biological systems such as skin wound healing and broken bone reconstruction. The goal of using self-healing polymers in various applications is to extend the service life of polymers without the need for replacement or human intervention especially in restricted access areas such as underwater/underground piping where inspection, intervention, and maintenance are very difficult. Through an industrial and scholarly lens, this paper provides: a) an overview of self-healing polymers; b) classification of different self-healing polymers and polymer-based composites; c) mechanical, thermal, and electrical analysis characterization; d) applications in coating, composites, and electronics; e) modeling and simulation; and f) recent development in the past 20 years. This review highlights the importance of healable polymers for an economically and environmentally sustainable future, the most recent advances in the field, and current limitations in fabrication, manufacturing, and performance.

Maleimide Self-Reaction in Furan/Maleimide-Based Reversibly Crosslinked Polyketones: Processing Limitation or Potential Advantage?

Polymers crosslinked via furan/maleimide thermo-reversible chemistry have been extensively explored as reprocessable and self-healing thermosets and elastomers. For such applications, it is important that the thermo-reversible features are reproducible after many reprocessing and healing cycles. Therefore, side reactions are undesirable. However, we have noticed irreversible changes in the mechanical properties of such materials when exposing them to temperatures around 150 °C. In this work, we study whether these changes are due to the self-reaction of maleimide moieties that may take place at this rather low temperature. In order to do so, we prepared a furan-grafted polyketone crosslinked with the commonly used aromatic bismaleimide (1,1'-(methylenedi-4,1-phenylene)bismaleimide), and exposed it to isothermal treatments at 150 °C. The changes in the chemistry and thermo-mechanical properties were mainly studied by infrared spectroscopy, 1H-NMR, and rheology. Our results indicate that maleimide self-reaction does take place in the studied polymer system. This finding comes along with limitations over the reprocessing and self-healing procedures for furan/maleimide-based reversibly crosslinked polymers that present their softening (decrosslinking) point at relatively high temperatures. On the other hand, the side reaction can also be used to tune the properties of such polymer products via in situ thermal treatments.

In-depth characterization of self-healing polymers based on π-π interactions.

The self-healing behavior of two supramolecular polymers based on π-π-interactions featuring different polymer backbones is presented. For this purpose, these polymers were synthesized utilizing a polycondensation of a perylene tetracarboxylic dianhydride with polyether-based diamines and the resulting materials were investigated using various analytical techniques. Thus, the molecular structure of the polymers could be correlated with the ability for self-healing. Moreover, the mechanical behavior was studied using rheology. The activation of the supramolecular interactions results in a breaking of these noncovalent bonds, which was investigated using IR spectroscopy, leading to a sufficient increase in mobility and, finally, a healing of the mechanical damage. This scratch-healing behavior was also quantified in detail using an indenter.

Using Synergistic Multiple Dynamic Bonds to Construct Polymers with Engineered Properties.

Dynamic bonds have achieved significant attention for their ability to impart fascinating properties to polymeric materials, such as high mechanical strength, self-healing, shape memory, 3D printability, and conductivity. Incorporating multiple dynamic bonds into polymer systems affords an attractive and efficient approach to endow multiple functionalities. This mini-review focuses on the use of complementary dynamic interactions to control the properties of soft materials. Owing to the diversity in dynamic chemistries that can be explored, the scope of this article is restricted to polymers and does not include colloids, amphiphiles, liquid crystals, or biological soft matter.

The Past 40 Years of Macromolecular Sciences: Reflections on Challenges in Synthetic Polymer and Material Science.

Technology and science are often successful in discontinuities ("disruptive innovations" or "leapfrogging"), in turn allowing true, big societal development by entire changes in technology rather than by minuscule stepwise improvements. Examples are the emergence of modern computer science by inventing the field-effect transistor rather than further fine-tuning the "Röhrentransistor"; the development of (organic) light-emitting diodes in advance of the "Gasglühstrumpf"; CRISPR/Cas exceeding any previous genetic method or Ziegler-Natta polymerization enabling stereoregular polypropylene (PP) and high-density polyethylene (HDPE) in advance of free-radical polymerization. Where may the frogs in polymer science in the future "jump" to? Contemplating past achievements in (synthetic) polymer science, such as living polymerization, "click" chemistry, supramolecular chemistry, the potentially "leaping" areas of self-healing and (bio)degradable materials, amyloids, and biomaterials are reflected upon.

Detailed Analysis of the Influencing Parameters on the Self-Healing Behavior of Dynamic Urea-Crosslinked Poly(methacrylate)s.

For this paper, the self-healing ability of poly(methacrylate)s crosslinked via reversible urea bonds was studied in detail. In this context, the effects of healing time and temperature on the healing process were investigated. Furthermore, the impact of the size of the damage (i.e., area of the scratch) was monitored. Aging processes, counteracting the self-healing process, result in a decrease in the mechanical performance. This effect diminishes the healing ability. Consequently, the current study is a first approach towards a detailed analysis of self-healing polymers regarding the influencing parameters of the healing process, considering also possible aging processes for thermo-reversible polymer networks.

Special Issue: "Smart and Functional Polymers".

Polymerization provides an efficient strategy for synthesizing macromolecules with versatile functionality [...].

Palladium-SCS Pincer Complexes as Cross-Linking Moieties in Self-Healing Metallopolymers.

Metallopolymers combine properties of metal complexes and polymers and are thus interesting materials for a wide field of different applications. One current major field is the utilization as self-healing polymers, and for this purpose, palladium-SCS pincer complexes are synthesized. Well-defined copolymers featuring those metal complexes in the side chain are obtained using the reversible addition-fragmentation chain-transfer polymerization technique. By the addition of a tetravalent pyridine cross-linkers, reversible cross-linked supramolecular networks are prepared, enabling self-healing properties. By utilizing density functional theory calculations, IR, and Raman spectroscopy, as well as isothermal titration calorimetry, the complex formation, reversibility, and stability are studied. The information of these experiments also enables further optimizations for the design of similar reversible systems in the future.

Environmentally Adaptable and Temperature-Selective Self-Healing Polymers.

Development of polymeric materials capable of self-healing at low temperatures is an important issue since their mechanical strength and self-healing performance are often in conflict with each other. Herein, random copolymers with self-healing capability in a wide temperature range prepared from 2-(dimethylamino)ethyl methacrylate (DMAEMA), glyceryl monomethacrylate (GlyMA), and butyl methacrylate monomers via free-radical polymerization and subsequent cross-linking with hexamethylene diisocyanate are reported. Wound closure is facilitated by swelling below the lower critical solution temperature or by heating above the glass transition temperature (T g ) of the polymer. GlyMA units form metal-ligand coordination complexes with dibutyltin dilaurate, leading to the formation of new carbonate bonds under ambient CO2 and H2 O conditions. Although swelling/heating reduces the polymer's mechanical strength, it is fully restored following chemical re-bonding/drying at room temperature. The swelling and degree of scratch healing are affected by pH, temperature, and the DMAEMA content.

Thermally Labile Self-Healable Branched Gel Networks Fabricated by New Macromolecular Engineering Approach Utilizing Thermoreversibility.

A new approach based on macromolecular engineering through thermoreversibility is reported to fabricate the engineered gel networks of thermally labile branched polymers exhibiting robust self-healing. This approach centers on the synthesis of linear polymers having Diels-Alder cycloadducts in the backbones (DALPs) through A2 + B2 step-growth polymerization of a difunctional furan and a difunctional maleimide. Reactive mixtures of the resulting DALP with a polyfuran at elevated temperature allow for the formation of engineered gel networks through random dissociation of backbone DA linkages of the DALPs by retro-Diels-Alder reaction, followed by their reconstruction in the presence of polyfuran (with functionality > 2) by Diels-Alder reaction. Optimizing the ratio of furan to DA linkages in the reactive mixtures yields thermally labile networks exhibiting excellent thermoreversibility. Effective self-healing demonstrated with reconstruction from two separate pieces and complete void filling on surface cuts as well as recovery of healing viscoelasticity suggest that the new macromolecular engineering approach offers versatility toward the development of thermally mendable thermosets.

Influence of Aspartate Moieties on the Self-Healing Behavior of Histidine-Rich Supramolecular Polymers.

Aspartate incorporated into the protein structure of mussel byssal threads is believed to play an important role, besides the reversible histidine-zinc interactions, in the self-healing behavior of mussel byssal threads. Therefore, copolymers containing both aspartate and histidine moieties are synthesized in order to investigate the influence of aspartate on the complexation of zinc(II) as well as on the self-healing behavior and the mechanical properties of the resulting supramolecular networks. For this purpose, isothermal titration calorimetry measurements of a model aspartate compound as well as of these copolymers are performed and the thermodynamic parameters are utilized for the design of self-healing copolymers. For this purpose, n-lauryl methacrylate-based copolymers containing histidine and aspartate are synthesized and crosslinked with zinc(II) acetate. The self-healing behavior of the supramolecular networks is investigated using scratch healing tests and the mechanical properties by nanoindentation.

A Self-Healing Aqueous Lithium-Ion Battery.

Flexible lithium-ion batteries are critical for the next-generation electronics. However, during the practical application, they may break under deformations such as twisting and cutting, causing their failure to work or even serious safety problems. A new family of all-solid-state and flexible aqueous lithium ion batteries that can self-heal after breaking has been created by designing aligned carbon nanotube sheets loaded with LiMn2 O4 and LiTi2 (PO4 )3 nanoparticles on a self-healing polymer substrate as electrodes, and a new kind of lithium sulfate/sodium carboxymethylcellulose serves as both gel electrolyte and separator. The specific capacity, rate capability, and cycling performance can be well maintained after repeated cutting and self-healing. These self-healing batteries are demonstrated to be promising for wearable devices.

Practical Applications of Self-Healing Polymers Beyond Mechanical and Electrical Recovery.

Self-healing polymeric materials, which can repair physical damage, offer promising prospects for protective applications across various industries. Although prolonged durability and resource conservation are key advantages, focusing solely on mechanical recovery may limit the market potential of these materials. The unique physical properties of self-healing polymers, such as interfacial reduction, seamless connection lines, temperature/pressure responses, and phase transitions, enable a multitude of innovative applications. In this perspective, the diverse applications of self-healing polymers beyond their traditional mechanical strength are emphasized and their potential in various sectors such as food packaging, damage-reporting, radiation shielding, acoustic conservation, biomedical monitoring, and tissue regeneration is explored. With regards to the commercialization challenges, including scalability, robustness, and performance degradation under extreme conditions, strategies to overcome these limitations and promote successful industrialization are discussed. Furthermore, the potential impacts of self-healing materials on future research directions, encompassing environmental sustainability, advanced computational techniques, integration with emerging technologies, and tailoring materials for specific applications are examined. This perspective aims to inspire interdisciplinary approaches and foster the adoption of self-healing materials in various real-life settings, ultimately contributing to the development of next-generation materials.

Light-Induced Living Polymer Networks with Adaptive Functional Properties.

The advent of covalent adaptable networks (CANs) through the incorporation of dynamic covalent bonds has led to unprecedented properties of macromolecular systems, which can be engineered at the molecular level. Among the various types of stimuli that can be used to trigger chemical changes within polymer networks, light stands out for its remote and spatiotemporal control under ambient conditions. However, most examples of photoactive CANs need to be transparent and they exhibit slow response, side reactions, and limited light penetration. In this vein, it is interesting to understand how molecular engineering of optically active dynamic linkages that offer fast response to visible light can impart "living" characteristics to CANs, especially in opaque systems. Here, the use of carbazole-based thiuram disulfides (CTDs) that offer dual reactivity as photoactivated reshuffling linkages and iniferters under visible light irradiation is reported. The fast response to visible light activation of the CTDs leads to temporal control of shape manipulation, healing, and chain extension in the polymer networks, despite the lack of optical transparency. This strategy charts a promising avenue for manipulating multifunctional photoactivated CANs in a controlled manner.

Impact of the Chemical Structure of Photoreactive Urethane (Meth)Acrylates with Various (Meth)Acrylate Groups and Built-In Diels-Alder Reaction Adducts on the UV-Curing Process and Self-Healing Properties.

A series of UV-curable urethane (meth)acrylates were obtained by copolymerization of the Diels-Alder adduct (HODA), isophorone diisocyanate, PEG1000, and various hydroxy (meth)acrylates. The aim of the present work was to determine the influence of the chemical structure of the introduced (meth)acrylic groups, i.e., hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, and hydroxypropyl methacrylate, on the UV-curing process and self-healing properties of cured coatings. The chemical structure of prepolymers was characterized by FTIR and NMR spectroscopy, whereas the UV-curing process was monitored in real time using FTIR and photo-DSC. In turn, the self-healing properties were characterized in relation to the thermally reversible mechanism, which was tested using the following methods: an FTIR spectroscope equipped with a heating attachment; DSC and TG apparatus; and an optical microscope equipped with a stage with programmable heating. The result of comprehensive research on the self-healing of photocurable coatings in the context of the presence of various photoreactive groups and the course of the curing process allows one to control the self-healing process by reducing the effective healing temperature. The self-healing properties, taken together with the fast UV curing of the coatings and excellent properties of cured coatings, make the material attractive for a variety of applications, in particular in cases where coatings are not repaired, e.g., for economic reasons or when it is not possible, such as in flexible electronic screens, car paint film, and aircraft interior finishes.

Development of a Resilience Parameter for 3D-Printable Shape Memory Polymer Blends.

The goal of this paper was to establish a metric, which we refer to as the resilience parameter, to evaluate the ability of a material to retain tensile strength after damage recovery for shape memory polymer (SMP) systems. In this work, three SMP blends created for the additive manufacturing process of fused filament fabrication (FFF) were characterized. The three polymer systems examined in this study were 50/50 by weight binary blends of the following constituents: (1) polylactic acid (PLA) and maleated styrene-ethylene-butylene-styrene (SEBS-g-MA); (2) acrylonitrile butadiene styrene (ABS) and SEBS-g-MA); and (3) PLA and thermoplastic polyurethane (TPU). The blends were melt compounded and specimens were fabricated by way of FFF and injection molding (IM). The effect of shape memory recovery from varying amounts of initial tensile deformation on the mechanical properties of each blend, in both additively manufactured and injection molded forms, was characterized in terms of the change in tensile strength vs. the amount of deformation the specimens recovered from. The findings of this research indicated a sensitivity to manufacturing method for the PLA/TPU blend, which showed an increase in strength with increasing deformation recovery for the injection molded samples, which indicates this blend had excellent resilience. The ABS/SEBS blend showed no change in strength with the amount of deformation recovery, indicating that this blend had good resilience. The PLA/SEBS showed a decrease in strength with an increasing amount of initial deformation, indicating that this blend had poor resilience. The premise behind the development of this parameter is to promote and aid the notion that increased use of shape memory and self-healing polymers could be a strategy for mitigating plastic waste in the environment.