Search

Self-healing polymers render their life cycle more sustainable by recovering their properties upon healing. Intrinsic self-healing polymers can be recycled, which further reduces waste production. Yet, despite these intrinsic benefits, several sustainability issues remain largely neglected, including the use of fossil-derived materials, hazardous chemicals, and the material management at the end of its life. Herein we report a series of castor oil-based self-healing elastomers that account for these challenges and show improved mechanical and self-healing capabilities compared to the other bio-based self-healing materials. Castor oil was functionalized using a simple, one-pot, solventless synthesis from renewable resources and crosslinked by Diels-Alder cycloaddition. They can be reprocessed and recycled, or hydrolytically degraded at the end of their service life. The mechanical properties of the materials can be tuned (Young’s modulus 0.5-20 MPa), with a fracture strain of up to 487%. A fracture strain of 100% could already be recovered after just 60 seconds at room temperature, and 75% of the mechanical properties after just 24 h. By taking advantage of these properties, a soft pneumatic gripper has been developed, capable of healing autonomously which is fully recyclable and degradable. Hence, we provide a sustainable alternative for soft-robotics applications and for self-healing elastomers in general.

Dielectric elastomer actuators rely on Coulomb forces for their actuation. They are promising candidates when it comes to the manufacture of electrically driven soft and lightweight robotic devices, which can undergo large movement. However, their commercial availability and hence their technological relevance are still rather limited due to several reasons: in-depth material knowhow in terms of electrical and mechanical behavior required; materials have to be available in thin sheets; complicated and error prone fabrication; limited design freedom. In order to change this, this paper presents a soft dielectric actuator fully produced by fused filament fabrication. Additive manufacturing with conventional fused filament fabrication machines offers the potential for the production of personalized dielectric actuators which are easily accessible and comparably cheap. The only requirement is a fused filament fabrication printer with multi-material capability. Focusing on a mass customization of the 3D printed actuators, exclusively commercially available devices and filaments without any modifications are used. In particular, the 3D printed prototypes are characterized in terms of the maximum displacement depending on the electrode printing direction. As a showcase example, a soft dielectric gripper made of two 3D printed actuators is developed. Comparable with other dielectric elastomer actuators, a maximum displacement of 42% is reached.

The thermophysical properties, mechanical behavior, and healing performance of Diels–Alder-based polymer networks can be tuned widely by changing the molar mass, functionality, and stoichiometric ratio of the multifunctional diene (e.g., furan) and dienophile (e.g., maleimide) monomers. Blending furan-functionalized monomers with a high and low molar mass opens a new dimension in tuning the properties of the resulting reversible polymer network blends (RPNBs), containing different ratios of elastomeric and thermoset phases, with low and high crosslink densities and glass transitions below and above room temperature, respectively. Increasing the thermoset content up to 20 wt % raises the gel transition temperature (Tgel), relaxation time (τ), rubbery plateau modulus, ultimate stress, and Young’s modulus of the stiffened elastomers, while the strain at fracture and self-healing efficiency remained mostly unchanged. Conversely, the addition of up to 20 wt % of the elastomer phase drastically toughens the thermosets because of stress dissipation, which can suppress the stress concentration in the phase-separated network blends. The blends showed higher stress and strain at break while retaining similar Young’s moduli. The toughness increases up to an order of magnitude at 50 wt %, compared to the pure thermoset. The phase-separated morphologies were evidenced by the presence of the Tg’s of the pure networks in DSC and DMA.

Magnetorheological (MR) elastomers are a class of stimuli-responsive materials of which the damping and stiffness can be reversibly tailored by applying magnetic fields. However, concerns such as fatigue damage, insufficient MR efficiencies with low loadings of magnetic particles or highly crosslinked elastomers, and lack of reprocessability remain unaddressed for conventional MR elastomers. To this end, a series of self-healing MR elastomers (SHMRE) were prepared based on thermoreversible Diels–Alder covalent crosslinks. The application of magnetic pulses yielded pre-aligned magnetic particles chains within the curing matrix which strongly influenced the SHMRE rheological properties. The resulting composites do not only exhibit a large MR effect but also efficient self-healing properties at room temperature. We found that the particle loading and the field-induced orientation of the aggregates affect the magnitude of the MR response, the mechanical strength and the healing efficiency. In addition, the MR response is also strongly influenced by the temperature. With a temperature increase from room temperature to 70 °C, a change in the MR response from 90% to 462% is observed while the SHMRE retain a solid viscoelastic state at 50 wt% particles loading. Interestingly, the thermoreversible features of the synthesized networks also allow potential reprocessability of SHMRE when heating these systems above the gel transition temperature (89 °C–90 °C). The final low viscous state makes it possible for the magnetic particles to be potentially restructured as chains by applying a magnetic field, which are retained upon cooling when the solid network state is recovered. The proposed SHMRE systems are shown to be a highly efficient and reprocessable solution to substitute classical MR elastomers in a wider context of generalized MR materials.

Acrylate-endcapped urethane-based precursors constituting a poly(D,L-lactide)/poly(ε-caprolactone) (PDLLA/PCL) random copolymer backbone are synthesized with linear and star-shaped architectures and various molar masses. It is shown that the glass transition and thus the actuation temperature could be tuned by varying the monomer content (0–8 wt% ε-caprolactone, T g,crosslinked = 10—42 °C) in the polymers. The resulting polymers are analyzed for their physico-chemical properties and viscoelastic behavior (G′ max = 9.6–750 kPa). The obtained polymers are subsequently crosslinked and their shape-memory properties are found to be excellent (R r = 88–100%, R f = 78–99.5%). Moreover, their potential toward processing via various additive manufacturing techniques (digital light processing, two-photon polymerization and direct powder extrusion) is evidenced with retention of their shape-memory effect. Additionally, all polymers are found to be biocompatible in direct contact in vitro cell assays using primary human foreskin fibroblasts (HFFs) through MTS assay (up to ≈100% metabolic activity relative to TCP) and live/dead staining (>70% viability).

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.

Chemical recycling of polymers is taking off as a circular technology, typically targeting pure recyclates. However, this is often not achieved efficiently due to high energy demand of separation and purification steps. In addition, many polymer applications have complex formulations that may be sensitive to impure feedstocks. Substitution of virgin feedstocks by complex recyclates (often containing impurities) requires a good knowledge of the structure/composition–property relations of polymer formulations. As this is often not the case, current practice relies on costly and rather inefficient enumeration experiments, or, at best, classical design-of-experiments approaches. We review the state-of-the art in structure–property modeling, present an example for polyurethane formulations, and propose a hybrid model-based framework. This involves a machine learning workflow for substitution problems in complex polymer formulations, combining existing data, novel reaction kinetics, structure–property models, molecular dynamics, and a minimum of experimental–analytical data where necessary, to build and validate the model.

The development of bio-based polymers displaying promising end-of-life opportunities would have an immense impact on the polymer industry but has remained largely unexplored. Therefore, in this work, self-healing bio-aromatic networks with lignin contents up to 29 wt % were designed based on the thermoreversible Diels−Alder reaction. The networks were developed by reacting a furan-functionalized solvent-extracted lignin and a furan-bearing polyether (F5000) with a bismaleimide (DPBM). The obtained networks were nanophase-separated between a flexible F5000-rich phase and a rigid lignin-rich phase, which was evidenced by (modulated temperature) differential scanning calorimetry and small angle X-ray scattering. Partially due to the phase-separation, improved mechanical properties were obtained, including a higher Young’s modulus and improved toughness. In addition, the thermal stability of the networks significantly increased due to the addition of lignin. Besides the thermomechanical improvement, the networks showed promising stress-relaxation, creep resistance, healing, and reprocessability. A macroscopic healing efficiency of 62% of the toughness was obtained. Moreover, the networks were completely resoluble, which allows for promising recycling possibilities.

Recent advances in soft continuum robots revealed the need for accurate models, required to develop advanced control strategies. In this paper a general methodology is presented that allows to create an accurate inverse kinematic control based on hyperelastic models, fitted on mechanical material properties. This methodology is based on finite element analysis (FEA) and links the mechanical properties of the material to the simulated behaviour of a Pneunet actuator. This procedure is valid for any sort of hyperelastic material used in soft robotics, however, as a case study, a specific material with a self-healing ability is considered. This elastomeric polymer can recover from macroscopic damages without losing its mechanical performances, after undergoing a heat treatment. In this article, an accurate characterisation of the mechanical behaviour of this material is provided, involving mechanical testing, both in tensile and compression. On this experimental characterisation data constitutive laws are fitted, using a FEA simulator. The robustness of this material modelling is shown, re-fitting the curve for material samples that were exposed to multiple damage-healing cycles. With this obtained constitutive model, a FEA simulation of a bending soft pneumatic actuator is developed. The simulation results are experimentally validated with a dedicated test bench consisting of a pressure control unit and a motion tracking camera. Using this validated model, an inverse kinematic control is developed, including the FEA simulation in the control scheme.

Soft robotics aims at creating systems with improved performance of movement and adaptability in unknown, challenging, environments and with higher level of safety during interactions with humans. This Roadmap on Soft Robotics covers selected aspects for the design of soft robots significantly linked to the area of multifunctional materials, as these are considered a fundamental component in the design of soft robots for an improvement of their peculiar abilities, such as morphing, adaptivity and growth. The roadmap includes different approaches for components and systems design, bioinspired materials, methodologies for building soft robots, strategies for the implementation and control of their functionalities and behavior, and examples of soft-bodied systems showing abilities across different environments. For each covered topic, the author(s) describe the current status and research directions, current and future challenges, and perspective advances in science and technology to meet the challenges., Multifunctional Materials, 5 (3), ISSN:2399-7532

Humins waste valorization is considered to be an essential pathway to improve the eco- nomic viability of many biorefinery processes and further promote their circularity by avoiding waste formation. In this research, the incorporation of humins in a Diels–Alder (DA) polymer network based on furan-maleimide thermoreversible crosslinks was studied. A considerable enhancement of the healing efficiency was observed by just healing for 1 h at 60 ◦C at the expense of a reduction of the material mechanical properties, while the unfilled material showed no healing under the same conditions. Nevertheless, the thermal healing step favored the irreversible humins polycondensation, thus strengthening the material while keeping the enhanced healing performance. Our hypothesis states a synergistic healing mechanism based on humins flowing throughout the damage, followed by thermal humins crosslinking during the healing trigger, together with DA thermoreversible bonds recombination. A multi-material soft robotic gripper was manufactured out of the proposed material, showing not only improved recovery of the functional performance upon healing but also stiffness- tunable features by means of humins thermal crosslinking. For the first time, both damage healing and zone reinforcement for further damage prevention are achieved in a single intrinsic self-healing system. ispartof: Polymers vol:14 issue:9 pages:1-19 ispartof: location:Switzerland status: Published online

Sensory receptors in biological systems are an essential part of natural organisms to perceive, understand and adapt to their environment and evaluate their internal state. They are interlinked with the senses of living organisms and are an important tool for the survival and evolution of species. Researchers have implemented the biomimetic receptor approach into the development of artificial sensors, in an attempt to mimic the sensory transduction of organisms, like self-healing ability and flexibility. However, aspects of biological transduction like selectivity and multi-sensing are still underdeveloped in the field of soft self-healing sensors (SSHS). The multi-sensing aspect is discussed in this review paper, focusing on resistive-based SSHS for detecting different stimuli, like strain, pressure, tactile, temperature, chemical species and structural damage. The inspiration from sensory transduction of biological systems will be a key factor for the further development of SSHS and their application in soft robotics, electronic skin, smart wearables and haptic devices.

International audience; The intrinsic compliance of soft robots provides safety, a natural adaptation to its environment, allows to absorb shocks, and protects them against mechanical impacts. However, a literature study shows that the soft polymers used for their construction are susceptible to various types of damage, including fatigue, overloads, interfacial debonding and cuts, tears and perforations by sharp objects. An economic and ecological solution is to construct future soft robotic systems out of self-healing polymers, incorporating the ability to heal damage. This review paper proposes criteria to evaluate the potential of a self-healing polymer to be used in soft robotic applications. Based on these soft robotics requirements and on defined performance parameters of the materials, linked to the mechanical and healing properties, the different types of selfhealing polymers already available in literature are critically assessed and compared. In addition to a description of the state of the art on self-healing soft robotics, the paper discusses the driving forces and limitations to spur the interdisciplinary combination between self-healing polymer science and soft robotics.

Selective Laser Sintering (SLS) is a well-known additive manufacturing technique that builds 3D complex structures by sintering powder particles together upon laser irradiation. In the polymer field, thermoset resins are a popular choice for a plethora of applications due to their great mechanical properties and thermochemical stability. However, due to their irreversible crosslinked nature, they are hard to be processed using SLS. Herein, a covalently crosslinked polymer thermoset based on thermally reversible Diels-Alder (DA) bonds has been printed using SLS. The thermoreversible network exhibits a glassy behavior due to its high crosslinked density and glass transition temperature and was successfully milled into a suitable particle size, morphology and flow for SLS applications. At low temperatures, the DA equilibrium shifts towards the formation of the DA crosslinks, while at high temperatures the DA adducts gradually dissociate, allowing flow and reprocessability at temperatures above 131˚C. Furthermore, the printed parts exhibit healing ability when heated above their glass transition temperature, reaching a fracture stress and Young’s Modulus of 25.4 MPa and 1416 MPa, respectively. Thus, the dynamic nature of the network extends the build lifetime upon damage and also brings the opportunity of recycling or reprocessing, enhancing circularity compared to classic thermosets.

A novel functional 2,6-substituted donor-acceptor anthracene derivative, bearing a long alkyl spacer and a polymerizable end-group, is synthesized from readily available compounds. This monomer possesses conjugated electron donor and acceptor moieties to achieve UV absorption and anthracene dimerization at higher wavelengths and under milder conditions, than anthracene and other reported anthracene derivatives. The compound was shown to absorb at higher wavelengths and dimerize much faster compared to most 9-substituted anthracenes. The fast photochemical and relatively slow thermal scission of the dimers were studied and related to the chemical structure, i.e. the 2,6-substitution.

The lifetime of soft robots is limited by their susceptibility to damage. Recent breakthroughs in healable soft robots have overcome this issue by manufacturing systems from self-healing polymers. However, to operate and recover autonomously, compatible sensors need to be embedded. We show that innovative healable soft sensors to measure strain, force, and damage, based on a self-healing conductive elastomeric composite, were developed and integrated with near-perfect interfacial strength in a healable soft gripper. The paper details the complete development of the gripper, including the selection of the conductive composite based on carbon black and nanoclay, sensor and actuator characterization, and its manufacturing. The gripper with embedded sensors recovers its actuation, sensor function, and damage detection after being severely damaged, and this multiple times.

The chemical structure and location of substituents on anthracene derivatives influence the electron balance of the aromatic system, thus determining the wavelengths at which light is absorbed, which results in the photochemically induced dimerization or monomerization. Here, the thermal dissociation kinetics of 7 photodimers of 9-substituted anthracene derivatives are studied using a combination of spectroscopic and calorimetric techniques in the condensed state and compared to scarce literature data on thermal dissociation of other anthracene derivatives. The length and chemical structure of the substituent chains have a clear impact on the melting temperatures of the anthracene derivatives and corresponding photodimers. The crystallinity of the photodimers and monomers in turn influences the thermal dissociation kinetics. The thermal dissociation behaviour and previously published photochemistry data are related to the electronic effects of the substituents by means of the Hammett parameters. Stronger electron-withdrawing effects result in larger red shifts of the maximum wavelength λmax for the photodimerization of the anthracene derivatives. It is also shown that for the studied substitutions on the 9-position of anthracene, the higher the magnitude of the electronic effect - both electron-donating and electron-withdrawing - the faster the thermal dissociation kinetics and thus the lower the thermal stability. The strong electronic effects of the substituents on the thermal and photochemical reactivity of the anthracene derivatives and their photodimers allow tuning of the thermal or photochemical responsiveness, e.g. for polymer networks.

Non-covalent interactions involving aromatic rings play a central role in many areas of modern chemistry. In medicinal and bioorganic chemistry, the intermolecular interactions between the aromatic side chains of amino acids, such as phenylalanine and tyrosine, are of great interest. To enhance the affinity between such aromatic side chains, halogenation is a promising modification strategy. In the current work, the nature and strength of halogenated p-p stacked phenylalanine (Phe) dimers have been investigated using density functional theory, energy decomposition analyses and the non-covalent interaction (NCI) method. Our analysis shows that increasing the degree of halogenation enhances the strength of the stacking interactions and, moreover, the heavier halides (Cl, Br and I) lead to stronger interactions compared to the lighter F. This effect was traced back to local secondary interactions of the halide with the aliphatic C-H bonds of the phenylalanine side chain. Based on the computational findings, a set of peptide hydrogelators was synthesized, and the resulting hydrogel properties were further investigated via dynamic rheometry. Experimental observations can be correlated to the trends found in the theoretical analysis, suggesting that local interactions indeed play a noticeable role in enhancing peptide-based hydrogel strength. This journal is

The Diels-Alder reaction between furan and maleimide is the most studied example of reversible covalent chemistries for creating self-healing materials. While scientific articles reporting the synthesis of new reversible polymer networks are numerous, accurate knowledge of the reaction kinetics and thermodynamics of the dynamically reversible equilibrium reaction and the structure and property development of derived stimuli-responsive materials are less widespread. The requirements for the material properties and behavior become more stringent when designing materials for dedicated applications, such as soft robotic structures. Optima need to be sought between reasonably fast reaction kinetics for fast and efficient damage healing at moderate temperatures and mechanical strength and structural stability on the other hand. Stress relaxation is desired to make materials tougher, relieving stress before defects can grow into cracks and ultimately lead to failure, while creep can’t be allowed. Recycling and reprocessing of materials are desirable from an ecological viewpoint, while the materials should also be able to withstand static and dynamic loading in a considerable range of environmental conditions. Accurate knowledge of the reaction kinetics and thermodynamics and an in-depth knowledge of structure-processing-property relations allow smart polymer network design with tailored stimuli-responsive behavior and use as self-healing materials for robotic applications.

Self-healing materials have been intensively studied as materials that can mimic healing properties of biological systems. Reversible polymer networks based on Diels-Alder thermoreversible covalent bonds exhibit great healing performance by controlling the temperature of the system. Despite the attractive applications of self-healing materials, most of them are restricted to the repair of narrow cracks due to their restricted mobility in the solid state. In this work, magnetite (Fe3O4) particles are used to create self-healing magnetic composites. The use of a conventional magnet to apply a magnetic driving force is proposed for the closure of wide damage gaps in the solid state without the need of either mechanical intervention or liquid-like flow inside the material, limiting the structural stability. Thermal, mechanical and chemical characterization of different composites are performed in this study and the healing efficiency is evaluated to assess their potential to close and heal large damage sizes.

Thermally and photochemically reversible functional groups, such as photodimers of anthracene derivatives, offer interesting stimuli-responsive behaviour. To evaluate their potential for application in reversible polymer networks, accurate kinetic parameters and knowledge of their thermophysical behaviour are required. Accurate kinetic studies of the thermal dissociation of the photodimers in the condensed state, thus without the influence of solvents on their reactivity, is still lacking. A methodology was set up to accurately evaluate the chemical reaction kinetics and complex phase behaviour during the thermal dissociation of photodimers into their corresponding monomers. Temperature-controlled time-resolved FTIR spectroscopy was used to determine the reaction progress, while non-isothermal DSC measurements were used to study the thermophysical changes, resulting from the thermal dissociation reaction. The thermal dissociation behaviour in the condensed state is more challenging than in the solution state due to the crystallinity of the dimers, stabilizing the dimers and thus slowing down the initial dissociation rates. Distinctly different sets of kinetic parameters were found for the dissociation from the molten and the crystalline state. For experiments performed below the melting temperature of the photodimer, the reaction rate changes abruptly as the dimer is partly dissociated and partly dissolved into the formed monomer. This methodology provides an accurate assessment of the reaction kinetics with detailed knowledge about the complex phase behaviour of the mixture of the anthracene photodimer and monomer during thermal dissociation.

In new-generation soft robots, the actuation performance can be increased by using multiple materials in the actuator designs. However, the lifetime of these actuators is often limited due to failure that occurs at the weak multi-material interfaces that rely almost entirely on physical interactions and where stress concentration appears during actuation. This paper proposes to develop soft pneumatic actuators out of multiple Diels&ndash, Alder polymers that can generate strong covalent bonds at the multi-material interface by means of a heat&ndash, cool cycle. Through tensile testing it is proven that high interfacial strength can be obtained between two merged Diels&ndash, Alder polymers. This merging principle is exploited in the manufacturing of multi-material bending soft pneumatic actuators in which interfaces are no longer the weakest links. The applicability of the actuators is illustrated by their operation in a soft hand and a soft gripper demonstrator. In addition, the use of Diels&ndash, Alder polymers incorporates healability in bending actuators. It is experimentally illustrated that full recovery of severe damage can be obtained by subjecting the multi-material actuators to a healing cycle.

The healing behavior of a dynamically reversible covalent polymer network with a Tg of 3 °C, based on the reversible Diels-Alder (DA) cycloaddition between furan and maleimide moieties is reported, for both room-temperature and heating-mediated healing. In previous work the dynamic character of the DA cycloaddition reaction in a polymer network was studied, those findings are used to understand its healing ability. The recovery of mechanical properties by intrinsic healing is studied by Dynamic Mechanical Analysis, quantifying the loss in properties due to incurred damage and assessing their recovery with respect to the original material. The DA cycloadduct bonds in the network can be mechanically activated to autonomously heal damage at room temperature if the damage surfaces remain activated. If the damage surfaces are not brought in contact fast enough they reestablish equilibrium (aging) and lose their autonomous healing potential. In the latter case, only heating-mediated healing can occur, reactivating the healing ability. Controlled damage formation and subsequent sealing of the damage of network coatings is monitored by Atomic Force Microscopy and optical microscopy. The repeatability for both mechanical activation, studied by tensile testing, and thermal cycling, studied by Differential Scanning Calorimetry, is reported.

Anthracene dimer diols having very different thermal stabilities have been incorporated in polyurethane networks. The thermal de-crosslinking of these networks was shown to be dependent on the anthracene dimer used, allowing tunable thermal degradation. This scission was studied using (HR-MAS) NMR, DSC and rheometry. The tunable thermal decrosslinking was validated as a technique to easily remove the coatings at their end-of-life. The UV irradiation of the thermally degraded materials allows for recuring the coatings to materials having similar properties. By simultaneous irradiation and heating of (partially) reversibly cross-linked networks, the networks were able to relief internal stress and adopt a new permanent shape.

In this work, we introduce single-component self-healing polyurethanes based on reversible maleimide/ furan Diels–Alder reactions with good physical integrity on healing. Diols bearing thermo-reversible furfuryl and maleimide moieties were synthesized by a four-step coupling strategy starting from commercially available 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) and thermo-responsive monoalcohol (with furfuryl or maleimide functions). These diols were reacted in presence of 1,6- hexamethylene diisocyanate and polypropylene glycol (PPG), in order to obtain amorphous polyurethanes with good chain mobility for the self-healing processes (Tg < RT). The cross-linking density, related to the healing process, could be readily tuned with the amount of thermo-responsive moieties as well as the PPG chain length. Swelling test and FTIR spectroscopy confirmed the formation of reversible networks. Qualitative micro-scratch test on thin films indicated tunable healing efficiencies based on the polyurethane composition as a function of the content, the chemical nature of the thermo-responsive moieties (reversible vs. irreversible), the PPG nature (hydroxyl or amino terminated) and chain length. This new structural concept is of great interest for thermally mendable, self-healing and adhesive polyurethanes.