Magnetic self-healing composites enable closing and healing of large damage

The evolution of material science and technology has underpinned the basis of evolution of humanity. Better comprehension of the complex behavior of materials gives rise to the possibility to modify their composition and physicochemical properties, tailoring their properties to suffice our needs. In a fast-changing world, the speed of evolution of materials directly impacts its assimilation by the populace. Presently, in the emerging field of smart materials, there has been much focus on stimuli-responsive materials that have the ability to reversibly modify one or more of their properties under the application of an external trigger in a controlled fashion. In such materials, the activation trigger can be based on changes in temperature, pH, application of an external mechanical force, irradiation with light at a certain wavelength or by the presence of an external electrical or magnetic field. In essence, the capability to heal damage extends their lifetime, reliability and safety, and decreases the need for costly maintenance. In this regard, thermo-reversible covalent bonds have received a great deal of attention over the past few decades, with a particular focus on reversible Diels-Alder (DA) crosslinks.

Self-healing material can either possess inherent ability to do so (be intrinsic) or can have the built-in self-healing capability (be extrinsic). Research has shown that reversible polymer networks based on Diels-Alder thermo-reversible covalent bonds exhibit great healing performance by controlling the temperature of the system. Despite the attractive applications of self-healing materials, it is rather unfortunate that most of them are restricted to the repair of narrow cracks due to their restricted mobility in the solid state. In this regard, it is desired to find new approaches for the closure of large damage in easily synthesizable and cost-affordable materials, so they are able to heal autonomously upholding their applicability. To this end, Dr. Guy Van Assche and Dr. Joost Brancart from the Vrije Universiteit Brussel, in collaboration with Dr. Kenneth Cerdan and Dr. Peter van Puyvelde from the Katholieke Universiteit Leuven, designed a novel approach to heal large damage by dispersing magnetite (Fe₃O₄) particles within an elastomeric thermo-reversible covalent network based on the Diels-Alder chemistry. Their work is published in the research journal Polymer.

Their goal was to assess the effect of different filler loadings of the magnetic particles on the thermomechanical, viscoelastic properties and on the self-healing behavior. To realize this, magnetic Fe₃O₄ nanoparticles were incorporated in an elastomeric polymer network with reversible Diels-Alder crosslinks to create self-healing magnetic elastomeric composites. Healing experiments were performed by placing a magnet next to a large damage and increasing the environmental temperature. Overall, thermal, mechanical and chemical characterization of different composites were performed and the healing efficiency evaluated to assess their potential to close and heal large damage sizes.

The authors reported that promising results were obtained for composites of certain compositions of magnetic nanoparticles, as the mechanical properties were similar to those of the bulk material in a considerable range of strains and stresses and exhibited a good magnetic response. An optimum was found between the increasing magnetic response and decreasing mobility required for self-healing, with increasing filler content. The researchers established that lower filler contents resulted in higher magnetically activated healing efficiencies around 100%. For higher filler loadings, the temperature needed to be raised too high to obtain a noticeable magnetic response, which resulted in viscous flow for the partial ferrous composite under the magnetic force.

In summary, the study presented a novel approach that was developed with the aim of improving the healing of large damage in self-healing materials by using a magnetic field as the driving force to close large damage sizes and to improve the contact between the broken surfaces. Based on the empirical work outlined, Fe₃O₄ can be stated as a promising, cheap and non-toxic alternative for the synthesis of magnetic composites. In a statement to Advances in Engineering, Dr. Joost Brancart explained their work opens up the use of such self-healing materials in other fields. In particular, he emphasized that by considering the ability to close and heal large damage, while preserving good mechanical properties and structural integrity, the demonstrated self-healing magnetic composites presents very interesting candidates for applications in soft robotics. Several paths are being investigated to improve both the magnetic response and the healing behaviour, while in parallel, applications in soft robotic actuation and manipulation, sensing capabilities and magnetic-field-assisted healing mechanisms are being explored.

Quote from the authors: As seen in nature, much of the intelligence is transferred (embodied) into the material and the geometrical structure of the robotic systems. The field of soft robotics is a rapidly developing field of research that continues to put forward ever more stringent requirements to materials used, expressing the need for the development of intelligent materials and systems that combine the properties demanded.