Self-healing materials have the ability to automatically repair damage, such as cracks or scratches, without external intervention. This capability could potentially extend the lifetimes of a wide range of products from buildings and bridges to plastics and coatings. Researchers are working on developing several different types of self-healing materials using various mechanisms to achieve repair. Some of the most promising approaches and their underlying scientific principles are described below.
Encapsulated healing agents: One effective method involves microcapsules embedded within a material that contain a liquid healing agent. When the material cracks or tears, the capsules break and release their contents to fill in and bond the damaged area. Scientists at the University of Illinois developed a prototype epoxy which contained microcapsules loaded with dicyclopentadiene (DCPD) healing agent. When the epoxy cracked, the microcapsules ruptured and the DCPD which acts as a hydrophobic liquid monomer came in contact with Grubbs’ catalyst already dispersed in the epoxy matrix. This caused a ring-opening metathesis polymerization reaction that polymerized the DCPD into a new crosslinked polymer which filled and bonded the crack together, restoring over 90% of the epoxy’s original mechanical properties. Other healing agents being researched are resorable waxes for concrete or asphalt and polyurethane prepolymers for plastics.
Many scientists are exploring ways to make microcapsules more robust and controlled-release. Methods to improve capsules include using layers of polymer shells, optimizing capsule thickness and mechanical properties, and incorporating stimuli-response triggers like temperature, pH, or magnetic fields to control the timing and location of agent release. Some self-healing cement prototypes use hollow fibers instead of microcapsules, allowing greater volumes of healing chemicals to be stored. Future enhancements may enable micro- and nano-sized encapsulation systems with better self-healing efficiency.
Vascular Healing: An alternative “vascular” approach takes inspiration from biological circulatory systems. Researchers inject self-healing thermosets with a healing agent and immobilize catalysts or polymerization initiators along branching microvascular networks embedded within the material. Upon damage, the vascular networks transport healing agents to the crack where they react with the immobilized species to repair the defect from within. One iteration by White et al. injected an epoxy material with a liquid prepolymer and dispersed Grubbs’ catalyst in the walls of a microfabricated vascular network. When cracked, the network transported the prepolymer which then polymerized wherever the catalyst was present. Other efforts explore vascular networks filled with supercritical fluids or gels to heal cracks from the inside out.
Continued efforts aim to design more intricate three-dimensional vascular networks capable of self-repairing complex damage from all directions. Integrated microfluidic approaches coupling micro/nanofluidic components like pumps, valves and reservoirs could enable controlled delivery of multiple heterogeneous healing agents. Multi-step sequential or cascade healing may provide enhanced repair. Advanced 3D printing techniques may offer routes to incorporate hollow, branching vascular networks within materials with complex shapes and internal architectures. Ultimately, vascular self-healing holds promise for durably repairing integral materials like concrete, composites or tissues from within.
Shape memory polymers: A different class of Smart materials exploits the shape memory effect where a temporary shape is fixed within a material which can be recovered to its original form upon application of an external stimulus like heat. Shape memory polymers undergo phase transitions from a hardened, glassy state to a rubbery state when heated above their transition temperature. Researchers have developed epoxy resins containing physically cross-linked networks of shape memory polymers which can reform cracks upon heating. The polymers are crosslinked with reversible bonds which dissociate at elevated temperatures, allowing the material to flow and reform its original shape. Upon cooling back below the transition temperature, the chemical bonds re-form and “freeze” the material into its healed state. This thermally activated self-healing mechanism enables repeatable repair of structural cracks or defects.
Recent advancements optimize shape memory formulations for glass transition temperatures suitable for different environments. Hybrid materials containing both reversible and permanent crosslinks improve mechanical properties in the healed state. Multishape capabilities enable programmed complex repair sequences. Shape memory alloys which transform between solid crystalline phases upon heating or mechanical loading offer alternatives to polymers for self-healing applications that require heating to much higher temperatures. A combination of shape memory effects with microencapsulation could potentially realize self-healing composites capable of autonomous damage detection and healing at room temperature. Overall, shape memory materials provide an energy-efficient thermal activation route to repairing microcracks and restoring material integrity.
Bioinspired Self-Healing: Nature serves as a fountain of inspiration, exemplified in the self-healing abilities of biological systems. Some scientists are trying to recreate biological self-repair mechanisms in synthetic materials through bio-inspired design principles. Examples include synthetic adhesives modeled after mussel adhesive proteins which polymerize irreversibly when brought together at defect sites. Other work uses enzyme-catalyzed reactions inspired by plants’ wound-healing pathways. Researchers dispersed haloalkane dehalogenase enzyme in a epoxy matrix to hydrolytically cleave incorporated haloester groups, generating reactive species to reform the polymer network around cracks upon damage. Light-activated self-healing uses photolabile “cages” to protect reactive groups which then uncage under illumination to crosslink and heal.
Moving forward, closer mimicking of complex biological wound healing cascades could lead to more robust self-repair akin to living tissues. This may involve multi-step autonomous signaling, inflammation, clotting and remodeling analogous to how skin heals cuts or breaks. Careful selection and controlled activation of catalysts, crosslinkers or chemistries modelled after biological processes could realize truly living material systems capable of repeated self-repair without external intervention. Synthetic biology and genetic engineering may assist in designing artificial self-healing materials that utilize enzymatic, genetic or cellular components to regenerate and remodel damage similar to biological wound healing. Overall this bioinspired route explores how to instill true autonomous living functionality into advanced smart repair materials.
With continued development self-healing materials could help address challenges in damage tolerance for innumerable applications from infrastructure to consumer goods. Concerted progress in encapsulation technologies, microfabrication, stimuli-responsive polymers and biologically inspired design is advancing the potential for fully autonomous and repeated self-repair. Careful control over healing agent delivery, activation and reaction kinetics will be key to optimizing repair efficiency as well as mechanical and physical properties in healed states. Ultimately self-healing functionality may enable extension of product lifetimes with a reduced need for repair or maintenance. Wider implementation could support more sustainable utilization of resources by material systems.
Andrew Stroud