Bioinspired Materials for Self-Cleaning and Self-Healing - Sottos ...

Bioinspired Materials for Self-Cleaning and Self-Healingand represents the first step in the developmentof materials systems with greatlyextended lifetimes.In biological systems, chemical signalsreleased at the site of fracture initiate asystemic response that transports repairagents to the site of injury and pro -motes healing (Figure 2a). The biologicalprocesses that control tissue responseto injury and repair are extraordinarilycomplex, involving inflammation, woundclosure, and matrix remodeling. 49 Coagulationand inflammation begin immediatelywhen tissue is wounded. After about24 hours, cell proliferation and matrixdeposition begin to close the wound.During the final stage of healing (whichcan take several days), the extracellularmatrix is synthesized and remodeled asthe tissue regains strength and function.Ideally, synthetic reproduction of the healingprocess in a material requires an initialrapid response to mitigate further damage,efficient transport of reactive materialsto the damage site, and structuralregeneration to recover full performance.Although no materials system yetaccomplishes this complete set of autonomichealing processes, crack healinghas been explored in a wide spectrum ofmaterials. 50 The efficiency of crack healingfor structural materials is defined as theability to recover the mechanical integrityof the virgin (undamaged) material,including such properties as fracturetoughness (K IC ), fracture energy (G c ), elasticstiffness (E), and strength (σ ult ). Forsome materials systems, healing isachieved autonomically (independentlyand automatically) without any externalintervention, whereas others require additionalenergy (such as heat) to heal.In polymers and polymer composites,two distinct approaches for self-healinghave emerged. In the first, the crackmendingprocess is initiated by an externalthermal-, photo-, mechanical- or chemicalinducedstimulus. Successful crack healinghas been achieved through both moleculardiffusion and thermally reversible solidstatereactions. In the second approach,damage in the form of a crack triggers therelease of healing agents stored in thematerial, as also occurs for fracture eventsin biological systems (Figure 2b). Bothcompartmentalized and continuous deliverystrategies have been demonstrated.Site-specific crack healing is achievedautonomically in this approach, withoutany external intervention.Thermally Induced Crack MendingCrack healing has been achieved in arange of polymer systems through the useof heat and pressure to promote diffusionabepidermiscrackgranulationtissuefibrin clotof polymer chains and/or chemicalchanges (Figure 3). In many cross-linkedpolymers, healing occurs through chaininterdiffusion if there is physical contactbetween the crack planes and sufficientviscoelastic deformation and wetting. 51,52An increase of the temperature above theglass transition, T g , and application ofpressure significantly enhance the mendingprocess. 51 The manual addition of solventalso promotes polymer healing byeffectively lowering T g . 53 For certainionomers, self-healing is triggered by thermalenergy transferred to the polymerduring a projectile puncture event. 54Thermally induced reversible polymerizations,in which the material contains areversible or dynamic covalent bond, havealso been exploited for crack healing.Chen and co-workers used a retroDiels–Alder strategy to prepare thermallyrepairable cross-linked polymers (Figure3). 55,56 With the application of heatbloodvesselsdermis1 2 3Figure 2. (a) Schematic of an intermediate stage of biological wound healing in skin.Tissue damage triggers bleeding, which is followed by the formation of a fibrin clot.Fibroblast cells migrate to the wound site enabling the creation of granulation tissue tofill the wound. 49 (b) Demonstration of bioinspired damage-triggered release of amicroencapsulated healing agent in a polymer specimen: ➀ schematic ofcompartmentalized healing agent stored in a matrix; ➁ release of dyed healing agent intothe crack plane, which leads to a synthetic clotting (polymerization) process to bond thecrack faces; and ➂ one-half of the fracture surface revealing ruptured capsules.(120–150°C) and modest pressure, thesepolymers demonstrate high crack-healingefficiencies based on recovery of fracturetoughness. 55–57 More recently, a thermoreversible,self-healing rubber based onhydrogen bonding has been reported. 58Details on the underlying chemistry ofthese and several new polymer systemsare described in the article by Williamset al. in this issue.Trigger and Release for AutonomicHealingPolymers that self-heal with no externalintervention rely on the quiescent, stablestorage of liquid healing agents and anactivator or catalyst within the material.The healing components must remainactive and separated until damage occurs,without significantly impacting the inherentproperties of the material. Effectivestorage and release has been achievedwith healing-agent-filled microcapsulesMRS BULLETIN • VOLUME 33 • AUGUST 2008 • www.mrs.org/bulletin 735