Self-healing material
Self-healing materials are artificial or synthetically created
Although the most common types of self-healing materials are
A material that can intrinsically correct damage caused by normal usage could prevent costs incurred by material failure and lower costs of a number of different industrial processes through longer part lifetime, and reduction of inefficiency caused by degradation over time.[2]
History
The
Materials science
Related processes in concrete have been studied microscopically since the 19th century.
Self healing materials only emerged as a widely recognized field of study in the 21st century. The first international conference on self-healing materials was held in 2007.
Biomimetics
Plants and animals have the capacity to seal and heal wounds. In all plants and animals examined, firstly a self-sealing phase and secondly a self-healing phase can be identified. In plants, the rapid self-sealing prevents the plants from desiccation and from infection by pathogenic germs. This gives time for the subsequent self-healing of the injury which in addition to wound closure also results in the (partly) restoration of mechanical properties of the plant organ. Based on a variety of self-sealing and self-healing processes in plants, different functional principles were transferred into bio-inspired self-repairing materials.[9][10][11] The connecting link between the biological model and the technical application is an abstraction describing the underlying functional principle of the biological model which can be for example an analytical model[12] or a numerical model. In cases where mainly physical-chemical processes are involved a transfer is especially promising.
There is evidence in the academic literature
Self-healing polymers and elastomers
In the last century, polymers became a base material in everyday life for products like plastics, rubbers, films, fibres or paints. This huge demand has forced to extend their reliability and maximum lifetime, and a new design class of polymeric materials that are able to restore their functionality after damage or fatigue was envisaged. These polymer materials can be divided into two different groups based on the approach to the self-healing mechanism: intrinsic or extrinsic.[25][26] Autonomous self-healing polymers follow a three-step process very similar to that of a biological response. In the event of damage, the first response is triggering or actuation, which happens almost immediately after damage is sustained. The second response is transport of materials to the affected area, which also happens very quickly. The third response is the chemical repair process. This process differs depending on the type of healing mechanism that is in place (e.g., polymerization, entanglement, reversible cross-linking). These materials can be classified according to three mechanisms (capsule-based, vascular-based, and intrinsic), which can be correlated chronologically through four generations.[27] While similar in some ways, these mechanisms differ in the ways that response is hidden or prevented until actual damage is sustained.
Polymer breakdown
From a molecular perspective, traditional polymers yield to mechanical stress through cleavage of
Homolytic bond cleavage
Polymers have been observed to undergo homolytic bond cleavage through the use of radical reporters such as DPPH (2,2-diphenyl-1-picrylhydrazyl) and PMNB (pentamethylnitrosobenzene.) When a bond is cleaved homolytically, two radical species are formed that can recombine to repair damage or can initiate other homolytic cleavages which can in turn lead to more damage.[28]
Heterolytic bond cleavage
Polymers have also been observed to undergo heterolytic bond cleavage through isotope labeling experiments. When a bond is cleaved heterolytically,
Reversible bond cleavage
Certain polymers yield to mechanical stress in an atypical, reversible manner.
Supramolecular breakdown
Intrinsic polymer-based systems
In intrinsic systems, the material is inherently able to restore its integrity. While extrinsic approaches are generally autonomous, intrinsic systems often require an external trigger for the healing to take place (such as thermo-mechanical, electrical, photo-stimuli, etc.). It is possible to distinguish among 5 main intrinsic self-healing strategies. The first one is based on reversible reactions, and the most widely used reaction scheme is based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions.[33] Another strategy achieves the self-healing in thermoset matrices by incorporating meltable thermoplastic additives. A temperature trigger allows the redispertion of thermoplastic additives into cracks, giving rise to mechanical interlocking.[34] Polymer interlockings based on dynamic supramolecular bonds or ionomers represent a third and fourth scheme. The involved supramolecular interactions and ionomeric clusters are generally reversible and act as reversible cross-links, thus can equip polymers with self-healing ability.[35][36] Finally, an alternative method for achieving intrinsic self-healing is based on molecular diffusion.[37]
Reversible bond-based polymers
Reversible systems are polymeric systems that can revert to the initial state whether it is
Polymer systems based on covalent bond formation and breakage
Diels-Alder and retro-Diels-Alder
Among the examples of reversible healing polymers, the
Cross-linked polymers
In this type of
The reversible DA/RDA reaction is not limited to furan-meleimides based polymers as it is shown by the work of Schiraldi et al. They have shown the reversible cross-linking of polymers bearing pendent anthracene group with maleimides. However, the reversible reaction occurred only partially upon heating to 250 °C due to the competing decomposition reaction.[39]
Polymerization of multifunctional monomers
In these systems, the DA reaction takes place in the backbone itself to construct the polymer, not as a link. For polymerization and healing processes of a DA-step-growth furan-maleimide based polymer (3M4F) were demonstrated by subjecting it to heating/cooling cycles. Tris-maleimide (3M) and tetra-furan (4F) formed a polymer through DA reaction and, when heated to 120 °C, de-polymerized through RDA reaction, resulting in the starting materials. Subsequent heating to 90–120 °C and cooling to room temperature healed the polymer, partially restoring its mechanical properties through intervention.[33][40] The reaction is shown in Scheme 4.
Thiol-based polymers
The thiol-based polymers have
Poly(urea-urethane)
A soft poly(urea-urethane) network uses the metathesis reaction in aromatic disulphides to provide room-temperature self-healing properties, without the need for external catalysts. This chemical reaction is naturally able to create covalent bonds at room temperature, allowing the polymer to autonomously heal without an external source of energy. Left to rest at room temperature, the material mended itself with 80 percent efficiency after only two hours and 97 percent after 24 hours.[citation needed] In 2014 a polyurea elastomer-based material was shown to be self-healing, melding together after being cut in half, without the addition of catalysts or other chemicals. The material also include inexpensive commercially available compounds. The elastomer molecules were tweaked, making the bonds between them longer. The resulting molecules are easier to pull apart from one another and better able to rebond at room temperature with almost the same strength. The rebonding can be repeated. Stretchy, self-healing paints and other coatings recently took a step closer to common use, thanks to research being conducted at the University of Illinois. Scientists there have used "off-the-shelf" components to create a polymer that melds back together after being cut in half, without the addition of catalysts or other chemicals.[42][43]
The urea-urethane polymers however have glassy transition temperatures below 273 K therefore at room temperature they are gels and their tensile strength is low.[44] To optimize the tensile strength the reversible bonding energy, or the polymer length must be increased to increase the degree of covalent or mechanical interlocking respectively. However, increase polymer length inhibits mobility and thereby impairs the ability for polymers to re-reversibly bond. Thus at each polymer length an optimal reversible bonding energy exists.[45]
Vitrimers
Vitrimers are a subset of polymers that bridge the gap between thermoplastics and thermosets.[46][47] Their dependence on dissociative and associative exchange within dynamic covalent adaptable networks allows for a variety of chemical systems to be accessed that allow for the synthesis of mechanically robust materials with the ability to be reprocessed many times while maintaining their structural properties and mechanical strength.[48] The self-healing aspect of these materials is due to the bond exchange of crosslinked species as a response to applied external stimuli, such as heat. Dissociative exchange is the process by which crosslinks are broken prior to recombination of crosslinking species, thereby recovering the crosslink density after exchange.[49] Examples of dissociative exchange include reversible pericyclic reactions, nucleophilic transalkylation, and aminal transamination. Associative exchange involves the substitution reaction with an existing crosslink and the retention of crosslinks throughout exchange.[49] Examples of associative exchange include transesterification, transamination of vinylogous urethanes,[50] imine exchange,[51] and transamination of diketoneamines.[49] Vitrimers possessing nanoscale morphology are being studied, through the use of block copolymer vitrimers in comparison to statistical copolymer analogues, to understand the effects of self-assembly on exchange rates, viscoelastic properties, and reprocessability.[52] Other than recycling, vitrimer materials show promise for applications in medicine, for example self-healable bioepoxy,[53] and applications in self-healing electronic screens.[54] While these polymeric systems are still in their infancy they serve to produce commercially relevant, recyclable materials in the coming future as long as more work is done to tailor these chemical systems to commercially relevant monomers and polymers, as well as develop better mechanical testing and understanding of material properties throughout the lifetime of these materials (i.e. post reprocess cycles).
Copolymers with van der Waals force
If perturbation of van der Waals forces upon mechanical damage is energetically unfavourable, interdigitated alternating or random copolymer motifs will self-heal to an energetically more favourable state without external intervention. This self-healing behavior occurs within a relatively narrow compositional range depended on a viscoelastic response that energetically favours self-recovery upon chain separation, owing to ‘key-and-lock’ associations of the neighbouring chains. In essence, van der Waals forces stabilize neighbouring copolymers, which is reflected in enhanced cohesive-energy density (CED) values. Urban etc. illustrates how induced dipole interactions for alternating or random poly(methyl methacrylate-alt-ran-n-butyl acrylate) (p(MMA-alt-ran-nBA)) copolymers owing to directional van der Waals forces may enhance the CED at equilibrium (CEDeq) of entangled and side-by-side copolymer chains.
Extrinsic polymer-based systems
In extrinsic systems, the healing chemistries are separated from the surrounding polymer in microcapsules or vascular networks which, after material damage/cracking, release their content into the crack plane, reacting and allowing the restoration of material functionalities.[58] These systems can be further subdivided in several categories. While capsule-based polymers sequester the healing agents in little capsules that only release the agents if they are ruptured, vascular self-healing materials sequester the healing agent in capillary type hollow channels that can be interconnected one dimensionally, two dimensionally, or three dimensionally. After one of these capillaries is damaged, the network can be refilled by an outside source or another channel that was not damaged. Intrinsic self-healing materials do not have a sequestered healing agent but instead have a latent self-healing functionality that is triggered by damage or by an outside stimulus.[58] Extrinsic self-healing materials can achieve healing efficiencies over 100% even when the damage is large.[59]
Microcapsule healing
Capsule-based systems have in common that healing agents are encapsulated into suitable microstructures that rupture upon crack formation and lead to a follow-up process in order to restore the materials' properties. If the walls of the capsule are created too thick, they may not fracture when the crack approaches, but if they are too thin, they may rupture prematurely.[60] In order for this process to happen at room
This process has been demonstrated with
In contrast, in multicapsule systems both the catalyst and the healing agent are encapsulated in different capsules.[65] In a third system, called latent functionality, a healing agent is encapsulated, that can react with the polymerizer component that is present in the matrix in the form of residual reactive functionalities.[66] In the last approach (phase separation), either the healing agent or the polymerizer is phase-separated in the matrix material.[67]
Vascular approaches
The same strategies can be applied in 1D, 2D and 3D vascular based systems.[68][69][15]
Hollow tube approach
For the first method, fragile glass capillaries or fibers are imbedded within a
Discrete channels
Discrete channels can be built independently of building the material and are placed in an array throughout the material.
Interconnected networks
Interconnected networks are more efficient than discrete channels, but are harder and more expensive to create.[71] The most basic way to create these channels is to apply basic machining principles to create micro scale channel grooves. These techniques yield channels from 600 to 700 micrometers.[71] This technique works great on the two-dimensional plane, but when trying to create a three-dimensional network, they are limited.[71]
Direct ink writing
The Direct Ink Writing (DIW) technique is a controlled extrusion of viscoelastic inks to create three-dimensional
Carbon nanotube networks
Through dissolving a linear polymer inside a solid three-dimensional epoxy matrix, so that they are miscible to each other, the linear polymer becomes mobile at a certain temperature[73] When carbon nanotubes are also incorporated into epoxy material, and a direct current is run through the tubes, a significant shift in sensing curve indicates permanent damage to the polymer, thus ‘sensing’ a crack.[74] When the carbon nanotubes sense a crack within the structure, they can be used as thermal transports to heat up the matrix so the linear polymers can diffuse to fill the cracks in the epoxy matrix. Thus healing the material.[73]
SLIPS
A different approach was suggested by Prof. J. Aizenberg from Harvard University, who suggested to use Slippery Liquid-Infused Porous Surfaces (SLIPS), a porous material inspired by the carnivorous pitcher plant and filled with a lubricating liquid immiscible with both water and oil.[75] SLIPS possess self-healing and self-lubricating properties as well as icephobicity and were successfully used for many purposes.
Sacrificial thread stitching
Organic threads (such as polylactide filament for example) are stitched through laminate layers of fiber reinforced polymer, which are then boiled and vacuumed out of the material after curing of the polymer, leaving behind empty channels than can be filled with healing agents.[76]
Self-healing fibre-reinforced polymer composites
Methods for the implementation of self-healing functionality into filled composites and fibre reinforced polymers (FRPs) are almost exclusively based on extrinsic systems and thus can be broadly classified into two approaches; discrete capsule-based systems and continuous vascular systems. In contrast to non-filled polymers, the success of an intrinsic approach based on bond reversibility has yet to be proven in FRPs. To date, self-healing of FRPs has mostly been applied to simple structures such as flat plates and panels. There is however a somewhat limited application of self-healing in flat panels, as access to the panel surface is relatively simple and repair methods are very well established in industry. Instead, there has been a strong focus on implementing self-healing in more complex and industrially relevant structures such as T-Joints[77][78] and Aircraft Fuselages.[79]
Capsule-based systems
The creation of a capsule-based system was first reported by White et al. in 2001,[60] and this approach has since been adapted by a number of authors for introduction into fibre reinforced materials.[80][81][82] This method relies on the release of an encapsulated healing agent into the damage zone, and is generally a once off process as the functionality of the encapsulated healing agent cannot be restored. Even so, implemented systems are able to restore material integrity to almost 100% and remain stable over the material lifetime.
Vascular systems
A vascular or fibre-based approach may be more appropriate for self-healing impact damage in fibre-reinforced polymer composite materials. In this method, a network of hollow channels known as vascules, similar to the blood vessels within human tissue, are placed within the structure and used for the introduction of a healing agent. During a damage event cracks propagate through the material and into the vascules causing them to be cleaved open. A liquid resin is then passed through the vascules and into the damage plane, allowing the cracks to be repaired. Vascular systems have a number of advantages over microcapsule based systems, such as the ability to continuously deliver large volumes of repair agents and the potential to be used for repeated healing. The hollow channels themselves can also be used for additional functionality, such as thermal management and structural health monitoring.[83] A number of methods have been proposed for the introduction of these vascules, including the use of hollow glass fibres (HGFs),[84] [85] 3D printing,[15] a "lost wax" process [86][87] and a solid preform route.[88]
Self-healing coatings
Coatings allow the retention and improvement of bulk properties of a material. They can provide protection for a substrate from environmental exposure. Thus, when damage occurs (often in the form of microcracks), environmental elements like water and oxygen can diffuse through the coating and may cause material damage or failure. Microcracking in coatings can result in mechanical degradation or delamination of the coating, or in electrical failure in fibre-reinforced composites and microelectronics, respectively. As the damage is on such a small scale, repair, if possible, is often difficult and costly. Therefore, a coating that can automatically heal itself (“self-healing coating”) could prove beneficial by automatic recovering properties (such as mechanical, electrical and aesthetic properties), and thus extending the lifetime of the coating. The majority of the approaches that are described in literature regarding self-healing materials can be applied to make “self-healing” coatings, including microencapsulation[89][60] and the introduction of reversible physical bonds such as hydrogen bonding,[90] ionomers [91][92] and chemical bonds (Diels-Alder chemistry).[93] Microencapsulation is the most common method to develop self-healing coatings. The capsule approach originally described by White et al., using microencapsulated dicyclopentadiene (DCPD) monomer and Grubbs' catalyst to self-heal epoxy polymer[60] was later adapted to epoxy adhesive films that are commonly used in the aerospace and automotive industries for bonding metallic and composite substrates.[94] Recently, microencapsulated liquid suspensions of metal or carbon black were used to restore electrical conductivity in a multilayer microelectronic device and battery electrodes respectively;[95][96] however the use of microencapsulation for restoration of electrical properties in coatings is limited. Liquid metal microdroplets have also been suspended within silicone elastomer to create stretchable electrical conductors that maintain electrical conductivity when damaged, mimicking the resilience of soft biological tissue.[97] The most common application of this technique is proven in polymer coatings for corrosion protection. Corrosion protection of metallic materials is of significant importance on an economical and ecological scale. To prove the effectiveness of microcapsules in polymer coatings for corrosion protection, researchers have encapsulated a number of materials. These materials include isocyanates[98][99] monomers such as DCPD[62][81] GMA[100] epoxy resin,[101] linseed oil[102][103] and tung oil.,[104][105] For encapsulation of core like as mentioned above, number of shell materials have been utilised such as phenol formaldehyde, urea formaldehyde [106] &,[107] dendritic or PAMAM,[108] melamine formaldehyde, etc. Each shell material has its own merits and demerits. Even these shell materials extended their applications in control delivery of pesticides [109] and drugs. By using the aforementioned materials for self healing in coatings, it was proven that microencapsulation effectively protects the metal against corrosion and extends the lifetime of a coating.
Coatings in high temperature applications may be designed to exhibit self-healing performance through the formation of a glass. In such situations, such as
Self-healing cementitious materials
Cementitious materials have existed since the Roman era. These materials have a natural ability to self-heal, which was first reported by the French Academy of Science in 1836.[112] This ability can be improved by the integration of chemical and biochemical strategies.
Autogenous healing
Autogenous healing is the natural ability of cementitious materials to repair cracks. This ability is principally attributed to further hydration of unhydrated cement particles and carbonation of dissolved calcium hydroxide.[112] Cementitious materials in fresh-water systems can autogenously heal cracks up to 0.2 mm over a period of 7 weeks.[113]
In order to promote autogenous healing and to close wider cracks, superabsorbent polymers can be added to a cementitious mixture.[114][115] Addition of 1 m% of selected superabsorbent polymer versus cement to a cementitious material, stimulated further hydration with nearly 40% in comparison with a traditional cementitious material, if 1 h water contact per day was allowed.[116]
Chemical additives based healing
Self-healing of cementitious materials can be achieved through the reaction of certain chemical agents. Two main strategies exist for housing these agents, namely capsules and vascular tubes. These capsules and vascular tubes, once ruptured, release these agents and heal the crack damage. Studies have mainly focused on improving the quality of these housings and encapsulated materials in this field.[117]
Bio-based healing
According to a 1996 study by H. L. Erlich in Chemical Geology journal, the self-healing ability of concrete has been improved by the incorporation of bacteria, which can induce calcium carbonate precipitation through their metabolic activity.[118] These precipitates can build up and form an effective seal against crack related water ingress. At the First International Conference on Self Healing Materials held in April, 2007 in The Netherlands, Henk M. Jonkers and Erik Schlangen presented their research in which they had successfully used the "alkaliphilic spore-forming bacteria" as a "self-healing agent in concrete".[119][120] They were the first to incorporate bacteria within cement paste for the development of self-healing concrete.[121] It was found that the bacteria directly added to the paste only remained viable for 4 months. Later studies saw Jonkers use expanded clay particles[122] and Van Tittlelboom use glass tubes,[123] to protect the bacteria inside the concrete. Other strategies to protect the bacteria have also since been reported.[124] Even microcapsule based self-healing applications has been extended on bio-based coating materials. These coatings are based on neem oil and possesses another bio-based character as it utilized vegetable oil as a core material.,[125]
Self-healing ceramics
Generally, ceramics are superior in strength to metals at high temperatures, however, they are brittle and sensitive to flaws, and this brings into question their integrity and reliability as structural materials.[126] phase ceramics, also known as
Self-healing metals
When exposed for long times to high temperatures and moderate stresses, metals exhibit premature and low-ductility creep fracture, arising from the formation and growth of cavities. Those defects coalesce into cracks which ultimately cause macroscopic failure. Self-healing of early stage damage is thus a promising new approach to extend the lifetime of the metallic components. In metals, self-healing is intrinsically more difficult to achieve than in most other material classes, due to their high melting point and, as a result, low atom mobility. Generally, defects in the metals are healed by the formation of precipitates at the defect sites that immobilize further crack growth. Improved creep and fatigue properties have been reported for underaged aluminium alloys compared to the peak hardening Al alloys, which is due to the heterogeneous precipitation at the crack tip and its plastic zone.[135] The first attempts to heal creep damage in steels were focused on the dynamic precipitation of either Cu or BN at the creep-cavity surface.[136][137] Cu precipitation has only a weak preference for deformation-induced defects as a large fraction of spherical Cu precipitates is simultaneously formed with the matrix.[138][139] Recently, gold atoms were recognized as a highly efficient healing agents in Fe-based alloys. A defect-induced mechanism is indicated for the Au precipitation, i.e. the Au solute remains dissolved until defects are formed.[140] Autonomous repair of high-temperature creep damage was reported by alloying with a small amount of Au. Healing agents selectively precipitate at the free surface of a creep cavity, resulting in pore filling. For the lower stress levels up to 80% filling of the creep cavities with Au precipitates is achieved[141] resulting in a substantial increase in creep life time. Work to translate the concept of creep damage healing in simple binary or ternary model systems to real multicomponent creep steels is ongoing.
Self-healing hydrogels
Self-healing organic dyes
Recently, several classes of organic dyes were discovered that self-heal after photo-degradation when doped in PMMA and other polymer matrices.[144] This is also known as reversible photo-degradation. It was shown that, unlike common process like molecular diffusion,[145] the mechanism is caused by dye-polymer interaction.[146]
Self-healing of ice
It has recently been shown that micrometer-sized defects in a pristine layer of ice heal spontaneously within a matter of several hours. The generated curvature by any defect causes a local increased vapor pressure and therefore enhances the volatility of the surface molecules. Hence, the mobility of the upper layer of water molecules increases significantly. The main mechanism, that dominates this healing effect is therefore sublimation from, and condensation onto the surface.[147] This opposes earlier work that describes sintering of ice spheres by surface diffusion.[148]
Self-healing of metal
In 2023 the Sandia National Laboratories reported the finding of self-healing of fatigue cracks in metal[149][150] and reported that the observations seems to confirm a 2013 study predicting the effect.[151]
Further applications
Self-healing epoxies can be incorporated onto metals in order to prevent corrosion. A substrate metal showed major degradation and rust formation after 72 hours of exposure. But after being coated with the self-healing epoxy, there was no visible damage under SEM after 72 hours of the same exposure.[152]
Assessment of self-healing efficacy
Numerous methodologies for the assessment of self-healing capabilities have been developed for each material class (Table 1).
Material class | Damage mechanism | Healing |
---|---|---|
Polymers | Razor blade/scalpel cut; Tensile test with rupture; Ballistic impact | Autonomic healing supramolecular networks |
Polymers | Razor blade/scalpel cut | Temperature triggered supramolecular networks |
Fibre Reinforced Composite | Delamination BVID (Barely Visible Impact Damage) | Vascular self-healing; Microcapsule self-healing |
Coatings | Microcutting with corrosion; Corrosion/erosion; Pull-out tests (adhesion); Microscratching | Molecular inter-diffusion (solvent); Encapsulated agent |
Concrete | Crack initiation by bending compression | Activation of microencapsulated agent |
Ceramic | Crack initiation by indentation | Temperature triggered oxidation reaction |
Ceramic coating | Crack initiation by indentation | Temperature triggered oxidation reaction |
Polyurethane foam coating | Puncturing with a spike | Reduction of the effective leakage area by negative strains pushing the walls of the fissure in the foam coatings to one another.[17] |
Hence, when self-healing is assessed, different parameters need to be considered: type of stimulus (if any), healing time, maximum amount of healing cycles the material can tolerate, and degree of recovery, all whilst considering the material's virgin properties.[153][154][90] This typically takes account of relevant physical parameters such as tensile modulus, elongation at break, fatigue-resistance, barrier properties, colour and transparency. The self-healing ability of a given material generally refers to the recovery of a specific property relative to the virgin material, designated as the self-healing efficiency. The self-healing efficiency can be quantified by comparing the respective experimental value obtained for the undamaged virgin sample (fvirgin) with the healed sample (fhealed) (eq. 1)[155]
-
η = fhealed/fvirgin
(1)
In a variation of this definition that is relevant to extrinsic self-healing materials, the healing efficiency takes into consideration the modification of properties caused by introducing the healing agent. Accordingly, the healed sample property is compared to that of an undamaged control equipped with self-healing agent fnon-healed (equation 2).
-
η = fhealed/fnon-healed
(2)
For a certain property Pi of a specific material, an optimal self-healing mechanism and process is characterized by the full restoration of the respective material property after a suitable, normalized damaging process. For a material where 3 different properties are assessed, it should be determined 3 efficiencies given as ƞ1(P1), ƞ2(P2) and ƞ3(P3). The final average efficiency based on a number n of properties for a self-healing material is accordingly determined as the harmonic mean given by equation 3. The harmonic mean is more appropriate than the traditional arithmetic mean, as it is less sensitive to large outliers.
Commercialization
At least two companies are attempting to bring the newer applications of self-healing materials to the market. Arkema, a leading chemicals company, announced in 2009 the beginning of industrial production of self-healing elastomers.[156] As of 2012, Autonomic Materials Inc., had raised over three million US dollars.[157][158]
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External links
- Media related to Self-healing material at Wikimedia Commons