Natural organisms have a remarkable, unique property, the ability to self-heal when certain damages and injuries occur. Their bodies are not over-dimensioned and fractures, ruptures and injuries will occur when a body part is overloaded in abnormal, extreme circumstances. The powerful biological healing function has inspired chemists to impart similar properties to synthetic materials to create “self-healing materials”. Since 2001, a broad range of self-healing (SH) materials has been developed. Self-healing mechanisms have been developed for metals and ceramics, but self-healing polymers showed laterly the largest evolution. Recent developments in the self-healing polymer technology have led to (commercial) applications.
Definition of self-healing material
There has been some discussions on the definition of a SH-material and therefore sometimes other denominations are used; eg. remendible materials. We follow a rather general definition; "Self-healing materials are polymers, metals, ceramics and their composites that when damaged through thermal, mechanical, ballistic or other means have the ability to heal and restore the material to its original set of properties." The healing procedure can be either spontaneous or take place with aid of a stimulus. Few materials intrinsically posses this ability.
Overview self-healing polymers
Because we introduced a self-healing mechanism in the soft robotics through the use of a SH-polymer we focus our research on polymers. Although SH-metal and ceramic applications exist, self-healing polymers recently have made the fastest evolution. In this section, a brief classification of SH-polymers is presented.
First two distinct classes can be dened: autonomic self-healing materials and non-autonomic systems. Autonomic systems require no stimulus (other than the formation of damage) for operation. These mechanisms do not require human intervention and are entirely self-contained. They most closely resemble biological systems, which deliver healing agents to compromised regions as soon as damage is inflicted. Non-autonomic systems on the other hand require some type of externally applied stimulus (such as heat or light) to enable a healing function. Yet this allows the healing process to be performed in a controlled way.
Autonomous self-healing polymers
A subcategory of autonomic systems are the self-healing mechanisms using encapsulation. In this case, reactive chemical reagents (so called healing agents) are stored through compartmentalization, usually in the form of microcapsules, inside the material. When damage is occurs, the capsules will break and the chemicals are released and interact in such a manner that void spaces become filled, and through a chemical reaction the surfaces bond together.
Application in soft-robotics: A disadvantage of systems working with a form of encapsulated reagents is that after the healing process the material properties will change locally. An even greater disadvantage is that the healing process can only operate once (or a limited number of times) at the same location because once the capsules are broken the healing mechanism is no longer present at that specific location. This means that the number of damage-healing cycles is strongly limited. Due to these two drawbacks this encapsulation self-healing system is not suitable for soft robotic applications. However encapsulation can be interesting for surface applications in (hard) robotics to protect, for example, the cover/skin against appearing scratch damages, which usually do not take place at the exact same location. New self-healing polymers are beeing developed which provide a solution to these drawbacks through the use of a vascular network instead of a discrete capsulation. However, this vascular network is yet too complex to be introduced in soft robotics.
Other autonomic systems are joined under the name: mechanochemical systems. These mechanisms rely on weak reversible interactions (weak metal-amine bonds or weak hydrogen bonds), in conjunction with a polymeric structure designed to bear and later release stress. These weak reversible interactions (which form a reversible network by cross-linking) will break under a sufficient mechanical stress. But in unstressed condition at ambient temperature, the bonds will be reconstructed again due to the reversible behavior of the reaction, regenerating the chemical bond, without the need of an external stimulus.
Application in soft-robotics: The mechanical properties of these polymers can be affected by the composition and the concentration of the weak bonds in the polymer structure. Up till now, mechanochemical polymers with mechanical properties adequate for soft robotics (high strength, low visco-elastic behavior,...) cannot be synthesized, due to the presents weak bonds, of which the reversible network is formed. However, these materials are very promising, since the ability to perform self-healing at ambient temperature, without an external stimulus or encapsulation, is a huge advantage in comparison with the other non-autonomous self-healing mechanisms.
Non-autonomous self-healing polymers
As already mentioned non-autonomic self-healing materials always require some type of external stimulus. This stimulus can be in the form of heat, light, a mechanical or chemical stimulus.
Light can be used to induce chemical based healing processes. An advantage of this mechanism is that photoreactions are usually very fast and can be selectively initiated by applying light of an appropriate wavelength (visible light or UV-light). Light also enables greater control over the healing process since it can be localized to specific sites.
Application in soft-robotics: A big drawback is that the healing process is usually only limited to the surface areas only, because of the limited penetration depth of the light, due to its absorption. Hence bulk or macroscopic healing is not possible photo-induced self-healing polymers are less interesting for their implementation in soft robotics. However, they show a very high potential for future coating applications in (hard) robotics.
Heat is used in most of the self-healing polymers already developed. Compared with the other non-autonomic self-healing systems, these mechanisms are most applicable in soft robotics design, because of their relatively easy thermal control. Based on their chemistry they can be classied as follows.
At room temperature, all these polymers consist of a thermo-reversible network, which is build up by cross-linking of thermo-reversible bonds. If heat is applied and the polymers are brought to elavated temperature these cross-linking bonds are reversibly broken. The network is broken (the polymer has an almost gel-like consistance) and polymer chains obtain enough mobility to slowly fill/seal/close microscopic and macroscopic damages, such as gaps, cuts, tears, cracks, perforations and punctures. After these damages are cured, the polymer is cooled down to room temperature and the reversible network is formed again. There exist already a broad number of thermo-reversible network polymers which rely on different chemical cross-link bonds; eg. covalent bond SH-polymers, coordination bond SH-polymers, hydrogen SH-polymers and Ionomers.
Application in soft robotics: Because there are a lot of different thermoreversible SH-polymers available, their differences are further elaborated in the next paragraphs.
Thermo-reversible self-healing polymers
Diels-Alder polymers are the most commenly used. Their thermo-reversible network is formed by a Diels-Alder (DA) cross-linking reaction between a furan ring and a maleimide ring.
Application in soft-robotics: Because their cross-linking is based on covalent bonds, the DA-polymers and covalent bond SH-polymers in general, posses engineering mechanical properties (high fracture strain and high ultimate tensile strength). In theory, there mechanical properties are almost completely recovered after healing, which means that the number of SH-cycles for a polymer part is not limited by the SH-mechanism. The non-autonomous self-healing process requires temperatures in the range of 70 to 120 °C.
Coordination bond polymers are composed of monomers that interact non-covalently. Common interactions include metal-ligand coordination, and van der Waals forces. Mechanical stress in these polymers causes the disruption of these specific non-covalent interactions, leading to monomer separation and polymer breakdown. These weak interactions are of particular interest for self-healing materials because of their reversible nature.
Application in soft-robotics: Because their network relies on non-covalent (weak) bonds, these materials lack good mechanical properties, a lot of them have a gelatinous behavior at ambient temperature. This disadvantage make coordination bond polymers less suitable for self-healing actuator application.
Like coordination bond SH-polymers, the thermo-reversible network relies on non-covalent bonds; hydrogen bonds.
Application in soft-robotics: Although the heat required for their SH-process is much lower than the one required for covalent bond SH-polymers, these H-bond SH-polymers lack adequate mechanical properties.
Ionomers are a class of polymers that contain as much as 20% of charged or ionic species as a part of their structures. These ions are able to interact with one another in a manner very similar to salt bridges seen in proteins, creating interactions or aggregates that have a profound effect upon their mechanical and physical properties. These aggregates consist of several ion pairs known as multipelts. When stressed, microscopic or macroscopic damages can occur in the material because these multipelts are broken. The non-autonomous healing process of these materials relies again on a heat stimulus: As heating occurs, whether through direct application or as a result of the friction resulting from damage, the multiplets dissociate (the thermo-reversible network is dissociated). This gives the material enough mobility to seal and close the micro/macroscopic damages. Cooling reverses the processes and the network is formed again. However, the material does not regain its native structure until the multiplets re-form, which can take days.
Application in soft-robotics: Because of their crystalline structure, SH-ionomers can be synthesized, having mechanical properties which are adequate for self-healing soft robotics. Their mechanical properties can be partially recovered after a SH-process, or fully if waited long enough. The non-autonomous self-healing process requires temperatures in the range of 110°C to 150 °C.
Although classied as non-autonomous, the damage event can provide enough energy, generally in the form of heat as a result of friction, so that the material eectively behaves as an autonomic self-healing material. This is what happens if the self-healing material is used for healing of ballistic impacts, the application where these polymers are used mainly. For example there exist already ionomer plates, which remain fully intact after a bullet impact. The bullet will penetrate the polymer and due to the heat induced by friction the hole, left behind by the bullet, will be closed immediately. These "Impact Seal Self-Healing Targets" are being commercialized and used as long lifetime targets on shooting ranges