Recent developments in material design have seen an exponential increase of polymers and polymer composites that can repair themselves in response to damage. In this review, a distinction is made between extrinsic materials, where the self-healing property is obtained by adding healing agents to the material to be repaired, and intrinsic materials, where self-healing is achieved by the material itself through its chemical nature. An overview of the crosslinking chemistries used in self-healing materials will be given, discussing the advantages and drawbacks of each system. The review is not only aiming to enable researchers to compare their ongoing research with the state-of-the-art but also to serve as a guide for the newcomers, which allows for a selection of the most promising self-healing chemistries.

Furan-based thermoset polyurethanes have been prepared in a one-pot fashion with the ability to selfmend under mild temperature conditions, by making use of a Diels−Alder shape-memory assisted self-healing (DASMASH) approach. For this, thermoreversible covalent bonds, obtained by Diels−Alder chemistry, are introduced as cross-linkers into a polycaprolactone (PCL) containing polyurethane material. It is demonstrated that, after introduction of a crack into the PUthermoset, Diels−Alder bonds preferentially break, regenerating free furan/maleimide functional groups, while the shape memory effect favors the crack closure at temperatures above the melting point of PCL, simultaneously resulting in a reformation of the reversible cross-links. The reversibility and shape memory ability of the materials were optimized and studied by FTIR, 1 H NMR and tensile measurements. Different compositions were used to properly understand the role and influence of each component. The polyurethane materials healed at 50°C after mechanical damage induced by either the application of a large tensile deformation or by performing controlled macro/micro scratches with a depth sensing indenter. Online FT-IR monitoring provided a kinetic description of the system reversibility for numerous cycles. Furthermore, mechanical recovery with complete disappearance of the microscratches was accomplished after multiple cycles of large tensile deformation. The results were not only confirmed by an optical inspection and scanning electron microscopy, but also with confocal microscopic mapping, by comparison of the cross-section profiles of the microscratches before and after healing.

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Thiol‐isocyanate chemistry, combined with a dual capsule strategy, is used for the development of extrinsic self‐healing epoxy materials. It is shown that the amine groups present in the matrix both serve as a catalyst for the addition reaction between a thiol and an isocyanate and as a way to covalently link the healed network structure to the surrounding resin. The tapered double cantilever beam (TDCB) geometry is used for evaluating the recovery of the fracture toughness at room temperature after different healing times. Using manual injection of the healing agents into the crack, a healing efficiency up to 130% is obtained for the EPIKOTE 828/DETA epoxy material. On the other hand, when two types of microcapsules, one containing a tetrathiol reagent and the other a low toxic isocyanate reagent, are incorporated into this epoxy thermoset (20 wt%), a recovery of more than 50% is reached. The influence of parameters such as the amount and core content of the microcapsules on the healing efficiency is investigated. Furthermore, the thiol‐isocyanate chemistry is also tested for an industrial cold‐curing epoxy resin (RIM 135/RIMH 137).

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This study evaluated thermoplastic polyurethanes (TPUR) as matrix excipients for the production of oral solid dosage forms via hot melt extrusion (HME) in combination with injection molding (IM). We demonstrated that TPURs enable the production of solid dispersions -crystalline API in a crystalline carrier -at an extrusion temperature below the drug melting temperature (Tm) with a drug content up to 65% (wt.%). The release of metoprolol tartrate was controlled over 24h, whereas a complete release of diprophylline was only possible in combination with a drug release modifier: polyethylene glycol 4000 (PEG 4000) or Tween 80. No burst release nor a change in tablet size and geometry was detected for any of the formulations after dissolution testing. The total matrix porosity increased gradually upon drug release. Oral administration of TPUR did not affect the GI ecosystem (pH, bacterial count, short chain fatty acids), monitored via the Simulator of the Human Intestinal Microbial Ecosystem (SHIME). The high drug load (65wt.%) in combination with (in-vitro and in-vivo) controlled release capacity of the formulations, is noteworthy in the field of formulations produced via HME/IM.

We report on a one-pot, facile approach for the encapsulation of the liquid hexamethylene diisocyanate isocyanurate trimer in polyurea microcapsules formed via the oil-in-water interfacial reaction of an uretonimine- modified diphenyl methane diisocyanate trimer with triaminopyrimidine, with in situ shell functionalization/modification using different types of hydrophobic agents. Remarkably, the use of hexa-methylenedisilazane resulted in microcapsules of about 70 mu m in diameter, with a smooth outer surface and a high isocyanate core content up to 85 wt% as determined by quantitative online FT-IR analysis of the extracted core. On the other hand, the use of an alkylamine, fluorinated aromatic amine and/or perfluoride amine provided microcapsules of approximately 100 to 150 mu m in diameter containing around 65-75 wt% of the isocyanate core content, with the outer shell surface bearing pendant hydrophobic groups as confirmed by SEM-EDX. The effects of the functionalizing compound on the microcapsule properties such as shell morphology, size distribution and stability were assessed. After one day immersion in water, the initial isocyanate content of the microcapsules with a non-functionalized shell dropped rapidly from 49 to 15 wt%, whereas the ones with the modified shell structure maintained their core content, suggesting a significantly enhanced microcapsule stability

The first step in the synthesis of functionalized polyamides consisted in the introduction of alkyne side groups in several types of linear polyamides by interfacial step-growth polymerization of hexane-1,6-diamine or 4,9-dioxadodecane-1,12-diamine in combination with on one hand adipoyl or sebacoyl chloride and on the other hand an alkyne-containing building block, i.e. 2-methyl-2-propargylmalonic acid dichloride. Both homo-and copolyamides were synthesized, creating a large range of alkyne-containing polyamides with degrees of functionalization ranging between 5 and 100 %. Subsequently, these polyamide chains have been modified with two types of linking chemistries, respectively with azides through the copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition or with thiols through the thiol-yne addition reaction.