Citation: Utrera-Barrios, S.; Verdejo, R.; López-Manchado, M.Á.; Santana, M.H. The Final Frontier of Sustainable Materials: Current Developments in Self-Healing Elastomers. Int. J. Mol. Sci. 2022, 23, 4757. https://doi.org/10.3390/ ijms23094757 Academic Editors: Ana MaríaDíez-Pascual and Ángel Serrano-Aroca Received: 13 April 2022 Accepted: 24 April 2022 Published: 26 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). International Journal of Molecular Sciences Review The Final Frontier of Sustainable Materials: Current Developments in Self-Healing Elastomers Saul Utrera-Barrios , Raquel Verdejo , Miguel Ángel López-Manchado * and Marianella Hernández Santana * Institute of Polymer Science and Technology (ICTP-CSIC), Juan de la Cierva 3, 28006 Madrid, Spain; sutrera@ictp.csic.es (S.U.-B.); r.verdejo@csic.es (R.V.) * Correspondence: lmanchado@ictp.csic.es (M.Á.L.-M.); marherna@ictp.csic.es (M.H.S.) Abstract: It is impossible to describe the recent progress of our society without considering the role of polymers; however, for a broad audience, “polymer” is usually related to environmental pollution. The poor disposal and management of polymeric waste has led to an important environmental crisis, and, within polymers, plastics have attracted bad press despite being easily reprocessable. Nonetheless, there is a group of polymeric materials that is particularly more complex to reprocess, rubbers. These macromolecules are formed by irreversible crosslinked networks that give them their characteristic elastic behavior, but at the same time avoid their reprocessing. Conferring them a self-healing capacity stands out as a decisive approach for overcoming this limitation. By this mean, rubbers would be able to repair or restore their damage automatically, autonomously, or by applying an external stimulus, increasing their lifetime, and making them compatible with the circular economy model. Spain is a reference country in the implementation of this strategy in rubbery materials, achieving successful self-healable elastomers with high healing efficiency and outstanding mechanical performance. This article presents an exhaustive summary of the developments reported in the previous 10 years, which demonstrates that this property is the last frontier in search of truly sustainable materials. Keywords: self-healing materials; self-healing rubbers; natural rubber; synthetic rubber; dynamic networks; supramolecular chemistry 1. Introduction In the actual environmental context, polymers like rubbers are particularly critical due to their reprocessing difficulties. These macromolecular materials are composed of irreversible crosslinked networks that act as “anchor points”, preventing the flow of polymeric chains. Consequently, the material cannot be reshaped [1], and a considerable amount of rubber waste could be generated. One of the strategies to solve this issue has been the recovery of end-of-life rubbers for their use as a diluent or reinforcing filler in new composite materials [2–6]. Also, the selective breaking of the crosslinking points, known as devulcanization [7–11], has been extensively studied; however, both strategies are considered insufficient. Thus, the redesign of crosslinked rubbers is mandatory. Most recent redesign strategies point toward building dynamic networks [1,12,13]. The creation of crosslinked polymers with dynamic networks has spawned a new gen- eration of polymers known as DYNAMERS (DYNAmic polyMERS)[14,15]. The construction of these networks is based on multiple dynamic bonds and/or supramolecular interactions, like hydrogen bonds [16,17], ionic interactions [18], metal–ligand coordination [19], disul- fide exchange [20], and Diels–Alder chemistry [21,22], among other covalent, non-covalent mechanisms and/or combinations between them [23–30]. The reversible nature of these networks can be controlled by an external stimulus, which can be temperature, pressure, electrical current, magnetic field, or further changes in the medium, such as pH [31–35]. In this way, the stimuli-responsive material would be able to release its “anchor points”, Int. J. Mol. Sci. 2022, 23, 4757. https://doi.org/10.3390/ijms23094757 https://www.mdpi.com/journal/ijms