Force-induced activation of covalent bonds in mechanoresponsive polymeric materials

Biology is replete with materials systems that actively and functionally respond to mechanical stimuli and thereby enable physiological processes such as the sense of touch, hearing or the growth of tissue and bone. In contrast, exposing polymers to large stresses tends to result in covalent bond rupture and hence damage or failure. Davis et al. now demonstrate that synthetic materials can be rationally designed to ensure that mechanical stress alters their properties in a useful manner. This is realized by incorporating a chemical group that responds to mechanical stress by changing its colour to red as it undergoes a ring-opening reaction, enabling the team to directly monitor the accumulation of plastic deformation. The principles underpinning this work should enable the development of other force-responsive chemical groups that could impart synthetic materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing. Exposing synthetic materials to large stresses tends to result in simple failure, unlike many biological systems, which respond by enabling physiological processes such as hearing and balance. But by incorporating a chemical group that responds to mechanical stress by changing its colour, it is possible to monitor the accumulation of plastic deformation directly in a synthetic polymer. This principle could be used to design synthetic materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing. Mechanochemical transduction enables an extraordinary range of physiological processes such as the sense of touch, hearing, balance, muscle contraction, and the growth and remodelling of tissue and bone1,2,3,4,5,6. Although biology is replete with materials systems that actively and functionally respond to mechanical stimuli, the default mechanochemical reaction of bulk polymers to large external stress is the unselective scission of covalent bonds, resulting in damage or failure7. An alternative to this degradation process is the rational molecular design of synthetic materials such that mechanical stress favourably alters material properties. A few mechanosensitive polymers with this property have been developed8,9,10,11,12,13,14; but their active response is mediated through non-covalent processes, which may limit the extent to which properties can be modified and the long-term stability in structural materials. Previously, we have shown with dissolved polymer strands incorporating mechanically sensitive chemical groups—so-called mechanophores—that the directional nature of mechanical forces can selectively break and re-form covalent bonds15,16. We now demonstrate that such force-induced covalent-bond activation can also be realized with mechanophore-linked elastomeric and glassy polymers, by using a mechanophore that changes colour as it undergoes a reversible electrocyclic ring-opening reaction under tensile stress and thus allows us to directly and locally visualize the mechanochemical reaction. We find that pronounced changes in colour and fluorescence emerge with the accumulation of plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the ring-opening reaction is an activated process. We anticipate that force activation of covalent bonds can serve as a general strategy for the development of new mechanophore building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to fully regenerative self-healing.