Mechanoresponsive Materials

“Smart” materials which change their properties on command, that is, upon exposure to a pre-defined stimulus in a highly selective and predictable manner, are currently of considerable interest because of their potential uses in applications that range from drug delivery to camouflage systems to artificial muscles. Many examples of chemically, thermally, electrically, optically, or otherwise responsive materials are known, but comparably few materials have been studied, which respond in a useful and controlled manner to the exposure of mechanical stress. This situation is in stark contrast to the diverse types of mechanically-responsive materials found in nature. This richness is rooted in the fact that essentially all living organisms rely on mechanochemical transduction processes, that is, cascades of events that translate macroscopic forces into chemical reactions. These, in turn, elicit electrical signals, open ion channels, promote cell growth, and enable many other vital functions. Chemical bond activation through mechanical force is also well known in artificial materials, but it has traditionally been regarded as a challenge rather than an opportunity; the state of research on the mechanical failure mechanisms of polymers is illustrative of this situation. It has long been known that mechanical force can alter the electronic states of chemical bonds and, thus, can lead to changes in chemical reactivity, optical properties, electrical conductivity, magnetic response, and other properties, but it has been a veritable challenge to harness this knowledge for the rational design of functional mechanoresponsive materials. The last decade though has witnessed tremendous progress. Researchers have been able to create mechanoresponsive systems in different materials families, including small organic molecules, polymers, and metal nanoparticles. These materials exploit a range of transduction mechanisms—not just activation of chemical bonds. Mechanical force has, for example, been used to assemble or disassemble small molecules in a controlled manner, break specific bonds in polymers, and alter the ion distribution in metal oxide lattices, all resulting in well-defined, pre-programmed responses. Approaches that rely on microscopic, rather than molecular-level, effects include the breaking of liquid-filled microcapsules or magnetomechanical coupling schemes. This themed issue of Journal of Materials Chemistry attempts to capture the excitement of this rapidly emerging, interdisciplinary field. Through a combination of feature articles and original research papers, it seeks to provide an overview of the current research activities and to highlight “hot” topical areas.

As reflected by the large numbers of contributions on this subject, mechanoresponsiveness in polymers is a very active area. In these materials, mechanical stress provides the activation energy for specific chemical reactions, which in turn enable pre-programmed functions. The feature article by Kucharski and Boulatov (DOI: 10.1039/c0jm04079g) provides an excellent summary of the physical chemical aspects of mechanochemical coupling and highlights the need for formalisms that connect molecular and macroscopic levels. Focusing on self-propelled gels that convert chemical energy into mechanical motion, a paper by Balazs and co-workers (DOI: 10.1039/c0jm03426f) illustrates strikingly how computational modeling can offer valuable insights into the dynamic coupling between chemical reactions and mechanical forces. Papers by White, Moore, Sottos and co-workers (DOI: 10.1039/c0jm04015k and DOI: 10.1039/c0jm03967e), Moore, Sottos, Craig and co-workers (DOI: 10.1039/c0jm04117c), Black et al. (DOI: 10.1039/c0jm03875j), Dopieralski et al. (DOI: 10.1039/c0jm03698f), and Bielawski and co-workers (DOI: 10.1039/c0jm03619f) summarize new experimental studies that address the activation of mechanochemically reactive units or “mechanophores” that were deliberately incorporated into linear and cross-linked polymers. These studies provide important insights on the structure–property relationships that will be important for the further design of such mechanoresponsive polymers. A paper by Matyjaszewski, Sheiko and co-workers (DOI: 10.1039/c0jm04152a) reports on the spontaneous degradation of bottle-brush macromolecules during spreading and shows how the bond-scission process can be directed on the molecular and macroscopic scales. Papers on elastic capacitive strain sensors (Kollosche et al.; DOI: 10.1039/c0jm03786a), liquid crystal elastomers (Amela-Cortés et al.; DOI: 10.1039/c0jm03691a), and mechanoresponsive polymer nano-objects as drug carriers and components of microfluidic devices (Zhang et al.; DOI: 10.1039/c0jm03634j) are illustrative of the potential applications and give a flavor for the broad usefulness of mechanoresponsive polymer systems.

Mechanochromic materials whose absorption and/or fluorescence color change upon deformation represent another prominent class of mechanoresponsive materials. It is obvious that such chromogenic effects are exceedingly useful for sensing and in particular the in situ monitoring of material failure due to stress fracture or fatigue. The feature articles by Roberts and Holder (DOI: 10.1039/c0jm04237d) and Pucci and Ruggeri (DOI: 10.1039/c0jm03653f) review fluorescent mechanochromic polymer systems that rely on the mechanically induced (dis)assembly of chromophores within polymer matrices. A paper by Ramachandran and Urban (DOI: 10.1039/c0jm03722b) reports a new family of mechanochromic cross-linked polymers and discusses the reversibility of mechanically-induced changes in these materials. Changing the molecular assembly through mechanical forces has also been the key to the recent development of small-molecule organic compounds that exhibit piezochromic fluorescence, also referred to as mechanochromic luminescence or mechanofluorochromic effect. Papers by Yoon and Park (DOI: 10.1039/c0jm03711g), Ooyama and Harima (DOI: 10.1039/c0jm03601c), and Araki and co-workers (DOI: 10.1039/c0jm03950k) report on three families of mechanofluorochromic dyes and discuss the relationships between chemical structure, molecular packing, solid-state luminescence, and the influence of pressure on the latter two properties. A series of papers by Fraser and co-workers (DOI: 10.1039/c0jm03871g; DOI: 10.1039/c0jm04326e; DOI: 10.1039/c1jm00067e) illustrates that mechanochromic luminescence is also observed in difluoroboron β-diketonate dyes, where mechanical perturbations also elicit structural changes, in this case from ordered to amorphous states.

A contribution by Sepelak et al. (DOI: 10.1039/c0jm03721d) shows that mechanoresponsive effects are not limited to the domain of organic or organometallic materials. Studying different types of spinel aluminates, the authors demonstrate that mechanical action can change the location of cations in the crystal lattice of spinel aluminates, such as ZnAl₂O₄.

Another subject of current interest is the in situ generation of mechanical forces within polymer systems by way of magneto-mechanical coupling. A paper by Schmidt and co-workers (DOI: 10.1039/c0jm03816d) shows how the local structure and dynamics of soft matter can be probed via magnetic nanoparticle based micro- and nanorheology. Going beyond the idea of probing soft systems, a contribution by Kim et al. (DOI: 10.1039/c1jm10272a) reports a biopolymer-ferromagnetic disk hybrid system capable of releasing drug molecules on demand through a magnetomechanically induced mechanism.

Aside from mechanoresponsive bulk materials and gels, mechanically sensitive surfaces are receiving growing attention. A paper by Mertz et al. (DOI: 10.1039/c0jm03496g) reports new biocatalytic assemblies based on a polyelectrolyte multilayer and enzymes. Mimicking the concept of cryptic sites found in natural proteins, a mechanically responsive barrier layer shields the enzyme from the substrates until it is rendered permeable through mechanical stress. In a related study Mjahed et al. (DOI: 10.1039/c0jm03457f) show that structural reorganizations observed in such polyelectrolyte multilayer films, for example in response to the ionic strength of solutions in contact with the films, are largely influenced by the history of applied stresses. Another illustrative example for mechanoresponsive surfaces are discretely functionalized gold nanoparticles, which allow for the detection of biomolecular activities by way of surface plasmon resonance measurements. This topic is discussed in a paper by Chak et al. (DOI: 10.1039/c0jm03709e).

Finally, a contribution of Kaupp et al. (DOI: 10.1039/c0jm03713c) takes us back to biological materials. It probes the mechanical response of nutshells from different species by nanoindentation and relates the behavior to the hierarchical structures of these nature-made composites.

As guest editor of this themed issue, I hope that this collection of outstanding papers conveys that the potential impact of mechanically-induced transduction processes is likely to go far beyond the examples discussed herein. It appears that the general concept can be adapted to create a plethora of novel stimuli-responsive materials in which the translation of stress into structural changes—ranging from the molecular to the macroscopic level—can be harnessed to create unusual but useful functionalities, such as mechanical morphing, mechanically-induced light generation, auto-lubricating behavior, and the mechanical release of small molecules such as drugs, fragrances and antiseptics.

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