TAMUKA CHIDANGURO, WENGUI WENG AND YOAN C. SIMON*
1.1 Introduction and Historical Perspective
Mechanochemistry from the contraction of ĀµĪ·ĻĪ±Ī½Ī¹ĪŗĆ³Ļ mÄkhanikos (mechanic) and ĻĪ·ĀµĆĪ± khÄmia (chemistry) is the study of the evolution of the formation and disruption of chemical bonds upon application of an external force. The terminology āmechanochemistryā was first coined by Ostwald as āthe coupling of mechanical energy and chemical energyā.1 Unlike electromagnetic force or gravity, mechanical force implies contact. Indeed, for a force to be applied on a given object or organism, it entails a connection or at least transmission of forces through a medium. Mechanical energy, often in the form of applied force, is a lesser-known way to initiate chemical reactions than conventional stimuli (e.g. heat, light and electricity). At first glance, it is especially interesting and somewhat intriguing to think of chemistry triggered by forces, as it is not something that is typically taught in your General Chemistry 101 class. However, bond ruptures upon mechanical action surround us from random chain scission as one tears through packaging material or simply as one presses on a surface and senses the mechanical deformation. In prehistory, our ancestors took advantage of mechanochemical phenomena for survival, e.g. drilling wood for fire. Theophrastus of Eresus (ca. 315 BC), Aristotleās student, first recorded that grinding cinnabar in a copper mortar using a brass pestle could reduce it to mercury.2 In the 19th century, Faraday and Lea applied sliding and grinding to study the chemical reactions of solid substances.3
In many other ways though, for the better part of its existence, humankind has tried to use materials that would minimize their alteration upon application of force. For instance, Damascus steel, which dates back to 900 AD, was a metal of choice for weaponry as it remarkably exhibited superplasticity along with incredible levels of hardness. Interestingly, it is also one of the early examples (though unwittingly) of nanotechnology and mechanochemistry as carbon nanotubes as well as cementite nanowires were proven to form during the forging and annealing of Indian wootz steel. It is indeed believed that the combination of heat and mechanical action along with impurities in the ore (including carburizing wood and leave additives) was responsible for the catalyzing the process responsible for the formation of these nanostructures and the characteristic wavy patterns.4 The quest for materials that would best resist mechanical constraints has been a technological driving force throughout the Anthropocene and the field of polymer science is no exception to this pursuit. For example, worldwide many groups have been looking at nanocomposites as a source of reinforcement of lightweight materials, with many of these solutions having made it to commercial products in the fields of transportation, construction or even appliances.5
From Staudinger to Melville,6,7 mechanical force was studied early on and the random scission often seen as a foe engendering undesirable chain scission and weakening or worse causing the rupture of polymeric materials. However, in the early 2000s, a new idea emerged in the realm of polymer science.8 Namely, it became apparent that one could utilize mechanical force to oneās advantage rather than combating it. Sensing, repair or self-stiffening were some of the many putative functionalities proposed in the context of mechanochemistry (which have been achieved today and are described in detail in the following chapters). Interestingly and regardless of whether Nature was a source of inspiration for some of the seminal work, it is worth noting that these very functions are ubiquitous in biological systems. Like in many other areas of the sciences, materials researchers recognized early on that one could learn a tremendous amount of information from studying biological materials and understanding how millennia of evolutions have shaped the functionalities of biological systems. Consequently, the present chapter focuses on the mechanochemical strategies developed by Nature to achieve some of the aforementioned functions. It aims to familiarize materials scientists (who may not always have the opportunity to investigate them) with some of the biomechanics and biophysics reports that cover these intricate (and sometimes not entirely understood) processes. This chapter was written from a polymer scientistās perspective and will therefore remain partial and sometimes simplistic. This āeditorial lineā is substantiated by two main reasons. First, the audience of this book is presumed to be mostly polymer researchers and it would be illusory to condense the complexity of biological mechanotransduction pathways and make it fully accessible in a few pages. Second, the point of this chapter is rather to inspire materials scientists, and provide them with a general overview of the strategies utilized by living organisms to sense and adapt to mechanical constraints. Consequently, a broad stroke approach seems more adequate as it would appear nonsensical and unachievable to reproduce exactly what Nature does to transduce mechanical forces. Instead, one can draw overarching guiding principles upon which to base his/her reflection and ultimately the design of biomimetic mechanotransducers. Interestingly, there are already several reports of materials which are (wittingly or not) akin to biological systems and, whenever suitable, we will draw a parallel between artificial and biological systems.
1.2 Biomimetism and Rationale for Emulating Mechanotransduction Pathways
Before delving deeper into the intricacies of mechanotransduction, it is worth doing two things: (i) remembering some of the key principles of how to approach biomimetism and (ii) explaining the reason why mechanotransduction is critical for life.
1.2.1 Principles of Biomimetism and Strategies to Implement It
In 1994, a team of researchers led by David Tirrell laid the foundation for the study of materials of biological origin, specifically hierarchically organized structures.9 Such hierarchy is also present in mechanotransduction schemes and it stands to reason that some of these general ideas developed in their report are readily applicable to mechanotransduction. Tirrell and coworkers highlighted some of the commonalities in materials of biological origin, which are worth recalling. Particularly, we will highlight the properties as they pertain to mechanotransduction.
Evolutionary engineering has advanced by means of an iterative process whereby the structure was refined slowly at each generation. As will become apparent, transduction pathways often find a great deal of commonalities, probably originating from shared ancestral strategies. Many of the elementary units (evidently nuclear bases and amino acids but not only, e.g. microfibrils) are recurrent such that function often comes from specific assembly rather than the building blocks themselves. This means for instance that orientation control plays a critical role in determining the assembly responsiveness. The latter is usually adapted and gradual, and varies according to the task performed (e.g. slow adaptation vs. imminent danger). Shape plays a pivotal role in dictating and modulating the response and shape complexity ensures the right response to the right cue. Since the (mechanotransduction) tasks often prove repetitive, resistance, durability and resiliency are essential. These properties are often predicated upon subjacent principles of reversibility or better yet of mendability. To achieve these paramount functions, Nature often capitalizes on the utilization of non-covalent forces as well as out-of-equilibrium dynamics.10 While the former are readily achievable and have been realized in the field of mechanochemistry (see Chapter 5), the latter is harder to implement in synthetic systems as dissipative structures are still a research curiosity. It is interesting to note that many of the biological systems involved in mechanotransduction in cells are either in kinetically trapped states (e.g. folded proteins, cell membranes) or simply in a dissipative state (e.g. the formation/rearrangement of the cytoskeleton to accommodate deformations).
Also of interest is the realization that the mechanical coupling usually occurs between objects of disparate sizes. Interestingly, the interactions maintaining these objects together are often weak and their chemical and thermal stabilities moderate. In that sense, artificial systems are likely to display superior performance and will therefore be more adapted to some of the harsh requirements of materials used in various applications (e.g. transportation or armament). By combining this enhanced resistance of man-made assemblies with the hierarchical notions of biological materials, one can anticipate the creation of tantalizing responsive structures. When considering the latter, one must also keep in mind the necessity to direct the self-assembly processes and to have them happen at a fast rate. While biological materials assemble at a slow speed (often due to concurrent interactions), any sort of technologically relevant artificial assembly would require high-throughput procedures to warrant economical viability. While Nature can afford long times for assembly to sustain life, it is indeed desirable to operate at greater celerity as one aims towards the widespread implementation of smart materials (i.e. materials capable of adapting their environment). Indeed, such materials typically require long fabrication time and are consequently rather costly. Of concern also when designing next-generation mechanoresponsive polymers is the introduction of environmental cost in the equation of the design of said materials. Much like Nature chose to use limited raw materials, it would make sense in the long run to limit oneself to non-deleterious starting materials and/or materials that can be easily recycled/reused or degraded.
Whenever possible, the best approach to ensure the success of biomimetic strategies takes advantage of the synergistic combination of diverse expertise (e.g. biophysicists, to synthetic chemists, material scientists, computational chemists and bioengineers). It is only through the combination of these know-hows that we will succeed in developing not only passively smart materials (i.e. that will be pre-programmed to respond by means of set transitions) but also actively smart materials (i.e. whereby a feedback loop serves to modulate the response as proposed in Chapter 7 of this monograph).
1.2.2 Introduction of the Importance of Mechanotransduction Pathways for Living Organisms
The ability to sense force is of paramount importance for the survival and the favorable evolution of complex organisms comprised of many cells. The latter themselves are constantly experiencing a slew of mechanical actions: flow, elongation, pressure and pressure waves. The ability to change as a response to the nature of their surrounding environment is capital in the development and the subsistence of living organisms. This feedback is ...
