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Overview
With tremendous growth over the last five years, mechanochemistry has become one of the most important topics in current polymer science research. With a particular focus on polymers and soft materials, Mechanochemistry in Materials looks at the subject from the application of macroscopic forces to solid systems of macroscopic dimensions.
The book has been divided according to length scale covering both experimental and theoretical considerations simultaneously. The first section of the book focuses on inspiration from nature, exploring and explaining multiple biological phenomena. The second section discusses molecular mechanochemistry, including the theoretical understanding of the transduction of mechanical force and its impact on covalent bonds cleavage and formation. The final section considers the implementation of these phenomena at the mesoscale and discusses the use of supramolecular/reversible aspects with similarities to biological systems.
The book provides a unique comparison with natural systems and contains all the important achievements in the area from the last decade. Appealing to a broad range of materials scientists, working in industry and academia, this well-presented and comprehensive title will be essential reading for researchers.
Product Details
| ISBN-13: | 9781788012393 |
|---|---|
| Publisher: | Royal Society of Chemistry |
| Publication date: | 10/24/2017 |
| Series: | ISSN |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 196 |
| File size: | 4 MB |
Read an Excerpt
CHAPTER 1
Mechanochemistry: Inspiration from Biology
TAMUKA CHIDANGURO, WENGUI WENG AND YOAN C. SIMON
University of Southern Mississippi, Hattiesburg, MS, USA
1.1 Introduction and Historical Perspective
Mechanochemistry from the contraction of [TEXT NOT REPRODUCIBLE IN ASCII] mekhanikos (mechanic) and [TEXT NOT REPRODUCIBLE IN ASCII] khemia (chemistry) is the study of the evolution of the formation and disruption of chemical bonds upon application of an external force. The terminology "mechanochemistry" was first coined by Ostwald as "the coupling of mechanical energy and chemical energy". 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), Aristotle's student, first recorded that grinding cinnabar in a copper mortar using a brass pestle could reduce it to mercury. In the 19th century, Faraday and Lea applied sliding and grinding to study the chemical reactions of solid substances.
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 nano-structures and the characteristic wavy patterns. 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.
From Staudinger to Melville, 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. Namely, it became apparent that one could utilize mechanical force to one's 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 scientist's perspective and will therefore remain partial and sometimes simplistic. This "editorial line" 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. 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. 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 necessary for a variety of key biological functions such as proliferation, differentiation, motility or cell death. It also plays a role in organ growth, bone adaptability and homeostasis (viz. the maintenance of parameters, e.g. pH, sugar, within a normal range for an organism). By analogy, for smart materials, one could anticipate systems that adapt to mechanical load or vibration, detect and report failure as well as initiate needed mending cycles. The implementation of feedback loops and graduated responses is a necessary condition for the future development of better smart materials. In living organisms, deficient feedback loops in mechanotransduction are responsible for numerous diseases such as cardiomyopathies, cancer metastasis or muscle degeneration. At the cellular level, mechanical deformation triggers diverse signaling pathways or induces changes in ionic concentrations, which in turn modulates the response appropriately (adjustment of the stiffness of the cell, its shape or even cross-talk with the extracellular matrix).
When one is thinking about the notion of mechanotransduction, the first things that probably come to mind are the senses of touch or hearing. Sensory cells are responsible for both. These cellular structures are hyper-specialized and have evolved to transduce mechanical inputs into given signals (e.g. flow of Ca2+ ions through transmembrane proteins to elicit an action potential in neurons). Sensory cells were logically amongst the first ones to be studied, as they offered a convenient model for mechanosensing. However, mechanotransduction goes far beyond the mere sensory pathways. It is important in the proper functioning of the vascular system and, in particular, in the role of the heart and its development. It shapes the bones and their strength as a result of muscle contractions and external forces (e.g. gravity or hits). Think of the ability of a martial arts expert to strengthen his/her shins or forearms to endure greater impact and administer harsher blows, or even patients encouraged to walk to limit osteoporosis and encourage bone growth. This self-strengthening mechanism is highly desirable and has been the source of inspiration for exquisite studies in the field of synthetic mechanochemistry. For instance, Black Ramirez et al. have demonstrated the possibility to induce mechanochemical strengthening as a response to shear forces in a poly(butadiene) system containing gem-dibromocyclopropane along the chain and infused with the ditetrabutylammonium salt of sebacic acid. Upon ring-opening of the three-membered propane cycle, one bromo-group thus formed can react with the dicarboxylate, thereby promoting the sought-after cross-linking reaction and subsequent reinforcement. Embryonal cell fate is also directed by external forces applied to the cell, which helps in the coordination of tissue growth. Also, said forces have been shown to influence stem-cell differentiation and have been proposed as a way to replace diseased tissues. Respiration cycles and the mechanical feedback loop associated with them contribute to the lung homeostasis.
Interestingly, in the body, all these circuits work in parallel, meaning that the overall response is the fruit of multiple mechanotransduction events. Wang et al. introduced the concept of gating in polymers as a means to control stress-response. The incorporation of cyclobutane served as a "gate" to control the activation of gem-dichlorodicyclopropanes. The single-chain force spectroscopy measurement indeed demonstrated the power of that gating mechanism for the design of next-generation materials. While this is rather limited in scope as compared to much more sophisticated biological systems, it is certainly a step in the right direction. Therefore, by combining mechanophores and/or specific architectures, one can readily envision creating systems that will get closer to emulating the intricate mechanotransduction pathways.
Mechanotransduction approaches can be broadly classified into those involved in sensory mechanisms (i.e. involving ion channels) and those mediated by more complex signaling pathways (i.e. entailing the coordination of a cascade of biochemical processes leading to the desired response). The former rely on the concerted rearrangement of vast supramolecular ensembles, such as lipid bilayers and pore-forming structures, while the latter are more often than not the consequence of protein unfolding and molecular conformational changes. The former are typically fast, while the latter often have latency times in excess of minutes. Interestingly, and as stated previously, both strategies involve non-covalent interactions. By reducing the number of bonds to break and form, Nature optimizes its energetic savings and limits the penalty associated with what would be a complex return to the pre-load state (equilibrium or metastable). Orr and coworkers have proposed the hypothesis that specialized cells employ mechanisms for mechanotransduction similar to the rest of the cells but obtain greater sensitivity by magnifying the strains experienced by the primary transducers.
(Continues…)
Excerpted from "Mechanochemistry in Materials"
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