Introduction

The quest to mimic nature is now reaching a new chapter. Whereas in the past, structural and functional characteristics served as inspiration for designs and materials, with attention given to mesoscopic and, perhaps, microscopic aspects, the arsenal of new experimental techniques and computational methods is descending to the nanometer scale. This is the scale at which atoms assemble into molecules, and molecules form molecular arrangements such as DNA, RNA, plasmids, and proteins.

Prominent characterization techniques available to modern researchers are (e.g. Gronau et al., 2012):

• transmission and scanning electron microscopy at higher and higher resolutions;

• micro and nano computerized tomography;

• atomic force microscopy;

• fluorescence spectroscopy;

• Fourier transform infrared spectroscopy (FTIR);

• dynamic light scattering;

• X-ray diffraction.

• small-angle X-ray scattering (SAXS);

• nanomechanical testing (nanoindentation);

The principal computational methods are:

• density functional theory (DFT);

• quantum-mechanical and ab initio methods;

• coupled cluster method (CCSD);

• reactive and nonreactive molecular dynamics;

• coarse graining;

• finite element method;

• computational fluid mechanics.

Early developments

For one brief moment, as one of us (MAM) walked into one of the fabled rooms of the British Museum, he was handed a tool used by early hominids two million years ago. The stone had a barely recognizable sharp edge but possessed a roundish side that fit snugly into the hand. It could have been used to cut through meat, scrape a skin, or crack a skull (Fig. 1.1). It was a brief but emotional event until the zealous anthropologist removed it from the hand that eagerly clasped the artifact and imagined himself deep in the Olduvai Gorge, slicing through the hide of a gazelle that had been hunted down by the group. This connection is at the heart of this book.

The first materials were natural and biological: stone, bones, antler, wood, skins. Figure 1.2(a) shows an Ashby plot of strength vs. density, for early neolithic materials. These natural materials gradually gave way to synthetic ones as humans learned to produce ceramics, then glass and metals. Some of the early ceramics, glasses, and metals are also shown in the plot, and they provide added strength. These synthetic materials expanded the range of choices and significantly improved the performance of tools. The long evolution of materials, from the stone shown in Fig. 1.1 to the cornucopia of materials developed in the past century, is shown in Fig. 1.2(b). Contemporary materials are of great complexity and variety, and they represent the proud accomplishment of ten thousand years of creative effort and technological development.

Introduction

Many biological structures require light weight. The prime examples are birds, whose feathers, beaks, and bones are designed to maximize performance while minimizing weight. Other examples are plants, where the same requirements operate. Figure 10.1 illustrates two examples of structures designed to have significant flexure resistance.

Plant stalks are composed of cellulose and lignin arranged in cells aligned with the axis of growth. The giant bird-of-paradise stem is lightweight and has a structure consisting of cells which have, on their longitudinal section, a rectangular (but interconnected in a staggered manner) shape (Meyers et al., 2013) (Fig. 10.1(a)). The cross-sectional shape is closer to elliptical. Thus, the cells have a cylindrical shape. If we look at the cell walls at a greater magnification, we observe that they are also cellular. This produces two levels of cellular structure, in a manner akin to the feather rachis in Falco sparverius, presented in Chapter 2 (see Fig. 2.1). The solution often found in nature is to use tailored structures to address the stress demands. The structure is designed to resist flexure stresses without buckling.

Introduction

There are numerous silicate- and calcium-carbonate-based biominerals in biological systems. Their function is, for most of them, protection. They provide a hard shell beneath which the softer organism can survive and prosper.

Exceptions are the sponges. We will provide illustrative examples and describe the structure-mechanical property relations for a few representative examples. We first cover the most important silicates and then move to shells. The number of different species is staggering: over 100 000 living species bear a shell, ranging from a single valve to eight overlapping valves. There are roughly 1000 species of mussel bivalves.

For shells, we rely here primarily on the ones studied by the UC San Diego group: abalone, conch, giant clam, and ocean and river bivalve clams. They possess quite different and unique structures, and are representative of the large number of shell species.

We also describe other carbonate-based hard materials such as the sea urchin, the smashing arm of the mantid shrimp, and the ubiquitous egg shell.

Introduction

Materials can be divided into two classes: structural and functional. Structural materials are the ones where mechanical properties are of foremost importance: strength, hardness, toughness, ductility, and density are some of their most important characteristics. Functional materials, on the other hand, have other primary attributes: optical, magnetic, electrical, superconducting, and energy storage are some of the more important properties for which they are developed.

In biological materials, these boundaries are not so clear because many are multifunctional. Nevertheless, some have as primary function the sustainment of the structure, while others have other functions as principal attributes.

In this chapter, we present the principles of attachment with emphasis on gecko feet, which are, not coincidentally, the subject of the cover of this book. This is a fascinating subject, and several research groups are feverishly working on it with the goal of creating synthetic reversible attachment devices. Next we present the superhydrophobic effect through which the surface of the lotus plant remains clean. Water simply rolls off the surface, taking with it all particles. Another interfacial property that is important is the shark skin; the scales have a morphology and configuration that generate microvorticity in the water, decreasing drag. Biological materials and systems also have outstanding optical properties: chameleons change color through a complex interplay of chromatophores, for example. Some butterflies have wings with features that act as photonic arrays. The deep blue of the Morpho butterfly is the result of the interaction of the light with nano-scale arrays. The iridescent feathers of some birds are the result of the interference of light with arrays on the barbules, which have a periodicity close to the wavelength of light. The iridescent throat patch of humming birds is an eloquent example. We review these and some other aspects in this chapter. The presentation is by no means exhaustive, but illustrates the nature of functional biological materials.

Introduction

With the rapid advances of materials used in science and technology, various intelligent materials that can sense variations in the environment, process the information, and respond accordingly are being developed at a fast pace. Shape-memory alloys, piezoelectric materials, etc., fall into this category of intelligent materials. Polymers have attractive properties compared to inorganic materials. They are lightweight, inexpensive, fracture tolerant, pliable, and easily processed and manufactured [1]. An organic polymer that possesses the electrical, electronic, magnetic and optical properties of a metal while retaining the mechanical properties and processability, etc., commonly associated with a conventional polymer, is termed an “intrinsically conducting polymer” (ICP), or more commonly, a “synthetic metal” [2]. The unique properties of these materials are highly attractive for a wide range of applications such as actuators, supercapacitors, batteries, etc. With the development of nanotechnology, these materials can be engineered to develop a variety of multifunctional active materials for intelligent applications that were previously imaginable only in science fiction. Figure 8.1 shows an artistic interpretation of the Grand Challenge for EAP actuated robotics.

Introduction

The cell is one of the most complex machines known. Life is associated with cells, and there are researchers who feel that we will never be able to create a cell. The analogy with a Boeing 747 illustrates this. This airplane, one of the most complex machines ever built, has approximately six million parts (including rivets, screws, etc.), whereas the typical mammalian cell has 10 000 different proteins (~50 000 of each), for a total of 500 million parts (P. LeDuc, Carnegie-Mellon University, private communication). Added to these are the ATP molecules, of which there are approximately six billion per cell. Thus, the task of reproducing a cell, close to 100 Boeings, in a volume of approximately hundreds of cubic micrometers, is daunting.

How was the first cell formed? This is indeed the definition of life, because a cell can undergo mitosis, a process by which it generates another cell. There are different hypotheses, none of which have yet been substantiated.

• Astrobiology, a new field of study, explores the possibility of life in outer space; it is possible that life could have been transported to earth embedded in meteorites.

• The classic Miller–Urey experiments were conducted, in which Miller and Urey created a “primeval soup” and passed a high current through it, synthesizing more complex organic molecules. However, there is evidence that the early atmosphere was quite different. As an encouragement to graduate students, Miller was a rather testy one who asked Urey, then already a well-known professor, to set up such an apparatus. Reluctantly, Urey agreed and, to his surprise, the success came early and easily. This paper had far-reaching consequences. Alas, Miller never got a Nobel Prize for his experiment, but Urey did. Another warning for graduate students.

• The vent holes in the bottom of the ocean. It has been proposed these vent holes could have given rise to life.

Organisms are classified into prokaryotic and eukaryotic types. Prokaryotes are all unicellular and are divided into archaea and bacteria. Eukaryotes are the other organisms, including us, homo sapiens. Prokaryotic cells are simpler and smaller than eukaryotic cells, and they do not have a nucleus. The eukaryotic cells have diameters 10–100 µm, whereas the prokaryotic cells have diameters of 1–10 µm. Whereas bacteria are unicellular, humans contain 1013 cells. As a comparison, there are approximately 1010 humans on this planet. These cells control all the processes in our body.

Introduction

A considerable number of books and review articles have been written on biological materials, and they constitute the foundation necessary to embrace this field. Some of the best known are given in Table 2.1. Table 2.2 lists some of the key review articles in the field. An important step was taken by D’ Arcy Thompson with his monumental book, On Growth and Form, initially published in 1917 (Thompson, 1917). This book still constitutes exciting reading material and is a valiant attempt at representing biological shapes mathematically.

Hierarchical structures

It could be argued that all materials are hierarchically structured, since the changes in dimensional scale bring about different mechanisms of deformation and damage. However, in biological materials this hierarchical organization is inherent to the design. The design of the structure and of the materials are intimately connected in biological systems, whereas in synthetic materials there is often a disciplinary separation, based largely on tradition, between materials (materials engineers) and structures (mechanical engineers). We illustrate this by presenting four examples in Fig. 2.1 (avian feather rachis), Fig. 2.2 (abalone shell), Fig. 2.3 (crab exoskeleton), and Fig. 2.4 (bone).

Introduction

Chapters 6 and 7 covered the structure, at different hierarchical levels, of biological mineral-based composites (see the Ashby–Wegst classification in Fig. 2.11), and explained the mechanical properties in terms of it. There are also a number of biological composites that do not contain minerals, or in which minerals appear only in small proportions. This is what will be studied in this chapter. We differentiate Chapter 8 by covering the biological polymers that have extensive “stretchability” in Chapter 9.

We start with the very important components, tendons and ligaments that join bone and muscle, and bone and bone, respectively. They are primarily composed of collagen. An extraordinary biological material, spider silk, is introduced next. The strength level can reach and exceed 1 GPa. If we normalize this by dividing it by the density (~1 g/cm3), we obtain a material that is considerably stronger than our strongest steels (~3 GPa, with density of 7.8 g/cm3). Molecular dynamics computations explain how this high strength is accomplished with the weak hydrogen bond.

Introduction

Although there are over 80 minerals present in biological systems, the most important are hydroxyapatite (HAP) (mammals and fishes), calcium carbonates (shells, arthropods, corals), and silica (diatoms, sponges). These minerals seem to have evolved from approximately 560 million years ago.

Minerals are essential for providing compressive strength to biological systems, whereas biopolymers are primarily responsible for tensile strength. The combination of minerals and biopolymers leads to the formation of biological materials with mechanical properties tailored in terms of hardness, toughness, and anisotropy. The formation of minerals involves nucleation and growth, both mediated by biological components. The organic matrix mediates nucleation in many ways: by providing nucleation sites and by controlling the polymorphs. The growth is also mediated by organic compounds, and illustrative examples are given in Chapters 6 and 7. For instance, the rapid direction of growth for aragonite is the c-direction, and long needles are formed. In nacre, this growth is regulated by the periodic deposition of organic layers. In bone, the HAP crystals nucleate in the interstices of collagen fibrils and growth is also regulated: they reach sizes on the order of nanometers: 40–60 nm long, 20–30 nm wide, and platelets of a few nanometers thickness are formed in such a fashion.

Introduction

A nanocomposite is a material in which the matrix contains reinforcement materials having at least one dimension in the nanoscale (<100 nm), wherein the small size offers some level of controllable performance that is expected to be better than in conventional composites. In another words, these nanocomposites should show great promise either in terms of superior mechanical properties, or in terms of superior thermal, electrical, optical and other properties, and in general, at relatively low-reinforcement volume fractions [1, 2]. The principal properties for such reinforcement effects are that (1) the properties of nano-reinforcements are considerably higher than the reinforcing materials in use and (2) the ratio of their surface area to volume is very high, which provides a greater interfacial interaction with the matrix [1]. Table 9.1 shows the geometries, types and surface-to-volume relations of reinforcements and their arrangement modes in fiber composites.

Table 9.2 lists the typical functional nanoparticles and matrices that have been used for the composites. Among all the nano-reinforcements, carbon nanotubes (CNTs), nanoclay, graphene and nanofibers are the most usually involved materials for the structural nanocomposites that are introduced in this chapter.

Polymer, metallic and ceramic materials in fibrous form are of fundamental importance in materials engineering. Fibrous materials are the basic building blocks for the backbone of most natural and man-made engineering structures, ranging from the skeletal structure of animals to advanced fiber-reinforced composites.

Fiber assemblies normally known as textile materials are unique in their combination of strength and toughness, lightweight, flexibility and cost effectiveness. As an essential requirement to fiber and fiber assemblies, mechanical properties are one of the most important properties that need to be characterized and investigated. In this chapter, we will consider the mechanical properties of fiber assemblies from single fiber to fiber assemblies in a hierarchical manner.

Structure of hierarchy of textile materials

Traditionally fibers are defined as soft materials with a length-to-diameter ratio above 103 and a diameter ranging from several to 100 microns. The emergence of nanofibers broadens the span of fibers to the nanoscale world.

For engineering applications, fibers are usually employed in different forms such as yarns/ropes, woven textiles and nonwoven textiles. The structure hierarchy of textile materials is shown in Fig. 5.1.