Biological materials

Nature has evolved a palette of biological materials to address different structural requirements such as:

• hardness,

• toughness,

• stretchability,

• light weight.

The intricate and ingenious hierarchical structure is responsible for the outstanding performance. Toughness is conferred by the presence of controlled interfacial features, buckling resistance can be achieved by filling a slender column with a lightweight foam, and armor protection is accomplished by small dermal plates with unique attachment arrangements, resulting in controlled and prescribed flexibility. In Chapters 6–10 we present and interpret selected examples of biological materials. In addition to the structural requirements, there are also functional requirements such as adhesion and optical properties.

The number of elements and compounds that can be synthesized at ambient temperature and in aqueous environments is limited, and therefore the architecture of the structure is of utmost importance.

We introduce the different classes of biological materials in these chapters, following the Wegst–Ashby classification. These were defined in Chapter 2 (Fig. 2.11) as:

• biominerals (Chapters 6 and 7),

• biopolymers (Chapter 8),

• bioelastomers (Chapter 9),

• biocellular materials (foams) (Chapter 10).

How big is a nanometer?

By definition, a nanometer, abbreviated as nm, is a unit for length that measures one billionth of a meter. (1 nm = 10−3 μm = 10–6 mm = 10−7 cm = 10−9 m.) Our hair is visible to the naked eye. Using an optical microscope we can measure the diameter of our hair, which is in the range of 20–50 microns (μm) or 20 000–50 000 nm. Blood cells are not visible to the naked eye, but they can be seen under the microscope, revealing a diameter of about 10 microns or 10 000 nm. The diameter of hydrogen atoms is 0.1 nm. In other words 10 hydrogen atoms can be placed side-by-side in 1 nm. Figure 1.1 provides an excellent illustration of the relative scales in nature. The discovery of nanomaterials ushered us to a new era of materials. We have progressed from the microworld to the nanoworld.

What is nanotechnology?

According to the National Science Foundation in the United States nanotechnology is defined as [1]:

Introduction

It is a misunderstanding to think that the ultimate goal of biomimetics is to reproduce living organisms. There are essential differences, as we list in the following.

What we try to do is to emulate the design and assembly principles used in natural materials. In this book we have seen many examples where superior properties are obtained through a hierarchical design and ingenious solutions. Bone, nacre, and dentin have toughnesses significantly superior to those of the mineral constituents, hydroxyapatite and calcium carbonate. Silk reaches strengths higher than 1 GPa using the weak hydrogen bond, through the existence of nano-scale β-sheet crystals with proper dimensions. Bioinspiration requires identification, understanding, and quantification of natural design principles and their replication in synthetic materials, taking into account the intrinsic properties (Studart, 2012). This approach is being pursued not only for structural materials, but also for functional materials and devices. The areas of sensing, optics, architecture, and robotics are exploring biological solutions.

The development of biomaterials

A biomaterial has been defined by Hench and Erthridge as a synthetic material used to replace a part or a function of the body in a safe, reliable, economic and physiologically acceptable manner. The Celmson University Advisory Board for Biomaterials has formally defined a biomaterial to be “a systemically and pharmacologically inert substance designed for implantation within or in a medical device, intended to interact with biological systems.” Biomaterials have been widely used in many areas, including replacement of damaged parts (artificial hip), assisting in healing (sutures), improving biological functions (pacemaker, contact lens), correcting abnormalities (spinal rods), cosmetics (augmentation mammoplasty), aiding diagnoses (probes) and aiding treatment (catheters). A material is considered biocompatible if it causes no irritation, allergic or toxic responses when used in a biological system [1]. Table 7.1 provides some examples of biomaterials used in the body.

Biotechnology and nanotechnology are the two of twenty-first century's most promising technologies. Convergence of these two technologies is expected to create innovations and play a vital role in various biomedical applications. The symposium in 2000 entitled “Nanoscience and Technology: Shaping Biomedical Research” held by the National Institutes of Health Bioengineering Consortium (BECON) addressed eight areas of nanoscience and nanotechnology, which include synthesis and use of nanostructures, applications of nanotechnology to therapy, biomimetic and biologic nanostructures, electronic–biology interface, devices for early detection of disease, tools for the study of single molecules, nanotechnology and tissue engineering [2].

Introduction

Hydroxylapatite or hydroxyapatite (HAP) is a calcium-phosphate-based mineral of the apatite family. Its chemical formula is Ca10(PO4)6(OH)2. It can be found widely in nature and is the major component of bone, enamel, and dentin in teeth, antler, ganoid fish scales (in alligator gar and Senegal bichir), turtle shells, and armadillo and alligator osteoderms. It exists in minute quantities in the brain (brain sand), without significantly affecting its function. Thus, the expression “having sand in the head” is not without reason. The density of HAP is 3.15 g/cm3. Nonstoichiometric minerals can exist with Ca10(PO4)6(OH, F, Cl, Br)2; if the OH group is replaced by F it is called fluoroapatite; if it is replaced by Cl, it is called chloroapatite. It can be occasionally used as a gem, and the cat’s eye is a commonly known use.

In this chapter, we will concentrate on bone and teeth with emphasis on their structure and mechanical properties. They are HAP–collagen composites and their mechanical properties are the result of the complex interplay and hierarchy built by these structures. Selected calcium-phosphate-based bony tissues with unique functionalities, such as antler, turtle shells, alligator osteoderms, and fish scales, will also be described in the second part of this chapter.

Nanofiber-forming technology

There are a number of techniques capable of fabricating nanofibers. These techniques include conjugate spinning, chemical vapor deposition, drawing, template synthesis, self-assembly, meltblown and electrospinning.

Conjugate spinning (island in the sea)

Sea–island-type conjugate spinning involves extruding two polymer components from one spinning die. The fiber islands are arranged in a sea component which is later removed by extraction. Nakata et al. reported that continuous PET nanofibers with a diameter of 39 nm could be obtained by sea–island-type conjugate spinning from the flow-drawn fiber with further drawing and removal of the sea component. Figure 3.1 shows a TEM image of a PET fiber island and Nylon-6 sea produced by conjugate spinning and flow-drawing [1].

Chemical vapor deposition (CVD)

In a CVD process, a substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface. The desired deposit is synthesized on the substrate surface. The volatile by-products are produced during the process and are removed by gas flow through the reaction chamber. The various forms of material that can be produced via CVD include monocrystalline, polycrystalline, amorphous, and epitaxial. Some examples of such CVD-fabricated materials are silicon, carbon fiber, carbon nanofibers, filaments and carbon nanotubes [2]. Figure 3.2 shows a schematic illustration of a plasma-enhanced CVD setup that can be used for fabricating single-walled carbon nanotubes.

Bioinspired materials and biomimetics

The ultimate goal for a materials engineer is to learn from the lessons of nature and to apply this knowledge to new materials and design. This is not a new quest, and humans have sought inspiration from nature since prehistory. The early materials used by humans were primarily natural: stones, bones, wood, skins, bark. The accelerating pace of the civilizing process has been attributed to the introduction of new synthetic materials; thus, the bronze and iron ages followed the stone age. We have now entered the brave new world of the silicon age, which is bound to produce unimaginable change. Homo silicensis, connected 24/7 to computerized contraptions, can already be seen on campuses and elite coffee houses, sipping lattes.

The constant quest for new materials and designs is leading us to a systematic inquiry of nature in order to unravel its secrets. This is the field of biomimetics, and VELCRO® is the standard example of bioinspired design. It is inspired by the burrs of plants that contain small hooks that attach themselves to animal wool or our clothes.

Future of nanotechnology

In 2000, Roco et al. [1] estimated that there would be two million nanotechnology workers worldwide (800 000 in the United States) and the product value would reach $1 trillion, of which $800 billion would be in the United States, by 2015, with a 25% rate growth. The initial estimation for the quasi-exponential growth in the nanotechnology workforce and the product value held up to 2008, as shown in Fig.10.1. The market is doubling every three years as a result of the successive introduction of new products, and new generations of nanotechnology products are expected to enter the market within the next few years [1]. So the estimated value of 2015 for both workforce and product value would have been realized as the 25% growth rate is expected to continue.

Nanotechnology is evolving toward new scientific and engineering challenges in areas such as assembly of nanosystems, nanobiotechnology and nanobiomedicine, development of advanced tools, environmental preservation and protection, and pursuit of societal implication studies. Key areas of emphasis in nanotechnology [1] over the next decade are as follows.

• Integration of knowledge at the nanoscale and of nanocomponents in nanosystems with deterministic and complex behavior, aiming toward creating fundamentally new products.

• Better control of molecular self-assembly, quantum behavior, creation of new molecules, and interaction of nanostructures with external fields in order to build materials, devices and systems by modeling and computational design.

• Understanding of biological processes and of nanobio interfaces with abiotic materials, and their biomedical and health/safety applications, and nanotechnology solutions for sustainable natural resources and nanomanufacturing.

Knowing the basic properties of nanofibers (such as morphology, molecular structure and mechanical properties) is crucial for the scientific understanding of nanofibers and for the effective design and use of nanofibrous materials. In order to evaluate and develop the manufacturing process, the composition, structure and physical properties must be characterized to decide whether the produced fibers are suitable for their particular application. Evaluation of the various production parameters in processes such as electrospinning is a critical step towards production of nanofibers commercially. Many common techniques used to characterize conventional engineering materials, as well as some not so common techniques, have been employed in the characterization of nanofibers. Table 6.1 shows the scales of fibers and the corresponding characterization techniques. To provide an overall understanding, some of the general characterization techniques for structural, chemical, mechanical, thermal and other properties will be introduced in this chapter.

Structural characterization of nanofibers

The morphological characterization techniques briefly discussed herein are: optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning tunneling microscopy (STM). These methods characterize the morphology and determine fiber diameter, pore size and porosity, all of which are necessary to evaluate the various production parameters. The techniques for characterization of order/disorder of molecular structures using X-ray diffraction (XRD) are also covered in this section. Furthermore, mercury porosimetry, a special technique for porosity measurement, is introduced.

In nanotechnology, polymers play a very important role as one of most often employed materials, especially in the fields of nanofibers and nanocomposites. Hundreds of polymers, including natural and synthetic polymers, have been fabricated into nanofibers and nanocomposites in the past 20 years. Thus a fundamental understanding of polymers, especially fiber-making polymers, is essential for people in various fields such as the biological, medical, electrical and material areas that are converging with nanotechnology.

Polymeric materials

The first polymers to be exploited were natural products such as wood, leather, cotton and grass for fiber, paper, construction, glues and other related materials. Then came the modified natural polymers. Cellulose nitrate was the one that first attained commercial importance for stiff collars and cuffs as celluloid in around 1885. Notably, cellulose nitrate was later used in Thomas Edison's motion picture film. Another early natural polymer material was Chardonnet's artificial silk, made by regenerating and spinning of cellulose nitrate solution, which eventually led to the viscose process that is still in use today. The first synthetic polymer was Bakelite, manufactured from 1910 onward for applications ranging from electrical appliances to phonograph records. Bakelite is a thermoset, that is, it does not flow after the completion of its synthesis. The first generation of synthetic thermoplastics (materials that could flow above their glass transition temperatures) are polyvinyl chloride (PVC), poly(styrene–stat–butadiene), polystyrene (PS), and polyamide 66 (PA66). Other breakthrough polymers include high modulus aromatic polyamides, known as Kevlar™, and a host of high temperature polymers. Table 2.1 lists some of the polymers currently often encountered.

Classification of fibers

A key objective in electrospinning is generating fibers of nanoscale diameter consistently and reproducibly. Considerable effort has been devoted to understanding how the parameters affect the spinnability and more specifically the diameter of the fibers resulting from the electrospinning process. Many processing parameters that influence the spinnability and the physical properties of nanofibers have been identified. These parameters include process parameters such as electric field strength, flow rate and spinning distance, spinning dope properties including concentration, viscosity and surface tension, etc., the spinning environment factors like humidity and the spinning setup factors such as the diameter of the orifice and the electrospinning angle. Through observation of the electrospinning process and analyzing these parameters, some governing models have been built and simulations of the motion of jet have been carried out. In this chapter, several main existing models and simulation works will be introduced to help readers gain an understanding of the concept of electrospinning.

Electrospinning mechanism

For a long time, the mechanism of electrospinning for forming nanoscaled fibers was believed to be a result of a “split” as seen by the naked eye (Fig. 4.1a). The “splitting” is explained by Doshi and Reneker [1, 2] that, as the jet diameter decreases, the surface charge density increases, resulting in high repulsive forces which split the jet into smaller jets splay. When a high-speed camera was used in the investigation of electrospinning jet, unstable bending, also known as “whipping” of jet, was observed, and the “bending instability” started being widely accepted as the electrospinning mechanism, as shown in Fig. 4.1b. As described [3, 4], the electrospun jet vigorously bent spirally and stretched inside a conical envelope resulting in a huge stretch ratio and a nanoscale diameter.