Structural Biological Materials

The ongoing process of bio-evolution has produced materials which are perfectly adapted to fulfil a specific functional role. The natural world provides us with a multitude of examples of materials with durability, strength, mechanisms of programmed self-assembly and biodegradability.

The materials industry has sought to observe and appreciate the relationship between structure, properties and function of these biological materials. A multidisciplinary approach, building on recent advances at the forefront of physics, chemistry and molecular biology, has been successful in producing many synthetic structures with interesting and useful properties.

Structural Biological Materials: Design and Structure-Property Relationships represents an invaluable reference in the field of biological materials science and provides an incisive view into this rapidly developing and increasingly important topic within materials science.

This book focuses on the study of three sub-groups of structural biological materials:

• Hard tissue engineering, focussing on cortical bone
• Soft tissue engineering
• Fibrous materials, particularly engineering with silk fibers.

The fundamental relationship between structure and properties, and certain aspects of design and engineering, are explored in each of the sub-groups. The importance of these materials, both in their intrinsic properties and specific functions, are illustrated with relevant examples. These depict the successful integration of material properties, architecture and shape, providing a wide range of optimised designs, tailored to specific functions.

Edited by Manuel Elices of the Universidad Politécnica de Madrid, Spain, this book is Volume 4 in the Pergamon Material Series.

David F. Williams
Wherever you look in science policy statements these days, you come across phrases such as 'multidisciplinarity', 'interdisciplinary', technology transfer, and so on. Scientists are being encouraged to work at the interfaces between disciplines or to engage in research and scholarship that encompasses more than their parent discipline. Quite specifically, there is a great deal of current attention being paid to the interface between engineering and the life sciences, and within small but significant sub-sectors, between materials engineering and structural biology and between materials chemistry and cell and molecular biology.

These are exciting areas and much is going on. This book, on Structural Biological Materials, edited by Manuel Elices addresses some of the aspects of the interface between biology and materials science by covering the characteristics of natures own materials and describing how they relate to parameters of materials that are traditionally discussed in the context of synthetic engineering materials. As the Editor points out, nature has tended to optimise the performance of its own materials, so there is much to learn. They have provided the blueprints for some of the concepts of smart materials and provide inspiration to materials scientists that try to emulate their performance in the design of novel, so-called biomimetic materials.

The book is divided into four sections, dealing with basic concepts, soft tissues, hard tissues and engineering with fibres.

Both chapters on concepts have been authored by Dr Jeronimidis, a well-respected scientists in the field of biomimetics. These underline the composite nature of most natural materials and introduce the principles of structure - property relationships, using several examples of natural materials for this purpose. The section on hard tissue is a chapter on bone by Ontanon et al. This covers the major structural features of bone and details of its histology. The main section of the chapter not surprisingly covers the mechanical properties of bone, especially the elastic constants and the fracture and fatigue properties. It is rightly concluded that, as much information as there is in the literature on these properties, it has still not been possible to develop models of the behaviour that predict failure.

The properties of soft tissue cover more space and more ground, with special reference to cartilage and tendon, and to so-called bioartificial implants and biomimickry. Bader and Lee have produced an excellent account of the characteristics and mechanical property of cartilage and used this as a basis to describe techniques of tissue engineering within joints. The area of tendon is also discussed by Bader, with a different co-author Schechtman, using the same strategy for construction of the chapter. Brown has produced a very interesting chapter on bioartificial implants, starting with the concepts of tissue repair, and then describing the design principles of bioartificial constructs, including materials for matrices and relevant cells. With all of the current interest in novel tissue engineering approaches to reconstructive surgery, some sound principles and factual sections can be found here. The biomimetic smart materials described by Otero include electroactive polymers, specifically with respect to artificial muscles, and organo-aqueous batteries and 'electric organs.'

The remainder of the book discusses engineering characteristics of fibres, with specific reference to a few fibrous biological systems. Renuart and Viney introduce the topic with a discussion of biological fibrous materials, self assembled structures and the optimised properties that can be achieved. It naturally concentrates on proteins and polypeptides and materials such as wool, hair, horn, beaks and claws. It is not surprising to note the prediction in the summary of this chapter that whole nanomachines will result from the examples provided by the self assembly protocols learned from nature when one considers the exquisite form and structure-property relationships in components such as insects antennae, which are self-assembled fibrous structures that are extremely small but mechanically robust, have the ability to undergo controlled and rapid changes in shape and orientation, are self repairing and can detect and process chemical and thermal information.

Silk is one of the most interesting of the natural fibres and is discussed by Viney in a separate chapter. The material has some attractive properties but as yet has attracted little engineering interest because of the unpredictable mechanical properties, poor heat tolerance and degradability. However, new biotechnological techniques open up channels for the reproducible synthesis of silk like structures that should have far better performance. The chapter has a fascinating section on the lessons to be learnt from silk for the molecular and microstructural design of engineering polymers, primarily based on the hierarchical structure. Also in this last group of chapters are two contributions from Termonia, one on computer models for the mechanical properties of fibres in general and one on the modelling of stress-strain behaviour of spider dragline. In the latter case, the material is described as one of the strongest of all known materials, with the added benefit of a very high strain at break and toughness. It is a strongly hydrogen bonded polymer in which small crystalline sheets create inside an amorphous phase a thin layer of very high modulus.

This book makes very interesting reading for biologists who are interested in structure - property relationships in the materials they work with and for materials scientists who wish to learn the lessons of natural history, where evolution has resulted in some exciting, smart, high performance materials.