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MATERIAL MATTERS COLUMN

Self assembly in nature

Professor Williams retired from the University of Liverpool, after 40 years, at the end of 2007. He retains the position of Emeritus Professor there and now has a series of professorial appointments in North Carolina, USA; Sydney, Australia; Cape Town, South Africa; and in China. He offers consulting services from his company Morgan & Masterson, based in Brussels, Belgium. He is Editor-in-Chief of Biomaterials, the leading journal in the biomaterials field. He is Scientific Director of STEPS, the European Commission Framework VI Programme on a Systems Approach to Tissue Engineering Products and Processes.

As an old fashioned metallurgist I was brought up in the world of top-down manufacturing. Alloys were prepared by mixing constituents in big containers, melted in furnaces and cast into ingots. If small intricate objects were required, then the big slabs of metal were reduced in size, usually by mechanical means, and pressed or rolled or extruded or otherwise converted into the required shape. Now as a biomaterials scientist/tissue engineer, I find that this approach does not always work and have had to adapt to the new world of bottom-up assembly in which we start with extremely small scale components, usually of molecular dimensions, and persuade them to arrange themselves into the structure or conformation that we seek.

I say that this is the “new world” because this bottom-up approach has become immensely important in the nanotechnology field, where it is often difficult to contemplate using the mechanical downsizing of materials to the nanoscale. In this field, far greater efficiency and complexity can be envisaged with chemically driven molecular self assembly. As usual, however, it is not that simple because the origins of bottom-up assembly do not involve the ingenuity of scientists, but are based on nature. Specifically, nature’s polymers, usually referred to as biopolymers such as the polypeptides, polysaccharides and nucleic acids, owe their organisation and their structure–property relationships to processes of self assembly. Indeed, many of nature’s engineering materials are derived from the self assembly of amino acids, which results in the formation of proteins. Although proteins are ubiquitous and of many diverse characteristics, some have powerful mechanical attributes to accompany their biological properties. Bone, dentine, tendon, muscle, ligaments and arterial walls all depend for their structural properties on the intrinsic characteristics and organisation of their own proteins; this is sometimes assisted by reinforcing minerals, as in the case with silk, one of nature’s most successful engineering materials.

It should not be surprising, and again this is not new, that materials scientists have recognised that they have a great deal to learn from these natural approaches to engineering materials. Several so-called biomimetic materials, which some prefer to describe as bioinspired materials, have been based on natural materials. For several years there have been attempts to make materials for medical devices from biomimetic materials and perhaps the best examples are seen in the areas of bone replacement. Here, some rather crude attempts to mimic the composite structure of bone have involved the use of a calcium phosphate that is analogous to the mineral phase and dispersed within a polymer matrix that is intended to mimic the collagen matrix of the bone. Attempts such as these, however, have largely a top-down manufacturing approach and do not involve the self assembly principle and do not result in the exquisite conformation and architecture of the natural materials.

More recently, and particularly in the context of scaffolds and matrices for tissue engineering products, we have seen far more inspirational attempts to mimic nature. For example, the work of the Stupp Laboratory at Northwestern University, Evanston, Illinois, USA, is leading the way in achieving the self assembly of a variety of molecules, including peptides, into hierarchically ordered membranes within which stem cells may grow and differentiate.1 There are many ways in which this type of approach may be used within medical nanotechnology. In this article I would like to address just one area and that involves the use of fibrous proteins.

Fibrous proteins

Proteins may be broadly divided into those that have a globular structure and those that are fibrous in character. Globular proteins involve chains of amino acids, the folding of which determines their shape and presents critical sequences to their immediate environment where they may exert some physiological or biochemical function. They are generally water soluble and functioning, for example, as enzymes or antibodies. They do not have mechanically robust characteristics. Fibrous proteins, however, as the name implies, are composed of filaments, generally involving smaller numbers of amino acid residues that are water insoluble. These filaments are able to form secondary structures such as a triple helix and individual chains may be crosslinked to give considerable strength, rigidity and stability. The keratin of hair, the elastin of arteries, the collagen of tendons and ligaments and the silk spun by spiders are all examples of fibrous proteins. It is important to note that although these proteins have mechanical properties that allow for weight bearing and physical protection functions, their amino acid sequences may still have biological properties so that they may also take part in physiological processes such as cell signalling. Moreover, this chemical structure may be capable of manipulation, either directly or through genetic manipulation to modify the properties. In other words, they are stable, versatile, multifunctional high performance materials; no wonder there is such strong interest in them.

Collagens are perhaps the best known and most widely examined of these fibrous proteins in the context of medical technology. There are more than 25 different types of collagen and they are the major feature of the extracellular matrix in animal tissues. Through the choice of collagen type and the manipulation of the structural hierarchy into different formats such as sponges, sheets and gels, different types of product from drug delivery systems to haemostatic agents and tissue engineering scaffolds can be produced. There are a few disadvantages of collagen materials that are sourced from animals, including contamination by other extracellular matrix proteins, which could stimulate immune and inflammatory systems, and contamination by infectious agents. Thus, a great deal of attention is being paid to alternative methods and sources, especially involving genetic engineering. These proteins are naturally derived from genetic blueprints and the formation of the different collagen architectures by self assembly is achieved through precise control of the amino acid sequences and chain lengths. The techniques of genetic engineering allow the introduction of defined alterations to these sequences, using transgenic animals and plants, to produce even more effective materials.

Elastin is an immensely important protein that imparts the essential elasticity to structures such as arteries. Because of difficulties in preparing pure native elastin, the focus of attention has been on synthetic and genetically engineered elastin-like polypeptides and some success has been achieved with injectable self-assembled elastomeric proteins.

Silk is an interesting material that is often described as one of the strongest of all natural materials. It was used as a suture material many decades ago. Recently, great strides have been made in understanding the mechanisms of silk fibre self assembly and ways to manipulate these in the construction of superior biomaterials. The self assembly of natural silk takes place within a water based environment and the water itself, through osmotic processes, regulates the structure and therefore the properties of the silk fibres. As fibres are produced under laboratory conditions, they can be processed through spinning techniques into a wide variety of structures. During these processes, variations in structure may be introduced. For example, it is possible to control the crystallinity, which in turn controls the stability, so that biodegradable silk scaffolds for tissue engineering may be produced. The dragline silk produced naturally by spiders can also be cloned and expressed within bacteria, which allows for the deliberate introduction of additional features by manipulation of the protein nucleating system. For example, domains can be introduced within the protein that facilitates biomineralisation so that bone-like structures of hydroxyapatite nanocrystals dispersed within the silk protein are generated.

These are just a few examples of the possibilities for new biomaterials based on the self assembly and genetic engineering of nature’s own fibrous proteins. The reader is referred to two recent reviews for more thorough coverage of this subject.2,3 There are obvious economic considerations to take into account because some of these developments and materials are expensive. However, some prices may well be worth paying if the potential attributes of these materials are fully realised and far more effective treatments for wide ranging diseases become available.

David Williams, Morgan & Masterson, Avenue de la Forêt 103, Brussels 1000, Belgium, tel. +32 4 7597 0556, e-mail: peggy@morgan-masterson.com, www.morgan-masterson.com.