THE HERITAGE OF FIBROUS POLYMERS R.J.C.Wilding. BDS, Dip Pros. M.Dent. Ph.D

Before the days of synthetic fibres, man relied on both plants and animals for clothing materials. These natural materials have a great deal in common. Cotton, and other plant fibres are all cellulose; wool, silk, furs and feathers are all keratins; leather hide is mostly collagen. The common factor shared by cellulose, keratin and collagen is that they all have similar molecular structures. They are all fibrous polymers.

It seems as though silk worms, cows and cotton plants, in spite of being very distantly related, have independently evolved the same recipe for making a tough fibre. First, you take some carbon atoms and make a long thin chain using the very best covalent bonds. Next, they need to be stacked alongside each other and packed tight using more covalent bonds. Then they must be tied in a bundle and the bundles positioned all along the line of tension. Lastly the free ends of each chain in the bundle need to be linked up to those in the bundle in front and behind. The recipe is the same whether its for a spider's web or a bird's feather. Darwin would surely have been delighted by this evidence of natural selection at work. Several unrelated organisms, with the same problem to solve, and quite independently, they arrive at the same answer! What works well, is worth keeping, and it survives.

Some of these natural materials have synthetic equivalents but it is still hard to find anything quite like Angora wool, or silk or just plain shoe leather. A craftsman who builds string instruments insists that there is nothing comparable with animal glues for joining the panels of a violin or a harpsichord. So he boils apart the collagen fibres from animal bones and puts up with the smell. There are others of us, perhaps just as old fashioned, who insist on cotton underwear.

Long before the complexity of the natural fibrous polymers was understood, chemists were taking the first steps towards copying their molecular structure. The first plastics were produced by joining together many small single units (mers) like ethylene into fewer large molecules (poly-mers) like poly-ethylene. The early synthetic polymers were brittle and affected by heat; they were called plastic, because they could be made plastic by heating, and while still hot, the material could be forced into a mould. Modern plastics are of sterner stuff and more properly called resins. The latest techniques used to make very strong, heat resistant resins, are based on principles of polymer mechanics which owe their heritage to living organisms.

Like the natural structural polymers, an essential requirement of a synthetic fibre is its ability to resist tension. If a material is being pulled out, it is less likely to break if it is made from a bundle of long fibres than if it is one solid block. A metal rod is more likely to snap under tension than a wire cable of the same dimension and made from the same metal. The components of the cable may in turn be smaller cables which in turn may be made from fine wire. This hierarchy from the smallest unit to the larger units of the structure is also found in animal tendons. The advantage of a hierarchy is that if one level breaks, the fault is contained there while the next higher level merely takes a bit more strain. A small fault in a mono-block material tends to spread, as a running crack through the entire structure.

The fibre, or cable, is strongest if all the units, even down to molecular size, are long chains all orientated along the lines of tension. Consider the strength of a piece of wood under tension along the grain; but if pulled across the grain it is very weak. These observation about fibre orientation and tensile strength of materials hold true at different scales of size, that is, they are also true at a molecular level.

The strongest bonds between molecules are those provided by the sharing of electrons which occurs in covalent bonds. The more electrons shared, the stronger they are, so elements like carbon, silica, nitrogen, oxygen and aluminium make strong covalent bonds. Examples are the ceramics, aluminium oxide (the basis of rubies and sapphires), silicon carbide (abrasives), silicon dioxide (glass) and of course the hardest material of all, diamond. Molecules linked together in a chain, by covalent bonds are very difficult to break if pulled along their length, so the orientation of the chain when tension occurs is crucial. If several chains are stacked next to one another, the bundle (or fibril) will be a useful unit for building a fibre, provided that the chains of molecules cannot slide past each other. In order to prevent this they need to be tied to each to each other by cross-linkages and at least reasonably long. It is worth looking closer at these three requirements for a strong molecular fibril; that is stacking, cross-linking and length.

If uncooked spaghetti is spilled on the table it is easily gathered and returned to the box; it would be impossible to bring such order to the pieces once you had cooked them. Stacking of long molecules cannot be achieved unless they are stiff, and cross-linkages are impossible to achieve unless the chains are packed close to each other.

The first formed molecular chains of collagen and keratin are stiffened in places by the rigid nature of some of the amino acids which form the links of the chain. The important one is proline, an imino acid with an aromatic ring which becomes part of the backbone of the chain. Commercially made polymers, like the aramids, also contain polyaromatic amide rings in the chain to make it rigid. But molecular stiffness is only the beginning. While they are still in the production line, within the cell, the collagen chains are woven together into a rope. Recall that if you unwind the end of a rope the three strands flop about and will not lie together again. You can reform the rope end, by twisting each of the strands. Strange, that you have to twist them in the opposite direction to the way they lie in the rope. Well, the primary unit of the collagen chain (protocollagen) has a twist to the right, and three of these chains join together to form a rope -like molecule (procollagen) with a twist to the left. No sooner is this done than some cross linking bonds are added to keep the coiled chain together.

The longer each chain unit, the greater the opportunity each has to bind against its neighbour and the greater the tensile strength of the entire material. However there is a diminishing return from excessive length. When chains get too long they are difficult to stack and orientate. The length:width ratio of the collagen molecule is about 100 to 1. Polymer chemists have calculated the ideal ratio between length and width of a molecular chain and, not surprisingly have come to the same conclusion; 100 to 1 is best! The collagen sub-units are 300 nanometres long but they are joined end to end with others making a continual chain throughout the entire length of a fibril. Keratin fibres are only one third the length and not surprisingly, keratin is less strong in tension than collagen.

If chains lying next to each other have cross linkages they are unable to slip past each other, and this increases their resistance to being physically torn from each other. Cross-linkages also occupy chemically reactive sites on the chain and this increases their resistance to being chemically separated by enzymes. Collagen chains are cross linked with the help of Vitamin C. If there is a dietary deficiency of vitamin C, as in scurvy, the newly formed collagen is weak and easily breaks down. Collagen is normally being constantly turned over; the older fibres are resorbed and replaced with new ones. The new defective collagen in scurvy, causes a break down of previously healed wounds which open, even years after having completely healed. The complex system of gingival and periodontal fibres also breaks down causing a gingivitis and tooth mobility.

Keratin chains are also frequently cross linked by di-sulphide bridges, which contribute to its extreme insolubility. Not surprisingly it is to be found as an all-weather covering for mammals in the form of hair, hooves and horns. Not the least of its vital roles is in skin where it provides an effective barrier to the invasion of microorganisms. The plant kingdom arrived at the same conclusions about cross-linking fibrous polymers.

The insolubility and strength of the cellulose in plant fibres is due to the frequent cross-linking which occurs between strait, long chains of sugar molecules. Anything less thorough, and cellulose would be structurally weak, and all too easily accessible as a source of food for animals, considering all the glucose it contains. The cross linkages of cellulose make it so tough and insoluble that only a few bacteria (sheltering in the stomachs of cows and termites) can break it down and get to the sugar. Starch has a very similar chemical make up to cellulose, but because the molecule curls up into a ball, there is no opportunity for chain alignment and cross linkage, so starch has no structural strength and is reasonably accessible as a food source. Finally mention must be made of chitin, the remaining great fibrous polymer, which is the main structural material of the insect world. Once again it consists of long fibrous polymers, closely packed and cross-linked.

Cross linkage also provides stability for synthetic and natural rubbers. Vulcanisation is a mechanism for cross linking natural rubber fibrils with sulphur, a process which was patented by the Goodyear rubber and tyre company in the 1850s. Until denture base polymers were cross-linked, they were affected by heat and the surfaces were crazed by organic solvents. Many commercially made polymers which lack cross linkages are easily softened by heat.

We have unpacked the chains of both living and synthetic polymers to find that they satisfy the mechanical requirements of strong fibres. The final step in reconstructing a cable or tendon is to orientate the fibres along the lines of tension.

Polymers which have long rigid chains may be aligned by squeezing them through a narrow opening, rather like logs in a river, which would tend to line up as they flowed fast down a narrow waterfall. Spiders squeeze out a long thin silk filament to construct their webs. They have no inner store of ready-to-use web but make it up from a bag of liquid containing long chained polymers. The liquid is forced through a fine tube which aligns the chains parallel to each other so that they can form cross-links and join head to tail. A fine filament emerges which has a tensile strength five time that of high tensile steel. Not content with this supreme feat of real- time material synthesis, the spider adds a touch more class by eating up unused web so as to recycle the raw materials. Polymer chemists have been successful in mixing two different polymers during the moulding process. The first material is extruded and then the second is placed over it, followed by a layer of the first and so on to form a laminated structure. Dentine is also extruded from the end of each tubular odontoblast and the collagen orientation tells us that it is laminated. The laminations of both living and synthetic polymer composites are specifically orientated to provide the most effective resistance to stresses which might disrupt the material.

The terminal peptides at the end of each tropocollagen molecule prevent any alignment from happening until the peptidase enzymes split them off outside the cell. Some factor in the extracellular environment (perhaps minute electric currents set up during stress) can control the alignment of the molecules into a fibril. This is clearly seen in tendons, where the lines of tension are most simple. But collagen fibres are also a vital strengthening element in bone and dentine. Their orientation in bone accounts for the plate like (lamella) structure of compact bone. Collagen fibres in dentine are most favourably orientated to prevent enamel cracks from continuing right through the tooth.

Fibrous polymers in tendons, bone and dentine, are blended, stiffened, stacked, packed, and orientated according to the stresses they must resist. Their organisation is reflected in the highest technology found in commercially produced polymers. And then some, for the living polymers are responsive to changes in their environment; they are forever remodelling in response to altered demands. The "heritage award" for fibrous polymers could certainly be given to collagen, but there is an even more deserving candidate. The fibroblast is the real source of the magic.

Produced by Moorland Dentistry . Further information from