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.
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.