Most synthesized materials including synthetic fibers have been developed by science and sometimes by chance in the past, whereas natural materials are produced as a consequence of biological processes. ' These approaches have now been integrated.


Plant fiber synthesized from carbon dioxide


Plants produce carbohydrates by photosynthesis. Air contains only about 0.3% carbon dioxide. Yet plants utilize this small amount of carbon dioxide with water to produce cellulose by photosynthesis. The structure of the resulting fiber cross-section is non-homogeneous, and is composed of complex multilayers, whereas that of the artificial fiber is homogeneous. Cellulose could be termed 'carbon dioxide fiber', and gives a hint as to how to produce an environmentally friendly fiber without using fossil energy if we could learn from nature.


Lessons from the silkworm


Rayon appeared about a century ago as the first chemical fiber mimicking silk. Rayon filament is made from wood pulp that is dissolved and wet-spun. Rayon is thus chemically composed of the same component (cellulose) as wood pulp. Then nylon appeared about a half a century later. Nylon was aimed to mimic silk chemically, and has similar amide groups.Fifty years after the invention of nylon (around 1988), a synthetic fiber reached a new stage of development when the combined yarn processing technology (the blends of filaments of different shrinkage characteristics) was developed to produce high bulky polyester fiber fabric with a characteristic feel different from natural silk. However, not all of silk's features were reconstructed. For example, the characteristic luster, moisture-absorbent characteristic and bright dyeability of silk have not yet been achieved.

All the organic constituents of a chrysanthemum, such as sugars, proteins, fats, cellulose, etc., contain the element carbon, and one function of photosynthesis is to bring new carbon into the plant. It has been estimated that 200 billion tons of carbon are taken from the air each year by the photosynthetic activity of plants. This is done by combining carbon dioxide from the air with water already in the plant, to form sugars.

Photosynthesis requires energy, for the sugars have higher energy content than the simple compounds from which they are formed, and this energy is obtained from light which is absorbed by the pigments (chlorophylls and carotenoids) in the leaves. Plants consist of more than sugars, however, and these compounds have then to be converted into structural materials such as cellulose and proteins. These conversions also require energy to drive them and this is obtained by breaking down some of the energyrich sugars into carbon dioxide and water again, in the presence of oxygen. This energy-releasing process is termed respiration and it is similar to the respiratory processes in animals. The second function of photosynthesis, therefore, is to capture energy and to store it in the form of sugars where it is available to power the process of growth.


Dr J. Magoshi at the National Institute of Agrobiological Science (NIAS) in Japan has elucidated the mechanism of in vivo synthesis of silk in silkworm. It was not previously known how the silkworm makes silk from mulberry leaves. Mulberry leaves are digested to amino acids, which are concentrated in the silk gland where a two-layered silk protein is produced at room temperature. Gel-like fibroin solution is formed by calcium ions in the silk gland. Gel is transformed to sol by carbon dioxide in air, and becomes liquid crystalline in a narrow tube. Sol is transferred slowly to the spinning tube and is spun to silk filament.