Silk fibres and biodegradable pearls

Silk and pearl have been elite ornaments for ages. While connoisseurs wonder about the smoothness of silk, and the smooth and shiny nature of the pearl, scientists have always wondered about its constituents.

Though imitation polymers of silk such as nylon or polycarbonate-based pearls have been made, mimicking their composite architecture with artificial materials has not been possible. All that may change now if the efforts of Chinese scientist, Hao Bai from the Zhejiang University come to fruition.

Bai and his team have used fibroin, the protein in the silk to manufacture a smooth and shiny but hard material similar to pearl. The 'silk nacre' prepared by Bai has not only the natural build of pearls but unlike the synthetic pearl material, is also highly biodegradable.

Nacre is the material that gives pearls their natural shine. They are, like silk fibroin, a composite of organic proteins and inorganic minerals. The nacre for the most part is plain calcium carbonate crystals known chemically as aragonite and cemented by a paste of organic materials. Under the microscope, it appears like neatly stacked bricks cemented by mortar.

Though aragonite by nature is brittle and powdery, mixed with the protein it forms a very strong material that is used by the shelled animals for protection. “Natural nacre is made of 95% of brittle aragonite, or calcium carbonate and the rest is organic material. Its toughness is reported to be more than 3000

times that of pure CaCO3, and its strength is found to be a few 1000 times higher than aragonite” says Dr Pattabhi Ramaiah, an Assistant Professor at IIT, Bhubaneswar who is also researching on nacre like composites.

Composites like this are engineers’ favourite because they are better than the materials which go to make them. Concrete is a common composite. Fibreglass is another. Though made up of a paste and a fibre, these composites are unlike either. The presence of hard material in a soft matrix is the key architecture of tough natural nacre,” explains Dr Pattabhi.

Concrete is not brittle like solid cement, nor does it bend like the steel rods that are in it. Composites made of metals and organics are also there. Though very useful, at the end of their life, synthetic composites turn to difficult to handle wastes. Because of a combination of different materials, it is not easy to either reuse or recycle such composites. Hence, the interest in natural composites. Pearl nacre tops the list because of its attractive look, and also strength.

Earlier attempts to produce synthetic nacre or synthetic have mostly been using the nacre itself. Waste shells are ground and mixed with polymers to create an almost pearl-like substance. Many artificial pearls are made this way. Such pearls are shiny but are brittle as their construction is not similar to natural pearls.

A few years ago another group of Chinese scientists had a go at it by dispersing aragonite crystals on sheets of chitosan and sticking these sheets together with silk fibroin. But now, Bai and his team have prepared nacre-like material using only silk fibroin.

While nacre is a favourite crystalline material for composites, fibroin has been the favourite fibre. It is thin but strong and thus can be manipulated easily. In India, much of the composite research has been with fibroin and nanocarbon. Bai’s team dissolved the long fibres of silk fibroin into a solution of small pieces.

The solution was poured into a mould made up of stacked sheets of a polymer and frozen. The polymer helped in cooling the solution from one end. Later the polymer was washed off to get thin sheets of fibroin. The sheets were then stuck together with water vapour, and then pressed together. The result was a thick sheet of nice, shiny sheets of silk fibroin which the team named 'silk nacre."

Under the microscope, Bai’s silk nacre appears similar in structure to natural nacre. Hence, the name. But there is some difference between the present silk nacre and natural nacre in their properties. “In the silk nacre, the brick structure is missing which is evident in natural nacre. As a result, the strength is comparable, but not the stiffness,” says Dr Pattabhi.

Because of this, the silk nacre plied into any required shape. Plain silk fibroin sheets, stacked this way, bend under pressure, while silk nacre stays tough. “The processed Silk nacre maintained its intriguing mechanical performance, as it could withstand around 1000 times of its own weight. In addition, the Silk nacre showed complete biodegradability due to the intrinsic nature of silk fibroin.” Reports Bai’s team. To bend silk nacre to the desired shape, one only needs to steam it for a while.

It may be too early to expect it to be of any use contends Prof. Pattabhi. “The manufacturing process is complex, involving bidirectional freezing, water vapour annealing, and densification processes. The key processes here are freezing and vapour annealing at temperatures around -196◦C, which is expensive.

However, the bio-degradable nature must be considered, as the structure may degrade when exposed to certain materials like enzyme pronase E.” says Prof. Pattabhi. The silk nacre disappears into a solution with the enzyme in just a month’s time. That and its hardness makes it an attractive alternative to plastics in composites such as artificial organ scaffolds, substrates for flexible electronics or as a replacement for reinforcing plastics in composites. The research was published in the latest issue of the journal Science Advances.