Silk-Based Filtration Material Breaks Barriers

Engineers find nanosized building blocks of silk hold the secrets to improved filtration membranes.

Silk-Based Filtration Material Breaks Barriers
A scanning electron micrograph shows a free-standing Bombyx mori silk nanofibril membrane. Image courtesy of the researchers.

Silk is a natural protein fiber, actually made from fibroin. It is generated by an insect known as larvae to form cocoons. The best quality of the silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity.

When silk was discovered by Chinese researcher it has superior quality and properties. It was reserved particularly for clothing the ruler, his relatives, and dignitaries. For above two millennia, the silk production mechanisms were a highly guarded secret.

Recently, researchers from MIT and Tufts University have discovered additional hidden secrets of silk, called Nanofibrils. This can be expertly drawn out and reorganize and also can be manufactured into advanced filtration membranes.

This research shows that how silk Nanofibrils (SNFs), can accelerate new naturally-based filters. This silk Nanofibrils (SNFs) are the key nanoscale building block of natural silk. The accelerated filters are too effective, cheap, and new than traditional commercial products. The discovery includes the new filter membranes, containing water treatment facilities, food manufacturers, and life sciences organizations.

Shengjie Ling, Civil, and Environmental Engineering (CEE) postdoc, “There has been a renewed focus recently on developing these types of ultrathin filtration membranes. It can provide maximum flow-through while retaining molecules or pollutants that need to be separated from the flow. The challenge has always been to create these new ultrathin and low-cost devices while retaining mechanical strength and good separation performance. Cast silk fibroin membranes aren’t an option because they do not have a porous structure. They dissolve in water if not pre-treated. We knew there had to be a better way.”

An insurmountable challenge — until now

For sharing ideas, working and reworking calculations, and experimenting in the lab, researchers have spent many months. The task was is to find out just the right solvent to melt the silk fibers into their most elemental compounds. The challenge was the solvent should not destroy the samples.

For developing the method for extracting the Nanofibrils from the natural silk fiber, scientists have spent too much time. This is a new idea, therefore scientists used trial and error before they got success. In the form of this new free-standing ultrathin filtration membrane and its innovative, advanced production technique, researchers find out the solution.

Small but powerful

Natural silk fibers, made up of pure protein, are famous for their incredible lightness, strength, and durability. The silk Nanofibrils used by the researchers were peeled from domesticated silkworm-produced fibers. This is the special feature of the silk Nanofibrils. It helps the creative membranes to maintain their excellent structure and superior physical properties.

The previous method which is used to extract or prepare these nanofibers has not always worked. The examples in the slideshow demonstrate the researchers’ unique four-step approach that proved effective by overcoming prior hurdles. The first two steps were used to peel the silk Nanofibrils from the silk fibers by exfoliating, washing, drying, and hatching them at a constant temperature. It is done before placing them in water and stirring or shaking them to remove any undissolved silk. The third step uses ultrasonic waves to drawn out the silk Nanofibrils, which remained stable over several months. The silk Nanofibrils had a diameter and curve length similar to the diameter of a single Nanofibril strand shown by scanning electron micrograph metaphors. In the last step, they combine the silk Nanofibrils into the ultrathin membranes using a vacuum filtration process.

Because of gathering and top three important membrane attributes scientists got success. Those attributes are thickness, superior water permeation, known as flux and superior broad-spectrum separation performance for most dyes, proteins, and nanoparticles. The thickness is 40-1,500 nanometres with narrowing pore sizes of 12-8 nm. All of these mechanical beneficial results are detracting to industry, especially for use in pressure-driven filtration operations, even at high applied pressures.

These new silk-based membranes offer important advanced functional effectiveness. Either it is while purifying waste water for drinking, or capturing the minuteness of blood clots in the human body. A Single piece of silk Nanofibrils membrane averages only near about $0.05-$0.51 than commercial filtration membrane which costs $1.20 per piece.

Silk Nanofibrils used in manufacturing hold other important benefits, too. As the by-products of silkworms, innovative manufacturers who influence silk’s natural properties can enhance their industrial ecology and generates less environmental stress. Once the filters have been replaced, the used one’s bio humiliate and departs non-lasting force.

A keen eye for detail

It is important for filters to work effectively to control the thickness of membranes and pore size distribution. For that, the researcher makes sure that the attached membrane pores generated in the lab were consistent and without cracks or pinholes. Additionally, scientists have noticed, the new membrane’s refusal of protein and gold nanoparticles in flow was higher than that of membranes with similar thickness. To assess size-selectivity, protein molecules, colloids, nanoparticles, small molecules, and ions all were used.

With members of different thicknesses which are 40-60 nm, scientists experimented commonly with water fluxes.

Jin said that the surprising fact was one flux was faster than that of most commercial materials. In some cases, it is above 1,000 times higher. The outcome was better than fluxes of the most advanced ultra thin membranes.”

The new membranes can be taken out without confirming to the supporting substrate. They are visible as homogeneous, was transparent with structural color on the surface. It could be cut and bent without damage, and most important, did not dissolve in water, a critical role in most filtration method.

Most positively charged molecules can be involved by the membranes via electrostatic interactions, due to silk Nanofibrils are negatively charged at neutral pH.

Professor Kristie J. Koski of Brown University’s Department of Chemistry, said, “These natural silkworm membranes have remarkable separation efficiency on par with current synthetic technologies. As a non-toxic, flexible, and tunable membrane, they have great potential for purification and recycling especially in applications where synthetic alternatives are not an option such as in biological systems.”

According to Professor Thomas Scheibel of The University of Bayreuth in Germany, “The most important parameters of filter materials is its effectiveness. This parameter is especially influenced by the structure of the filter material. Nano silk filters are consistently filled and therefore authorize the custody of quite small particles. New filter devices based thereon should allow threating the overall energy absorption in water and in air filtration at constant or even higher filter efficiencies than existing ones.”