How to spin synthetic spider silk | Science News for Students

How to spin synthetic spider silk

New method combines protein parts from two species to mimic spiders
Jan 30, 2017 — 7:10 am EST
Holding spiders

Anna Rising (left) and Jan Johansson (right) hold up golden silk orb-weaver spiders, also called Nephila clavipes, at their lab at the Karolinska Institute in Stockholm, Sweden. 

Kerstin Nordling

This is one in a series presenting news on technology and innovation, made possible with generous support from the Lemelson Foundation.

In the comics, Spiderman hurls his spider silk at will. In real life, making artificial — or synthetic — spider silk is not nearly as easy. But researchers now have found a way to make the flexible yet super-strong strands.

Spiderman Marvel
Although Spiderman's web-slinging powers are make-believe, scientists have now managed to do the real thing on a much smaller scale, in the lab. The results could have many possible uses.
Courtesy of Marvel

“Now we can do it the way the way that spiders do it,” says Anna Rising.

She works at the Swedish University of Agricultural Sciences in Uppsala and the Karolinska Institute in Stockholm, Sweden. As a medical biochemist, she studies chemical processes important in living things. Rising initially became a veterinarian. Then she got interested in the challenge of making synthetic spider silk. It could be a big help for medicine, for manufacturing and even for producing gear to protect soldiers.

Rising teamed with up Jan Johansson, another medical biochemist at those schools. Spider silk proteins interested him because of his work on Alzheimer’s disease. In that illness, a protein clumps in the brain. Those clumps block normal brain functions and damage nerve cells. And how the proteins come together is similar in both cases.

The idea for the new process developed as the scientists studied what spiders do naturally.

Each molecule of spider silk protein is like a long chain with three basic parts. The longest part has segments that repeat over and over (which are known as “repeats”). If you look at that long part under a high-powered microscope, it would look like towers of stacked Lego blocks connected by springs, explains Randy Lewis. He’s a biochemist at Utah State University in Logan and didn’t work on the project. The Lego-stack areas provide strength, he notes. The springy sections give the material elasticity, or stretch.

A separate part of the spider silk protein is found at the start of that long portion. Another part attaches to the end. Silk proteins can hook onto each other when the silk is spun. That makes long lengths of silk fibers.

Spiders have glands in their abdomens that contain the silk-making proteins in a water-based solution. To make synthetic spider silk, Rising’s team needed building blocks that would form a similar starting protein.

Rising started by collecting fishing spiders from South Africa. (Their scientific name is Euprosthenops australis.) Then she, Johansson and other researchers studied the spider’s silk and its genes. From this, they figured out which part of the spider’s genetic code would make the silk protein. To make lots of copies of those DNA segments, they used a process called the polymerase (Puh-LIM-ur-ace) chain reaction, or PCR. 

Araneus ventricosus
Araneus ventricosus is one of two species whose DNA was used by researchers to design a new synthetic spider protein.
Masaki Ikeda/Wikimedia (Gnu Free Documentation License)

Rising and Johannsson’s group then put that genetic material into bits their DNA that would enter into bacteria easily. The bacteria added these bits to its DNA and could now make parts of the natural silk. But there was one problem. The small amounts they made were not very soluble in water. That meant the team wouldn’t be able to mix it into a water-based solution like that contained in the spiders’ silk glands.

Meanwhile, Chinese researchers did similar work with an Asian spider, Araneus ventricosus. The two groups joined forces and designed a hybrid protein. They chose the parts from each spider species that would be most soluble in water. The starting part came from the African spider’s silk. The end part was from the Asian spider. For the middle, the researchers used two repeats from the African spider. (That spider’s natural silk protein has about 100 such repeats.)

The team coached bacteria to make this hybrid protein. Then they made a solution of the protein in water, concentrated at up to 50 percent. That’s similar to the concentration in spider glands.

Getting the proteins to make fibers

Next came the challenge of spinning the proteins into fibers. As a spider’s gland pumps out the solution, the solution’s pH falls. (The pH scale measures how acidic something is. The lower the pH, the more acidic it is.) Rising’s group figured it needed to do something similar.

Silk spools
Synthetic spider silk can be wound onto spools after it’s formed.
Marlene Andersson

To mimic the way spider silk becomes more acidic as it’s spun out, the group’s new process pumps the solution through a thin tube. The diameter of the tube’s tip narrows at the end. That forces the protein solution into a jet stream. The stream empties into a beaker with an acidic, water-based solution. As the jet stream of protein goes through that liquid, its pH drops. The individual proteins then link up. This makes them fall out of the solution as fibers. The resulting strand of synthetic silk can be pulled out of the beaker and wound onto a spool or card.

The team’s study appears in the January 9 issue of Nature Chemical Biology.

Toward even stronger silk

Lewis’s group at Utah State had already managed to dissolve spider silk proteins in water. In 2015, these researchers reported making them into a silk using a different method. However, the protein level in that solution was much lower than what Rising’s group achieved.

Lewis notes that the silk protein made by Rising and Johansson’s group has only a couple of repeats. More repeats in that silk would strengthen the strands, he suspects.

Silk close-up
This close-up shows how synthetic spider silk forms after a solution with protein hits an acidic bath and the pH falls.
Marlene Andersson

Johansson agrees it might be better to have more repeats. What's more, he thinks that keeping the protein highly soluble also is important. And the shorter repeating section probably helps with that. But silk made with their new process is already about one-third as strong as natural spider silk. Yet it has just two percent as many repeats as those in the South African spider’s silk.

The new work is important, Lewis says. “It provides an interesting opportunity for maybe simplifying the spinning process significantly.” And, he adds, if it works for large proteins, "it is a possible major advance.”

After all, raising spiders to gather natural silk is impractical. Each would have to be raised alone or they might eat each other. And there would be other challenges.

A synthetic silk could have lots of uses. “Spider silk has a unique combination of both strength and elasticity,” Lewis notes. In medicine, spider silk could work as sutures. It could repair tendons. It might help damaged nerves repair themselves. It might even form a framework for growing replacement tissues in a lab.

For the military, synthetic spider silk could go into protective gear. For instance, the strong fibers might help keep tiny fragments of explosive devices from penetrating to the skin and causing infections. In industry, spider-like silk could be used to make strong, lightweight parts for airplanes or cars. “One of the things we’ve discovered is you don’t even have to use [the silk] to make fibers,” Lewis says. The proteins could go into coatings, gels, films or adhesives.

More work must be done before this synthetic silk is ready for mass production. Yet after 13 years, Rising is glad her international team finally found a way to mimic how spiders spin their own silk. “It’s been one of the projects where everything basically just works,” she says.

Power Words

(for more about Power Words, click here)

acidic     An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.

Alzheimer’s disease     An incurable brain disease that can cause confusion, mood changes and problems with memory, language, behavior and problem solving. No cause or cure is known.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

concentration     (in chemistry) A measurement of how much of one substance has been dissolved into another.

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

dissolve     To turn a solid into a liquid and disperse it into that starting liquid. For instance, sugar or salt crystals (solids) will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

fiber     Something whose shape resembles a thread or filament of some kind. (in nutrition) Components of many fibrous plant-based foods. These so-called non-digestible fiber tends to come from cellulose, lignin, and pectin — all plant constituents that resist breakdown by the body’s digestive enzymes.

gel     A gooey or viscous material that can flow like a thick liquid.

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for producing a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

genetic     Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

gland     A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hybrid     An organism produced by interbreeding of two animals or plants of different species or of genetically distinct populations within a species. Such offspring often possess genes passed on by each parent, yielding a combination of traits not known in previous generations. The term is also used in reference to any object that is a mix of two or more things.

infection     A disease that can spread from one organism to another. It’s usually caused by some sort of germ.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nerve     A long, delicate fiber that communicates signals across the body of an animal. An animal’s backbone contains many nerves, some of which control the movement of its legs or fins, and some of which convey sensations such as hot, cold, pain.

pH     A measure of a solution’s acidity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.

polymerase chain reaction  (PCR)    A biochemical process that repeatedly copies a particular sequence of DNA. A related, but somewhat different technique, copies genes expressed by the DNA in a cell. This technique is called reverse transcriptase PCR. Like regular PCR, it copies genetic material so that other techniques can identify aspects of the genes or match them to known genes.

protein     Compound made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins. Medicines frequently work by latching onto proteins.

silk     A fine, strong, soft fiber spun by a range of animals, such as silkworms and many other caterpillars, weaver ants, caddis flies and — the real artists — spiders.

soluble     Some chemical that is able to dissolve some liquid. The resulting combo becomes a solution.

solution     A liquid in which one chemical has been dissolved into another.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

spider     A type of arthropod with four pairs of legs that usually spin threads of silk that they can use to create webs or other structures.

suture     A stitch or row of stitches holding together the edges of a wound or surgical incision.

synthetic     An adjective that describes something that did not arise naturally, but was instead created by people. Many have been developed to stand in for natural materials, such as synthetic rubber, synthetic diamond or a synthetic hormone. Some may even have a chemical makeup and structure identical to the original.

tendon     A tissue in the body that connects muscle and bone.

tissue     Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.

unique     Something that is unlike anything else; the only one of its kind.

veterinarian     A doctor who studies or treats animals (not humans).

Citation

Journal: M. Andersson et al. Biomimetic spinning of artificial spider silk from a chimeric minispidroin. Nature Chemical Biology. Published online January 9, 2017 . doi: doi:10.1038/nchembio.2269.

Journal: J. Jones et al. More than just fibers: An aqueous method for the production of innovative recombinant spider silk protein materials. Biomacromolecules. Vol. 16, March 19, 2015, p. 1418. 10.1021/acs.biomac.5b00226.