A small purse made of silk leather. Photo: Laia Mogas Soldevilla.
A small purse made of silk leather. Photo: Laia Mogas Soldevilla.

Leather is an ever growing multi-billion-dollar industry requiring more than 3.8 billion bovine animals to sustain production each year. And while the products made from leather – clothing, shoes, furniture and more – can be quite elegant and durable, the environmental impact of leather production has been severe, leading to deforestation, water and land overuse, environmental pollution and greenhouse gas emissions.

With this in mind, researchers at Tufts University School of Engineering set out to find an alternative to leather, with similar texture, flexibility and stiffness but made from materials that are sustainable, non-toxic and friendly to the environment. It turns out we have been wearing such a material all along – it's silk. But instead of weaving the silk into fabric, the Tufts engineers were able to break down the fibers from silkworm cocoons into their protein components, and re-purpose the proteins to form a leather-like material. The researchers report their process for making silk-based leather in a paper in Materials & Design.

The silk-based leather can be printed into different patterns and textures, and has similar physical properties to real leather. It can also withstand the folding, piercing and stretching typically used to create leather goods, including the ability to stitch together pieces of material and attach hardware such as rivets, grommets, handles and clasps.

"Our work is centered on the use of naturally-derived materials that minimize the use of toxic chemicals while maintaining material performance so as to provide alternatives for products that are commonly and widely used today," said Fiorenzo Omenetto, professor of engineering at Tufts School of Engineering, director of the Tufts Silklab where the material was created and corresponding author of the paper. "By using silk, as well as cellulose from textile and agricultural waste and chitosan from shell-fish waste, and all the relatively gentle chemistries used to combine them, we are making progress towards this goal."

There is of course already an existing portfolio of alternative leathers developed by industry and the research community, with a focus on using agricultural by-products or regenerated materials that have a reduced impact on the environment and animal raising. These include leather-like materials made from petroleum (polyurethane leather or 'pleather'), tree bark, pineapple husks, plant oils, rubber, fungi, and even cellulose and collagen produced by bacterial cultures.

The silk-based leather made at Tufts offers some unique advantages over these other approaches. In addition to being derived from dissolving silk fibers, the manufacturing process is water based, using only mild chemicals. It is also conducted at room temperature and produces mostly non-toxic waste.

What is more, the silk leather can be fabricated using computerized 3D printing, which provides the ability to create regular micropatterns that can tune the material's strength and flexibility. It can also print macropatterns for aesthetics (e.g. a basket weave) and non-regular geometrical patterning to mimic the surface texture of real leather.

Like leather, the resulting materials are strong, soft, pliable and durable, and biodegrade once they enter the waste stream. In fact, the silk-leather products can be re-dissolved and regenerated back into the gel-like stock matter to be re-printed into new products.

The process of making the silk leather starts with silk fibers that are commonly used in the textile industry. These fibers are made up of silk fibroin protein polymers, which are broken down into their individual protein components in a water-based slurry.

A base layer of chitosan containing a non-toxic plasticizer such as glycerol and a dye is printed by extrusion through a tiny bore nozzle onto a surface to provide flexibility and strength. Chitosan is itself derived from natural sources such as the shells of crabs, lobsters and shrimp. A layer of the silk fibroin, combined with a plasticizer and a thickener (from vegetable gum), is then printed on top of this base layer.

Extruding the fibroin slurry through the printer nozzle creates shear forces that may contribute to arranging the proteins in a way that strengthens the material, making it ductile rather than brittle, and mimics the natural extrusion that occurs in the silk gland of a worm or spider. Changing the printed pattern of the silk layer can alter the appearance, tunable strength and other physical qualities of the material.

The Silklab at Tufts has developed a wide range of other products from silk, from implantable medical devices to architectural materials that can sense and respond to the environment by changing color. In fact, much of the technology that has been developed in the lab to derivatize silk proteins can be applied to the silk-based leather, including attaching and embedding molecules that can sense and respond to the surrounding environment.

"That's the advantage of using silk protein over other methods – it has a well-established, versatile chemistry which we can use to tune the qualities of the material and embed smart elements like sensing molecules," said Laia Mogas-Soldevila, a former research fellow in the Silklab who is currently assistant professor of architecture at the University of Pennsylvania and first author of the paper. "So while there may be many options for leather-like materials, silk-based leather has the potential to be most amenable to innovative designs."

This story is adapted from material from Tufts University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.