Leather is a steadily growing multibillion dollar industry that requires more than 3.8 billion cattle – or one for every two people on earth – to sustain production each year. And while the products – clothes, shoes, furniture and more – can be quite stylish and durable, the environmental impact of leather production has been severe, resulting in deforestation, overuse of water and land, pollution. environment and greenhouse gas emissions.
Researchers at Tufts University School of Engineering sought an alternative to leather, with similar texture, flexibility and stiffness, while focusing on materials that are durable, non-toxic and environmentally friendly. It turns out we’ve been wearing this material from the start – it’s silk, but instead of weaving silk into fabric, Tufts engineers were able to break down the fibers of the silkworm cocoons into their protein components and reuse proteins. to form the leather-like material. The process of making leather from silk is described in a study published in the journal Materials and design.
Silk-based leather can be printed in different patterns and textures, has similar physical properties to real leather, and can withstand the bending, piercing and stretching typically used to create leather goods, including the ability to sew pieces of material and tie them. hardware such as rivets, eyelets, handles and clasps.
“Our work centers on the use of naturally occurring materials that minimize the use of toxic chemicals while maintaining the performance of the materials to provide alternatives to products commonly and widely used today,” said Fiorenzo Omenetto, Frank C. Doble professor of Engineering at the Tufts School of Engineering, director of the Tufts Silklab where the material was created and corresponding author of the study. “By using silk, as well as cellulose from textile and agricultural waste and chitosan from shellfish waste, and all the relatively mild chemistries used to combine them, we are making progress towards this goal.”
There is of course already a portfolio of alternative leathers developed by industry and the research community, with an emphasis on the use of agricultural by-products or reclaimed materials that have a reduced impact on the environment and livestock. . These include leather-like materials made from petroleum (polyurethane or “pleather”), tree bark, pineapple pods, vegetable oils, rubber, mushrooms and even cellulose and collagen produced by bacterial cultures.
The silk-based leather made at Tufts offers unique advantages to all of these approaches. In addition to being derived from dissolving silk fibers, the manufacturing is water-based, uses only mild chemicals, takes place at room temperature, and produces mostly non-toxic waste. The silk leather material can be made using computerized 3D layers with the ability to create regular micropatterns that can adjust the strength and flexibility of the material, print macropatterns for aesthetics (e.g. basket weave ) as well as irregular geometric patterns to mimic the texture of the real leather surface. The resulting materials, like leather, are strong, soft, flexible and durable, and like natural leather, they are biodegradable once they enter the waste stream.
In fact, silk leather products could be re-dissolved and regenerated into its gel-like base material to be reprinted in new products.
The process of making silk leather begins with silk fibers commonly used in the textile industry. These fibers are made from silk fibroin protein polymers, and they can be broken down into its individual protein components in a water-based suspension. A chitosan basecoat containing glycerol and a non-toxic plasticizer dye is extruded through a small bore nozzle onto a surface to provide flexibility and strength to the material. Chitosan itself is derived from natural sources such as the shells of crabs, lobsters and shrimp. A layer of silk fibroin combined with a plasticizer and thickener (from vegetable gum) is printed on top of the base layer.
The extrusion of the fibroin slurry through the printer nozzle creates shear forces that can help organize proteins in a way that strengthens the material, making it ductile rather than brittle, and mimics natural extrusion which occurs in the silk gland of a worm or spider. Changing the printed pattern of the silk layer can provide a range of appearance, tunable strengths, and other physical qualities.
The printing method, also called “additive manufacturing”, is known to be very conservative in the use of materials and waste produced compared to other methods such as injection molding or subtractive manufacturing (such as sculpting or shaving from a block).
Tufts Silklab has developed a wide range of other products ranging from silk, implantable medical devices to architectural materials capable of sensing and responding to the environment by changing color. In fact, much of the technology developed in the laboratory to derive silk proteins can be applied to silk-based leather, including the attachment and incorporation of molecules capable of sensing and reacting to the surrounding environment.
“This is the advantage of using silk protein over other methods – it has a well-established, versatile chemistry that we can use to fine-tune the qualities of the material and integrate smart elements like sensing molecules.” said Laia Mogas-Soldevila, a former Silklab research fellow, currently an assistant professor of architecture at the Stuart Weitzman School of Design at the University of Pennsylvania and the study’s first author. “So while there may be many options for leather-like materials, silk-based leather has the potential to be the most conducive to innovative designs.”