3D printing leather-like materials derived from silkworms into products such as shoes offers new pathways for reducing industry's carbon footprint in the fight against global warming, U.S. researchers argue in a new paper.

The paper, published March 2 in Materials & Design, builds on a growing trend worldwide to harness the potential of biomaterials. While the mechanical properties and comfort parameters of silk-based leather-like materials are still being studied, the researchers from Massachusetts-based Tufts University say that their results hold promise for viable alternatives to traditional leather for upholstery and fashion, which could help to alleviate climate change issues. 

"It's pretty impressive," lead study author Fiorenzo Omenetto said of what the research team observed with its silk-based material. "It compares favorably to all the materials that are out there." 

The materials still lose to sheep and cows in terms of material failures such as disintegration in the rain, noted Omenetto, who is also the dean for research at Tufts' School of Engineering. Yet biomaterials such as silk protein blends from silk cocoons are already being used in the fashion industry due to their environmental friendliness, among other reasons related to global warming.

Omenetto, who runs a biomaterials lab at Tufts focused on the links between technology and biology, pointed to his findings regarding how the silk-based materials held up in dry and wet conditions, as compared with actual leather.  

"These are natural building blocks you can assemble so that when they come together, the properties of the materials that come out are favorable, pliable, water-resistant, they do not fall apart, are bendy, you can stretch them, they're resistant and so on," he said.

The team converted silk from the cocoon, a protective casing spun by the Bombyx mori silkworm, into a fibrous silk protein to create the biomaterial blend. The silkworm changes into a pupa, emerging from the cocoon as an adult moth after about two months. The market for such cocoons is booming, particularly in places like India. 

The researchers used computer-aided design software and instructions for digital manufacturing that were translated with custom coding. The blend was a mixture of silk, a salt mainly extracted from brown algae known as sodium alginate, a sugar contained in shellfish waste known as chitosan and water. The team then applied the biomaterials blend to digital printing to enable large-scale production of the materials. 

Findings outlined in the new paper show that they stayed supple, formable and pleasant to the touch for longer than traditional leather, even when sent to museums and handled by non-scientists outside the Tufts lab's purview. 

Because the silk-based formulations presented in the paper are also manageable through additive manufacturing — industry lingo for 3D printing — they can be produced at a larger scale and in a sustainable way, because they are renewable resources.

Past research from Omenetto and colleagues shows that Bombyx mori silkworms' silk cocoons have been used in textiles and medical tools such as sutures and scaffolds for years. 

But there is a growing movement to push the use of biomaterials beyond medical applications. Omenetto explains that this is due to their potential for lowering the carbon footprint of both people and producers by reducing the amount of fossil fuels burned to create consumer products such as shoes and jackets.

Other research has reported the carbon footprint of silkworm cocoon to be 6.17 kilograms of carbon dioxide equivalent for each kilogram of cocoon production. Another study, meanwhile, indicated that the carbon footprint of finished leather is about 15,190 kilograms of carbon dioxide equivalent per 100 square meters of leather for shoe uppers, or about 152 kilograms per square meter.

The researchers cite previous literature noting that biomaterials — including cellulose from corn, beet, rice or wheat and chitosan, or raw protein such as silk fibroin — are among the most abundantly produced renewable resources on Earth. 

The team created its blend by leveraging a toolkit used for past research. The team has previously transformed biomaterials in many formats, ranging from electronics that are edible and implantable in the body to tapestries with sensing capabilities and biological clothing that responds to fatigue levels. 

As far as the new paper's particular area, the team's next focus will be on different manufacturing scales and the role of production processes. The researchers also intend to delve deeper into smart materials, or intelligent or responsive materials, which are designed with properties to be tuned in a controlled way through external stimuli, including stress, moisture, magnetic fields, light, temperature or chemical compounds.

The paper, "Additively manufactured leather-like silk protein materials," published March 2 in Materials & Design, was authored by Laia Mogas-Soldevila, Giuseppina Matzeu, Marco Lo Presti and Fiorenzo Omenetto, Tufts University.