A team of researchers from Tufts University have developed a leather-like 3D printing material made of silk.

As well as coming from a sustainable source, the material is entirely recyclable and biodegradable, all while offering mechanical properties comparable to those of real leather. The scientists have already used their new formulation to 3D print a usable purse, with other fashionable consumer products also in the pipeline.

Fiorenzo Omenetto, a co-author of the study, explains, “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.”

A purse 3D printed using silk leather. Photo via Tufts University.

The damaging effects of leather

Leather is characterized by its durability and flexibility. A staple in the fashion industry, the material is usually created by tanning the hides of animals such as cattle. As useful as it is, the sourcing of leather is unfortunately not very environmentally-friendly, as cattle farms take up copious amounts of land while requiring tons of water every year (not to mention, the ethics of mass slaughter).

Beyond this, cows themselves produce an abundance of methane, which has more than 80x the warming power of carbon dioxide, resulting in polar bears having to go for a swim.

There is also the issue of leather processing, which relies on harsh chemicals to produce the end-product. The non-exhaustive list of tanning agents includes chromium salts, formaldehyde, and a range of heavy oils – all of which can have damaging effects on flora and fauna.

The material can be 3D printed with feature sizes down to 0.25mm. Photos via Tufts University.

Biomaterials: a sustainable alternative

So it’s not hard to see how there might be a need to find greener alternatives. Silk on its own is nothing like leather, but the Tufts team found a way to process silk to make it stronger, tougher, and compatible with a custom-built pneumatic extrusion 3D printer. The system itself featured a 3-axis CNC gantry, a 1m x 1m print bed, and a Nordson EFD ValveMate dispense valve controller.

The formulation process involved mixing silk fibers from silkworm cocoons into a mud-like blend. Broken down into their base protein components, the fibers were then mixed with a plasticizer and a vegetable gum thickener, which gave the resulting material its extrudability. To add strength to the material, the scientists also integrated a base layer of chitosan, the sugar that gives shellfish exoskeletons their hardness. Best of all, the process only relies on mild chemicals and can be done in a standard lab environment.

Omenetto adds, “By using silk, as well as cellulose from textile and agricultural waste and chitosan from shellfish waste, and all the relatively gentle chemistries used to combine them, we are making progress towards our goal.”

According to the study, the final leather-like material can be 3D printed into almost any geometry, with feature sizes ranging from 0.25mm to 5mm. With some tinkering around, the material can also be deposited with varying flexibilities and opacities, lending itself to a wide variety of fashion applications. Once it reaches the end of its useful life, the material can even be recycled back into a silky mix for reuse, or discarded altogether as it is completely biodegradable.

The material also allows for a range of flexibilities and opacities. Photos via Tufts University.

Further details of the study can be found in the paper titled ‘Additively manufactured leather-like silk protein materials’. It is co-authored by Fiorenzo Omenetto et al.

The 3D printing of biomaterials like silk can do wonders for the circular economy. Scientists from the Delft University of Technology (TU Delft) recently used a novel 3D bioprinting technique to create a living ‘artificial leaf’ material made of algae cells. The research involved 3D printing microalgae directly into a non-living bacterial cellulose – an organic excretion compound produced by bacteria – to create a tough and flexible material capable of photosynthesis.

Elsewhere, researchers at the Singapore University of Technology and Design (SUTD) have previously explored the 3D printing of chitin-based bio-composites sourced from urban food waste. Specifically, the team used black soldier flies to bioconvert organic food waste into a 3D printable chitin, which was used to fabricate biodegradable natural-form objects.