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Towards Sustainable 'Biofriendly' Materials for Additive Manufacturing (Part 2 of 3)

A spot on the wing of the butterfly Bicyclus anynana; scale: image width = 1.7 mm

Image (cropped here) courtesy of Gilles San Martin (CC BY-SA 2.0)
By Susan Gladwin and Michael Floyd

Consider the humble mollusk and its byssus. If you’ve ever eaten a mussel and removed its “beard” (the byssus), you might consider it a nuisance. But these essential silky filaments are what the creature produces to attach itself to home base and avoid drifting all over the ocean. Transforming foam into thread, mollusks manufacture an amazing adhesive that holds its shape and stickiness in water, and is unlike anything humans can currently fabricate. 

It’s this kind of everyday manufacturing miracle that inspired researchers in the Fall 2015 Greener Solutions class at the University of California Berkeley Center for Green Chemistry (UCBCGC). In pursuit of more sustainable additive manufacturing and a better 3D printing user experience, Autodesk supported this class, in which researchers were tasked to use biomimicry as a lens for sustainable materials innovation through green chemistry[1]. Equipped with 11 bio-inspired design strategies borrowed from the natural world, the class spent the fall semester developing promising avenues for the development of high performance 3D printing materials that are “biofriendly” and sustainable. You can explore the 11 design strategies in detail here.

The UCBCGC research team began by asking: What can we learn from nature’s way of 3D printing? The natural world presents us with 3.8 billion years of macro- and micro-biological problem solving from which to borrow inspiration. “What always drew me to the fundamental sciences was a fascination with the natural world, and a deep appreciation for how elegant its solutions are,” says team member Coleman Rainey, a researcher at Lawrence Berkeley National Laboratory. In 3D printing, we might investigate how nature generates solid forms from liquid sources, creates adhesion, and builds in layers. Once we have understood how living systems perform these functions, we can fold our insights back into the technology we create, learning from natural systems to transform human systems.

Earlier research commissioned by Autodesk yielded 11 bio-inspired design strategies, all identified as especially relevant to 3D printing. Looking into these stimulated new ideas about materials for stereolithography (SLA) processes, enabling the researchers to explore fresh ground. “Previously, the industry has been thinking about chemical bonding as a way to transition liquid resin to a solid product,” says Chen Cheng, a Chemistry PhD student, “but that is first of all not reversible, and secondly it involves hazardous chemicals. Today we have been thinking about reversible phase changes that don't really require any chemical bonding… and could allow us to explore a much wider range of chemical resins.”

Of the 11 strategies, several stand out for the research team as especially compelling.

Hierarchy Across Linear Scales

When we first glance at an object in nature, like a butterfly’s wing, we may see only its bulk form. But closer examination reveals breathtaking layers of structural detail. If we zoom in to the microscopic level, we discover intricate forms whose design helps define how the wing functions as a whole. Life shapes matter by starting from the tiniest level, and building up materials that function at the bulk scale. This is a totally different approach to materials than we’re used to with, say, cast metal or injection-molded plastic.

“My favorite [design strategy] to think about recently has been a hierarchy across linear scales,” says Rainey, “recognizing that there is a profound relationship between the molecular scale, the nano scale, the meso scale, and the bulk scale. The way those are related, and the ways that a lobster… or a mollusk shell produces and processes its materials across those scales… usually involves just one or two materials. But the rich diversity of ways in which you can structure that material on all those scales produces an incredibly wide range of material properties. It's just something that humans have not been able to do. That's why additive manufacturing is so exciting! It starts to open up and loosen the constraints of how we can manipulate matter, not on just one particular scale, but actually tuning the architecture at both the molecular and mesoscopic scale.”

The Optimal Activator: The Environment is the Catalyst

Within the field of additive manufacturing there are many different ways to build up material layer by layer. All of them require some process for manipulating that material - temporarily melting it, binding it together, or turning it from a liquid into a solid. The potential activators for these processes include light, temperature, and pressure. In the typical stereolithography (SLA) process, projected near-UV light (405 nm wavelength) is a common activator that initiates polymerization, “curing” liquid resin and transforming it into a solid polymer. There are several other environmental conditions that could be used or combined with this process to achieve a phase change, and pH is one such condition that has a great deal of unexplored potential. “We’re looking a little bit more into pH change,” says researcher Lee Ann Hill, a Master of Public Health candidate. “In terms of biomimetic principles, when a substrate that's in an acidic environment hits an alkaline environment like seawater, it goes through a phase change. So that's one of the main strategies we're looking at, and it was inspired by the environment itself as a catalyst.”

Functionally Graded Material

“Functionally-graded materials,” Rainey explains, “are those whose chemical or material functionality changes dependent on your position in that material. So you can have something that transitions from being super rigid to being super flexible, or something that transitions from being super conductive to being super insulating, or something of that nature." The beak of the Humboldt squid is often cited as an example of this design strategy at work in nature. It’s the hardest non-mineral material found in nature yet this same sharp appendage smoothly transitions through a gradient to the soft jelly-like body of the animal, all as a function of how it is made. Rainey states that when it comes to 3D printing, “the nature of the technology implies that you are constructing something from the bottom up, and you're dictating the properties at each layer.” This enables the creation of new kinds of objects with finely tuned mechanical properties, like the squid’s beak, which would be impossible to create with traditional fabrication processes.

Water is the Universal Medium

The existing liquid resins used in SLA 3D printing are chemically derived from petroleum, a non-renewable resource, and require cautious handling. But their formulations do have the benefit of being highly reactive, allowing for tightly controlled chemical reactions that can create high-resolution 3D prints. Achieving the same quality of result with a safer, non-petroleum formulation (such as a water-based material) is no easy task.

Using water as a medium in the SLA process would mean a paradigm shift in how chemistry is performed within the 3D printer. Despite the challenges associated with such an approach, the likely benefits of an aqueous medium are too enormous to ignore: extreme recyclability, multiple functionality, and common, local, and cheap sourcing, to name a few. Cheng further elaborates, “A lot of chemical processes are incompatible with water or oxygen, which acts as an inhibitor of the current SLA process, and this can be a challenge or limitation. So the way that nature makes things allows us to rethink the whole process, and that allows us to completely bypass those restrictions.” Water-based processes could also reduce or eliminate the use of synthetic solvents, which come with their own set of human and environmental problems in the cleaning of SLA prints.

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Through biomimicry, the UCBCGC researchers are seeking pathways to safer and more sustainable material solutions. By helping materials researchers explore digital fabrication techniques that are more in tune with natural processes, a biomimetic approach is more likely to deliver results that respect the needs of living organisms and the constraints of our finite planet. As Cheng puts it, “If the end goal is to minimize the impact on the environment, just taking what nature is already using is probably the best way.” Where better to look for inspiration than at the materials, forces, and processes that have powered 3.8 billion years of 3D printing in the natural world?

In the first installment of this series we examined how biomimicry and green chemistry can work together to support biofriendly innovation for SLA resins. In the next and final installment, we will further explore the research insights the students gained and the promising directions for continued investigation they proposed.

[1] http://www2.epa.gov/greenchemistry/basics-green-chemistry