Image (cropped here) courtesy of Gilles San Martin (CC BY-SA 2.0)
By: by Tom McKeag, Berkeley Center for Green Chemistry
Prepared for: the Autodesk Green Resin Project / the Biomimicry Institute
April 23, 2015
We will consider several themes from nature concerning the manufacture of materials in order to recommend new approaches to material selection for the most common SLA printer resins. Additive manufacturing shows great promise for making things in a way that more closely resembles natural form, but some of the below strategies are worth investigating to reduce or eliminate toxicity and to make more functional products.
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Many of the forms seen in nature are a result of millions of years of natural selection towards structural advantage. Often that advantage means the saving of material and energy by using a better shape. A common example is the bone structure of birds, where lightness and strength are required and stresses are resisted by the geometry of the bone. Moreover, extra material is grown only in those places experiencing greater stress and as a result of that stress.
This sort of shape optimization is being employed currently in many of the CAD files that direct the objects made in AM machines. It is also being investigated in the manufacturing process of SLA with a recent method by Formlabs to use a natural shape algorithm in printing the expendable object supports in their 3D printer (3). In the AM realm this means a savings in material and time, money for customers and the burden on the earth.
Shape optimization could be investigated in other aspects of the SLA process. For example, objects are currently being cured in solid horizontal layers and any voids are the result of direction from the STL file and occur at the 50 micron scale and above. It may be possible to initiate voids within the liquid resin itself at a smaller linear scale. It may also be possible to create new shapes by how the resin is laid up, using a “cookie batter” approach, or tacky pull away method, for example. By way of comparison, the injection molding industry has long demonstrated the ability to make structural foams by injecting gas into the polymer mix before molding in order to make a stronger, more durable and lightweight product.
Recently Lawrence Livermore Laboratory and MIT have demonstrated the ability to make an ultra lightweight, strong foam using SLA methods (4). They constructed a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. They reported that all arrays exhibit ultrastiff properties across more than three orders of magnitude in density, regardless of the constituent material (5). This is ground-breaking work and appears to be a continuation of the trend to push the performance limits of both materials and methods.
This suggests re-evaluating some of the assumptions held in liquid resin methodology. For example, that photo initiated near-instant polymerization should be the only time-related criterion considered when selecting new resin formulas. Tunability of the cure time, therefore, might be included in considerations. Density of the material and microarchitecture, as suggested above, are others.
Another natural strategy that could be developed is self-assembly and this strategy has structural implications, particularly at the smaller scales. For example, tuning the way the same material will self-assemble will determine different desired structures, satisfying our mini/max model by using shape rather than more or different material. Molecular recognition is the key concept in this process. The physics of attraction and repulsion are employed typically, such as results from the polarity of molecules.
An example of self-assembly in your body is the DNA base pairing discovered famously by Watson and Crick. Another example can be seen in our hemoglobin example above in the processes that fold this protein into its shape and allow enzymes to bind to it.
Photo-initiated polymerization in SLA can be seen as a form of self-assembly at the molecular scale. Further development would mean investigating the ability to finetune this process to produce a variety of results and to replicate those results consistently at different scales. Ordered structures have been built in the laboratory using functionalized organic molecules able to recognize each other and self-assemble (6). So-called Janus molecules, possessing different chemical properties on opposite sides, have been used to create complex arrays (7).
Nature builds things from the bottom up, using information to build in ever more complex arrangements. Emulating this type of construction is not easy; technological systems typically lack the sophisticated information control that makes growth, repair and adaptation possible in the natural world. This means research into not only how material can be made at the molecular level but how that level can be intelligently combined at larger linear scales to produce emergent properties. The field of supramolecular chemistry investigates how molecules can be organized in an articulated and well-defined way. The forces used are typically non-covalent bonding such as the hydrogen bonding commonly seen in complex organic structures.
Hair is a great example of solving structural problems across linear scales, employing shape for strength at many different sizes. Material is made therefore with many interfaces and these interfaces are capable of more control of certain problems like tearing. The Keratin protein is coiled and braided and sheathed and these strands are then bundled and this bundle is in turn sheathed before becoming part of an even larger bundle, eventually forming a scale-protected diameter that is still less than 100 microns. All of this intricate, hierarchical construction serves to make hair strong and durable, able to resist tensile stresses while being very thin and supple.
The typical SLA system currently polymerizes a batch of resin monomers in one exposure, into one material type and that material phase change is irreversible. If we were to use hair as a model, we might consider how ingredients within the resin bath could be sequentially polymerized and in different ways, and these components combined, possibly through self-assembly.
Hierarchically structured materials have yet to be scaled to large volume manufacturing although some examples are available from scientific and engineering laboratories. One interesting example germane to the liquid resin SLA method is the use of emulsions containing nanoparticles that self-assemble into complex hierarchical structures. This structural change can even be reversed using the external stimulus of pH change with an appropriately responsive polymer (8) (9).
There is a pressing need for new lightweight structural materials that are able to support more efficient technologies that serve a variety of strategic fields, such as transportation, buildings, and energy storage and conversion. It seems evident that as AM techniques become more sophisticated, 3DP will be used increasingly to create these materials. Any printing approach would have to incorporate nano-, micro- and macroscale features, and thus involve the so-called mesoscale approach (8).
Functionally graded means that a recipe of constituent parts can be subtly changed across space in order to respond to the stresses present or the job needing to be done. The basic ingredients do not change, but their relative ratios do, as well as how the material is arrayed, as discussed above. Plants are very good at this, often functionally grading leaf petioles or stems for the purpose of controlled deformation. Here, their parts will bend from the stresses of wind, say, and save the plant from catastrophic structural failure.
Functionally graded material (FGM), also known as Local Composition Control (LCC), can save material and energy in manufacturing by eliminating the need for separate parts and fittings (and the expense of making them). A simple example is a clothespin made of a bendable polymer rather than separate wood parts and a metal spring.
More sophisticated examples are being manufactured currently using the process of SLA. For example metal and ceramics have been successfully combined for high-performance materials in an alternative to the more traditional vapor deposition method for over 15 years. (10) Selective Laser Sintering of coaxial metal powders into FGM has also been demonstrated (11). The search for a more benign resin for SLA ought to include consideration of this very important application strategy.
Although nature is able to manufacture many different forms from one substance, structural challenges often present contradictions that cannot be solved with one material. The strategy of choice typically combines a hard mineral substance like hydroxyapatite with a flexible one like a protein or a polysaccharide. This composite construction can be done at different scales. A macro example is a tooth in which hard enamel covers soft dentine, affording an impact resistant surface that is also resilient. Eggshells, insect bodies, and the nacre of mollusks all exhibit this composite construction at the micro and nano scales.
As one might imagine this composite construction is typically combined with many of the other attributes listed above, especially hierarchical construction. Technology has for a long time produced simple composites like steel reinforced concrete. More recently biomineralized protein scaffolds have been used in biomedical applications. The synthetic replication of mollusc shell, a composite of calcium carbonate and proteins, has also been an area of sustained and active research, as has the embedment of stiff nanofibers in a gel matrix, much like fiberglass. A team at Case Western Reserve University, for instance, has even been experimenting with a way to tune the orientation of collagen nanofibers in a soft matrix in order to create either a stiff or flexible material on demand. This work had originally been inspired by the sea cucumber (12).
The market development trajectory of AM seems certain to include a shift in dominance from non-structural prototyping to higher performance working parts and objects. Any SLA material research that looks to the future should include this strategy for structural integrity.
Biology is a water-based realm, and living things both benefit and are limited by the properties of water. In the search for benign materials for SLA it cannot be ignored as a base or solvent. Water is unique: a covalently bonded polar molecule with a singular ability to form three-dimensional lattices of molecules using many mutual and relatively weak hydrogen bonds. It is the most common compound found on earth and the only common substance to occur naturally in all three states, solid, liquid and gas. It serves many physical functions beyond being a solvent, such as transporter, temperature regulator and structural component. Water also has a direct chemical role in biological functions such as the stabilizing of proteins and in the catalysis of enzymes.
Water fulfills these roles within a life-friendly range of temperature and pressure, and serves life at all scales. For example, its structural properties give support within the vacuole of a plant cell, maintaining hydrostatic pressure or turgor, while water’s high specific heat capacity makes it a critical moderating influence on the temperatures of the entire biosphere. Its importance cannot be overstated.
From a perspective of materials and mechanical performance, water has several additional properties germane to our discussion above. Water within a substance most often will impart a suppleness to the material that is absent without it. More precisely, it changes the modulus of elasticity of the material, thus making it more resilient. In material manufacture both strength and durability are desired and calibrating water content is one path toward this balance.
Taken further, water can be used as a medium with which to functionally grade a material or composite. In nature, one of the most impressive examples of this is the beak of the squid. The squid is a predatory soft-bodied mollusk with an extremely hard beak. The material of this beak is made of four ingredients, chitin, proteins, water and pigment, and it is functionally graded for stiffness in an elegant fashion, from the very soft to the very hard. In fact, the beak is the hardest wholly organic material known.
Using a water-based material in the SLA process would mean a paradigm shift in how chemistry is performed within the device. The ability of water to change states within naturally occurring temperature and pressure regimes demands a different approach that might have distinct advantages. Similarly, shrinkage and deformation of the final object would probably have to be controlled in a different way. Indeed, ways might be found to fine tune deformation toward a positive outcome, as nature often does. Despite the challenges of a new approach, the benefits of this medium are too enormous to ignore: extreme recyclability, multiple functionality, and common, local, and cheap sourcing are just a few.
Currently much work is being done in the development and application of hydrogels, particularly in the biomedical field. Hydrogels are crosslinked polymer networks that are highly hydrated and possess tissue-like elasticity. In particular, poly(ethylene glycol) (PEG),a synthetic hydrogel, has been used widely because of its hydrophilicity, biocompatibility, and ability to be chemically tailored. Hydrogels can also be made from natural materials like chitosan. Typically, they have been modified for use in SLA and other AM processes by the cross-linking with acrylates and methacrylates. Applications include tissue engineering, drug delivery vehicles, and cell carriers among many others. They are well suited to biomedical applications: permeable to oxygen, nutrients, and other water-soluble metabolites and have mechanical properties similar to many soft tissues. PEG for instance, is highly biocompatible and with added ingredients can be biodegradable. Natural material based hydrogels hold even more promise for sustainability (13).
In directing further research and development paths to obtain a greener resin for SLA printers, the industry can benefit from some lessons from nature. Ideally, the bio-inspired printer of the future will be able to make many different materials from just a few components, substitute shape for material in making strong parts, impart some information into the material and organize it hierarchically, and do all of this with a water-based recipe that is locally acquired, completely recyclable and benign to manufacture and use. Achieving this without the benefits of nature’s sophisticated information control machinery will not be easy, but even simple approximations of some of these strategies will yield appreciable benefits.