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Researchers Review 3D Printing with Biomass-Derived Composites

Researchers from State University of New York College of Environmental Science and Forestry, University of Tennessee, Oak Ridge National Laboratory, and The University of Tennessee Institute of Agriculture have come together to research materials for 3D printing, specifically with a focus on composites made from biomass.

Composites are being used more widely as users explore materials that are magnetic, PLA additives, mixtures of recycled polymers, graphene, and more. More natural materials like wood, and specifically lignin, have been a source of experimental studies, too, in efforts to strengthen certain materials for given applications.

In this study, the authors focus on lignocellulosic biomass and derivatives, reviewing and analyzing a variety of additives for 3D printing. Their findings have been released in the recently published ‘3D printing of biomass-derived composites: application and characterization approaches.’

The study of materials is often just a means to an end for the manufacturing of high-performance parts; however, there is also a need for more biocompatible and environmentally-friendly polymers. Because lignocellulosic biomass is natural—and plentiful—it is used in creating bio-fuels, paper, and more, composed of:

  • Cellulose
  • Hemicellulose
  • Lignin
  • Proteins
  • Other extractives

“The development of biomass-derived materials using 3D printing technology as an alternative to fossil oil-based plastics will provide an opportunity to achieve sustainable and renewable bioeconomy,” explained the authors.

Number of patents for cellulose and biomass-derived in 3D printing.

Renewable materials have been a substantial source of study, especially in terms of cellulose.

(a) Schematic of the tree hierarchical structure illustrating the role of cellulose. Reprinted with permission from ref. 44. Copyright 2011 Chemical Society Reviews. (b) SEM image of the CNF, scale bar 6 μm. Reprinted with permission from ref. 30. Copyright 2019 Advanced Functional Materials. (c) TEM image of CNCs, scale bar 100 nm. Reprinted with permission from ref. 30. Copyright 2019 Advanced Functional Materials. (d) SEM image of BC produced by Komagataeibacter xylinus. Scale bar 5 μm. Reprinted with permission from ref. 46. Copyright 2017 RSC Advances.

There have been 5,100 patents for 3D printing with cellulose since 2015:

“This trend implies that the application of biomass and its components in 3D printing has become a hot topic and cellulose for 3D printing has been widely used,” stated the authors.

(a) Mechanism of FDM/FFF. (b) “Printing zone” defined in an FDM printing (ABS, HIPS and NBR41–HW represent acrylonitrile–butadiene–styrene, high impact polystyrene and acrylonitrile butadiene rubber with 41 mol% of nitrile contents, respectively). Reprinted with copyright permission from ref. 25. Copyright 2018 Science Advances.

Common methods of 3D printing with composites include fused deposition modeling (FDM), direct-ink writing (DIW), stereolithography (SLA), direct light processing (DLP), and binder jetting.

Mechanism of SLA/DLP. Redrawn based on ref. 86. Copyright 2019 ACS Omega.

DIW 3D printing is considered by the authors to be a good candidate for use with cellulose, due to its fluidic properties—meaning that it disperses well in water, acting as a suspension. Hydrogels can be created this way for a variety of bioprinting endeavors, especially due to good rheological properties like shear-thinning, good yield stress for encouraging stability, and both finite elastic modulus and rapid elastic recovery. Previous research recommends CNF hydrogels for use in neural bioprinting, while other additives like chitin (added between the gaps of fibers) may be used in applications like building wind turbine blades. Further crosslinking and ‘enhancing of mechanical performance’ can also result in smart materials, able to respond, and deform to changes in environment like temperature or moisture levels.

Various studies on 3D printing of cellulose and cellulose derivates. (a) The alignment of cellulose nanofibers (CNF) and nanocrystals (CNC) controlled by the flow in a DIW printing (left), leading to a strong aerogel hook (right). Reprinted with permission from ref. 30. Copyright 2019 Advanced Functional Materials. (b) DCW printing of cellulose composites with other biomasses. Reprinted with permission from ref. 36. Copyright 2019 Advanced Materials Technologies. (c) Wood cell mimicking structure combined FDM printing structure with UV-cured resin and CNC. Reprinted with permission from ref. 67. Copyright 2018 Materials & Design. (d) SLA printing of CNC reinforced structure that can be used in medical fields. Reprinted with permission from ref. 39. Copyright 2017 ACS Applied Materials & Interfaces. (e) MC-assisted ceramic slurry showed unique rheology behavior on printing. Below two images showed the prototype (left) and the sintered counterpart (right). Reprinted with permission from ref. 76. Copyright 2019 Journal of Alloys and Compounds. (f) CA-based oil/water separation mesh and its anti-oil-fouling property. Reprinted with permission from ref. 77. Copyright 2019 ACS Applied Materials & Interfaces.

In SLA 3D printing, researchers have also been using resin/cellulose blends more often—and especially in medical applications when materials are biocompatible.

Studies on lignin 3D printing. (a) FDM printing process of lignin-included composite that owns the highest reported lignin contents (60 wt%) and the printed oak leaf. Reprinted with permission from ref. 93 Copyright 2018 Science Advances. (b) SLA printing of lignin-included resin that showed an improvement of the tensile strength. Reprinted with permission from ref. 86. https://pubs.acs.org/doi/abs/10.1021/acsomega.9b02455, Copyright 2019 ACS Omega. Further permissions related to the material excerpted should be directed to the ACS. (c) Modified lignin in SLA printing can be printed with the highest concentration of 15 wt%. Reprinted with permission from ref. 41. Copyright 2018 ACS Applied Materials & Interfaces. Further permissions related to the material excerpted should be directed to the ACS.

“Lignin, the second most abundant terrestrial biopolymer after cellulose, has been under-utilized and, to date, is mostly used for direct combustion,” stated the authors. “Therefore, the valorization of lignin has drawn great attention in the current biorefinery process. Given that lignin contributes to the hydrophobicity, antimicrobial, and antioxidant activities of the plant cell wall, it can be a reinforcing agent in 3D printing composites.”

3D printing of lignin composites

Lignin offers a range of potential uses, but especially in flame retardant products, anti-aging, and absorption of UV rays. Previous research has also proved its uses for improving tensile performance with better layer adhesion, improvement in drug delivery systems with 3D printed PLA/lignin/tetracycline materials, as well as decreases in warpage and shrinkage. Binder jetting was also used in experimentation with starch-based drug delivery systems.

In contrast to production and uses of cellulose and lignin, whole biomass is simpler to deal with—lacking any requirements for complicated processing, whether physical or chemical. Researchers have created materials like for mud-straw walls in construction of basic structures, with wood emerging as ‘one of the most popular biomasses in 3D printing applications.’

3D printing of wood composites

As researchers continue to create new wood composites, FDM 3D printing is being used for increased tensile strength, along with other properties; however, experimenting with printing parameters has also been critical in printing materials like PLA with a recycled pine wood additive.

“… understanding the structure of biomass component(s) remains important, along with the utilization of the supramolecular structures, like crystallinity, material anisotropy and interfacial interactions need to be well-studied to help reach the target property of the 3D printed biomass-derived materials,” concluded the authors. “For the processing, various parameters such as the printing resolution and part production rate are the areas that have to undergo further engineering for making 3D printing competitive with conventional material fabrication technologies.

“In the future, the use of compatibilizers and modification of interfacial chemistry may enhance bonding and distribution of biomass-derived fillers with plastics, which could significantly improve the concentration of the fillers with moderate strength to alleviate depletion, at least partially, of the petroleum-based materials. Moreover, characterization techniques that are tailored to the final commercial application can be valuable to better assess the strengths and weaknesses of printed materials. An in-site characterization that has been applied to metal 3D printing is also a possible approach in this field to promote the printing quality.”

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[Source / Images: ‘3D printing of biomass-derived composites: application and characterization approaches’]

 

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MIT Developments: A Faster 3D Printer and Antibacterial 3D Printed Cellulose

(L to R) Adam Stevens and A. John Hart [Image: Stuart Darsch]

Researchers at MIT have developed a new 3D printer print head that can deposit material at extremely high speed, creating objects in minutes instead of hours. A. John Hart, a professor of mechanical engineering and director of the Laboratory for Manufacturing and Productivity and the Mechanosynthesis Group at MIT, is an expert on 3D printing and is working to advance the development and adoption of the technology. One thing 3D printing needs to be, he believes, is faster.

Hart worked with doctoral student Adam Stevens and graduate Jamison Go, who now works as a mechanical engineer at Desktop Metal, to study several commercial FFF desktop 3D printers. The team concluded that the 3D printers’ volumetric building rates were limited by three factors: how much force the print head could apply as it pushed the material through the nozzle; how quickly it could transfer heat to the material to get it to melt; and how fast the printer could move the print head.

They then developed a 3D printer that circumvents all three of those limitations. The design involves a filament with a threaded surface that goes into the top of the print head between two rollers that keep it from twisting. It then enters the center of a rotating nut, which is turned by a motor-run belt and has internal threads that mesh with the external threads on the filament. As the nut turns, it pushes the filament into a quartz chamber surrounded by gold foil. A laser then enters from the side and is reflected multiple times by the gold foil, passing through the filament and heating it.

The filament then enters a hot metal block where it is heated to a temperature above its melting point. It continues to melt and narrow as it descends and is eventually extruded.

It sounds complicated, but it’s much faster than standard 3D printers, in which the filament is pushed by two small, rotating wheels. If you try to speed things up by adding more force, the wheels lose traction and the filament stops moving. That’s not an issue with the MIT design; the matching threads on the filament and nut ensure maximum contact between them, and the system can transfer a high force to the filament without losing grip.

Typical 3D printers also rely on thermal conduction between the moving filament and a heated block. A higher feed rate may not completely melt the filament, but preheating the filament with a laser ensures that it is entirely melted by the time it gets to the nozzle. Tests showed that the researchers’ print head can deliver at least two and a half times more force to the filament than standard desktop 3D printers can, achieving an extrusion rate 14 times greater.

Because the extrusion rate was so high, the researchers needed to find a way to move the print head fast enough to keep up. They designed an H-shaped metal overhead suspension gantry that has a continuous belt that travels around pulleys powered by two motors mounted on the stationary frame. The print head sits on top of a stage that is connected to the belt and is carried quickly and smoothly through the prescribed positions within each plane.

The researchers 3D printed a series of test objects, including a pair of eyeglass frames, which took 3.6 minutes; a small spiral cup, which took just over six minutes; and a helical bevel gear in just over 10 minutes. The printed layers were highly uniform, and the parts showed themselves to be strong and robust in tests of their mechanical properties. The researchers also 3D printed the same object with their printer and several commercial models: a triangular prism 20 mm tall. For a comparable resolution, the printer achieved an average volumetric build rate up to 10 times higher than the other printers.

The printer wasn’t without its issues; for example, the high build rates resulted in layers that did not adhere well, as well as distortion. These problems were solved, however, by directing a controlled flow of cooling air onto newly extruded material. The researchers are also working on improving the printer’s accuracy by coordinating the extrusion rate and print head speed, as well as implementing new control algorithms.

MIT’s 3D printer prototype cost about $15,000, making it an unlikely candidate for replacing most desktop models. However, it could be competitive with some higher-end professional 3D printers.

Hart and his team are also working on developing new 3D printing materials that are environmentally friendly and easy to source – like cellulose. Cellulose has many advantages: it’s inexpensive, biodegradable, renewable, robust and chemically versatile. It’s difficult to 3D print, however, because it tends to decompose when heated.

Hart and former postdoc Sebastian Pattinson worked with cellulose acetate, a chemically treated form of cellulose that has fewer hydrogen bonds and thus makes it less prone to decomposition. First, the cellulose acetate was dissolved in an acetone solvent to form a viscous material that flows easily through a printer nozzle at room temperature. As the mixture spreads across the print bead, the acetone solvent quickly evaporates, leaving the cellulose acetate behind. Immersing the printed object in sodium hydroxide removes the acetate and restores the full network of hydrogen bonds that give cellulose its strength.

The researchers were able to 3D print complex objects with good mechanical properties from the material. Their strength and stiffness were even found to be superior to objects printed from common 3D printing materials. The researchers then began experimenting further.

“You can modify cellulose in different ways, for example, to increase its mechanical properties or to add color,” said Hart.

The researchers modified the cellulose acetate by adding antimicrobial properties. They 3D printed a series of disks, some from plain cellulose acetate and some with antimicrobial dye added, and deposited a solution containing E. coli bacteria on each one. They left some of the disks in the dark and exposed others to light from a fluorescent light bulb. After 20 hours, analysis showed that the disks made with dye and exposed to the light had 95 percent fewer bacteria than the others. They then 3D printed surgical tweezers as an example of a tool that could be made with the valuable antimicrobial properties.

Hart believes that there is commercial potential for their cellulose 3D printing process. Cellulose is inexpensive and widely available, and can be printed at room temperature, eliminating the need for a costly heat source. As long as the acetone is captured and recycled, it’s also an environmentally friendly process.

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[Source: MIT]