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Empa: Airy cellulose from the 3D printer

Cellulose aerogel is versatile, lightweight, thermally insulating, and biodegradable. Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) have succeeded in using 3D printing to mold this natural material into complex shapes that could serve as precision insulation in microelectronics or as personalized medical implants.

Biodegradable materials, inks for 3D printing, and aerogels may not seem to have much in common at first glance. However, all three are promising for the future because

• "Green" materials do not harm the environment.

• with a 3D printer, complex structures can be created without wasting material, and

• according to researchers, the ultra-light aerogels are excellent thermal insulators.

The Empa researchers, led by Deeptanshu Sivaraman, Wim Malfait, and Shanyu Zhao from the Empa laboratory "Building Energy Materials and Components," in collaboration with the "Cellulose & Wood Materials" and "Advanced Analytical Technologies" laboratories and the Center for X-ray Analytics, have succeeded in combining all these advantages in a single material.

As a starting material, the researchers chose the most common biopolymer on Earth: cellulose. From this plant material, different nanoparticles can be obtained through simple processing steps. Two types of such nanoparticles—cellulose nanocrystals and cellulose nanofibers—were used by doctoral student Deeptanshu Sivaraman to create the "printer ink" for the bio-aerogel.

Ink made from cellulose and over 80 percent water

The flow behavior of the ink is crucial in 3D printing: It must be sufficiently viscous to maintain a three-dimensional shape before curing. At the same time, it should liquefy under pressure so that it can flow through the printer nozzle. Sivaraman succeeded in achieving this with a combination of nanocrystals and nanofibers: The long nanofibers give the ink high viscosity, while the shorter crystals ensure that it reacts as shear-thinning, thus becoming temporarily liquid during printing.

The ink contains about twelve percent cellulose and 88 percent water. "We were able to achieve the required properties with cellulose alone, without any additives or fillers," says Sivaraman. This is not only beneficial for the biodegradability of the finished aerogel but also for its heat-insulating properties. For the ink to become an aerogel after printing, the researchers replace the solvent water in the pores first with ethanol and finally with air—without deforming the printed object. "The less solid material the ink contains, the more porous the resulting aerogel," explains Zhao.

This high porosity and the small size of the individual pores make all aerogels extremely effective thermal insulators. However, the researchers have discovered a special property in the printed cellulose aerogel: it is anisotropic. This means that its strength and thermal conductivity depend on the direction. Thus, researchers can control in which axis the printed aerogel piece should be particularly stable or insulating. Such precisely insulating components could be used, for example, in microelectronics, where heat is allowed to be conducted in only one specific direction.

Great potential in medicine

Although the original research project, funded by the Swiss National Science Foundation (SNF), primarily focused on thermal insulation, the researchers quickly identified another application for their printable bio-aerogel: medicine. Since it is made from pure cellulose, the new aerogel is biocompatible with living tissue. Its porous structure can absorb medications and then release them over an extended period in the body. Additionally, 3D printing offers the capability to produce precise shapes that could serve as scaffolds for cell growth or as implants.

Particularly advantageous: The printed aerogel can be rehydrated and dried multiple times after the drying process without losing its shape or porous structure. In practical applications, this would simplify handling: the material could be stored and transported in dry form and rehydrated shortly before use. When dry, it is not only lightweight and manageable but also less susceptible to bacteria – and does not require elaborate protection from drying out.  (mi)

“This article was first published on our sister portal 
"MM Maschinenmarkt" (German Edition), Vogel Communications Group.“