What are aerogels and what are they made of?
Aerogels are nanostructured, open-porous solids produced with the sol-gel method: Firstly, several materials are combined to create a clear liquid in which they react with each other to form solid, nanoscale particles, so-called colloids. The particles bond to chemically form a 3D particle network structure. In this moist gel, which consists of the particle network and the pore liquid, the liquid is step by step replaced by CO2 in a pressure container. The container is heated, the pressure increases and the CO2 becomes overcritical. In short: CO2 shows characteristics between gas and liquid which means the ultrafine gel structure can dry without collapsing.
Conventional aerogels measure between 10 and 100 nm. New definitions of aerogels include nanofoams and pseudo-nanofoams that measure several hundred nanometres or even some micrometres.
In general everything that can gel can be turned into an aerogel, be it polymers, carbons, metal oxides or other mineral materials. Aerogels made from silica are by far the most widely used ones.
Where are aerogels used?
Aerogels are highly porous materials and depending on their basic component they can be used in different applications. Most of the aerogel material classes (with the exception of silica and selected polymer aerogels) are not yet commercially available.
Today, silica aerogels are mostly used for thermal insulation purposes as they are heat resistant up to 600 °C, ultralight, not toxic and water repellent. They insulate steam pipes or deep sea pipelines, in which the oil has to have a high temperature in order for it to flow easily from one pump station to the next. Other aerogel classes can be used in catalysis or to filter pollution particles from the environment or to remove oil spills.
And in medicine?
In medicine, biopolymers such as pectin, cellulose and chitosan are interesting since they are natural and biodegradable substances. Aerogels can also be used for so-called timed-release products: When for example an agent such as paracetamol is built into an aerogel, it cannot crystallize. Minute amorphous clusters are formed in nanoscale pores, i.e. the agent assumes different solution and release kinetics. In other words: The moment an agent is locked into an aerogel, the time of its release can be controlled.
In pulmonary applications, the agent docks onto an aerogel and can be used in respirators such as sprays.
You printed a 3D silica aerogel: What was special about this project?
In one of the best known 3D printing processes thermoplastic materials are melted and then cooled. Since the molten material has the desired viscosity the object to be printed layer by layer. This does not work with aerogels. In order to print them we need ink or another printable formula which maintains the same degree of viscosity for many hours. During the printing process, ink must no gel but has to remain a thin liquid. However, if it is too thin it flows all over the surface. This predicament had to be solved.
We use the direct-ink-writing method which allows us to precisely control the flow and gel properties of the silica ink in a way that both discrete structures and wafer-thin membranes can be printed. We take aerogel powder particles and suspend them in a sol which subsequently gels. This involves a liquid phase which contains solids in particle form and flow properties similar to toothpaste. This allows printing.
Where can printed aerogels be used?
Thanks to our 3D printed gels we can now also produce very delicate structures with aerogels without having to cut them from an aerogel block, which in most cases is next to impossible anyway. This means we now have entirely new possibilities to prepare complex 3D aerogel shapes.
The printed structures can have a thickness of as little as a 1/10th of a millimetre. Thermal conductivity of silica aerogel is about 16 mW/(m*K). Thus we can thermally separate even minute electronic components or layers. We can use conductive aerogel inks to print and embed structures in 3D objects. Another possibility is the use of conductive sensor elements for on-body sensors or wearable bioelectronics.
Another field of application is the insulation of heat sources in implants. Electrical currents create heat above body temperature. A local hot spot develops which can be a problem for tissue. In order to protect the tissue the surface of implants should never be more than 37 °C. Aerogels can thermally insulate implants and either discharge heat or distribute it across a larger surface.
Biopolymers can be used in scaffolding to grow tissues, cell cultures and at some point even organs. 3D printing of bone material is also interesting. Calcium apatite might be suitable as is gels in a biopolymer sol.
The basic principles can also be applied to other material class combinations. Think of hybrids of silica and biopolymers or different inks that contain different material systems and create 3D networks. In short, 3D printed aerogel structures can be integrated in many new high-tech applications.
The research "Additive manufacturing of silica aerogels" was published in "Nature".
Dr Matthias Koebel received his PhD in physical and surface chemistry from Brown University followed by a two year postdoctoral fellowship at the University of California, Berkeley with Professor Gabor Somorjai. His research activities range from preparative sol-gel chemistry to scale-up of soft-chemistry derived nanomaterials. He currently heads the Building Energy Materials and Components laboratory at Empa, targeting next generation, nanoporous aerogel materials for thermal insulation and energy applications. He is a known expert in aerogels, published close to 100 scientific articles, numerous patents and is co-editor of the “Aerogels Handbook”.
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