Researchers at the National Institute of Standards and Technology (NIST) have developed a new method for 3D printing of gels and other soft materials. Published in a new paper, it has the potential to create complex structures with precision at the nanometer scale. Because many gels are compatible with living cells, the new method can start the production of soft little medical devices such as drug delivery systems or flexible electrodes that can be inserted into the human body.
A standard 3D printer creates solid structures by creating sheets of material ̵
Using a 3D printer to make an object made of gel is “a bit more of a sensitive cooking process”, says NIST researcher Andrei Kolmakov. In the standard method, the 3D printer chamber is filled with a soup of long-chain polymers – long groups of molecules bound together – dissolved in water. Then “spices” are added – special molecules that are sensitive to light. When light from the 3D printer activates the special molecules, they sew together the chains of polymers to form a fluffy web-like structure. This scaffolding, still surrounded by liquid water, is the gel.
Normally, modern 3-D gel printers have used ultraviolet or visible laser light to initiate the formation of the gel stand. However, Kolmakov and his colleagues have focused their attention on another 3D printing technique for making gels using electron beams or X-rays. Because these types of radiation have a higher energy or shorter wavelength than ultraviolet and visible light, these rays can be more focused and therefore produce gels with finer structural detail. Such detail is exactly what is needed for tissue engineering and many other medical and biological applications. Electrons and X-rays offer a second advantage: they do not need a special set of molecules to initiate the formation of gels.
But at present, the sources of this tightly focused short-wave radiation – scanning electron microscope and X-ray microscope – can only work in one vacuum. This is a problem because the liquid in each chamber evaporates in a vacuum instead of forming a gel.
Kolmakov and his colleagues at NIST and at Elettra Sincrotrone Trieste, Italy, solved the problem and demonstrated 3D gel printing in liquids by placing an ultra-thin barrier – a thin sheet of silicon nitride – between the vacuum and the liquid chamber. The thin sheet protects the liquid from evaporating (as it normally would in a vacuum) but allows X-rays and electrons to penetrate into the liquid. The method allowed the team to use the 3-D printing method to create gels with structures as small as 100 nanometers (nm) – about 1000 times thinner than a human hair. By refining their method, the researchers expect to imprint structures on the gels as small as 50 nm, the size of a small virus.
Some future structures made with this approach may include flexible injectable electrodes to monitor brain activity, biosensors for virus detection, soft microrobots, and structures that can emulate and interact with living cells and provide a medium for their growth.
“We are bringing new tools – electron beams and X-rays that work in liquids – to 3D printing of soft materials,” says Kolmakov. He and his staff described their work in an article published September 16 in ACS Nano.
Electron motions in liquid measured in super slow motion
Tanya Gupta et al, electron- and X-ray-focused radiation-induced crosslinking in liquids: Towards fast continuous 3D nanoprinting and interfaces with soft materials, ACS Nano (2020). DOI: 10.1021 / acsnano.0c04266
Provided by the National Institute of Standards and Technology
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