Existing techniques for creating nano-structures are limited in what they can accomplish. Etching patterns onto a surface with light can produce 2D nano-structures but doesn’t work for 3D structures. It is possible to make 3D nano-structures by gradually adding layers on top of each other, but this process is slow and challenging. And while methods exist that can directly 3D-print nanoscale objects, they are restricted to specialized materials like polymers and plastics that lack the functional properties necessary for many applications. Also, they can only generate self-supporting structures; for example, the technique can yield a solid pyramid, but not a linked chain or a hollow sphere.
A technique for high-resolution imaging of brain tissue was adapted to address this problem. This technique, known as expansion microscopy, involves embedding tissue into a hydrogel and then expanding it, allowing for high-resolution imaging with a regular microscope. Using the technique, any shape and structure can be created by patterning a polymer scaffold with a laser. After attaching other useful materials to the scaffold, it is shrunk, generating structures one-thousandth the volume of the original. These tiny structures could have applications in many fields, from optics to medicine to robotics.
By reversing this process, large-scale objects embedded in expanded hydrogels can be created and then shrunk to the nanoscale, an approach called implosion fabrication. A very absorbent material made of polyacrylate, commonly found in diapers, is used as the scaffold for the nanofabrication process. The scaffold is bathed in a solution that contains molecules of fluorescein that attach to the scaffold when they are activated by laser light.
Using two-photon microscopy that allows for precise targeting of points deep within a structure, the researchers attach fluorescein molecules to specific locations within the gel. The fluorescein molecules act as anchors that can bind to other types of molecules that are added.
Once the molecules are attached in the right locations, the researchers shrink the entire structure by adding an acid. The acid blocks the negative charges in the polyacrylate gel so that they no longer repel each other, causing the gel to contract. Using this technique, the researchers can shrink the objects tenfold in each dimension (for an overall 1,000-fold reduction in volume). This ability to shrink not only allows for increased resolution, but also makes it possible to assemble materials in a low-density scaffold. This enables easy access for modification, and later the material becomes a dense solid when it is shrunk.
Currently, the researchers can create objects that are around 1 cubic millimeter, patterned with a resolution of 50 nanometers. There is a tradeoff between size and resolution: If the researchers want to make larger objects, about 1 cubic centimeter, they can achieve a resolution of about 500 nanometers; however, that resolution could be improved with further refinement of the process.