‘Implosion fabrication’ technique shrinks objects to nanoscale size

December 31, 2018

By Rich Pell

Medical Electronics Analog Flexible Electronics Materials & processes Test and Measurement Optoelectronics Sensing / Conditioning Electromechanical

The system, say the researchers, produces 3-D structures one thousandth the size of the originals. It can be used to pattern objects with a variety of useful materials, including metals, quantum dots, and DNA.

“It’s a way of putting nearly any kind of material into a 3-D pattern with nanoscale precision,” says Edward Boyden, the Y. Eva Tan Professor in Neurotechnology and an associate professor of biological engineering and of brain and cognitive sciences at MIT, as well as a senior author of a paper on the research.

Using the new technique, the researchers say they can create any shape and structure they want by first patterning a polymer scaffold with a laser. After attaching other useful materials to the scaffold, they shrink it, generating structures one thousandth the volume of the original.

The nanoscale structures could have applications in many fields, from optics to medicine to robotics, say the researchers. The technique uses equipment that many biology and materials science labs already have, making it widely accessible for researchers who want to try it.

The new approach overcomes limitations of existing techniques for creating nanostructures. For example, etching patterns onto a surface with light can produce 2-D nanostructures but doesn’t work for 3-D structures. And while it is possible to make 3-D nanostructures by gradually adding layers on top of each other, this process is slow and challenging.

And, while methods exist that can directly 3-D print nanoscale objects, they are restricted to specialized materials like polymers and plastics, which lack the functional properties necessary for many applications. Furthermore, say the researchers, they can only generate self-supporting structures.

The new technique was adapted from an approach the researchers had developed years earlier for high-resolution imaging of brain tissue. Known as expansion microscopy, it involves embedding tissue into a hydrogel and then expanding it, allowing for high resolution imaging with a regular microscope.

Expansion microscopy is now being used by hundreds of research groups in biology and medicine to enable 3-D visualization of cells and tissues with ordinary hardware. By reversing this process, the researchers found that they could create large-scale objects embedded in expanded hydrogels and then shrink them to the nanoscale – an approach that they call “implosion fabrication.”

The researchers used a very absorbent material made of polyacrylate – commonly found in diapers – as the scaffold for their nanofabrication process. The scaffold is bathed in a solution that contains molecules of fluorescein, which attach to the scaffold when they are activated by laser light.

Using two-photon microscopy, which 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 the researchers add.

“You attach the anchors where you want with light, and later you can attach whatever you want to the anchors,” says Boyden. “It could be a quantum dot, it could be a piece of DNA, it could be a gold nanoparticle.”

Graduate student Daniel Oran adds, “It’s a bit like film photography – a latent image is formed by exposing a sensitive material in a gel to light. Then, you can develop that latent image into a real image by attaching another material, silver, afterwards. In this way implosion fabrication can create all sorts of structures, including gradients, unconnected structures, and multimaterial patterns.”

Once the desired molecules are attached in the right locations, the entire structure is shrunk 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 10-fold 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.

“People have been trying to invent better equipment to make smaller nanomaterials for years,” says graduate student Samuel Rodriques, “but we realized that if you just use existing systems and embed your materials in this gel, you can shrink them down to the nanoscale, without distorting the patterns.”

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, the researchers say.

The researchers are now exploring potential applications for the technology. They anticipate that some of the earliest applications might be in optics – for example, making specialized lenses that could be used to study the fundamental properties of light.

The technique might also allow for the fabrication of smaller, better lenses for applications such as cell phone cameras, microscopes, or endoscopes, say the researchers say. Looking farther in the future, the researchers say that the approach could be used to build nanoscale electronics or robots.

“There are all kinds of things you can do with this,” says Boyden. “Democratizing nanofabrication could open up frontiers we can’t yet imagine.”

For more, see “3D nanofabrication by volumetric deposition and controlled shrinkage of patterned scaffolds.”