Honey, I shrunk everything! Nanoscale 3D objects set for the big time
Image credit: Rex Features
Researchers at MIT have developed a new nano-shrinking technique that can fabricate nanoscale 3D objects, with potential applications in semiconductors and even DNA.
If you’ve ever seen a nappy drenched in water – or other less palatable fluids – you’ll know that they can expand quite a lot. What you might not know is that the same material can now be used, in reverse, to shrink objects to nano-scales. In fact, nappies might be about to blow the world of nano-technology wide open.
A new nano-shrinking technique using a polymer found in nappies was recently developed by a team of brain researchers at the Massachusetts Institute of Technology (MIT). The process, called implosion fabrication, involves taking the polymer, called sodium polyacrylate, and adding a fluid to make it swell. Lasers are then used to attach fluorescein molecules, creating anchor points to which the structure to be shrunk is fixed. This could be anything from a metal to a semiconductor or even a piece of DNA. The object can be shrunk to a thousandth of its original size while maintaining its overall shape and integrity.
The idea was developed from a process called expansion microscopy that the team already uses to swell brain tissue to sizes that can be studied through conventional microscopes. “The new process is very much in reverse,” says lead researcher Professor Edward Boyden. “Rather than forming a polymer then swelling it, we form the polymer, swell it, then shrink it again. We thought this might be a cheap way of making nano-technology, and that is what we found.”
If reversing an existing process sounds simple, it wasn’t. It took Boyden and his team four years to perfect the technique. “There was no guarantee that it wouldn’t shrink down to form incoherent nonsense,” says Boyden. “So, it’s a kind of fundamental discovery that we have here – that if you shrink something down, it preserves the nano-scale information.”
Sodium polyacrylate is a charged polymer that expands through the repulsion of its negative charges. Once swollen, fluorescein molecules are attached via a process called two-photon microscopy, which allows for precise targeting of points deep within a structure. Molecules of the object to be shrunk are attached to the fluorescein molecules, forming a kind of scaffold that maintains the overall pattern during shrinkage. Finally, an acid is added to the polymer which blocks the negative charges in the polyacrylate gel, causing it to contract again.
The process shrinks the structure 10-fold in each dimension (1,000-fold overall) creating objects as small as one cubic millimetre, patterned with a resolution of 50nm. Not only does this preserve the integrity of the overall structure but assembling materials in such a low-density scaffold allows for easy modification before shrinking occurs. Also, importantly, the process can produce non-self-supporting structures such as linked chains or hollow spheres – results that have so far proved unachievable in other nano-fabrication techniques.
In fact, implosion fabrication surpasses other methods in many ways. Existing techniques such as etching patterns onto a surface with light can only produce 2D structures. These can be built up into 3D elements by gradually adding layers, but the process is difficult and slow. Other methods can directly print nanoscale objects, but the materials they use are limited to polymers and plastics. Implosion fabrication can work with all kinds of materials including metals, semiconductors and even biological matter such as brain tissue or DNA.
This flexibility opens up a wide range of possible applications. One that looks immediately promising is in the field of optics. “If we can make conductors, like metals, and non-conductors, like insulators, in clever patterns,” says Boyden, “then maybe you can make new kinds of lenses or so-called meta-materials.”
Meta-materials are substances that have nano-structures built into their very fabric in order to enhance some particular property. Meta-materials in optical devices could be used to form a so-called ‘superlens’, which could go beyond the diffraction limit, enabling finer resolutions than conventional lenses. “You could in principle do some really clever things,” says Boyden. “So, in an era where cameras and self-driving cars and all sorts of things need lots and lots of lenses and cameras, this could be very interesting over a broad range of sizes.”
‘The idea that anybody could make a prototype in their garage is sweeping many industries now... So what if you could make nano-technology in the same way?’
Another exciting application might be in computing, where the cheap nature of the process could revolutionise how microchips are made. “Right now, most people can’t afford a billion-dollar factory to make microchips,” says Boyden. “But in the future if we could make computer chips, that would be interesting.”
“The PC started when the founders of Apple and other people were making computers in their garage. Imagine an era where you could make not just the computer in your garage, but the computer chip too.”
It is the affordability of the technique and the accessibility of the tools that are perhaps the most exciting things about implosion fabrication. The technique of two-photon microscopy was developed in the 1990s and the lasers required can be found in the materials science and biology labs of most universities.
Boyden and his team have already taught the reverse process of expansion microscopy to hundreds of research groups, and the materials required are commonplace – as commonplace as nappies. “The rest is just dehydrating it and shrinking it down,” says Boyden. “So no equipment is needed there either.”
Perhaps it is no surprise then that Boyden’s ultimate goal is to democratise nano-technology, turning it into the kind of industry that even hobbyists could get involved in. He sees a parallel with the development of 3D printing. “The idea that anybody could make a prototype in their garage is sweeping many industries now,” he says. “From automotive to mechanical engineering to space travel and everything in between. So what if you could make nano-technology in the same way?”
Boyden is already in talks with several entrepreneurs and venture capitalists about how best to bring the technology to market. With the cheapness and simplicity of the process, its breakout could come relatively soon and on a broad front. Boyden says: “In our group we tend to focus on revolutionary technologies that are also really cheap. Revolutions should be inexpensive.
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