Self-assembling magnetic microparticles mimic biological error correction

Everybody makes mistakes. Biology is no different. However, living organisms have certain error-correction mechanisms that enable their biomolecules to assemble and function despite the defective slough that is a natural byproduct of the process.

A Cornell-led collaboration has developed microscale magnetic particles that can mimic the ability of biological materials such as proteins and nucleic acids to self-assemble into complex structures, while also selectively reducing the parasitic waste that would otherwise clog up production.

This magnetic assembly platform could one day usher in a new class of self-building biomimetic devices and microscale machines.

The findingswere published Aug. 29 in Proceedings of the National Academy of Sciences. The paper's lead authors are postdoctoral researchers Zexi Liang and Melody Lim.

For years, Itai Cohen, professor of physics, and Paul McEuen, the John A. Newman Professor of Physical Science, Emeritus, both in the College of Arts and Sciences, have been exploring how the binding power of magnetscan be leveraged for designing micro- and nanoscale self-assembling systems.

It all began with a popular toy.

"Paul would just come into my office all the time and pitch crazy ideas, and I would always say, 'No, no, no,'" Cohen said. "And then one day, he came into my office with a set of Magna-Tiles. And I was like, yeah, this we can do."

The idea was to essentially miniaturize the magnetic building set by fabricating micron-scale structures and machines at the Cornell NanoScale Science and Technology Facility(CNF) and investigate what happens when information is encoded into individual micrometer-size building blocks.

"We wanted to see how far we can take that," Cohen said. "This is the culmination of that work."

One of the sticking points in self-assembly systems is just that: The systems often get stuck in deformed, intermediate states - so-called "parasitic products" that gunk up the works and drive down efficiency. However, biological systems have a way to compensate: By pumping energy in the form of enzymes and adenosine triphosphate (ATP), they can make it tougher for the intermediates to form, and easier to break them up. This error-correction mechanism enables the system to attain the correct structure with high fidelity.

"We've introduced a new 'knob,' which is magnetic fields, as a way of implementing an error-correction strategy," Cohen said.

The researchers created flat monomer panels adorned with hundreds of magnets, each approximately 500 nanometers long and 50 nanometers wide, so that they all have the same magnetic strength.

Liang and Lim spent three "grueling" years developing a recipe to make the panels by lithography, Cohen said. While the magnetic dipoles are all the same magnitude, the direction of the dipoles can be controlled so that when driven by a rotating magnetic field, particles spin around, bump into each other and eventually bind to make dimers, trimers and tetramers.

Because the magnetic dipoles cancel each other out when the particles form tetramers, these target structures are impervious to the magnetic field, while the monomer, dimer and trimer panels continue to spin.

But that process results in only half the panels forming their intended tetramer structure, with the other half floating loose as parasitic products.

Working with a group led by Professor Michael Brenner of Harvard University, Liang and Lim determined there is a sweet spot at which enough magnetic mixing will melt the in-between states, but not so much as to melt the tetramers.

"What we're doing with this cycling is figuring out a way to destabilize the intermediate states really thoroughly so that the system has kind of no choice except to accumulate a large number of tetramers instead of getting stuck," Lim said.

Cycled enough times, the yield rockets from 50% to upward of 95%, with few parasitic products remaining.

This error-correction mechanism, also known in biology as "proofreading," could make self-assembly efficient, accurate and productive enough for engineering miniature technology, such as microLEDs, according to Liang.

"That's when we need this kind of a self-assembly approach with very good accuracy, so that the yield is high enough for an actual manufacturing purpose," he said.

Co-authors include postdoctoral researchers Jason Kim, Conrad Smart, Ph.D. '23, and Tanner Pearson, Ph.D. '21; and researchers from Harvard.

The research was supported by the National Science Foundation, the Alfred P. Sloan Foundation and the Cornell Center for Materials Research(CCMR).

The researchers made use of CNF and the CCMR.