By taking advantage of bottle-brush polymers (left), researchers from Drexel University have discovered a way to grow hollow crystal spheres. They have also managed to pause the symmetrical growth of these crystal spheres, so they form with holes. Image: Drexel University.From snowflakes to quartz, nature’s crystalline structures form with a reliable, systematic symmetry. Researchers at Drexel University studying the formation of crystalline materials have now shown that it’s possible to control how crystals grow – including interrupting the symmetrical growth of flat crystals and inducing them to form hollow crystal spheres. This discovery is part of a broader design effort focused on encapsulating medicine for targeted drug treatments.
The new development, reported in a paper in Nature Communications, was led by Christopher Li, a professor in Drexel's College of Engineering whose research has centered around engineering polymer structures for special applications. Li has been collaborating with Bin Zhao, a professor in the Chemistry Department at the University of Tennessee, Knoxville. Their work shows how these polymer structures, including polymer crystal spheres, can be formed by simply mixing chemicals in a solution – rather than by physically manipulating their growth.
"Most crystals grow in a regular pattern; if you think about snowflakes, there is a translational symmetry that guides the unit cell repeating throughout the crystalline flake," said Li. "What we've discovered is a way to chemically manipulate the macromolecular structure so that this translational symmetry is broken when the molecule crystallizes. This means we can control the overall shape of the crystal as it forms – which is a very exciting development, both for its scientific significance and the implications it could have for mass production of targeted therapies."
The technique Li uses to compel what would normally be a flake-like crystal to draw itself up into a sphere builds on his previous work with polymers that look like brushes and polymer crystals formed from emulsion droplets. Incorporating these pliable ‘bottle-brush’ polymers as the structural system of the crystal allows Li to shape its growth by adjusting the ‘bristles’ of the brush.
"A bottle-brush polymer has lining bristles surrounding a spine. What we discovered is that we can make that spine bend upon crystallization by packing bristles on to one side of it," Li said. "This sets the pattern that is repeated as the crystal grows – so instead of growing flat it curves three-dimensionally to form a sphere." This means the amount of bristle polymers in a solution will determine how much the bottle-brush spine bends, and thus the shape and size of the resulting crystal sphere.
Li's team also reports on how to pause the formation of the crystal, which leaves holes in the sphere that could be useful for inserting a medicinal payload during the manufacturing process. Once filled, the crystal sphere can be closed with polymers tailored to help direct it to a specific target in the body.
"We have been working toward this achievement for some time," Li said. "This spherical crystallography manifests itself in robust structures that we see in nature, from eggshells to virus capsids, so we believe it is the ideal form to survive the rigors of delivering medication in the body. Being able to control the properties of the crystal as it forms is an important step toward realizing this application."
This story is adapted from material from Drexel University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.