Complex coacervation droplets as model material


image: Alamgir Karim, Dow Chair and Welch Foundation Professor of Chemical and Biomolecular Engineering, led the research team.
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Credit: University of Houston

The manipulation of solid particles of a few micrometers using an electric field is of great interest to physicists. These controllable particles can be assembled into dynamic chains capable of effectively controlling the flow of liquids in thin tubes like capillaries. Replacing these solid particles with liquid droplets would allow previously unfeasible electrorheology applications in biotechnology, as liquid droplets can store and utilize various biomolecules such as enzymes. Until now, it was not possible to use liquid droplets for electrorheology because they tend to fuse together or deform, making them ineffective as electrorheological fluids.

New research led by the University of Houston Cullen College of Engineering* in conjunction with the National Institute of Standards and Technology (NIST) and the University of Chicago, has shown a simple pathway to stabilize polyelectrolyte coacervate droplets that do not fuse or deform under an electric field. The study has just been published in the Proceedings of the National Academy of Sciences of the United States of America (PNAS).

Activated by the high polarizability and the residual surface charge, these “stabilized” droplets can be directed into an aqueous environment using a low voltage source, for example a 9V battery. Known as coacervates, these droplets contain charged polymers that allow the encapsulation of biologically relevant charged species such as proteins and genes. Thus, they have the potential to transport and deliver a variety of goods useful in the manufacturing and medical industries.

Coacervation droplets form when two oppositely charged polymers, also called polyelectrolytes, co-assemble as a condensate in a salt solution. Specifically, the solution often quickly transforms into a two-phase system, with the polymer-rich coacervate droplets suspended in the surrounding solution. The droplets are tens of microns in size, about the size of typical biological cells. In fact, these droplets have been shown to perform a variety of biologically relevant reactions. However, coacervate droplets have a major drawback – they coalesce with each other to form larger and larger droplets as they coalesce until all the droplets coalesce to form a sedimented macroscopic layer due to gravity sedimentation .

“Consider mixing a spoonful of olive oil in a cup of water and shaking it vigorously. Initially, you will see small droplets that make the mixture cloudy, but over time these droplets coalesce to form layers. similarly, droplet bioreactors or electrorheological fluids made from coacervates fail over time as the droplets coalesce to form layers,” said Alamgir Karim, Dow Chair and Welch Foundation Professor from the University of Houston, who led the research project, in collaboration with Jack F. Douglas, a longtime colleague and polymer physicist at NIST, with insights provided by polyelectrolyte coacervate expert Matthew Tirrell , dean of the Pritzker School of Molecular Engineering at the University of Chicago.

“Scientists solved the problem of oil droplet coalescence by adding surfactant molecules that go to the oil droplet interface, preventing the oil droplets from coalescing,” Douglas said. He continued: “Recently, a similar technology has been applied to coacerbate droplets where specialized polymer chains have been used to coat the interface of the droplets, thus prohibiting their coalescence. However, these molecular coatings prohibit material transport in and out of the droplets, making them ineffective for bioreactor applications.

“I wanted to stabilize these droplets without introducing any additional molecules,” said Aman Agrawal, the graduate student from the Karim Research Group leading the project. After months of research, Agrawal found that “when coacervation droplets are transferred from their native salt solution to distilled water, their interface tends to acquire a strong resistance to coalescence.” The researchers propose that this droplet stability is due to a loss of ions from the droplet interface in distilled water driven by an abrupt change in ion concentration. Agrawal then studied these stable droplets under an electric field, demonstrating how to form chains of droplets under an alternating field and then moving them with a DC field.

“This new development in the field of coacervates,” said Tirrell, “has potential applications in drug delivery and other encapsulation technologies. In basic biology, this mechanism may explain why intracellular organelles and condensates biological, and prebiotic protocells (possible life-giving agents) have the stability they have.Recent measurements have shown that cells of different types can be manipulated quite similarly to coacervate droplets stabilized with l application of electric fields, suggesting that the polarizability of coacervate droplets could have important ramifications for the manipulation of many biological materials composed of charged polymers.

*This research was supported by the Welch Foundation through grant number E-2105-20220331.

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