Complex coacervate droplets as a model material for studying the electrodynamic response of biological materials
Manipulating solid particles of a few micrometers using an electric field has been of great interest to physicists. These controllable particles can be assembled into dynamic chains that can effectively control fluid flow in thin tubes such as capillaries. Replacing these solid particles with liquid droplets would enable previously unfeasible electrorheological applications in biotechnology, as liquid droplets can store and utilize various biomolecules such as enzymes. Until now, it has not been possible to use liquid droplets for electrorheology because they tend to fuse or deform, making them ineffective as electrorheological liquids.
New research led by the University of Houston Cullen College of Engineering* in collaboration with the National Institute of Standards and Technology (NIST) and the University of Chicago has demonstrated a simple path to stabilize polyelectrolyte coacervate droplets that do not coalesce or deform under an electric field. The research was recently published in the Proceedings of the National Academy of Sciences (PNAS).
Thanks to the high polarizability and residual surface charge, these “stabilized” droplets can be directed in an aqueous environment using a low voltage source, for example a 9V battery. These droplets, known as coacervates, contain charged polymers that allow for the encapsulation of biologically relevant charged species such as proteins and genes. Thus, they have the potential to carry and deliver a variety of cargo useful in the manufacturing and medical industries.
Coacervate droplets are formed when two oppositely charged polymers, also called polyelectrolytes, assemble together in a condensed state in a salt solution. More specifically, the solution is often rapidly converted into a two-phase system, with the polymer-rich coacervate droplets suspended in the surrounding solution. The droplets are the size of tens of microns, about the size of typical biological cells. In fact, these droplets have been shown to carry out several biologically relevant reactions. However, coacervate droplets have a major drawback: they fuse with each other to form larger and larger droplets by fusing together until all droplets coalesce into a macroscopically settled layer due to gravity settling.
“Remember to mix a spoonful of olive oil in a cup of water and shake it vigorously. At first you will see small droplets that make the mixture cloudy, but over time these droplets fuse to form separate oil and water layers. Likewise, droplet bioreactors or electrorheological fluids made from coacervates fail over time when the droplets coalesce to form layers,” said Alamgir Karim, Dow Chair and Welch Foundation Professor at the University of Houston, who led the research project, in collaboration with Jack F. Douglas, a long-time colleague and polymer physicist at NIST, with insights provided by polyelectrolyte coacervate expert, Matthew Tirrell, the dean of the Pritzker School of Molecular Engineering at the University of Chicago.
“Scientists have solved the problem of oil droplet fusion by adding surfactant molecules that travel to the interface of oil droplets, preventing the oil droplets from fusing together,” Douglas says. He continued: “Similar technology was recently applied to coacervate droplets, using specialized polymer chains to coat the droplet interface, effectively preventing their fusion. However, such 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 an extra molecule,” said Aman Agrawal, the graduate student in the Karim Research Group who is leading the project. After months of research, Agrawal found that “when coacervate droplets are transferred from their original saline 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 the distilled water, driven by an abrupt change in ion concentration. Agrawal then studied these stable droplets under an electric field and demonstrated how to form droplet chains under an alternating current field and then move them with a direct current field.
“This new development in coacervate,” Tirrell said, “has potential applications in drug delivery and other encapsulation technologies. In basic biology, this mechanism may explain why intracellular organelles and biological condensates and prebiotic protocells (potential agents in the origin of life) can have the stability they have.” Recent measurements have shown that cells of different types can be manipulated similarly to the stabilized coacervate droplets with the application of electric fields, suggesting that the polarizability of the coacervate droplets may have significant implications for the manipulation of numerous biological materials composed of charged polymers.
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Aman Agrawal et al, Electric field coacervate droplet manipulation, Proceedings of the National Academy of Sciences (2022). DOI: 10.1073/pnas.2203483119
Quote: Complex coacervate droplets as a model material for studying the electrodynamic response of biological materials (2022, August 4), retrieved August 4, 2022 from https://phys.org/news/2022-08-complex-coacervate-droplets- material-electrodynamic .html
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