Complex coacervate droplets as a model material to study the electrodynamic response of biological materials

The manipulation of solid particles of a few microns in size with an electric field is of great interest to physicists. These controllable particles can be assembled into dynamic chains that can effectively control the flow of liquids in thin tubes such as capillaries. Replacing these solid particles with liquid droplets would enable unprecedented 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 as they tend to coalesce or deform, rendering them ineffective as electrorheological liquids.

New research led by the University of Houston’s Cullen College of Engineering* in collaboration with the National Institute of Standards and Technology (NIST) and the University of Chicago has revealed a simple way to stabilize polyelectrolyte coacervate droplets that do not form under a merge or deform electric field. The study was recently published in Proceedings of the National Academy of Sciences (PNAS).

Enabled by the high polarizability and residual surface charge, these “stabilized” droplets can be placed in an aqueous environment using a low voltage source, e.g. a 9V battery. Known as coacervates, these droplets contain charged polymers that allow for 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.

Coacervate droplets form when two oppositely charged polymers, also called polyelectrolytes, combine to form a condensate state in a salt solution. More specifically, the solution often rapidly transforms into a two-phase system with the polymer-rich coacervate droplets suspended in the surrounding solution. The droplets are tens of micrometers in size, about the size of typical biological cells. In fact, these droplets have been shown to carry out various biologically relevant reactions. However, coacervated droplets have one major disadvantage – they fuse together to form larger and larger droplets by coalescing until all the droplets merge into a macroscopic settled layer due to gravitational settling.

“Remember to mix a spoonful of olive oil in a cup of water and shake vigorously. Initially you will see small droplets making the mixture cloudy, but over time these droplets will merge into separate layers of oil and water. Likewise, droplet bioreactors, or electrorheological fluids made from coacervates, fail over time as the droplets fuse into 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 and longtime colleague Polymer physicist at NIST, with insights from 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 interface of oil droplets and prevent the oil droplets from merging,” Douglas said. He continued: “Recently, a similar droplet coacervation technology has been applied, using special polymer chains to coat the droplet interface, effectively preventing their coalescence. However, such molecular coatings prevent material transport in and out of the droplets, making them ineffective for bioreactor applications.”

“I wanted to stabilize these droplets without introducing additional molecules,” said Aman Agrawal, the PhD student in Karim’s research group who is leading the project. After months of research, Agrawal found that “when coacervated droplets are transferred from their original saline solution to distilled water, their interface tends to acquire a strong resistance to coalescence.” The researchers propose that this droplet stability is due to ion loss from the droplet interface into the distilled water caused by an abrupt change in ion concentration. Agrawal then studied these stable droplets under an electric field and demonstrated how to form chains of droplets under an alternating field and then move them with a constant field.

“This new development in the field of coacervates,” Tirrell said, “has potential applications in drug delivery and other encapsulation technologies (live) have the stability they have.” Recent measurements have shown that cells of different types stabilized quite similarly to the coacervate -droplets can be manipulated by applying electric fields, suggesting that the polarizability of the coacervate droplets could have significant implications for the manipulation of numerous biological materials composed of charged polymers.