Amoeba-like oscillating materials synthesized in lab Fluids may provide clue to develop sci-fi movie-like soft machines

A research group at the University of Tokyo and their collaborators have created for the first time amoeba-like fluids that oscillate spontaneously between liquid "sol" and semisolid "gel" states coupled with significant fluctuations in viscosity. These novel fluids may provide clues for studying the autonomy of living organisms, like the motion mechanism of amoeba, in the near future, as well as help realize the creation of so-called soft machines, similar to those appearing in science fiction movies, which exhibit supple movements resembling living creatures.

The diverse range of biological phenomena occurring in living organisms is achieved through a complex interaction of various substances. For example, constant changes in viscosity attained through the self-assembly and -disassembly by a biopolymer, a large molecule found in living things, called actin strongly supports amoeba movement.

This sol-gel oscillation derived from actin is not only important for amoeba motility, but also plays a crucial role in spreading cancer cells from one part of the body to another, the development of immune cells, cell division, wound healing, and other mechanisms. However, reports of studies on the artificial reproduction of these autonomous dynamic behaviors have been scarce due to the experimental difficulty of replicating this activity.

In the current study, the research group led by Professor Ryo Yoshida and graduate student Michika Onoda of the Graduate School of Engineering at the University of Tokyo created fluids, consisting of polymers, that exhibit amoeba-like spontaneous sol-gel oscillation without applying any external stimuli such as an electric field, heat, or light, by developing a mechanism capable of spontaneous assembly and disassembly in a synthetic polymer.

The researchers incorporated a unique mechanism that can drive an oscillating chemical reaction called the Belousov-Zhabotinsky (BZ) reaction to the ABC triblock copolymer, a polymer with a special molecular arrangement, that plays a central role in realizing the sol-gel oscillation. During the course of the BZ reaction, also known as a model of metabolic reactions in living organisms, the oxidation-reduction states—in which electrons are lost or gained—of the metal complex changed periodically. The group modified the polymer chemically by introducing the metal complex, and designed the molecules so they would be a sol at the oxidized state and a gel at the reduced state. When the researchers applied the BZ reaction to the polymer solution, they successfully yielded spontaneous amoeba-like sol-gel oscillation in which the molecules metabolize the reactants—the substances involved in the chemical reaction—autonomously.

The period or amplitude of the sol-gel oscillation is easily controlled by changing the temperature or the concentration of the polymer or BZ substrates. The current outcome could also be described as the first report of artificially reproducing an autonomous behavior observed in living organisms derived from actin.

"The creation of the amoeba-like fluids exhibiting spontaneous sol-gel oscillation was one of our major goals," says Yoshida. He continues, "We proved that autonomous bioinspired materials can be produced by extracting the essence of biological phenomena and using it deftly."

"I will not forget the excitement I felt when I observed the first-ever spontaneous sol-gel oscillation in the lab," says Onoda. He continues, "Based on this achievement, we will ramp up research on bioinspired materials with autonomous dynamic function."

This research outcome is the result of a collaboration with Senior Researcher Takeshi Ueki, National Institute for Materials Science (NIMS); Professor Mitsuhiro Shibayama, Institute for Solid State Physics, the University of Tokyo; and other researchers.