Active system is formed by individual elements that, when interacting between themselves, show complex collective behaviors. In nature this behavior is seen in starling flocks, bacteria colonies, or in cells, in tissue growth and in cytoskeleton. In recent years, synthetic analogous active matter has been designed in these natural systems, aiming to analyze the most fundamental traits. In these materials, we can find the active gels, that, unlike common liquids, have a certain internal structure and own dynamics at the same time. Now, a team of the University of Barcelona could control the order and flow of one active matter putting it in contact with a passive liquid crystal controlled by a magnetic field.

“The material we used is formed by cell extracts. We could actually say it is the “minimum” copy of the cytoskeleton. What we saw is that when putting it in contact with a liquid crystal in a stage called smectite (with a marked rheological anisotropy), the material orders according to the preferred movement directions”, said Francesc Sagués, Professor at the Department of Materials Science and Physical Chemistry, and member of the Institute of Nanosciences and Nanotechnology of the University of Barcelona (IN2UB).

The studied active matter consists of a watery suspension of filamentary proteins (tubulin) and motor proteins (kinesin), which, in presence of adenosine triphosphate (ATP), form elongated self-assembled fibers. When they condensate on a passive liquid crystal under an external magnetic field, fibers get organized and oriented. This active matter was developed by Brandeis University (MA, USA) less than five years ago, and although its properties were already known, the dynamics could not be controlled. According to Pau Guillamat, researcher in training of the same group and first author of the article, “orienting a biology material and controlling it in the direction you want, has been an achievement, for it has a complex and unpredictable dynamics”.

The results of this work, which has the participation of Jordi Ignés-Mullol, lecturer of the same department and member of the IN2UB, have been published in the journal Proceedings of the National Academy of Sciences (PNAS) and commented on in an article of the journal Nature Reviews Materials, where they highlight that this mechanism allows creating a reversible cycle that can be easily controlled rotating the magnetic field, which is in charge of orienting the supporting liquid crystal.

“The following step, on which we are already working, -added Guillamat-, is to apply this methodology with cells, to condition their movement in vitro”.