Under a weak magnetic field, large-scale organizational features can be attained in an active gel interfaced with a passive liquid crystal layer, as Francesc Sagués and co-workers report in PNAS.

Credit: George Clerk/Getty stock photo

Active matter systems comprise individual constituents that consume energy, leading to complex, non-equilibrium behaviour. In nature, this can be seen in the flocking or swarming behaviour of animals, as well as on a cellular level. Synthetic analogues of such systems have recently emerged as ideal platforms to study the fundamental mechanics of living cells and may lead to the development of unique biomimetic materials. One prominent example is active liquid crystals — viscous suspensions of cross-linked microtubule bundles self-propelled by motor protein complexes to establish supramolecular nematic order. The motor (for example, ATP-fuelled cytoskeleton proteins, as used in this study) is located at one end of each polar microtubule, which causes microtubules of opposing polarity to slide past each other within the bundles. This motion results in a bulk active gel that dynamically reforms or a 2D nematic phase at an oil/water interface.

In contrast to conventional passive liquid crystals, the orientations of which can be easily switched between predesigned configurations, it has proven difficult to control the behaviour of active nematics through external stimuli. This represents a bottleneck in the development of in vitro models and new biomimetic functional materials based on these systems. To overcome this, Sagués and colleagues drew inspiration from nature; specifically, they mimicked how the cytoskeleton of living cells adapts to its mechanical microenvironment. Their strategy involves interfacing the active biomaterial with a passive, thermotropic liquid crystal (similar to those used in flat-panel displays), the flow directions of which can be magnetically controlled. As Jordi Ignés-Mullol, a co-author of this work, explains, “we took advantage of the giant viscous anisotropy of the thermotropic oil, and the fact that it can be easily aligned with magnetic fields to establish a template to guide the spontaneous flow of the active gel in contact with the passive liquid crystal through a biocompatible interface.”

The researchers selected the thermotropic oil octo-cyanobiphenyl (8CB) as the passive liquid crystal, because its two liquid-crystal phases (nematic and smectic) exist at temperatures compatible with the protein. Above a critical temperature, 8CB exists in the nematic phase, and the active nematic underneath exhibits characteristic self-driven turbulent flow. Application of a magnetic field causes alignment of the 8CB molecules but has no perceptible effect on the underlying active nematic film. When the temperature was lowered to induce the smectic phase, and under application of a magnetic field, the active nematic reorganised to form parallel stripes aligned perpendicular to the field. “We succeeded in aligning the chaotic flow patterns that typically arise in these preparations by applying a modest magnetic field,” summarizes Sagués. Their system could be cycled reversibly and the orientation could be easily controlled by rotating the magnetic field.

We succeeded in aligning the chaotic flow patterns that typically arise in these preparations by applying a modest magnetic field

This ability to switch between turbulent and laminar flow regimes is attributed to rheological phenomena associated with the structure of the smectic phase, whereby the passive layer assumes a ‘bookshelf’ geometry, with planes of molecules oriented perpendicular to the magnetic field. This configuration allows liquid to flow easily when sheared parallel to the planes but behaves like a solid material when the material flows in the perpendicular direction. The anisotropy in interfacial viscosity experienced by the active nematic results in its periodic, striped pattern.

Using tracer microparticles, the researchers observed that the lanes between the stripes of aligned microtubules could be used to actively transport biocompatible cargo. In future work, they plan to extend their method to study more sophisticated biomaterials. “Eventually one could envisage a similar strategy to direct and control the motion of individual cells or the proliferation of growing tissues,” says Sagués.