Active materials are capable of converting free energy into mechanical work to produce autonomous motion, and exhibit striking collective dynamics that biology relies on for essential functions. Controlling those dynamics and transport in synthetic systems has been particularly challenging. Here, we introduce the concept of spatially structured activity as a means to control and manipulate transport in active nematic liquid crystals consisting of actin filaments and light-sensitive myosin motors. Simulations and experiments are used to demonstrate that topological defects can be generated at will, and then constrained to move along specified trajectories, by inducing local stresses in an otherwise passive material. These results provide a foundation for design of autonomous and reconfigurable microfluidic systems where transport is controlled by modulating activity with light.

These data are images of a two dimensional nematic liquid crystal formed by crowding short, labelled actin filaments onto an oil-water interface using methylcellulose as the crowding agent. Images from an upright nikon microscope with a spinning disk confocal head. The laser used to excite the fluorophore is polarized such that labelled actin filaments that point in the y direction in the laboratory frame fluoresce more than those with an orthogonal orientation. 

 These liquid crystals are driven out of equilibrium by custom Myosin motors. These motors, when stimulated with ~470nm light, can undergo a conformational change that results in a higher gliding speed along the filament.  In these experiments this stimulation wavelength is targeted to just one region of the sample using a digital micromirror array. "Figure_1_ROI.roi" is the stimulation region for the data labelled "Figure_1_raw.tiff" and likewise "Stimulation_ROI_F5.rgn" is the stimulated region for the data of the format "Figure_5_xx.tiff". 

In the data from figure 1, one pixel is 0.1083 microns. In the data from figure 5, one pixel is 0.1625 microns.