In designing new motile materials, much can be learned by studying the
physical mechanisms underlying cell crawling. One important form of
cell crawling is driven by self-assembly of the protein actin. In this
process, energy is supplied and various proteins cooperate to assemble
actin from small proteins into a branched network. We have conducted
the first physically-consistent simulations of this process and have
discovered that the mechanism driving motion of the cell boundary
(modeled in our case by a flat disk) is very simple: the buildup of the
branched actin network behind the disk drives the disk forwards because
the disk is repelled by actin. This is reminiscent of the old joke
about why bagpipe players always walk while they play (to get away from
the noise). Understanding of this mechanism opens the way to the
development of new materials that can move, such as
asymmetrically-coated beads in self-generated concentration gradients.
Figure: Snapshot from simulation of the disk (purple) moving to the
right due to assembly of a branched actin network to its left. The
green spheres represent actin proteins in filaments.
