  Modeling approaches of suspended, rod-like particles and recent experimental
data have shown that depletion forces display different signatures depending on
the orientation of these particles. It has been shown that axial attraction of
two rods yields contractile forces of 0.1pN that are independent of the
relative axial shift of the two rods. Here, we measured depletion-caused
interactions of actin bundles extending the phase space of single pairs of rods
to a multi-particle system. In contrast to a filament pair, we found forces up
to 3pN . Upon bundle relaxation forces decayed exponentially with a mean decay
time of 3.4s . These different dynamics are explained within the frame of a
mathematical model by taking pairwise interactions to a multi-filament scale.
The macromolecular content employed for our experiments is well below the
crowding of cells. Thus, we propose that arising forces can contribute to
biological force generation without the need to convert chemical energy into
mechanical work.
