  In biological systems, molecular-scale forces and motions are pivotal for
enabling processes like motility, shape change, and replication. These forces
and motions are organized, amplified, and transmitted across macroscopic scales
by active materials such as the cytoskeleton, which drives micron-scale
cellular movement and re-organization. Despite the integral role of active
materials, understanding how molecular-scale interactions alter macroscopic
structure and force propagation remains elusive. This knowledge gap presents
challenges to the harnessing and regulation of such dynamics across diverse
length scales. Here, we demonstrate how mediating the bundling of microtubules
can shift active matter between a global force-transmitting phase and a local
force-dissipating phase. A fivefold increase in microtubule effective length
results in the transition from local to global phase with a hundredfold
increase in velocity autocorrelation. Through theory and simulation, we
identify signatures of a percolation-driven transition between the two phases.
This provides evidence for how force propagation can be generated when local
molecular interactions reach a sufficient length scale. We show that force
propagation in the active matter system enables material transport.
Consequently, we demonstrate that the global phase is capable of facilitating
millimeter-scale human cell transport and manipulation, as well as powering the
movement of aqueous droplets. These findings underscore the potential for
designing active materials capable of force organization and transmission. Our
results lay the foundation for further exploration into the organization and
propagation of forces/stresses in biological systems, thereby paving the way
for the engineering of active materials in synthetic biology and soft robotics.
