Biopolymer network geometries: Characterization, regeneration, and elastic properties
Researchers in George Whitesides and David Weitz's groups designed a model system of a polymer network with active constituents to explore the way that cells control their mechanical properties. Cells use networks composed of active filaments controlled by molecular motors. Cells use networks composed of active filaments controlled by molecular motors. The system in question is a filamentous actin (F-actin) network, made active via myosin molecular motors anchored by the cross-linking agent filamin A (FLNa). By inducing internal stress in the network, researchers were able to manipulate the elastic properties of the gel over 3 orders of magnitude. It has been demonstrated that systems containing only a few percent protein by volume can experience a transition from liquid-like to solid-like mechanical behavior. This fact, coupled with the tuning of active components opens up the possibility of active biomaterials that adapt mechanically to their environment.
A number of properties of the system were explored. Some key findings are listed below:
Effects of System Components on Stiffness
Researchers isolated the effects of each component of the system. It was found that FLNa crosslinking only increased the networks stiffness slightly. According to the researchers, the relative softness, or "floppiness" of these crosslinks caused them to dominate the mechanical behavior in the linear regime. Once active myosin II thick filaments (molecular motors, which cause anti-parallel movement of network filaments, hence "contracting" the network and inducing internal stress) were included in the network, the stiffness and the ratio of loss to storage decreased, implying more solid-like behavior. When blebbistatin was used to de-activate the myosin motors, all effect on stiffness disappeared. To create the aforementioned stiffening effects, all three components (filaments, cross-linkers, and molecular motors) must be present. A threshold effect was noted: A slight increase in loss with increasing concentration was noted. Figure 1 shows the associated data.
Role of long-range structure and network alignment
Local alignment is present without or with FLNa cross-linkers, but the long-range structure showed no overall alignment. When myosin molecular motors formed permanent bonds (due to an ATP-free environment), significant filament "bundling" and alignment was noted. Both FLNa and myosin are required for local "bundling", and network "contraction". Figure 2 shows the bundling effect.
Comparison of active networks under internal stress to passive networks under external stress
By comparing active networks under internal stress to passive networks under external stress, researchers were able to approximate the internal stress induced by myosin motor activity. For the highest stress values, the ratio of the loss modulus to the storage modulus increased slightly for the active networks for frequencies below 0.2 rad/s, while passive networks showed frequency-independence of the loss modulus in high external stress states. These results imply that stress relaxation is mediated by a process with a characteristic time scale of larger than 5 seconds--a time scale consistent with measured myosin motor release rates. Results also demonstrated that myosin motor activity can make actin networks behave more like a solid by increasing the internal stress. Effect of tension sensitivity and unbinding 1. Myosin motors must be active 2. f-actin cross-linkers are required as anchor points, otherwise the myosin motors permit sliding One order of magnitude lower elastic moduli, possibly due to filament length (dependence supported by these experiments) Specific alignment or ordering might not be required to see stress stiffening.