Cyclic hardening in bundled actin networks.

Area of Science:

• Biophysics
• Materials Science
• Soft Matter Physics

Background:

• Nonlinear deformations can permanently change material properties.
• Most soft materials, including biological tissues and rubber, soften with cyclic deformation (Mullins effect).
• Actin networks are crucial cytoskeletal components in cells, influencing cell mechanics.

Purpose of the Study:

• To investigate the mechanical response of reconstituted actin networks to cyclic shear.
• To determine if actin networks exhibit softening or hardening under cyclic deformation.
• To elucidate the role of network architecture in this mechanical response.

Main Methods:

• Preparation of reconstituted networks of crosslinked, bundled actin filaments.
• Application of cyclic shear deformation.
• Simultaneous macrorheology measurements and confocal microscopy.
• Varying crosslinker concentrations to probe network architecture effects.

Main Results:

• Reconstituted actin networks exhibit hardening under cyclic shear, a stark contrast to typical soft materials.
• A significant stress barrier is generated at the maximum strain, indicating a mechano-memory effect.
• This hardening response is dependent on crosslinker concentration; lower concentrations lead to softening (Mullins effect).
• Cyclic shearing induces structural reorganization within the actin network, encoding the maximum strain into its architecture.

Conclusions:

• Actin networks possess a unique mechano-memory capability, hardening rather than softening under cyclic shear.
• The observed hardening and mechano-memory are governed by the network's crosslinker density and resulting architecture.
• These findings challenge conventional understanding of soft material mechanics and have implications for cellular mechanics and biomaterials.