An AFM-based stiffness clamp for dynamic control of rigidity

Area of Science:

• Biophysics
• Materials Science
• Cell Biology

Background:

• Atomic force microscopy (AFM) is crucial for measuring material properties and applying mechanical forces to biological samples.
• Current AFM methods allow control over force or position, but not the dynamic control of cantilever stiffness.
• The Hookean relation (F=kx) describes cantilever deflection, where stiffness (k) has been a static parameter.

Purpose of the Study:

• To introduce and demonstrate a novel 'stiffness clamp' for AFM that allows dynamic control of cantilever stiffness.
• To enable decoupling of stiffness from force and deformation in mechanical measurements.
• To investigate cellular and material responses to controlled, varying stiffness environments.

Main Methods:

• Developed a 'stiffness clamp' by modifying AFM feedback control to dynamically adjust cantilever stiffness.
• Applied the stiffness clamp to a contracting fibroblast cell and an expanding polyacrylamide hydrogel.
• Measured traction rate (dF/dt) and contraction velocity (dx/dt) in response to step changes in stiffness (1-100 nN/µm).

Main Results:

• Fibroblast cells showed rapid changes in traction rate and contraction velocity in response to stiffness changes, independent of absolute force or height.
• This indicates cells can directly sense and react to stiffness variations.
• The passive hydrogel maintained constant expansion velocity but showed altered traction rates with changing stiffness, demonstrating distinct material behavior.

Conclusions:

• The AFM stiffness clamp provides a new capability for dynamic mechanical testing on diverse samples.
• This method allows for the investigation of cellular mechanotransduction under precisely controlled stiffness conditions.
• The technique is applicable to both biological and non-biological materials, opening new avenues for mechanical studies.