Analysis of indentation: implications for measuring mechanical properties with atomic force microscopy

Area of Science:

• Biophysics
• Materials Science
• Mechanical Engineering

Background:

• Atomic force microscopy (AFM) is valuable for probing micromechanical properties of soft biological tissues.
• Interpreting AFM indentation data is challenging due to probe tip geometry and indentation depth effects.
• Existing models often simplify material behavior, leading to inaccuracies for complex biological samples.

Purpose of the Study:

• To investigate the influence of indentation depth, tip geometry, and material properties on AFM indentation responses.
• To evaluate the accuracy of infinitesimal strain models versus finite element models (FEMs) for biological materials.
• To develop a method for accurate quantitative analysis of AFM indentation data for nonlinear and heterogeneous materials.

Main Methods:

• Finite element models (FEMs) were employed to simulate AFM indentation on various material models.
• Simulations explored the impact of varying indentation depths and probe tip geometries.
• Comparison of FEM results with traditional infinitesimal strain models was performed.

Main Results:

• Infinitesimal strain models showed significant errors when estimating properties of nonlinear elastic materials.
• FEMs accurately captured the effects of material nonlinearity and heterogeneity on indentation response.
• Calculating an apparent elastic modulus considering indenter geometry identified material nonlinearities.

Conclusions:

• Accurate AFM indentation analysis requires accounting for finite deformation and indenter geometry.
• FEMs provide a robust framework for understanding micromechanical properties of complex biological materials.
• Combining indentation with other techniques like biaxial stretch can enable quantitative material characterization.