Measuring the elastic modulus of soft biomaterials using nanoindentation

Area of Science:

• Biomaterials Science
• Materials Characterization
• Mechanical Engineering

Background:

• Measuring the elastic modulus of soft biomaterials is crucial for their application.
• Non-cantilever nanoindentation systems, designed for hard materials, face challenges in accurately determining contact points for soft samples.
• Accurate zero-point determination is essential for reliable depth-of-penetration measurements in nanoindentation.

Purpose of the Study:

• To investigate the efficacy of non-cantilever based nanoindentation for differentiating soft biomaterials (hydrogels) with low elastic moduli.
• To develop and apply a bespoke soft contact protocol and low instrument stiffness for enhanced characterization of hydrogels.
• To assess the repeatability and accuracy of nanoindentation measurements for hydrogels in the kilopascal range.

Main Methods:

• Fabrication of polyethylene glycol (PEG) hydrogels with varying elastic moduli by adjusting polymer concentration and crosslinking.
• Utilizing non-cantilever based nanoindentation with a specialized soft contact protocol and low flexural stiffness.
• Calculating elastic modulus using the Oliver-Pharr method, Hertzian contact model, and a viscoelastic model.
• Validating stiffness measurements by testing cantilever beams with equivalent flexural stiffness.

Main Results:

• Demonstrated high repeatability in nanoindentation measurements of PEG hydrogels.
• Successfully differentiated between hydrogels with elastic moduli ranging from single kilopascals (kPa) to hundreds of kPa.
• The bespoke protocol and low instrument stiffness enabled accurate characterization of very soft materials.

Conclusions:

• Non-cantilever based nanoindentation, when adapted with a soft contact protocol and low instrument stiffness, is capable of characterizing the elastic modulus of soft biomaterials.
• This method provides a reliable approach for differentiating hydrogels with elastic moduli in the single to hundreds of kPa range.
• The findings support the use of this technique for quality control and material development in soft biomaterials research.