Space-resolved quantitative mechanical measurements of soft and supersoft materials by atomic force microscopy

Abstract

Atomic force microscopy (AFM) has proven to be a valuable instrument to characterize quantitatively the mechanical and morphological properties of soft materials. For medium and hard samples (E>1 MPa), the nanomechanical accuracy of AFM is well established and ascertained. However, for soft samples, the experimental setup and data analysis for AFM are not yet firmly established. A calibration obtained for homogeneous samples with a Young’s modulus ranging from 100 Pa to a few kPa will prove its usefulness for nanomechanical AFM investigations of soft biological specimens, such as living cells and extracellular matrices. For this purpose, poly(N-isopropylacrylamide) (PNIPAM) hydrogels were synthesized in different methanol−water mixtures to produce a series of homogeneous samples with finely tunable mechanical properties. These samples allowed the comparison and validation of AFM force spectroscopy results using macroscopic and rheological techniques. In AFM measurements, the geometry of the indenter is fundamental to the model used for data interpretation; therefore, experiments were carried out using spherical micrometric and standard pyramidal sharp probes. Moreover, a PNIPAM gel embedded with hard microspheres was analyzed, which showed the capability of AFM for measuring the local mechanical properties of heterogeneous samples. Soft and supersoft materials can now have their nanomechanical properties gauged quantitatively using atomic force microscopy (AFM). Understanding how properties such as cell elasticity affect biological behaviour is a key goal for researchers, but when sharp AFM probes deform rubbery, wet biomaterials the results are often imprecise. Florian Stadler and colleagues from Shenzhen University in China addressed this challenge by examining how the probe's shape, stiffness and contact point properties change when characterizing a series of poly(N-isopropylacrylamide) (PNIPAM) hydrogels with systematically different elastic deformations. Atypical probes made from spherical colloidal particles yielded the best agreement with independent rheological measurements, due in part to the lower applied pressure they imparted. The team validated their calibrations by successfully characterizing the mechanical properties of a more complex heterogeneous PNIPAM gel embedded with nanoaggregates. Morphology and nanomechanical properties by atomic force microscopy (AFM) in 1:1 correspondence for the poly(N-isopropylacrylamide) (PNIPAM) hydrogel in a water environment. Measurements were performed in force volume mode at medium resolution (64 × 64) using a 2550 nm spherical tip: morphology (A) and Young’s modulus mechanical map (B). Force (nN) vs indentation (nm) graph (C) that highlights the raw data and Hertz fit. Histogram of Young’s modulus values in log-normal scale with Gaussian distribution fits (D) for the quantitative analysis and error calculation.