Mechanical properties of soft hydrogels: assessment by scanning ion-conductance microscopy and atomic force microscopy.

The growing interest in biomimetic hydrogels is due to their successful applications in tissue engineering, 3D cell culturing and drug delivery. The major characteristics of hydrogels include swelling, porosity, degradation rate, biocompatibility, and mechanical properties. Poor mechanical properties can be regarded as the main limitation for the use of hydrogels in tissue engineering, and advanced techniques for its precise evaluation are of interest. The current research aims to demonstrate the suitability of scanning ion conductance microscopy (SICM) for assessing the stiffness of various hydrogels - Fmoc-FF peptide hydrogel, polyacrylamide and gelatin, - which differ by two orders of magnitude in Young's modulus (E). We provide a direct comparison between SICM measurements and atomic force microscopy (AFM) data, the latter being a widely used method for assessing the mechanical properties of scaffolds. The results of these methods showed good agreement, however, for materials with various stiffness two SICM-based approaches - application of hydrostatic pressure and application of intrinsic force - should be used. For hydrogels with Young's modulus of more than 2.5 kPa the application of SICM using hydrostatic pressure is recommended, whereas for soft materials with E ∼ 200-400 Pa the technique using intrinsic force can also be applied. We have shown that SICM and AFM methods can be used for the evaluation of the mechanical properties of soft hydrogels with nanometer resolution, while SICM is a completely non-invasive method, which requires a minimum influence on the sample structure.