Review Article

Physical, Spatial, and Molecular Aspects of Extracellular Matrix of In Vivo Niches and Artificial Scaffolds Relevant to Stem Cells Research

Table 2

Methods to assess physical characteristics of matrices and tissues, used in stem cell studies.

Method/deviceDescription Application variants and aspectsStudies using this method

Young’s modulus and complex shear modulus assessment
Note: different methods can give different results for the same tissue

Macromicroscale (whole tissue specimen elasticity is measured)
Micromechanical testing systemsMeasuring applied stress and strain during deformation of the specimen of millimeter size (compression, tension, or indentation test); to measure viscoelastic properties (storage and loss modulus) dynamic mechanical analysis is used; that is, sinusoidal stress of certain frequency is applied and the strain in the material is measuredCompression test
Tensile test
Indentation
[53, 55, 132]
[61, 78]
[56, 58]
Indentation methodIn indentation experiments, a rigid indenter (e.g., a plane ended cylinder, a cone shaped tip, or a sphere) is pressed against the tissue and shear moduli are calculated from the applied load and extent of tissue deflectionNano- and macroscopic indentation[33]
RheometerTwo parallel plates or two coaxial cylinders with a narrow gap in-between are moving relative to each other, imposing shear stress on material squeezed in the gap; displacement and force are measured; thus, shear storage modulus and loss modulus are calculated and usually used to measure viscosity or rheology of fluidsLiquefaction stress of thixotropic gels
Viscoelasticity measurement
[87]
 [12]
Magnetic resonance elastographyShear waves inside the sample are induced by sonic mechanical vibrator on the surface of the sample; then, the shear wave propagation is recorded with a magnetic resonance technique and the image is assessed to generate a shear stiffness mapNon-invasive method for measuring stiffness in small samples [76]
Ultrasound elastographyAn external force is applied to the studied tissue and the resulting displacement and the generated strain are then mapped by ultrasound imaging; the external force can be static (compression, shear) and dynamic (shear waves propagation, whose speed is directly related to the medium shear modulus)Non-invasive method for measuring tissues stiffness in patients, for instance, suffering from liver fibrosis[44]

Nanoscale (local elasticity is measured)
Atomic force microscopy (AFM)The specimen is subjected to indentation by nanometer size indenter; force-indentation distance profiles are collected and analysed with a Hertz cone model to compute the elastic moduliNanoindentation[10, 49, 52, 68, 89]
Micropipette aspirationThe tip of a small micropipette is brought in contact with a sample and a series of equal steps in pressure are applied; the length of the sample aspiration representing an equilibrium deformation is determined for each pressure Young’s modulus is calculated from the experimental length–pressure data (using particular theoretical model)Used for thin matrix samples to assess local characteristics of pericellular matrix[37]

Pore size assessment

Advanced microscopy methodsThe porosity and pore sizes of dried hydrogels are examined by microscopic imaging Scanning electron microscopy (SEM)[49, 51]
Confocal microscopy[51]
DNA electrophoresisThe radius of gyration of extended DNA may be used to estimate the effective maximum pore size of the hydrogel[49]
Pore size through permeability measurementMeasuring permeability of the gel, that is, fluid flow velocity through the gel under certain pressure, allows calculating mean pore size[89]