What Caging Force Cells Feel in 3D Hydrogels: A Rheological Perspective

Abstract

It has been established that the mechanical properties of hydrogels control the fate of (stem) cells. However, despite its importance, a one‐to‐one correspondence between gels' stiffness and cell behavior is still missing from literature. In this work, the viscoelastic properties of poly(ethylene‐glycol) (PEG)‐based hydrogels are investigated by means of rheological measurements performed at different length scales. The outcomes of this work reveal that PEG‐based hydrogels show significant stiffening when subjected to a compressional deformation, implying that conventional bulk rheology measurements may overestimate the stiffness of hydrogels by up to an order of magnitude. It is hypothesized that this apparent stiffening is caused by an induced “tensional state” of the gel network, due to the application of a compressional normal force during sample loading. Moreover, it is shown that the actual stiffness of the hydrogels is instead accurately determined by means of both passive‐video‐particle‐tracking (PVPT) microrheology and nanoindentation measurements, which are inherently performed at the cell's length scale and in absence of any externally applied force in the case of PVPT. These results underpin a methodology for measuring hydrogels' linear viscoelastic properties that are representative of the mechanical constraints perceived by cells in 3D hydrogel cultures.

A Hydrogel Platform that Incorporates Laminin Isoforms for Efficient Presentation of Growth Factors – Neural Growth and Osteogenesis

“…Mechanical properties of LM/PEG hydrogels were characterised by shear bulk rheology and nanoindentation ( Figure ,C); these techniques allow investigation of distinct aspects of the mechanical properties of hydrogels, due to i) the length scale at which they are performed (bulk vs nanoscale), ii) the nature of the measurement (i.e., dynamic vs static), iii) the mode of deformation of the material (shear vs axial). [ 45 ]…”

“…Mechanical properties of LM/PEG hydrogels were characterised by shear bulk rheology and nanoindentation (Figure 2A,C); these tech-niques allow investigation of distinct aspects of the mechanical properties of hydrogels, due to i) the length scale at which they are performed (bulk vs nanoscale), ii) the nature of the measurement (i.e., dynamic vs static), iii) the mode of deformation of the material (shear vs axial). [45] Shear rheology measurements were performed to study the bulk viscoelastic (i.e., dynamic) properties of the gels which arise both from the intrinsic polymeric nature of the material as The upper plate of radius R (7.5mm) oscillates at a constant frequency (10 rads -1 ) and increasing shear stress, τ, resulting in an increasing oscillatory strain γ (0.01 to 1%). The phase lag, δ, between the applied stress and the resulting strain defines the viscoelastic behavior of the sample.…”

Mechanical Characterization of Multilayered Hydrogels: A Rheological Study for 3D-Printed Systems

“…However, the measured stiffness can be affected by the induced compressional normal force during measurements. 61 For gels formed using FmocFF, there is a dependence of the compressional force being applied to the gel before measurements on the resulting storage modulus ( Figure S12a ). As such, we used a setup where the PP geometry was lowered to a position where the detected normal force was 0.05 N. This force is low enough to detect the gel and stop the measuring system without compressing the gel significantly ( Figure 3 b).…”

“…It is common to find in the literature the rheological properties of hydrogels being measured using a PP measuring system. 59 , 61 , 64 , 65 …”

Non-contact elastography methods in mechanobiology: a point of view

“…A broad comparison of methods for cell mechanics has been recently carried out, showing that different methods might provide differences up to two orders of magnitude in the evaluation of the same elastic modulus (Wu et al 2018a). This striking result clearly highlights that the measurement of cell mechanics based on deformation devices is highly dependent on the pattern of force application, both for inherent technical caveats and for the emergence of apparent stiffening phenomena in the material (Ciccone et al 2020).…”