Engineering hydrogel viscoelasticity

Abstract

The aim of this study was to identify a method for modifying the time-dependent viscoelastic properties of gels without altering the elastic component. To this end, two hydrogels commonly used in biomedical applications, agarose and acrylamide, were prepared in aqueous solutions of dextran with increasing concentrations (0, 2 and 5% w/v) and hence increasing viscosities. Commercial polyurethane sponges soaked in the same solutions were used as controls, since, unlike in hydrogels, the liquid in these sponge systems is poorly bound to the polymer network. Sample viscoelastic properties were characterised using the epsilon-dot method, based on compression tests at different constant strain-rates. Experimental data were fitted to a standard linear solid model. While increasing the liquid viscosity in the controls resulted in a significant increase of the characteristic relaxation time ( ), both the instantaneous () and the equilibrium ( ) elastic moduli remained almost constant.However, in the hydrogels a significant reduction of both and was observed. On the other hand, as expected,an indicator of the equilibrium elastic behaviour after the occurrence of viscoelastic relaxation dynamicswas found to be independent of the liquid phase viscosity.Therefore, although the elastic and viscous components of hydrogels cannot be completely decoupled due to the interaction of the liquid and solid phases, we show that their viscoelastic behaviour can be modulated by varying the viscosity of the aqueous phase. This simple-yet-effective strategy could be useful in the field of mechanobiology, particularly for studying cell response to substrate viscoelasticity while keeping the elastic cue (i.e. equilibrium modulus, or quasi-static stiffness) constant.

Cell Mechanical and Physiological Behavior in the Regime of Rapid Mechanical Compressions that Lead to Cell Volume Change

“…The cell is modeled as a Maxwell viscoelastic material with dynamic viscosity µ = T v E , where E is the Young's Modulus and T v is the viscous time constant as measured by AFM (Figure S2, Supporting Information). Viscous force is dependent on cell compression velocity ( V = Δ LT c −1 where T c is the compression time measured by video analysis) and a characteristic length ( L is the relaxed cell diameter, Δ L = L – compression gap).…”

Engineering Gels with Time-Evolving Viscoelasticity

“…A high τ indicates that the material presents a tendency towards an elastic or 'solid-like' behaviour; conversely, a low τ indicates a tendency toward viscous or 'liquid-like behaviour'. In particular, when τ → ∞ the material is referred as pure elastic and when τ → 0 it is considered as pure viscous [25,43].…”

“…While the role of stiffness in orchestrating cell behaviour has been widely studied, the effect of viscoelastic properties, or cell response to viscotaxis [19] is still scarcely investigated [20,21]. Some recent reports describe how gel viscoelastic properties have been modulated both by varying the polymer concentration, molecular weight and crosslinking [22][23][24] or by altering the liquid phase viscosity [25] to obtain viscoelastic substrates (i.e., with a time dependent behaviour characterised by constant viscoelastic properties over the culture period). Charrier and colleagues showed that the loss modulus (G") of PAAm gels can be modulated by entrapping high molecular weight linear polymers in the gels [23].…”

“…Moreover, Chaudhuri and co-workers demonstrated that the characteristic relaxation time of alginate gels can be modified by varying the polymer molecular weight or with the addition of PEG spacers to provide a steric hindrance to alginate crosslinking [24]. Finally, the relaxation time of agarose and PAAm gels was modulated by varying the viscosity of the gel liquid phase with different dextran concentrations while maintaining a constant equilibrium modulus [25].…”

Alternative Form of Standard Linear Solid Model for Characterizing Stress Relaxation and Creep: Including a Novel Parameter for Quantifying the Ratio of Fluids to Solids of a Viscoelastic Solid

“…The standard linear solid model is much simpler compared to other more general models like the generalized Maxwell model and fractional-order viscoelastic model (Xiao et al, 2016). Nevertheless, the standard linear solid model is still useful for analyzing viscoelastic behaviors of many biomaterials like polycaprolactone scaffolds (Sethuraman et al, 2013) and hydrogels (Shazly et al, 2008;Feng et al, 2010;Tirella et al, 2014;Cacopardo et al, 2019). In addition, the results by using the standard linear solid model is easier to interpret since the model produces only three parameters (Braunsmann et al, 2014).…”