Thermoresponsive Stiffening with Microgel Particles in a Semiflexible Fibrin Network

Abstract

We report temperature-responsive soft composites of semiflexible biopolymer networks (fibrin) containing dispersed microgel colloidal particles of poly(N-isopropylacrylamide) (pNIPAM) that undergo a thermodynamically driven de-swelling transition above a Lower Critical Solution Temperature (LCST). Unlike standard polymer-particle composites, decreasing the inclusion volume of the particles (by increasing temperature)is concomitant with a striking increase of the overall elastic stiffness of the composite. We observe such a behavior over a wide composition space. The composite elastic shear modulus reversibly stiffens by up to 10-fold over a small change in temperature from 25-35°C. In isolation, the fibrin network and microgel suspension both soften with increased temperature, making the stiffening of the composites particularly significant. We hypothesize that stiffening is caused by contracting microgel particles adsorbing on the fibrin filaments and modifying the structure of the semiflexible network. We develop two phenomenological models that quantify this hypothesis in physically distinct manners, and the derived predictions are qualitatively consistent with our experimental data.

Linear and nonlinear viscoelasticity of concentrated thermoresponsive microgel suspensions

“…As a result, the volume of solvent a 'free' microgel can imbibe in the equilibrium configuration changes drastically [28]. A modest hysteresis in the hydrodynamic diameter while cooling and heating the suspension is observed, consistent with prior studies [12], [29]. Microgels prepared by such a crosslinker free preparation protocol are known to be highly deformable and ultra-soft [24].…”

“…Microgels prepared by such a crosslinker free preparation protocol are known to be highly deformable and ultra-soft [24]. Our prior estimates for the individual particle modulus is ~ 1.5 kPa in the fully swollen state [9], [12].…”

“…Additional complications arise for soft and deformable microgels [9]- [11] given their internal structure is crosslinked polymer chains imbibed by solvent. This allows such particles to change their volume (and hence the effective suspension packing fraction) and degree of interpenetration depending on factors such as temperature, polymer-solvent interactions, microgel concentration (loading), and osmotic pressure [9], [10], [12], [13].…”

Nonaffinity controls critical slowing down and rheology near the onset of rigidity

“…This also suggests new avenues for tuning the dynamics of stress relaxation; for example, in fibrous networks, microscopic components that generate localized internal stresses can drive macroscopic stiffening transitions and thus precisely control the nonaffinity [71]. Relevant examples include molecular motors in the cellular cytoskeleton [72][73][74], contractile cells in the extracellular matrix [75][76][77], and embedded particles driven to shrink due to changes in temperature [78] or rearrange under applied magnetic fields [79].…”

Exploiting Nonlinear Elasticity for Anomalous Magnetoresponsive Stiffening

“…The variability and the nonmonotonicity in the values of G 0 are consistent with other composites of fibrin studied earlier. 15,16 To make a quantitative comparison of the MR effect at various compositions, we compare the relative stiffening of shear modulus G(B)/G 0 . In both composites, several shared signatures in the MR response are observed.…”