Mechanics of composite hydrogels approaching phase separation

Abstract

For polymer-particle composites, limited thermodynamic compatibility of polymers and particles often leads to poor dispersal and agglomeration of the particles in the matrix, which negatively impacts the mechanics of composites. To study the impact of particle compatibility in polymer matrices on the mechanical properties of composites, we here study composite silica- protein based hydrogels. The polymer used is a previously studied telechelic protein-based polymer with end groups that form triple helices, and the particles are silica nanoparticles that only weakly associate with the polymer matrix. At 1mM protein polymer, up to 7% of silica nanoparticles can be embedded in the hydrogel. At higher concentrations the system phase separates. Oscillatory rheology shows that at high frequencies the particles strengthen the gels by acting as short-lived multivalent cross-links, while at low frequencies, the particles reduce the gel strength, presumably by sequestering part of the protein polymers in such a way that they can no longer contribute to the network strength. As is generally observed for polymer/particle composites, shear-induced polymer desorption from the particles leads to a viscous dissipation that strongly increases with increasing particle concentration. While linear rheological properties as function of particle concentration provide no signals for an approaching phase separation, this is very different for the non-linear rheology, especially fracture. Strain-at-break decreases rapidly with increasing particle concentration and vanishes as the phase boundary is approached, suggesting that the interfaces between regions of high and low particle densities in composites close to phase separation provide easy fracture planes.

Hydrogels as Potential Nano-, Micro- and Macro-Scale Systems for Controlled Drug Delivery

“…The inclusion of nanomaterials in the HG polymer network is an interesting way to adjust the mechanical properties of HG and/or to provide the composite with sensitivity to external stimuli [192,196,197,199]. Different nanomaterials have been immobilized in polymeric networks, including inorganic nanoparticles [200,201], carbon nanomaterials [202], and lipids [99]. Nanoparticle systems have gained considerable attention by being one of the most interesting and promising biomedical materials with the exceptional physicochemical properties, controlled shapes, nano-sized characteristics, comprehensive modification options and well-defined multi-functionality [83].…”

Hybrid Complex Coacervate

“…Remarkably, the addition of the silica nanoparticles has an unexpected negative effect on the non-linear mechanical properties, decreasing both fracture resistance and toughness. This behavior has been observed in hybrid hydrogels in which the nanoparticles interact weakly with the matrix [54]: the filler in this case can be embedded in the network only up to a certain threshold (7% volume fraction), after which the system phase separates because of the aggregation of the nanoparticles. Below that critical concentration, the fracture resistance decreases as a function of the filler content, as observed in our work.…”

Fibrin Matrices as (Injectable) Biomaterials: Formation, Clinical Use, and Molecular Engineering

“…Importantly, phase separation most commonly occurs during the conversion of fibrinogen to fibrin. The criticality of this point is that phase separation and above all its kinetics are rather difficult to precisely control, which on its turn may lead to poor control over structural and mechanical properties of the materials.It must be noted that in some cases phase separated fibrin‐containing hybrids have been produced intentionally and in a controlled fashion. This applies, for example, to fibrin‐collagen composites prepared using multilayered constructs, or microfluidic‐produced fibrin microbeads dispersed in collagen .…”

“…Importantly, phase separation most commonly occurs during the conversion of fibrinogen to fibrin. The criticality of this point is that phase separation and above all its kinetics are rather difficult to precisely control, which on its turn may lead to poor control over structural and mechanical properties of the materials.…”