Influence of Interfacial Layer Between Nanoparticles and Polymeric Matrix on Viscoelastic Properties of Hydrogel Nanocomposites

Nanocomposite Hydrogels as Functional Extracellular Matrices

Over recent years, nano-engineered materials have become an important component of artificial extracellular matrices. On one hand, these materials enable static enhancement of the bulk properties of cell scaffolds, for instance, they can alter mechanical properties or electrical conductivity, in order to better mimic the in vivo cell environment. Yet, many nanomaterials also exhibit dynamic, remotely tunable optical, electrical, magnetic, or acoustic properties, and therefore, can be used to non-invasively deliver localized, dynamic stimuli to cells cultured in artificial ECMs in three dimensions. Vice versa, the same, functional nanomaterials, can also report changing environmental conditions-whether or not, as a result of a dynamically applied stimulus-and as such provide means for wireless, long-term monitoring of the cell status inside the culture. In this review article, we present an overview of the technological advances regarding the incorporation of functional nanomaterials in artificial extracellular matrices, highlighting both passive and dynamically tunable nano-engineered components.

Defect generation surrounding nanoparticles in a cross-linked hydrogel network

A detailed understanding of polymer-nanoparticle interactions is a key element in demystifying the reinforcement mechanism for nanocomposites. To decouple the effects of the polymer-nanoparticle interactions from the particle distribution, we utilized polymerized crystalline colloidal arrays based on a thermosensitive hydrogel, poly(N-isopropylacrylamide) (pNIPAAm). First, the hydrogel network structure in the vicinity of the nanoparticles was investigated by the deswelling behavior of particle-filled hydrogels. The addition of nanoparticles led to an increased rate of deswelling when the particle-filled hydrogel was heated beyond the lower critical solution temperature (32 degrees C). To interpret this observation, we have suggested that the polymer network has a significant increase in defects (e.g., dangling chain ends) in the vicinity of the nanoparticles. The apparent percolation threshold associated with the interaction of the nanoparticles was about 20 times smaller than the theoretical percolation threshold of spherical particles. As a consequence, we have determined the thickness of this defect zone to be about 85 nm. This is much larger than the size of the unperturbed linear pNIPAAm chains, suggesting that the polymers that play a role in the adsorption are not constrained segments of polymers bound between cross-link junctions but relatively free chains. This finding enabled us to emulate the adsorption behavior of pNIPAAm hydrogels on the particles by simply adding linear pNIPAAm chains to the particle suspensions. We then prepared silica and polystyrene suspensions with free pNIPAAm chains at a concentration much lower than the overlap concentration c*. A rheological study was conducted to determine the adsorption thickness of linear polymer chains on both silica and polystyrene nanoparticles. No significant adsorption was observed on silica, whereas the resultant thickness of the polymer was 8 nm on polystyrene.