Number Of Physical Crosslinks Research Articles

Protein-Nanoparticle Hydrogels That Self-assemble in Response to Peptide-Based Molecular Recognition.

Recently, supramolecular hydrogels assembled through nonspecific interactions between polymers and nanoparticles (termed PNP systems) were reported to have rapid shear-thinning and self-healing properties amenable for cell-delivery applications in regenerative medicine. Here, we introduce protein engineering concepts into the design of a new family of PNP hydrogels to enable direct control over the polymer-nanoparticle interactions using peptide-based molecular recognition motifs. Specifically, we have designed a bifunctional peptide that induces supramolecular hydrogel assembly between hydroxy apatite nanoparticles and an engineered, recombinant protein. We demonstrate that this supramolecular assembly critically requires molecular recognition, as no assembly is observed in the presence of control peptides with a scrambled amino acid sequence. Titration of the bifunctional peptide enables direct control over the number of physical cross-links within the system and hence the resulting hydrogel mechanical properties. As with previous PNP systems, these materials are rapidly shear-thinning and self-healing. As proof-of-concept, we demonstrate that these materials are suitable for therapeutic cell delivery applications in a preclinical murine calvarial defect model.

Investigations on mechanical properties of PI–PS multigraft copolymers

The stress–strain behaviour of multigraft copolymers consisting of a polyisoprene (PI) backbone and grafted polystyrene (PS) arms have been characterized by applying models of rubber elasticity such as Mooney–Rivlin, slip-tube and the extended non-affine tube model. Additionally, the range of low deformation has been investigated by relaxation tests for determining the stress relaxation. Multigraft copolymers show high strain at break and low residual strain caused by the large number of physical cross links resulting from several grafted PS side chains. From the model fits the material parameters G c ST and G e ST of the slip-tube model, representing the influence of chemical cross links and entanglements effects, respectively, and the n e / T e -value ( n e – number of statistical segments between two successive entanglements, T e – Langley trapping factor) of the extended non-affine tube model, are used to describe the tensile behaviour of these thermoplastic elastomers. The PS content was considered as filler phase taking into account the effect of hydrodynamic amplification. The influence of functionality and the number of branch points per molecule on the strain at break and the tensile strength is explained by the model parameters describing the stress–strain curve at low to medium (⩽400%, slip-tube, Mooney–Rivlin) and low to high (⩽900%, extended non-affine tube) elongations. It was observed that for the material with a spherical morphology G e ST is increasing with the number of branch points β (each branch point consists of a PI backbone segment, depending on the functionality one, two or four grafted PS arms). For cylindrical and lamellar morphologies the G e ST was decreasing with increasing β, which could be reconfirmed by applying the extended non-affine tube model where the n e / T e -parameter is increasing with β.

Aggregation and gelation in hydroxypropylmethyl cellulose aqueous solutions

In this work we present an analysis of the thermal behavior of hydroxypropylmethyl cellulose aqueous solutions, from room temperature to higher temperatures, above gelation. We focus on significant aspects, essentially overlooked in previous work, such as the correlation between polymer hydrophobicity and rheological behavior, and the shear effect on thermal gelation. Micropolarity and aggregation of the polymer chains were monitored by both UV/vis and fluorescence spectroscopic techniques, along with polarized light microscopy. Gel formation upon heating was investigated using rheological experiments, with both large strain (rotational) tests at different shear rates and small strain (oscillatory) tests. The present observations allow us to compose a picture of the evolution of the system upon heating: firstly, polymer reptation increases due to thermal motion, which leads to a weaker network. Secondly, above 55 °C, the polymer chains become more hydrophobic and polymer clusters start to form. Finally, the number of physical crosslinks between polymer clusters and the respective lifetimes increase and a three-dimensional network is formed. This network is drastically affected if higher shear rates, at non-Newtonian regimes, are applied to the system.

Electrospinning and Stabilization of Fully Hydrolyzed Poly(Vinyl Alcohol) Fibers

Fully (99+ %) hydrolyzed poly(vinyl alcohol) (PVA) was electrospun from water using Triton X-100 surfactant to lower the surface tension. The diameter of the electrospun PVA fibers ranged from 100 to 700 nm. Treatment of the PVA fiber mats with methanol for 8 h stabilized the fibers against disintegration in contact with water. In addition, the mats showed increased mechanical strength due to increased crystallinity following post-spinning treatment with methanol. We suggest that methanol treatment serves to increase the degree of crystallinity, and hence the number of physical cross-links in the electrospun PVA fibers. This may occur by removal of residual water within the fibers by the alcohol, allowing PVA−water hydrogen bonding to be replaced by intermolecular polymer hydrogen bonding resulting in additional crystallization. Potential applications of electrospun PVA include filters, precursors to graphitic fibers, and biomedical materials.