Light-activated hydrogel formation via the triggered folding and self-assembly of a designed peptide

Photopolymerization can be used to construct materials with precise temporal and spatial resolution. Applications such as tissue engineering, drug delivery, the fabrication of microfluidic devices and the preparation of high-density cell arrays employ hydrogel materials that are often prepared by this technique. Current photopolymerization strategies used to prepare hydrogels employ photoinitiators, many of which are cytotoxic and require large macromolecular precursors that need to be functionalized with moieties capable of undergoing radical cross-linking reactions. We have developed a simple light-activated hydrogelation system that employs a designed peptide whose ability to self-assemble into hydrogel material is dependent on its intramolecular folded conformational state. An iterative design strategy afforded MAX7CNB, a photocaged peptide that, when dissolved in aqueous medium, remains unfolded and unable to self-assemble; a 2 wt % solution of freely soluble unfolded peptide is stable to ambient light and has the viscosity of water. Irradiation of the solution (260 < lambda < 360 nm) releases the photocage and triggers peptide folding to produce amphiphilic beta-hairpins that self-assemble into viscoelastic hydrogel material. Circular dichroic (CD) spectroscopy supports this folding and self-assembly mechanism, and oscillatory rheology shows that the resulting hydrogel is mechanically rigid (G' = 1000 Pa). Laser scanning confocal microscopy imaging of NIH 3T3 fibroblasts seeded onto the gel indicates that the gel surface is noncytotoxic, conducive to cell adhesion, and allows cell migration. Lastly, thymidine incorporation assays show that cells seeded onto decaged hydrogel proliferate at a rate equivalent to cells seeded onto a tissue culture-treated polystyrene control surface.     

Temperature-triggered micellization of block copolymers on an ionic liquid surface

In situ neutron reflectivity was used to study thermally induced structural changes of the lamellae-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer thin films floating on the surface of an ionic liquid (IL). The IL, 1-butyl-3-methylimidazolium trifluoromethanesulfonate, is a nonsolvent for PS and a temperature-tunable solvent for P2VP, and, as such, micellization can be induced at the air-IL interface by changing the temperature. Transmission electron microscopy and scanning force microscopy were used to investigate the resultant morphologies of the micellar films. It was found that highly ordered nanostructures consisting of spherical micelles with a PS core surrounded by a P2VP corona were produced. In addition, bilayer films of PS homopolymer on top of a PS-b-P2VP layer also underwent micellization with increasing temperature but the micellization was strongly dependent on the thickness of the PS and PS-b-P2VP layers.     

In vivo assessment of blood-spinal cord barrier permeability: serial dynamic contrast enhanced MRI of spinal cord injury

Serial in vivo dynamic contrast enhanced (DCE) MRI studies were performed on spinal cord injured rats on post-injury Days 0, 10, 20 and 30 to determine the distribution of gadopentetate-dimeglumine (Gd) concentration in injured cord tissue. A two-compartment pharmacokinetic model was fitted to the time course of the concentration data at the epicenter of injury for each post-injury day. From these fits, the rates of the Gd transport between plasma and injured cord tissue were determined as a measure of blood-spinal cord barrier (BSCB) permeability. The results indicated that Gd transport rates decrease steadily with a concomitant improvement in motor functions of the rats with post-injury time. Specifically, the rates of Gd accumulation in injured SC tissue and its clearance correlated with the neurobehavioral scores with correlation coefficients of rho = -0.96 and -0.79, respectively, suggesting a significant link between the neurobehavioral function and the restoration of BSCB integrity as a result of the ongoing repair and recovery processes within the injured cords.     

Molecular dynamics simulations of He bubble nucleation at grain boundaries

The nucleation behavior of He bubbles in single-crystal (sc) and nano-grain body-centered-cubic (bcc) Mo is simulated using molecular dynamics (MD) simulations, focusing on the effects of the grain boundary (GB) structure. In sc Mo, the nucleation behavior of He bubbles depends on irradiation conditions. He bubbles nucleate by either clustering of He atoms with pre-existing vacancies or self-interstitial-atom (SIA) punching without initial vacancies. In nano-grain Mo, strong precipitation of He at the GBs is observed, and the density, size and spatial distribution of He bubbles vary with the GB structure. The corresponding He bubble density is higher in nano-grain Mo than that in sc Mo and the average bubble size is smaller. In the GB plane, He bubbles distribute along the dislocation cores for GBs consisting of GB dislocations and randomly for those without distinguishable dislocation structures. The simulation results in nano-grain Mo are in agreement with previous experiments in metal nano-layers, and they are further explained by the effect of excess volume associated with the GBs.