Alexander V. Korobko

Dextran based photodegradable hydrogels formed via a Michael addition

A photodegradable, covalently crosslinked hydrogel system has been constructed from the biocompatible polymers dextran and poly(ethylene glycol) using the acrylate–thiol Michael addition as the crosslinking method. Light sensitivity of the hydrogel was introduced by placing a non-toxic photolabile o-nitrobenzyl moiety in between dextran backbone and acrylate group. Hydrogels were prepared under physiological conditions without the need of any additional reagents by mixing solutions of dextran functionalized with acrylate-modified o-nitrobenzyl moieties (Dex-AN) and dithiolated poly(ethylene glycol) (DSPEG). The degradation of the hydrogels due to UV irradiation was investigated with scanning electron microscopy, infrared and UV-vis spectroscopy. Using green fluorescent protein (GFP) as a model protein, light triggered protein release from the obtained gel matrices was investigated in different forms. Furthermore, photodegradation of the hydrogel via two photon excitation was also examined using focused pulsed near infrared (NIR) laser beam as a light source.

Noncovalent triblock copolymers based on a coiled-coil peptide motif.

The formation of a noncovalent triblock copolymer based on a coiled-coil peptide motif is demonstrated in solution. A specific peptide pair (E and K) able to assemble into heterocoiled coils was chosen as the middle block of the polymer and conjugated to poly(ethylene glycol) (PEG) and polystyrene (PS) as the outer blocks. Mixing equimolar amounts of the polymer-peptide block copolymers PS-E and K-PEG resulted in the formation of coiled-coil complexes between the peptides and subsequently in the formation of the amphiphilic triblock copolymer PS-E/K-PEG. Aqueous self-assembly of the separate peptides (E and K), the block copolymers (PS-E and K-PEG), and equimolar mixtures thereof was studied by circular dichroism, dynamic light scattering, and cryogenic transmission electron microscopy. It was found that the noncovalent PS-E/K-PEG copolymer assembled into rodlike micelles, while in all other cases, spherical micelles were observed. Temperature-dependent studies revealed the reversible nature of the coiled-coil complex and the influence of this on the morphology of the aggregate. A possible mechanism for these transitions based on the interfacial free energy and the free energy of the hydrophobic blocks is discussed. The self-assembly of the polymer-peptide conjugates is compared to that of polystyrene-b-poly(ethylene glycol), emphasizing the importance of the coiled-coil peptide block in determining micellar structure and dynamic behavior.

Cyclodextrin–dextran based in situ hydrogel formation: a carrier for hydrophobic drugs

A rapid in situ hydrogel forming system composed of thiol functionalized β-cyclodextrin and maleimide functionalized dextran has been prepared and the in vitro release profile of the hydrophobic drug all-trans retinoic acid was studied.

Controlled liposome fusion mediated by SNARE protein mimics

The fusion of lipid membranes is essential for the delivery of chemicals across biological barriers to specific cellular locations. Intracellular membrane fusion is particularly precise and is critically mediated by SNARE proteins. To allow membrane fusion to be better understood and harnessed we have mimicked this important process with a simple bottom-up model in which synthetic fusogens replicate the essential features of SNARE proteins. In our fusogens, the coiled-coil molecular recognition motif of SNARE proteins is replaced by the coiled-coil E/K peptide complex, which is one-ninth the size. The peptides are anchored in liposome membranes via pegylated lipids. Here we discuss how the liposome fusion process is controlled by different parameters within the minimal model. The lipopeptide fusogens form specific coiled coils that dock liposomes together, resulting in the merging of membranes via the stalk intermediate. Unusually for model systems, the lipopeptides can rapidly lead to fusion of entire liposome populations and the liposomes can undergo many rounds of fusion. The rate and extent of fusion and the number of fusion rounds can be manipulated by adjusting the fusogen and liposome concentrations. For example, these parameters can be tuned such that tens of thousands of ∼100 nm liposomes fuse into a single giant liposome ∼10 μm in diameter; alternatively, conditions can be selected such that only two liposomes fuse. The improved understanding of membrane fusion shows how application-specific fusion attributes can be achieved, and paves the way for controlled nanoreactor mixing and controlled delivery of cargo to cells.

Mechanical and thermal properties of polymer micro- and nanocomposites

Abstract We have analyzed the thermal conductivity and the tensile modulus of composite materials within the framework of the Halpin-Tsai and Lewis-Nielsen models. The parameter linking thermal conductivity and tensile modulus together is the shape factor of the (nano)filler. Model analysis based on experimental data shows that particle aggregation into a weak mechanical network may be required to achieve good correlation between thermal conductivity and the Young’s modulus when analyzing the data within the framework of a single model and requiring the same value of the shape factor. We believe this approach will make quantitative analysis of nanocomposite thermal properties possible.

Fast and controlled fabrication of porous graphene oxide: application of AFM tapping for mechano-chemistry

This paper describes a novel method to fabricate porous graphene oxide (PGO) from GO by exposure to oxygen plasma. Compared to other methods to fabricate PGO described so far, e.g. the thermal and steam etching methods, oxygen plasma etching method is much faster. We studied the development of the porosity with exposure time using atomic force microscopy (AFM). It was found that the development of PGO upon oxygen-plasma exposure can be controlled by tapping mode AFM scanning using a Si tip. AFM tapping stalls the growth of pores upon further plasma exposure at a level that coincides with the fraction of sp2 carbons in the GO starting material. We suggest that AFM tapping procedure changes the bond structure of the intermediate PGO structure, and these stabilized PGO structures cannot be further etched by oxygen plasma. This constitutes the first report of tapping AFM as a tool for local mechano-chemistry.

Changes of the Molecular Mobility of Poly(ε-caprolactone) upon Drawing, Studied by Dielectric Relaxation Spectroscopy

Dielectric relaxation spectroscopy (DRS) of poly(ε-caprolactone) with different draw ratios showed that the mobility of polymer chains in the amorphous part decreases as the draw ratio increases. The activation energy of the α process, which corresponds to the dynamic glass transition, increases upon drawing. The enlarged gap between the activation energies of the α process and the β process results in a change of continuity at the crossover between the high temperature a process and the α and β processes. At low drawing ratios the a process connects with the β process, while at the highest drawing ratio in our measurements, the a process is continuous with the α process. This is consistent with X-ray diffraction results that indicate that upon drawing the polymer chains in the amorphous part align and densify upon drawing. As the draw ratio increases, the α relaxation broadens and decreases its intensity, indicating an increasing heterogeneity. We observed slope changes in the α traces, when the temperature decreases below that at which τα ≈ 1 s. This may indicate the glass transition from the ‘rubbery’ state to the non-equilibrium glassy state.

Near-second-order transition in confined living-polymer solutions.

We analyze a near-second-order transition occurring in solutions of living polymers confined by two parallel surfaces in equilibrium with a reservoir solution. The molecular self-consistent field theory in the regime of weak adsorption or depletion is mapped to phenomenological Landau theory, where the order parameter is the average degree of polymerization or, equivalently, the normalized chain-end concentration. The distance between two surfaces at which the transition occurs scales as ℓ(c)(2)|c| where ℓ(c) is the correlation length of the polymer solution in the reservoir and c(-1) is de Gennes adsorption length. In the second half of the paper we focus on experimentally observable features. The predicted transition can be detected experimentally by probing the living-polymer mediated disjoining potential between surfaces by means of, e.g., colloidal probe atomic force microscopy. To facilitate experimental investigations we derive simple explicit expressions for the disjoining potential for several regimes. By comparison with full numerical calculations it was verified that these are quite accurate.