Mechanical Properties of Composite Hydrogels for Tissue Engineering.

Tissue engineering provides solutions that require medicine to restore damaged tissues or even complete organs. This discipline combines biologically active scaffolds, cells and molecules; being the addition of nanoparticles into the scaffolds, one of the techniques that is attracting more interest these days. In this work, Hydroxyapatite Nanorods (HA) were added to the network of Gelatin hydrogel (GE), and the particular properties resulting from their interaction were studied. Specifically, viscoelastic properties were characterized as a function of gel and nanoparticle concentration, varying ratios and temperatures. Oscillatory Time Sweeps (OTS) provided the necessary information about how the timeresolved material property/structure alteration. A wide variety of Continuous Flow Tests and Frequency Sweeps were used to describe the mechanical properties of the material, proving that the presence of nanoparticles led to a reinforcement of the gel network, mechanical stiffness and strength. The thixotropic nature of the gels was also evaluated and the most common theoretical models were described and commented. The attributes inferred from the data, showed a material that can allow the natural growth of bone tissue whilst withstanding properly the mechanical efforts; resulting in a material with an outstanding suitability to be used in regenerative medicine.

Shear-induced structural and thermodynamic phase transitions in micellar systems.

In this contribution a methodology to compute and classify shear-induced structural and phase transitions in surfactant/water mixtures from rheological measurements is presented. Non-linear rheological experiments, considering variations in surfactant concentration and temperature, are analyzed. In particular, the parameters of the BMP (Bautista-Manero-Puig) model, obtained from the fitting of the shear stress versus shear rate data, which are functions of surfactant concentration and temperature, allow classifying structural and phase transition boundaries. To test this methodology, we consider the analysis of the shear-induced structural and phase transitions of two micellar systems, cetyltrimethylammonium tosylate (CTAT)/water as a function of CTAT concentrations and Pluronics P103/water as a function of temperature. We found that the CTAT/water system presents a first-order phase transition at 30 ° C, and around 31 to 32 wt.% from isotropic to nematic phases, whereas a 20 wt.% Pluronics P103 aqueous micellar solution has two second-order (structural) phase transitions, one from spherical to cylindrical micelles at 33.1 ° C, and another one from cylindrical micelles to a nematic phase at 35.8 ° C and one first-order phase transition around 37.9 ° C at high shear rates near to the cloud point previously reported. The proposed methodology is also able to identify the instability regions where the wormlike micelles are broken, producing the typical shear banding behavior.

Lenghty reverse poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) polymeric micelles and gels for sustained release of antifungal drugs.

In this work, we present a detailed study of the potential application of polymeric micelles and gels of four different reverse triblock poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) copolymers (BOnEOmBOn, where n denotes the respective block lengths), specifically BO8EO90BO8, BO14EO378BO14, BO20EO411BO20 and BO21EO385BO21, as effective drug transport nanocarriers. In particular, we tested the use of this kind of polymeric nanostructures as reservoirs for the sustained delivery of the antifungals griseofulvin and fluconazole for oral and topical administration. Polymeric micelles and gels formed by these copolymers were shown to solubilize important amounts of these two drugs and to have a good stability in physiologically relevant conditions for oral or topical administration. These polymeric micellar nanocarriers were able to release drugs in a sustained manner, being the release rate slower as the copolymer chain hydrophobicity increased. Different sustained drug release profiles were observed depending on the medium conditions. Gel nanocarriers were shown to display longer sustained release rates than micellar formulations, with the existence of a pulsatile-like release mode under certain solution conditions as a result of their inner network structure. Certain bioadhesive properties were observed for the polymeric physical gels, being moderately tuned by the length and hydrophobicity of the polymeric chains. Furthermore, polymeric gels and micelles showed activity against the yeast Candida albicans and the mould demartophytes (Trichophyton rubrum and Microsporum canis) and, thus, may be useful for the treatment of different cutaneous fungal infections.

DNA/chitosan electrostatic complex.

Up to now, chitosan and DNA have been investigated for gene delivery due to chitosan advantages. It is recognized that chitosan is a biocompatible and biodegradable non-viral vector that does not produce immunological reactions, contrary to viral vectors. Chitosan has also been used and studied for its ability to protect DNA against nuclease degradation and to transfect DNA into several kinds of cells. In this work, high molecular weight DNA is compacted with chitosan. DNA-chitosan complex stoichiometry, net charge, dimensions, conformation and thermal stability are determined and discussed. The influence of external salt and chitosan molecular weight on the stoichiometry is also discussed. The isoelectric point of the complexes was found to be directly related to the protonation degree of chitosan. It is clearly demonstrated that the net charge of DNA-chitosan complex can be expressed in terms of the ratio [NH3(+)]/[P(-)], showing that the electrostatic interactions between DNA and chitosan are the main phenomena taking place in the solution. Compaction of DNA long chain complexed with low molar mass chitosan gives nanoparticles with an average radius around 150nm. Stable nanoparticles are obtained for a partial neutralization of phosphate ionic sites (i.e.: [NH3(+)]/[P(-)] fraction between 0.35 and 0.80).

Complex self-assembly of reverse poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) triblock copolymers with long hydrophobic and extremely lengthy hydrophilic blocks.

Amphiphilic block copolymers have emerged during last years as a fascinating substrate material to develop micellar nanocontainers able to solubilize, protect, transport, and release under external or internal stimuli different classes of cargos to diseased cells or tissues. However, this class of materials can also induce biologically relevant actions, which complement the therapeutic activity of their cargo molecules through their mutual interactions with biologically relevant entities (cellular membranes, proteins, organelles...); these interactions at the same time, are regulated by the nature, conformation, and state of the copolymeric chains. For these reasons, in this paper we investigated the self-assembly process and physico-chemcial properties of two reverse triblock poly(butylene oxide)-poly(ethylene oxide)-poly(butylene oxide) block copolymers, BO14EO378BO14 and BO21EO385BO21, which have been recently found to be very useful as drug delivery nanovehicles and biological response modifiers under certain conditions (A. Cambón et al. Int. J. Pharm. 2013, 445, 47-57) in order to obtain a clear picture of the solution behavior of this class or block copolymers and to understand their biological activity. These block copolymers are characterized by possessing long BO blocks and extremely lengthy central EO ones, which provide them with a rich rheological behavior characterized by the formation of flowerlike micelles with sizes ranging from 20 to 40 nm in aqueous solution and the presence of intermicellar bridging even at low copolymers concentrations as denoted by atomic force microscopy. Bridging is also clearly observed by analyzing the rheological response of these block copolymers both storage and loss moduli upon changes on time, temperature, and or concentration. Strikingly, the relatively wide Poisson distribution of the polymeric chains make the present copolymers behave rather distinctly to conventional associative thickeners. The observed rich rheological behavior and their tunability also make these copolymers promising materials to configure drug gelling depots.