SCREENING FOR OXIDATIVE STRESS ELICITED BY ENGINEERED NANOMATERIALS: EVALUATION OF ACELLULAR DCFH ASSAY

The DCFH assay is commonly used for measuring free radicals generated by engineered nanomaterials (ENM), a well-established mechanism of ENM toxicity. Concerns exist over susceptibility of the DCFH assay to: assay conditions, adsorption of DCFH onto ENM, fluorescence quenching and light scattering. These effects vary in magnitude depending on ENM physiochemical properties and concentration. A rigorous evaluation of this method is still lacking. The objective was to evaluate performance of the DCFH assay for measuring ENM-induced free radicals. A series of diverse and well-characterized ENM were tested in the acellular DCFH assay. We investigated the effect of sonication conditions, dispersion media, ENM concentration, and the use of horseradish peroxidase (HRP) on the DCFH results. The acellular DCFH assay suffers from high background signals resulting from dye auto-oxidation and lacks sensitivity and robustness. DCFH oxidation is further enhanced by HRP. The number of positive ENM in the assay and their relative ranking changed as a function of experimental conditions. An inverse dose relationship was observed for several Carbon-based ENM. Overall, these findings indicate the importance of having standardized assays for evaluating ENM toxicity and highlights limitations of the DCFH assay for measuring ENM-induced free radicals

Multicore–Shell PNIPAm-<i>co</i>-PEGMa Microcapsules for Cell Encapsulation

The overall goal of this study was to fabricate multifunctional core–shell microcapsules with biological cells encapsulated within the polymer shell. Biocompatible temperature responsive microcapsules comprised of silicone oil droplets (multicores) and yeast cells embedded in a polymer matrix (shell) were prepared using a novel microarray approach. The cross-linked polymer shell and silicone multicores were formed in situ via photopolymerization of either poly(<i>N</i>-isopropylacryamide)(PNIPAm) or PNIPAm, copolymerized with poly(ethylene glycol monomethyl ether monomethacrylate) (PEGMa) within the droplets of an oil-in-water-in-oil double emulsion. An optimized recipe yielded a multicore–shell morphology, which was characterized by optical and laser scanning confocal microscopy (LSCM) and theoretically confirmed by spreading coefficient calculations. Spreading coefficients were calculated from interfacial tension and contact angle measurements as well as from the determination of the Hamaker constants and the pair potential energies. The effects of the presence of PEGMa, its molecular weight (<i>M</i>_n 300 and 1100 g/mol), and concentration (10, 20, and 30 wt %) were also investigated, and they were found not to significantly alter the morphology of the microcapsules. They were found, however, to significantly improve the viability of the yeast cells, which were encapsulated within PNIPAm-based microcapsules by direct incorporation into the monomer solutions, prior to polymerization. Under LSCM, the fluorescence staining for live and dead cells showed a 30% viability of yeast cells entrapped within the PNIPAm matrix after 45 min of photopolymerization, but an improvement to 60% viability in the presence of PEGMa. The thermoresponsive behavior of the microcapsules allows the silicone oil cores to be irreversibly ejected, and so the role of the silicone oil is 2-fold. It facilitates multifunctionality in the microcapsule by first being used as a template to obtain the desired core–shell morphology, and second it can act as an encapsulant for oil-soluble drugs. It was shown that the encapsulated oil droplets were expelled above the volume phase transition temperature of the polymer, while the collapsed microcapsule remained intact. When these microcapsules were reswollen with an aqueous solution, it was observed that the hollow compartments refilled. In principle, these hollow-core microcapsules could then be filled with water-soluble drugs that could be delivered in vivo in response to temperature

Thermoresponsive Semicrystalline Poly(ε-caprolactone) Networks: Exploiting Cross-linking with Cinnamoyl Moieties to Design Polymers with Tunable Shape Memory

The overall goal of this study was to synthesize semicrystalline poly­(ε-caprolactone) (PCL) copolymer networks with stimuli-responsive shape memory behavior. Herein, we investigate the influence of a cinnamoyl moiety to design shape memory polymer networks with tunable transition temperatures. The effect of various copolymer architectures (random or ABA triblock), the molecular weight of the crystalline domains, PCL diol, <i>(M</i>_w 1250 or 2000 g mol^–1) and its composition in the triblock (50 or 80 mol %) were also investigated. The polymer microstructures were confirmed by NMR, DSC, WAXS and UV–vis spectroscopic techniques. The thermal and mechanical properties and the cross-linking density of the networks were characterized by DSC, tensile testing and solvent swelling, respectively. Detailed thermomechanical investigations conducted using DMA showed that shape memory behavior was obtained only in the ABA triblock copolymers. The best shape memory fixity, <i>R</i>_f of ∼99% and shape recovery, <i>R</i>_r of ∼99% was obtained when PCL diol with <i>M</i>_w 2000 g mol^–1 was incorporated in the triblock copolymer at a concentration of 50 mol %. The series of triblock copolymers with PCL at 50 mol % also showed mechanical properties with tunable shape memory transition temperatures, ranging from 54 °C to close to body temperature. Our work establishes a general design concept for inducing a shape memory effect into any semicrystalline polyester network. More specifically, it can be applied to systems which have the highest transition temperature closest to the application temperature. An advantage of our novel copolymers is their ability to be cross-linked with UV radiation without any initiator or chemical cross-linker. Possible applications are envisioned in the area of endovascular treatment of ischemic stroke and cerebrovascular aneurysms, and for femoral stents

Thermoresponsive Semicrystalline Poly(ε-caprolactone) Networks: Exploiting Cross-linking with Cinnamoyl Moieties to Design Polymers with Tunable Shape Memory

The overall goal of this study was to synthesize semicrystalline poly­(ε-caprolactone) (PCL) copolymer networks with stimuli-responsive shape memory behavior. Herein, we investigate the influence of a cinnamoyl moiety to design shape memory polymer networks with tunable transition temperatures. The effect of various copolymer architectures (random or ABA triblock), the molecular weight of the crystalline domains, PCL diol, <i>(M</i>_w 1250 or 2000 g mol^–1) and its composition in the triblock (50 or 80 mol %) were also investigated. The polymer microstructures were confirmed by NMR, DSC, WAXS and UV–vis spectroscopic techniques. The thermal and mechanical properties and the cross-linking density of the networks were characterized by DSC, tensile testing and solvent swelling, respectively. Detailed thermomechanical investigations conducted using DMA showed that shape memory behavior was obtained only in the ABA triblock copolymers. The best shape memory fixity, <i>R</i>_f of ∼99% and shape recovery, <i>R</i>_r of ∼99% was obtained when PCL diol with <i>M</i>_w 2000 g mol^–1 was incorporated in the triblock copolymer at a concentration of 50 mol %. The series of triblock copolymers with PCL at 50 mol % also showed mechanical properties with tunable shape memory transition temperatures, ranging from 54 °C to close to body temperature. Our work establishes a general design concept for inducing a shape memory effect into any semicrystalline polyester network. More specifically, it can be applied to systems which have the highest transition temperature closest to the application temperature. An advantage of our novel copolymers is their ability to be cross-linked with UV radiation without any initiator or chemical cross-linker. Possible applications are envisioned in the area of endovascular treatment of ischemic stroke and cerebrovascular aneurysms, and for femoral stents

Improving Charge Carrier Mobility of Polymer Blend Field Effect Transistors with Majority Insulating Polymer Phase

The key approach to achieve high performance field effect transistor fabricated from semiconducting/insulating polymer blends with majority insulating polymer phase is the formation of connected fibrous structures of semiconducting polymer and good interfacial interaction of semiconducting polymer with the dielectric layer. Herein, tetrahydrofuran (THF) as a marginal solvent was used as an additive in marginal/good solvent mixtures to control the crystallite structure, phase segregation, and hole transport properties of poly­(3-hexylthiophene)/poly­(styrene) (P3HT/PS; weight ratio: 1/4) blends, with the advantage that marginal/good solvent mixture gives P3HT sufficient time for phase segregation and relatively better solvent quality to aggregate to more stable structures compared to other reported strategies as bad/good solvent mixtures or directly marginal solvents. Incorporation of THF reduces the P3HT solubility, forming connected fibrous structures as observed in both neat P3HT and blend films; it appears these structures are responsible for improved charge transport. Furthermore, enhanced molecular ordering, π–π stacking and conjugation length are observed with increasing THF amount. THF promotes the edge-on orientation and more stable crystal structures in P3HT, while the lattice spacing remains the same. Finally, the added THF increases hole mobility for P3HT/PS blend FETs, reaching a maximum value of 4 0.0 × 10^–3 cm²/(V s) with 20 vol % THF and being comparative to neat P3HT; however, THF has an insignificant influence on the hole mobility for neat P3HT FETs. Morphological characterization supports the idea that differential solubility creates both enhanced chain ordering and vertical phase segregation that both improve FET performance. These results are promising for the development of environmentally stable and lower cost polymer electronics

Effects of Particle Surface Charge, Species, Concentration, and Dispersion Method on the Thermal Conductivity of Nanofluids

The purpose of this experimental study is to evaluate the effects of particle species, surface charge, concentration, preparation technique, and base fluid on thermal transport capability of nanoparticle suspensions (nanofluids). The surface charge was varied by changing the pH value of the fluids. The alumina (Al2O3) and copper oxide (CuO) nanoparticles were dispersed in deionized (DI) water and ethylene glycol (EG), respectively. The nanofluids were prepared using both bath-type and probe sonicator under different power inputs. The experimental results were compared with the available experimental data as well as the predicted values obtained from Maxwell effective medium theory. It was found that ethylene glycol is more suitable for nanofluids applications than DI water in terms of thermal conductivity improvement and stability of nanofluids. Surface charge can effectively improve the dispersion of nanoparticles by reducing the (aggregated) particle size in base fluids. A nanofluid with high surface charge (low pH) has a higher thermal conductivity for a similar particle concentration. The sonication also has a significant impact on thermal conductivity enhancement. All these results suggest that the key to the improvement of thermal conductivity of nanofluids is a uniform and stable dispersion of nanoscale particles in a fluid