Bridgette M. Budhlall

Microwave, Photo- and Thermally Responsive PNIPAm−Gold Nanoparticle Microgels

Microwave-, photo- and thermo-responsive polymer microgels that range in size from 500 to 800 microm and are swollen with water were prepared by a novel microarray technique. We used a liquid-liquid dispersion technique in a system of three immiscible liquids to prepare hybrid PNIPAm- co-AM core-shell capsules loaded with AuNPs. The spontaneous encapsulation is a result of the formation of double oil-in-water-in-oil (o/w/o) emulsion. It is facilitated by adjusting the balance of the interfacial tensions between the aqueous phase (in which a water-soluble drug may be dissolved), the monomer phase and the continuous phase. The water-in-oil (w/o) droplets containing 26 wt% NIPAm and Am monomers, 0.1 wt% Tween-80 surfactant, FITC fluorescent dye and colloidal gold nanoparticles spontaneously developed a core-shell morphology that was fixed by in situ photopolymerization. The results demonstrate new reversibly swelling and deswelling AuNP/PNIPAm hybrid core-shell microcapsules and microgels that can be actuated by visible light and/or microwave radiation (<or=1,250 nm) and/or temperature. This is the first study to demonstrate that incorporating AuNPs speeds up the response kinetics of PNIPAm, and hence enhances the sensitivity to external stimuli of PNIPAm. These microgels can have potential applications for microfluidic switches or microactuators, photosensors, and various nanomedicine applications in controlled delivery and release.

Pickering Emulsion as a Template to Synthesize Janus Colloids with Anisotropy in the Surface Potential

A versatile new concept is presented for the synthesis of Janus colloids composed of Laponite nanoclay armored poly(divinylbenzene) with an anisotropic surface potential via a double Pickering emulsion template. First, polystyrene or poly(divinylbenzene) colloids stabilized with Laponite nanoclay are synthesized via a Pickering miniemulsion approach. These nanoparticle-stabilized colloids were then templated at a wax-water interface in a second Pickering emulsion in order to chemically modify one hemisphere of the colloids. Janus modification of the colloids was accomplished by cation exchange of sodium ions, originally present on the surface of the Laponite with various salts of modifying ions (Ca(2+), Fe(2+), and Fe(3+)) in the suspension. The zeta potential of the chemically modified and unmodified colloids was compared. The maximum change in the zeta potential was given by the calcium ions, Ca(2+)-adsorbed modified colloids as compared to unmodified sodium ions, Na(+)-adsorbed colloids. The distribution of charges on the Janus colloids results in a nonuniform zeta potential. XPS and optical microscopy were used to verify the successful chemical modification by the cation exchange of Na(+) for Ca(2+) ions on one hemisphere of the Janus colloids.

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, (M(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, R(f) of ~99% and shape recovery, R(r) of ~99% was obtained when PCL diol with M(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.

Multicore–Shell PNIPAm-co-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(N-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 (M(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.

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 ( Al 2 O 3 ) 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.

Microgels or microcapsules? Role of morphology on the release kinetics of thermoresponsive PNIPAm-co-PEGMa hydrogels†

The effect of morphology of PNIPAm, PEGMa and PNIPAm-co-PEGMa hydrogels on the uptake and delivery (release kinetics) of a model drug (FITC–dextran) was investigated. Two types of hydrogel architectures: microgels and microcapsules, without and with core–shell morphology, were synthesized. The microcapsules had 30–50% greater uptake compared to the corresponding microgel architecture. The estimated pore size for the PNIPAm, PNIPAm-co-PEGMa and PEGMa hydrogels were 78, 92 and 130 A, respectively. The drug release was performed at 25, 37 (physiological temperature), and 45 °C (targeted stimulating temperature). Diffusion coefficients at temperatures below VPTT of the microgels showed close correlation with the pore size, but this was not the case for the microcapsules. The release kinetics is dominated by temperature responsiveness at T greater than the VPTT and by hydrogel morphology at T VPTT. More than 80% of the drug was released in the first 10 min using the temperature responsive microcapsule, compared to 1 h for the corresponding microgel. Unlike prior reports in the literature, the release of FITC–dextran is characteristic of a Super Case II Fickian diffusion and Anomalous release mechanism for the copolymer microgels when T > VPTT and for the PNIPAm and PEGMa microgels when T < VPTT. These results demonstrate the feasibility of modulating the release profile of encapsulated proteins (for tissue repair), chemotherapeutics (for drug delivery) and nucleic acids (for gene delivery) by tailoring the polymer morphology.

Temperature dependence of serum protein adsorption in PEGylated PNIPAm microgels

The effect of PEGylation on the thermal response and protein adsorption resistance of crosslinked PNIPAm microgels was investigated. It was found that the presence of PEG, its molecular weight (M(n) 300 and 1100 g/mol) and its concentration (10, 20, and 30 wt.%) each significantly influenced both the value and breadth of the volume phase transition temperature (VPTT) and the adsorption of bovine serum albumin (BSA) on the surface of the microgels. Specifically, as the degree of PEGylation increased, the value and breadth of the VPTT increased, and the adsorption of BSA decreased significantly. The critical concentration that minimizes protein adsorption on PNIPAm-co-PEGMa microgels was found to be 20 wt.% of PEGMa. This critical concentration was confirmed qualitatively using laser scanning confocal microscopy (LSCM). Evidence for the effect of the molecular weight of PEG on the structure of PNIPAm-co-PEGMa microgels was provided by thermal analysis using differential scanning calorimetry. The VPTT study revealed significant differences in the composition of the microgels when PEGMa samples with two different molecular weights were used as comonomers with PNIPAm. It was determined that the molecular weight and concentration of PEGMa controls the structure of the microgels, which in turn influences their temperature response and protein adsorption resistance properties of the microgels. Our work establishes specific design concepts for controlling the molecular architecture of the hydrogels in order to tune their temperature response and biocompatibility for use in a variety of biomedical applications such as, cell encapsulation, drug delivery and tissue engineering applications.

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.

High refractive index immersion fluids for 193 nm immersion lithography

For the next-generation immersion lithography technology, there is a growing interest in the immersion fluids having a refractive index larger than 1.5 and low absorbance at 193nm wavelength. In this paper, we report our effort in identifying new immersion fluid candidates. The absolute refractive index values and thermo-optic coefficients, dn/dT, were measured with 1x10-4 and 1x10-5 accuracy respectively at 193nm wavelength. The results showed promising candidates having refractive index ranging from 1.5 to 1.65 with low absorbance at 193nm wavelength. Preliminary imaging results with a new immersion fluid gave good 65nm Line/Space patterns. However, the minimum exposure time of 20sec is about ten times as needed for water, indicating the need to further reduce the absorbance of the immersion fluid.

Tuning oxygen permeability in azobenzene-containing side-chain liquid crystalline polymers

A series of poly[4-(4-cyanoazobenzene-4′-oxy)alkyl methacrylate]s, side-chain liquid crystalline polymers (azoLCPs) were synthesized with methylene groups as spacers varying from 5 to 12. The thermal properties and phase transition temperatures of the polymers were characterized with differential scanning calorimetry and polarized optical microscopy and a relationship between spacer lengths, glass transition (Tg) and clearing temperatures (Tc) of the polymers was established. The Tg decreased with increasing spacer length, while the Tc exibited an odd–even effect with varying spacer length. X-ray diffraction was used to determine the degree crystallinity above and below Tc. The azoLCPs exhibited smectic ↔ nematic ↔ isotropic phases. Increasing crystallinity correlated linearly with decreasing gas permeability as measured using an oxygen permeation analyzer, which was used to measure the films’ permeability to oxygen (O2) gas. Switching of the azoLCPs from a liquid crystalline to an isotropic state was accomplished by heating the films above their Tc, which resulted in at least a 10-fold increase in the O2 permeability coefficient (PO2). Increasing the methylene spacer length of the azoLCP had little or no effect on gas permeability however it did decrease the Tc, allowing fine control of the temperature at which the switch in PO2 takes place by tuning the mesophase between nematic and isotropic states.