Improved antimicrobial potency through synergistic action of chitosan microparticles and low electric field.

Techniques to inhibit gram-negative bacteria such as Shiga toxin-producing Escherichia coli are valuable as the prevalence of large-scale industrial food preparation increases the likelihood of contamination. Chitosan, the deacetylated derivative of chitin, has been demonstrated to inhibit bacteria growth in acidic environments, but is significantly less effective in preventing bacteria grown at pH >7.0. Pulsed electric fields, constituting another method of bacteria inhibition, are difficult to generate at sufficient strength due to the high electric potentials required. This study utilizes adsorption of particulate chitosan in a very low electric field for an increased inhibition of gram-negative bacteria in neutral or alkaline pH conditions. Chitosan microparticles are demonstrated to flocculate E. coli, inhibit growth, and exhibit increased efficacy when combined with a low voltage electric field applied over 2-min intervals. Using sustained pulses of approximately 100 V/cm, it is demonstrated that bacteria viability is reduced by several orders of magnitude. The degree of bacterial inhibition is increased when chitosan microparticles are introduced to the system prior to imposing a small electric field.

Biocompatible nanoparticles trigger rapid bacteria clustering.

This study reveals an exciting phenomenon of stimulated bacteria clustering. Rapid aggregation and microbial arrest are shown to occur in Escherichia coli solutions of neutral pH when chitosan nanoparticles with positive zeta potential are added. Because chitosan nanoparticles can easily be dispersed in aqueous buffers, the rapid clustering phenomenon requires only minuscule nanoparticle concentrations and will be critical in developing new methods for extricating bacterial pathogens. This work establishes the dominant role of electrostatic attraction in bacteria-nanoparticle interactions by varying the nanoparticle zeta potential from highly positive to strongly negative values, and by exploring concentration effects. For strongly negative nanoparticles, no clusters form, while aggregates are small and loose at intermediate conditions. In addition, optical density measurements indicate that over 90% of the suspended bacteria flocculate within seconds of being mixed with chitosan nanoparticles of a highly positive surface charge. Finally, the nanoparticles are significantly more efficient as a clustering agent compared to an equal mass of molecular chitosan in solution, as the bacteria-nanoparticle clusters formed are substantially larger. The bacteria-nanoparticle aggregation effect demonstrated here promises a rapid separation method for aiding pathogen detection and for flocculation of bacteria in fermentation processes.

Imaging contrast effects in alginate microbeads containing trapped emulsion droplets.

This study focuses on spherical microparticles made of cross-linked alginate gel and microcapsules composed of an oil-in-water emulsion where the continuous aqueous phase is cross-linked into an alginate gel matrix. We have investigated the use of these easily manufactured microbeads as contrast agents for the study of the flow properties of fluids using nuclear magnetic resonance imaging. Results demonstrate that combined spin-spin (T(2)) relaxation and diffusion contrast in proton NMR imaging can be used to distinguish among rigid polymer particles, plain alginate beads, and alginate emulsion beads. Multi-echo CPMG spin-echo imaging indicates that the average spin-lattice (T(1)) and spin-spin (T(2)) relaxation times of the plain alginate and alginate emulsion beads are comparable. Meanwhile, diffusion-weighted imaging produces sharp contrast between the two types of alginate beads, due to restricted diffusion inside the embedded oil droplets of the alginate emulsion beads. While the signal obtained from most materials is severely attenuated under applied diffusion gradients, the alginate emulsion beads maintain signal strength. The alginate emulsion beads were added to a suspension and imaged in an abrupt, annular expansion flow. The emulsion beads could be clearly distinguished from the surrounding suspending fluid and rigid polystyrene particles, through either T(2) relaxation or diffusion contrast. Such a capability allows future use of the alginate emulsion beads as tracer particles and as one particle type among many in a multimodal suspension where detailed concentration profiles or particle size separation must be quantified during flow.

Stream spreading in multilayer microfluidic flows of suspensions.

The goal of this experimental study is to quantify the spreading of parallel streams with viscosity contrast in multilayer microfluidic flows. Three streams converge into one channel where a test fluid is sheathed between two layers of a Newtonian reference fluid. The test fluids are Newtonian fluids with viscosities ranging from 1.1 to 48.2 cP and suspensions of 10-mum-diameter PMMA particles with particle volume fractions phi = 0.16-0.30. The fluid interface locations are identified through fluorescence microscopy. The steady-state width of the center stream is strongly dependent on the viscosity ratio between the adjacent fluids and exhibits a near power-law relationship. This dependence occurs for both the Newtonian fluids and the suspensions, although the slopes differ. The high-concentration suspension (phi = 0.30) diverges from Newtonian behavior, while the low-concentration suspensions (phi = 0.16, 0.22) closely approximate that of the Newtonian fluids. The observed suspension behavior can be attributed to shear-induced particle migration.

Dialysis without membranes: how and why?

Dialysis between two flowing, miscible fluids without an intervening membrane enhances both the transport rate and biocompatibility. Unfortunately, it also presents serious challenges, including the loss of pressure as a driving force for volume transport, the need for sterile dialysate in greater quantity than in conventional dialysis, the possibility of unacceptable protein loss, and even the possibility of blood cell loss. This paper quantifies these advantages and disadvantages, and evaluate the means by which the latter might be surmounted. Preliminary data are provided to show that stable flows of one fluid sheathing another, miscible fluid are achievable and that molecular exchange between the fluids is orderly and in qualitative agreement with the theory. Extension of the concept to other blood purification tasks, especially in the treatment of liver failure and various macromolecular separations, is also discussed. In conclusion, membraneless separations will require a secondary process and a recycle stream. Under these conditions, its advantages can be preserved and its disadvantages controlled.