Design and fabrication of a multilayered polymer microfluidic chip with nanofluidic interconnects via adhesive contact printing

The design and fabrication of a multilayered polymer micro-nanofluidic chip is described that consists of poly(methylmethacrylate) (PMMA) layers that contain microfluidic channels separated in the vertical direction by polycarbonate (PC) membranes that incorporate an array of nanometre diameter cylindrical pores. The materials are optically transparent to allow inspection of the fluids within the channels in the near UV and visible spectrum. The design architecture enables nanofluidic interconnections to be placed in the vertical direction between microfluidic channels. Such an architecture allows microchannel separations within the chip, as well as allowing unique operations that utilize nanocapillary interconnects: the separation of analytes based on molecular size, channel isolation, enhanced mixing, and sample concentration. Device fabrication is made possible by a transfer process of labile membranes and the development of a contact printing method for a thermally curable epoxy based adhesive. This adhesive is shown to have bond strengths that prevent leakage and delamination and channel rupture tests exceed 6 atm (0.6 MPa) under applied pressure. Channels 100 microm in width and 20 microm in depth are contact printed without the adhesive entering the microchannel. The chip is characterized in terms of resistivity measurements along the microfluidic channels, electroosmotic flow (EOF) measurements at different pH values and laser-induced-fluorescence (LIF) detection of green-fluorescent protein (GFP) plugs injected across the nanocapillary membrane and into a microfluidic channel. The results indicate that the mixed polymer micro-nanofluidic multilayer chip has electrical characteristics needed for use in microanalytical systems.     

Fluidic communication between multiple vertically segregated microfluidic channels connected by nanocapillary array membranes

Hybrid microfluidic/nanofluidic devices offer unique capabilities for manipulating and analyzing minute volumes of expensive or hard-to-obtain samples. Here, multilayer poly-(methyl methacrylate) microchips, with multiple spatially isolated microfluidic channels interconnected by nanocapillary array membranes (NCAMs), are fabricated using an adhesive contact printing process. The NCAMs, positioned between the microfluidic channel layers, add functionality to the inter-microchannel fluid transfer unit operation. They do so because the transport of specific analytes through the NCAM can be controlled by adjusting the ionic strength, the polarity of the applied bias, the surface charge density, and the pore size. A simplified, floating injection technique for NCAM-coupled nanofluidic devices is described and compared with conventional biased injection. In the floating injection approach, a voltage is applied across the injection channel and the slight electric field extension at the cross-section is used to transfer analytes through the nanopores to the separation channel. Floating injection excels in plug reproducibility, separation resolution, and operation simplicity, although it decreases assay throughput relative to biased injection. Floating injection can avoid the uneven distribution of analytes in the microfluidic channel that sometimes results from biased injection because of the volume mismatch between NCAM nanopore transport capacity and the supply of fluid. Moreover, the pressure-driven flow caused by the mismatch of the EOFs in the microfluidic channels connected by an NCAM must be considered when using NCAMs with pore diameters below 50 nm.     

Electrostatic interactions in molecular dynamics simulation of a three-dimensional system with periodicity in one direction

The Mellin transform and Poisson summation formula are used to derive an expression for the Coulomb interaction energy of a three-dimensional system with periodicity in one direction. Initially, calculations are performed for interactions characterized by any inverse power and, using the analytical continuation of the energy function, one obtains the final expression for the interaction energy of charges. We consider also a special case when two different charges are located on a line parallel to the periodicity direction. The energy and force expressions are identical to those obtained from the Lekner summation which is simply a sum over reciprocal lattice terms. The convergence behaviour of the Lekner summation is compared with that based on the Ewald type approach.     

Summation methods of Coulomb interactions in computer simulations of a system with one—dimensional periodic boundary conditions

The Ewald-type method, its modified version and the Lekner-type method for summing Coulomb interactions in a system periodic along one direction are presented and compared. Advantages and disadvantages of these methods are discussed, and the methods are tested in molecular dynamics simulations of acetone molecules confined to cylindrical silica pores.     

Immobilization of DNAzyme catalytic beacons on PMMA for Pb2+ detection

Due to the numerous toxicological effects of lead, its presence in the environment needs to be effectively monitored. Incorporating a biosensing element within a microfluidic platform enables rapid and reliable determinations of lead at trace levels. A microchip-based lead sensor is described here that employs a lead-specific DNAzyme (also called catalytic DNA or deoxyribozyme) as a recognition element that cleaves its complementary substrate DNA strand only in the presence of cationic lead (Pb(2+)). Fluorescent tags on the DNAzyme translate the cleavage events to measurable, optical signals proportional to Pb(2+) concentration. The DNAzyme responds sensitively and selectively to Pb(2+), and immobilizing DNAzyme in the sensor permits both sensor regeneration and localization of the detection zone. Here, the DNAzyme has been immobilized on a PMMA surface using the highly specific biotin-streptavidin interaction. The strategy includes using streptavidin physisorbed on a PMMA surface to immobilize DNAzyme both on planar PMMA and on the walls of a PMMA microfluidic device. The immobilized DNAzyme retains its Pb(2+) detection activity in the microfluidic device and can be regenerated and reused. The DNAzyme shows no response to other common metal cations and the presence of these contaminants does not interfere with the lead-induced fluorescence signal. While prior work has shown lead-specific catalytic DNA can be used in its solubilized form and while attached to gold substrates to quantitate Pb(2+) in solution, this is the first use of the DNAzyme immobilized within a microfluidic platform for real time Pb(2+) detection.     

Coulomb interactions in a computer simulation of a system periodic in two directions

The integral representation of the gamma function and the Poisson summation formula are used to calculate the interaction energy of charged particles in a 3-dimensional system periodic in two directions. A parallelogram shape simulation box is considered. Calculations are carried out for interactions described by any inverse power, and analytical continuation of the energy function leads to the final expression for the Coulomb interaction energy. Summation over the simulation box replica along one or the other side of the box base is replaced by summation in reciprocal space. Therefore there are two equivalent formulas for the potential energy that offer the possibility of avoiding slowly convergent series. The energy expressions are identical to those obtained from the Lekner method. The special case is considered where the functions defining the energy are infinite, i.e. when two charges lie on a line parallel to the simulation box side that was chosen to convert real space summation into reciprocal space.