No abstract

Selective trapping of nanoscale bioparticles (size <100 nm) is significant for the separation and high-sensitivity detection of biomarkers. Dielectrophoresis is capable of highly selective trapping of bioparticles based on their characteristic frequency response. However, the trapping forces fall steeply with particle size, especially within physiological media of high-conductivity where the trapping can be dissipated by electrothermal (ET) flow due to localized Joule heating. Herein, we investigate the influence of device scaling within the electrodeless insulator dielectrophoresis geometry through the application of highly constricted channels of successively smaller channel depth, on the net balance of dielectrophoretic trapping force versus ET drag force on bioparticles. While higher degrees of constriction enable dielectrophoretic trapping of successively smaller bioparticles within a short time, the ETflow due to enhanced Joule heating within media of high conductivity can cause a significant dissipation of bioparticle trapping. This dissipative drag force can be reduced through lowering the depth of the highly constricted channels to submicron sizes, which substantially reduces the degree of Joule heating, thereby enhancing the range of voltages and media conductivities that can be applied toward rapid dielectrophoretic concentration enrichment of silica nanoparticles (∼50 nm) and streptavidin protein biomolecules (∼5 nm). We envision the application of these methodologies toward nanofabrication, optofluidics, biomarker discovery, and early disease diagnostics.

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We present a quantitative design methodology for optimizing insulator gap width, gap resistivity, and collector to needle height for the alignment of sub-100 nm electrospun nanofibers at insulator gaps of metal collectors. Enhancement of the spatial extent of alignment forces at insulator gaps, due to the concerted action of attractive stretching forces from the modified electric fields and repulsive forces from residual charges on undischarged fibers in the gap, is studied. At gap widths considerably smaller than the collector to needle height (<2%), the spatial extent of stretching forces is large as evidenced by successive reduction in nanofiber size with gap width; however, the low magnitude of repulsive forces limits the degree of nanofiber alignment. At successively larger gap widths less than the needle height, the spatial extent of the stretching forces is gradually restricted toward the metal-insulator edges, while the influence of repulsive forces is gradually extended across the rest of the spatial extent of the gap, to cause enhanced nanofiber alignment through the concerted action of these forces. At gap widths greater than the needle height, the limited spatial extent and lowered maximum value of the stretching forces at the metal-insulator edge reduces their influence on fiber stretching and alignment. The collection of sub-100 nm electrospun poly(lactic acid-co-glycolic acid) nanofibers with a good degree of alignment (≤10° deviation) is found to require intermediate size gaps (∼2% of needle height) of high resistivity (≥10(12) ohm-cm), to enhance the spatial extent of stretching forces while maintaining the dominance of repulsive forces due to residual charge across a majority of the spatial extent of the gap.