Scaling down constriction-based (electrodeless) dielectrophoresis devices for trapping nanoscale bioparticles in physiological media of high-conductivity.

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.

Nano-constriction device for rapid protein preconcentration in physiological media through a balance of electrokinetic forces.

We describe a methodology to steeply enhance streptavidin protein preconcentration within physiological media over that achieved by negative dielectrophoresis (NDEP) through utilizing a DC offset to the AC field at nanoscale constriction gap devices. Within devices containing approximately 50-nm constriction gaps, we find that the addition of a critical DC field offset (1.5 V/cm) to the NDEP condition (∼200 V(pp) /cm at 1 MHz) results in an exponentially enhanced extent of protein depletion across the device to cause a rapid and steeply rising degree of protein preconcentration. Under these conditions, an elliptical-shaped protein depletion zone that is extended along the device centerline axis forms instantaneously around the constrictions to result in protein preconcentration along the constriction sidewall direction. Through a potential energy diagram to describe the electrokinetic force balance across the device, we find that the potential energy barrier due to NDEP is gradually tilted upon addition of DC fields, to cause successively steeper potential wells along the sidewall direction for devices containing smaller constriction gaps. Hence, for approximately 50-nm constriction gaps at a critical DC field, the ensuing narrow and deep potential energy wells enable steep protein preconcentration, due to depletion over an exponentially enhanced extent across the device.

Nanofiber size-dependent sensitivity of fibroblast directionality to the methodology for scaffold alignment.

The sensitivity of fibroblast guidance on directional cues provided by aligned nanofibers is studied for scaffolds of successively smaller fiber sizes (740±280, 245±85, 140±40, and 80±10 nm) fabricated using mandrel and electrical alignment methodologies for electrospun nanofibers (∼10° angular deviation (AD)), as well as nanoimprint methodologies for perfectly aligned fibers (0° AD). On aligned scaffolds of large fibers (∼740 nm) cell directionality closely follows the underlying fibers, irrespective of the alignment method. However, on mandrel aligned scaffolds of successively smaller fibers the cell directionality exhibits greater deviations from the underlying fiber alignment due to the higher likelihood of interaction of cell lamellipodia with multiple, rather than single, nanofibers. Using electrically aligned scaffolds, fibroblast directionality deviations can be maintained in the range of nanofiber alignment deviation for fiber sizes down to ∼100 nm. This improvement in cell guidance is attributed to molecular scale directional adhesion cues for cell receptors, which occur within electrically aligned scaffolds due to fiber polarization parallel to the geometric alignment axis of the nanofiber under the modified electric field during electrospinning. While fibroblast directionality is similar on electrically aligned vs. nanoimprinted scaffolds for fiber sizes >100 nm, cell directionality is influenced more strongly by the perfect alignment cues of the latter on ∼100 nm fiber scaffolds. The scaffold alignment methodology is hence highly significant, especially for tissue engineering applications requiring sub-100 nm aligned fibers.

Floating-electrode enhanced constriction dielectrophoresis for biomolecular trapping in physiological media of high conductivity.

We present an electrokinetic framework for designing insulator constriction-based dielectrophoresis devices with enhanced ability to trap nanoscale biomolecules in physiological media of high conductivity, through coupling short-range dielectrophoresis forces with long-range electrothermal flow. While a 500-fold constriction enables field focusing sufficient to trap nanoscale biomolecules by dielectrophoresis, the extent of this high-field region is enhanced through coupling the constriction to an electrically floating sensor electrode at the constriction floor. However, the enhanced localized fields due to the constriction and enhanced current within saline media of high conductivity (1 S/m) cause a rise in temperature due to Joule heating, resulting in a hotspot region midway within the channel depth at the constriction center, with temperatures of ∼8°-10°K above the ambient. While the resulting vortices from electrothermal flow are directed away from the hotspot region to oppose dielectrophoretic trapping, they also cause a downward and inward flow towards the electrode edges at the constriction floor. This assists biomolecular trapping at the sensor electrode through enabling long-range fluid sampling as well as through localized stirring by fluid circulation in its vicinity.

Interplay of electrical forces for alignment of sub-100 nm electrospun nanofibers on insulator gap collectors.

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.

Enhancing DNA hybridization kinetics through constriction-based dielectrophoresis.

The enhancement of signal sensitivity through the scaling-down of sensors presents mass transport limitations that can arrest the sensitivity gains obtained as a result of miniaturization. To alleviate these limitations, we study the application of constriction-based dielectrophoresis methods to enhance transport through pre-concentration of target DNA in the vicinity of the diffusion layer of the sensor, on which capture probe DNA molecules were immobilized. We demonstrate that constriction-based DEP pre-concentration was not impeded by scaling-down of the sensor, as long as the sensor electrode was composed of nanostructured edges and was coupled to an equally scaled down insulating constriction within a microfluidic channel to enhance the focusing effects of the constrictions and edges. Furthermore, as a result of the high focusing fields, pre-concentration of single-stranded target DNA occurred in the vicinity of the sensor pad within the relatively high ionic strength buffers required for DNA hybridization, with minimal degradation of capture probe molecules. Finally, constriction-based DEP resulted in an almost immediate pre-concentration of target DNA in the vicinity of the sensor electrode diffusion layer, resulting in a ten-fold enhancement of the DNA hybridization kinetics at target concentration values down to the sensitivity limit of 10 pM for the sensor platform.