Multiplexed actuation using ultra dielectrophoresis for proteomics applications: a comprehensive electrical and electrothermal design methodology.

In this work, we present a methodological approach to analyze an enhanced dielectrophoresis (DEP) system from both a circuit analysis and electrothermal view points. In our developed model, we have taken into account various phenomena and constraints such as voltage degradation (due to the presence of the protecting oxide layer), oxide breakdown, instrumentation limitations, and thermal effects. The results from this analysis are applicable generally to a wide variety of geometries and high voltage microsystems. Here, these design guidelines were applied to develop a robust electronic actuation system to perform a multiplexed bead-based protein assay. To carry out the multiplexed functionality, along a single microfluidic channel, an array of proteins is patterned, where each element is targeting a specific secondary protein coated on micron-sized beads in the subsequently introduced sample solution. Below each element of the array, we have a pair of addressable interdigitated electrodes. By selectively applying voltage at the terminals of each interdigitated electrode pair, the enhanced DEP, or equivalently 'ultra'-DEP (uDEP) force detaches protein-bound beads from each element of the array, one by one, without disturbing the bound beads in the neighboring regions. The detached beads can be quantified optically or electrically downstream. For proof of concept, we illustrated 16-plex actuation capability of our device to elute micron-sized beads that are bound to the surface through anti-IgG and IgG interaction which is on the same order of magnitude in strength as typical antibody-antigen interactions. In addition to its application in multiplexed protein analysis, our platform can be potentially utilized to statistically characterize the strength profile of biological bonds, since the multiplexed format allows for high throughput force spectroscopy using the array of uDEP devices, under the same buffer and assay preparation conditions.

Smart surface for elution of protein-protein bound particles: nanonewton dielectrophoretic forces using atomic layer deposited oxides.

By increasing the strength of the negative dielectrophoresis force, we demonstrated a significantly improved electrokinetic actuation and switching microsystem that can be used to elute specifically bound beads from the surface. In this work using atomic layer deposition we deposited a pinhole free nanometer-scale thin film oxide as a protective layer to prevent electrodes from corrosion, when applying high voltages (>20 V(pp)) at the electrodes. Then, by exciting the electrodes at high frequency, we capacitively coupled the electrodes to the buffer in order to avoid electric field degradation and, hence, reduction in dielectrophoresis force due to the presence of the insulating oxide layer. To illustrate the functionality of our system, we demonstrated 100% detachment of anti-IgG and IgG bound beads (which is on the same order of magnitude in strength as typical antibody-antigen interactions) from the surface, upon applying the improved negative dielectrophoresis force. The significantly enhanced switching performance presented in this work shows orders of magnitude of improvement in on-to-off ratio and switching response time, without any need for chemical eluting agents, as compared to the previous work. The promising results from this work vindicates that the functionality of this singleplexed platform can be extended to perform a multiplexed bead-based assay where in a single channel an array of proteins are patterned each targeting a different antigen or protein.

Microfluidic diagnostic tool for the developing world: contactless impedance flow cytometry.

In this work, we demonstrate a novel and cost-effective approach to implement a disposable microfluidic contactless impedance cytometer. Conventional methods for single cell impedance cytometry use microfabricated electrodes in direct contact with the buffer to measure changes of its electrical impedance when cells pass through the applied electric field. However, this approach requires expensive microfabrication of electrodes, and also, the fabricated electrodes cannot be reused without thorough and time-consuming cleaning process. Here, we introduce a novel approach to allow for single cell impedance cytometry using electrodes that can be reused, without the need for microfabrication of the electrodes. This disposable device can be potentially inserted onto a printed circuit board (PCB) which has a non-disposable, yet inexpensive, electronic reading apparatus. This significantly reduces the manufacturing costs, making it suitable for low resource settings, such as point-of-care testing in the developing countries.

Control of DNA capture by nanofluidic transistors.

We report the use of an array of electrically gated ~200 nm solid-state pores as nanofluidic transistors to manipulate the capture and passage of DNA. The devices are capable of reversibly altering the rate of DNA capture by over 3 orders of magnitude using sub-1 V biasing of a gate electrode. This efficient gating originates from the counter-balance of electrophoresis and electroosmosis, as revealed by quantitative numerical simulations. Such a reversible electronically tunable biomolecular switch may be used to manipulate nucleic acid delivery in a fluidic circuit, and its development is an important first step toward active control of DNA motion through solid-state nanopores for sensing applications.

Use of negative dielectrophoresis for selective elution of protein-bound particles.

In this paper with the aid of negative dielectrophoresis force in conjunction with shear force and at an optimal sodium hydroxide concentration we demonstrated a switchlike functionality to elute specifically bound beads from the surface. At an optimal flow rate and sodium hydroxide concentration, negative dielectrophoresis turned on results in bead detachment, whereas when negative dielectrophoresis is off, the beads remain attached. This platform offers the potential for performing a bead-based multiplexed assay where in a single channel various regions are immobilized with a different antibody, each targeting a different antigen. To develop the proof of concept and to demonstrate the switchlike functionality in eluting specifically bound beads from the surface we looked at two different protein interactions. We chose interactions that were in the same order of magnitude in strength as typical antibody-antigen interactions. The first was protein G-IgG interaction, and the second was the interaction between anti-IgG and IgG.

Prediction of protein orientation upon immobilization on biological and nonbiological surfaces.

We report on a rapid simulation method for predicting protein orientation on a surface based on electrostatic interactions. New methods for predicting protein immobilization are needed because of the increasing use of biosensors and protein microarrays, two technologies that use protein immobilization onto a solid support, and because the orientation of an immobilized protein is important for its function. The proposed simulation model is based on the premise that the protein interacts with the electric field generated by the surface, and this interaction defines the orientation of attachment. Results of this model are in agreement with experimental observations of immobilization of mitochondrial creatine kinase and type I hexokinase on biological membranes. The advantages of our method are that it can be applied to any protein with a known structure; it does not require modeling of the surface at atomic resolution and can be run relatively quickly on readily available computing resources. Finally, we also propose an orientation of membrane-bound cytochrome c, a protein for which the membrane orientation has not been unequivocally determined.

Method for sensitivity analysis of photonic crystal devices.

We present a new method for sensitivity analysis of photonic crystal devices. The algorithm is based on a finite-difference frequency-domain model and uses the adjoint variable method and perturbation theory techniques. We show that our method is highly efficient and accurate and can be applied to calculation of the sensitivity of transmission parameters of resonant nanophotonic devices.

Effects of scaling on the SNR and speed of biosensors.

This work presents a stochastic model for the observed signal of biosensors, a model that predicts the signal fluctuation of the system and the SNR associated with it using a Markov chain process. In the process, transition probabilities are derived from the target and probe binding kinetics in view of statistical motion and random walk events. Based on this model, we are able to estimate the settling time, power-spectral density (PSD), and signal to noise ratio (SNR) of general affinity-based biosensors. The effects of scaling from macroscopic to microscopic regimes are also studied, which indicate a fundamental tradeoff between settling time (speed) and signal fluctuation (noise). The model is also applied to analyze the behavior of a DNA hybridization electronic detector.