Ashutosh Shastry

Controlling Liquid Drops with Texture Ratchets

Controlled vibration selectively propels multiple microliter-sized drops along microstructured tracks, leading to simple microfluidic systems that rectify oscillations of the three-phase contact line into asymmetric pinning forces that propel each drop in the direction of higher pinning.

Measurement of the Cell-Substrate Separation and the Projected Area of an Individual Adherent Cell Using Electric Cell-Substrate Impedance Sensing

This work presents an electrical technique called electric cell-substrate impedance sensing to measure the cell-substrate separation and the projected area of an individual adherent cell. Cell adhesion and cell spreading are fundamental processes of adherent cells. By recording changes in the cell-substrate separation, the projected area, or both properties with time, the dynamics of cell spreading and cell adhesion can be studied. The advantage of this electrical technique is that it enables a measurement of many individual cells simultaneously. This is a great benefit to the study of heterogeneity in cell populations. The research consisted of building a custom impedance sensing setup, designing an in vitro assay to record an impedance spectrum of an individual living cell, and developing a data analysis method to obtain two properties of the cell from curve-fitting of the impedance spectrum. The values of the cell-substrate separation and the projected area of an individual cell were within the expected ranges and in agreement with those obtained from optical microscopy.

Engineering surface roughness to manipulate droplets in microfluidic systems

Systematic variation of surface roughness has been employed to create microstructured guide rails for droplets propelled by vibration. The wetting mechanism of rough hydrophobic surfaces has been utilized to design surface energy gradients that form the guide-rails. Micro fabricated pillars in silicon have been employed to control the roughness which determines the contact angle and thus the surface energy. Droplets were moved down the surface energy gradient overcoming hysteresis by supplying energy through mechanical vibration. This work introduces roughness as a new control variable in any scheme of manipulating droplets, and presents fabricated structures and experimental results that validate the approach.

Selective attachment of multiple cell types on thermally responsive polymer

Programmable surface chemistry has been achieved by depositing a temperature sensitive polymer onto arrays of micro-fabricated metallic heaters. Activating a single heater causes a localized change in the device surface chemistry from non-fouling to fouling in an aqueous environment. Two types of proteins and two types of cells were used to demonstrate localized immobilization of proteins and cells on such surface. These experiments show, for the first time, selective cell attachments on thermally responsive polymer controlled by a micro heater array. It suggests a new approach to realize proteomic chips and cell chips.

Protein Patterning with Programmable Surface Chemistry Chips

Programmable surface chemistry has been achieved by depositing thermally responsive polymer (plasma polymerized N-isopropylacrylamide, ppNIPAM) onto arrays ofmicro-fabricated metallic heaters. Activating a single heater causes a localized change in the device surface chemistry from non-fouling to fouling in aqueous environment. Various proteins were used to demonstrate localized immobilization of proteins on the surface ofcoated micro-heater arrays. Additional uses ofthis technique include applications such as cell patterning, tissue engineering, self-assembly, etc.

Contact Angle Hysteresis Characterization of Textured Super-Hydrophobic Surfaces

This paper presents the fabrication of rough super-hydrophobic surfaces, dynamic high-speed measurements of sliding angles of water droplets, and develops a mechanistic understanding of contact angle hysteresis - the major dissipative mechanism in droplet based microfluidic systems. We investigate texture-dependence of hysteresis, evaluate the current model, propose a modification, and observe that the two models - current and proposed - are useful bounds on hysteresis of the surface except in ultra- hydrophobic regime where observed hysteresis is significantly higher than predictions of either model.

Using Electric Cell-Substrate Impedance Sensing to Measure Changes in the Projected Area of Individual Cells

We used an electrical cell-substrate impedance sensing (ECIS) technique to measure changes in the projected area of individual cells. The projected area of the cell plays a significant role in cellular behavior, including cell spreading, cell growth, cell death and cell adhesion. We built a custom impedance sensing setup; we recorded real time impedance spectra of individual adherent cells; and we used two parameters extracted from each impedance spectrum to measure the changes in the projected area of the cell. The projected area of the cell determines how much electrical current was measured using ECIS; therefore, it directly governed the values of two extracted impedance parameters.

Lower limits of detection for single biological particles using impedance spectroscopy

Single-cell impedance spectroscopy integrated with lab-on-a-chip systems provides a direct and minimally invasive approach for monitoring and characterizing properties of individual cells in real-time. Here we investigate the theoretical potential and limitations of this technique for analyzing single membrane-bound particles as small as 100 nm in diameter. Our theoretical model suggests a lower limit of detection for single cells on the order of a few microns.Copyright