Sanhita Dixit

Nanodroplet real-time PCR system with laser assisted heating

We report the successful application of low-power (approximately 30 mW) laser radiation as an optical heating source for high-speed real-time polymerase chain reaction (PCR) amplification of DNA in nanoliter droplets dispersed in an oil phase. Light provides the heating, temperature measurement, and Taqman real-time readout in nanoliter droplets on a disposable plastic substrate. A selective heating scheme using an infrared laser appears ideal for driving PCR because it heats only the droplet, not the oil or plastic substrate, providing fast heating and completing the 40 cycles of PCR in 370 seconds. No microheaters or microfluidic circuitry were deposited on the substrate, and PCR was performed in one droplet without affecting neighboring droplets. The assay performance was quantitative and its amplification efficiency was comparable to that of a commercial instrument.

Light-Driven Formation and Rupture of Droplet Bilayers

We demonstrate the optical manipulation of nanoliter aqueous droplets containing surfactant or lipid molecules and immersed in an organic liquid using near-infrared light. The resulting emulsion droplets are manipulated using both the thermocapillary effect and convective fluid motion. Droplet-pair interactions induced in the emulsion upon optical initiation and control provide direct observations of the coalescence steps in intricate detail. Droplet-droplet adhesion (bilayer formation) is observed under several conditions. Selective bilayer rupture is also realized using the same infrared laser. The technique provides a novel approach to studying thin film drainage and interface stability in emulsion dynamics. The formation of stable lipid bilayers at the adhesion interface between interacting water droplets can provide an optical platform on which to build droplet-based lipid bilayer assays. The technique also has relevance to understanding and improving microfluidics applications by devising Petri dish-based droplet assays requiring no substrate fabrication.

Droplet Shape Analysis and Permeability Studies in Droplet Lipid Bilayers

We apply optical manipulation to prepare lipid bilayers between pairs of water droplets immersed in an oil matrix. These droplet pairs have a well-defined geometry allowing the use of droplet shape analysis to perform quantitative studies of the dynamics during bilayer formation and to determine time-dependent values for the droplet volumes, bilayer radius, bilayer contact angle, and droplet center-line approach velocity. During bilayer formation, the contact angle rises steadily to an equilibrium value determined by the bilayer adhesion energy. When there is a salt concentration imbalance between droplets, there is a measurable change in the droplet volume. We present an analytical expression for this volume change and use this expression to calculate the bilayer permeability to water.

Inspiratory contrast for in vivo optical imaging

We demonstrate the use of inspired oxygen and carbon dioxide as a possible route to increase contrast in optical imaging of cancerous tissue. Differential imaging in human xenograft rodent models of cancer exhibits significant variation in signal between normal and cancerous tissue. This differential cancer-specific contrast is stronger and more consistent than the conventional static contrast. This differential technique exploits the response of abnormal tumor vasculature to inhaled gases and could provide a promising alternative to supplement mainstream cancer imaging modalities such as x-rays and MRI.

Near infrared transillumination imaging of breast cancer with vasoactive inhalation contrast

Inhalation of vasoactive gases such as carbon dioxide and oxygen can provide strong changes in tissue hemodynamics. In this report, we present a preliminary clinical study aimed at assessing the feasibility of inhalation-based contrast with near infrared continuous wave transillumination for breast imaging. We describe a method for fitting the transient absorbance that provides the wavelength dependence of the optical pathlength as parametrized by tissue oxygenation and scatter power as well as the differential changes in oxy- and deoxy-hemoglobin. We also present a principal component analysis data reduction technique to assess the dynamic response from the tissue that uses coercion to provide single temporal eigenvalues associated with both oxy- and deoxy-hemoglobin changes.

Cancer detection using infrared transillumination

The viability of a new infrared imaging modality for the detection of breast cancer is tested via whole animal imaging studies. The potential of this technique for imaging human breast tissue is also demonstrated.

Infrared Laser-Based Aqueous Droplet Manipulation and Biomimetic Droplet Bilayers

Lipid bilayers are formed when pairs of lipid-coated aqueous droplets in a mineral oil matrix are brought into contact via laser-induced thermal motion of the matrix. We characterize these bilayers for biomimetic platforms.

Thermal Manipulation of Water Droplets - A Path to Droplet Microfluidics

Optical manipulation of water droplets in an oil matrix is important in the development of droplet microfluidics. In this report we demonstrate infrared laser based droplet manipulation using the thermal Marangoni effect and thermal convection

Optical Temperature Measurement in Aqueous Nanodroplets for PCR

We are investigating temperature measurements of nanoliter droplets to support polymerase chain reaction (PCR) assays. A fluorescence quenching method appears most promising for this application.

Rapid Nanodroplet-Based Real-Time PCR System with Laser Heating

We report the first use of laser heating for rapid PCR to our knowledge. Real-time PCR DNA analysis is performed using nanoliter droplets dispersed in oil as independent assay chambers.