Sudheer S. Sridharamurthy

Programmable autonomous micromixers and micropumps

Programmable autonomous micromixers and micropumps have been designed and realized via a merger between MEMS and microfluidic tectonics (/spl mu/FT). Advantages leveraged from both fabrication platforms allow for relatively simple and rapid fabrication of these microfluidic components. Nickel (Ni) microstructures, driven by an external rotating magnetic field, are patterned in situ and serve as the microactuators in the devices. /spl mu/FT permits in situ patterning through the use of a step-and-repeat fabrication process known as liquid-phase photopolymerization (LP/sup 3/). LP/sup 3/ is a polymer-based fabrication process tool that offers additional fabrication materials, including responsive hydrogels that expand and contract under different stimuli. Using pH- and temperature-sensitive hydrogels as clutches, autonomous micromixers and micropumps have been fabricated and tested that perform as closed-loop microsystems. The step-and-repeat fabrication process allows pre-programming of the system, like a programmable read-only memory chip, to be sensitive to a desired stimulus. Different Ni blade designs, and pH-sensitive hydrogel geometries and dimensions have been designed and tested to better ascertain their effects on micromixing efficiency and response times of hydrogels (related to the autonomous functionality), respectively. Temperature-responsive hydrogels have allowed for development of temperature-sensitive micromixers and micropumps with applications in areas demanding temperature control. [1498].

A microfluidic chemical/biological sensing system based on membrane dissolution and optical absorption

A microfluidic system to sense chemical and biological analytes using membranes dissolvable by the analyte is demonstrated. The scheme to detect the dissolution of the membrane is based on the difference in optical absorption of the membrane and the fluidic sample being assayed. The presence of the analyte in the sample chemically cleaves the membrane and causes the sample to flow into the membrane area. This causes a change in the optical absorption of the path between the light source and detector. A device comprising the microfluidic channels and the membrane is microfabricated using liquid-phase photopolymerization. A light emitting diode (LED) and a detector with an integrated amplifier are positioned and aligned on either side of the device. The state of the membrane is continuously monitored after introducing the sample. The temporal dissolution characteristics of the membrane are extracted in terms of the output voltage of the detector as a function of time. This is used to determine the concentration of the analyte. The absorption spectra of the membrane and fluidic sample are studied to determine the optimal wavelength that provides the maximum difference in absorbance between the membrane and the sample. In this work, the dissolution of a poly(acrylamide) hydrogel membrane in the presence of a reducing agent (dithiothreitol—DTT) is used as a model system. For this system, with 1 M DTT, complete membrane dissolution occurred after 65 min.

A Microfluidic Device to Acquire Gaseous Samples Via Surface Tension Held Gas-Liquid Interface

We present a relatively simple and effective method for acquiring gaseous samples into microfluidic channels. Hydrophobic polymers are photopatterned on hydrophilic substrates. Due to surface tension, aqueous liquid is confined by the hydrophobic polymers, but not completely blocked by a physical wall, thus allowing an interface for gas-liquid interaction. Here, the mechanism is demonstrated by using hydrophobic (poly)iso-bornyl acrylate polymer patterns on hydrophilic glass substrates, and through a Nesslers reagent-ammonia reaction that exhibits changes in color and electrical resistance.

Dissolvable membranes as sensing elements for microfluidics based biological/chemical sensors

We demonstrate a chemical and biological sensing mechanism in microfluidics that transduces chemical and biological signals to electrical signals with large intrinsic amplification without need for complex electronics. The sensing mechanism involves a dissolvable membrane separating a liquid sample chamber from an interdigitated electrode. Dissolution of the membrane (here, a disulfide cross-linked poly(acrylamide) hydrogel) in the presence of a specific target (here, a reducing agent-dithiothreitol) allows the target solution to flow into contact with the electrode. The liquid movement displaces the air dielectric with a liquid, leading to a change (open circuit to approximately 1 kOmega) in the resistance between the electrodes. Thus, a biochemical event is transduced into an electrical signal via fluid movement. The concentration of the target is estimated by monitoring the difference in dissolution times of two juxtaposed sensing membranes having different dissolution characteristics. No dc power is consumed by the sensor until detection of the target. A range of targets could be sensed by defining membranes specific to the target. This sensing mechanism might find applications in sensing targets such as toxins, which exhibit enzymatic activity.

An on-chip autonomous microfluidic cooling system

An on-chip self-contained autonomous microfluidic cooling system, driven by a constant external rotating magnetic stirrer, has been developed using liquid-phase photopolymerization and nickel electroplating. A temperature-sensitive hydrogel, that acts in a way similar to an automotive clutch, provides the autonomous functionality. By controlling the rotation of the nickel impeller, the hydrogel effectively controls the pumping of cold water to cool the system when temperatures are high. Once cooled, the system autonomously stops pumping. The autonomous functionality and cooling effect of the system were observed at various heater temperature setpoints. Cooling temperatures ranging between 1.6/spl deg/C and 4.0/spl deg/C were exhibited by the system.

A Liquid Crystal Based Gas Sensor Using Microfabricated Pillar Arrays as a Support Structure

We present principles for the design of a vapor-phase chemical sensor based on orientational transitions of nematic liquid crystals (LC) supported by a microfabricated pillar array structure. These principles are demonstrated by using the structure to detect vapor-phase Di-methyl methyl phosphonate (DMMP) via change in the intensity of light transmitted through the LC (5CB: 4-pentyl-4-cyanobiphenyl). The micro-pillar support structure stabilizes a thin (-22 mum) LC film by capillary force. The structure offers easier filling of LC, reduced susceptibility to gravity and shock, and is capable of embedding different thicknesses of LC films. Concentrations down to 10 ppm of DMMP were detected.

A fluidic chemical and biological sensing mechanism with high transduction based on dissolvable membranes

We demonstrate an elegant chemical and biological sensing mechanism that transduces chemical and biological signals to electrical signals with large intrinsic amplification not requiring complex on-chip microelectronics. The sensing mechanism employs dissolvable membranes separating a fluid chamber from an interdigitated capacitor initially in air. Dissolution of the membrane (here, a hydrogel membrane) in the presence of the target species (here, dithiothreitol) allows the target species to flow into the capacitor bringing about a change in its impedance. Using this mechanism, a simple circuit can generate 2.9 V DC output with 2.94 V DC supply. No DC power is consumed until the detection of the target species. A range of species can be sensed by defining membranes specific to the target species. The fabrication process is compatible with conventional IC fabrication technologies and is applicable to wireless microsensor networks.

On-chip integration of a microfluidic valve and pump for sample acquisition and movement

We report on the on-chip integration of a valve and pump for acquiring microfluidic samples and moving them through micro-channels. The valve employs temperature-sensitive hydrogels which are controlled by micro-heaters. The pump is a nickel rotor actuated magnetically by an external rotating magnet. The valve is fabricated as a series of hydrogel rings spaced within microfluidic channels. The expanded state of the hydrogel cylinders at low temperatures blocks liquid flow. Upon application of heat, the hydrogels contract in volume allowing liquid to flow through them. The pump brings about a recirculating movement of the liquid within the microchannel due to the rotation of the nickel rotor. The device is fabricated by combining liquid phase photopolymerization of structural polymers and temperature responsive hydrogels, with nickel electroplating. The valve has a response time of ~45 s and the pump generates a flow rate of ~1 μL/min.

A wireless chemical and biological microsensor based on dissolvable membranes

We demonstrate a wireless bio/chemical microsensor based on dissolvable membranes. The dissolution of a target specific membrane enables fluid flow into an on-chip microcapacitor, thereby drastically changing its impedance. Here, this fact is used to generate large intrinsic amplification and thus, a large output voltage, which allows the microsensor to directly drive a microcontroller operating at 3.6964 MHz, without complex electronics. The electronic modules, operating at 5 V, have been integrated onto a printed circuit board of dimension 10.2 cm times 10.2 cm, consuming 135 mW of power. An automated procedure to estimate the concentration of the sensed species (here, dithiothreitol) and interface circuitry for wireless transmission at 916.48 MHz with a range of 150 ft in closed premises are realized