Praveenkumar Pasupathy

High-Throughput Universal DNA Curtain Arrays for Single-Molecule Fluorescence Imaging

Single-molecule studies of protein-DNA interactions have shed critical insights into the molecular mechanisms of nearly every aspect of DNA metabolism. The development of DNA curtains-a method for organizing arrays of DNA molecules on a fluid lipid bilayer-has greatly facilitated these studies by increasing the number of reactions that can be observed in a single experiment. However, the utility of DNA curtains is limited by the challenges associated with depositing nanometer-scale lipid diffusion barriers onto quartz microscope slides. Here, we describe a UV lithography-based method for large-scale fabrication of chromium (Cr) features and organization of DNA molecules at these features for high-throughput single-molecule studies. We demonstrate this approach by assembling 792 independent DNA arrays (containing >900,000 DNA molecules) within a single microfluidic flowcell. As a first proof of principle, we track the diffusion of Mlh1-Mlh3-a heterodimeric complex that participates in DNA mismatch repair and meiotic recombination. To further highlight the utility of this approach, we demonstrate a two-lane flowcell that facilitates concurrent experiments on different DNA substrates. Our technique greatly reduces the challenges associated with assembling DNA curtains and paves the way for the rapid acquisition of large statistical data sets from individual single-molecule experiments.

Low-cost passive sensors for monitoring corrosion in concrete structures

A passive sensor platform has been developed at the University of Texas at Austin to monitor corrosion of embedded reinforcement in concrete structures. The sensors are powered and interrogated in a wireless manner. Initial sensor designs used a sacrificial corroding steel wire to indicate the risk of corrosion within concrete. The wire was physically connected to the sensor circuitry and passed through the circuit protection layer. Consequently, it allowed contaminants to reach the circuit electric components causing corrosion and limiting the service life of the sensor. A novel sensor configuration that relies on wireless inductive coupling between a resonant circuit and the transducer element is presented. The non-contact design eliminates the breach concern and enhances the durability of the senor. Preliminary test results of the new design will be discussed in this paper.

Resonant Sensors for Detecting Corrosion in Concrete Bridges

A passive sensor platform was developed to detect the onset of corrosion in concrete bridge decks. The term “passive” is used to describe the sensors because they do not include onboard processing capabilities or sources of power. The sensors are powered and interrogated in a wireless manner by measuring the impedance through an external reader coil, which is magnetically coupled to resonant circuits within the sensor. The sensors are designed to be embedded in a concrete bridge during construction and interrogated periodically over its service life. Reinforced concrete slabs were subjected to long-term exposure tests to demonstrate the reliability of the prototype sensors. The middle region of each slab was exposed to alternating moisture cycles with saltwater. Sensors in this region indicated that threshold amounts of corrosion occurred, whereas sensors located in regions that were not exposed to saltwater indicated the absence of corrosion. At the conclusion of the tests, the sensor readings were confirmed by removing the concrete cover and examining the condition of the embedded reinforcement. One flaw in the initial design of the sensors, however, was that the transducing element (a corroding steel wire) passed through the epoxy potting that protected the circuit components. This configuration provided a pathway for contaminants to infiltrate the sensors, causing corrosion. To improve the durability of the passive sensors, a new configuration is being developed in which the corroding element is physically isolated from the resonant circuits within the sensor.

Magneto-inductive waveguide as a passive wireless sensor net for structural health monitoring

This paper summarizes ongoing work to develop low-cost, wireless, resonant sensor nets that can be used to monitor corrosion in infrastructure systems. A magnetically coupled sensor array is analyzed using a circuit model. The array acts as a magneto-inductive waveguide and the impedance discontinuities caused by corrosion (or other defects) lead to reflection. The relationship between the relative position of defects and pass band ripples is investigated, providing a technique to determine the location of targets. A configuration for increased sensitivity and a method for defect localization are presented.

Improved reading techniques for electronic structural surveillance tags

Results from our efforts to improve the performance of low-cost, unpowered, wireless, resistance based Electronic Structural Surveillance tags (ESS) will be presented. The ESS tags use an unpowered embedded sensor read by an external reader using an inductively coupled impedance measurement. Read range of coupled tags is largely dependent on the strength of inductive coupling which is influenced by the relative shape and size of the coils. Reader coil geometries can be optimized to increase read range. Additionally, an enhanced circuit model, for data extraction, is developed and tested with the corrosion sensor. The model provides increased information about the sensor and its surroundings. Better coil design and circuit model based data extraction methods can improve the reliability in reading the sensor. Recommendations for design and analysis resulting from this study can be extended to optimize other electronic structural surveillance tag sensors.

Passive Sensors for Detecting Corrosion in Concrete Structures

This paper describes a prototype passive sensor that can be powered and interrogated in a wireless manner to monitor the conditions inside structural concrete members. The term “passive” is used because the sensors do not include any on-board processing capabilities or sources of power (batteries). The sensors are designed to be embedded in the concrete during construction and interrogated sporadically over the life of the structure. The response of an embedded sensor is determined by measuring the impedance of an external reader coil that is magnetically coupled to the sensor. To date, the research has focused on detecting the initiation of corrosion within concrete structures. Accelerated corrosion tests were used to evaluate the reliability of the passive sensors. Sensors were embedded in reinforced concrete prisms and successfully detected the onset of corrosion in the reinforcement. Unlike the traditional measurements, such as half-cell potentials, the passive sensor readings did not fluctuate with changes in the temperature or moisture content of the concrete.

Detection of multiple corrosion thresholds in reinforced concrete structures using passive sensors

This paper describes the ongoing research efforts to develop a novel class of low-cost, unpowered, wireless sensors for detecting corrosion of reinforcement in concrete structures. The sensors are powered through magnetic coupling between an external reader coil and an embedded sensor. Measured AC impedance is used to interpret the state of the embedded sensor. The sensors are envisioned to be placed during construction and interrogated as part of routine inspections. The sensor prototype incorporates a sacrificial corroding element that is placed entirely outside the sensor components and interacts with the resonant circuit by inductive coupling and shielding of the magnetic fields. As the resistance of the sacrificial element increases due to corrosion, the measured frequency response changes gradually indicating corrosion initiation within concrete. In this paper the potential for detecting multiple levels of corrosion damage is demonstrated.

Enhanced resolution of passive wireless conductivity sensors

A passive, wireless sensor has been developed at the University of Texas at Austin to monitor the insitu conductivity of concrete within civil infrastructure systems. Electrical conductivity is one possible indicator of corrosion of embedded reinforcement and thereby provides information on structural performance. The sensors would be attached to the reinforcement cages before placement of the concrete and interrogated as part of a routine inspection over the service life. A new sensor design, a non-contact conductivity sensor, is being developed to minimize the likelihood of damage to the sensor during placement of the concrete; a metal element is positioned above the sensor body but is not connected to the resonant circuit within the sensor. In order to verify the response of the non-contact conductivity sensors, they were submerged in liquids of increasing conductivity. Analysis of the measured data demonstrated that the noncontact conductivity sensors successfully detected conductivity variations in liquids.

Microfabricated self-resonant structure as a passive wireless chemical sensor

This paper describes ongoing work to develop a low cost, passive wireless chemical sensor using a microfabricated inductor with interdigitated capacitors (IDC). A self-resonant-structure (SRS) is designed by incorporating IDC electrodes in the inter-winding space of the inductor. The distributed IDC capacitance is affected by dielectric constant and conductivity of its environment or material under test (MUT). This serves as a capacitive transducer changing the resonant frequency of the SRS. The SRS is interrogated using a non-contact inductively coupled reader coil. The shift in resonance frequency of the SRS is used to detect material properties of the environment/MUT. The dielectric constant (ε) and conductivity (σ) can be used to provide information about the surrounding environment. The ε and the σ are determined by fitting and extraction from circuit models of the IDC. Relationship between sensor layout and coupling factor between sensor and reader is investigated. Optimizations of the coupling factor based on this relationship are discussed. IDC design trade-offs between the sensors sensitivity and coupling factor are investigated. The sensors response to variety of liquid MUTs with a wide range of dielectric constant and conductivity is presented.

Embedded passive wireless sensors for detecting conductivity within RC structures

A passive, wireless and inexpensive sensor has been developed to monitor the conductivity of concrete and thereby provide information on the progress of chloride-induced corrosion of the embedded reinforcement in concrete structures. Sensors are designed to be attached to the reinforcement cages before placement of the concrete in new construction or in portions of rehabilitated structures. Sensors will then be interrogated intermittently over the service life during routine inspections. The results of two experimental investigations are discussed in this paper. In the first, conductivity sensors were submerged in liquids of increasing conductivity. In the second, conductivity sensors were embedded in concrete cylinders and interrogated over a 25-week period during initial set and curing of the concrete. Analysis of the measured data shows that the passive conductivity sensors were successful in detecting a variety of conductivity levels in the concrete.