S.B. Prakash

University of Maryland, College Park, College Park, MD, United States

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Publications (18)11.9 Total impact

  • S.B. Prakash, P. Abshire
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    ABSTRACT: This paper presents a fully differential capacitance sensor employing the CBCM technique to map differential input capacitances into rail-to-rail differential output voltages. The circuit has been designed for measuring capacitances in the plusmn25-fF range, appropriate for sensing live cells using on-chip microelectrodes. An array architecture based on a shielded current routing bus has been developed for incorporating the capacitance measurement circuit into sensor arrays, with each pixel comprising four minimum-size digital transistors, enabling high-density integration. In addition to improving spatial resolution, the shielded current bus also eliminates the need for individual pixel calibration, conserves sensor evaluation speed, and provides protection from junction leakage. The sensor employs a 3-phase clocking scheme that enables on-chip gain tuning. The paper also presents a modified version of the sensor circuit incorporating floating-gate transistors for mismatch compensation and output offset cancellation, performed using a combination of impact-ionized channel hot electron injection and Fowler-Nordheim tunneling mechanisms. Chips comprising both versions of the sensor circuits in test arrays employing the shielded current routing bus were fabricated in a commercially available 2-poly, 3-metal, 0.5-mum CMOS process. The sensor operation was demonstrated by measuring on-chip test capacitances comprising single and interdigitated metal electrodes, configured using different capacitance compensation schemes. The differential sensor in combination with the shielded current bus exhibits a maximum sensitivity of 200 mV/fF, a resolution of 15 aF, and an output dynamic range of 65 dB.
    Circuits and Systems I: Regular Papers, IEEE Transactions on 06/2009; · 2.24 Impact Factor
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    ABSTRACT: We describe the design, fabrication, and performance of a class of simple handheld fluorometers. The devices consist of a sensor along with an integrated optical filter packaged in a handheld format. The sensor is a differential active pixel sensor with in-pixel correlated double sampling fabricated in a 0.5-mu m 2-poly 3-metal complementary metal-oxide semiconductor process and has a readout noise of 175.3 muV, reset noise of 360 muV, dynamic range of 59 dB, and conversion gain of 530 nV/e<sup>-</sup> . The filter is a high rejection chromophore embedded in a polymer film which is cast onto the chip. We show the results of bioassays utilizing two different single color fluorometers constructed by using the chromophores 2-(2'-hydroxy 5'-methylphenyl) benzotriazole and Sudan II with long-pass wavelengths of 400 nm and 540 nm, respectively. The bioassays measures metabolic activity and viability of biological cells, which are useful for cytotoxicity and pathogen detection applications.
    IEEE Transactions on Biomedical Circuits and Systems 05/2009; · 2.74 Impact Factor
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    ABSTRACT: We demonstrate the application of a handheld fluorometer optimized for UV excitable assays. We demonstrate the measurement of metabolic products as yeast cells germinate in dextrose solution. In particular we measure NADH which is produced during cellular respiration. The handheld fluorometer consists of a CMOS active pixel sensor with in-pixel CDS, coupled with a custom chromophore-polymer emission Alter and a UV LED as the excitation source. The handheld fluorometer is able to detect as little as 10 muM of NADH, and in its present format should be applicable to any fluorescence assay with UV excitation and visible emission wavelengths.
    Circuits and Systems, 2008. ISCAS 2008. IEEE International Symposium on; 06/2008
  • Somashekar Bangalore Prakash, Pamela Abshire
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    ABSTRACT: We report a novel technique for assessing cell proliferation that employs integrated capacitance sensors for monitoring the growth of anchorage-dependent living cells. The sensors measure substrate coupling capacitances of cells cultured on-chip in a standard in vitro environment. The biophysical phenomenon underlying the capacitive behavior of cells is the counterionic polarization around the insulating cell bodies when exposed to weak, low frequency electric fields. The sensors employ charge sharing for mapping sensed capacitance values in the fF range to output voltage signals. The sensor chip has been fabricated in a commercially available 0.5microm, 2-poly 3-metal CMOS technology. We report experimental results demonstrating sensor response to the adhesion of MDA-MB-231 breast cancer cells followed by their proliferation on the chip surface. On-chip capacitance sensing offers a non-invasive, label-free, easy-to-use, miniaturized technique with real-time monitoring capability for tracking cell proliferation in vitro.
    Biosensors and Bioelectronics 06/2008; 23(10):1449-57. · 5.44 Impact Factor
  • Somashekar Bangalore Prakash, Pamela Abshire
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    ABSTRACT: The paper presents a fully differential capacitance sensor employing the CBCM technique to map differential input capacitances to rail-to-rail differential output voltages. The circuit has been designed for measuring capacitances in the plusmn20 fF range, appropriate for sensing live cells using on-chip microelectrodes. The paper also proposes an array architecture based on a shielded current routing bus that allows for a single measurement circuit to be shared by all the sensor pixels without compromising performance. This eliminates the need for individual pixel calibration. Each sensor pixel comprises 6 minimum size digital transistors, enabling high density integration. The sensor employs a 3-phase clocking scheme that enables gain tuning and also limits output voltage offsets. The paper presents data obtained from 5 chips fabricated in a commercially available 2- poly, 3-metal, 0.5 mum CMOS technology, each of them comprising individual circuits measuring the substrate-coupling capacitances of metal3 electrodes of varying sizes. The test data indicates successful sensor operation with a maximum sensitivity of 126 mV/fF, a maximum achievable resolution of 14 aF and an output dynamic range of 69.4 dB.
    International Symposium on Circuits and Systems (ISCAS 2008), 18-21 May 2008, Sheraton Seattle Hotel, Seattle, Washington, USA; 01/2008
  • International Symposium on Circuits and Systems (ISCAS 2008), 18-21 May 2008, Sheraton Seattle Hotel, Seattle, Washington, USA; 01/2008
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    ABSTRACT: The paper reports, for the first time, an integrated potentiostat for driving electrochemical actuation reactions on a CMOS chip. The integrated microsystem comprised on-chip working, counter, and quasi-reference electrodes fabricated on top of the necessary control circuitry. To demonstrate its capability, the potentiostat chip was used to electrodeposit films of polypyrrole/dodecylbenzenesulfonate, PPy(DBS), in situ onto the working electrodes. It was also used to cycle PPy(DBS) films between the fully oxidized and fully reduced states. This was confirmed by video recordings of the distinctive electrochromic changes, as well as by the shapes of the cyclic voltammograms. Both showed good repeatability. The demonstrated approach of integrating driver circuitry along with actuation electrodes on a common CMOS substrate eliminates the need for an external potentiostat instrument, and can be widely employed for the fabrication and control of electrochemical microactuators in integrated lab-on-a-chip systems.
    Sensors and Actuators B: Chemical. 01/2008;
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    ABSTRACT: We report the development of a handheld fluorometer for UV excitable fluorescence assays. The handheld detector serves as a demonstration platform for an integrated fluorescence sensor and comprises a CMOS detector coated with a polymer based optical filter and placed in close proximity to a UV LED which is used as an excitation source. The sensor function has been validated for metabolic activity and cytotoxicity assays. The fluorometer was able to determine NADH concentration as low as 17 muM and was able to track NADH production in live yeast cells over time and as the yeast cell concentration varied. The sensor was also used to discriminate the viability of human intestinal adenocarcinoma cells (Caco-2 cell line) using a live/dead stain after exposure to toxic and benign nanoparticles. The integrated fluorescence sensor is suitable for microscale fluorescence detection in lab-on-a-chip applications.
    Biomedical Circuits and Systems Conference, 2007. BIOCAS 2007. IEEE; 12/2007
  • S.B. Prakash, P. Abshire
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    ABSTRACT: The paper describes a fully differential CMOS circuit for integrated capacitance sensing of living cells. The proposed circuit is based on the charge based capacitance measurement (CBCM) technique which maps differential input capacitances linearly to rail-to-rail differential output voltages. The paper also proposes a shielded current routing bus architecture which will enable the measurement circuit to be incorporated into sensor arrays. The circuit has been designed in a commercially available 1-poly, 8-metal, 130-nm CMOS technology and has been simulated for different input capacitance ranges on the fF scale, appropriate for sensing cell layers or individual cells cultured on-chip. The simulated static response curves and computed calibration curves have been used to evaluate sensitivity and linearity performance metrics of the sensor circuit. The fully differential capacitance measurement approach increases sensor dynamic range and improves output noise resolution thereby providing better discrimination of cell-related phenomena.
    Sensors, 2007 IEEE; 12/2007
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    ABSTRACT: Cell-based sensors are being developed to harness the specificity and sensitivity of biological systems for sensing applications, from odor detection to pathogen classification. These integrated systems consist of CMOS chips containing sensors and circuitry onto which microstructures have been fabricated to transport, contain, and nurture the cells. The structures for confining the cells are micro-vials that can be opened and closed using polypyrrole bilayer actuators. The system integration issues and advances involved in the fabrication and operation of the actuators are described.
    Proc SPIE 04/2007;
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    Somashekar Bangalore Prakash, Pamela Abshire
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    ABSTRACT: We describe an integrated circuit for sensing the substrate coupling capacitance of anchorage-dependent living cells in a standard culture environment. Capacitance is measured using charge redistribution in response to weak, low frequency electric field excitations. The underlying biophysical phenomenon results primarily from the insulating nature of the cell structure and the counterionic polarization in the surrounding aqueous medium. The measured capacitance depends on a variety of factors related to the cell, its growth environment and the supporting substrate. These include membrane integrity, morphology, extracellular ionic concentration, adhesion strength, and substrate proximity. The measured capacitance is an indication of both the interaction between cells and substrate and cell health. The capacitance sensor uses the principle of charge sharing and translates sensed capacitance values to output voltages. The sensor chip has been fabricated in a commercially available 0.5-mum, 2-poly 3-metal CMOS technology. The sensing technique does not require direct electrical connection to the aqueous culture medium. We report results from experiments demonstrating on-chip tracking of cell adhesion as well as long term monitoring of cell viability in vitro
    IEEE Sensors Journal 04/2007; · 1.48 Impact Factor
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    ABSTRACT: We present a 1.5V low power CMOS temperature sensor for detection of on-chip temperature gradients. The temperature sensor has measured accuracy of+/- 0.25degC over a temperature range from 25degC to 97degC. The power consumption of a single temperature sensor is 96nW and the layout occupies 64.8mum<sup>2</sup> in a 0.5mum CMOS technology. A chip with spatially non-uniform heat generation devices and an array of temperature sensors was fabricated and tested. Experimental results demonstrate that the sensors successfully detect the induced temperature gradient on the chip
    Circuits and Systems, 2006. ISCAS 2006. Proceedings. 2006 IEEE International Symposium on; 06/2006
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    ABSTRACT: The cell clinics microsystem is a platform for long-term cell monitoring for such applications as cell-based sensing. This system includes microvials with individually actuated lids, integrated circuits that monitor cells and control the position of the vial hinges, and an automated cell loading mechanism that relies on dielectrophoresis to manipulate individual cells into the vials
    01/2006;
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    ABSTRACT: Cell clinics, CMOS/MEMS hybrid microsystems for capturing and in-situ investigation of living cells, aims at providing high-speed, automated, and economical cell monitoring. Integrated sensors are being developed for extracellular signal amplification, cell-substrate capacitance sensing, contact imaging, and fluorescence detection. We describe the methodology for characterizing the responses of these sensors to biological cells. We also present results obtained from the long-term monitoring of cells cultured on-chip using two of the sensors: (i) a bio-amplifier, used for amplifying weak extracellular potentials from electrically active cells, and (ii) a cell-substrate capacitance sensor, used for tracking cell adhesion and assessing cell viability
    Life Science Systems and Applications Workshop. 01/2006;
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    ABSTRACT: We describe a potentiostat designed for in situ electrochemical control of MEMS actuators. This module is tailored for integration into a hybrid CMOS-MEMS system-on-a-chip to confine cells and measure signals from them. The design has been fabricated in a commercially available 0.5 mum CMOS process. The fabricated chip has been employed for the control of off-chip electroactive polymer films and micro-actuators
    International Symposium on Circuits and Systems (ISCAS 2006), 21-24 May 2006, Island of Kos, Greece; 01/2006
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    S.B. Prakash, P. Abshire
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    ABSTRACT: We describe a CMOS capacitance sensor for measuring the capacitive behavior of living cells in a culture environment, in the presence of weak electric fields. The underlying physical phenomenon results primarily from polarization of the ionic cloud surrounding the cell in aqueous medium. The measured capacitance depends on a variety of factors including cell morphology, membrane integrity, medium pH and extra cellular ionic concentration and serves as an indicator of cell health. The capacitance sensor uses the principle of charge sharing and maps sensed capacitance values to voltages. The sensor chip has been fabricated in a commercially available 0.5 mum, 2-poly 3-metal CMOS technology. The sensors have been successfully used for long term monitoring of cell viability in vitro
    Sensors, 2005 IEEE; 12/2005
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    ABSTRACT: We present the use of electroactive polymer actuators as components of a biolab-on-a-chip, which has potential applications in cell-based sensing. This technology takes full advantage of the properties of polypyrrole actuators as well as the wide range of CMOS sensors that can be created. System integration becomes an important issue when developing real applications of EAP technologies. The requirements of the application and the constraints imposed by the various components must be considered in the context of the whole system, along with any opportunities that present themselves. In this paper, we discuss some of these challenges, including actuator design, the use of complementary actuation techniques, miniaturization, and packaging.
    Proc SPIE 05/2005;
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    ABSTRACT: We describe a CMOS capacitance sensor for measuring the capacitive coupling between living cells and the underlying substrate, a quantity that can be used to characterize cell adhesion strength and cell health. The capacitance sensor operates on the charge sharing principle, mapping sensed capacitance values to voltages. The sensor has been fabricated in a commercially available 0.5 μm, 2-poly 3-metal CMOS technology. Experimental results are presented for bench tests using a micropositioned electrode and in vitro tests with cells cultured directly on the chip surface. The sensor achieves an empirical distance resolution of 3 nm and capacitance resolution of 135 aF. The sensors have been successfully used for long term monitoring of cell viability in vitro.
    International Symposium on Circuits and Systems (ISCAS 2005), 23-26 May 2005, Kobe, Japan; 01/2005