Joseph A. Potkay

High-performance temperature-programmed microfabricated gas chromatography columns

This paper reports the first development of high-performance, silicon-glass micro-gas chromatography (/spl mu/GC) columns having integrated heaters and temperature sensors for temperature programming, and integrated pressure sensors for flow control. These 3-m long, 150-/spl mu/m wide and 250-/spl mu/m deep columns, integrated on a 3.3 cm square die, were fabricated using a silicon-on-glass dissolved wafer process. Demonstrating the contributions to heat dissipation from conduction, convection, and radiation with and without packaging, it is shown that using a 7.5-mm high atmospheric pressure package reduces power consumption to about 650 mW at 100/spl deg/C, while vacuum packaging reduces the steady-state power requirements to less than 100 mW. Under vacuum conditions, 600 mW is needed for a temperature-programming rate of 40/spl deg/C/min. The 2300 ppm//spl deg/C TCR of the temperature sensors and the 50 fF/kPa sensitivity of the pressure sensors satisfy the requirements needed to achieve reproducible separations in a /spl mu/GC system. Using these columns, highly resolved 20-component separations were obtained with analysis times that are a factor of two faster than isothermal responses.

A Low-Power Pressure- and Temperature-Programmable Micro Gas Chromatography Column

This paper presents the theory, fabrication, and experimental results for a high-performance low-power micro gas chromatography column. The suspended-dielectric 1-m-long column is split into two sections, permitting independent pressure and temperature programming. Integrated column heaters have a mean resistance of 16.8 kOmega and a temperature coefficient of resistance of 431 ppm/degC. The suspended column requires 11 mW to raise its temperature by 100degC in vacuum (1 mtorr). The column separates ten volatile organic compounds in 52 s and four chemical warfare agent simulants and an explosive simulant in 60 s.

An Integrated Micro-Analytical System for Complex Vapor Mixtures

A micro gas chromatograph (muGC) capable of quantitatively analyzing the components of complex vapor mixtures at trace concentrations is described. The muGC features a micro- preconcentrator/focuser (muPCF), dual-column pressure- and temperature-programmed separation module, and an integrated array of nanoparticle-coated chemiresistors. The latest design modifications and performance data are presented. Highlights include a 4-min separation of a 30-component mixture with a 3-m DRIE Si/glass microcolumn, a 14-sec separation of an 11-component mixture on a 25-cm microcolumn, a complete multi-vapor analysis from a hybrid microsystem that combines analytical, rf- wireless, and microcontroller modules, and a rapid analysis driven by a 4-stage peristaltic micropump.

The effects of PEG-based surface modification of PDMS microchannels on long-term hemocompatibility

The current study demonstrates the first surface modification for poly(dimethylsiloxane) (PDMS) microfluidic networks that displays a long shelf life as well as extended hemocompatibility. Uncoated PDMS microchannel networks rapidly adsorb high levels of fibrinogen in blood contacting applications. Fibrinogen adsorption initiates platelet activation, and causes a rapid increase in pressure across microchannel networks, rendering them useless for long term applications. Here, we describe the modification of sealed PDMS microchannels using an oxygen plasma pretreatment and poly(ethylene glycol) grafting approach. We present results regarding the testing of the coated microchannels after extended periods of aging and blood exposure. Our PEG-grafted channels showed significantly reduced fibrinogen adsorption and platelet adhesion up to 28 days after application, highlighting the stability and functionality of the coating over time. Our coated microchannel networks also displayed a significant reduction in the coagulation response under whole blood flow. Further, pressure across coated microchannel networks took over 16 times longer to double than the uncoated controls. Collectively, our data implies the potential for a coating platform for microfluidic devices in many blood-contacting applications.

A high-performance microfabricated gas chromatography column

This paper presents the theory, fabrication, and experimental results for a microfabricated gas chromatography column capable of performing multi-component separations using air as a carrier gas and vacuum at the outlet. These features are necessary to the realization of a wristwatch-size gas chromatograph (/spl mu/GC), but pose significant challenges in the design of a high-performance column. Columns 0.9 m and 3.0 m long were fabricated using a deep reactive ion etch (DRIE) and were sealed with a glass cap. The 3.0 m-long non-polar column occupies 3.2 cm/spl times/3.2 cm, has separated 16 gaseous components in 75 seconds, and achieves approximately 4900 plates (about 45% of theoretical predictions). The fabrication process has been extended to minimize the mass of the columns and facilitate low-power operation.

Characterization of an S-nitroso-N-acetylpenicillamine–based nitric oxide releasing polymer from a translational perspective

ABSTRACT Due to the role of nitric oxide (NO) in regulating a variety of biological functions in humans, numerous studies on different NO releasing/generating materials have been published over the past two decades. Although NO has been demonstrated to be a strong antimicrobial and potent antithrombotic agent, NO-releasing (NOrel) polymers have not reached the clinical setting. While increasing the concentration of the NO donor in the polymer is a common method to prolong the NO release, this should not be at the cost of mechanical strength or biocompatibility of the original material. In this work, it was shown that the incorporation of S-nitroso-penicillamine (SNAP), an NO donor molecule, into Elast-eon E2As (a copolymer of mixed soft segments of polydimethylsiloxane and poly(hexamethylene oxide)), does not adversely impact the physical and biological attributes of the base polymer. Incorporating 10 wt% of SNAP into E2As reduces the ultimate tensile strength by only 20%. The inclusion of SNAP did not significantly affect the surface chemistry or roughness of E2As polymer. Ultraviolet radiation, ethylene oxide, and hydrogen peroxide vapor sterilization techniques retained approximately 90% of the active SNAP content and did not affect the NO-release profile over an 18-day period. Furthermore, these NOrel materials were shown to be biocompatible with the host tissues as observed through hemocompatibility and cytotoxicity analysis. In addition, the stability of SNAP in E2As was studied under a variety of storage conditions, as they pertain to translational potential of these materials. SNAP-incorporated E2As stored at room temperature for over six months retained 87% of its initial SNAP content. Stored and fresh films exhibited similar NO release kinetics over an 18-day period. Combined, the results from this study suggest that SNAP-doped E2As polymer is suitable for commercial biomedical applications due to the reported physical and biological characteristics that are important for commercial and clinical success. GRAPHICAL ABSTRACT

Thermal behavior of high-performance temperature programmed microfabricated gas chromatography columns

This paper reports the thermal behavior of high-performance Si-glass /spl mu/GC separation columns having integrated heaters and temperature sensors. These 3m-long columns, integrated on a 3.25 cm/spl times/3.25 cm die, were fabricated using a DRIE process and sealed with a glass cap. Demonstrating the contributions to heat dissipation from conduction, convection, and radiation with and without packaging, it is shown that using a 7.5 mm-high atmospheric package reduces power consumption to less than 650 mW at 100/spl deg/C, while vacuum packaging reduces power requirements to less than 100 mW. The paper points the way toward low-power temperature-programmed devices and identifies the limits for ultra-low power applications.

An electrostatically latching thermopneumatic microvalve with closed-loop position sensing

This paper presents a microvalve that combines thermopneumatic drive with an electrostatic hold, harnessing the advantages of both actuation mechanisms in a structure believed to be the first of its kind. A heater grid, elevated 10/spl mu/m above the cavity floor, energizes a working fluid, raising the pressure in the cavity and deflecting the corrugated diaphragm and valve plate. A capacitive pressure sensor reads out the cavity pressure and valve seat position and provides feedback to determine when to apply the electrostatic hold voltage. Once latched, the power can be removed from the heater and, thus, the valve only requires power to transition from open to closed and consumes no static power. The thermopneumatic/electrostatic actuator combination provides high force, high speed, and low power in an 8/spl times/8mm die and is being developed for a low-power wireless gas chromatography system. The thermopneumatic actuator closes the valve in 1s at 200mW and the valve position is determined with a sensitivity of 4.3fF/Torr. The leak rate is less than 0.02sccm and the open flow is 3.3sccm at 130Torr. The electrostatic hold mechanism latches above 40V for thermopneumatic pressures greater than 600Torr.

A Hybrid Thermopneumatic and Electrostatic Microvalve with Integrated Position Sensing

This paper presents a low-power hybrid thermopneumatic microvalve with an electrostatic hold and integrated valve plate position sensing. This combination of actuators in a single structure enables a high throw and force actuator with low energy consumption, a combination that is difficult to otherwise achieve. The completed 7.5 mm × 10.3 mm × 1.5 mm valve has an open flow rate of 8 sccm at 600 Pa, a leak rate of 2.2 × 10 −3 sccm at 115 kPa, a open-to-closed fluidic conductance ratio of nearly one million, an actuation time of 430 ms at 250 mW, and a required power of 90 mW while closed. It additionally requires no power to open, and has a built-in capacitive position sensor with a sensitivity of 9.8 fF/kPa. The paper additionally presents analytical models of the valve components, design tradeoffs, and guidelines for achieving an optimized device.

Stability of Polyethylene Glycol and Zwitterionic Surface Modifications in PDMS Microfluidic Flow Chambers

Blood-material interactions are crucial to the lifetime, safety, and overall success of blood contacting devices. Hydrophilic polymer coatings have been employed to improve device lifetime by shielding blood contacting materials from the natural foreign body response, primarily the intrinsic pathway of the coagulation cascade. These coatings have the ability to repel proteins, cells, bacteria, and other micro-organisms. Coatings are desired to have long-term stability, so that the nonthrombogenic and nonfouling effects gained are long lasting. Unfortunately, there exist limited studies which investigate their stability under dynamic flow conditions as encountered in a physiological setting. In addition, direct comparisons between multiple coatings are lacking in the literature. In this study, we investigate the stability of polyethylene glycol (PEG), zwitterionic sulfobetaine silane (SBSi), and zwitterionic polyethylene glycol sulfobetaine silane (PEG-SBSi) grafted by a room temperature, sequential flow chemistry process on polydimethylsiloxane (PDMS) over time under ambient, static fluid (no flow), and physiologically relevant flow conditions and compare the results to uncoated PDMS controls. PEG, SBSi, and PEG-SBSi coatings maintained contact angles below 20° for up to 35 days under ambient conditions. SBSi and PEG-SBSi showed increased stability and hydrophilicity after 7 days under static conditions. They also retained contact angles ≤40° for all shear rates after 7 days under flow, demonstrating their potential for long-term stability. The effectiveness of the coatings to resist platelet adhesion was also studied under physiological flow conditions. PEG showed a 69% reduction in adhered platelets, PEG-SBSi a significant 80% reduction, and SBSi a significant 96% reduction compared to uncoated control samples, demonstrating their potential applicability for blood contacting applications. In addition, the presented coatings and their stability under shear may be of interest in other applications including marine coatings, lab on a chip devices, and contact lenses, where it is desirable to reduce surface fouling due to proteins, cells, and other organisms.