Phillip Zellner

A fabrication technology for three-dimensional micro total analysis systems

This paper presents a new fabrication technique capable of creating three-dimensional (3D) buried microchannels in a silicon substrate. With a single mask and a single etch of the substrate, silicon microstructures are created with control in all three dimensions by utilizing reactive ion etch (RIE) lag. The microstructures are then sealed with plasma enhanced chemical vapor deposition (PECVD) dielectrics. By depositing up to 6.3 µm of PECVD oxide, rectangular openings in the masking layer ranging in size from 2 µm × 2 µm to 4 µm × 10 µm microchannels were sealed. Using these mask openings, microchannels were created with depths ranging from 4 µm to 200 µm. In addition channels with controlled transition between depths and transition slopes ranging from 40° and 60° were created. Furthermore, the flexibility of this technique allows for the creation of predictable nano-scaled holes on the substrate surface. The entire process is fabricated on silicon and CMOS compatible, thus allowing for 3D buried channel devices to be integrated with microelectronics. To show the impact of this technique, practical microfluidic devices with a wide range of applications are demonstrated.

Silicon insulator-based dielectrophoresis devices for minimized heating effects.

Concentration of biological specimens that are extremely dilute in a solution is of paramount importance for their detection. Microfluidic chips based on insulator‐based DEP (iDEP) have been used to selectively concentrate bacteria and viruses. iDEP biochips are currently fabricated with glass or polymer substrates to allow for high electric fields within the channels. Joule heating is a well‐known problem in these substrates and can lead to decreased throughput and even device failure. In this work, we present, for the first time, highly efficient trapping and separation of particles in DC iDEP devices that are fabricated on silicon using a single‐etch‐step three‐dimensional microfabrication process with greatly improved heat dissipation properties. Fabrication in silicon allows for greater heat dissipation for identical geometries and operating conditions. The 3D fabrication allows for higher performance at lower applied potentials. Thermal measurements were performed on both the presented silicon chips and previously published PDMS devices comprised of microposts. Trapping and separation of 1 and 2 μm polystyrene particles was demonstrated. These results demonstrate the feasibility of high‐performance silicon iDEP devices for the next generation of sorting and concentration microsystems.

3D Insulator-based dielectrophoresis using DC-biased, AC electric fields for selective bacterial trapping

Insulator‐based dielectrophoresis (iDEP) is a well‐known technique that harnesses electric fields for separating, moving, and trapping biological particle samples. Recent work has shown that utilizing DC‐biased AC electric fields can enhance the performance of iDEP devices. In this study, an iDEP device with 3D varying insulating structures analyzed in combination with DC biased AC fields is presented for the first time. Using our unique reactive ion etch lag, the mold for the 3D microfluidic chip is created with a photolithographic mask. The 3D iDEP devices, whose largest dimensions are 1 cm long, 0.18 cm wide, and 90 μm deep are then rapidly fabricated by curing a PDMS polymer in the glass mold. The 3D nature of the insulating microstructures allows for high trapping efficiency at potentials as low as 200 Vpp. In this work, separation of Escherichia coli from 1 μm beads and selective trapping of live Staphylococcus aureus cells from dead S. aureus cells is demonstrated. This is the first reported use of DC‐biased AC fields to selectively trap bacteria in 3D iDEP microfluidic device and to efficiently separate particles where selectivity of DC iDEP is limited.

A Single-Mask Process for 3-D Microstructure Fabrication in PDMS

This paper reports a single-mask process technique to develop 3-D structures in polydimethylsiloxane (PDMS) finding a wide variety of applications in microfluidics. This technique enables the fabrication of channels and cavities having round corners and many other customized shapes in PDMS in a predictable manner. The process relies on reactive-ion-etching lag to form 3-D channels and cavities in silicon in a single-etch process. The negative replica of patterns is then transferred from the silicon substrate to a glass master by using anodic bonding under vacuum, glass reflowing at temperatures above 700 °C for about 5 h, and complete removal of silicon in KOH. Finally, soft lithography is exploited to transfer the structures to PDMS maintaining the same aspect ratio and feature sizes of the original patterns in silicon. As a case example, an insulator-based dielectrophoresis (iDEP) device with 3-D constrictions has been developed that can operate at lower applied potentials compared with previously reported 2-D iDEP designs. Using the 3-D iDEP device, trapping of 2-μm and 500-nm polystyrene beads was achieved with an applied potential of 150 and 350 V, respectively, with more than 80% trapping efficiency.

Interchannel Mixing Minimization in Semi-Packed Micro Gas Chromatography Columns

Semi-packed columns containing an array of micropillars embedded within an open rectangular column structure are a new class of micro gas chromatography (μGC) columns introduced to provide higher separation efficiency and higher sample capacity. Three different semi-packed column configurations are evaluated with respect to pillar spacing along the flow direction and number of pillars across channel. The efficiencies of semi-packed columns, in terms of height-equivalent-to-a-theoretical-plate (HETP), are compared with two microfabricated open-rectangular columns. According to simulation results, arranging the pillars in a symmetrical configuration with spacing equal to the post dimension can suppress multi-path flows. Experimental results confirm the simulation prediction as the design with 2 μm post spacing (SP1) demonstrates the highest performance among other designs. SP1 is found to have HETP of 0.010 cm (15 000 plates/m) at an optimal velocity of 18 cm/s. An open channel design with comparable channel width yields an HETP of 0.025 cm (6000 plates/m) at an optimal velocity of 45 cm/s. The pressure drop in semi-packed columns is experimentally measured to be ~ 9 kPa which falls within the practical range of microfabricated pumps.

Three dimensional passivated-electrode insulator-based dielectrophoresis

In this study, a 3D passivated-electrode, insulator-based dielectrophoresis microchip (3D πDEP) is presented. This technology combines the benefits of electrode-based DEP, insulator-based DEP, and three dimensional insulating features with the goal of improving trapping efficiency of biological species at low applied signals and fostering wide frequency range operation of the microfluidic device. The 3D πDEP chips were fabricated by making 3D structures in silicon using reactive ion etching. The reusable electrodes are deposited on second glass substrate and then aligned to the microfluidic channel to capacitively couple the electric signal through a 100 μm glass slide. The 3D insulating structures generate high electric field gradients, which ultimately increases the DEP force. To demonstrate the capabilities of 3D πDEP, Staphylococcus aureus was trapped from water samples under varied electrical environments. Trapping efficiencies of 100% were obtained at flow rates as high as 350 μl/h and 70% at flow rates as high as 750 μl/h. Additionally, for live bacteria samples, 100% trapping was demonstrated over a wide frequency range from 50 to 400 kHz with an amplitude applied signal of 200 Vpp. 20% trapping of bacteria was observed at applied voltages as low as 50 Vpp. We demonstrate selective trapping of live and dead bacteria at frequencies ranging from 30 to 60 kHz at 400 Vpp with over 90% of the live bacteria trapped while most of the dead bacteria escape.

CMOS-compatible three dimensional buried channel technology (3DBCT)

This paper reports the development of a single-mask CMOS-compatible process for creating three dimensional buried channels (3DBCT). The structures are formed in silicon using isotropic SF6 plasma etching in a deep reactive ion etcher and are then sealed by depositing a low-stress dielectric material using low-temperature plasma enhance chemical vapor deposition. Utilizing reactive ion etch lag, this bulk micromachining technique creates silicon channels with 3D variability. With a single mask and a single etch step, silicon microchannels are created with control in all three dimensions to form complex μTAS comprising microchannels and cavities with varying depths and width. This technique also allows for creating microfluidic access ports to the microchannels.

Selective E. coli trapping with 3D insulator-based dielectrophoresis using DC-biased, AC electric fields

We present the development of a batch trapping, insulator-based dielectrophoretic (iDEP) device with three-dimensional design. The microfluidic devices use DC-biased, AC electric fields to selectively manipulate biological particles based on their electric properties. The mold for the polymer microdevices is fabricated using an RIE-lag technique which creates microchannels with varying depths using a single etch process. The resulting three-dimensional insulating constrictions permit operation at low applied voltages. By varying both the applied frequency and the ratio of AC to DC electric fields, the iDEP device can trap and separate polystyrene beads and E. coli cells.

Off-chip electrode insulator based dielectrophoresis

We present the first reported off-chip electrode, insulator-based dielectrophoresis microchip (ODEP). In contrast to previous off-chip DEP efforts, the DEP forces are enhanced by the insulating structures within the channel, enabling higher sensitivity and throughput as well as low frequency operation. The device was tested by selectively concentrating Escherichia coli (E. coli) and Salmonella typhimurium, two known waterborne pathogens, from water samples at flow rates as high as 1200 μl/hr. In order to demonstrate the ability to selectively concentrate bacteria, separation of bacteria and polystyrene beads was performed.

Mammary cancer cell manipulation with embedded passivated-electrode insulator-based dielctrophoresis (EπDEP)

In this paper, we introduce a new embedded passivated-electrode insulator-based dielectrophoresis (EπDEP) device for cell manipulation. This device maximizes the electric field strength in the microfluidic channel by reducing the thickness of the passivation layer to 5μm. The devices are made by polymer molding using 3D glass molds fabricated by melting glass into features created by the RIE-lag technique on silicon. This paper demonstrates the trapping of MDA-MB-468 mammary cancer cells using EπDEP technology with very high efficiency (97%).