Marc Madou

Photoresist‐Derived Carbon for Microelectromechanical Systems and Electrochemical Applications

Photopatterned resists pyrolyzed at different temperatures and different ambient atmospheres can be used as a carbonaceous material for microelectromechanical systems. Carbon films were prepared by pyrolysis of photoresists at temperatures ranging from 600 to 1100°C. The carbon films were characterized by several analytical techniques, viz., profilometry, thermogravimetric analysis. four-point probe measurements, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. In addition, cyclic voltammetry was performed on the carbon film electrodes, and the carbon films were compared to glassy carbon (GC for their electrochemical behavior. Electron-transfer rate constants for the benchmark Fe(CN) 3-/4- 6 and Ru(NH 3 ) 3+/2+ 6 redox systems increased with increasing heat-treatment temperature and approached those observed on GC following-treatment at 1100°C. The pyrolyzed films have low capacitance and background current, approximately one-fourth of that observed on GC. The oxygenicarbon atomic ratio determined from XPS was low (∼1% for I 100°C pretreatment), and increased more slowly upon exposure to air than that for GC treated under identical conditions. Pyrolysis of photoresist films permits photolithographic fabrication of carbon electrode devices, and also appears to yield a carbon film with a smooth surface and unusual surface chemistry.

Design and Fabrication of CD-like Microfluidic Platforms for Diagnostics: Microfluidic Functions

In this paper, the design of a polymer based microfluidic compact disk (CD) platform is presented. Several microfluidic functions such as flow sequencing, cascade micro-mixing, and capillary metering can be integrated into the CD by balancing the centrifugal force and the capillary force. These functions are demonstrated experimentally. For flow sequencing, a two-point calibration design is used as an example to show how the release and flow of fluids can be precisely controlled by the rotation speed of the CD. For cascade micro-mixing, a typical application is reconstituting lyophilized protein. A simple metering technique based on bubble snap-off in the two-phase flow is also described.

A novel method for the fabrication of high-aspect ratio C-MEMS structures

A novel fabrication process was developed to create high aspect ratio (>10:1) carbon posts, all-carbon suspended bridges and wires, self-organized bunches of carbon posts, and carbon plates supported by carbon beams. The structures are all made from a two-step pyrolysis process with SU-8 photoresist as the starting material. In this paper we describe the fabrication of these various new C-MEMS structures and detail an important application of the high aspect ratio carbon posts arrays. The carbon post arrays can be reversible charged/discharged with Li ions, an application that may greatly impact the application of C-MEMS in three-dimensional microbatteries. Complex suspended C-MEMS structures, such as wires, plates, ribbons, and self-organized bunches of posts, were built. Methods to accurately and repeatedly fabricate all the above 3-D C-MEMS structures are given.

C-MEMS for the manufacture of 3D microbatteries

We have demonstrated that carbon-microelectromechanical systems (C-MEMS), in which patterned photoresist is pyrolyzed in inert environment at high temperature, constitutes a powerful approach to building 3D carbon microelectrode arrays for 3D microbattery applications. High aspect ratio carbon posts (>10:1) are achieved by pyrolyzing SU-8 negative photoresist in a simple one step process. Lithium can be reversibly charged and discharged into these C-MEMS electrodes. Because of the additional volume of the posts, higher capacities are achieved with the 3D array electrodes as compared to unpatterned carbon films with the same projected electrode area. These novel electrode arrays represent one of the critical components for 3D batteries, which may be interconnected with C-MEMS leads to enable smart power management schemes.

Microactuators toward microvalves for responsive controlled drug delivery

A responsive controlled drug release system in which the delivery of drugs is achieved by actuating miniature metal or polymeric valves has been introduced. These valves might be actuated under the control of a sensor responding to a specific biological stimulus. This approach offers better reproducibility and easier control than drug release achieved by passive diffusion out of a polymer host matrix. The metal valve systems, which are irreversible, consist of thin suspended non-porous layers that can be electrochemically dissolved or disintegrated by water electrolysis. The reversible polymeric valve systems, also called ‘artificial muscle’, are prepared from a blend of redox polymer and hydrogel that swells and shrinks either by applying a suitable bias or through a specific chemical reaction. In one of the several possible configurations for the artificial muscle valves such as the sphincter configuration, the blend is electropolymerized within an array of holes, which open and close corresponding to the shrinking and swelling of the polymer actuator. The swelling and shrinking property of the blends is characterized by video monitoring and by in situ conductivity measurements. Significantly larger magnitude of swelling and shrinking were observed for the blend than the redox polymer itself. The blend also appeared smoother and more voluminous. The largest actuation was obtained for the blend consisting of polyaniline and poly(2-hydroxyethylmethacrylate)-poly(N-vinylpyrrolidinone). The results demonstrate that it is possible to apply artificial muscle for the fabrication of microactuator valves for responsive controlled drug delivery.

A pH Electrode Based on Melt-Oxidized Iridium Oxide

Fabrication and characterization of a novel potentiometric pH electrode based on melt-oxidized iridium oxide film is presented. The oxide film produced in a lithium carbonate melt has the composition of and shows high chemical stability. The electrode based on this oxide film exhibits very promising pH sensing performance, with an ideal Nernstian response in the tested pH range of 1 to 13. The potential response is fast, with a 90% response time obtained in less than 1 s for all pH changes. The open-circuit potential of the electrode is almost drift-free, with an average variation over time in a pH 6.6 solution as small as 0.1 mV/day. Furthermore, the potential/pH slopes and the apparent standard electrode potentials show excellent agreement among electrodes from the same batch. A comparison is made of the present electrode and those reported in the literature with respect to fabrication method and pH sensing characteristics.

Cell lysis on a microfluidic CD (compact disc)

Cell lysis was demonstrated on a microfluidic CD (Compact Disc) platform. In this purely mechanical lysis method, spherical particles (beads) in a lysis chamber microfabricated in a CD, cause disruption of mammalian (CHO-K1), bacterial (Escherichia coli), and yeast (Saccharomyces cerevisiae) cells. Interactions between beads and cells are generated in the rimming flow established inside a partially filled annular chamber in the CD rotating around a horizontal axis. To maximize bead-cell interactions in the lysis chamber, the CD was spun forward and backwards around this axis, using high acceleration for 5 to 7 min. Investigation on inter-particle forces (friction and collision) identified the following parameters; bead density, angular velocity, acceleration rate, and solid volume fraction as having the most significant contribution to cell lysis. Cell disruption efficiency was verified either through direct microscopic viewing or measurement of the DNA concentration after cell lysing. Lysis efficiency relative to a conventional lysis protocol was approximately 65%. In the long term, this work is geared towards CD based sample-to-answer nucleic acid analysis which will include cell lysis, DNA purification, DNA amplification, and DNA hybridization detection.

Controlled Continuous Patterning of Polymeric Nanofibers on Three-Dimensional Substrates Using Low-Voltage Near-Field Electrospinning

We report on a continuous method for controlled electrospinning of polymeric nanofibers on two-dimensional (2D) and three dimensional (3D) substrates using low voltage near-field electrospinning (LV NFES). The method overcomes some of the drawbacks in more conventional near-field electrospinning by using a superelastic polymer ink formulation. The viscoelastic nature of our polymer ink enables continuous electrospinning at a very low voltage of 200 V, almost an order of magnitude lower than conventional NFES, thereby reducing bending instabilities and increasing control of the resulting polymer jet. In one application, polymeric nanofibers are freely suspended between microstructures of 3D carbon on Si substrates to illustrate wiring together 3D components in any desired pattern.

Electrochemical Studies of Carbon Films from Pyrolyzed Photoresist

Carbon film electrodes were prepared by pyrolysis of photoresists on silicon wafers at temperatures ranging from 600 to 1,100 C. The physical properties of the carbon films were characterized by scanning and transmission electron microscopies, thermal gravimetric analysis, and four-point probe electrical resistivity measurements. The electrochemical properties of the carbon films were investigated by cyclic voltammetry to observe the kinetics of the Fe(CN){sub 6}{sup 4{minus}}/Fe(CN){sub 6}{sup 3{minus}} redox couple. The carbon film electrodes prepared at temperatures {ge} 700 C showed electrochemical behavior similar to that of glassy carbon. Better electrocatalytic behavior was obtained with carbon films prepared at the higher pyrolysis temperatures, which is attributed to different film compositions at different pyrolysis temperatures. The electrochemical properties of the carbon film electrodes are very stable, exhibiting reproducible behavior even after storing at room temperature in air for 3 months.

Pyrolysis of Negative Photoresists to Fabricate Carbon Structures for Microelectromechanical Systems and Electrochemical Applications

Carbon structures were fabricated by the pyrolysis of photopatterned negative photoresists (SU-8 and photosensitive polyimide) on silicon and fused silica wafers. Results here are compared with those of positive resists published earlier by this group. Negative resist films need exposure to ultraviolet light prior to pyrolysis to produce carbon films. The pyrolysis was carried out in a closed quartz tube furnace in a forming gas (95% N 2 , 5% H 2 ) atmosphere. The pyrolysis process was characterized using a combination of thermogravimetric analysis and differential thermal analysis. The pyrolysis of SU-8 involved gas evolution in a narower range of temperature than polyimide, The adhesion of the carbon film was found to depend on the resist, the substrate, and the heating cycle used. The carbon structures were characterized in terms of their shrinkage during the pyrolysis, the resistivity, the degree of crystallinity and the peak separation in cyclic voltammetry. Carbons derived from pyrolysis of negative resists showed higher resistivity, vertical shrinkage, and peak-to-peak separation voltage than positive resists. Transmission electron microscope results showed a distinct lack of crystallinity even after pyrolysis at 1100°C, unlike the positive resist derived carbon.