Ciprian Iliescu

A practical guide for the fabrication of microfluidic devices using glass and silicon

This paper describes the main protocols that are used for fabricating microfluidic devices from glass and silicon. Methods for micropatterning glass and silicon are surveyed, and their limitations are discussed. Bonding methods that can be used for joining these materials are summarized and key process parameters are indicated. The paper also outlines techniques for forming electrical connections between microfluidic devices and external circuits. A framework is proposed for the synthesis of a complete glass/silicon device fabrication flow.

Cell culture on MEMS platforms: a review.

Microfabricated systems provide an excellent platform for the culture of cells, and are an extremely useful tool for the investigation of cellular responses to various stimuli. Advantages offered over traditional methods include cost-effectiveness, controllability, low volume, high resolution, and sensitivity. Both biocompatible and bio-incompatible materials have been developed for use in these applications. Biocompatible materials such as PMMA or PLGA can be used directly for cell culture. However, for bio-incompatible materials such as silicon or PDMS, additional steps need to be taken to render these materials more suitable for cell adhesion and maintenance. This review describes multiple surface modification strategies to improve the biocompatibility of MEMS materials. Basic concepts of cell-biomaterial interactions, such as protein adsorption and cell adhesion are covered. Finally, the applications of these MEMS materials in Tissue Engineering are presented.

Fabrication of a dielectrophoretic chip with 3D silicon electrodes

This paper describes a device in which the DEP electrodes form the channel walls. This is achieved by fabricating microfluidic channel walls from highly doped silicon so that they can also function as DEP electrodes. The device is fully enclosed and there is no fluidic leakage due to lead-outs. The electrode arrangement minimized the electrical dead volumes such that the DEP force is always sufficient to overcome Stokes force and concentrate the cells and beads at the nominal operating potential of 25 Vp–p. The device has been tested successfully with yeast cells. When the actuation signal was increased to 13 Vp–p, cells began to move towards the tip of the DEP electrodes, where the electric field gradient was highest. As the actuation voltage increased, the cells moved faster. For 25 Vp–p, a stable equilibrium of cell concentration pattern was achieved in 10–13 s.

Microfabricated microneedle with porous tip for drug delivery

This paper presents a novel approach to fabrication of a silicon microneedle array with porous tips. Dry etching technology with SF6/O2 gas by STSs inductively coupled plasma (ICP) etch tool was used to achieve the pyramidal needle structure. A thin silicon nitride layer was deposited after a thick photoresist layer was coated and reflowed at 120 °C. The silicon nitride layer and residual photoresist on the tips of the pyramidal structures were removed using reactive ion etching (RIE). Electrochemical etching in MeCN/HF was carried out to generate porous silicon on the tips of the microneedles. The fabricated microneedle array has potential applications in drug delivery, since the porous tips can be loaded with a high molecular weight drug. Analytic solutions to the critical loadings of the fabricated microneedle structure are also presented. The variations of the square cross-section were expressed as a function of the axial coordinate to analyze the bending normal stress and critical buckling loading. This analytic method can also be used for other microneedle structures with different cross-sections.

A robust high-throughput sandwich cell-based drug screening platform

Hepatotoxicity evaluation of pharmaceutical lead compounds in early stages of drug development has drawn increasing attention. Sandwiched hepatocytes exhibiting stable functions in culture represent a standard model for hepatotoxicity testing of drugs. We have developed a robust and high-throughput hepatotoxicity testing platform based on the sandwiched hepatocytes for drug screening. The platform involves a galactosylated microfabricated membrane sandwich to support cellular function through uniform and efficient mass transfer while protecting cells from excessive shear. Perfusion bioreactor further enhances mass transfer and cellular functions over long period; and hepatocytes are readily transferred to 96-well plate for high-throughput robotic liquid handling. The bioreactor design and perfusion flow rate are optimized by computational fluid dynamics simulation and experimentally. The cultured hepatocytes preserved 3D cell morphology, urea production and cytochrome p450 activity of the mature hepatocytes for 14 days. When the perfusion-cultured sandwich is transferred to 96-well plate for drug testing, the hepatocytes exhibited improved drug sensitivity and low variability in hepatotoxicity responses amongst cells transferred from different dates of perfusion culture. The platform enables robust high-throughput screening of drug candidates.

Adhesive bonding with SU-8 at wafer level for microfluidic devices

The present work proposes an adhesive bonding technique, at wafer level, using SU-8 negative photoresist as intermediate layer. The adhesive was selective imprint on one of the bonding surface. The main applications are in microfluidic area where a low temperature bonding is required. The method consists of three major steps. First the adhesive layer is deposited on one of the bonding surface by contact imprinting from a dummy wafer where the SU-8 photoresist was initially spun, or from a Teflon cylinder. Second, the wafers to be bonded are placed in contact and aligned. In the last step, the bonding process is performed at temperatures between 100 O C and 200 O C, a pressure of 1000 N in vacuum on a classical wafer bonding system. The results indicate a low stress value induced by the bonding technique. In the same time the process presents a high yield: 95-100%. The technique was successfully tested in the fabrication process of a dielectrophoretic device.

Exploitation of physical and chemical constraints for three-dimensional microtissue construction in microfluidics

There are a plethora of approaches to construct microtissues as building blocks for the repair and regeneration of larger and complex tissues. Here we focus on various physical and chemical trapping methods for engineering three-dimensional microtissue constructs in microfluidic systems that recapitulate the in vivo tissue microstructures and functions. Advances in these in vitro tissue models have enabled various applications, including drug screening, disease or injury models, and cell-based biosensors. The future would see strides toward the mesoscale control of even finer tissue microstructures and the scaling of various designs for high throughput applications. These tools and knowledge will establish the foundation for precision engineering of complex tissues of the internal organs for biomedical applications.

Dielectrophoretic separation of biological samples in a 3D filtering chip

A field-flow dielectrophoretic separation method in a 3D filtering chip has been developed in this work. The separation method was possible due to the special configuration of the DEP filtering chip, which has a structure similar to a classical capacitor with two parallel plate electrodes (realized by using a stainless steel mesh) and a dielectric medium (defined by a suspension of 100 µm diameter silica beads in buffer solution). The dielectrophoretic phenomenon is generated by the non-uniformities of the dielectric media, which produce a gradient of the electric field and, as a consequence, a DEP force. If a suspension medium with cells flows through the filter, the DEP force can trap these cells around the contact points between the silica beads (if the cells exhibit positive DEP) or they are repelled into the space between the beads (if the cells exhibit negative DEP). It is shown that for two different cell populations, the frequency of the electric field and permittivity of the media can be tuned in such a way that one population will exhibit positive DEP and the other one negative DEP. The population that expresses negative DEP can be easily flushed out due to the hydrodynamic force which is larger at the center point between the beads. In such a way two cell populations can be separated. The working principle was verified with both live and dead yeast cells. Best results for the separation of viable and nonviable cell populations were achieved at an applied voltage of 150 V in a frequency range between 10 kHz and 20 kHz for flow rates of 0.1 ml min−1 and 0.2 ml min−1. With a few of these devices cascaded in series, higher efficiency could be achieved. As a result, this device and the associated proposed separation method can be very useful tools for bio-pharmaceutical industries since continuous flow separation at relatively high flow rates is both time and cost saving.

Microfabricated Silicon Microneedle Array for Transdermal Drug Delivery

This paper presents developed processes for silicon microneedle arrays microfabrication. Three types of microneedles structures were achieved by isotropic etching in inductively coupled plasma (ICP) using SF6/O2 gases, combination of isotropic etching with deep etching, and wet etching, respectively. A microneedle array with biodegradable porous tips was further developed based on the fabricated microneedles.

A Dielectrophoretic Chip With a 3-D Electric Field Gradient

This paper presents the design, fabrication and testing of a new structure of dielectrophoresis (DEP) chip having a three-dimensional (3-D) electric field gradient and an asymmetric distribution of the electric field in the vertical plane. This achievement was possible due to the special configuration of the electrodes: a bulk silicon electrode and a thin amorphous silicon electrode. The thick electrode defines, at the same time, the walls, while the two glass dies form the ceiling and floor of the microfluidic channel. The top glass die presents two etch-through inlet/outlet holes of the microfluidic channel. In the bottom glass die, isotropic via-holes are performed through the glass for the lead-outs. For this reason, the lead-outs does not generate fluidic leakage. Using a single material for the electrodes, the electrochemical effect for conventional multilayer metal electrodes is eliminated. The proposed DEP structure, with thin and thick electrodes, generates in the vertical plane an asymmetric distribution of the electric field and, therefore, an enhanced electric field gradient. As a result, for positive DEP, the particles are trapped near the thin electrode, while for negative DEP the particles are levitated. Compared with typical planar DEP devices, the proposed DEP structure, presents an increased DEP force in the vertical direction. As a result, the same trapping or levitation effect can be achieved at a lower voltage and, in this way, with a reduced heating of the solution. Using the bulk electrode for definition of the microfluidic channel, the need for a separate channel wall material is eliminated. The DEP device is wafer level packaged (being fully fabricated at wafer level using batch processes) therefore, can be consider as a low cost solution. The good isolating properties of the glass confer the opportunity of working at a high frequency range. Yeast cells have been used to successfully test the performance of the device: the trapping using positive DEP occurred on the bottom of the channel near the thin electrode, while the negative DEP generate a suspension of the cells 35-40 mum from the bottom