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Abstract

Free-standing microstructures such as cantilevers, membranes or microchannels are building blocks of microfluidic systems and MEMS. As a complement to silicon, the large family of polymers offers many opportunities for micro and nanotechnologies. Their low temperature processing and the planarizing properties of many resists is a definitive advantage for system integration, paving the way to complete lab-on-chips. In this article, we investigate a fabrication process of polymeric free standing structures based on the lamination of SU-8, a thick epoxy photoresist. Our motivation is the hybrid integration of polymer microfluidic or MEMS components with silicon chips (e.g., integrated circuits or sensors). Compared to rigid substrates used in more conventional SU-8/SU-8 bonding process, the flexible photosensitive films used within this lamination technique allows a more homogeneous and reliable bonding at low pressure and temperature, and a 3D fabrication with an excellent levelto- level alignment. A parametric optimization of the lamination process is presented. The fabrication of a leakage-free 3D microfluidic network is demonstrated by stacking up to five layers. A polyethylene terephtalate layer has been employed to easily release the SU-8 devices. We show that this release layer also significantly decrease the curvature of the substrate by 32% and the related residual stress in a 100 lm SU-8 layer by at least 10%. Finally, we briefly describe the hybrid integration of a silicon sensor in a microfluidic network as a direct application of our lamination process to the fabrication of lab-on-chips.

Abstract

In a first step towards chemical sensors using molecular imprinted materials, the complexing characteristics of diethyl 4-nitrobenzylphosphonate, an organophosphate pesticide analogue, have been studied. Two molecules have been assessed as potential interacting moieties, specifically a fluoroalcohol and an aromatic acid. The interactions have been first characterized by regular methods, such as 1H, 31P NMR and IR spectroscopy. These showed a stoichiometry 1/1 for both complexes and association constants, respectively, close to 40 ± 10 and 12 ± 2 M1. In a second step, isothermal titration calorimetry was used and a method was developed to obtain low-association constants. The association constant could be obtained for the fluoroalcohol ligand and was found equal to 63 ± 0.7 M1. For the acidic molecule, an appropriate model could not be found, preventing the evaluation of this constant.

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We report on a novel approach for controlling nanohydrodynamic properties at the solid-liquid interfaces through the use of stimuli-responding polymer coatings. The end-tethered polymers undergo a phase separation upon external activation. The reversible change in the thickness and polarity of the grafted polymers yields in a dynamic control of the surface-generated, electrokinetic phenomena. Nonactivated, swollen polymers are thicker than the electrical double layer (EDL) and prohibit the development of an EOF even on charged surfaces. On the other hand, activated polymer chains shrink and become thinner than the EDL and allow for the EOF to build up unimpeded. We show here that, for given experimental conditions, the EOF velocity on the shrunken surface is 35 times greater than the one on the nonactivated surface. Furthermore, we reveal that coupling of such surfaces with dense arrays of thermal actuators developed in our laboratory can lead to novel micro- and nanofluidic devices.

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This paper describes a new technology permitting a hybrid integration of silicon chips in polymer (PDMS and SU8) microfluidic structures. This two-step technology starts with transferring the silicon device onto a rigid substrate (typically PCB) and planarizing it, and then it proceeds with stacking of the polymer-made fluidic network onto the device. The technology is low cost, based on screen printing and lamination, can be applied to treat large surface areas, and is compatible with standard photolithography and vacuum based approaches. We show, as an example, the integration of a thermal sensor inside channels made of PDMS or SU8. The developed structures had no fluid leaks at the Si/polymer interfaces and the electrical circuit was perfectly tightproof.

Abstract

We report here on the development of an integrated device for sample desalting and pre-concentration for nanoLC / ESI-MS analysis combining poly-(N-isopropyl acrylamide) (PNIPAM) grafted microbeads and the means to dynamically control their temperature. Thermoresponsive properties of PNIPAM induce switchable hydrophobic/hydrophilic surfaces on which peptides can reversibly adsorb and desorb. The device is fabricated on a glass or pyrex substrate with deposited Ti/Au electrodes serving as built-in resistive heating sources. Premolded microfluidic channels and reservoirs made in PDMS are eventually assembled. Electrical and thermal characterization together with multiphysics modeling have been performed. The SiO2 surfaces of the channels and silica beads used as carriers of the stationary phases have been end-grafted with PNIPAM and employed to study the reversible adsorption and release kinetics of albumin-fluorescein conjugates by fluorescence video microscopy. It is clearly shown albumin-fluorescein complexes adsorb on beads surfaces above the transition temperature of PNIPAM (hydrophobic state), and are released when the temperature decreases (hydrophilic state), yet not fully reversibly.

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Since its introduction in the nineties, the negative resist SU-8 has been increasingly used in micro- and nanotechnologies. SU-8 has made the fabrication of high-aspect ratio structures accessible to labs with no high-end facilities such as X-ray lithography systems or deep reactive ion etching systems. These low-cost techniques have been applied not only in the fabrication of metallic parts or molds, but also in numerous other micromachining processes. Its ease of use has made SU-8 to be used in many applications, even when high-aspect ratios are not required. Beyond these pattern transfer applications, SU-8 has been used directly as a structural material for microelectromechanical systems and microfluidics due to its properties such as its excellent chemical resistance or the low Young modulus. In contrast to conventional resists, which are used temporally, SU-8 has been used as a permanent building material to fabricate microcomponents such as cantilevers, membranes, and microchannels. SU-8-based techniques have led to new low-temperature processes suitable for the fabrication of a wide range of objects, from the single component to the complete lab-on-chip. First, this article aims to review the different techniques and provides guidelines to the use of SU-8 as a structural material. Second, practical examples from our respective labs are presented.

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