A Simple and Reliable PDMS and SU-8 Irreversible Bonding Method and Its Application on a Microfluidic-MEA Device for Neuroscience Research

Polydimethylsiloxane (PDMS) and SU-8 are currently two very commonly used polymeric materials in the microfluidics field for biological applications. However; there is a pressing need to find a simple, reliable, irreversible bonding method between these two materials for their combined use in innovative integrated microsystems. In this paper; we attempt to investigate the aminosilane-mediated irreversible bonding method for PDMS and SU-8 with X-Ray Photoelectron Spectroscopy (XPS) surface analysis and bonding strength tests. Additionally; the selected bonding method was applied in fabricating a microelectrode array (MEA) device, including microfluidic features, which allows electrophysiological observations on compartmentalized neuronal cultures. As there is a growing trend towards microfluidic devices for neuroscience research, this type of integrated microdevice, which can observe functional alterations on compartmentalized neuronal culture, can potentially be used for neurodegenerative disease research and pharmaceutical development.

Polymer microchips bonded by O2-plasma activation

Hubert Girault, Joël Stéphane Rossier, Nicolas Xanthopoulos

This paper presents a fabrication of polymer microchips with homogeneous material technique due to surface treatment by plasma before sealing. UV laser photoablation was used for fast prototyping of microstructures, and oxygen plasma was used as a surface treatment for both the microfabricated substrate and the polymer cover. It was found that with an oxidative plasma treatment, successful bonding could be achieved without adhesive material between polymer sheets substantially below the glass transition temperature of the polymer. Homogeneous polyethylene terephthalate (PET) microstructures were characterized by scanning electron microscopy (SEM) and analyzed by X-ray photoelectron spectroscopy (XPS) surface analyses after different surface treatments. The electroosmotic flow characteristics including the velocity and the stability over 20 days have been tested and compared to composite channels, in which the cover presents a polyethylene (PE) adhesive layer. Capillary zone electrophoresis in both homogeneous and composite microanalytical devices were then performed and compared in order to evaluate the separation efficiency. In preliminary experiments, a plate height of 0.6 m has been obtained with homogenous microchannels. The surface analysis pointed out that the surface chemistry is of prime importance for the performance of microfluidic separation.

2002

Low stress acrylated hyperbranched polymers

Lars Schmidt

The objective of this study was to investigate the behavior of highly functional acrylates, during isothermal ultraviolet (UV) curing. The materials included a pentafunctional acrylate and two acrylated hyperbranched polymers, one with a stiff polyester core and one with a more flexible polyether core. In particular, the influence of UV intensity and reactive blend composition on structural transitions, such as gelation and vitrification, and on the dynamics of internal stress was considered. Curing kinetics were studied with photo differential scanning calorimetry. The chemical conversion was analyzed using an autocatalytic model and a criterion for identifying vitrification directly from photocalorimetric experiments was proposed. It was observed that reactive blends containing HBPs had a higher conversion at vitrification, compared to the pure penta-functional acrylate. Strong intensity dependence of the maximum conversion rate and a weak intensity dependence of the ultimate conversion were observed. The latter was found to be controlled by the conversion at vitrification. The structural transitions and the modulus build-up during UV polymerization were determined by photorheology. A refined data processing algorithm was developed, that allows monitoring the shear modulus over 5 orders of magnitude within a short experimental time scale, with millisecond time resolution. Gelation – the liquid-solid transition – was found to be below 5 % conversion for all acrylates investigated. In contrast, the conversion at vitrification was strongly dependent on the actual monomer and increased with increasing UV intensity. The results of the photo DSC and the photorheology study were synthesized in the form of timeintensity- transformation diagrams. The dynamics of internal stress and cure shrinkage were studied using beambending and an interferometry-based method, respectively. The internal stress of the acrylated HBPs was largely reduced compared to the standard highly functional acrylate monomer. Moreover, in the case of one HBP with a polyester core and a reactive blend of the HBP with the standard highly functional acrylate, the stress reduction was obtained with a combined increase of Young's modulus, which was attributed to retarded modulus build-up and a higher final conversion. It was found that curing at a lower UV intensity led to earlier vitrification, hence earlier internal stress build-up, but limited maximum conversion thus limited final stress. Curing at a higher intensity led to later stress build-up but higher final stresses. Polymer microstructures were fabricated from the different acrylates in a photolithographic process and compared to SU-8, an epoxy frequently used for this kind of application. It was shown that the shape accuracy is linked to the processinduced internal stresses: the best result for thick and high aspect ratio microstructures – as used for example for microfluidic devices – was obtained for the acrylated HBP with a polyether core.

EPFL2006

Nanocomposites based on luminescent colloidal nanocrystals in polymers: new opportunities for micro and nano fabrication

Jürgen Brugger, Vahid Fakhfouri

In recent years a wide range of materials including metals, semiconductors and metal oxides have been synthesized with nanometer dimensions. In particular II–VI semiconductor nanocrystals (NCs) are receiving considerable attention for fundamental studies as zerodimensional quantum confined material and for their potential application in optoelectronics and photonics. The colour tunability of the emission of semiconductor NCs as a function of size is one of their most attractive properties. A current bottle-neck of these materials is their poor compatibility with micropatterning methods in general and MEMS processes in particular. Inorganic colloidal nanoparticles encapsulated in polymer matrix may lead to versatile hybrid micro- and nanoscale devices made by simple techniques. Colloidal methods provide effective routes to prepare semiconductor NCs that are soluble in organic solvents with a narrow size distribution. Moreover the ability to manipulate their surface chemistry is a powerful tool to place the NCs in almost any chemical environment, due to the possibility of properly engineering their surface by ligand exchange techniques. Recent efforts have been directed towards the fabrication of polymer nanocomposites containing nanometer sized inorganic semiconductor. Such interest is due to the combination the processability and the structural properties of the polymeric matrix, i.e. thermoplastic or photosensitive materials, and the unique optoelectronic characteristics of the inorganic moiety, that can be thus used as emitters for many purposes such as optical switches, sensors, electro-luminescent devices, lasers and biomedical tags. In particular, microelectromechanical systems (MEMS) that are currently built mainly from Silicon, dielectrics, polymers and some selected metals will benefit from new functional polymeric nanocomposit materials, which combine simple UV processing and superior material properties such as biocompatibility, chemical stability, luminescence, photochromism and more. This paper describes the combination of synthesis and functionalization of luminescent colloidal NCs for polymer modification, with micro- and nanofabrication methods to form basic elements of advanced polymer MEMS/NEMS with increased inherent functionality. We prepared nanocomposites based on highly luminescent II-VI semiconductors colloidal CdS, CdSe, and CdSe@ZnS, core shell type. The NCs were incorporated in poly-methyl methacrylate (PMMA) a thermoplastic material, and SU-8, a photosensitive polymer with superior lithographic properties, respectively. Spectroscopical (UV-vis absorption and emission), structural (TEM) and morphological analysis were performed on both types of nanocomposite solutions as well as the thin films spin coated onto silicon and quartz substrates. The effect of NC composition, concentration, size, and surface chemistry was evaluated in order to understand the role played by these factors on the optical properties of the investigated nanocomposites. The presence of organic ligand shell at colloidal NC surface shows to be critical for their incorporation either into SU8 and PMMA. Incorporation of the different types of luminescent NCs in PMMA and SU-8 did not show any relevant change of the optical properties of the NCs. The excitonic peaks as well as the emission properties were retained. Preliminary spinning, UV exposure and development experiments were performed on the SU-8 based nanocomposites. The obtained structures were inspected by optical microscope and confirmed that the overall UVstructuring capability of the modified polymer was not macroscopically affected by the NC incorporation in the polymer.