Self-induced patterning of PDMS microstructures by photorefractive effect on Lithium Niobate crystals

Abstract

A new method for direct patterning of Poly(dimethylsiloxane) (PDMS) structures is presented. This technique will be named Light Induced Patterning (LIP) as it's induced by laser light through a functionalized substrate of Lithium Niobate (LN) crystal. The process is a full field technique that, conversely to the point wise ones, allows PDMS patterning with a single laser light exposure. PDMS is a polymeric matter largely used in the fields of micro/nano fluidics, lab-on-chip and micro/nanoelectromechanical (MEMS/NEMS) devices. Interest in its employment is due to some properties as the high moulding handiness, biocompatibility and transparency in visible light. PDMS prototyping is usually realized by soft-lithography, that is, fabrication and/or replication of predefined structures using stamps, molds, or photo-masks. Here, a different method for direct patterning of PDMS microstructures is developed by taking advantage of photorefractive effect in a functionalized substrate. In particular the substrate we investigate is iron doped LN crystal 1cm 5× 1cm in y and z axes while thickness (x-axis) is 500µm (dopant level 0.05% weight). When the crystal is exposed to spatially modulated laser light a space-charge field arise inside the crystal [1,2] and it generates an electric field gradients patterns on its surface. Crystal is covered by a thin liquid film of PDMS, the electric field generated on its surface, causes a reorganization of the PDMS matter due to dielectrophoretic forces [3]. The liquid PDMS follows the geometric pattern of laser light illumination pattern. As final step, an appropriate thermal treatment is applied to the crystal while the light source is generating the PDMS structure. Heating is responsible of the PDMS cross-linking that leads a stable and reliable PDMS microstructure. Argon laser emitting at 514nm is used and the light distribution on the crystal sample is obtained by means of an 100µm amplitude grating (AG), inserted in the setup as depicted in Fig.1(a). The AG was imaged by a lens (L) whose focal length is 25.4mm. On the LN sample, a replica of the AG spatial intensity distribution, is obtained. The final structure correspondent to the setup of Fig.1(a) is displayed in Fig.1(b) where linear PDMS grating with 100µm period is imaged by a bright field microscope (5× magnification). Two dimensional structures of PDMS are realized substituting the linear AG (Fig.1(a)) with different ones of desired geometry. An example of the allowed patterns is showed in Fig.1(c) where an optical microscope image of radial channels is presented.