Wet Bonding of Soft Elastomeric Microchannels

Abstract

In microfluidics small amounts of fluids are circulating in channels with maximum dimensions in hundreds of micrometers. Poly(dimethylsiloxane) (PDMS) elastomers and especially commercial Sylgard 184 (Dow Corning) is widely used for fabrication of microfluidic devices due to its advantageous chemical and mechanical properties. As stretchability and softness of Sylgard 184 are limited, softer elastomers from Dragon Skin and Ecoflex product families (Smooth-On) are commonly used in applications that demand more deformability from the material. Fabrication of soft microchannels from these silicone elastomers is conducted with cast molding and layer-by-layer stacking. To create closed channels, the open channels are sealed to a flat elastomer substrate. Sylgard 184 can be sealed with oxygen plasma bonding where the increased number of functional groups form the bond between oxygen plasma treated surfaces. This method is not applicable for most softer elastomers due to silicone oils that are diffusing to the surface preventing the bonding. As an alternative, a thin layer of uncured elastomer can be used as adhesive between the layers. This bonding technique is relatively unreliable as the uncured elastomer easily flows to the open channels blocking the channel. The capillary action can be reduced by decreasing the thickness of the layer and partially curing it before bonding. Softness of the elastomer also effects on the dimensions of producible microchannels. The ceiling of the channel can collapse, or the channel walls can pair before bonding if the dimensions are not suitable for the elastomer. In this work, the soft microchannels are fabricated from commercial elastomers Dragon Skin 30, Ecoflex 00-50 and Ecoflex 00-20 by bonding the elastomer substrates with the adhesive layer to enhance reliability and reproducibility of the fabrication technique. The adhesive layer is spread evenly with a spin coater. Thus, spin speed and spin time together with viscosity of the elastomer are determining the thickness of the layer. The layer is partially cured in an oven at elevated temperature. Different spin speeds and pre-curing times are tested to find suitable parameters for each elastomer for the reliable fabrication process. Different channel widths and separations are used to discover the range of producible microchannel dimensions for these soft elastomers. The study revealed that the capillary action can be decreased but not totally prevented. Based on the results, 2000 RPM spin speed and 20-40 seconds pre-curing in oven at 60°C are proposed for the reliable and reproducible fabrication process with Ecoflex 00-50 and Ecoflex 00-20. The same spin speed is recommended for Dragon Skin 30, but pre-curing the layer is not required. Microchannels with 50 μm width are producible with Dragon Skin 30 as well as 50 µm thick channel walls. That narrow and dense channels were unfeasible with softer Ecoflex 00-50 and Ecofelx 00-20. In addition, a soft pneumatic strain sensor was fabricated from Ecoflex 00-50 to show a practical example of the studied bonding technique. The sensor changes its pneumatic resistance as response to deformation of the embedded microchannel while external force is applied. The fabricated soft strain sensor can tolerate at least 300% strain, has negligible hysteresis, and can be used to measure large strains with gauge factor close to 1.