Fabrication of polymer micro-devices based on organic sacrificial pastes

We investigate in this work the fabrication of polymer micro-fluidic and micro-mechanical devices based on sublimable compounds as sacrificial layers, using a thick-film screen-printing deposition process. Fabrication of printed polymer microstructures featuring properties such as low cost, transparency and biocompatibility is particularly attractive and suitable for disposable biomedical devices. Channel, cavities and spacings for micro-devices are first defined by sacrificial material layers. This is followed by polymer resin deposition and polymerization. Removal of the sacrificial layers is then accomplished by heating at their decomposition / evaporation / sublimation temperature (Fig 1). This process does not require an ulterior dissolution step, and systems exist where the sacrificial layer molecules escape by diffusion through the polymer at high temperature: we are therefore not limited to open structures. Furthermore, the use of printed compositions is very advantageous regarding process cost and flexibility. Although several authors already reported micro-structures based on organic sacrificial layers, such as the combination of SU-8 (over-layer) and polypropylene carbonate (sacrificial material) [1], the required processing temperatures remain too high for most polymer applications. In this work, we introduce materials with significantly lower sacrificial layer decomposition temperatures, thereby considerably extending the application field of this process. The key here is naturally the formulation of the sacrificial pastes. It must be stable at the overlayer processing temperature, yet decompose / evaporate cleanly at a temperature low enough to avoid degradation of the polymer. Also, the sacrificial materials must have a low toxicity, and ideally degrade in the solid state to avoid collapse of fine structures by capillary effects. The selected sacrificial material consists of heat-degradable polymers or sublimable polyol-type compounds, formulated as thick-film pastes. Studies were therefore performed with several solvents to determine the most suitable formulation to obtain the correct rheology for screen-printing (ca. honey-like consistency). In this work, we use various epoxy resins and silicones as thermosetting polymers, optionally filled with graphite to impart electrical conductivity and improve mechanical stability. Formulation of this over-layer should also require careful attention. A too strong interaction between the sacrificial paste and the top layer could result in potential destruction of the sacrificial material. Based on the investigated structuration methods, the operation of several devices, such as simple fluidic circuits (Fig 2) or micro-mechanical bridges / cantilevers, is demonstrated.