Design and characterization of a planar microfabricated liquid filter for implantable drug delivery applications is presented. An on-chip liquid filter is required at the inlet of a micromachined membrane pump [1] to ensure trouble-free operation of pump components. The filter, bulk micromachined in silicon, consists of a slightly etched long filtration barrier within a deep wet-etched cavity with asymmetrically offset inlet and exit encapsulated by bonding a pyrex coverslip over the silicon substrate. Minimum filtrate size is thus controlled by the depth h of the barrier etch and can be as slim as 0.1 μm. Flow resistance of the filter is minimized by winding the filtration barrier into a zig-zag. Effects observed during priming of the filters are described here and a model is presented which led to a filter design with vastly improved priming characteristics. The filter fulfills design specifications with respect to proper operation of the micropump, and successfully blocks the transport of 0.48 μm particles as visualized using blue fluorescent microbeads. The gap size achieved in practice was of 0.18 μm for a desired gap size of 0.2 μm. The filter design can also be used as a test structure for the exploration of liquid transport effects in submicron channels. Preliminary results show that in accordance with previous theoretical and experimental work [2,3], there is an increase in apparent viscosity for the flow of aqueous NaCl solutions through submicron-sized gaps.

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