A Scalable, Hierarchical Rib Design for Larger-Area, Higher-Porosity Nanoporous Membranes for the Implantable Bio-Artificial Kidney

Silicon nanoporous membranes provide the fundamental underlying technology for the development of an implantable bio-artificial kidney. These membranes, which are comprised of micromachined slit-pores that are nominally 10 nm wide, allow for high-efficiency blood filtration as well as immunoprotection for encapsulated cells. Our approach takes advantage of well-established semiconductor fabrication technology to give us precise dimensional control over pore widths, thereby enabling a highly selective filtration function and a clear path towards further miniaturization. This work builds on our prior results on "ribbed nanoporous membranes" by adding a second-level hierarchy of significantly taller "mega-ribs" to further strengthen the membranes. Relying on a two-step Deep Reactive Ion Etch (DRIE) process, we etch 4 μm-deep as well as 40 μm-deep trenches into a silicon substrate, grow a thermal oxide liner, and deposit a layer of polysilicon into this "mold" to form membranes which, when released after a backside DRIE etch, feature a network of reinforcing ribs on the underside. We have fabricated and tested freestanding membrane spans that are up to 14 times wider than before, with approximately double the measured permeability per unit area. The new architecture can also improve cross-membrane mass-transfer rates and reduce chip-fabrication costs.