Size-Selective Mass Transport Via a Block Polymer / Micromachined Silicon Composite Membrane

One challenge in implantable chemical sensing is blocking immunoproteins and other fouling agents from entering the device while simultaneously facilitating fast diffusion of target solute into the device. As the immunoproteins are typically much larger than the target solute, one strategy is size exclusion, but this requires short, straight, densely-packed pores tens of nanometers across to simultaneously block large molecules while offering a short, wide cross-section for solute transport.

By integrating a triblock polymer film with a microfabricated silicon support, we have constructed an asymmetric membrane in which the polymer forms a size-selective layer less than 100 nm thick with densely-packed, uniform 40 nm pores. This polymer film is supported by 100 um thick silicon with uniform 20 um wide pores, providing mechanical support while also keeping a short, wide path for solute transport. Diaphragm cell studies were performed to establish the size selectivity of the membrane for two model solutes, methyl orange (MW = 367 Da) and blue dextran (MW = 2 MDa). The fifty-fold favoring of the smaller solute confirms the membrane's size selectivity, while the microfabricated support provides a convenient handle for incorporating the membrane into MEMS-style sensors and drug-delivery systems.