An important challenge for the membrane community is to mimic natural protein channels that outperform, by orders of magnitude, man-made systems based on pore size and coarse chemical selectivity. To mimic protein channel pumping on a robust engineering membrane platforms, applied bias can be used to actuate charged gatekeepers and induce ionic pumping. Carbon nanotubes have three key attributes that make them of great interest for novel membrane applications 1) atomically flat graphite surface allows for ideal fluid slip boundary conditions and extremely fast flow rates [1,2] 2) the cutting process to open CNTs inherently places functional chemistry at CNT core entrance for chemical selectivity and 3) CNT are electrically conductive allowing for electrochemical reactions and application of electric fields gradients at CNT tips. The CNT membrane, with tips functionalized with charged molecules, is a nearly ideal platform to induce electro-osmotic flow with high charge density at pore entrance and a nearly frictionless surface for the propagation of plug flow [3,4]. Use of the electro-osmotic phenomenon for responsive/programmed transdermal drug delivery devices for nicotine addiction [5]. Electrophoresis and electroosmosis phenomena are discussed in detail through CNT conduits. Another approach is to mimic natural protein channel transport cycles with binding/transport/release/reset events. Porous alumina (AAO) membranes have top and bottom electrodes coated with thin Au layers with pore dimension tuned to match protein dimensions. At this thin layer at pore entrances, Ni-NTA is able to bind to hys-tag residue on target protein, as is commonly employed in chromatography. A binding voltage pulse attracts anionic target protein to top electrode and blocking the pore, while repelling the cationic imidazole release agent. The second voltage cycle attracts cationic release agent to top of membrane while pumping anionic target protein to bottom permeate and resetting the pumping cycle. The separation efficiency of 1cm2 of membrane was comparable to convention 1cm3 volumes used in chromatography [6]. Further modeling of system kinetics shows order of magnitude increase in throughput with sub-second cycle times enabled with high imidazole concentrations in a continuous cross flow operation.
1 “Nanoscale hydrodynamics: Enhanced flow in carbon nanotubes” Majumder, M.; Chopra, N.; Andrews, R; Hinds, B.J Nature 2005, 438, 44.
2 ‘Mass Transport through Carbon Nanotube Membranes in three different regimes: ionic diffusion, gas, and liquid flow’ Mainak Majumder, Nitin Chopra, B.J. Hinds ACS Nano 2011 5(5) 3867-3877
3 ‘Highly Efficient Electro-osmotic Flow through Functionalized Carbon Nanotubes Membrane’ Ji Wu, Karen Gerstandt, Mainak Majunder, B.J. Hinds, RCS Nanoscale 2011 3(8) 3321-28
4 “ Programmable transdermal drug delivery of nicotine using carbon nanotube membranes” J. Wu, K.S. Paudel, C.L. Strasinger, D. Hamell, Audra L. Stinchcomb, B. J. Hinds Proc. Nat. Acad. Sci. 2010 107(26) 11698-11702.
5 “Electrophoretically Induced Aqueous Flow through sub-Nanometer Single Walled Carbon Nanotube Membranes” Ji Wu, Karen Gerstandt, Hongbo Zhang, Jie Liu, and B. J. Hinds Nature Nano 2012 7(2) 133-39
6 “Dynamic Electrochemical Membranes for Continuous affinity protein separation”, Z. Chen, T. Chen, X. Sun and B.J. Hinds. Advanced Functional Materials 2014 24(27) 4317-23
