Towards mimicking natural protein channels with aligned carbon nanotube membranes for active drug delivery

Abstract

Aims: Carbon nanotube (CNT) membranes offer an exciting opportunity to mimic natural protein channels due to 1) a mechanism of dramatically enhanced fluid flow 2) ability to place 'gatekeeper' chemistry at the entrance to pores 3) the ability for biochemical reactions to occur on gatekeeper molecules and 4) an ability to chemically functionalize each side of the membrane independently.

Main methods: Aligned CNT membranes were fabricated and CNT pore entrances modified with gatekeeper chemistry. Pressure driven fluid flow and diffusion experiments were performed to study the mechanisms of transport through CNTs.

Key findings: The transport mechanism through CNT membranes is primarily 1) ionic diffusion near bulk expectation 2) gas flow enhanced 1-2 orders of magnitude primarily due to specular reflection 3) fluid flow 4-5 orders of magnitude faster than conventional materials due to a nearly ideal slip-boundary interface. The transport can be modulated by 'gatekeeper' chemistry at the pore entrance using steric hindrance, electrostatic attraction/repulsion, or biochemical state. The conformation of charged tethered molecules can be modulated by applied bias setting the stage for programmable drug release devices.

Significance: The membrane structure is mechanically far more robust than lipid bilayer films, allowing for large-scale chemical separations, delivery or sensing based on the principles of protein channels. The performance of protein channels is several orders of magnitude faster than conventional membrane materials. The fundamental requirements of mimicking protein channels are present in the CNT membrane system.

Fig. 1

CNT membrane microstructure. (a) Cross sectional SEM image of aligned polymer film with CNTs aligned across film thickness. Shown is cleaved film (lower bright area is bottom membrane surface) with aligned CNTs slightly pulled out from surface. The polymer matrix is polystyrene. (b) Schematic of target membrane structure. With a polymer impregnation between CNTs, a viable membrane structure can be readily produced with the pore being the rigid inner tube diameter of the CNT.

Fig. 2

Processing steps involved in aligned CNT membrane fabrication process.

Fig. 3

Separation factor of MV/Ru as a function of bias applied to CNT membrane with flow-grafted charged gatekeeper functionality (below).

Fig. 4

Nicotene flux through skin/patch (medifilm) assembly with and without CNT membrane barrier layer.