Water flow across NTs has been extensively investigated theoretically. In their molecular dynamics simulations, Hummer et al observe spontaneous and continuous filling of a CNT 0.8 nm in diameter with a single-file chain of water molecules, illustrated in Figure 3. In the absence of an applied pressure, and with no ions present, water is found to flow in both directions at a rate of water molecules per nanosecond. Moreover, the filling or emptying of these NTs occurs by sequential adding or removing of water molecules to or from the single-file hydrogenbonded chain, and this is shown to occur rapidly.39 As a comparison, aquaporin-1 conducts at a rate of 3 water molecules per nanosecond. This initial finding, that a CNT is populated with a single-file chain of water molecules, has been confirmed in ab initio simulations. Mann and Halls observe a single-file chain of water molecules inside a CNT 0.8 nm in diameter. In contrast to the biological water channel aquaporin-1, proton conduction can occur across CNTs 0.8 nm in diameter under the influence of an applied electric field.41 Similar results have been obtained by Garate et al. Aquaporin-1 allows the rapid flow of water with no proton transport, which is postulated to result from the transient reversal of the water chain due to residues lining the pore.