 Moreover, membranes made out of carbon nanotube arrays have been suggested as switchable molecular sieves, with sieving and permeation features that can be dynamically activated/deactivated by either pore size distribution (passive control) or external electrostatic fields (active control).Carbon nanotubes have been implemented in nanoelectromechanical systems, including mechanical memory elements (NRAM being developed by Nantero Inc.) and nanoscale electric motors (see Nanomotor or Nanotube nanomotor).Carboxyl-modified single-walled carbon nanotubes (so called zig-zag, armchair type) can act as sensors of atoms and ions of alkali metals Na, Li, K. In May 2005, Nanomix Inc. placed on the market a hydrogen sensor that integrated carbon nanotubes on a silicon platform.