This led to an exchange of letters in the ACS publication Chemical & Engineering News in 2003.[29] Though biology clearly demonstrates that molecular machine systems are possible, non-biological molecular machines are today only in their infancy. Leaders in research on non-biological molecular machines are Dr. Alex Zettl and his colleagues at Lawrence Berkeley Laboratories and UC Berkeley. They have constructed at least three distinct molecular devices whose motion is controlled from the desktop with changing voltage: a nanotube nanomotor, a molecular actuator,[30] and a nanoelectromechanical relaxation oscillator.[31] See nanotube nanomotor for more examples.