84 CHAPTER 3 Nanomorphic electronics Note that for dimensional scaling with barrier height/operational energy held constant, the switching time will remain constant, i.e. devices using heavier particles should not be inferior to electron-based devices. Several recent demonstrations indeed suggest the possibility of physical realizations for a sub-5 nm binary switch. The atomic-scale switch reported in [5, 23], for example, opens or closes an electrical circuit by the controlled reconfiguration of silver atoms within an atomic-scale junction. Such `atomic relays' operate at room temperature and the only moveable parts of the switch are the contacting atoms, which open and close a nm-scale gap. Experimentally, a critical device size (gap) of 1 nm was reported [23]. The atomic relay operates at a relatively low voltage of 0.6 V. The experimentally measured switching time was 1 m s, though the authors projected the switching time for optimized devices will be in the range of 1 ns [23]. Moving atoms/ions also plays a key role in the mechanism for the operation of a recently reported `memristor', utilizing TiO 2 thin films, where the switching occurs due to ionic motion of oxygen vacancies [8, 24, 25]. Memristor-type devices may have a potential for extreme scaling. Also, some researchers believe that memristors could offer a new fundamental circuit element, which could allow for realization of complex functions with lower device count. The latter is very important for nano- morphic cell applications, where volume is one of the primary concerns. The model of memristive behavior has recently been proposed as a possible mechanism in the adaptive behavior of unicellular