1. An apparatus including a memristor, the memristor comprising: a first electrode; a second electrode spaced apart from the first electrode; and a first active material and a second active material in contact with the first active material disposed between the first and second electrodes, each of the active materials characterized by a respective change in electrical resistance in response to a change in oxygen ion content, the memristor characterized by a non-volatile electrical resistance that is adjustable by way of programming voltages, the first active material decreasing in electrical resistance with decreasing oxygen ion content, the second active material decreasing in electrical resistance with increasing oxygen ion content.