Sensor

A sensor comprising:

• a plurality of sensor elements arranged in an array;
• wherein each of the sensor elements is a memristor that has an electrical resistance characteristic related to exposure of the memristor to a species to be sensed;
• wherein the array comprises at least one memristor group;
• wherein each said memristor group comprises four of the memristors, wherein the four memristors are connected as a first pair of two memristors in series, and a second pair of two memristors in series, and with the first pair and the second pair connected in parallel between first and second connection points;
• and
• wherein a resistance characteristic of the array between the first and second connection points is related to exposure of the memristors in the array to the species to be sensed.