Atomic number: 49
Used in: touchscreens, solar cells, medicine
Criticality rating: critical
We spend a lot of time looking at indium, yet rarely see it. The alloy indium tin oxide (ITO) possesses the rare combination of being both electrically conductive and optically transparent. That makes it essential for flat screen displays and televisions, where it forms the see-through front electrode controlling each pixel. A layer of ITO on a smartphone's screen gives it touch-sensitive conductivity, enabling the device to detect taps, swipes and pinches.
When mixed with other metals, indium loses its transparency and instead becomes a light-collector. Alongside cadmium telluride cells, solar cells made of copper, indium and selenium, sometimes with a sprinkle of gallium, are starting to challenge silicon's pre-eminence.
And that's not all. When medics need to follow what happens to blood platelets or neutrophils they label them with 111 Indium (a slightly radioactive form) and follow them with a gamma camera. In the same way they can image the pancreas, a very difficult organ to visualize.