Ultem, or PEI (Poly Ether Imide) is a lightweight, heat-resistant material widely used in medical devices that is a great choice for an eyewear material. It has excellent resilience due to its strong flexibility and elasticity. Some of PEI's characteristics that make it a great material for glasses is its ultralight weight, high temperature tolerability, and superior resistance to stress and chemicals. On top of this, PEI is a material that maintains a high level of sterility.