Safe and readily available blood substitutes remain a critical unmet need in emergency medicine, trauma care and regions with limited access to blood donation. Inspired by the oxygen-carrying proteins found in nature, we are developing fully synthetic oxygen carriers based on metal–peptide complexes. Using computer-aided design, we create cyclic peptides that hold a metal center in a stable, precisely engineered environment. The goal is a carrier that binds oxygen under physiological conditions and releases it where it is needed most: in oxygen-deprived tissues. Unlike donated blood, such carriers require no blood-type matching, carry no risk of infection, and can be stored long-term and produced at scale. This approach could pave the way for a new generation of artificial blood and expand access to life-saving oxygen therapy worldwide.