Scarlet Therapeutics Ltd. has demonstrated its manufactured red blood cells (RBCs) act in the same way as their natural counterparts in vivo, opening the way for the cells to be used as drug delivery vehicles and raising the possibility they could replace conventional blood transfusions. To build on this, Scarlet has closed a $4 million seed round to work on the first clinical application, in which RBCs loaded with therapeutic proteins will be used to treat rare metabolic diseases.