Sad sack or serious salvation? A small group of scientists is angling to replace laboratory animals with living “organ sacks” grown from human cells. R3 Bio, a billionaire-backed biotech startup, is committed to designing “complete organ systems” to be used in the research and development of drugs and potentially, as a source of tissues and organs for humans. Based in San Francisco, R3 aims to eliminate animal suffering by replacing the use of lab animals with these systems. The company takes