Singularity Hub here looks at some of the research work that will lead to the ability to generate blood as needed: "Researchers [have] found a way to hunt down and isolate the stem cells from which your entire blood supply is derived. Until now, these hematopoietic stem cells (HSC) have been remarkably hard to track and isolate ... researchers were able to identify the CD49f protein as a key surface marker for hemotopoietic stem cells. Single CD49f HSCs were placed inside immunosupressed mice, and monitored to see how they developed. The entire spectrum of blood cells were produced, and just as important: they were self-renewing. The CD49f HSC wasn't just creating blood, it was creating an expanding and sustaining blood supply that should theoretically survive long term in the body." This will lead to a number of potential ways to generate sufficient quantities of blood to remove the need for blood donations, and ultimately will allow a patient's own cells to be used to generate blood on demand.

Link: http://singularityhub.com/2011/08/03/scientists-bag-and-tag-the-stem-cell-that-may-create-an-endless-supply-of-blood/