Someday, doctors would like to grow limbs and other body tissue for soldiers who have lost arms in battle, children who need a new heart or liver, and many other people with critical needs. Today, medical professionals can graft cells from a patient, deposit them onto a tissue scaffold, and insert the scaffold into the body to encourage the growth of bone, cartilage, and other specialized tissue. But researchers are still working toward building complex organs that can be implanted into patients.