Chemical and Engineering News surveys work aimed at replicating the ability of lower animals to regenerate whole limbs and organs: "complicating the attempt to unravel regeneration is the fact that these capabilities change over the lifetime of a single organism. A tadpole, for example, can generally replace a missing tail or limb but loses this ability after its metamorphosis into a frog. Likewise, higher vertebrates such as mammals can regenerate much better during their embryonic and fetal stages than after they have become adults. ... Levin and his collaborator David Kaplan [are] now testing whether a replicated amniotic environment can promote regeneration in adult mammals. Kaplan has already developed a small, cylindrical 'regenerative sleeve' that can be filled with an aqueous solution and fastened onto the stump of a rat's amputated limb. The sleeve is fitted with a variety of ports and electrical connections so the researchers can sample and alter the container's chemical contents ... the researchers hope to create a regenerative current at the stump's surface by adjusting the ionic composition of the solution inside the sleeve and by adding drugs that open or close ion channels in the membranes of the cells at the wound site. The sleeve will offer some additional benefits. The aqueous environment it provides will prevent the scarring that normally develops in a mammalian wound exposed to air. The researchers might also use it to bathe the wound with scar-reducing compounds, immune-modulating drugs, and more traditional growth factors."