[1] MIT, Dept Mech Engn, Cambridge, MA 02139 USA
[2] Brigham & Womens Hosp, Harvard Med Sch, Div Gastroenterol Hepatol & Endoscopy, Boston, MA 02115 USA
[3] MIT, David H Koch Inst Integrat Canc Res, Cambridge, MA 02139 USA
[4] MIT, Div Comparat Med, Cambridge, MA 02139 USA