Ex vivo rheology of spider silk N. Kojic (1,2), J. Bico (1), C. Clasen (3) & G.H. McKinley (1) (1) Hatsopoulos Microfluids Laboratory, Department of Mechanical Engineering, MIT, Cambridge, Massachusetts 02139, USA. (2) Harvard-MIT Division of Health Sciences and Technology,Cambridge, Massachusetts 02139, USA. (3) Institut für Technishe und Macromolekulare Chemie, 20146 Hamburg, Germany. (3) Abstract: The spinning process developed by spiders over millions of years provides a great example of a natural microfluidic system and remains a considerable source of questions1-3. In order to probe the rheological properties of minute amounts of the spinning material extracted ex vivo from the major ampullate gland of a Nephila clavipes spiderwe have developed two new micro-rheometric devices4-6. The present study shows that the spinning liquid is a complex viscoelastic fluid. Its shear viscosity (resistance to flow) decreases ten-fold as it is pushed through the narrow spinning canals of the spider, whereas its extensional viscosity (resistance to stretching) increases more than one hundred-fold during the spinning process. Quantifying the properties of native spinning solutions provides new guidance for adjusting the spinning processes of synthetic or genetically-engineered silks to match those of the spider.