UP to Material Notes
2 Part Urethane rubbers start off as syrup consistency liquids and cure to amazingly tough rubbery solids ranging from skin-soft to heel-hard.  Check out Smooth-On.com for materials.

High Abrasion Strength urethane rubber surfaces can survive a hard life of contacting other things, e.g. skateboard wheels. 

High Tear Strength meaning that tears won't easily "spread".  To better understand this, imagine the following scenario:  a paper thin sheet is held stretched in a frame and a cut is made in the middle with a razor blade.  With urethane, the cut wouldn't grow on its own.  With silicone rubber, on the other hand, the cut would quickly zip across the whole sheet.

Unified Casting of Different Hardnesses.  Most urethane rubbers bond well to each other.  This allows the creation of objects with different hardnesses in different zones.  Such a capability opens the way for new thinking in design.  

Easy to work with.  Urethane rubbers mix easily, and if bubble-free castings are desired can readily be de-aired under vacuum.
Robot Finger Tip, 2008, C. Pisaturo.  Steel parts are well anchored into harder urethane rubber, which is abutted by a softer "pad zone"  (border between hard and soft zones is visible).
works of  Carl C Pisaturo