Eric Drexler indicates in a new post that stiffness matters for nanotechnology.
Drexler also continues his discussion on stiffness of materials and different proteins at e-drexler.com
Drexler also has an article about molecular assembly lines and getting away with less programmability. In the comments, Chris Phoenix notes that it is easy to add a useful level of programmability and flexibility.
A 1996 paper that discusses the structural strength of meat.