Crosslinked networks of stiff filaments exhibit negative normal stress
  Motivated by recent experiments showing that stiff biopolymer gels exhibit highly unusual negative normal elastic stresses, we develop a computational model for stiff polymer networks subject to large strains. In all cases, we find that such networks develop normal stresses that are both negative and of magnitude comparable to the corresponding shear stress. We find that these normal stresses coincide with other nonlinearities in our networks, and specifically with compressive bucking of the individual filaments. Our results suggest that negative normal stresses are a characteristic feature of stiff (bio)polymer gels that have been shown to exhibit strong nonlinear elastic properties. 
Enrico Conti and F.C. MacKintosh