Formalin did not work for us because it doesn't physically crosslink the collagen. Gluteraldehyde does crosslink the collagen and will likely work better to maintain the deformed state, i believe.

We did this previously for aortic valve leaflets. We stretched them to 10 or 20% in a bioreactor and fixed overnight with gluteraldehyde. Worked really well and the TEM turned out nice in order to visualize the cell and nuclear deformation. The images can be seen in Figure 4 in Journal of Biomechanics 43 (2010) pp. 87-92.

Occlude the vessel on both sides of the region to be studied. Inflate the vessel in the region to be studied with PBS at the desired pressure. Immerse the specimen in the some suitable fixative (10% formalin should work) that will maintain the tissue in the deformed state and monitor the pressure during the fixation period. When the pressure no longer changes, prepare the tissue for study. Some stress relaxation, ie, return of the tissue toward its unstressed state state, is to be expected, but the direction of the deformation induced in the deformed vessel can still be studied.

Formalin did not work for us because it doesn't physically crosslink the collagen. Gluteraldehyde does crosslink the collagen and will likely work better to maintain the deformed state, i believe.

Thanks for the interesting responses, I've tried something similar to the method proposed by Robert, however the results were not satisfactory. As David stated, the crosslinks are not well preserved with formalin.

By the way, I'm extremely interested in quantification of the collagen fibers within soft bio tissues.

I've started a new research on the tendons and ligaments and trying to identify the interaction of the collagen fibers with mineral molecules in the interface of the these soft tissues with the bone. any suggestions?