Thank you.

It states that:

"The large variation of Er was mainly caused by three factors:

1. Different individual fibrils.

2. Different dehydration state of the fibrils.

3. Uncertainties of the indentation process, mainly the contact area determination. "

However, the Young's modulus report in the article and in the table presenting the previous measurements) are in GPa range!

Dear Hanie Kavand,

The Young's modulus of collagen fibrils varies a lot in the literature  as there is no consensus about the transverse (indentation/squeezing the fibrils) or longitudinal (tensile/3 point bending). Besides this,  a lot of publications have used models that do not reflect the nature of collagen and therefore this results in a  greater distribution of reported values. 

The first answer quoted one of our earlier paper, but since we have refined our approach . Provided that one uses the correct model/stiffness of cantilevers/sample preparation, we have found that the two main variables are : hydration (previously stated) but also the level of cross-linking of the fibrils. In fact, it is the latter that plays the most important role regardless of any other parameters. We will be soon share our  evidence for this. Can a collagen fibril have a modulus in the GPa range ? - have you tried to bend a dehydrated collagen fibrils ? - it breaks like glass. Hydrate the same fibril and it turns all floppy. If you add a variable level of cross-links to this and non-accurate modelling  - there you have your answer for the huge variations in moduli reported.

If you have any specific questions about the mechanical properties of collagen do not hesitate to contact me directly - [email protected]