Determining the tensile modulus of collagen fibrils by bending on a flexible substrate.

Session

Poster Session Two

Authors

Miss Holly Barter (1), Zhen Bai (1), Ellen Oudkerk-Sodia (1), Emilie Gachon (1), Dr Patrick Mesquida (1)

Affiliations

1. King's College London

Abstract text

Collagen fibrils constitute the most important and basic building-blocks of the extracellular matrix, where they provide mechanical strength and structural integrity to biological tissues. Hence, we need experimental methods to determine their mechanical properties, such as their tensile, elastic modulus (Young’s modulus). This is challenging because of the typical diameter of collagen fibrils of only a few tens to a few hundreds of nanometers. An approach that has been taken for many years is nanoindentation with an Atomic Force Microscope (AFM) tip, where fibrils are deposited on a hard surface and loaded perpendicularly to their axis by the tip. However, this does not constitute a uniaxial mechanical test and determining moduli in such a way relies on a number of assumptions. The result is, therefore, influenced by a superposition of several, often unknown factors, such as the exact shape of the tip-fibril contact area, etc.  

In the present study, we replaced the hard surface on which the fibrils are deposited with a highly flexible substrate of polydimethylsiloxane (PDMS). PDMS has an elastic modulus which is orders of magnitude lower than that of collagen fibrils. This allows us to completely change the mechanical test geometry from a complex indentation deformation to a much simpler bending deformation. When loaded vertically with the AFM tip, the fibrils are now behaving as a Winkler’s beam on an elastic foundation. Such a behaviour can be described by a simple, analytical model, which only requires easily accessible parameters such as the fibril diameter or the Young’s modulus of PDMS. It can be used to determine the beam’s tensile modulus without resorting to guesswork.  

  

A caveat is, however, that the AFM only provides a force-distance curve as the tip loads the fibril. We cannot see the actual deformation during the test. It is conceivable that the tip still indents the collagen fibril to a certain degree while, at the same time, bending the fibril into the PDMS. In order to assess how much of the deformation is indentation and how much is bending we performed finite element simulations of the test and adjusted the input parameters (elastic moduli, geometry) so that the simulations gave the same force curves as the experiments. With this approach, we could determine that the amount of indentation is negligible compared to the amount of bending within the inevitable uncertainty of the experiments. 

As a result, we applied the analytical Winkler formula to experimentally determine the tensile moduli of collagen fibrils and obtained values that are corroborated by previously determined data using other methods. A great, practical advantage of the Winkler method is that no special hardware nor gripping of the fibrils is needed. Force curves can be performed with any commercial AFM and the collagen fibrils can simply be deposited manually on a PDMS film, which is easy and inexpensive to produce.