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Note: the numbers
in square brackets refer (and link) to the List of
Publications
Mechanical Strength of Nanofibres
Method
Development
Mechanical measurements of properties such as
elasticity, Young’s modulus and bending, shear and rupture behaviour are
routinely measured on biological and man-made fibres on the macro- and
micro-scale. However, the existing methods cannot be easily extended to the
investigation of the nanofibrils of which
biological fibres often are composed. Investigation of these smallest
units, on the other hand, would provide important insight into the
molecular mechanism of the fibres’ phenomenal mechanical properties, which
in turn might help with the development of methods for modification of
these properties. Here, we are developing methods which will progress such
mechanical measurements to samples on the nanometer
size scale, using an atomic force microscope (AFM).
For this purpose, nanofibrils
are deposited on a surface with a repeating pattern with dimensions on the
order of few 100 nm. Individual fibrils are localised and visualised using
the AFM. The AFM tip then is used to push down on a fibril hanging over a
hole/trough or pull the fibril sideways along a trough while recording the
forces exerted on the fibril as it is being stretched. Performing such
measurements over a range of forces allows the determination of the
elastic Young’s modulus as well as the observation of non-elastic extension
and ultimately the measurement of the rupture force of protein fibrils
with diameters as small as a few nanometers.
 
Mechanical
Properties of Collagen Nanofibrils
Collagen is the most prevalent structural material
used in biological organisms. It consists of long proteins which aggregate
into linear trimers (tropocollagen),
adopting a triple helix structure where the three proteins are twisted
around each other. These units then self-assemble into nanofibrils,
which in turn aggregate to form larger objects. The mechanical properties
of collagen fibres with diameters on the order of 100 nm have been reported
previously. Although much smaller (nano-)fibrils (with a diameter of only 5 nm) can be prepared
from the same material, it had not been possible to investigate their
mechanical properties until now.
Our Young’s modulus measurements on dry
collagen nanofibrils yield a value of approx. 15 GPa [Abstract15].
This is significantly larger than the Young’s modulus of 5-7 GPa which has been measured on collagen fibres with
diameters on the order of 100 nm, indicating that the major contribution to
the elasticity of these larger fibres arises from slippage of bundles of tropocollagen units relative to each other, which most
likely arises from the limited cross-linking between these units.
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