Mechanical Properties of Collagen Fibrils

1
EDST - UL
Ecole Doctorale des Sciences
et de Technologie
Université Libanaise
Nanotechnologie et Nano-Biomécanique des
Systèmes Biologiques Complexes
Submitted to Dr. Karim EL KIRAT
February 19th, 2016
Sarah Hussein Master TIS

A G E N D A
Article 1
§  Authors: Marco P. E. Wenger
Laurent Bozec
Michael A. Horton
Patrick Mesquidaz
§  Journal: Biophysical Journal
Volume 93
August 2007
Article 2
§  Authors: Colin A. Grant
David J. Brockwell
Sheena E. Radford
Neil H. Thomson
Volume 97
December 2009
“Mechanical Properties of
Collagen Fibrils”
“Tuning the Elastic Modulus of
Hydrated Collagen Fibrils”
2

A G E N D A
Article 1
Laurent Bozec
Michael A. Horton
Patrick Mesquidaz
Volume 93
August 2007
Article 2
David J. Brockwell
Sheena E. Radford
Neil H. Thomson
Volume 97
December 2009
Collagen Fibrils”
3

O U T L I N E S
INTRODUCTION METHODOLOGIES RESULTS CONCLUSIONS
1 2 3 4
4

INTRODUCTION
¨  Context
¨  Purpose
1
5

I N T R O D U C T I O N
Context
q  Collagen is a very abundant structural protein in mammals
q  It is a major component of connecting tissue, skin, bone, cartilage and tendons
q  The development of collagen model is important
q  There is a lack of convincing and comprehensive structural model for fibrils
q  The biological function of collagen lies in its mechanical properties
q  There is a necessity to determine these mechanical properties at different scales
6

Context (Cont.)
q  These information are important to explain the macroscopic biophysics of
different tissues
q  In addition, it contribute to the understanding of the microscopic structure of
collagen fibrils
q  Conventional macroscopic technical tools are not applicable to Nano fibrils
q  Other techniques are used, the Nano scale indentation is one such a technique
7

Purpose
“To determine the mechanical properties of
individual type I collagen fibrils of rat tail tendon in
air & at room temperature”
“The Young’s modulus is determined
quantitatively using sharp AFM tip in combination
with the Olivier and Pharr indentation model”
8

MATERIALS & METHODS
¨  Sample preparation
¨  AFM Imaging
¨  Nano-Indentation by AFM2
9

M A T E R I A L S & M E T H O D S
Sample Preparation
q  Type I collagen fibrils extracted from rat tail tendons and stored at 4°C
q  The sample is sectioned with a scalpel and washed in deionized water
q  A microscope glass slide was used as sample substrate
q  It was cleaned with DI water in an ultrasonic bath and rinsed with ethanol and
dried in a stream of nitrogen
q  A few samples deposited on the glass slide and smeared out using tweezers
q  The sample was then dried in a gentle stream of nitrogen
10

AFM Imaging
q  AFM imaging and indentation of
collagen fibrils using a Nanowizard AFM
q  All measurements were taken in air and
at room temperature
q  Aluminum-coated, silicon AFM tips of
150 kHz resonance frequency and 4.5
N/m nominal spring constant is used
q  Cantilever chosen to match stiffness of
collagen for optimizing sensitivity & SNR
11

Nano Indentation By AFM
12

Nano Indentation By AFM (Cont.)
13

RESULTS
¨  Young’s Modulus
¨  Anisotropy of Collagen Fibrils
¨  Fibrils Dehydration & Tip Shape
¨  Accuracy of Reduced modulus3
14

R E S U L T S
Anisotropy of Collagen Fibrils
q  Two fibrils used from the same sample,
similar in diameter, and aligned
perpendicular to each other
q  The same AFM tip used for experiments
and its orientation was not changed
q  Reduced modulus was found to be in
the range from 5 GPa to 11.5 Gpa
q  Non-uniform shape of the imprints
indicates different material properties in
longitudinal and transversal directions
15

R E S U L T S
Fibrils Dehydration & Tip Shape
q  Tendency of the Er to increase slightly
from 6 GPa to 7.5 GPa over three days
q  The indentation depth decreases from
3.5 nm to 2.5 nm
q  Dehydration of collagen fibrils
contributes with 30% to the broad
distribution of fibril moduli
q  Tip shape by AFM imaging revealed a
spherical tip apex
16

R E S U L T S
Accuracy of The Reduced Modulus Determination
q  The accuracy of reduced modulus depends on the experimental accuracy of the
determination of the contact area (A) and of the fibril stiffness (SF)
q  Stiffness is directly proportional to the cantilever spring constant: 5% error
q  For contact area, AFM tip radius and indentation depth is considered : 20% for
the radius determination and 10% for the indentation depth à 30%
q  Another possible source of error of contact area are attractive interfacial forces
between AFM tip and sample surface
q  The effects discussed above are assumed to be less relevant in the present work
and were not taken into account
17

CONCLUSIONS
¨  Brief Summary
4
18

C O N C L U S I O N S
Brief Summary
q  The great advantage of Nano-Indentation is the small size of the indenter
q  Using an indenter with tip apex smaller than the collagen fibril diameter,
indentation caused small imprints
q  The non-uniform shape of these imprints indicates an anisotropic material
structure of collagen fibrils
q  This anisotropy can be neglected as its contribution to the overall experimental
error is smaller than the variation of the stiffness upon fibril dehydration
19

A G E N D A
Article 1
Laurent Bozec
Michael A. Horton
Patrick Mesquidaz
Volume 93
August 2007
Article 2
David J. Brockwell
Sheena E. Radford
Neil H. Thomson
Volume 97
December 2009
Collagen Fibrils”
20

O U T L I N E S
INTRODUCTION METHODOLOGIES RESULTS CONCLUSIONS
1 2 3 4
21

INTRODUCTION
¨  Context
¨  Purpose
1
22

Context
q  Huge importance of collagen fibrils
q  Collagen has a structure of triple helix of left-handed polypeptide chains coming
together to form right-handed twisted molecule (tropocollagen)
q  The generalized well-known formula for collagen amino acid sequence is Gly-X-
Y, where X is proline (Pro) and Y a hydroxyproline (Hyp) residue
q  Glycine, which occurs at every third residue, is found at the center of the coiled
peptide chain to allow a close packing of the triple helix
23

Context (Cont.)
q  Charged residues make up ~15–20% of residues in tropocollagen, and ~40% of
Gly-X-Y sequences contain at least one charged residue
q  Electrostatic interactions are of great importance for the stability of the triple-
helical conformation with tropocollagen
q  Studies have shown that hydrated collagen fibrils under bulk aqueous solution
conditions have a reduction in modulus compared to the anhydrates
q  Measured hydrated shear modulus changes very little when a cross-linker is
applied to collagen fibrils
24

Purpose
“To highlights the range of elastic response of
collagen type I fibrils that can be achieved in liquid by
altering the environment”
&
“To discuss the finding that the mechanical properties
of hydrated collagenous fibrils can be tuned by
adding salts, by changing the solution pH, or by
changing the solvent”
25

MATERIALS & METHODS
¨  AFM & Nano-Indentation
2
26

AFM & Nano-Indentation
q  AFM imaging and force measurements were made using silicon nitride
cantilevers with integral tips with spring constants of the order k ~ 0.3 N/m
q  Force volume (FV) imaging was carried out on isolated fibrils using arrays of 50
50 pixels, with each pixel representing a single force-distance measurement
q  The built-in software is used to calculate both the cantilever spring constant and
the reduced elastic modulus (Er), which uses a Hertzian contact theory
27

RESULTS
¨  Effect of Fibril Swelling in
Aqueous buffer
¨  Effect of Salt
Concentration
¨  Effect of Altering The
Cation Species
¨  Effect of Lowering pH
¨  Effect of Ethanol
3
28

R E S U L T S
Effect of Fibril Swelling in Aqueous buffer
q  Measurements of the same collagen fibrils
demonstrate that stable AFM imaging can be
achieved in liquids
q  Swelling occurs in the hydrated form but does
not change significantly among different
conditions of salt and pH
q  Collagen fibril mechanics are influenced by the
liquid phase of this biocomposite material
29

R E S U L T S
Effect of Salt Concentration
q  NaCl was added to the 100 mM sodium
phosphate buffer to concentrations up to 1 M
q  Modulus value showed no appreciable
change up to 500 mM NaCl, but 2.3 times
increase in modulus detected at 1 M NaCl
q  Imaging of the fibril in 1 M NaCl showed no
noticeable change in the morphology
30

R E S U L T S
Effect of Altering The Cation Species
q  Various monovalent chloride salt solutions
were used at concentrations up to 1 M
q  Cation species does not influence final value
of elastic modulus at 1 M monovalent
chloride salt
q  Increase in modulus at higher salt
concentration appears to be related to ionic
strength of solution surrounding collagen
31

R E S U L T S
Effect of Lowering pH
q  Effect on modulus of adding salt (1 M KCl)
greatly enhanced when the pH was lowered
q  The combined effects of lowering pH to 5
and increasing salt concentration to 1 M
raises the modulus by 10-fold compared
with its value at pH 7 with no salt
q  Increasing the ionic strength gradually
through the use of two different potassium
acetate buffers and and increasing amounts
of KCl, gave a gradual increase in the
measured modulus
32

R E S U L T S
Effect of Ethanol
q  Freshly prepared collagen fibril were placed
under an increasing concentration of ethanol
in 100 mM sodium phosphate
q  Modulus steadily increases with increasing
ethanol concentration up to 50%
q  Large increase in modulus was found when
scanning medium increased to 100% ethanol
q  Indentation depths measured from the FV
analysis in 100% ethanol are lower than
those measured in buffer
33

CONCLUSIONS
¨  Brief Summary
4
34

C O N C L U S I O N S
Brief Summary
q  When salt concentration or ionic strength is increased, hydration or solvation forces
dominate the response
q  When the pH is lowered, ion pair interactions would seem the most likely to
dominate
q  It is also conceivable that hydrophobic effects play a role in all these scenarios but
likely at a lower magnitude than the hydrophilic forces described in each case
q  A better understanding of which forces modulate the elastic response when one
parameter is changed should aid rational design of new materials based on collagen
and analogous synthetic peptides
q  All these effects appear to be fully reversible, which may indicate that it is possible
to modulate tissue elasticity in vivo by directed therapeutics, whether the tissue in
question is natural or bioengineered implants
35

Mechanical Properties of Collagen Fibrils

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