This is a presentation I and my group made for NSMS 510.

1. Mechanical Principles of Biological
Nanocomposites
Greg Orlicz  Introduction / Fracture Mechanics
George Keller  Mechanics employed by nature / Role of “nano”
Anthony Salvagno  Biological examples / Hierarchy in nature
Jingshu Zhu  Characterization / Fabrication of synthetic materials

2. What can we learn from biological materials?
• Biological materials (e.g. bone, tooth, shell, and wood)
-- excellent mechanical properties: strength, toughness,
and resistance to fracture
• Much is attributed to nanostructure
• Want to understand what factors attribute to strength
and toughness by understanding the mechanical forces
that result due to the structure
• Perhaps we can synthesize materials to meet and
surpass the same robustness of biological materials
(biomimicking)
• Emphasis is material strength and resistance to fracture
– fracture mechanics

3. Setting the foundation for fracture mechanics…
F
A
ΔL
L0
Stress: σ = F/A
related by Hooke’s Law: σ = E ε
Strain: ε = ΔL/L0 Young’s Modulus (Stiffness): F L0
E = Tensile Stress/Tensile Strain =
A L
Analog to spring force: Fs= -kx
Strain energy – energy stored in plate like a spring during elastic deformation
(can return to same position) (like pushing fist into car door)
* When material is unloaded, the strain energy can do work

4. What is plasticity?
• Plastic deformation - if stress is high enough, material is strained beyond elastic approximation
- internally damaged and deformation is irrecoverable
(like punching car door – denting)
- linear relationship between stress and strain is lost
• Stress-Strain relationship determined by tensile test
• Yield Stress = stress under which material no longer deforms elastically
(deforms plastically – irrecoverable)
• Ultimate Tensile Strength = max stress that material can withstand
Extensometer

5. Toughness vs. Strength
Strength – how much stress a material can support without failure
(usually defined as σUTS = max stress on stress-strain curve)
Toughness – amount of energy per unit volume a material can
absorb before rupturing energy
f
toughness d
volume 0
σ
ε

6. A very close look at elastic fracture
Bond energy:
Approximate force as half of sine wave:
For small displacements:
k is analogous to spring constant:
Pc
Considering all bonds per unit area, Pc E
 σc; k  E (Young’s modulus) :
Surface energy – energy required to break a
plane of atomic bonds to create two free
surfaces (energy per surface area):
Arrive at critical stress:

7. Cracks and defects in a material magnify the local stress
da Vinci – strength of iron wires varied inversely to the wire length -- flaws
Stress concentration at crack tip – stress is higher at crack tip than externally applied stress
Stress intensity factor k = σA/σ
Recall:
Location, A
Highest local stress
Bonds break when σA=σc
(describes crack nucleation)
Materials do not begin to fail at yield stress (theoretical strength of material) –
fail at lower values because flaws create higher local stresses

8. Griffith’s Energy Balance – describes crack propagation
Griffith, Irwin, and Orowan Energy balance criteria
dE d dW s The total energy must remain the same for a given increase in
0
dA dA dA crack size (strain energy in plate plus work input)
2 2
a B potential energy due to strain energy
0
E
Ws 4 aB s work that goes into creating two new surfaces
Therefore,
2
dW s
R is the energy required to break atomic bonds with further crack
d a
G = R
dA
2 s extension (create two new surfaces). It is a measure of the
dA E
toughness of a material.
G is called the Energy Release Rate, which is thought of
as the driving force trying to extend the crack
So if the driving force equals the resistance
(G=R), then the crack will grow.
i.e.
* Modification  substitute wf s p for plasticity effects
(more energy)

9. R-curves help predict fracture resistance and material toughness
• R-curves are studied in literature so we can predict:
(1) the conditions under which a crack will extend
(2) if crack growth occurs whether it will result in failure of the material
(unstable growth)
• R-curves can take on different shapes and values, depending on the material, its
microstructure, geometry, temperature etc…
A flat R-curve indicates a brittle material. A rising R-curve indicates materials that undergo
R is ideally an invariant material property. plastic deformation. (e.g. ductile metals)
(e.g. glass)

10. Testing resistance to fracture
• Test specimens are used to determine resistance to fracture of various materials
Both are Mode I loading
(load is normal to crack)
• A stress intensity factor is associated with each kind of specimen
• Usually use displacement control
• Arrive at the fracture toughness (how much stress is needed for the crack to grow)

11. Nature has found a way to improve the strength of materials
Biological materials can have greater mechanical properties than the individual
components that make them up (strength, toughness, fracture resistance)
We want to understand nature’s approach to material structuring!

12. • Hard, brittle mineral crystals embedded in
soft, elastic protein matrix
• The load transfer is accomplished largely by
the shearing of the protein matrix between
the long sides of mineral platelets
• The TSC can be regarded as the primary
structure of biological materials

13. • Affects the mechanical
properties of the
nanostructure such as,
load transfer, stiffness,
strength and elastic
stability
• Large ratios make up for
softness in the protein
matrix
• Aspect ratio cannot be
infinitely large
h

14. • Why is the structure of
biological materials always at
the nanoscale?
• Length Scale:
• When mineral exceeds the
length scale material is
sensitive to crack-like flaws
• When mineral drops below
the length scale failure is
governed by the theoretical
strength of material

15. • Protein effectively stabilizes
mineral crystals
• At a given volume
concentration of protein, the
critical stress approaches a
constant limited value as the
aspect ratio becomes
sufficiently large
• Buckling stress in composite
nanostructure is proportional
to the geometric means of
Young’s moduli of protein and
mineral

16. • For nanocomposites, as the
mineral bits have nanoscale
size, the protein-mineral
interfacial area can be
enormous
• Interface strength depends on
both size and geometry
• The chain structure of proteins
is a crucial factor for the
strength of the protein-mineral
Figure. Atomistic modeling of protein-mineral interface
interface strength showing the mechanical
behavior of chain molecules and their interaction
with the substrate during interface failure.

17.  Whether or not a bone splits or breaks depends on how
efficiently short cracks can be prevented from growing into
longer ones
 Role of micro cracks
 Crack deflection and crack bridging
 5X greater toughness in the transverse orientation
compared to the longitudinal orientation
http://www.lbl.gov/publicinfo/newscenter/
features/assets/img/MSD-bone-
tough/Bone-Transverse-Koester.mov

18. • hierarchy is inherent in
nature
o DNA, proteins, cells,
organisms, ecosystems,
planets, etc.
• differing structures at the
meso, micro, and nano
scales all play a role
hierarchy of crab exoskeleton
Example: Crab Exoskeleton
-layers of brittle mineral rods organized in a helix
-each rod is made of softer protein which are comprised of smaller fibrils

19. • fractal-bone model
o self-similar layers repeated
N times
• bottom-up design process
o design lowest level
structure first
o next level structure
determined from current
level and characteristics
wanted

20. • strength
o by combining different compounds,
shapes, and structures in a material,
strength limitations can be
exceeded
• toughness
o shielding of crack initiation and
propagation
• flaw-tolerance
o lots of small structures handle flaws
better than one large structure
• stiffness
o more hierarchy leads to higher
stiffness
• effects of more levels of hierarchy:
o decrease in strength
o increase in fracture energy and flaw-
tolerance

21. Bone
• platelets in protein matrix bone
Wood
• complexity from cellular
construction
wood
Seashell (Nacre)
• layers of tiles in brick-and-
mortar fashion
nacre
Tendon
• tightly packed arrays of
collagen

22. Mechanical Properties Hierarchical Organization
• multifunctional material • compact bone exterior; spongy
• compact bone for strength and interior
toughness (structural support for • osteons are concentric rings of
body) mineral and collagen
• spongy bone for bone marrow • each ring has parallel sheets of fibrils
and living cells; also allows for and mineral plates
compression in other bone types • tropocollagen forms larger fibrils that
• can withstand crack-like flaws at act as a protein matrix
many levels of hierarchy • nanocrystals mineralize into plate-
like structures

23. Mechanical Properties
• high strength due to
brick-like arrangement
• high resilience due to organic
Hierarchy
matrix • staggered-tile structure
• toughness similar to silicon • mineral "bricks" in an
o addition of water enhances organic "mortar"
toughness • organic matrix made up of
• low crack propagation thin layers of elastic
• can undergo microbuckling biopolymers

24. Hierarchy Mechanical Properties
• cellulose packed into microfibrils • specific stiffness and strength
• bundles of microfibrils packed into comparable to steel
larger macrofibrils • microfibril angle plays a large part in
o contains regions of crystalline the strength and stiffness
structure and amorphous regions o young trees are more flexible and
• large fibrils supported by amorphous have larger angles
matrix of lignin and hemicellulose o older trees have small MFA and
• these fibers organize into a number thus stiffer trunks
of cell walls surrounding a given cell • high toughness similar to nacre
o cracks don't easily propagate
perpendicularly

25. Hierarchy Mechanical Properties
• similar to bone's organization • connects muscle to bone
• repeat layers of larger and larger • has elastic properties
structures • stiffness increases with strain
• collagen molecules self assemble • up to 300x stronger than muscle
o allows for small sizes
into fibrils
• fibers provide flexibility
• fibrils decorated with proteoglycans
and grouped to form fascicles

26. Spider Silk
• in nature, needs to absorb high momentum without recoil
and endure high stress from impact
• high toughness and extensibility(ability to endure strain
without failure)
• strength comparable to steel
Teeth
• two layers of protection: enamel and dentin
• enamel has high hardness (hardest material in vertibrates)
• dentin (similar in design to bone) has high toughness
Feathers
• require stiffness and flexibility to endure flight
• hollow shaft reinforced with "foam" structure
• very light but strong

27.  Chemical properties behind biomimicking
nanocomposites
 Method to fabricate nanocomposites
 Characterization Techniques for Nanocomposites
 Limitations

28.  The structure-function harmony of nacre and other hard biological
tissues has inspired a large class of biomimetic advanced materials and
organic/inorganic composites.
 The addition of inorganic components, such as clays, to organic
polymers noticely improves the mechanical, barrier and thermal
properties of polymers and rubbers.
 Finding a synthetic pathway to artificial analogs of nacre and bones
represents a fundamental milestone in the development of composite
materials.

29.  In the case of organic-inorganic nanocomposites, the strength or
level of interaction between the organic and inorganic phases is an
important issue.
1. hydrogen bonding, van der Waals forces covalent or ionic-covalent
bond
2. polarity, molecular weight, hydrophobicity, reactive groups, and so on of
the polymer
3. type of solvent and clay mineral type

30. Chemistry Properties behind nanocomposites
Extensive coiling of the polyelectrolyte leads to
the formation of loops with macromolecular
segments linked together by van der Waals and
ionic interactions.
One surface charge on clay can attract positive
headgroups from different parts of the chain
resulting in loops. Gradually, the polyelectrolyte
molecules become significantly deformed due
to sliding of the clay platelets over each other
to involve ionic bonds.

31.  More generally, molecular self-
assembly seeks to use concepts
of supramolecular chemistry
and molecular recognition in
particular, to cause single-molecule
components to automatically
arrange themselves into some
useful conformation.
Photograph of the inner side of a green abalone
(Haliotis fulgens) shell, showing the iridescent nacre.
Shell diameter is ~20 cm.

32.  Ease of preparation
 Versatility
 Capability of incorporating high loadings of different types
of biomolecules in the films
 Fine control over the materials’ structure
 Robustness of the products under ambient and
physiological conditions

33.  Biomimetics
 Biosensors
 Drug delivery
 Protein and cell adhesion
 Mediation of cellular functions
 Implantable materials

34. Schematic view of the interface
bottom-up synthesis method
of crystalline rubeanic acid copper.
Layer-by-layer assembly and supramolecular
chemistry were used to create an
ultrathin-film platform technology for small-
molecule delivery using a hydrolytically
degradable polyion (see picture, blue waves)
and a polymeric cyclodextrin (see picture, red
cups).

35. The layered organic-inorganic composites
were made from montmorillonite clay
platelets (C) and polyelectrolytes (P) by the
well-established technique of sequential
adsorption of organic and inorganic
dispersion, often called layer-by-layer
assembly (LBL). The general film structure
can be represented by the schematic in
Fig. 1c.
In nacre, mineral platelets, which are a few
hundreds of nanometers in thickness,
interlock to form sheets that are stacked on
top of each other in a staggered formation.

36.  Atomic force microscope (AFM)
 Scanning electron microscopy (SEM)
 Transmission electron microscopy (TEM)
 Wide-angle X-ray diffraction (WAXD)
 Small-angle X-ray scattering (SAXS)

37. a, Phase-contrast AFM image of a (P/C) film
on Si substrate.b, Enlarged portion of the
film in a showing overlapping clay platelets
marked by arrows.

38. e and f,Topographic AFM images of PDDA molecules adsorbed between
the clay platelets. Elevated areas of irregular shape represent PDDA coils
adsorbed to montmorillonite platelets. Arrows track the partially decoiled
macromolecules stretched between the clay platelets.
poly(diallyldimethylammonium chloride) (PDDA)

39. Scanning electron microscopy (SEM)
examination (a) of the (P/C)100 film
cross-section revealed a layered
structure which was conceptually
similar to that of nacre.The film was
dense and uniform in thickness.
Scanning process and image formation
In a typical SEM, an electron beam is thermionically
emitted from an electron gunelectron fitted with
a tungsten filament cathode.

40. TEM images showed that the film
remained continuous and retained its
integrity even when local stress had torn
away the epoxy resin serving as an
embedding media (b). Perpendicular
sectioning slightly expanded the
multilayers (c).

41. Challenging problems in the biomimicking synthesis:
 Control the size
 Geometry
 Alignment of nanostructure
 Higher levels of hierarchy