BIOMATERIALS- CLASSIFICATION
When a synthetic material is placed within the human
body, tissue reacts towards the implant in a variety of
ways depending on the material type.
The mechanism of tissue interaction depends on the
tissue response to the implant surface.
Biomedical materials can be divided roughly in to
three main types governed by the tissue response.
LECTURE 2
BIOMATERIALS
1
BIOMATERIALS- CLASSIFICATION
Biomaterials are widely classified as
Bioinert Biomaterials
Bioactive Biomaterials
Bioresorbable Biomaterials
LECTURE 2
BIOMATERIALS
2
BIOINERT BIOMATERIALS
The term bioinert refers to any material that once
placed in the human body has minimal interaction
with its surrounding tissue.
Examples of these are stainless steel, titanium,
alumina, partially stabilized zirconia, and ultra high
molecular weight polyethylene.
Generally a fibrous capsule might form around bioinert
implants hence its biofunctionality relies on tissue
integration through the implant.
LECTURE 2
BIOMATERIALS
3
BIOACTIVE BIOMATERIALS
Bioactive refers to a material, which upon being placed
within the human body interacts with the surrounding
bone and in some cases, even soft tissue.
This occurs through a time –dependent kinetic
modification of the surface, triggered by their
implantation within the living bone .
LECTURE 2
BIOMATERIALS
4
BIOACTIVE BIOMATERIALS
An ion-exchange reaction between the bioactive implant
and the surrounding body fluids-results in the formation
of a biologically active carbonate apatite (CHAp) layer on
the implant that is chemically and crystallographically
equivalent to the mineral phase in bone.
Prime examples of these materials are synthetic
hydroxyapatite [Ca 10 (PO4)6(OH)2], glass ceramic and
bioglass.
LECTURE 2
BIOMATERIALS
5
BIORESORBABLE BIOMATERIALS
Bioresorbable refers to a material that upon placement
within the human body starts to dissolve and slowly
replaced by advancing tissue (such as bone).
Common examples of bioresorbable materials are
tricalcium phosphate [Ca3(PO4)2] and polylacticpolyglycolic acid copolymers.
Calcium oxide, calcium carbonate and gypsum are other
common materials that have been utilized during the
last three decades.
LECTURE 2
BIOMATERIALS
6
COMPARISON OF PROPERTIES
The wide application of biomaterials in medicine
depends on the properties of these materials.
Different biomaterials should have different properties
depending on the high end applications.
The surface properties, mechanical properties and the
thermal properties which are important are discussed.
LECTURE 2
BIOMATERIALS
7
SURFACE PROPERTIES
The surface properties for the biomaterials which are being
considered for discussion are
Surface Energy
Contact Angle
Critical Surface Tension
LECTURE 2
BIOMATERIALS
8
SURFACE ENERGY
Surface energy quantifies the disruption of
intermolecular bonds that occurs when a surface is
created.
In other words surface energy is a measure of the extent
to which bonds are unsatisfied at the surface of
material.At the surface, there is an asymmetric force
field, which results in an attraction of atoms which are
there on the surface in to the bulk.
This tends to deplete the surface of atoms putting the
surface in tension.
LECTURE 2
BIOMATERIALS
9
SURFACE ENERGY
Metals and ceramics have surfaces with high surface
energies ranging from 102 to 104 ergs/cm2.
In contrast, most polymers and plastics have much
smaller surface energies, usually <100 ergs/cm2.
The surface energy values are subject to much
experimental variation due to adsorption of gases or
organic species.
LECTURE 2
BIOMATERIALS
10
CONTACT ANGLE
The contact angle is the angle at which a liquid/vapor
interface meets the solid surface.
The contact angle is specific for any given system and is
determined by the interactions across the three
interfaces.
LECTURE 2
BIOMATERIALS
11
CONTACT ANGLE
When a liquid drop is placed on to the surface of a solid
or the surface of the liquid, the processes which occur
are:
1.The liquid may sit on the surface in the form of a
droplet or
2. It may spread out over the entire surface
depending on the interfacial free energies of the
two substances.
At equilibrium contact angle or Young Dupree equation
is given by
s / g = s / l + l / g cos 
LECTURE 2
BIOMATERIALS
12
CONTACT ANGLE
Where s / g, s / l and l / g are the interfacial free energy
between the solid and gas; solid and liquid, liquid and gas
respectively and  the contact angle.
LECTURE 2
BIOMATERIALS
13
CONTACT ANGLE
The wetting characteristic can be generalized as  = 0,
complete wetting ;   0  900, partial wetting ;  > 900 ,
no wetting.
The contact angle can be affected greatly by the surface
roughness and adsorption of polar gases or organic
species or contamination by dirt.
LECTURE 2
BIOMATERIALS
14
CRITICAL SURFACE TENSION
The critical surface tension is defined as that value of
surface tension of a liquid below which the liquid will
spread on a solid and is expressed in dynes/cm.
The critical surface tension of a material is determined
by measuring the different values of contact angle 
formed by liquids with different values of l / g.
A plot of cos  versus l / g is usually a straight line
LECTURE 2
BIOMATERIALS
15
CRITICAL SURFACE TENSION
The l / g at which cos  =1 is defined as the critical surfacetension (c).
LECTURE 2
BIOMATERIALS
16
CRITICAL SURFACE TENSION
Blood compatibility of material surfaces has been shown
to vary in the same order as the critical surface tension.
It is found that the amount of thrombus formation
increases and blood clotting time decreases as c
increases.
LECTURE 2
BIOMATERIALS
17
MECHANICAL PROPERTIES
The Mechanical Properties which will be considered are
• Youngs and Rigidity Modulus
• Poisson’s Ratio
• Hardness
• Isotropy
• Creep and Viscous Flow
• Fatigue
LECTURE 2
BIOMATERIALS
18
YOUNGS AND RIGIDITY MODULUS
By using the definitions of stress and strain, Hooke’s law
can be expressed in quantitative terms:
=E , ( tension or compression )
= G , ( shear )
E and G are proportionality constants that may be
likened to spring constants.
The tensile constant, E is the tensile (or (young’s)
modulus and G is the shear modulus
LECTURE 2
BIOMATERIALS
19
YOUNGS AND RIGIDITY MODULUS
These moduli are also the slopes of the elastic portion of
the stress versus strain curve.
Since all geometric influences have been removed, E
and G represent inherent properties of the material.
These two moduli are direct macroscopic manifestations
of the strengths of the interatomic bonds.
Elastic strain is achieved by actually increasing the
interatomic distances in the crystal (i.e., stretching the
bonds).
LECTURE 2
BIOMATERIALS
20
YOUNGS AND RIGIDITY MODULUS
For materials with strong bonds (e.g., diamond, Al2O3,
tungsten), the moduli are high and a given stress
produces only a small strain.
For materials with weaker bonds (e.g., polymers and
gold), the moduli are lower.
Cobalt Chromium Alloy is found to have high young’s
modulus whereas SS316L(class of Stainless Steel) and
Cobalt Chromium Alloy is found to have high shear
modulus.
LECTURE 2
BIOMATERIALS
21
POISSON’S RATIO
Poisson's ratio is the ratio of the relative contraction
strain, or transverse strain (normal to the applied load),
divided by the relative extension strain, or axial strain (in
the direction of the applied load).
It is found that Tantulum has higher poisson ratio than
SS316L(class of Stainless Steel) ,Cobalt
Chromium,Nitinol (alloy of Ni and Ti-designated as
Shape Memory Alloy).
LECTURE 2
BIOMATERIALS
22
HARDNESS
The resistance of a material to permanent deformation
of its surface is called Hardness.
The hardness of a material is very important property
since in any way it decides the life of a biomaterial.
The hardnesss is generally tested by Vickers hardness
test and is represented in terms of Vickers hardness
number.
It has been found that the Cobalt Chromium Alloys have
higher hardness number than the other major implant
counter parts like Stainless steel,Tantulum,Nitinol.
LECTURE 2
BIOMATERIALS
23
COMPARISON OF POISSONS RATIO
LECTURE 2
BIOMATERIALS
24
COMPARISON OF YOUNGS AND SHEAR MODULUS
LECTURE 2
BIOMATERIALS
25
COMPARISON OF HARDNESS
LECTURE 2
BIOMATERIALS
26
ISOTROPY
The two constants, E and G, are all that are needed to
fully characterize the stiffness of an isotropic material
(i.e., a material whose properties are the same in all
directions).
Single crystals are anisotropic (not isotropic) because
the stiffness varies as the orientation of applied force
change relative to the interatomic bond directions in the
crystal.
In polycrystalline materials (e.g., most metallic and
ceramic specimens), a great multitude of grants
(crystallites) are aggregated with multiply distributed
orientations
LECTURE 2
BIOMATERIALS
27
ISOTROPY
• On the average, these aggregates exhibit isotropic
behavior at the macroscopic level, and values of E and
G are highly reproducible for all specimens of a given
metal, alloy, or ceramic.
• On the other hand, many polymeric materials and most
tissue samples are anisotropic (not the same in all
directions) even at the macroscopic level.
• Bone, ligament, and sutures are all stronger and stiffer in
the fiber (longitudinal) direction than they are in the
transverse direction.
• For such materials, more than two elastic constants are
required to relate stress and strain properties.
LECTURE 2
BIOMATERIALS
28
CREEP AND VISCOUS FLOW
Creep is the term used to describe the tendency of a
material to move or to deform permanently to relieve
stresses.
Material deformation occurs as a result of long term
exposure to levels of stress (physics) that are below the
yield strength or ultimate strength of the material.
Creep is more severe in materials that are subjected to
heat for long periods and near melting point.
Creep is often observed in glasses.
LECTURE 2
BIOMATERIALS
29
CREEP AND VISCOUS FLOW
It this been assumed that when a stress is applied, the
strain response is instantaneous.
For many important biomaterials, including polymers
and tissues, this is not a valid assumption.
If a weight is suspended from an excised ligament, the
ligament elongates essentially instantaneously when
the weight is applied.
This is an elastic response. Thereafter the ligament
continues to elongate for a considerable time even
though the load is constant (Fig.A).
LECTURE 2
BIOMATERIALS
30
CREEP AND VISCOUS FLOW
• This continuous, time-dependent extension under load is called
"creep."
• Similarly, if the ligament is extended in a tensile machine to a
fixed elongation and held constant while the load is monitored,
the load drops continuously with time (Fig.B). The
continuous drop in load at constant extension is called stress
relaxation.
• Both these responses are the result of viscous flow in the
material.
• The mechanical analog of viscous flow is a dashpot or cylinder
and piston (Fig.C). Any, small force is enough to keep the
piston moving. If the load is increased, the rate of displacement
will increase.
LECTURE 2
BIOMATERIALS
31
CREEP AND VISCOUS FLOW
A Elongation Vs Time at constant load of ligament
B Load Vs Time at constant elongation for ligament
LECTURE 2
BIOMATERIALS
32
CREEP AND VISCOUS FLOW
C Dash pot or Cylinder and Piston model of viscous flow
D Dash pot and spring model of viscoelastic material
LECTURE 2
BIOMATERIALS
33
CREEP AND VISCOUS FLOW
Despite this liquid-like behavior, these materials are
functionally solids.
To produce such a combined effect, they act as though they
are composed of a spring (elastic element) in series with a
dashpot (viscous element) (Fig.C).
Thus, in the creep test, instantaneous strain is produced
when the weight is first applied (Fig.A).
This is the equivalent of stretching the spring to its
equilibrium length (for that load).
LECTURE 2
BIOMATERIALS
34
CREEP AND VISCOUS FLOW
Thereafter, the additional time-dependent strain is modeled
by the movement of the dashpot.
Complex arrangements of springs and dashpots are often
needed to adequately model the actual behavior of
polymers and tissues.
Materials that behave approximately like a spring and
dashpot system are viscoelastic.
One consequence of viscoelastic behavior can be seen in
tensile testing where the load is applied at some finite rate.
LECTURE 2
BIOMATERIALS
35
CREEP AND VISCOUS FLOW
• During the course of load application, there is time for some
viscous flow to occur along with the elastic strain.
• Thus, the total strain will be greater than that due to the
elastic response alone.
• If this total strain is used to estimate the Young's modulus of
the material (E = /), the estimate will be low.
• If the test is conducted at a more rapid rate, there will be less
time for viscous flow during the test and the apparent
modulus will increase.
LECTURE 2
BIOMATERIALS
36
CREEP AND VISCOUS FLOW
If a series of such tests is conducted at ever higher loading
rates, eventually a rate can be reached where no detectable
viscous flow occurs and the Modulus determined at this
critical rate will be the true elastic modulus.
For all viscoelastic materials, moduli determined at rates
less than the critical rate are "apparent" moduli and must be
identified with the strain rate used.
Failure to do this is one reason why values of tissue moduli
reported in the literature may vary over wide ranges.
LECTURE 2
BIOMATERIALS
37
CREEP AND VISCOUS FLOW
 Finally, it should he noted that it may be difficult to
distinguish between creep and plastic deformation in
ordinary tensile tests of highly viscoelastic materials (e.g.,
tissues).
 For this reason, the total nonelastic deformation of tissues
or polymers may at times be loosely referred to as plastic
deformation even though viscous flow is involved.
LECTURE 2
BIOMATERIALS
38
FATIGUE
• It is not uncommon for materials, including tough and ductileones like 316L, stainless steel, to fracture even though the
service stresses imposed are well below the yield stress.
• This occurs when the loads are applied and removed for a
great number of cycles, as happens to prosthetic heart valves
and prosthetic joints.
• Such repetitive loading can produce microscopic cracks that
then propagate by small steps at each load.
LECTURE 2
BIOMATERIALS
39
FATIGUE
 Fatigue, then, is a process by which structures fail as a
result of cyclic stresses that may be much less than the
ultimate tensile stress.
 Fatigue failure plagues many dynamically loaded structures,
from aircraft to bones to cardiac pacemaker leads.
LECTURE 2
BIOMATERIALS
40
FATIGUE
 The stresses at the tip of a crack, a surface scratch, or a
sharp corner are locally enhanced by the stress-raising
effect.
 Under repetitive loading, these local high stresses ally
exceed the strength of the material over a small region.
 This Phenomenon is responsible for the stepwise
propagation of the cracks, Eventually, the load-bearing
cross-section becomes so small that the part finally fails
completely .
LECTURE 2
BIOMATERIALS
41