INTRODUCTION TO

BIOMATERIALS
 Definition
• Biomaterial can be simply defined as a synthetic material used
to replace part of a living system or to function in intimate
contact with living tissue
• A biomaterial is defined as any systemically, pharmacologically
inert substance or combination of substances utilized for
implantation within or incorporation with a living system to
supplement or replace functions of living tissues or organs. In
order to achieve that purpose, a biomaterial must be in contact
with living tissues or body fluids resulting in an interface
between living and non-living substances.

• Bruck, 1980 - materials of synthetic as well as of natural origin in

contact with tissue, blood, and biological fluids, and intended for
use for prosthetic, diagnostic, therapeutic, and storage
applications without adversely affecting the living organism and
its components
 Biomaterial vs Biological material
• A biological material is a material such as skin
or artery, produced by a biological system.
 What can be considered as
Biomaterial?
• Artificial materials that simply are in contact
with the skin, such as hearing aids and
wearable artificial limbs, are not included in
our definition of biomaterials since the skin
acts as a barrier with the external world
• Are drugs considered as Biomaterials ???
Classification of biomaterials
 First Generation Implants
• “ad hoc” implants
• specified by physicians using common and borrowed materials
• most successes were accidental rather than by design

Examples
• gold fillings, wooden teeth,
PMMA dental prosthesis
• steel, gold, ivory, etc., bone
plates
• glass eyes and other body parts
• dacron and parachute cloth
vascular implants
 Second generation implants
• engineered implants
• developed through collaborations of physicians
and engineers
• built on first generation experiences
• used advances in materials science (from other
fields)

Examples
• titanium alloy dental and orthopaedic implants
• cobalt-chromium-molybdinum orthopaedic implants
• UHMW polyethylene bearing surfaces for total joint
replacements
• heart valves and pacemakers
 Third generation implants
• bioengineered implants using bioengineered materials
• few examples on the market
• some modified and new polymeric devices
• many under development

Examples
•tissue engineered implants designed to regrow rather than replace tissues
•Integra LifeSciences artificial skin
•Genzyme cartilage cell procedure
•some resorbable bone repair cements
•genetically engineered “biological” components (Genetics Institute and
Creative Biomolecules BMPs)
 Skin/cartilage
Drug Delivery
Devices
Ocular
Polymers implants

Orthopedic Bone
screws/fixation replacements

Heart
valves
Metals Synthetic Ceramics
BIOMATERIALS

Dental Implants Dental Implants

Semiconductor
Materials Biosensors
Implantable
Microelectrodes
 Atomic Bonding
• All solids are made up of atoms held together by the interaction of
the outermost (valence) electron
• In metallic bonds the electrons are loosely held to the ions, which
makes the bond nondirectional. Therefore, in many metals it is easy
for plastic deformations to occur (i.e., the ions can rearrange
themselves permanently to the applied external forces).
• The ionic bonds are formed by exchanging electrons between
metallic and nonmetallic atoms. The metallic atoms, such as Na,
donate electrons, becoming positive ions (Na+), while the
nonmetallic atoms (e.g., Cl) receive electrons, becoming negative
ions (Cl–).
• The valence electrons are much more likely to be found in the
space around the negative ions than the positive ions, thus
making the bonds very directional.
• The covalent bonds are formed when atoms share the valence
electrons to satisfy their partially filled electronic orbitals.
• The greater the overlap of the valence orbitals or shells, the stronger
the bonds become, but bond strength is limited by the strong
repulsive forces between nuclei.
• Covalent bonds are also highly directional and strong, as can be
attested by diamond, which is the hardest material known.

Structure of Diamond
• Metallic bonds allow high electrical and heat conductivity due to the
free electrons, which act as the medium.
• Ionic materials are insulators of heat and electricity since their
electrons are tightly held by the ions
• Covalent bonds share valence electrons with neighboring atoms.
Generally covalent compounds show poor electrical and thermal
conductivity as for ionic bonding.
• Primary bonds the secondary bonds such as dipole dipole,
interactions, hydrogen bonds, van der Waals interactions also play
the major role in determining the properties.
 Mechanical properties
• Stress is defined as a force per unit area, which is
usually expressed in Newtons per square meter
(Pascal, Pa)

• Load - tension, compression, and shear

• Tension- loads (forces) that pull an object
apart
• Compressive stresses squeeze it together
• Shear stresses resist loads that deform or separate by sliding
layers of molecules past each other on one or more planes
 Strain
• The deformation of an object in response to
an applied load is called strain:
Stress–Strain Behavior
 Hooke's law
• In the elastic region, the strain Ɛ increases in
direct proportion to the applied stress σ

Young's modulus or the modulus of elasticity - The slope (E) or

proportionality constant of the tensile/compressive stress–strain
curve
Shear modulus (G) is defined as the initial slope of the curve of
shear stress versus shear strain.
The unit for the modulus is the same as that of stress since strain
is dimensionless.
 Plastic deformation
• In the plastic region, strain changes are no
longer proportional to the applied stress.
• Further, when the applied stress is removed,
the material will not return to its original
shape but will be permanently deformed
which is called a plastic deformation
Two-dimensional atomic model after
elastic and plastic
deformation.
 Stress-strain curves of materials
Ductile material exhibiting different characteristics under
stress. The areas underneath the curves
are the measure of toughness
 Engineering stress vs True Stress
• True Stress- Stress calculated based on a specimen's
true cross-sectional area
 • Toughness: The fracture toughness of a material
can be characterized by the amount of energy
per unit volume required to produce the failure.

A material that can withstand high stresses and can

undergo considerable plastic deformation (ductile-tough
material) is tougher than one that resists high stresses but
has no capacity for deformation (hard-brittle material) or
one with a high capacity for deformation but can only
withstand relatively low stresses (ductile-soft or plastic
material).
 Visco elasticity- refer Park book
• Viscoelastic materials are those for which the
relationship between stress and strain
depends on time.
• In such materials the stiffness will depend on
the rate of application of the load. In addition,
mechanical energy is dissipated by conversion
to heat in the deformation of viscoelastic
materials.

• Characterization of viscoelastic materials

– Creep and Stress relaxation
 Creep
• Creep is a slow, progressive deformation of a
material under constant stress.
 Stress Relaxation
Stress relaxation is the gradual decrease of stress when the
material is held at constant extension
 Mechanical Model
Spring – Purely elastic

The stress is proportional to the strain , The constant of

proportionality k is the spring constant.
 Mechanical Models
Dashpot – Purely Viscous

The stress is proportional to the time derivative of

the strain (the strain rate). The constant of
proportionality K is a viscosity
 Kelvin (Voigt) model
• Spring in parallel with a
dashpot
• The strain is the same in both
elements, but the stress in the
Kelvin model is the sum of the
spring and dashpot stresses
Differential form of the stress–strain
relation
 Maxwell model
• If the spring and dashpot are arranged in series
• If we hold the Maxwell model after
instantaneous deformation, the dashpot will
react due to retraction of the spring, and this
will take time (t = finite). The foregoing
description can be expressed concisely by a
simple mathematical formulation. The
deformation response to stress
by the Maxwell model is the sum of
deformations, since the displacements are
additive

Differentiating both sides, replace spring

constant by Young’s Modulus
Biomaterial performance
 Body response to implants
• Biocompatibility has been defined as “the ability of a
material to perform with an appropriate host response in a
specific application”
• A typical tissue response is the appearance of
polymorphonuclear leukocytes near the implant followed by
macrophages.
• If the implant is inert to the tissue, only a thin collagenous
layer encapsulates the implant.
• If the implant is chemically or physically irritating to the
surrounding tissue, then inflammation occurs at the implant
site. The inflammation delays normal healing process,
leading to the formation of granular tissues.
• Porous implants are fixed by ingrowth of surrounding
tissues. Some implants may cause necrosis of tissues by
chemical, mechanical and thermal trauma
Four major types of biomaterials in term
of interfacial response of tissues
• Type 1: nearly inert, smooth surface;
• type 2: nearly inert, microporous surface;
• type 3: controlled reactive surface
• type 4: Resorbable
Summary of the tissue response to implants
Wound healing – refer Park book
Schematic representation of the cell populations occupying epidermal wounds at
different phases of wound repair
 Blood compatibility – Ref Park book
• Any implant material should not damage the proteins,
enzymes, and formed elements of blood
Factors affecting surface roughness
1. Surface roughness
2. Surface wettability
3. Surface Charge

• The surface of intima is negatively charged largely due to

the presence of polysaccharides, especially chondroitin
sulfate and heparin sulfate
 Approaches to producing
thromboresistant surfaces
Nano scale phenomena
 Nano scale phenomena- Ref
Biomaterials- A Nano approch book
• Nanomaterials, also called nanostructured
materials, are single phase or multiphase
materials with size less than 100 nm at least in
one of the dimensions.
One dimensional- ultrathin coatings
Two dimensional- nanotubes and nanowires
Three dimensional- nanoparticles
Comparison of nanoparticles compared with relative biological systems.
 Properties at Nanoscale
• Increased surface area of grains- The surface
area of NPs increases with a decrease in size.
This is because a NP of 1-nm size will have
100% of its atoms on its surface, while a 3-nm
size NP of spherical shape will have 50% of its
atoms on its surface
• Increased volume of grain boundaries- smaller grain size, have a
large amount of grain boundary area. In conventional micron grain
size materials, impurities preferentially diffuse to the grain
boundary region and may lead to intergranular fracture. Since
nanophase material has a large amount of grain boundaries to
diffuse, there may be homogenization of impurities and hence less
susceptibility to failure
• Increased amount of surface atoms

Percentage of bulk and surface atoms as a function

of particle size
• Formation of discrete electronic energy levels
(quantum effect)
Nano-Silver