Transcript Slide 1

POLYMERS
The structures of polymers determine their utilization in
various medical domains like in surgery, dermatology,
ophthalmology, pharmacy,depending on chemical and
physical properties.
The stability and lifetime of polymers in long-term
implantation depend not only on chemical structure of
the
material employed but also on the conditions under
which they are utilized.
Biomedical polymers can be classified into either
elastomers or plastics.
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POLYMERS
Elastomers are able to withstand large deformations and
return to their original dimensions after releasing the
stretching force.
Plastics on. the other hand are more rigid materials and
can
be classified into two types:
Thermoplastic
Thermosetting.
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POLYMERS
Thermoplastic polymers can be melted, reshaped and
reformed.
The thermosetting plastics can be remelted and reused,
since the chemical reactions that have taken are
irreversible.
The thermoplastic polymers used as biomaterials include
polyolefins, Teflon (fluorinated hydrocarbons), Poly
(methyl methacrylate (PMMA), Polyvinyl chloride (PVC)
Polycarbonate, nylon, polyester (Dacron ® ) etc.
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POLYMERS
A number elastomers have been tried as implant
materials.
These include, butyl rubber, chlorosulfonated
polyethylene,epichlorohydrin rubber,polyurethane,natural
rubber and silicone rubber.
The fact that should be kept in mind when implanting
plastics is the toxicity of these additives and the ease
with they may be released into the surrounding tissues.
Residual monomers due to incomplete polymerization
and catalyst used for polymerization may cause
irritations.
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POLYMERS
Polymer processing into a wide variety of shapes is
carried out using extrusion, molding, spinning, weaving,
knitting and casting techniques.
Polymeric materials can also be processed using lathes,
grinders and shapers in similar manner to metals.
Polymeric materials have a wide variety of applications
for implantation, as they can be easily fabricated into
many forms: fibers, textiles, films, foams, solid, rods,
powders, liquids etc.
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Polymer
Specific Properties
Biomedical uses
Polyethylene
Low cost, easy Possibility excellent Tubes for various
electrical
insulation
properties,
catheters,
hip
excellent
chemical
resistance,
joint, knee joint
toughness and flexibility even at low
prostheses
temperatures
Polypropylene
Excellent chemical resistance, weak Yarn for surgery,
permeability to water vapors good
sutures
transparency and surface reflection.
Tetrafluoroethylene Chemical
inertness,
exceptional Vascular
and
weathering and heat resistance,
auditory
nonadhesive, very low coefficient of
prostheses,
friction
catheters tubes
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Polymer
Specific Properties
Biomedical uses
Polyvinylchloride Excellent resistance to abrasion, good Flexible
or
semidimensional
stability,
high
flexible
medical
chemical resistance to acids,
tubes, catheter, inner
alkalis, oils, fats, alcohols, and
tubes components of
aliphatic hydrocarbons
dialysis installation
and temporary blood
storage devices
Polyacetals
Stiffness,
fatigue
endurance, Hard tissue replacement
resistance to creep, excellent
resistance to action of humidity
gas and solvents
Polymethyl
methacrylate
Optical
properties,
exceptional Bone
cement,
transparency,
and
thermo
intraocular lenses,
formation and welding
contact lenses,
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Polymer
Specific Properties
Polycarbonate Rigidity and toughness upto
1400C transparency, good
Electricalinsulator,
physiological inertness
Biomedical uses
Syringes, arterial tubules,
hard tissue replacement
Polyethylene Transparency, good resistance to Vascular,
laryngeal,
terephythalate
traction and tearing, resistance
esophageal prostheses,
to oils, fats, organic solvents
surgical sututes, knitted
vascular prostheses.
Polyamide
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Very good mechanical properties, PA 6 tunes for intracardiac
resistance to absrasion and
catheters,
urethral
breaking, stability to shock and
sound; surgical suture,
fatigue,
low
friction
films for packages,
coefficient,
good
thermal
dialysis
devices
properties,
components,
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Polymer
Specific Properties
Biomedical uses
Polyurethane
Exceptional resistance to Adhesives, dental materials,
abrasion, high resistance
blood pumps, artificial hear
to breaking, very high
and skin
elasticity modulus at
compression,
traction
and sheering remarkable
elongation to breaking.
Silicone
rubber
Good thermal stability, Encapsulant for pacemakers,
resistance
to
burn
treatments,
shunt,
atmospheric
and
Mammary prostheses, foam
oxidative
agents,
dressing, valve, catheter,
physiological inertness
contact lenses, membrances,
maxillofacial implants.
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Polyethylene and Polypropylene
The first polyethylene [PE,(-CH2-CH2-)n] was made by
reacting ethylene gas at high pressure in the presence of
a perioxide catalyst for starting polymerization.
This process yields low density polyethylene.
By using a Zigler-Natta catalyst, high-density
polyethylene can be produced at low pressure; unlike the
former, high-density polyethylene does not contain
branches.
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Polyethylene and Polypropylene
This results in better packing of the chains, which
increases density and crystallinity.
The crystallinity usually is 50-70% for low density PE to
70-80% for high density PE.
Several densities of polyethylene are available with the
tensile strength, hardness, and chemical resistance
increasing with the density.
The grade of polyethylene which has the major impact
upon surgery is referred to as ultra high molecular weight
polyethylene (UHMWPE)
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Polyethylene and Polypropylene
Polypropylene (PP) having repeating units of [-CH(CH3)CH2-)]n can have two ordered conformation, one in which
all methyl groups lie on the same side (isotactic), the
other in which they alternate (syndiotactic).
These structural regularities permit long-range order
among assemblies of molecules and hence the close
packing for crystallinity.
Other arrangement called atactic form is also possible.
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Polyethylene and Polypropylene
Suture materials of monofilament polypropylene (Prolene
®) are used clinically.
Compared with metal wire, catgut, silk, and polyglycolic
acid sutures, propylene product exhibits least fibroblastic
response and silk the most in the nerve tissues of
rabbits.
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ACRYLIC RESINS
Simple acrylates have relatively high toughness and
strength.
The most widely used polyacrylate is poly(methyl
methacrylate,PMMA).
The features of acrylic polymers are
 brittle in comparison with other polymers
 excellent light transparency
 high index of refraction.
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ACRYLIC RESINS
This transparent material is sometimes referred to as
organic glass.
It has excellent chemical resistivity and is highly
biocompatible in the pure form.
Therefore, this polymer is used extensively in medical
applications such as contact lenses,implantable ocular
lenses (IOL),bone cement for joint fixation,dentures and
maxillofacial prostheses.
Acrylic resins can be cast molded or machined with
conventional tools.
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ACRYLIC RESINS
Most medical and dental acrylic resins are available as a
two component system,powder which consists of polymer
(PMMA) liquid containing the monomer (MMA).
The powder and the liquid are mixed in a ratio 2:1 and a
moldable dough is obtained which cures in about 10 min
or more quickly and then injected in the femur as shown
in the figure.
The monomer polymerizes and binds together the
preexisting polymer particles.
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ACRYLIC RESINS
The disadvantage of using acrylic resins is that they
cause allergic reactions.
In orthopedic surgery PMMA is used in hip arthroplastics.
It is also suitable for the repairs of cranial defects.
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BONE CEMENT MIXING AND INJECTION
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HYDROGELS
Hydrogels find their name from their affinity for water and
incorporation of water into their structure.
The concentration of water in the hydrogel can affect the
interfacial free energy of the hydrogel,as well as the
biocompatibility.
Hydrogels have inherently weak mechanical properties.
Hence for some applications they are often attached to
tougher materials such as silicone rubber, polyurethane
or PMMA.
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HYDROGELS
• Hydrogels may be attached to conventional polymer
substrates by a number of surfaces grafting techniques.
These procedures include chemical initiation such as the
irradiation with electrons accelerated by high voltages,
high-energy Co-Gamma rays and microwave discharge.
• Many different chemical structures can be classified as
hydrogels.
• These varied structures have in common a strong
interaction with water, however, they are not soluble in
aqueous media.
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HYDROGELS
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HYDROGELS
The interest in hydrogels as biomaterials stems from a
number of advantages such as
(1)The soft, rubbery nature of hydrogels minimize
mechanical and frictional irritation to the surrounding
tissues.
(2)These polymers may have low or zero interfacial
tension with surrounding biological fluids and tissues,
thereby, minimizing the driving force for protein
adsorption and cell adhesion
(3) Hydrogels allow the permeating and diffusion of low
molecular weight metabolities,waste products and salts
as do living tissues.
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HYDROGELS
Poly (hydroxyethyl methacrylate) (PHEMA) is a rigid
acrylic polymer when dry, but it absorbs water when
placed in aqueous solution and changes into and elastic
gel.
Depending on the fabrication techniques,3 to 90% of its
weight can be made up of water.
Usually PHEMA Hydrogel takes up approximately 40%
water, and it is transparent when wet.
Since it can be easily machined while dry, yet is very
pliable when wet, it makes a useful contact lens
material.
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POLYURETHANES
The polyurethanes consist essentially of varied
arrangements of polymeric molecules, which share a
common urethane linkage (-O-CO-NH-).
Thermoplastic segmented polyurethanes have been
valuable in producing such medical items as extruded
blood tubing’s while the cross linked polyurethanes have
received more attention for long term surgical implants.
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POLYURETHANES
 Polyther-urethanes are block copolymers consisting of
the variable length blocks that aggregate in phase
domains giving rise to microstructure responsibility for the
physical and mechanical characteristics of the polymer.
 The PEG based polyether urethanes are hydrophilic
polymers.
 The most recent generation of polyurethanes is based on
cycloliphatic polyether urethanes and is soluble inorganic
solvents like tetrahydrofuran and dimethlacetamide.
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POLYURETHANES
Vascular tubes made of these are used as aortic patch
grafts.
Polyurethane copolymer is the natural choice for long
implant use because of its greater hydrolytic heart assist
devices.
This gives rise to minimal inflammatory reaction.
This polymer shows good blood compatibility.
It is also noncytotoxic and does not give rise to adverse
tissue reactions.
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POLYAMIDES
Polyamides are obtained through condensation of
diamine and diacid derivative.
These polymers are known as nylons and are
designated by the number of carbon atoms in the parent
monomers.
These polymers have excellent fiber forming properties
due to inter-chain hydrogen bonding and high degree of
crystallinity, which increases the strength in the fiber
direction.
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POLYAMIDES
 Since the hydrogen bonds play a major role in
determining properties, the number and distribution of
amide bonds are important factors.
 Nylon tubes find applications in catheters.
 The coated nylon sutures find wide biomedical
applications.
 Nylon is also utilized fabrication of hypodermic syringes
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