LECTURE 5BIOMATERIALS1 POLYMERS The structures of polymers determine their utilization in various medical domains like in surgery, dermatology, ophthalmology,

LECTURE 5 BIOMATERIALS 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.


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.


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.


POLYMERSA 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.


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.


Polymer Specific Properties Biomedical uses

Polyethylene Low cost, easy Possibility excellent electrical insulation properties, excellent chemical resistance, toughness and flexibility even at low temperatures

Tubes for various catheters, hip joint, knee joint prostheses

Polypropylene Excellent chemical resistance, weak permeability to water vapors good transparency and surface reflection.

Yarn for surgery, sutures

Tetrafluoroethylene Chemical inertness, exceptional weathering and heat resistance, nonadhesive, very low coefficient of friction

Vascular and auditory prostheses, catheters tubes



Polyvinylchloride Excellent resistance to abrasion, good dimensional stability, high chemical resistance to acids, alkalis, oils, fats, alcohols, and aliphatic hydrocarbons

Flexible or semi-flexible medical tubes, catheter, inner tubes components of dialysis installation and temporary blood storage devices

Polyacetals Stiffness, fatigue endurance, resistance to creep, excellent resistance to action of humidity gas and solvents

Hard tissue replacement

Polymethylmethacrylate

Optical properties, exceptional transparency, and thermo formation and welding

Bone cement, intraocular lenses, contact lenses,



Polycarbonate Rigidity and toughness upto1400C transparency, goodElectricalinsulator,physiological inertness

Syringes, arterial tubules, hard tissue replacement

Polyethyleneterephythalate

Transparency, good resistance to traction and tearing, resistance to oils, fats, organic solvents

Vascular, laryngeal, esophageal prostheses, surgical sututes, knitted vascular prostheses.

Polyamide Very good mechanical properties, resistance to absrasion and breaking, stability to shock and fatigue, low friction coefficient, good thermal properties,

PA 6 tunes for intracardiac catheters, urethral sound; surgical suture, films for packages, dialysis devices components,



Polyurethane Exceptional resistance to abrasion, high resistance to breaking, very high elasticity modulus at compression, traction and sheering remarkable elongation to breaking.

Adhesives, dental materials, blood pumps, artificial hear and skin

Siliconerubber

Good thermal stability, resistance to atmospheric and oxidative agents, physiological inertness

Encapsulant for pacemakers, burn treatments, shunt, Mammary prostheses, foam dressing, valve, catheter, contact lenses, membrances, maxillofacial implants.


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.



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)



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.



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.


ACRYLIC RESINSSimple 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.


ACRYLIC RESINSThis 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.


ACRYLIC RESINSMost 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.


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.


BONE CEMENT MIXING AND INJECTION


HYDROGELSHydrogels 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.


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.


HYDROGELS


HYDROGELSThe 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.


HYDROGELSPoly (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.


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.


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.


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.


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.


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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LECTURE 5BIOMATERIALS1 POLYMERS The structures of polymers determine their utilization in various medical domains like in surgery, dermatology, ophthalmology,