uhmwpe- ultra high molecular weight polyethylene__as a biomaterial

UHMWPE ultra high molecular weight poly

ethylene

Lukkumanul hakkim N.lukku007@gmail.com

UHMWPE

• Linear type PE has a branching chain loosely contain in one chain

• Mol.wt 2-6 million (regular 20000-30000)• Due to high mol wt , shows superior

properties

Physical propertiesPROPERTIES HDPE UHMWPE

Mol.wt(million) .05-.2 2-6

MP( oC) 130-137 125-135

Density (g/cc) 0.95-0.96 0.93-0.94

Tensile yield (Mpa) 26.2-33.1 19.3-23

Elongation at break(%) 10-1200 250-450

Tensile modulus(Mpa) 400-4000 600-1500

Izod ipact(Jm-1) 21-210 <1070 no break

Shore d hardness 67-73 60-65

Synthesis

• By use of Ziegler Natta coordination catalyst under low pressure(TiCl-Al.Alk)

• Heterogeneous system(hexane solvent , ethylene gas , catalyst )

• Press. 0.4-0.6 Mpa• Temp. 66-90 oC• Results white granular powder• Calcium sterate is added for isolation

Processing Polymerized

UHMWPE resin

Direct molding

Ram extruded rod

Compression molded block

machining

sterilizationGamma raye- beamPlasma gasEt o

Processing

• Unlike HDPE,LDPE even heating above 200 oC UHMWPE shows high granular structure leads to difficulty in processing

• Proper temp. and high press. Required for achieving proper plastication

• Three methods are 1. Ram extrusion 2. Compression molding 3. Direct compression molding

Ram extrusion

• Temp 180-220 oC

Compression moulding

• Temp. 200-230 oC• press. 2-10 MPa

Direct compression moulding

• 200-240 oC• 5-10 MPa

Sterilization

• Performed after packaging• Available methods are Dry heat, steam under

press. , steam formaldehyde , et.oxide , irradiation…

• Mostly using methods are 1.Gamma radiation . 2.Ethylene oxide

• Gamma radiations can destroy micro organisms and reduce mechanical properties

Oxidation • Occurs after gamma radiation• Gama ray causes breaking of C-H bonds , produces free

radicals• These radical leads further chain scission and introduction of

oxygen containing group• Leads –changes in physical and chemical properties-increase

in modulus and brittleness-increase in density over surfaces-decreace mechanical properties

• Finally turns to regular PE• Prevention – oxidation carried under vacuum or inert gas

atmosphere

Wear properties

• UHMWPE used as bearing surface for artificial joints due to 1 . High toughness . 2. low friction coefficient . 3. low wear in body

• If wearing occurs the generated partcles can cause adverse cellular effects

• Coefficient of friction is very low (0.10-0.22) compared to other materials

Fatigue properties

• Flaking type of wear or delamination is observed

• Initiated by high stress in subsurfaces• Leads to cracking

Modifications

Cross linking

• Increased cross linking results improvement in wear resistance

• vacuum treated implants shows greater cross linking and wear resistance

• UHMWPE is cross linked by low doss gamma radiation in vacuum and directly compression molded at 200 oC and 20 Mpa leads increased orientation and wear resistance

Vitamin E addition

• Prevents crack propagation due to reduced hardness and grain boundaries

• Gamma irradiated vitamin E contain UHMWPE is promising material to prevent flaking like distruction of joints

Bio medical applications

• UHMWPE has been used as the bearing material in total joint prosthesis ( hip , knee , ankle , shoulder , elbow and wrist ) because of unique material properties such as

Low friction coefficientHigh resistance to wearHigh impact resistanceHigh ductilityStability in body

HIP REPLACEMENT INTER VERTEBRAL

ANKLE REPLACEMENT

SHOULDER REPLACEMENT

ELBOW REPLACEMENT

THE END