Supercritical CO2 impregnation of polyethylene components for medical purposes

Abstract

Modem hip and knee endoprosthesis are produced in titanium and to reduce the friction at the contact area polymer parts, mainly ultra-high molecular weight polyethylene (UHMW-PE), are installed. The polyethylene is impregnated with a-tocopherol (vitamin E) before processing for remarkable decrease of oxidative degradation. Cross linked UHMW-PE offers much higher stability, but a-tocopherol cannot be added before processing, because a-tocopherol hinders the cross linking process accompanied by a heavy degradation of the vitamin. The impregnation of UHMW-PE with a-tocopherol has to be performed after the cross linking process and an accurate concentration has to be achieved over the cross section of the whole material. In the first tests UHMW-PE-cubes were stored in pure a-tocopherol under inert atmosphere at temperatures from 100 to 150 ?C resulting in a high mass fraction of a-tocopherol in the edge zones and no constant concentration over the cross section. For better distribution and for regulating the mass fraction of a-tocopherol in the cross linked UHMW-PE material supercritical CO2 impregnation tests were investigated. Again UHMW-PE-cubes were impregnated in an autoclave with a-tocopherol dissolved in supercritical CO2 at different pressures and temperatures with variable impregnation times and vitamin E concentrations. Based on the excellent results of supercritical CO2 impregnation standard hip and knee cups were stabilized nearly homogeneously with varying mass fraction of a-tocopherol.Savremene endoproteze kukova i kolena proizvode se od titana, a da bi se smanjilo trenje kontaktnih povrsina ugradjuju se polimerni delovi, i to uglavnom od polietilena velike molekulske tezine (UHMW-PE). Pre obrade polietilen se impregnise sa a-tokoferolom (vitamin E) koji zna-cajno smanjuje degradaciju oksidacijom. Poprecno vezivanje UHMW-PE omogucava vecu stabilnost, ali se a-tokoferol ne moze dodavati pre obrade, zato sto a-tokoferol sprecava procese poprecnog vezivanja, pracene izrazenom degradacijom vitamina. Impregnacija UHMW-PE sa a-tokoferol mora se izvesti posle procesa poprecnog vezivanja i tacna koncentracija mora se postici duz celokupnog poprecnog preseka materijala. U prvim ispitivanjima, UHMW-PE kocke su drzane u cistom a-tokoferolu u inertnoj atmosferi na temperati od 100 do 150 ?C, sto je rezultiralo u velikom masenom udelu a-tokoferola u ivicnim zonama i promenljivoj koncentraciji duz poprecnih preseka. U cilju bolje raspodele kao i regulacije masenog udela a-tokoferola u poprecnim vezivanju UHMW-PE materijala, ispitivana je nadkriticna CO2 impregnacija. Ponovo UHMW-PE kocke su impregnisane u autoklavu sa a-tokoferolom rastvorenim u nadkriticnom CO2 na razlicitim pritiscima i temperaturama sa razlicitim vremenima impregnacije i koncentracijama vitamina E. Na osnovu odlicnih rezultata impregnacijom nadkriticnim CO2, casice vestackih kukova i kolena su priblizno homogeno stabilisane sa razlicitim masenim udelima a-tokoferola.