The influence of molecular weight on the propertie

Authors: Kinga Pielichowska

Publish Date: 2012/02/22

Volume: 19, Issue: 2, Pages: 9788-

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Abstract

A series of polyoxymethylene POM/hydroxyapatite HAp nanocomposites for longterm bone implants were prepared by melt processing using POM with different molecular weight Bioactivity was examined by incubating the samples in simulated body fluid Moreover in vitro stability wettability thermal stability and formaldehyde release process were investigated Results of in vitro investigations show that an increase in HAp content in POM nanocomposites facilitates the formation of an apatite layer on the sample surface There is no significant influence of HAp concentration on the contact angle values as revealed by wettability studies Thermogravimetric analysis results indicate that with an increase in HAp content the thermal stability of POM matrix decreases significantly depending upon the POM molecular weight Finally it was found the amount of formaldehyde leached out of the samples during the incubation period depends on the HAp content amounts in excess of 3 ppm were emitted for the samples containing only 10 HApPolyoxymethylene POM belongs to the group of engineering thermoplastics with properties that include excellent “metallike” machining characteristics high mechanical strength excellent abrasion fatigue resistance moldability etc POM has a long history of use in animal studies and as a longterm implant material in a variety of medical applications These include cardiac valve prostheses eg the Björk–Shiley valve tiltingdisk style prosthesis 1 2 dental implants 3 and prosthetics as well as orthopedic implants 4 5 6 7The use of POM Delrin® in total hip prostheses was first suggested in the mid1960s and since 1970 Delrin acetal polymer has been used in the Christiansen total hip prosthesis and endoprosthesis Up to the end of 1977 approximately 7000 surgical procedures had been carried out with the prosthesis in the USA POM components of the Christiansen prosthesis were implanted in about 20000 patients worldwide in the 1970s and 1980s 8 The prosthesis durability was assessed as high ie prostheses removed in cases of inflection or loosening did not show wear on the articulating Delrin components Furthermore histological studies of the surrounding tissue show a benign tissue response similar to that seen around polyethylene implants Hence the Christiansen Delrin prosthesis has been recommended as a Class II device by the US Food and Drug Administration 9POM as a mediawetted component in a tissueengineering bioreactor was applied by Penick et al 10 who found that the use of POM appears to be innocuous at least with respect to human mesenchymal stem cells MSCbased tissue engineering even after machining and repeated autoclavingVitral and coworkers 11 investigated the cellular viability and nitric oxide production by J774 macrophages with ceramic polycarbonate and POM brackets Their conclusion was that in the bracket group there was no significant difference compared with the control groupAccording to Kokubo and Takadama’s definition of bioactivity a bioactive material is a material on which bonelike hydroxyapatite HAp will form selectively after it is immersed in a serumlike solution 12 However HAp and βtricalcium phosphate are difficult to shape in the complex forms required for bone treatment because of their hardness and brittleness The concept of using bioactive HAp particles/modified polymer composites as implant materials for bone replacement was introduced by Bonfield in the early 1980s 13 Since then HApmodified polymerbased biocomposites have been widely studied for bone tissue replacement The polymer matrix provides the ductility and other associated properties that are required for hard tissue replacement materials 14 15 The bioactivity of these composites is rendered by the bioactive component which when implanted into the human body interacts with the surrounding bone and in some cases even with the soft tissue This occurs through a timedependent modification of the surface triggered by its implantation within the living bone An ion exchange reaction between the bioactive implant and the surrounding body fluids results in the formation of a biologically active carbonate apatite cHAp layer on the implant that is chemically and crystallographically equivalent to the mineral phase of bone 16In this study nanosized HAp was introduced into the POM matrix to produce a new highquality bioactive material for bone implants Results of the bioactivity assessment thermal stability longterm stability and formaldehyde release of POM/nanoHAp nanocomposites are presentedThree commercial grades of POM copolymers T200 T300 and T411 Tarnoform® T2 T3 and T4 respectively with melt flow rate MFR 190/216 ISO 1133 of 25 90 and 13 g/10 min respectively were acquired from Zaklady Azotowe w TarnowieMościcach SA Molecular weight characteristics of the POM copolymers are given elsewhere 17

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