Plasma immersion ion implantation of UHMWPE

Abstract

Ultrahigh molecular weight polyethylene (UHMWPE) has been the material of choice in orthopaedic joint prostheses for over three decades owing to its proven low friction and excellent wear resistance when coupled with smooth Co-Cr-Mo alloy or austenitic stainless steel surfaces in vivo [1]. However, it is gradually recognized that the principal constraint on the longevity of total joint replacements is the body’s reaction to debris, which is mostly UHMWPE [2]. Therefore, how to improve the wear resistance of UHMWPE and thus reduce the volume of wear debris, which promotes implant loosening, is a timely task from both a scientific and technological point of view. Nitrogen ion implantation of UHMWPE has been proved to improve its tribological properties in terms of low friction and extremely low wear rate [3], however, it is difficult in practice to uniformly treat such three-dimensional objects as hip joint sockets without using sophisticated manipulating devices. Recently, an innovative hybrid technology, plasma immersion ion implantation (PI3), has been developed, which can, to a large extent, address the problems with ion implantation of components with complex shapes [4]. However, there appears to have been little, if any, work reported on the surface modification of UHMWPE using the innovative, hybrid PI3 technology with a view to improve its mechanical and tribological properties [5]. In the present investigation, an attempt has been made to explore the feasibility of surface modification of UHMWPE using the novel PI3 technology. The material used in the study was medical grade UHMWPE (GUR-412) in the form of 12.5 diameter, extruded and annealed rod which was obtained from Poly Hi Solidur Ltd (GB). The density of the homogeneous material and the average molecular weight are 0.932 g cm−3 and 3.6× 106 g mol−1, respectively. Specimens were treated in a Mark 1 PI3 unit at the Technical University of Clauthal with the pulse source being (PI3 treatment) or not being (plasma treatment) applied. Specimens were immersed in a low pressure (0.17 Pa) radio frequency plasma discharge, which was generated by a 300 W r.f. power at 13.56 MHz. Table I lists the treatment conditions for plasma treatment (PT) and PI3 treatments. By selecting appropriate pulse length and pulse frequency, the treatment temperature was controlled at below 90 ◦C. Although it has been claimed that ion implantation can decrease the surface roughness of polymeric materials [6], such an improvement was not observed in the present study. Indeed, surface relief appeared on PI3-H specimens largely because of the ion etching effect resulting from nitrogen ion bombardment. Slight surface discoloration was observed for all surface treated specimens, increasing in the order of PT, PI3-L and PI3-H. In view of the fact that the surface modified layers of UHMWPE material usually are very thin and the substrate very soft, the mechanical properties of the untreated and treated UHMWPE specimens were measured using a NanoTest 500. A standard procedure developed by Oliver and Pharr was adopted to process the load-depth data [7]. Fig. 1 depicts characteristic load versus depth hysteresis curves, showing the loadingunloading response of the PI3 treated as well as untreated material. It can be clearly seen that, to reach the same penetration depth (250 nm), a higher load was need for the treated surface relative to the untreated surface. It also demonstrated that PI3 treated material exhibited a higher elastic recovery and less creep during the dwelling time (10 s) than the untreated material.