Tribological Performance of Medical Grade UHMWPE Polymer at Egg Albumen Lubricated Condition

In this experimental study, the friction and wear performance of medical grade UHMWPE polymer under egg albumen lubrication condition are evaluated. The sliding experiments were carried out on a pin-on-disc wear tester. The contact configuration used was a polymer pin on a rotating stainless steel disc. Tests conditions were 2000m travelling distance, room temperature, 40 to 120N load and 0.5 m/s sliding speed. The results show that the coefficient of friction and specific wear rate increase with the increase in load and speed values. The coefficient of friction of UHMWPE under egg albumen 20g/l lubricant conditions is lower than that of under egg albumen 10g/l lubricant conditions. Moreover, for the range of load and speed values of this study the specific wear rate using egg albumen lubricant registered lower values than that of the dry conditions. Finally the specific wear rate values for medical grade UHMWPE polymer at egg albumen lubricant conditions are at the order levels of 0.5x10m/N respectively.


Introduction
In prosthesis technology, knee replacements are usually the only solution for patients whose joints are totally worn out. In knees the loads on the prosthesis can reach peak values of three times the body weight during normal walking conditions [1] and can peak between four to five times the body weights during more stressful surface activities, such as walking upstairs. This is of concern as contact stresses can exceed the compressive yield stress of the prosthesis material [2][3][4]. The geometry of the prosthesis and the type of loading determine the contact area and size of the contact stresses in the prosthesis component [5][6][7]. On the other hand the tribological behavior of prostheses material is important such as wear. In such cases wear should be low and the debris produced must not be toxic. Excessive wear can cause the prostheses to work loose within the bone, resulting in pain, reduce life and instability. Ultra-high molecular weight polyethylene (UHMWPE) is a useful thermoplastic polymer for biomaterials applications. This is due to its excellent properties, such as bio-compatibility, chemical stability, high impact strength, high wear resistance, and low friction [8,9]. On the other hand, that, UHMWPE has low hardness and creep resistance, which may lead to excessive permanent deformation of the bearing surface, and ultimately affect the surface geometry. Understanding, enhancing tribological and mechanical properties of UHMWPE will be important to prolong the longevity of joint replacement components and alleviate pain of the patients. In past, studies have been carried out on wear characteristics of UHMWPE and point out the recognition of multidirectional motion in wear mechanisms and the development on an orientation-softening wear concept [10,11]. Furthermore, extensive studies were conducted to improve UHMWPE wear resistance, such as ion implantation, fiber reinforcement, cross-linking [12][13][14][15] and grafting [16]. Apart from molecular weight role of UHMWPE, factors affecting the wear characteristics of UHMWPE include prostheses geometry [3], presence of third party, topography, speed, loading and lubrication [17][18][19][20][21].
The aims of this study are to clarify the friction and wear performance of GUR 1020 medical grade UHMWPE polymer sliding against AISI 316L stainless steel under egg albumen lubricated conditions and to evaluate the level of influence of applied load and sliding speed values. Friction and wear tests against stainless steel disc were carried out on a pin-on-disc arrangement. These tests were at room temperature under 40, 80 and 120 N applied load values and at 0.5, 1.0 and 2.0 m/s sliding speeds.

Materials
Pin samples were machined from a non-sterilized Chirulen®1020 UHMWPE medical grade rod with 6mm diameter and 50mm length for tribological tests, respectively. AISI 304L stainless steel discs were machined to 10 mm thickness and 100 mm diameter, and was grind to 0.09 μm Ra surface roughness and with a hardness value of 297 HV. Material properties and the specific wear test conditions (that is, materials, ambient temperature, speed and humidity) are summarized in Table 1. The tribometer and tests A pin-on-disc apparatus connected to a computer was used to evaluate the friction coefficient of the GUR 1020 medical grade UHMWPE polymer against steel under egg albumen lubricated conditions. Before each tests the flat-ended polymer pins and AISI 304L stainless steel discs were cleaned with alcohol and acetone and then installed in the pin-on-disc apparatus. The friction and wear tests were performed at room temperature, sliding speeds of 0.5 m/s and applied load values from 40 to 120 N. These wide ranges of test condition were decided to cover severe working condition of knee joint such as jogging. These tests were carried out under egg albumen lubricated conditions. The lubricants were added to the rubbing surfaces at a rate of 10 and 20 drops per minute to ensure the presence of lubricant media during the test period.  Figure 1 shows a schematic diagram of the pin-on-disc wear test machine that was designed and used for this work. As shown in this figure, the rig consists of a stainless steel table which is mounted on a turntable, a variable speed motor which provides the rotational motion to the turntable, hence to the disk sample and a pin sample holder which is rigidly attached to a pivoted loading arm.
This loading arm is supported in bearing arrangements to allow loads to be applied to the specimen. During the test, friction force was measured by a transducer mounted on the loading arm. The friction force readings were taken as the average of 35 readings every one sec for a period of a 66 min test time which ensures the sufficient sliding distance within the steady state wear region. For this purpose a microprocessor controlled data acquisition system was used. Finally, the specific wear rates were calculated from mass loss and the represented. Sliding wear data is the average of at least three runs.

Results and Discussion
Figures 2a,b,c presents the variation of under lubricated coefficients of friction of UHMWPE polymer with sliding distance, tested at 0.5m/s sliding speed and 40, 80, 120N applied load. It is clear from these figures that there is a running-in stage for about 600 m sliding distance followed by a steady state condition. The lowest friction coefficient is for egg albumen 2 condition followed by egg albumen 1 condition. Figure 3 presents the variation of under lubricant friction coefficients of UHMWPE polymer with applied load tested at 0.5 m/s condition. It is clear from figure 3 that the coefficient of friction value for UHMWPE polymer decreases linearly with the increase in load value. There is an average decrease of 40% in friction coefficient for 200% increase in load value. As it is known that UHMWPE polymer is a visco-elastic material which it's deformation under load is visco-elastic. Therefore, the variation of friction coefficient with the load follows the equation µ = KxN (n-1) where µ is friction coefficient, N is the applied load, K and n are constants, n constant value is between 0.66 and 1 [22]. According to this equation, the friction coefficient decreases with the load increase. It is also clear from figure 3 that the friction coefficient for UHMWPE polymer is the lowest under egg albumen 2 lubrication conditions. This could be explained that the egg albumen as a lubricant might function to significantly hinder the friction induced thermal effect. Figures 4 shows the variation of specific wear rate of UHMWPE polymer with applied load, tested at 0.5m/s and under lubricant condition. This figure shows that, the average specific wear rates for UHMWPE polymer is in the order of 7.5x10 -15 m 2 /N to 5.0x10 -15 . The highest wear rate is for UHMWPE polymer under egg albumen 1 followed by egg albumen 2 lubricant. The lowest wear rate is 5.0x10 -15 m 2 /N using egg albumen 2 lubricant. In all cases the specific wear for GUR 1020 medical grade UHMWPE is linearly decreasing with the increase in applied load values. This is a decrease of 33% in specific wear rate for 200% increase in applied load values. In lubrication process, egg albumen lubricant enhances boundary lubrication. The minimum wear rate is obtained with the increment of egg albumen amount in the lubricants.

Conclusions
The following conclusions can be drawn from the present study.
The friction coefficient of GUR 1020 UHMWPE polymer under egg albumen 20g/l lubricant conditions is lower than that of under egg albumen 10g/l lubricant conditions. The highest wear rate is for UHMWPE at 0.5m/s sliding speed and under 40N applied load under both under egg albumen 20g/l lubricant conditions and egg albumen 10g/l lubricant conditions with a value of 7.5x10 −15 m 2 /N The lowest wear rate is 4.5x10 −15 m 2 /N for UHMWPE under egg albumen 20g/l lubricant conditions at 0.5 m/s sliding speed and under 120N applied load value. For the range of lubricants used in this investigation, the specific wear rate is highly influenced by the change in applied load and the egg albumen lubrication media. For the range of lubricants used in this investigation, egg albumen 20g/l lubricant condition is the most effective lubricant.