Testing implants using a joint simulator is complex, time consuming and very expensive. Ducom BioTribometer is widely used as a precursor to joint implant simulators, primarily for fundamental research on biomaterials that replace biointerfaces in orthopedic and dentistry.
The BioTribometer consists of a single station or up to six stations. Each station is equipped with sensors to measure friction and compound wear, a lubricant cup and a heating unit (see Fig. 1). The normal load is applied independently on each station and it is fully automated. This allows the generation of physiological gait load profiles (in compliance with the ISO 14242 test standard), as well as fixed load profiles (as per ASTM F732 test standard).
The X-Y stage is capable of multidirectional motion along both axis, like elliptical, rotation, square, and figure 8 wear profiles. In case of linear reciprocating motion (along X or Y axis), the cross-shear is generated by using the pin rotation feature up to 2 Hz or square type motion profile without pin rotation.
Ducom BioTribometer has been benchmarked against simulators and retrieved implants to establish the in-vivo wear mechanisms and wear rates observed for UHMWPE-CoCrMo hip prostheses materials. Ripples associated with fatigue-wear mechanism (gait loading) cannot be reproduced by conventional multidirectional pin on disc tribometers.
Table 1. Comparison of wear rate and mechanism for UHMWPE as determined by simulator, retrieval study and Ducom BioTribometer.
In this regard, we present several case studies related to friction, wear and lubrication behavior of biomaterials and biofluids tested in Ducom BioTribometer.
List of Case Studies
SIMULATE ISO 14242 GAIT LOAD Profile. As shown in Figure 2B, we were able to reproduce the gait load profile with intervals as close to the ISO 14242. The total duration of one cycle was below 8 seconds. Furthermore, each station in the BioTribometer has a pair of load cells to acquire friction force along X and Y axis for all test duration. The friction coefficient is automatically calculated and displayed in the WinDucom software and MOOHA platform.
WEAR RESISTANT UHMWPE GRADES FOR DOUBLING IMPLANT LIFE. Two different grades of ultra-high-molecular polyethylene (UHMWPE) pins have been tested against Cobalt Chromium Molybdenum (CoCrMo) disks. The load profile was a sine wave from 88 to 186 N over 0.5 million cycles (MC). The motion profile was linear reciprocation with 1 Hz of pin rotation and the test samples were submerged in bovine calf serum at 37 °C. In situ friction profile was acquired throughout the test lasting several days. The gravimetric weight loss analysis or wear was measured at every 0.25 MC.
As shown in Figure 3A, the wear rate of the crosslinked UHMWPE (0.64 mm3 / MC) was almost an order of magnitude lower compared to the uncrosslinked UHMWPE (5.19 mm3 / MC). In fact, crosslinking of polyethylene reduces the mobility of adjacent carbon-carbon chains by reinforcing their bonds, hereby rendering the polyethylene more resistant to wear. Also, severe scratches were observed on both uncrosslinked UHMPWE and its counterbody CoCrMo disk (see Fig. 3B).
| Professor Kantesh Balani and his research team (Department of Materials Science and Engineering at Indian Institute of Technology - Kanpur), have recently published an interesting article on the Tribological properties of SS 304 and Ti6Al4V using four reciprocating geometries. They used Ducom BioTribometer (six-stations) to classify the severity of friction and wear of two triboparis under four different reciprocating geometries – linear, circle, square and eight/butterfly. |
FRICTION AND WEAR COMPARISON BETWEEN Ti6Al4V and CoCrMo. Standard UHMWPE pins in contact with Ti6Al4V and CoCrMo disks were tested over 1.2 million cycles (MC) using a walking gait load profile (400N as maximum load). The test samples were submerged in bovine calf serum at 37 °C. Gravimetric weight, friction and wear were acquired and displayed up to 1.2 MC.
Friction coefficient of UHMWPE and Ti6Al4V tribopair was lower than UHMWPE and CoCrMo tribopair (see Fig. 4A). It can be attributed to a thicker oxide layer on Ti6Al4V compared to CoCrMo. In general, wear rate of UHMWPE increased linearly over an increase in the number of cycles up to 1.2 MC (see Fig. 4B). The cumulative wear of UHMWPE -Ti6Al4V tribopair and UHMWPE-CoCrMo was 8.35 mm3/MC and 9 mm3/MC, respectively.
Qualitiative wear analysis using scanning electron microscopy (SEM) (see Fig. 5) indicated higher wear on UHMWPE (against Ti6Al4V), as there were severe fiber pull out (or delamination), a polished surface, deeper cracks and interestingly more ripples. There were also more scratches on the Ti6Al4V disk compared to the CoCrMo disk. These surface characteristics usually are observed on retrieved implants or joint simulators. Several clinical studies have proven that entrapped third-body wear particles (including CoCrMo particles) are responsible for scratches on CoCrMo articulating against UHMWPE. Further study is needed to understand the oxide layer formation and corrosion resistance of the metal surfaces using the tribocorrosion setup in Ducom BioTribometer.
WHY ASTM F732 IS ANTIQUATED. Dynamic loads and heat generated (up to 45°C) during the physical activities like walking and stair climbing can influence wear of UHMPWE. However, the present standard test method ASTM F732 does not include these factors. Here, we modify the ASTM F732 by changing the fixed load into dynamic loads (A), and the effect of temperature on wear of UHMWPE (B). The test samples were submerged in bovine calf serum and the duration of the test was 0.6 million cycles in both cases.
A. Dynamic load effect
Walking and stair climbing gait load cycles (see Fig. 6) have been reproduced in Ducom BioTribometer. CoCrMo pins in contact with UHMWPE doped with Vitamin E disks were used as tribopair.
Stair climbing shows high run in friction (see Fig. 7A), severe multidirectional scratches, deep grooves and spalling (see Fig. 7B) compared to walking. Gravimetric weight loss after the test was 3.6 mm3/MC and 1.6 mm3/MC for stair climbing and walking, respectively.
B. Temperature effect
Standard UHMWPE and UHMWPE doped with Vitamin E pins in contact with CoCr disks were tested at 37°C and 45°C during walking gait cycle. As shown in Figure 8, the low friction wear behavior of UHMWPE doped with vitamin E at 37 °C is not applicable at 45 °C, due to denatured Vitamin E at elevated temperatures.
Furthermore, large ripples were observed on UHMWPE doped with vitamin E at 45 °C due to low fatigue wear resistance (see Fig.9).
WHY BIOTRIBOLOGY TESTS AT STATIC LOAD IS UNREALISTIC?Friction and wear behavior of UHMWPE materials is often investigated at fixed load that do not represent the variable physiological stress conditions. This difference can adversely affect the selection of UHMWPE materials for the component testing with an aim to improve the lifespan of joint implants. Here we have tested UHMWPE pin (8 mm in diameter) against CoCrMo disk (29 mm in diameter) at fixed load (225 N, according to the ASTM F732) and using the walking gait cycle (please refer to Fig. 6A). The test lubricant was Bovine Serum Albumin (BSA) diluted in phosphate-buffered saline (5 g/l PBS). Friction and wear behavior were monitored for 1.26 million cycles.
During dynamic load, the friction coefficient was stable compared to the fixed load. Furthermore, the UHMWPE showed lower volume loss during dynamic load compared to fixed load.
Our results indicate that the wear multiple cracks for both fixed and dynamic load profiles, deep grooves for fixed load and fatigue wear visible for dynamic load (ripples). Our results indicate that the wear mechanism of UHMWPE in ASTM F732 (fixed load) is qualitatively and quantitatively different compared with ASTM F732 (dynamic load).
DO ANTIBACTERIAL COATINGS AFFECT WEAR AND METAL TOXICITY? In this study, we investigated the wear behavior of two different antibacterial coatings on CoCr. The counter body was a standard UHMWPE pin and the lubricant used was Newborn Calf Serum (Australian Origin) with a protein concentration of 20 g/l. The testing chambers were covered with balloons in order to avoid contamination and evaporation of the serum (see Fig. 12). We have tested 5 identical tribopairs simultaneously. Friction and wear behavior of these coatings were monitored for 1 million cycles. A “Butterfly” profile as well as a sine load curve going from 250 to 400 N was be used as test profile. The testing chambers were heated at 37°C.
In general, the deformation and wear rate of UHWMPE was in the same order as that of clinical reports on UHMWPE cup penetration and wear rate. The friction coefficient was in the range of 0.04 to 0.10. Results from the BioTribometer showed the differences between coating A and B.
Coating A had increased the wear rate of UHMWPE by a factor of three compared with coating B (see Fig. 13A). Furthermore, the post-test wear images showed severe scratches on the coating A compared to coating B (see Fig. 14). However, these coatings showed no significant differences in the Co or Cr ion release from CoCrMo substrate (see Fig. 13B). This study shows that BioTribometer can mimic the wear behavior of implant materials, it was used for selection of wear resistant antibacterial coating, that was coating A.
CALF vs FOETAL SERUM - WHICH LUBRICANT SHOULD WE USE? In this study, we investigated the lubrication effect on the wear and friction of UHMWPE. The counter body was steel alloy disk and the lubricants used were Newborn Calf Serum (Australian Origin) and Foetal Bovine Serum, both with a protein concentration of 30 g/l. Friction and wear behavior of were monitored for 1 million cycles. A “Butterfly” profile was used as test profile at a fixed load of 650 N. The testing chambers were heated at 37°C. We followed the cycle intervals as mentioned in the ASTM F732 (i.e., 50000, 200000, 500000, 1000000).
In general, the wear rate of UHMWPE lubricated by both serum increased over the 1 million cycle. After 1 million cycles, the wear rate of UHMWPE lubricated by newborn calf serum was surprisingly three times higher compared to the previous measurement (i.e., at 500,000 cycles). The wear rate of UHMWPE lubricated by calf and foetal serum was 18.8 mm3 and 10.3 mm3, respectively. In general, the friction coefficient decreased with the number of cycles, indicating a smoothening effect of the polyethylene surfaces. The average friction coefficient for the tribopairs lubricated by calf and foetal serum was 0.014 and 0.019, respectively. This study shows that BioTribometer can be used for screening of different biolubricants. This preliminary study suggests the use of new born calf serum compared to foetal serum for accelerated wear test.
IMPORTANCE OF FRICTION IN DENTAL IMPLANT WEAR. Titanium alloys are widely used as dental biomaterials in the form of abutments and screws. Although they are biocompatible, they have a poor wear resistance behaviour against ceramics. Hard coatings could represent a solution to improve the wear resistance of titanium-based materials. The effectiveness of such coatings should be tested in friction and wear tests able to reproduce physiological conditions occurring in dental implants. In tis study, Ti6Al4V disks, with and without DLC coating, were evaluated for their friction and wear properties under physiological chewing loads (see Fig. 16) using Ducom BioTribometer.
The disk specimens were reciprocated along a 2 mm stroke length against a yttria-stabilized zirconia ball (3Y-TZP) 6 mm in diameter. The tests were run in a PBS solution bath at a temperature of 37°C, for a total duration of 1 hour (2610 cycles). Optical microscopy was used to quantify the wear of dental materials, post friction test.
DLC protective coating reduced the friction for Ti-6Al-4V by more than 80% (see Fig. 17). As a result, the wear of Ti-6Al-4V dropped 99%. Under physiological load condition, an increase in friction translated in an increase in wear, for Ti-6Al-4V (Figure 18). Finally, friction has a direct influence on wear of dental materials. DLC coating protects Ti-6Al-4V against wear due to its low friction behavior.
TRIBOCORROSION STUDY. Ducom BioTribometer can be equipped with a tribocorrosion module (see Fig. 19) to evaluate the electrochemical behavior of materials during friction and wear tests.
The tribocorrosion module consists of a three-electrode electrochemical cell setup to measure potential and current. In this study, we evaluated the tribocorrosion behavior of a Zirconia ball (10 mm in diameter) loaded against a disk made of SS316L (60 mm in diameter, 15 mm thick). The normal load was 10 N, the stroke length was 10 mm, and the sliding velocity was kept at 20 mm/s (1 Hz frequency). The medium used was NaCl (3.5 wt%) at ambient temperature. The tribocorrosion test protocol was validated with multiple tests on same sample (see Fig. 20).
During the Stage I (about 5 minutes), only corrosion potential was measured (stabilization / passivation). During Stage II, the disk started reciprocating against the Zirconia ball initiating the destruction of the passive layer, enlargement of the wear track, enhancing corrosion and wear. After 30 minutes of friction and wear test, the relative motion stopped and a re-formation of the passive film was observed (Stage III).
Physiological load profiles (e.g., walking, stair climbing, running, etc.) of daily activities and cross shear effect of UHMWPE must be considered in the screening process of new biomaterials. Dynamic load profiles, multidirectional motion, and temperature control features in Ducom BioTribometer offer realistic test platform for simulating in-vivo conditions and reproduce the wear mechanisms and wear rates observed from clinical studies.