Le PDMS comme modèle de substitution pour la friction du cartilage articulaire

(En Anglais seulement)

The Mach-1 v500ct (Biomomentum Inc.) can conduct rotational friction testing to measure the coefficient of friction (COF) between a pair of materials for a given lubricant at a controlled temperature. Such tests may be useful for characterizing interarticular supplements. However, testing these supplements with articular cartilage samples require many of these samples to overcome their variabilities in shape and mechanical properties. An alternative is to test with polydimethylsiloxane (PDMS) plugs. Various studies have reported low COFs using PDMS substrates in the 5×10^-3 to 5×10^-2 range with values comparable to human cartilage-on-cartilage COFs [1]. Furthermore, PDMS plugs are inexpensive to produce, and their shapes and physical properties are easily controllable.

As a demonstration, this case study presents the feasibility of using the Mach-1 v500ct (Biomomentum Inc.) for pin-on-disk friction experiments. In the tests, a PDMS plug (height of 10.6mm ± 1.2mm and diameter of 15.5mm) was fixed to a multi-axis load cell and compressed onto a glass disk. It was deformed by 3% strain, which is the lowest strain guaranteed to be within the PDMS’s linear elastic region (data not shown for simplicity). Then, the glass disk was rotated, either at an effective speed of 1mm/s or 7mm/s (note that the effective speed is calculated as the rotational velocity of the motor multiplied by the effective radius (2/3 x radius) of the PDMS plug). Moreover, to reduce uncertainty, the PDMS sample was allowed to relax back to its undeformed state between each test. As the glass disk rotates, it exerts a frictional torque (Tz) that is measured by the multi-axis load cell. The ratio between this frictional torque (Tz) and the effective moment arm (Reff) provides the effective frictional force that acts tangential to the direction of rotation. The disks were rotated for 720° so that the system could stabilize during the first 360°, and data analyzed during the second revolution (360-720°). The equations used to calculate the static and kinetic coefficients of friction are given in Figure 1. The PDMS-glass interface was lubricated by 4mL of either phosphate buffered solution (PBS) or equine synovial fluid (SF), with n=4 PDMS-glass samples tested per lubricant. To simulate physiological temperatures, the tests were conducted in a chamber submerged in a 37°C PBS bath. Figure 2 features a schematic of the test setup.

The resulting friction curves demonstrate consistency between n=4 samples—particularly, in the static friction region, as shown in Figure 3a. The curves have similar shapes and peak at similar angular positions. The kinetic friction curves also remain relatively stable and fall within ±35% of the overall average, as illustrated in Figure 3b. Furthermore, the data reveals that static COFs (Figure 4a) and kinetic COFs (Figure 4b) are statistically different between lubricants (p=0.003 and p=0.0004, respectively), even with only n=4 samples. No statistical differences were found between the testing speeds for both static and kinetic COFs (p=0.95 and p=0.12, respectively) supporting the homogeneity in mechanical properties of PDMS samples. The statistical analysis was performed using two-way ANOVA tests with a 95% confidence interval.

These results demonstrate the viability of measuring coefficients of friction between PDMS and glass as a less expensive and more convenient alternative to cartilage-on-cartilage testing. The method could be developed into a larger-scale study with a larger sample size to evaluate the effectiveness of future interarticular supplements.

Case study and analysis prepared by Richard Sheng, engineering intern at Biomomentum.

[1] Lin, W., Klein, J., Recent Progress in Cartilage Lubrication. Adv. Mater. 2021, 33, 2005513.