Traditionally we've used the tester to study non-living tissues to calculate friction coefficients but we are significantly expanding the tester's capabilities to test living tissues and to evaluate biological interactions. The friction testing device delivers reciprocal translating motion and compressive load to contacting surfaces of living tissue explants. The device is modular, allowing for the testing of various biological counterfaces.
The method could provide insight into how frictional forces affect the mechanical and mechanicobiological responses of living cartilage and synovium, which may lead to new strategies for maintaining joint health. Begin by harvesting the juvenile bovine synovium. Using a scalpel blade, trace the outline of the synovium region of interest.
Using forceps, grasp one end of the synovium and gently lift to stretch the synovium distal to the underlying bone. Use a scalpel blade to remove the synovium from the bone and place the tissue in appropriate culture media or testing bath solution. Then harvest the juvenile bovine cartilage by securing the tibia in an adjustable holder.
Remove the meniscus carefully while avoiding contact with the cartilage surface. On the outer edges of the tibial plateau, use a box cutter to cut perpendicular to the cartilage toward the bone. Cut completely through the cartilage to make straight edges or sides.
Remove excess tissue. On the outside edges, use the box cutter to make a clean cut at the interface between the bone and the cartilage. To remove the tibial strip from the plateau surface, gently insert a flathead screwdriver below the cut and gently rotate to loosen the articular cartilage from the subchondral bone.
As the sample loosens, slowly push the screwdriver forward until the cartilage strip detaches from the bone. Ensure that the screwdriver is pushed towards the bone, not the cartilage. Using a box cutter, cut the tibial plateau surface to produce rectangular samples of desired size and thickness.
Place tissue in appropriate culture media or testing bath solution. If the tibial strip is used as the bottom counterface, remove the detachable magnetic base and glue a 60-millimeter diameter Petri dish to the top surface of the detachable base. With the Petri dish glued in place, attach the detached base to the fixed base and mark the Petri dish to indicate a sliding direction.
Apply a small amount of cyanoacrylate to the center of the dish. Align the tibial strip with the sliding direction of the stage. Gently press the cartilage strip onto the dish.
Restore the removable magnetic base to its paired fixed base in the friction tester. Fill the Petri dish with the desired testing bath solution. If the synovium is used as the top counterface, remove the loading platen and support rod from the friction tester.
Place the synovium on top of the circular platen. To secure the synovium, spread an O-ring over its circumference. Using forceps, gently pull at the synovium to stretch tissue taut and flat beneath the O-ring.
Trim excess tissue with surgical scissors. Restore the loading platen and support rod to the friction tester. Adjust the vertical height of the loading platen such that the synovium hovers over the bottom counterface and is submerged in the testing bath.
Insert the mounted specimens into the friction tester device. Open the analog data build MF DAQ. Initialize Load PID and Trigger Dynamic Caller Windows in the program.
Run the analog data build MF DAQ and initialize load PID window by pressing the Run button. Navigate to the Stepper tab in the Trigger Dynamic Caller window. Specify the acceleration, speed, and distance of the translation stage in the user input boxes.
To input the test duration, click on the Open Folder button at the bottom right of the Time State table and select the Stepper Time Index file path. Then specify the test duration again in the Voice Coil tab. Select the Voice Coil Index file path by clicking on the Open Folder button at the bottom right of the Time State table and select the file.
This must be performed whether the voice coil is used or not. Apply the normal load. If using deadweights, place desired weights on the linear bearings above the loading platen.
Ensure that the load applied plus the weight of the loading platen and support rod do not surpass the load cell-rated capacity. Select the path and file name for data storage using the Open Folder button to the right of the Right to File box. Save the file with a txt extension.
Center the bottom counterface underneath the top counterface and set this as the zero X position. To do this, run the Trigger Dynamic Caller window by pressing the Run button. In the Stepper tab click on the Home button to move the stage to the last saved zero X position.
If counterfaces are not aligned, move the stage by clicking the green left and right arrow buttons. When the desired location is reached, click on the zero button to save the current stage location as the new zero X position. Stop the Trigger Dynamic Caller window by clicking on the Stop button.
Once the top and bottom counterfaces are centered, initiate friction testing of the samples by starting the cyclic movement of the stage. Once the stage moves, slowly bring the top counterface into contact with the bottom. Let the test run, collecting the friction testing data.
After the desired testing duration, stop the test by pressing the Stop button and by unloading the specimens by raising the top counterface and moving it out of contact with the bottom counterface. Use the custom code to calculate the friction coefficient and the hysteresis per cycle. Ensure a single folder contains all relevant files.
Open the Friction Cycle run. m file. Click on the Run button in the script.
Select the raw data file to analyze and the desired save location. The friction coefficient and hysteresis plots will be output by MATLAB. A synovium-on-cartilage configuration was used to friction test juvenile bovine explants where the synovium was mounted on a 10-millimeter diameter acrylic loading platen such that the intimal layer would be in contact with the underlying cartilage.
A tibial strip was used as the cartilage counterface. An effective friction coefficient was calculated from the average of FT divided by FN over each reciprocating cycle and then plotted against test duration to yield a friction coefficient-versus-time plot. For each test, friction coefficient values were averaged over the entire test.
In a PBS testing bath, the average friction coefficient values increased as the contact stress increased. Conversely, the average friction coefficient values remained similar as the contact stress increased in a bovine synovial fluid bath. Remember to start collecting load data before bringing the surfaces into contact.
This ensures a proper tare load can be calculated. Tissue and lubricating bath components can be assessed before and after testing to evaluate the biological changes imparted by a given experimental regimen.
