Mechanical Bioreactor for Uniaxial Loading of Articular Cartilage
Background Information & Big Picture
Articular cartilage is a unique connective tissue that lines the ends of long bones, serving as a bearing material that allows for smooth, pain-free joint motion. In response to aging, injury, or mechanical overloading, the tissue will often break down, leading to the painful, debilitating condition of osteoarthritis (OA). No effective treatment of OA currently exists, and extensive research is needed to understand the onset of OA in order to develop strategies to halt or reverse the progression of the disease.
One procedure for OA research involves explanting small living pieces of cartilage tissue from animal joints and maintaining them in a Petri dish for long-term examination. Mechanical bioreactors are a key requisite for this procedure, as they are capable of applying dynamic loads to these cartilage explants. By subjecting cartilage specimens to dynamic loading, research staff at Professor Albro's Boston University laboratory will be able to stimulate what is known as a "growth factor". Ultimately, this will provide them with valuable information about the onset of OA in humans.
Specific Task & Motivation
The group was tasked with building a device capable of applying physiologic mechanical loads to live cartilage samples at Professor Albro’s lab. The objective of our project is to provide our customer, Professor Albro, with a scaled prototype of a mechanical bioreactor that will conduct uniaxial strain tests on articular cartilage samples in a sterile environment. The device should be permanently placed in an incubator, with the ability to withstand the internal environment and function without losing sterility. The well tray containing cartilage samples should be easily removable and able to be loaded in fume hood. No pressure/force measurements are required for the scope of this project. A chart of all the quantitative and qualitative requirements for this projects is shown below:
Figure 1: The qualitative and quantitative specifications for the mechanical bioreactor, as stated by Professor Michael Albro.
The tare and dynamic strains were two important engineering concepts that the future bioreactor had to account for. Tare strain refers to the initial strain applied which serves to account for the variability in cartilage sizes. Because the bovine cartilage specimens were clipped by hand, they were not of the same size, height and flatness; the specimens had a variation of 3 mm ± 0.05 mm. As a result, the initial tare strain (5-30% of the cartilage size, or 100-500 microns) ensured that all samples are in equal contact before the dynamic strain testing had been initiated. A sample drawing of the cartilage samples explaining the concept of tare strain is shown below:
Figure 2: A drawing of the cartilage samples, explaining the concept of tare strain.
Following the initial application of the tare strain, a cyclical, up-and-down dynamic strain (5-30% of the cartilage size) needed to be performed on the cartilage for 3-4 hour time frames. The dynamic strain would serve to stimulate the growth factor in the cartilage samples.
The bioreactor design also had to account for the relatively small cartilage sizes (2-3 mm in diameter and height) and the need for these samples to be submerged in DMEM - a common culture fluid used in biomedical labs - during their testing. In order to ensure proper testing, the culture fluid volume had to be 10 times that of the cartilage volume.
Another important engineering specification that had to be considered was the elastic modulus of the cartilage. Prior to building the bioreactor, our group conducted testing during which we found that the cartilage had an elastic modulus of 1 MPa when dry, and 50 MPa when saturated with DMEM fluid. The bioreactor had to be able to perform testing on these samples, with an increasing frequency from 0.1 Hz during initial cycles to 1 Hz with subsequent cycles. The testing had to run for approximately 3-4 hours at a time.
The machine also had to account for the sterility of the cartilage. In order to prevent contamination, the samples had to to be loaded under a fume hood by a lab assistant. This process is pictured below:
Figure 3 (left): A research student loading the cartilage specimens in a well tray, under a fume hood. Figure 4 (right): Two live cartilage specimens in a Petri dish with DMEM culture fluid.
Figures 5-6: The outside (left) and inside (right) of the incubator used in Professor Albro's research lab.
Design Considerations Based on the engineering specifications provided to our group, we concluded that our final bioreactor design would likely employ a stepper motor with a driver and a microprocessor, likely an Arduino. The design would also have to feature an actuator to stimulate the cartilage specimens and some sort of measurement device (either a digital encoder or a manual dial indicator) to track how far vertically the actuator has moved. The bioreactor would have to test multiple cartilage specimens at once, so a micro plate was deemed appropriate for holding many samples in one place.
Following the initial planning, a Gantt chart was created with hopes of organizing the team and creating important deadlines:
Figure 7: A Gantt chart of the bioreactor project.
Design Process The design of the mechanical bioreactor began with a preliminary sketch of a weighted lever-camshaft design. This design incorporated a stepper motor attached to a weighted lever which, in turn, was affixed to a pivot point. The rotational movements of the stepper motor would translate to uniaxial movement of the lever, pushing the plunger piece up and down. The vertical movement of the plunger piece would then stimulate the cartilage specimens, placed in a microplate underneath the plunger. A sample sketch of this design is shown below:
Figure 8: Sketch of the first, weighted lever-camshaft design.
While this design above was relatively simple and doable, it had one major flaw - the user would have to manually adjust the pivot location of the weighted lever in order to switch between different tare strains and dynamic strains functions. By switching the pivot location, the user could proportionally vary the vertical distance that the plunger moved. The mechanical nature of this design was a major drawback, as we wished to design a machine that was fully autonomous.
Our next idea solved the issue of manual adjustment by incorporating three lead screws driven by three NEMA 17 stepper motors with 200 steps/revolution. These lead screws would be attached to an actuator bridge, which would press on the plunger piece, stimulating the cartilage. This design is shown below:
Figure 9: Sketch of the second, lead screw-stepper motor design.Figure 9: Sketch of the second, lead screw-stepper motor design.
To further simplify the electronics and operation of the device, we decided to reduce the number of lead screws and motors to one. We finally compared the first design and the modified second design using the Pugh chart shown below:
Table 1: Pugh chart comparing the first and second bioreactor designs.
In order to produce micron-scale movements while using a stepper motor with only 200 steps/revolution, we needed a lead screw with an appropriate diameter and lead. Using an online "Prusa steps/mm calculator for lead screw-driven systems", we determined that we need a lead screw with a 2 mm diameter and a 0.5 mm lead.
However, we also needed to ensure that the lead screw would not buckle during the actual testing of the cartilage. In order to figure out the maximum number of cartilage samples that we can simultaneously load using our lead screw, we used a sensitive force measurement apparatus to test the loading per sample in Professor Albro's lab. The resulting data is shown in the table below:
Table 2: Testing single samples of cartilage for force load estimations.
As can be seen in the table, we measured 200 grams or 1.96 N of force for a single sample loaded at 9% strain. This roughly translates to 94 N for all 48 samples. Considering that we would have a strain of up to 30%, we expected roughly 300 N of force when all 48 samples were loaded.
To calculate the maximum buckling load of our 2 mm diameter/0.5 mm lead lead screw, we performed the following calculations:
Figure 10: Calculations for the maximum buckling load of our 2 mm diameter/0.5 mm lead lead screw.
From the above calculations, our lead screw could only withstand 243 N of force. Therefore, at the maximum 30% dynamic strain testing, we could only test 38 cartilage samples. Any more than 38 would cause the lead screw to bend.
Design Results Once the design had been finalized, all relevant calculations performed, and all component dimensions measured, we created a SolidWorks assembly of our mechanical bioreactor. The finalized version, featuring all components, is shown below:
Figures 11-12: Isometric (left) and front (right) SolidWorks assembly views of our mechanical bioreactor.
As seen in the CAD assemblies above, the final design relies on a NEMA 17 stepper motor (labeled as "1" in the front view above) and a lead screw (2) to move an acrylic actuator bridge (3) up and down several micrometers at a time (depending on the dynamic strain selected). The design has 2 stainless steel guide rods (4) parallel to the lead screw to ensure the uniaxial vertical motion of the actuator bridge. The bridge, in turn, translates that movement to the polypropylene sterile cover (5), which presses the 48-plunger Teflon piece (6). Finally, this plunger piece presses the cartilage samples, located in a 48-well microplate inside the Teflon base tray (7).
The stepper motor is attached on the acrylic roof (8), while the guide rods are held in place by bushings which were press-fit into the bottom acrylic sheet (9).
To maintain the structural integrity of our design and to keep the bending of the sub-assembly parts under 100 microns, we decided to reinforce our bioreactor with aluminium 80/20s. Because of weight issues and sterility requirements, we changed our base tray (7) and plunger piece (6) to Teflon, as it is an autoclavable material and can withstand our requirements for strength and stiffness. Furthermore, we changed our initial sterile cover (5) material from acrylic to polypropylene because polypropylene is also autoclavable and easily machinable.
The acrylic roof (8) and bottom acrylic sheet (9) were laser cut at the Boston University Engineering Product Innovation Center (EPIC). Below is the SolidWorks drawing of the acrylic roof:
Figures 13: SolidWorks drawing of the acrylic roof, used for laser cutting.
The holes of the 48-plunger Teflon piece were machined by a 3-axis Haas CNC mill located in EPIC. A Teflon rod was then cut into 48 smaller cylinders, which were later press-fitted inside the holes to create plungers. Below is a SolidWorks drawing of the plunger piece, which was used for creating a model in GibbsCAM. Using this model, the G-code was generated, which was later uploaded to the CNC machine to mill the holes properly.
Figure 14 (left): SolidWorks drawing of the plunger piece. Figure 15 (right): GibbsCAM model of the Teflon plunger piece holes featuring the CAM tool paths in orange.
Finally, we created a GibbsCAM model of the Teflon base tray, which we used for machining at the CNC mill.
Figure 16: GibbsCAM model of the Teflon base tray showing the CAM tool paths in orange.
Once all components had been machined, we assembled the prototype. Below is an image of the first prototype, featuring a complex stepper motor with 52,000 micro-steps/revolution from Schneider Electric:
Figure 17: First prototype of the mechanical bioreactor.
To simplify the above design, the Schneider Electric motor was replaced with a simpler, 200 steps/revolution NEMA-17 motor connected to a DRV8824 driver and an Arduino. Additionally, we programmed an LCD with buttons, which enables the user to select a tare strain and dynamic strain combination from the LCD menu. Below is a picture of the second prototype, with no lead screw:
Figure 18: Second prototype of the mechanical bioreactor, featuring an interactive LCD with menu options.
A dial indicator was then added to the design to indicate when contact with the cartilage has been made. Additionally, small pieces of 80-20 were bolted to the acrylic roof to minimize any potential deflection, as this could alter the testing results. The final version of the mechanical bioreactor is shown below:
Figure 19: Third and final version of the mechanical bioreactor, featuring a dial indicator.
Design Evaluation A comprehensive bill of materials detailing the price of all components used in the construction of the mechanical bioreactor is shown below. The total money our team spent on the bioreactor project was $579.92. However, we did not use the $90.80 steel block, intended for the base tray, as stainless steel is relatively difficult to machine. Therefore, excluding the block, the total cost of the mechanical bioreactor was $489.12.
Figure 20: Bill of materials for the mechanical bioreactor.
Several improvements to this projects can be made by future teams. For one, the NEMA 17 motor can be replaced with a more sophisticated motor having a greater resolution, which will allow future teams to swap out the current 2 mm diameter lead screw with a thicker one and still achieve micron movements. Consequently, this will enable future users to test all 48 cartilage samples simultaneously, instead of a maximum of 38 at a time.
Additionally, an absolute encoder can be used instead of a dial indicator to provide more-accurate feedback of the location of the actuator bridge. This will ensure that the cartilage samples are tested properly.
When taking into account production volume, the cost per unit can be lowered to around $400. This is significantly more affordable than current, commercially-available bioreactors, which are often $10,000 or more. While our design is not ideal due to having only a semester to work on it, if improved by future teams, it can serve as an economical alternative for Professor Albro's Lab.