The defined objective was that the mold must be intuitive to researchers, regardless of experience level. One mold can make six PDMS frames in one round of PDMS curing. Common laboratory practice of making PDMS is intuitive, and a 10mL syringe can be used to inject the PDMS into each well. The mold is easy to handle, and can be moved from benchtop to oven, without concern. Compared to the current model assembly, the mold itself combines four steps into one. Previously, users had to start from a sheet of PDMS, trim the edges of the PDMS sheet, punch six evenly spaced holes, cut squares around each hole, and make those squares into
octagons. With the PDMS mold, the user will only have to remove the PDMS frame and begin cutting slits and placing fibrin microthreads into the slits. The mold produces identical frames without relying on the user to properly shape them. The mold has a built-in groove for holding the Thermanox® coverslip, eliminating the need for the user to make the Thermanox® stage.
Once the PDMS frames are carefully removed from the mold using a microspatula , the mold can be cleaned and used again. Figure 39 displays the steps for both the Pins Model and ML Model.
Figure 39. Steps of both the Pins Lab Model (left) and ML Model (right). The ML Model decreased number of steps from 15 to 8.
The assembly times to create the Pins Model and the ML Model were recorded and compared. Each of the members of the design team (N=4) were timed as they completed each part of the Pins Model assembly (Appendix G) and the ML Model assembly (Appendix H). The assembly steps were categorized into four different parts: PDMS frame fabrication, Thermanox® ® placement, fibrin microthread fixturing slit cutting, and fibrin microthread placement.
Although times for all four steps were recorded, the team decided that only three parts, PDMS frame fabrication, Thermanox® placement, and fibrin microthread fixturing slit cutting, would be analyzed and compared between models. This is because the mold, cutting tool, and cutting
guide were designed to decrease user-to-user variability and ease of use in these steps, and did not address fibrin microthread placement.
The average time of each team member for assembly of the Pins Model and ML Model were recorded in Tables 19 and 20.
Table 19. Assembly times for four assembly steps: Pins Lab Model
Assembly Step Team Member 1 Team Member 2 Team Member 3 Team Member 4 Average Standard Deviation PDMS Frame 6.5 min 8.5 min 9 min 8.2 min 8.05 min ± 1.08 min Thermanox®
Stage 11.9 min 12.2 min 12.5 min 12.7 min 12.3 min ± 0.35 min Fixturing Slits 1.5 min 1.1 min 1.25 min 1.5 min 1.3 min ± 0.20 min Thread
Placement 33.12 min 19 min 22.2 min 26.4 min 25.2 min ± 6.09 min
Table 20. Assembly times for four assembly steps: ML Model
Assembly Step Team Member 1 Team Member 2 Team Member 3 Team Member 4 Average Standard Deviation PDMS Frame 5 min 6 min 5.2 min 5.2 min 5.35 min ± 0.44 min Thermanox®
Stage 4.4 min 4.7 min 5.7 min 5.5 min 5.08 min ± 0.62 min Fixturing
Slits 1.4 min 0.9 min 0.8 min 0.75 min 0.96 min ± 0.29 min Thread
Placement 25.1 min 23.4 min 32.2 min 14.9 min 24.1 min ± 7.15 min
Figure 40 displays the average times for the three analyzed parts of the Pins and ML Model assemblies.
Figure 40. Assembly time differential between Pins Model and ML Model.
A paired one tail T-test was performed on the collected data to determine if the amount of time it took to assemble the different parts of the ML Model was significantly shorter than the time it took to assemble the Pins Model. A P-value of <0.05 (*) was considered to be statistically significant. The average times to create the PDMS frames for the Pins Model and ML Model were 8.05±1.08 minutes and 5.35±0.44 minutes, respectively. The P-value for the PDMS frame fabrication step is 0.00559. The average times to place the Thermanox® stages for the Pins Model and ML Model were 12.3±0.35 minutes and 5.08±0.62 minutes, respectively. The P-value for the PDMS frame fabrication step is 1.317×10-5. Statistically, it takes significantly less time to
create the PDMS frames using the ML Model. The average times to create the fibrin microthread fixturing slits for the Pins Model and ML Model were 1.3±0.20 minutes and 0.96±0.29 minutes, respectively. The P-value for the PDMS frame fabrication step is 0.0407. Statistically, it takes significantly less time to create the fixturing slits using the ML Model. A decreased assembly time will allow the user of the ML Model to create more models per day in preparation to run the
cellular outgrowth assay. In turn, this will increase data throughput of the assay results, expediting the in vitro testing process.
6.2.1.2 Cutting Guide and Cutting Tool for Fibrin Microthread Fixation Slits
The cutting guide makes cutting slits in the PDMS frame much more user-friendly and reproducible. The old model required the use of a single razor blade to cut three slits in each ring based on the user’s best guess of where to place them and how deep to cut. The cutting tool cuts three slits (on each side of the frame) in three frames at once. The cutting guide ensures that all slits are the same depth. The tool/guide combo is intuitive and decreases the rate of failure caused when the PDMS is cut too deep and the frame breaks. With one motion and cut, the user is able to accomplish making identical slits (six slits per frame, three on each side) for three frames at once.