Test Tube and Cap Holding Racks for COVID-19 Rapid Testing

Due to supply chain problems that still exist in the US, a local lab reached out in need of test tube and cap holding racks for rapid COVID-19 tests. These racks hold specialized test tubes which lock into place while being unscrewed using a semi-automatic decapper. After being unscrewed, the caps are then transferred to cap holding racks and placed into holding individual slots. This then allows the contents of the test tube to be extracted for testing and/or research.

To combat shortages, these 3D printed racks were developed to try and help out. While this design looks relatively simple, there is little margin for error due to the precision needs with the equipment being used to align with the 3D printed racks.

As this design requires specialized equipment, instead of explaining how to build these exact test tube racks, I instead wanted to showcase how the design thinking process was used in this project. I felt this provides a real life demonstration of many principles showcased in a previous Instructable called Teacher Professional Development: Design Thinking for Schools.

After following this process and getting approval from the lab staff, the racks have been printed and donated for free to help with rapid COVID-19 testing where they have been in use.

*Disclaimer: Do not attempt to create laboratory or medical equipment without consulting proper professionals.

To start this project, I needed to gather info and figure out problems which may be encountered later on. I was not just looking for details on the equipment itself, but also to gain an understanding on how lab staff would be interacting with everything on a daily basis.

This particular decapper consists of 6 "arms" that lower and grab onto the top of each test tube. They are then spun in individual slots until they lock onto small tabs that extend slightly from the lower portion of each slot. When the test tubes spin and lock, the caps are then able to loosen and can be lifted upwards by the decapping arms.

I met with laboratory staff and examined their machine and chatted about how they would utilize it through the entire testing process. (I also took a ton of measurements for later use).

For this particular design I talked with lab staff, expert 3D printers, etc. and found myself asking questions such as:

• How durable do the racks need to be?
• How will these racks physically be handled day in and day out?
• What temperatures might the racks need to be able to tolerate?
• What (if any) are the viral contamination risks?
• How will the racks be sanitized?

During these meetings I came up with a clearly defined problem:

Staff need to be able to be quickly load racks and have them function properly during the decapping process because of the rapid nature of testing needs.

Some notes relating to this problem statement that I felt might be relevant later in the process were:

• Precision was key as there was going to be little room for error if the equipment didn't align correctly.
• The bar codes need to be easily scannable for staff during the process when needed
• Access may be needed underneath the racks to scan bar codes on the bottom of test tubes.
• The racks need to be easy to sanitize for lab technicians.
• They will need to be structurally sound to handle repeated use.

While thinking of design ideas, everything naturally kept coming back to trying to keep the design as simple as possible. Even so, there were areas which needed to be addressed to make that happen. I constantly ran these ideas by others to get opinions and viewpoints. During the ideation phase I was throwing questions around such as:

How precise might measurements need to be to ensure successful removal and transfer? Is there any room for errors in measurements or printing?

When the 6 decapping arms came down, I wasn't sure if the arms would have any movement from side to side. I assumed the quickest path would be to simply build and test to learn more and get the design dialed in.

If measurements or prints ended up slightly off, how might this affect performance and would it cause problems? If yes, how so?

I thought about how these issues should affect decisions like print speed, layer height, etc. when slicing prototypes. I also thought about the bar codes on the side of the test tubes and the need for a design which would not cause damage.

What might be the best design for sanitization?

After the staff told me how they were going to be sanitizing after each use, a focal point was coming up with a design that would be easy to clean. Having done spray testing with PPE last spring, a design which was smooth and avoided as many crevices as possible seemed like the way to go.

Would the locking tabs break under the force applied when spinning?

I debated this and brainstormed possible solutions. I felt that adding an inclined plane in the bottom section of each slot would add reinforcement to the back side of the tabs while the test tubes were locking. I also thought this would allow for the test tubes to slide down into and better fit their slots.

It was finally time to design the first prototype. I chose to first tackle the test tube racks.

I designed a holding rack which was rectangular and had slightly curved edges to fit the decapper's plate dimensions. Within this rack I added slots which would hold 48 test tubes which was the max for this particular decapping machine.

In approaching this, my plan was to design and then print:

1. A single slot for one test tube to check fit.
2. A single row for 6 test tubes which could be used to test with the decapper.
3. A whole rack which would be tested with all 48 test tubes making sure every row was aligned properly.

Step 1: The single slot prototypes immediately revealed problems. The diameter of the test tubes was 12.2mm and the initial distance I chose from the slot's outer "posts" (as seen in the previous cutaway picture) to the test tube was too close. It was tough to insert and remove each test tube, which made it hard to spin inside the slot and resulted in damaged bar codes. After a few tweaks, I made the diameter of the posts smaller, and I ended up with a .3mm gap allowance from the test tube to each post which seemed fix the problem.

Step 2: The design then moved to a 6 slot, single row that spanned the width of the decapping machine base. I added spacing from the end of the rack to this first row as there was room that the manufacturer had incorporated into their design. I had previously done caliper measurements from the edge of the machine base to the decapping arms, but figured I may need to rework this to get this proper alignment. Fortunately, the measurements were close to spot on and the decapping arms lined up well with all of the caps.

Step 3: With everything squared away it was time to begin a full prototype. The previous steps seemed to have dialed in all the measurements correctly. The full prototype fit exactly as planned and didn't look like it needed any tweaking as the decapping arms lined up properly with each row.

The cap holding racks used the same distancing as the test tube rack, so my initial plan was to simply raise the cylinders, or "posts", under the assumption that the caps would lower into their individual spots. I planned to use the same 3 step process as the test tube rack design.

However, after speaking with lab staff, I found out the test tube caps weren't the original manufacturer's version and didn't always align properly when lowered. During Step 2 with the single row test, this caused some of the caps to misalign and not lower into their slots (as seen in the picture). Going back to the ideation stage, I realized there was a need to have the posts angled to help guide any caps which may not be lining up perfectly.

Ultimately, a complete redesign was done which shifted away from a perfect cylindrical post to a diamond shaped column that had a slight bevel at the top leading to the lower portion of the column. These beveled sections were meant to act as a ramp and guide every cap into place.

This took a lot more design and prototyping effort than originally expected, but eventually the columns worked and both the test tube and rack holders appeared complete after final prototyping.

The past steps covered some testing and issues which were resolved, but the final prototypes still needed to be fully tested and given the thumbs up by laboratory staff.

After placing all 48 test tubes into their racks, all of the caps were successfully unscrewed from their respective tubes. The locking tabs were functioning as intended and every cap was able to be lifted up away from the rack.

Upon transfer to the cap holding racks, all of the caps were lowered into their slots without issue. These tests were performed multiple times using different prints to ensure they worked as intended.

However, even after these successful tests, the lab staff noted an issue they were encountering. They had no way of identifying which direction the racks were aligned, so a simple indent on the top left corner was added for orientation to fix this problem. It's a great example of empathy for end users and also showcases how constantly using design thinking is an important tool.

These iterations are not uncommon when following the design thinking process and you'll likely notice that I reverted back to previous stages, would make adjustments, and then continue on in progressing to the desired end goal. Using this process allowed the project to move from concept to reality in under a week's time. With these completed designs, the printed racks have now been in use to help with testing efforts.