Inside the Cue COVID-19 Test Cartridge

My employer generously provided me with a Cue reader and COVID-19 test cartridges.

The Cue reader is a bluetooth-enabled device that connects to my phone. In the accompanying app I can then start a test by inserting a cartridge. The reader heats up the cartridge (at least it says so). Once the cartridge is heated up, I have to take a nasal sample with the included swab. The swab goes into the end of the cartridge that sticks out of the reader.

Then magic happens and after ~10 minutes I get the result in the app.

The cartridges are made of plastic and are not reusable. I have to use a new one every time I take a test. According to the vendor the cartridges can be safely disposed off via household trash. After today's test I was curios what's inside the cartridges and how the test might work. Instead of throwing it away, I decided to open the used cartridges and see what they had inside.

I opened the cartridge and found a PCB inside. I also spilled a bit of fluid, likely the speciment I painfully extracted from my nose earlier. 🥲 I noticed that there was a mechanic inside the cartridge that seems to open three small fluid containers that were sealed with aluminum foil. This migh explain why the app warns of moving or tilting the reader once the cartridge is inserted. It would risk spilling the fluid. I believe the reader even has an angular rate sensor (gyro sensor) that can detect the tilt of the reader, but I am not entirely sure.

With the PCB freed from its case I was able to inspect it a bit closer. I don't know how exactly the measurement is done. I imagine the following process:

• Inserting the swab activates the switch, letting the reader now that it is time to start the procedure.
• The liquid containers are unsealed releaseing the liquids and enabling them to mix with each other but also get in contact with the swab tip.
• There seems to be a pipe built into the transparent plastic covering the PCB. The sponge creates some sort of negative pressure attracting the mix of fluids. On their way to the sponge, they pass a series of contacts which do the measurement via gauging a change in resistance.
• The mix of fluids behaves measurably different whether or not it was able to attach to COVID-19 receptors. Maybe there are molecules that change charge when attaching to receptors. Pure speculation, I have no idea what I am talking about.

The EEPROM is a 24C04WP memory chip which can be write protected. Maybe this is to make sure it is a original cartridge? Something like DRM to protect the business model and ensure the quality of the results? Someone on Hackernews suggested it contains a serial number and calibration information to make up for production differences of each cartridge.

The back of the PCB is rather boring. Some resistors and markings.

The cartridges come in a sealed pouch. The pouch looks air and water tight to me.

That's all for now. I did expect a mini laboratory in the cartridge. However, I did not expect to find an EEPROM and a switch.

Tue 18. Jan 2022 Update: From the FDA emergency use authorization I learned:

Your product is an automated assay that utilizes isothermal nucleic acid amplification technology for the qualitative detection of SARS-CoV-2 viral nucleic acids.

My initial guess about change in molecualr charge or resistance wasn't too far off:

Heating, mixing, amplification, and detection take place within the cartridge. The current flow from the electrodes is converted to a positive or negative result (based on a pre-determined cutoff).

Fri 21. Jan 2022 Update: Folks seem to be invested in tearing open Cue cartridges and learning how they work. I got several messages regarding the inner workings and other people's adventures in dismantling cartridges. I also received quite a few pointers to the related US Patent US20170043335A1 which describes the cartridge internals. The protocol they use is called Loop Mediated Isothermal Amplification (LAMP) and nicely explained in this video.

Here are two interesting observations around the cartridge's inner processes:

• How did the cartridge open the valves between the different liquid containers?

  The conduit between the containers are sealed with a meltable wax or polymer. There are resistors that are used to heat and melt the "valves" open and the little "teeth" on the mobile frame tear open the membrane to let the wells drain. Pretty elegant!

• How did the cartridge ensure the correct mixing and movement of the sample and the analytical liquids in just 30 minutes?

  When you look at the little "bump" inside the reader where the "dent" in the cartridge clicks into, it is not a heater. It is a electromagnet! Cue binds nanomagnetic particles to the chemically active bits, and then the reader uses varying magnetic fields to move the bits around. Think of it as an artificial Brownian motion, or a kid waving a magnet to make iron particles wiggle.

Thank you Malcolm and Tim for helping me learn more about this mini lab on a PCB.