Medical Device: Dried Blood Spot Sampling

Patients with challenged motor control (such as pediatrics or ALS) have poor dexterity to fill blood samples using the conventional capillary tubes. The blood samples are applied to specially treated sampling paper, allowed to dry, and delivered to a laboratory for monitoring treatments and drug dosages. The device includes a preloaded lancing device (to prick the finger) which is also under the patent.

I developed the functional concept of this device and designed it to production prototype.

Engineering Requirements:

• Collect a specific volume of blood and deposit it onto a specially treated sampling card

• Biohazard and contaminant secure

• Extremely user friendly

• Small, Inexpensive, Disposable

Challenge

I began by conducting research on capillary flow and experimenting with different wicking materials, capillary geometries, and surface energies. The crux of the project was measuring a specific volume of fluid and then delivering it to the blotting paper in a passive or nearly passive device.

Two leading concepts emerged from a dozen brainstormed ideas. Eventually the final mechanism was a hybrid of these two:

Concept 1: When the lanced finger is engaged with the device, a shuttle depresses allowing blood to fill a capillary tube. Due to the physics at this scale the capillary tube cannot overfill, thus saturating to a specific volume. The finger is then released and the shuttle is sprung upward, contacting the capillary tube onto the blotting paper where it then drains. The device is filled with dessicant to fully dry the sample.

Concept 2: This method pulls excess blood from the blotting paper in a sort of ‘capillary action tug-of-war’. It begins by the pricked finger draining blood into the receiver where it directed to the blotting paper. Beneath the blotting paper is a dense porous compound that has a high capillary potential, thus wicking away the excess blood. By characterizing the capillary potential of the blotting paper, a Porex foam was selected to pull the attenuate the appropriate amount of blood.

Some takeaways on Capillary Potential

• Porous compounds are not readily described when researching capillary dynamics so I’ll save you some time. One thing I discovered is that smaller tighter pores have greater pull. This is due to the ‘radius’ being smaller, thus increasing the capillary height in the above equation. The interesting thing to think about is that these smaller pores will saturate faster and thus retain less fluid overall. Here’s an example:

You spill beer on your friend’s carpet and take a sponge to sop it up (after grabbing yet another beer, of course). Squeezing the sponge into the carpet will increase the capillary potential although it will decrease the total volume of the sponge. Thus, a method of a squeezed sponge and rinsing it out and repeating is the recommendation.

• I explored an idea that used wedge shaped foams to modulate the wicking force. This did not affect it. The capillary potential is not affected by overall volume. The way to do this would be to use variable pore sizes.