Getting the sweat out
Recently, I got to be part of a really cool project to develop a system for recovering sweat from a body patch. The way it works is an athlete wears an absorbent patch (kind of like a band-aid), and it collects sweat during a workout. Then the patch can be sent off to a lab to analyze the sweat. This is non-invasive alternative blood testing with several applications. Sports leagues can easily test for performance-enhancing drugs. Plus, the analysis provides athletes with information about their performance based on the biomarkers present in their sweat.
The problem was, it’s really difficult to extract such a minute amount of liquid from a small body-worn patch. The patch only absorbed about 2 ul of sweet to start with. So, we needed to recover every drop of liquid. It’s important to build quick prototypes before launching into a full system design. Keeping the model as a benchmark until the bugs can be tweaked is critical for an early stage product designer. So I brainstormed some ideas and came up with two viable options. Then I created prototypes for testing.
A quick brainstorm left me with a few interesting concepts paths.
Pressure, Compression, Evaporation, Centrifuge
Prototype 1: The Rollers
The first prototype worked similar to a juicer. The patch was folded and pressed through two rollers that squeezed the fluid into a collection area. This first prototype was less than Ideal. It only collected around 80% of the sweat—decent results, but I knew I could do better. Plus, this method destroyed the absorbent patch during the extraction process and fibers ended up in the collection area, contaminating the sample. Time to move on to my next idea!
Prototype 2: The Centrifuge
For my next prototype, I went with a system similar to the current blood separating methods. The centrifuge creates a high force that drives every molecule to an outside collection chamber. I went with the standard centrifuge machine already used in labs for blood testing and adapted the idea for our purposes. The pad was retained in an Eppendorf tube while sweat was gathered lower in the tube. I used injection molding to built a unique carrier that went inside the Eppendorf tube to prevented the patch from sticking to the walls. This way the fluid would not get stuck in the patch and the patch would not be driven to the bottom of the tube.
The results were much better with this prototype. The centrifuge recovers 99% of the initial sweat in the pad. Plus, it uses standard lab equipment with a custom Eppendorf tube. Companies could mass-produce these custom-built tubes for easy use in analysis machines. With this method, customers can send in their sweat for analysis. Athletes and personal trainers can get better performance information.
This was a really fun project to work on, and I’m glad I got to be a part of it.
A future concept
the challenge with this project was to collect the fluid from a porous fabric. A possible solution to this problem would be to create microwells which would wick the fluid into them retaining it and making it easier for extraction.