What do fitness trackers have to do with bacterial cultures in the lab? Absolutely nothing, unless and until someone turns a fitness band into a general-purpose optical densitometer for the lab.
This is one of those stories that shows that you never know from where inspiration is going to come. [Chinna Devarapu] learned that as a result of playing around with cheap fitness bands, specifically an ID107HR. A community has built up around hacking these bands; we featured a similar band that was turned into an EEG. With some help, [Chinna] was able to reflash the microcontroller and program it in the Arduino IDE, and began looking for a mission for the sensor-laden platform.
He settled on building a continuous optical densitometer for his biology colleagues. Bacterial cultures become increasingly turbid as the grow, and measuring the optical density (OD) of a culture is a common way to monitor its growth phase. This is usually done by sucking up a bit of the culture to measure, but [Chinna] and his team were able to use the hacked fitness band’s heartrate sensor to measure the OD on the fly. The tracker fits in a 3D-printed holder where an LED can shine through the growing culture; the sensor’s photodiode measures the amount of light getting through and the raw data is available via the tracker’s Bluetooth. The whole thing can be built for less than $20, and the plans have been completely open-sourced.
We really like the idea of turning these fitness bands into something completely different. With the capabilities these things pack into such a cheap and compact package, they should start turning up in more and more projects.
It’s worth noting that optical density is somewhat wavelength dependent with 600nm / yellow light being the common choice. Certain species give more consistent results under different wavelengths. Since heart rate monitors are generally optimised for detecting red and near IR light, I wonder how this compares to a more purpose built spectrophotometer.
Hi Leithoa,
It’s a very good point. It was addressed in the publication. An external 600nm LED was added to the fitness sensor so that results can be compared with a com and the results are compared with a professional spectrophotometer. Please find the details in the below peer-reviewed publications.
https://pubs.acs.org/doi/abs/10.1021/acs.analchem.9b02628
Seems like they did address this. From the linked article: “One important modification we did to the fitness is adding a circuit to drive an LED at 600nm. This is important because ODs are traditionally measured at 600nm, where most bacteria has low absorption of light.”
I appreciate seeing a project that is relevant to both the DIY community and the scientific-research community! IMO each group could learn lots from the other; projects like this one demonstrate that. Great work!
This is typically done by drinking up a bit of the ethos to measure, but China and his side were able to use the chopped fitness band’s heartrate sensor to amount the OD on the hover.