The 15-day limit is due to the enzyme degradation on the electrode.
> While GOx maintains this level of activity in vitro, its stability in vivo is of the order of 10–14 days, necessitating novel immobilization and enzyme modification strategies to extend the functional lifetime of these oxygen-sensitive sensors.
Coincidentally, when I was doing this field of research, I wanted a hardware/software design that could algorithmically compensate for the sensor degradation. I even wanted to mathematically model the electrochemistry to find a clean solution. My electrode was nanoporous platinum, which then was considered more advanced/fashionable/versatile than glucose oxidase coating. In the end, I decided on reading off some cached table, not unlike a consumer electronics estimating remaining battery life.
On a separate note, you can design a fuel cell out of blood glucose. So, your body glucose generates electricity to fuel your electronics. The amount of electricity generated is not too off the target power for ultra-efficient circuits, and we even conducted in-vitro experiments as a proof of concept.
All of this was 20 years ago. I am glad that there are commercial solutions now but a bit saddened that the field hasn't progressed as fast as other areas of technology.
You know better than almost anybody just how hard it is to insert sensors in the body. Optical techniques let you bypass that degradation, and a company actually shipped a portable raman-based glucometer something like a decade ago. Its battery life wasn't good enough, but making a wearable, miniaturized Raman spectrometer was crazy impressive.
The biggest reason for sensor life is likely due to the sensor (needle) being attacked by immune cells, inflammation or fibrotic scarring. This type of foreign body reaction causes the amount of glucose available to the sensor to drop and reach a point where it just isn't accurate/sufficient anymore
This likely why the sensor life is advertised as ~12-15 days:
"*Sensor Survival Clarifying Statement: A study was conducted to assess the sensor life where 77.9% of sensors lasted the full 15 days. In other words, when using the product per the package labeling, approximately 20% of sensors may not last for the full 15 days, 10% of these sensors may last less than 12 days."
If you like this, you might like the Omnipod teardown as well. It's what pairs with continuous glucose monitors. It's an entire insulin pump that you throw away every three days, so the engineering to make the parts that cheap is pretty amazing. They don't waste money on any motors. they just ratchet a worm gear with something called muscle wire. It's a nicanol alloy that shrinks when current heats it. You hear a click click click as it injects a 20th of a unit of insulin at a time. https://www.youtube.com/watch?v=e2MQUUkubgs
SMA wires are not particular cheap. Rather, due to mass production of cellphones, small "pager" motors are incredibly cheap (tens of cents). From the video, it seems that the SMA wires used for precise control & counting of the gear teeth.
It's not the wire that's precise here - it just provides enough force to get a the stamped metal piece over the tooth of the plastic gear, then it slams into a hard stop, then the button batteries dump current into the other wire, which pulls it the other way, pushing the gear one tooth width, and slamming into the other hard stop. Each back and forth ratchet of the tooth turns the worm gear on the plunger the precise 0.05 units.
Another cool part of these is the initial insertion - there's a huge torsion spring that stabs a hollow needle into the skin then fully retracts it, leaving the rubber tube.
Then there's a little corner of the PCB with a cutout and power trace, you can stab with a paper clip to disable the pod in an emergency.
Then there's this membrane that equalizes the air pressure in the pod if you're flying with it. It lets air through but not water. So you can dive in a lake with it too.
I love clipping these things open and just marveling at the clever engineering for cheap mass production of such a sophistocated device.
I agree. The wire itself is not where the precision is. It's the reciprocating mechanism done by the wire.
Regarding price, my point still stands that wire is not cheap enough to motivate its use. Your link actually proves my point (1.6 USD/30cm). Pager/toy motors are costing cents.
The margin on this stuff can easily be 1000% when sold to hobbyists. Or rather costs of cutting the wire to length, packaging it, warehousing, etc. For a small number of orders, I wouldn't be surprised if this was done mostly by hand. Plus there is some risk the stock sells too slow, since it is so niche (compared to ESP32 or sensors). That's all priced in there, plus some healthy margin.
While the pump manufacturer just puts a 1000m spool in his assembly line every few hours.
It seems absurd that hair thin thread costs more than a whole assembly of permanent magnets and copper coiled in a stator. Just thinking from first principles. Whats driving the cost? The processing? For scale, Omnipod only makes this one product and they're on the S&P 500 because they sell so many of them. I'd guess 10 cents in bulk but who knows. It's a SparkFun link for DIY hobbyists. They're not charging the bulk rate. They have to make a margin.
But maybe it is more expensive. Then they probably choose it for the reliability, and a simple and quiet design. Or perhaps the button batteries don't have the energy to drive a pager motor.
At high enough volumes, cost becomes the cost of the raw materials. NiTi wires are obviously made of Ni and Ti, both of which are ~3x the cost of copper.
If you can design the wire to require 3x less material than the motor approach, it will win.
> Stelo, created by Dexcom, was the first of these sensors to receive OTC approval from the FDA. This small, completely disposable device uses the same hardware as the Dexcom G7. The main difference between it and the prescription-only device is the software that pairs with it. While the sensor has the same capabilities as the one marketed for people on insulin therapy, the OTC software restricts much of this functionality.
> Lingo.. uses the same hardware and housing as the [prescription-only] Freestyle Libre 3. Like Stelo, the software system is where the real differences exist... Abbott’s second OTC CGM is Libre Rio. This sensor looks identical to the Lingo and Freestyle Libre 3. The main difference is in the marketing
My understanding is that the Stelo doesn't offer an API, like the Dexcom G6/7 does.
That is really unfortunate, because the default Dexcom apps are terrible (the Stelo app is slightly better / more modern, but has all the limitations of the Dexcom app, plus no calibration).
So xDrip was created to completely skip the Dexcom APIs. With G4 we used a wixel microcontroller, to pass the proprietary radio signal to bluetooth. We'd carry it in our pockets in a tiktac case with a lipo. A graph of our blood sugar would show on our Pebble smartwatches at the time. Once Dexcom built Bluetooth into their transmitters, xDrip was then able to directly pair. There's a lot of smart people on the project, reverse engineering all the Dexcom protocols, and even some other brands of CGM. It does work with Stelo too. xDrip can then sync the data it collects to various APIs/websites. I've been a dexcom user since G4 and never once signed up for a Dexcom account or installed a Dexcom app - the open source stuff was always so much better.
Between insurance since I couldn't afford the Cobra costs when my last job ended. I've been using these for the past month and they work okay.
The only down side is the software states it's not for t2d on insulin or 5qd for that matter. It only shows input from 70-240, outside that range all you know is it's above or below. So it's hard to tell how much correction you need at times.
It really sucks how much medications and cgm cost over the counter.
Wouldn't 1 per 5 minutes suffice? Just a couple of bytes should be fine for the reading. What other data could possibly be needed?
To me this smells of it sending a huge json payload with software version numbers and other unnecessary junk simply because the interface between the hardware and app wasn't designed with power efficiency in mind.
> Imagine a permanent CGM, powered simply by going outside in the sun for 15 minutes a day. What a life!
Type 1 diabetic here: one of the reasons for changing the insertion site is because of the risk of infection / rejection. It's not just about the power requirements.
As an alternative, Eversense is a long term CGM inserted by a healthcare provider that's supposed to last a year. I've heard mixed reviews.
Speaking of mixed reviews, I have mixed feelings about non diabetics using CGMs. But if it lowers the price for all of us, then I'm on board.
Yeah, battery life was never a problem. The Dexcom G4 battery lasted like seven months. I've heard of some people claim they can keep a sensor going a few months, by restarting the transmitter, but they're definitely outliers. For most people biofilm builds up and the sensor data gets really noisy. Massaging the flesh around the sensor can help extend it. I typically go ~20 days per sensor.
No restarts required if you use the Anubis transmitter. The battery is replaceable too. I get 14 days out of most but by day 10 the readings are lower than actual, I have to finger stick test and calibrate.
There are no downsides, really. It's mostly an emotional thing that comes from dealing with an awful chronic disease. So I mostly keep it to myself.
e.g., I feel sad when a non diabetic uses a CGM and says something like "omg I had a milkshake and my blood sugar spiked all the way to 120!". There's nothing wrong with them saying that, of course. It's just challenging to hear when you have a body that can't produce insulin at all.
I used one recently on suspicion of a problem based on an odd but not typically related test result. Fasting glucose and a1c on bloodwork were fine. The CGM however showed blood sugar ping ponging from 120 to 200 all day, barely getting back to 120 by dinner after a lunch spike, and lows overnight, even worse < 70 when the dawn phenomena should be pumping it higher. The CGM has saved an unknown period of time by providing detailed metabolic data that traditional tests couldn’t
I understand your feelings. One upside that I see from more people using it for non-medical purposes, is more data. That could potentially lead to new research and insights about diabetes, nutrition, and health in general.
Yes definitely! Overall I see it as a win, which is why I don't say these things to non diabetics using CGMs that I meet IRL. I think it's a win for diabetics and non diabetics, and it's more about my internal grief of living with an incurable chronic illness (that I can take to my therapy session).
Is depletion of the glucose oxidase reagent the limiting factor, versus the ~~17 week estimated battery life? One would expect an item with a 17 week battery that also has the glucose oxidase reagent electrode that also lasts ~~17 weeks to have a higher service life - since it does not, I expect the electrode is the limiter and it might have perhaps 30 days? Of course, it has a clock and a limited life to guarantee correct service beyond the 15 day clockout. It will take a better hacker than me to amend the software to explore these electrode limits in a safe manner. One would expect a foreign body might face attack by the body that inexorably acts to cover/passivate this foreign body.
> The sensor life is limited by the amount of chemical coating on the probe and the amount of glucose in the interstitial fluid. It could last longer for a person with low and steady BG than for one with chronically high, so the sensor life is a conservative compromise between battery size, Bluetooth wake cycles, and probe life. If the probe had a larger supply of reactant or its dispersal rate could be slowed, the sensor life could be extended to the battery capacity or the max that the FDA would permit.
On the G4 through G6 generation of Dexcom sensor, there is a trick to restart the timer. So I regularly get about 20 days per sensor. The limiting factor is really just the sticker peeling off. So you have to put some kinesiology tape over the whole thing to keep it on. The other limiting factor is the noise in the data gets bad. By massaging the flesh around the sensor, I can make the noise go away. so i don't think the reactant is running out, i think it's biofilm building up so it can't interact with the interstitial fluid as directly.
Restart? For the idiots. The smart among us purchased the Anubis transmitter. No restarts required, the sensor stays in until you determine it is no longer accurate or it stops working. The battery is also replaceable.
The 15-day limit is due to the enzyme degradation on the electrode.
> While GOx maintains this level of activity in vitro, its stability in vivo is of the order of 10–14 days, necessitating novel immobilization and enzyme modification strategies to extend the functional lifetime of these oxygen-sensitive sensors.
https://pmc.ncbi.nlm.nih.gov/articles/PMC3879770/
Coincidentally, when I was doing this field of research, I wanted a hardware/software design that could algorithmically compensate for the sensor degradation. I even wanted to mathematically model the electrochemistry to find a clean solution. My electrode was nanoporous platinum, which then was considered more advanced/fashionable/versatile than glucose oxidase coating. In the end, I decided on reading off some cached table, not unlike a consumer electronics estimating remaining battery life.
On a separate note, you can design a fuel cell out of blood glucose. So, your body glucose generates electricity to fuel your electronics. The amount of electricity generated is not too off the target power for ultra-efficient circuits, and we even conducted in-vitro experiments as a proof of concept.
All of this was 20 years ago. I am glad that there are commercial solutions now but a bit saddened that the field hasn't progressed as fast as other areas of technology.
You know better than almost anybody just how hard it is to insert sensors in the body. Optical techniques let you bypass that degradation, and a company actually shipped a portable raman-based glucometer something like a decade ago. Its battery life wasn't good enough, but making a wearable, miniaturized Raman spectrometer was crazy impressive.
Recent discussion on optical glucose sensors, https://news.ycombinator.com/item?id=43124436
The biggest reason for sensor life is likely due to the sensor (needle) being attacked by immune cells, inflammation or fibrotic scarring. This type of foreign body reaction causes the amount of glucose available to the sensor to drop and reach a point where it just isn't accurate/sufficient anymore
This likely why the sensor life is advertised as ~12-15 days:
"*Sensor Survival Clarifying Statement: A study was conducted to assess the sensor life where 77.9% of sensors lasted the full 15 days. In other words, when using the product per the package labeling, approximately 20% of sensors may not last for the full 15 days, 10% of these sensors may last less than 12 days."
https://www.stelo.com/faqs/using-stelo/how-long-can-i-wear-m...
If you like this, you might like the Omnipod teardown as well. It's what pairs with continuous glucose monitors. It's an entire insulin pump that you throw away every three days, so the engineering to make the parts that cheap is pretty amazing. They don't waste money on any motors. they just ratchet a worm gear with something called muscle wire. It's a nicanol alloy that shrinks when current heats it. You hear a click click click as it injects a 20th of a unit of insulin at a time. https://www.youtube.com/watch?v=e2MQUUkubgs
SMA wires are not particular cheap. Rather, due to mass production of cellphones, small "pager" motors are incredibly cheap (tens of cents). From the video, it seems that the SMA wires used for precise control & counting of the gear teeth.
Thanks for the awesome video.
It's pretty cheap https://www.sparkfun.com/muscle-wire-0-005-diameter-1-foot.h...
It's not the wire that's precise here - it just provides enough force to get a the stamped metal piece over the tooth of the plastic gear, then it slams into a hard stop, then the button batteries dump current into the other wire, which pulls it the other way, pushing the gear one tooth width, and slamming into the other hard stop. Each back and forth ratchet of the tooth turns the worm gear on the plunger the precise 0.05 units.
Another cool part of these is the initial insertion - there's a huge torsion spring that stabs a hollow needle into the skin then fully retracts it, leaving the rubber tube.
Then there's a little corner of the PCB with a cutout and power trace, you can stab with a paper clip to disable the pod in an emergency.
Then there's this membrane that equalizes the air pressure in the pod if you're flying with it. It lets air through but not water. So you can dive in a lake with it too.
I love clipping these things open and just marveling at the clever engineering for cheap mass production of such a sophistocated device.
I agree. The wire itself is not where the precision is. It's the reciprocating mechanism done by the wire.
Regarding price, my point still stands that wire is not cheap enough to motivate its use. Your link actually proves my point (1.6 USD/30cm). Pager/toy motors are costing cents.
The margin on this stuff can easily be 1000% when sold to hobbyists. Or rather costs of cutting the wire to length, packaging it, warehousing, etc. For a small number of orders, I wouldn't be surprised if this was done mostly by hand. Plus there is some risk the stock sells too slow, since it is so niche (compared to ESP32 or sensors). That's all priced in there, plus some healthy margin.
While the pump manufacturer just puts a 1000m spool in his assembly line every few hours.
It seems absurd that hair thin thread costs more than a whole assembly of permanent magnets and copper coiled in a stator. Just thinking from first principles. Whats driving the cost? The processing? For scale, Omnipod only makes this one product and they're on the S&P 500 because they sell so many of them. I'd guess 10 cents in bulk but who knows. It's a SparkFun link for DIY hobbyists. They're not charging the bulk rate. They have to make a margin.
But maybe it is more expensive. Then they probably choose it for the reliability, and a simple and quiet design. Or perhaps the button batteries don't have the energy to drive a pager motor.
At high enough volumes, cost becomes the cost of the raw materials. NiTi wires are obviously made of Ni and Ti, both of which are ~3x the cost of copper.
If you can design the wire to require 3x less material than the motor approach, it will win.
I wrote a thing a while ago about my experience with continuous glucose monitors.
This post about the Stelo is fascinating.
https://josh.works/cgm
Will we see open-source software that can interact with these sensors?
https://sequenex.com/the-otc-cgm-market-comparing-stelo-ling...
> Stelo, created by Dexcom, was the first of these sensors to receive OTC approval from the FDA. This small, completely disposable device uses the same hardware as the Dexcom G7. The main difference between it and the prescription-only device is the software that pairs with it. While the sensor has the same capabilities as the one marketed for people on insulin therapy, the OTC software restricts much of this functionality.
> Lingo.. uses the same hardware and housing as the [prescription-only] Freestyle Libre 3. Like Stelo, the software system is where the real differences exist... Abbott’s second OTC CGM is Libre Rio. This sensor looks identical to the Lingo and Freestyle Libre 3. The main difference is in the marketing
I believe xDrip works with it. I use it for my G6. The app is amazing, SO much better than the official Dexcom app.
My understanding is that the Stelo doesn't offer an API, like the Dexcom G6/7 does.
That is really unfortunate, because the default Dexcom apps are terrible (the Stelo app is slightly better / more modern, but has all the limitations of the Dexcom app, plus no calibration).
So xDrip was created to completely skip the Dexcom APIs. With G4 we used a wixel microcontroller, to pass the proprietary radio signal to bluetooth. We'd carry it in our pockets in a tiktac case with a lipo. A graph of our blood sugar would show on our Pebble smartwatches at the time. Once Dexcom built Bluetooth into their transmitters, xDrip was then able to directly pair. There's a lot of smart people on the project, reverse engineering all the Dexcom protocols, and even some other brands of CGM. It does work with Stelo too. xDrip can then sync the data it collects to various APIs/websites. I've been a dexcom user since G4 and never once signed up for a Dexcom account or installed a Dexcom app - the open source stuff was always so much better.
xDrip is the GOAT. It's also amazingly stable for an open source project with frequent updates. It's the #1 reason I don't switch to iPhone.
Stelo is just a vaguely firmware-modified G7 that skips offering calibration and has slightly longer sensor life.
xDrip's got 'em figured out and they'll happily fire up without an issue there.
https://navid200.github.io/xDrip/docs/Dexcom/G7.html
Between insurance since I couldn't afford the Cobra costs when my last job ended. I've been using these for the past month and they work okay.
The only down side is the software states it's not for t2d on insulin or 5qd for that matter. It only shows input from 70-240, outside that range all you know is it's above or below. So it's hard to tell how much correction you need at times.
It really sucks how much medications and cgm cost over the counter.
The sensor is the same. I wonder if xDrip can read a wider range?
Why is this thing sending 50 bluetooth packets?
Wouldn't 1 per 5 minutes suffice? Just a couple of bytes should be fine for the reading. What other data could possibly be needed?
To me this smells of it sending a huge json payload with software version numbers and other unnecessary junk simply because the interface between the hardware and app wasn't designed with power efficiency in mind.
The sensor likely needs to send all of this info for compliance reasons. Each measurement needs to be tied to the sensor directly I think.
> Imagine a permanent CGM, powered simply by going outside in the sun for 15 minutes a day. What a life!
Type 1 diabetic here: one of the reasons for changing the insertion site is because of the risk of infection / rejection. It's not just about the power requirements.
As an alternative, Eversense is a long term CGM inserted by a healthcare provider that's supposed to last a year. I've heard mixed reviews.
Speaking of mixed reviews, I have mixed feelings about non diabetics using CGMs. But if it lowers the price for all of us, then I'm on board.
Yeah, battery life was never a problem. The Dexcom G4 battery lasted like seven months. I've heard of some people claim they can keep a sensor going a few months, by restarting the transmitter, but they're definitely outliers. For most people biofilm builds up and the sensor data gets really noisy. Massaging the flesh around the sensor can help extend it. I typically go ~20 days per sensor.
No restarts required if you use the Anubis transmitter. The battery is replaceable too. I get 14 days out of most but by day 10 the readings are lower than actual, I have to finger stick test and calibrate.
> I have mixed feelings about non diabetics using CGMs
What downsides do you see?
I've seen stock-outs on free freestyles, following “influencer” videos on tiktok. That's a problem.
It's a problem that solves itself:
- If more people buy something, in the short term, you have shortages.
- In the long term, you have better economies of scale and prices go down.
These things should be available for maybe $15 at most, generically, off-brand.
Personally, I think this is the kind of exercise most people should go through perhaps annually.
There are no downsides, really. It's mostly an emotional thing that comes from dealing with an awful chronic disease. So I mostly keep it to myself.
e.g., I feel sad when a non diabetic uses a CGM and says something like "omg I had a milkshake and my blood sugar spiked all the way to 120!". There's nothing wrong with them saying that, of course. It's just challenging to hear when you have a body that can't produce insulin at all.
I used one recently on suspicion of a problem based on an odd but not typically related test result. Fasting glucose and a1c on bloodwork were fine. The CGM however showed blood sugar ping ponging from 120 to 200 all day, barely getting back to 120 by dinner after a lunch spike, and lows overnight, even worse < 70 when the dawn phenomena should be pumping it higher. The CGM has saved an unknown period of time by providing detailed metabolic data that traditional tests couldn’t
I understand your feelings. One upside that I see from more people using it for non-medical purposes, is more data. That could potentially lead to new research and insights about diabetes, nutrition, and health in general.
Yes definitely! Overall I see it as a win, which is why I don't say these things to non diabetics using CGMs that I meet IRL. I think it's a win for diabetics and non diabetics, and it's more about my internal grief of living with an incurable chronic illness (that I can take to my therapy session).
Is depletion of the glucose oxidase reagent the limiting factor, versus the ~~17 week estimated battery life? One would expect an item with a 17 week battery that also has the glucose oxidase reagent electrode that also lasts ~~17 weeks to have a higher service life - since it does not, I expect the electrode is the limiter and it might have perhaps 30 days? Of course, it has a clock and a limited life to guarantee correct service beyond the 15 day clockout. It will take a better hacker than me to amend the software to explore these electrode limits in a safe manner. One would expect a foreign body might face attack by the body that inexorably acts to cover/passivate this foreign body.
Some discussion in this 2018 article, https://hackaday.com/2018/12/03/why-is-continuous-glucose-mo...
https://idoroseman.com/freestyle-libre-blood-glucose-monitor...
> The sensor life is limited by the amount of chemical coating on the probe and the amount of glucose in the interstitial fluid. It could last longer for a person with low and steady BG than for one with chronically high, so the sensor life is a conservative compromise between battery size, Bluetooth wake cycles, and probe life. If the probe had a larger supply of reactant or its dispersal rate could be slowed, the sensor life could be extended to the battery capacity or the max that the FDA would permit.
Thanks for that +
On the G4 through G6 generation of Dexcom sensor, there is a trick to restart the timer. So I regularly get about 20 days per sensor. The limiting factor is really just the sticker peeling off. So you have to put some kinesiology tape over the whole thing to keep it on. The other limiting factor is the noise in the data gets bad. By massaging the flesh around the sensor, I can make the noise go away. so i don't think the reactant is running out, i think it's biofilm building up so it can't interact with the interstitial fluid as directly.
Restart? For the idiots. The smart among us purchased the Anubis transmitter. No restarts required, the sensor stays in until you determine it is no longer accurate or it stops working. The battery is also replaceable.
Thanks for the pointer.
https://bionicwookiee.com/tag/anubis/
https://www.loopnlearn.org/anubis/
https://www.facebook.com/groups/310545804061135
Idiot seems like a harsh word to use