How do plutonium-powered pacemakers work? | Hacker News Reader

How do plutonium-powered pacemakers work?

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172 pointsluu81 comments

FTA:

> I wondered if there wasn't also worry about plutonium being recovered for weapons use, but the risk seems much smaller

The answer here is a simple "nope." Plutonium in radioisotopic thermal generators is always Plutonium-238 with a 87.7 year half-life. You can't make bombs out of it. The fissile isotopes of Plutonium are 239 (24,000 year half-life) and 241 (14 year half-life, but a beta emitter instead of alpha). The fission cross section (probability) of Pu-238 is >100 less than Pu-239 for a "normal-speed" neutron and also a lot less at fission-spectrum energies.

It sounds to me like it's only a simple "nope" if the only weapons you are worried about are nuclear weapons.

Pu-238 could still be useful for radiological weapons such as dirty bombs. If you're worried about them then it becomes a (slightly) more nuanced, "How many morgues do I have to raid, and can I do it before the authorities catch on to the Crematorium Bandit?"

From what I could gather, pacemakers contain up to 4 curies of Pu-238 which is less than 300mg of pure metal. Enough to poison a few people but nowhere near enough to create WMD considering the effort. Industrial radiation sources, in contrast, typically contains tens and sometimes hundreds of curies of mostly undecayed cobalt or iridium isotopes, and is usually much less guarded.

Covered in the article, though the author suggests even less Pu.

A supplier could have a few few devices in a warehouse, but nowhere near the numbers described.

> a pacemaker has only around 135 mg, if I did the conversion from curies correctly. Even so, if I were in charge of keeping plutonium out of the wrong hands, I would still worry about this. It does not seem totally out of the realm of possibility that someone could collect 25,000 pacemakers. Opening 25,000 titanium capsules does sound rather tedious.

I wouldn't be enough to kill many people, but if a certain amount (even non-dangerous, like 1µS/h) of radiations were detected in all Manhattan, the panic would cause lasting damages.

The threat of dirty bombs is pretty low, there's a reason they aren't used. They are extremely hard to obtain, and don't add a significant amount more deaths (if the blast didn't kill you, the radiation will probably not. Though definitely send you to the hospital). They work great as weapons of terror, and there is a lengthy cleanup process, but there's a reason that they aren't common. The people working on the device are also more likely to die, and more easily caught. The materials are difficult to obtain, but definitely not impossible. It just probably isn't worth the time and effort when you could more easily make and/or obtain more conventional explosives.

This is all assuming the smaller type of explosives used in acts of terror.

In fact to create weapons-grade Pu, you need to remove as much Pu-238 as you can, because if Pu-238 is contaminating your Pu, its high fission rate means the bomb explodes too early in the implosion process.

Pu 238 doesn’t undergo spontaneous fission. It’s an alpha emitter.

How do you emit alpha particles without a nuclear process?

It’s a nuclear reaction. It just isn’t fission.

Right. Pu238 alpha decay is aneutronic so it doesn't contribute to a chain reaction. However heating from too much Pu238 (or more importantly, Pu240) would be a heat removal/thermal challenge.

Quite so, I was actually thinking of Pu-240. One heavier rather than one lighter!

(like Pu-238, Pu-240 is an alpha emitter. But unlike Pu-238, Pu-240 also undergoes spontaneous fission).

Isn't a 14 year half life very little for a bomb that should continue to work for decades?

Or isn't it actually used for it, but just CAN be used for it?

Yeah 241 is not commonly or ever used for weapons, it just could be. it's always easier and more practical to use u235 or pu239. Other fissile nuclides no one makes bombs out of include Americium-242m and Curium-244. Technically, sophisticated weaponeers can work with non-fissile but still fissionable nuclides like Pu240 but it's harder. But that's why the IAEA defines significant quantities of plutonium without specifying the isotopics.

Dumb question, but would it be possible to have a nuclear-powered object in your body producing energy for your body to use?

Keeping nuclear material (safely) in your body is the easy part, the hard part is converting that energy into something your body can use. The issue is, our (badly designed) meat-hardware runs on glucose instead of heat or electricity, and there's no easy (or any way that I know of really, but then I'm not a scientist) to convert heat/electricity into glucose.

That's true but isn't 80% of our energy usage used to heat and regulate our body temp? I was wondering if we would then need less "food" for daily calories...

Isn't heat essentially a by-product of muscles "burning" the glucose? I'm not sure we actually need more heat besides what's already being generated as a by-product.

No, you can actually induce greater caloric expenditure by lowering the ambient temperature (within reason), if I’m not misremembering.

This is, after all, the distinguishing characteristic of mammals. Reptiles have no shortage of muscle mass (just look at aligators or crocs!) but still can’t raise their body temperature to compensate for cooler weather or environments.

You're correct, mammals have systems to convert energy in to heat without movement and regulate this very effectively.

This isn't a distinguishing characteristic of mammals though - all birds and a few fish can regulate their body temperature. The distinguishing feature of mammals is the mammary gland - the production of milk for young.

It's possible that a factor in the raised temperature is that it deters fungal infections.

https://www.newscientist.com/article/mg21228411-700-killer-f...

See https://en.wikipedia.org/wiki/Brown_adipose_tissue

You need more if you're cold enough!

Go out in frigid weather with a T-shirt on. You'll soon shiver - your body is recruiting muscles to generate heat. But also, your "brown fat" cells will start pumping out heat as well.

Liver also speeds up metabolism creating yet more heat.

Where I live there is a rather large temperature change between summer and winter (usually 60 K, sometimes higher), and during the summer I definitely feel the need to eat less calories. However this may just be an emotional response rather than anything biological.

Ambient temperature definitely affects calorie consumption. No doubt about it. Body burns more calories keeping organs at 37C if the exterior is 10C, compared to 40C.

Don't you start to get overheating phenomena at 40C?

If you count "sweating", then sure.

I went to a lecture recently by a researcher who is investigating the mechanisms of photosynthesis. The chlorophyll uses the energy from the photon to pump an electron, which (after a long and poorly-understood chain of reactions, which is what he was studying) creates an energy gradient which powers ATP synthase to produce ATP. Presumably if we understood and could recreate this reaction, we could create the electrochemical gradient directly, and then use it to power the ATP synthase and subsequent glucose-production mechanisms.

What kinda gaps in our understanding do we have that limit the reserach for this idea. I know I've seen some news or buzz about creating electricity with "green solar panals": basically using photosynthesis instead of photovoltaic cells. Maybe you can figure out a way to manage the gradient in photosynthesis by trying to see which proteins and complexes and pathways we can fiddle with to regulate it the way we want. But still, even to get to that point we need to map out and understand a lot about the reaction and system before we can begin to probe and see what we can do.

Biochemists at Caltech were studying photosynthesis back in the 70s, with an eye towards artificially replicating the process.

I see not much progress has been made.

This is a paper from two years ago but it shows that progress is still being made in this field. http://science.sciencemag.org/content/352/6290/1210

> Presumably if we understood and could recreate this reaction, we could create the electrochemical gradient directly

We don't need to reproduce the reaction, we know how to create a potential (in Volt) gradient between two faces of a membrane : you just need electrodes for that purpose.

You could "just" make a nuclear liver that snapped metabolic byproducts back together.

I guess you might not directly target the production of glucose, but there should be some places in the metabolism where food energy could be replaced with energy from the plutonium.

I don’t think that’s dumb, and the answer is... sort of. The easiest way to make it work would be to have the device somehow help to heat and cool you internally, which would save you energy. You might be able to use the power to produce ATP or some other metabolic necessity, and deliver it where needed.

The problem is that you’d need a respectable amount of radioactive material in you, waaaaay more than you’d need for a pacemaker. It would probably make more sense just to keep some glucose syrup and a hand warmer around.

I look forward to the day nuclear powered sustenance becomes a fad diet, and accidentally creating immortal zombies whose minds have expired before their bodies.

This might interest you: https://en.wikipedia.org/wiki/Radiotrophic_fungus

The ability to metabolize radiation does exist (https://en.wikipedia.org/wiki/Radiotrophic_fungus) so we might be able to modify our genes such that our bodies can use this energy directly.

I'm under the impression that most nuclear powered pacemakers were not based on RTGs which is what the article describes but on betavoltaics (which does not use heat of decay as energy source, but converts beta radiation directly to electricity).

How long to these pacemakers last? Have people had to get the plutonium "batteries" replaced?

http://www.prutchi.com/pdf/implantable/nuclear_pacemakers.pd...

Last paragraph on page 2... quite a long damned time; by 35 years it was still going strong.

The half-life of Pu-238 (the isotope used in these) is 87.7 years [1]. So they pretty much last a lifetime. For the same reason, Voyager 1 is still kicking even though it's out of the solar system [2].

[1] https://en.wikipedia.org/wiki/Plutonium-238

[2] https://en.wikipedia.org/wiki/Voyager_1

Still, Voyager 1 is slowly shutting down its scientific instruments due to decreased power supply.

I highly recommend this podcast episode with one of the Voyager project managers: https://www.listentospacepod.com/episodes/2016/8/5/episode-5... (about 20 minutes long, iirc)

(In fact, I'd highly recommend the whole podcast, not just this episode, but this one actually talks about power consumption and battery life of Voyager).

The project manager Suzy Dodd talks a lot about shutting down instruments in order to save the power. But the power output of radioisotope thermoelectric generator does not depend on how much power is drawn. Is there a conventional electric battery on the Voyagers that the instruments draw their power from instead?

By saving power she means leaving enough margin to keep the supply voltage stable, running the machine near the limit could cause voltage sag when another device like the tape drive is turned on.

Just like apple limits power draw of it's devices to avoid possible reboots when too much current is demanded from an old battery.

That makes sense. I still wonder if there is battery that would be recharged by the RTG. Seem like it could save some useful energy for the future time when it is needed.

I don't know, but it wouldn't surprise me if no such battery was on the spacecraft.

You have to remember that Voyager is way past its originally planned mission life span, and that the nuclear battery was more than fine for the planned life span.

Batteries tend to be heavy, not operate well at very low temperatures, and wear down quicker than the 40+ years since construction of the space craft.

From the perspective of the original mission design, even with incredible foresight, it seems to be not worth the trade-offs.

Easier to replace the whole pacemaker than the battery, for a couple of reasons:

* Pacemakers are hermetically sealed, usually laser-welded in a titanium case. Adding a replaceable battery with seals would complicate this arrangement.

* By the time the battery winds down, there may be a newer, better pacemaker on the market that fits the patient's needs.

* Since a battery replacement necessitates surgery, you might as well replace the whole unit and get all new parts rather than put an old one back in that may be reaching MTTF.

As for how long the cells last, modern Lithium Thionyl Chloride cells last 5-10 years depending on the pacemaker. They probably actually last longer, manufacturers are pretty conservative with lifetime estimates.

As for plutonium supplies like those in the article, they don't really die since they're thermoelectric. The amount of current you can draw will be proportional to the amount of heat generated by the isotope and the temperature gradient. Since the heat is related to the amount of isotope remaining, the power available will follow an exponential decay. The half-life of Pu-238 is about 88 years, so the battery will last a very long time. The exact lifetime depends on how much current the pacemaker takes to operate.

Most people had these devices replaced with more modern versions, but there are still a few people who have the old plutonium devices which were implanted decades ago.

> The exact lifetime depends on how much current the pacemaker takes to operate.

Not so sure. I don't think the plutonium decay is affected by the current drawn via the thermocouple - it'll just keep putting out the same power (well, the same decaying power curve anyway) no matter how much current you try and draw. I suspect the thermocouple's voltage will just sag so the maximum drawn power will be the thermal output less the efficiency of the conversion (which, I guess, might be non-linear with current?).

But I'm pretty sure whether you draw zero Amps or short circuit the output, you'll still have precisely 50% of your plutonium left after 88 years, right?

> But I'm pretty sure whether you draw zero Amps or short circuit the output, you'll still have precisely 50% of your plutonium left after 88 years, right?

True.

The power requirements can still vary between patients (due to varying quality of the electrical contacts, which tends to deteriorate over time), so there is an individual component, it's just not based on the power drawn from the battery.

Correct, the decay is not affected by the amount of charge you've pulled out of the cell.

What I meant (and should have expressed more clearly) is that the pacemaker will have some minimum current it needs to be able to draw from the TEG to operate, and after the plutonium has decayed to a certain point it won't be able to source enough current to operate, so the pacemaker will brown out.

Somebody in my family has a pacemaker, and is fully dependent on it. I can confirm this part:

> Easier to replace the whole pacemaker than the battery

unfortunately, this statement:

> As for how long the cells last, modern Lithium Thionyl Chloride cells last 5-10 years depending on the pacemaker. They probably actually last longer, manufacturers are pretty conservative with lifetime estimates.

only partially applies in this case. That pacemaker has a lot of work to do (stimulate every single heart beat), and depending on the quality of the electrodes on the heart, and the cables that lead to them, some needed to be replaced after two or three years. If one lasts 7 years, that is a very welcome respite, but quite the exception.

That said, pacemakers have come a long way. The first on that this person had implanted had a fixed beat. The current generation has sensors for oxygen saturation and movement, and be configured and maintained through a wireless interface, they log unusual events etc.

I have one. 100% dependent, dynamic rate, wireless interface, logging, runs about 7 years. "Battery low" state is addressed with progressively more annoying behavior (fixed 60 bpm, beat skipping, etc) that both saves power and urges patient to doctor. Whole unit indeed gets replaced (yes, the hard way), no point in fiddling with maintaining old tech and increased point-of-failure interfaces.

And with it, I'm likely healthier than without it & the underlying issue.

Wouldn't wireless charging pad work well for this? Or is it unnecessary complicated considering how long batteries last?

I think the main problem is the giant meat sack sitting between the charging pad and battery :)

The main problem might be patents :)

Samsung includes charging of medical device[1] in their wireless charging patent. Although the patent[2] seems to focus on the charging method/devices, rather than where they are located.

1. http://patentimages.storage.googleapis.com/US20140084858A1/U...

2. http://www.google.com/patents/US20140084858

Chest cavities aren't especially space-constrained, and another commenter mentioned that electrodes wear out fairly quickly (single-digit years), so it appears that a fixed non-rechargeable battery has enough capacity to last the lifetime of the device.

Wireless charging (for a pacemaker) has the issue that you've got to push the energy through quite a lot of flesh, with consequent efficiency and heat issues.

Wireless charging is absolutely an option for other devices implanted closer to the skin, though.

The issue being rather close to my heart, I'd rather have a single-use 7-year battery + full modern replacement of unit, than have to fiddle with periodic recharging just to prolong use of aging technology. Nobody much wants their life relying on >10 year old computers. (Remember: it's already "old technology" when you first get it implanted.)

Rumor is Medtronic pacemakers still use 6502 processors (albeit on modernized custom silicon) with code written in the 80s. Big medical OEMs are loathe to change.

You don’t fix what isn’t broken when it’s a life critical system.

I'm sure that in 1985, plutonium is available in every corner drugstore, but in 1955, it's a little hard to come by.

>Wikipedia says the technique was abandoned because of worries that the capsule wouldn't be absolutely certain to survive a cremation. (Cremation temperatures go up to around 1000°C; titanium melts at 1668°C.)

You'd think they could switch to tungsten if that was the issue?

Titanium is nicely biocompatible. Tungsten can be poisonous. https://patient.info/doctor/tungsten-poisoning

Could you have an inner layer of tungsten to seal the plutonium, and an outer layer of titanium to avoid poisoning the host?

I guess that'd work - the patient is going to stop worrying about biocompatibility long before the titanium gets near its melting point!

I did not know about these. They are so cool!

I wonder how long a plutonium-powered cell phone could operate...

It wouldn’t really be feasible, the power requirements would require a large amount.

It probably wouldn't be feasible, mostly because if it got smashed by a car, a fairly potent alpha emitter would possibly be released, which is an ingestion and inhalation hazard. The radioisotope usually used (Pu238) is also not produced very much right now so it's in short supply. You make it in special Neptunium-237 targets inside nuclear reactors. In terms of power you can get well up into the hundreds of Watts fairly easily but it can get big. Here's the one used in Mars Curiosity rover [1].

[1] https://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_The...

People talk about using isotopes from nuclear waste for this kind of thing a lot but they're usually considered too expensive.

Definitely agree. I didn’t know about the supply/production issue, though. Thanks for that.

My answer was of course a pure hypothetical, focusing on the pragmatism in a world where the safety issues were non-existent, presuming demand would drive supply.

I wish it was easier to get your hands on refined isotopes. I believe it'd spawn quite a bit of interesting industrial research that's being held back right now. A lot of this waste is a nuisance, but I believe if it was easier to get access to it, we'd see someone start to solve the waste problem.

Not that long... or rather, it could work for a very long time but wouldn't be that useful because of the low power provided.

From Wikipedia - https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

Plutonium-238 has a half-life of 87.7 years, reasonable power density of 0.54 watts per gram - that's heat energy.And then there's math on that which has a rather low efficiency (somewhere around 23% for a stirling engine approach). And well... its hot. And how old is your phone? I don't think my 20 year old phone would still work... why charge power it with something that would last 30 years when the technology that drive it is gone in half a decade.

Those numbers aren't that bad, actually. A cell phone's peak power draw isn't much more than 5W, so you'd need 10g / efficiency. At 10% efficiency, that's 100g, which is 5 cm^3 for plutonium. A cell phone battery is about 3 times that size, assuming 3000 mAh and an energy density of 600 Wh/L. Of course you'd have to build in a heat engine, but perhaps even a peltier would work given the generous efficiency allowance.

At 10% efficiency you'd still be dumping out around 45 watts of heat in your pocket. We'll just need some aluminum pants for the heatsink!

Saying along those lines used to be a popular joke among Soviet nuclear scientists.

Also you don't need peak power all the time, you just need more than your average power draw combined with a battery big enough to smooth out the gaps. I'd think the biggest problem would be your phone being dangerously hot all the time.

It's better than that. Normal iPhones have less than 8 watt hours of battery life, they simply don't need peak power much. 8/24 = 0.333w, you lose some to overhead but you also reduce power draw by 0.33w so that's a reasonable estimate. @ 20% efficient that's 1.65w of heat, but your also storing .3w so really only 1.35w of waste heat. That's a 30% heat production increase at peak which doesn't seem like a deal killer.

7 Plus has a ~30% larger battery, but can dump heat ever more area so not a big deal.

I can see unscrupulous sports people having a plutonium powered lactic acid scrubbing device, or an extra lung.

Uh, does it run, like, on regular unleaded gasoline?

Unfortunately, no. It requires something with a little more kick. Plutonium.

author seemed a bit concerned about Pu-238 being used for weapons, but I think that's the wrong isotope (238 being not fissile, as well as physically hot)

I got curious when i misread pacemaker as pEacemaker

rofl, sounds like they need some peace over there EVERY THING EVAPORATING

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