The Power Supply Problem (TPSP)

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FUTURE PROJECT (maybe): piezoelectric flex implant or rigid solar implant ??

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I don’t know if this project is a pipe dream, or whether anyone has attempted a similar device in the past (although I have been asking around). I welcome all critiques and redirects, as I don’t want to be reinventing the wheel over here…

So, from my vantage point, it seems that one of the critical issues surrounding the development of implantable technology is POWER. In the case of Dangerous Things’ technologies, the solution to the power supply problem is capacitance — the device is only powered when in the field of a reader, if I understand everything correctly. In medically necessary devices like pacemakers and insulin pumps, the “powered” portion of the device is usually kept outside of the body, and any implanted components requiring power are hooked up via transdermal leads, which seems unviable for “consumer” implants.

However, despite the hurdles involved in powering implants, I have seen a lot of interest in things like always-on lights (see the discontinued xGLO) or tiny e-ink or backlit screens.

I am wondering what options might be possible for powering small-draw consumer use cases like this, and in my discussions and research the two potential options that have stood out as the safest and most accessible are: 1) a small subdermal solar panel, or 2) a piezoelectric flex component implanted across a joint, such that bending the joint powers the circuit.

Has anything like this been attempted before? Am I correct in interpreting that The Power Supply Problem is a big hurdle facing implant development? Or, is TPSP a wild goose chase, as no matter how power is supplied there would still need to be a power storage solution?

Please let me know your thoughts. I have worked with piezoelectrics before (many many years ago) and have a concept of how to make that component, at least. The implantation site for the piezoelectric option seems less than ideal, however…

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Off the top of my head I can think of a few things that may be possible:

  • A solid state thermoelectric generators could be implanted and use body heat for a source of constant power, negating a need for a battery (if it stops working either it is broken or you are dead, right?)

  • Converting mechanical motion to electricity by use of an unbalanced wheel: Take a disc and cut it in half and put it on a spindle and it will spin around as you move. This could be used to generate small amounts of power, but it involves moving parts.

  • Chemical generator: use body chemistry to perform a chemical reaction and convert that to electricity. This is does not exist as far as I know, but it is possible.

  • Induction transfer: basically a electrical transformer cut in half. In a transformer you wind two coils next to each other and put a current through one and it induces current in the other. I suppose this is pretty much exactly what RFID is doing so it isn’t very novel.

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Seebeck generators are the first thing that came to mind. A small enough one with the cold side facing up completely encapsulated would probably work. But man the health risks if the encapsulation fails…

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Pretty sure there is a thread on here documenting tests of subdermal solar panels.

Also pacemakers have internal batteries. But they are also quite large. Actually a friend works for a company that makes them and they have one that can be placed via a catheter.

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My first idea, which in my relatively uninformed opinion seemed mostly stupid but also not entirely, was a set of pads intended to mate with a matching pogo-pin style cable. The pads being encased in bodysafe plastic all the way up to the outside, with biobond around the topmost bits where it passes through the skin to the outside(the purpose of this in my head being to encourage skin growth around the connector, thus keeping it from moving around too much in the area, and keeping it sealed from the outside). This could connect either directly to the device or in the future to a power storage solution of some sort(though at this point, just make it charge wirelessly).

Again, my very uneducated idea for a possible solution, albeit one with quite a few immediately visible flaws, and even more that I probably haven’t even considered.

and hey, I never said it was a good idea :rofl:

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This is where I started, as I have more of a biochem bg… but it seems exceptionally difficult even conceptually, without trying to pack all of the necessary reactions into a tiny capsule.

Induction or something like what @NatTheCat is proposing feel like a great solution for things that we can’t wait to develop specialized technology for, but it seems like a good “moonshot” to aim for a fully contained system… again, perhaps it is a pipe dream :sweat_smile:

Amazing! Exactly what I had in mind, thanks for bringing that to the surface. Amal does bring up an important point in that thread though:

Perhaps the piezoelectric supply would result in a more accessible end technology?

Yeah I am still unsure about batteries, at least in myself… If someone needs a pacemaker I think the risk of a deep-tissue battery is probably less relevant to the decision. But perhaps they are still viable for consumer implants, they could simply have a shorter lifespan or something.

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Induction or something like what @NatTheCat is proposing feel like a great solution for things that we can’t wait to develop specialized technology for, but it seems like a good “moonshot” to aim for a fully contained system… again, perhaps it is a pipe dream

Induction would be super easy. You can play around with it using a kit like this:

or this

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Speaking of a crazy couple days out in the desert, I’m at Grindfest 9 right now and we’ve been working on some solutions to this problem. For power delivery we’ve been using Qi charging and wireless power transfer from a distance, and for energy storage we’ve been characterizing and exploiting supercaps and testing different lithium ion battery topologies. We also took apart a Boston Scientific spinal cord stimulator to document some of their methods.

Transdermal power transfer with wires is extremely difficult. You can do it for short stints but if you try to heal around a transdermal connection the immune system in that area will remain on constant high alert and cause tons of irritation and damage until you remove the intruder. There’s been testing with hydroxyapatite coatings to promote tissue infiltration, but they haven’t been very successful.

A solar panel or even just discreet photodiodes would be interesting, but the body significantly attenuates all wavelengths of visible light until you get into the red spectrum. With red being the lowest energy, the whole system would be very inefficient. It would be less ambient light energy harvesting (which would probably only get you ~100uA at ~2V) and more carrying around your phone or flashlight for short charging bursts. We already have to do that with inductive coupling though, and using magnetic fields is more efficient because your body doesn’t attenuate them at all.

Piezoelectrics are interesting, but present two unique problems not seen in any other energy harvesting scenario. I’m going to lump MEMS compliant mechanism devices in with piezoelectrics because they’re functionally very similar. First they are entirely dependant on mechanical motion to produce energy, which gives them an inherent lifespan depending on the fatigue characteristics of the materials. Second they produce very high voltage but low power spikes of energy at very long intervals (1 second is an eternity in electronics design). It’s difficult to even make an energy harvesting or storage circuit that can handle that type of generator, and making it efficient is even harder. They’re suitable for harvesting from consistent sources, like a vibrating engine, but hard to implement for implants.

Thermal energy generation from thermopiles or peltier elements has always been a very interesting concept because our body generates heat, but they only operate on a temperature gradient, and only efficiently on a pretty significant one. You are warmer near your core, but even the maximum differential between core and skin would only be 4°C. That might get you 200uA at 0.5V. Transdermals would help if you could get them to work, but air is a terrible thermal medium, so even with transdermals you’d want some kind of wearable cooling block. At that point though what are we even doing here.

You could use body chemistry to generate very small amounts of electricity. There have been some interesting advancements in glucose fuel cells recently. Unfortunately they’re very weak (maybe 50uA at 2V in an implant form factor). This method also introduces new problems. They can become biofouled by interacting with the interstitial fluid and blood, and if they rely on chemical reagents they’ll run out.

Inductive coupling (and eventually resonant inductive coupling at a greater distance) are really appealing because of the higher power density and the complete lack of interaction with the body. Betavoltaic generators are another cool option, but they’re pretty tightly controlled.

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While piezoelectric implants are very limited in what they can do, they do exist.
They make them for fish:
https://www.energy.gov/technologytransitions/articles/powered-life-self-charging-tag-tracks-fish-long-they-swim-long

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This. Pegleg is a good example of how this has been utilsed.

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My mind is blown and it seems that I am not alone, thank you so much for taking the time to share all of this information! I have a lot to catch up on it seems. (: I am glad to hear that there is work being done on this problem, and it’s almost funny that it all comes back to optimizing supercaps and Lithium. No matter how the power is delivered - Qi charging or something internal - power storage becomes the design challenge…

If I understand this thread so far, it sounds like due to various hurdles from both a design and power efficiency standpoint that both solar and PE are off the table… The inability to stand up to wear is a solid disqualifier for piezo-e regardless of power conversion rates, and solar is a bit better because it has better power harvesting and no moving parts… but induction is mechanically a more promising option than either of the other two and is already in use to some degree. (Thank you @NatTheCat @Plato @hadoukenboy628 )

TEGs are perhaps the most frequent recommendation I have received, and I understand that they are fairly well studied in the realm of electronics, but I have not seen reports of implantable devices which utilize them. I also foresee a lot of difficulty finding a reasonable temperature differential within the human body using a relatively accessible install location.

Implants located in the subdermal fascia are relatively accessible for a piercer or body mod artist to install, but as they lay parallel to the dermis in relatively homogeneous tissue they are unlikely to experience a significant temperature differential. Placing the implant such that it experienced a temperature differential would require a legitimate deep tissue tunnel perpendicular to the surface of the dermis… or a transdermal component which has many issues.

The biobattery is what I would consider perhaps the holy grail solution of this problem – perfect integration with the biochemical energy flux already happening… like a tiny cybernetic organ. But it’s going to be a tough solution to crack I think… it would require harvesting some source chemical (without disrupting normal metaboliosm), making a small enough (and non-consumable) generation system to process it, and returning any waste (again without disrupting normal metabolism); and on top of all that how do you keep the system vascularized/pumping??

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Well first determine what you want to do and we’ll find a suitable power source for it. There’s no one size fits all solution. You can justify plenty of less ideal options for specific use cases.

For example Betavoltaics provide absolutely miniscule amounts of power, but they do it continuously for a decade. That makes it a good fit for a real time clock project like the Endochron.

Say you have implanted photodiodes. They may be inefficient for power generation within the skin, but maybe your applications requires very little power and is only used when you have sunlight available. You make a light sensor powered by the sunlight which monitors the intensity and duration of your sun exposure and when you’re at risk of a sunburn it vibrates or gently shocks you to warn you.

Say you have the opportunity to design a device that will be attached to a knee implant which uses piezoelectrics in a very reproducible bend motion. Storing small amounts of energy capacitively within the implant, you could occasionally do a spectroscopy measurement of the meniscus and send out a burst of Bluetooth communication about it’s physical wear.

Say you’re a professional swimmer and you want to be able to time yourself. You get a completely subdermal implant powered via thermoelectric generators. When you’re in the water the differential between your epidermis and deep tissue may be 5-7°C and it produces enough power to run a timer. Once you get out and the power level drops it dumps all the remaining stored power onto a small LED display of the runtime.

Honestly the bio battery solutions are the ones I’m least optimistic about (right below piezoelectrics). They provide vanishingly small amounts of power and have to contend with interfacing directly with the body, which is a goopy corrosive mess that you can’t afford to damage. Glucose fuel cells are interesting, but really only for applications surrounding diabetes where when you have increased glucose levels you need to deliver insulin, so the highest generation peak is also the highest demand.

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Hmm I think I see what you’re getting at. I’m almost attempting to engineer a function without really being set on the use case that will dictate its constraints. Picking a use case will dictate the final ideal setup, but I also do find a lot of value in looking at this hypothetical of whether a given approach is even viable— constraining the problem a bit makes it easier to focus in on what’s possible.

I suppose my mental model of this problem used the xGLO as inspiration because the use case was trivial and relatively simple as a starting point— say I wanted to have an LED that glows all the time. That’s why I was considering solar at first, as a panel could both charge the system when in sunlight, and activate the light when it’s dark… like a tiny garden ornament.

Eventually this could have a more practical application, like your sun protection notification idea, or perhaps some other haptic notification paired to a sensor or an external device like a phone… but I was still wrapping my head around what has already been attempted and what hasn’t but is viable, so I hadn’t really moved beyond power supply/storage yet :thinking:

And I do agree that the bio battery is a low hope option, it’s just the most sci-fi option. :sweat_smile: I will look into glucose supply though, that’s a good lead even if it’s a long shot…

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If we could afford Betavoltaics we could set up basically a little joule thief circuit to power an LED continuously from that for decades

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I’d love to try betavoltaic batteries — BetaVolt claimed to make one called BV100 that’s 15x15mm, outputting 100uW at 3V for 50 years, but I wasn’t able to get any response from emailing them and there has been no media progress on their site since January, which is when all their hype stuff came out.

I have also been looking into solar. In direct sunlight I think you could get 200uW per cm^2 of solar panel through 1mm of skin — then it becomes a question of how big a solar panel you need to power your application. The most optimized Bluetooth microcontrollers need only ~100uJ to read a sensor and fire off a packet, meaning if you get 30 min of direct sun a day on a 1cm^2 implant you could send 3600 packets a day (1 every 24 sec). I think this is pretty doable, and stacks up too with time spent in indirect sun (which is about 5x worse than direct but causes less skin cancer). I ordered some parts to try this out and I want to use some pig skin from the butcher, but I think the optical properties of skin change really fast post-mortem so it may not be a realistic test.

I also thought a bit about discriminatory tech concerns re:skin tone. I was surprised to see this, but the difference in optical transmittance at IR wavelengths is actually pretty small between light/dark toned skin. Here’s a figure from this paper

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The gap in absorption really narrows as the wavelength gets higher than ~850nm or so, which is also the range where solar cells are most efficient. Here’s a graph of different solar cell types and their efficiencies. The most common ones are mc-Si and c-Si, which peak around 1000nm.
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Anyway, I think the skin tone differences are important to address, but they may be more similar than we imagined.

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Just a few comments… I’m not trying to be Negative Nancy over here, but no way 100uW is enough to read a sensor and transmit a ble packet… unless you meant to write 100uJ which is not even the same unit of measure as the 100uW the betavoltaic outputs… also 100uW at 3v means it’s putting out 33.3uA … ble chips need around 13mA to 21mA peak to do their thing generally speaking.

Finally infrared can penetrate skin pretty well but no typical solar panel chemistries operate using infrared light. Infrared photons are too low energy to knock an electron off the silicon atoms. There are methods of converting to infrared photons to a higher energy photon so that it can be used to produce work, but you’ve already hacked the efficiency down by 50%. The graphs are interesting but real world experiments with commonly available chemistries don’t show much promise.

If we were going to explore subdermal solar, the first thing to do would be to try to source some silicon chemistries that can be used efficiently with the same infrared ranges that do penetrate skin pretty well.

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In summary, the factors which influence the performances of different power harvesters. With the consideration in system level, implantable PV cells are more promising in harvested energy, smaller size, less complexity in power conversion, and flexible configurations. Considering the electrical performance, the implantable PV cells are also advantageous for stable output voltage and hundreds of mA current. With the development of two decades, the great achievement has been made not only in the off-chip instrument but on-chip measurement or even in vitro and in vivo biocompatibility tests. The implantable PV energy harvesting system is finalized with device fabrication, on-chip power management circuitry and encapsulations. The polymer encapsulation and hermetic package are applied to protect the PV cell from subcutaneous fluids. The mono-crystalline silicon is mostly used in the implantable PV cell fabrication, while the polysilicon and a-Si are advantageous for slower dissolution. Without encapsulation, the bio-performance of silicon is much better than the other materials. The a-Si is promising with the best performance in device viability, followed by mono-Si and polysilicon. Nevertheless, the electrical output characteristic of mono-silicon is much better than a-Si, and the polysilicon is advantageous in material cost reduction. Turning the point to the package and encapsulation, the cell viability of previous studies shows that the implantable PV cell with encapsulation is highly biocompatible. Two types of encapsulation structure can be used, which are hermetic package and polymer encapsulation. The hermetic structure is proposed with long-term stability in in vivo test, while there is lack of evidence to show the stability of polymer encapsulation in the implantable applications. It must be emphasized that the comfortability and flexibility of the polymer encapsulation is much better than the hermetic, while the hermetic structure provide the better protection for the devices. The other factors such as light source and optical skin loss can also affect the performance of PV cell. The NIR light is confirmed as a nice input power source of the implantable PV cells if the heating problem can be solved. The human tissue various with different ethnic group, different location, different age and even different part of body in same person is summarized. The previous research shows the African skin will cause more losses than that of Caucasian and Asian. Moreover, it is also shown that the PV cell in dermis can harvest move power than that in hypodermis, while the cells in hypodermis can provide more stable energy. The hand skin is one of the best parts to implant PV cells because of the lower thickness. In popular believe, the in vivo test of human implantation is far away from the PV cells now. The best alternative subject is pork skin because of the similar thickness and skin properties. In conclusion, the implantable PV cell is advantageous in supplying the sufficient power within small area compared with the other power harvesting techniques. The circuitry of the PV cell is not as complex as the other AC source. However, there are still some constrains of implantable PV cells: Low feasibility and intensity of the light, rigid and expensive materials, penetration depth for PV cell to implant in surface area of tissue and still need investigation on long term performance after implantation.

Based on the literature review in this article, it is clear that there has been plenty of interest in using PV cells for powering implantable electronic devices. These cells have their limitations and advantages. Their main advantage is the large power density that can be generated. Their main limitation is the difficulty in harnessing light due to tissue losses. However, since all energy harvesters have their limitations, we believe that using a hybrid harvester can help remedy this problem. Hybrid energy harvesters are capable of scavenging energy from multiple sources, thereby offsetting any limitations caused by the unavailability of energy from one source. In this case, the system is guaranteed to receive energy in case one or more energy sources are unavailable.

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Reading more of the paper… considering revisiting this;

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Totally valid power concerns, you’re right that 100uW is definitely not sufficient alone to power BLE continuously. However, if you’re cool with discontinuous (sleep for 10 minutes, wake up record data), there’s some energy harvesting chips which will continuously steal away little bits of power from a PV cell or other source until their capacitor is full, then activate something bigger very briefly. Here’s a graph from the R1801K DC-DC converter chip. Even from 200nA, they can eventually get enough power to turn on a 13mA thing very briefly, and PV cells easily reach 3V at even low lighting conditions. I think this enables a lot of energy harvesting applications because even with almost nothing, you can eventually get something.

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Regarding solar — I might just think of it from a strange angle, but when I hear 50% efficiency I just think of doubling the size. What’s a extra square cm here and there?
I’m not sure I understand your point about the IR photons. There are groups who’ve already shown pretty sufficient power transmission through skin using existing PV cell tech. For instance, I think these people are exaggerating their numbers a bit, but using standard sunlight strength (~1000W/m^2) they claim to get 9mW/cm^2 of under-skin solar panel. I’m pretty excited to try this using the random photodiodes I got off Mouser. They have reception into the 950nm range, so there should be some power at least.

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