Monday, February 18, 2019

Clever Idea for Powering Medical Implants

Do you have a cardiac pacemaker?  Someone you know?  How about an implanted defibrillator?  

All of the current implants are battery operated, which means the batteries need to be replaced.  Battery replacement is generally minor surgery, but it's still surgery and cutting someone open brings risk of infection.

But this is the era of energy scavenging.  For example, hybrid and battery electric cars use regenerative braking.  A motor and a generator are virtually the same arrangement of parts looked at from different ways.  When you no longer want to power the motors but want to stop you can get power out of the motors (now generators) and dump it back into the batteries.  The battery charging load on the generator slows the motor shaft, slowing the car, and some energy that might go into heating the brake pads and wheels goes back into the battery.

There are energy sources in our bodies; can anything be tapped to provide power for pacemakers or other implantable electronics?  Could we harvest body heat?  What about motion of arms or legs?  In what might be the most ironic arrangement, Engineers at the Thayer School of Engineering at Dartmouth have designed a way to get energy from the beat of the heart itself to keep the pacemaker battery recharged.  The charger is:
... a dime-sized piezoelectric cantilever that is both flexible and porous. The new design incorporates a polyvinylidene fluoride-trifluoroethylene thin film within a dual-cantilever structure wrapped around a pacemaker lead with two free ends. The free ends extend out from the pacemaker to harvest energy from the heart’s motion.
It turns out it doesn't harvest much energy, at least in its current version.
The initial test results demonstrated a maximum electrical output yield of 0.5 V and 43 nA under the frequency of 1 Hz. The team found that adding a proof mass of 31.6 mg on the dual‐cantilever tip results in a 1.82 times power enhancement.
The initial values of 0.5V at 43 nanoAmps (billionths of an ampere) is a mere 21.5 nanowatts (nW).  Adding that 31.6 mg mass got that up to 38.7 nW.  That naturally made me wonder what the power requirements really are.  How big of a battery does an implantable pacemaker carry and how long does it last?  I like to bound problems with numbers to help see how big those problems are.  I found this article on Trends in Pacemaker Batteries from 2004.  The first thing I learned is that batteries aren't replaced like you'd replace batteries in your mouse or flashlight: most often, the whole assembly is replaced.
The most important factor for a cardiac pacemaker battery is its reliability. Unlike many consumer products, batteries in implantable devices cannot be replaced. They are hard wired at the time of manufacture before the device is hermetically sealed. From that point on, the battery is expected to power the device during final testing at the factory, during the shelf life and throughout the useful life of the device while it is implanted. In general the power source of the implantable device is the only component, which has a known predictable service life, which in turn determines the service life of the implanted device itself.
Since the first (experimental) pacemaker was implanted in 1958, batteries have changed quite a bit.  That survey article goes through various battery chemistries from Nickel Cadmium through various Lithium battery chemistries to Lithium Iodine, which it implies is the current technology.  It lists some characteristics of a current battery.
  1. Open Circuit Voltage: 2.8 Volt
  2. Control Circuit minimal voltage: 2.2 Volt
  3. Control Circuit current drain: 10 μA
  4. EOL battery resistance: 10 k Ohms
  5. Chold: 10 μF
  6. Oscillator frequency: 167 Hz
  7. Ah rating: 2 Ah (typical rating)
The Amp Hour rating of 2 Ah is a little surprising, but think of it this way: that's total use for 8 years, or a bit over 70,000 hours.  2 Ah spread out over 70,000 hours is 28.6 micro amps per hour, and 477 nA per minute.  The output voltage of the little scavenger is too low, but that could be stepped up to what's required with a switching regulator at a loss of about 5 to 10% of the energy being harvested.  That looks like if the person's pulse was 72 beats/minute, there's about twice as much energy in the heartbeats as the pacemaker needs.  In rough numbers, it seems like there's enough energy recovered to be useful in these very low energy devices.

In reality, though, it seems this would lead to total redesign of pacemakers because of the change in what they're doing.  According to that survey article, the pacemaker's controller literally counts every pulse it applies to keep tabs on battery life.  Consider a pacemaker implanted with a fully charged battery that is then kept constantly topped off rather than discharging day after day.  It would be a rechargeable battery because if the person's heart stops, the little circuit wouldn't supply energy to the pacemaker when it most needs it.

The research team is currently working under a National Institute of Health funding grant and has two years left to finish the pre-clinical process and obtain regulatory approval. Engineers on the program have also started to investigate how the technology can be used to charge other implanted devices as well.

 a.) Concept of piezoelectric thin film energy harvester for implantable cardioverter defibrillator and a flexible porous PVDF‐TrFE dual‐cantilever energy harvester on the AICD lead. b.) Video analysis of chronically implanted pacemaker leads from a dog. c.) A dual‐cantilever energy harvester within a soft tube on the AICD lead. (Image credit: Dartmouth College) 


  1. Johnny Quest episodes clearly promised me a heart powered by safe, clean plutonium. Sigh.

  2. They've probably already thought of this and rejected for some reason, but...electricity is generated by moving wires past a magnet, and it doesn't matter which is the stationary part. The diaphragm moves up and down ~14/minute 24X7X365 (lower during rest, higher during extertion) and since the necessary current to keep the batteries charged is so low, I wonder if that could be used to generate enough juice to maintain the minimum charge, or, more likely, extend the battery life before replacement is required.

    If the charging mechanism was inadequate (too low a respiration rate, not enough amperage produced due to component aging, etc.) it would seem no harm/no foul because the original battery service life would prevail.

    1. I think that's an interesting idea. I'm not visualizing how to implement that - where to put the two parts to get relative motion - but my not seeing it doesn't mean much.

      In the weird world of the internet, there's some chance researchers find this and get the idea.

  3. And then the AI machine overlords realize we're too valuable to destroy, and they implant a probe into our heads with a digital matrix, while harvesting our bodies' production of heat and electricity.

    Until the Chosen One arrives.

    But seriously, it's probably going to be doable.

    Next use: internally generated electricity used to zap over-stimulated neurotransmitters, and kill chronic debilitating musculo-skeletal pain, and then eliminate phantom pain from surgical and traumatic amputations, thus eliminating long-term opiate Rx use.

    The step beyond that: we kill the drive for opiate drug abuse in a similar fashion.

    The step after than: we microminituarize chemical compounding factories to implantable device size, enabling things like insulin synthesis to cure diabetes, and microdoses of endogenous compounds to fight disease and enable analgesia, micro-steroid therapy, and a virtual pharmacopeia of substances directly, internally, making the corner pharmacy go the way of the buggy whip factory.

    And that's just off the top of my head.

    1. Aesop, Your question about a possible future device that would
      "kill chronic debilitating musculo-skeletal pain, and then eliminate phantom pain from surgical and traumatic amputations, thus eliminating long-term opiate Rx use."
      is currently humming away in my back.
      It is called a "Spinal Simulator" It has twin wires that run along both sides of the spine and are attached to the controller/battery pack that is implanted on my left hip. There are two different types of batteries, the rechargeable and the replace when empty version. The replace version had an expected life span of around ten years (that was in 2010 when I decided on the recharge version) but did not require the time and hassle of recharging. My recharge version is done with an external "puck" and while a charge lasted about three weeks when it was new it now is just short of two weeks and it is now nine years old, so I think it should last another 3-5 years before it will have to be replaced.

      Having said all that, this device does NOT "kill" or "eliminate" pain. The best that I can describe it is like turning up the radio in the car to drown out or make less noticeable the "bad" road noises. While it has been an amazingly good treatment for chronic pain without the side effects of more narcotics, there are down sides to this device. First there is a limit to how high the sound can be turned up and second there is also a point at which I become "tired" of the stimulus and must turn it down or off for a short time.

      Interesting times, and there is so much potential still to be realized in the medical field for these types of devices.

      MSG Grumpy

    2. still seems like a hard way to get power into a device. we've used thru glass inductively coupled coils to get power into a device situated in a salt water aquarium. seems like everything these days has a no-conact charger - phones, toothbrushes and so on. just lay down on a charging pad periodically or wear it on you shirt, whatever. so long as a sub-cutaneous coil could be situated long term, i fail to see the self generation advantages.

  4. The advantage is set and forget, once implanted. Forever.
    Not everybody lives with electricity for pennies, 24/7/forever.

    And Grumpy, your device as it is isn't the ultimate version; the one that's self-powered is.

  5. I had the same thought as bryanb - an induction chargeable battery. I'm wholly ignorant on the design of medical implants but my gut feeling is you'd want to put a minimum of stuff into someone. Less stuff that can break or cause complications.

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