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Ultracapacitors for Medical Implants

Posted July 20, 2010 12:00 AM by Jaxy

Engineers from MIT report that they may have found a feasible way for ultracapacitors to be used in place of batteries. They have invented an energy-storing chip that could help ultracapacitors conquer one of the last technical obstacles, so that they can replace batteries in tiny electronics.

Background

Ultracapacitors have a high-energy density, quick recharge, and a seemingly endless number of charge-recharge cycles. But because voltage decreases as their state-of-charge drops, they haven't been able to replace batteries which produce a relatively secure voltage. At 25% charge, the voltage of ultracapacitors is half of its full charge capacity, compared to the 5% drop in voltage at ΒΌ charge in lead-acid batteries.

The Chip

This 1.3mm x 1.4mm chip proposes to get the most charge out of ultracapacitors, while maintaining a constant voltage using four parallel, connected 2.5-volt 250-mF ultracapacitors. When the charge drops to below 25% charge, the energy chip redirects the connections so that two ultracapacitors are in series. The sum of these two series ultracapacitors makes 2.5V.

Once the two pairs drop down to supplying less than 1.25V, there is more rerouting until all of the ultracapacitors are in series, which causes the voltage to rise back to 2.5V. This configuration will cause 98% of the original charge to be utilized.

Future Improvements

The next stage of the project focuses on making tiny, energy chip-powered, implantable medical devices. These devices will focus on supervising patients with neurological conditions that cause tremors.

William Sanchez, the lead graduate student of the project, suggests that there are a few improvements yet to be made. The efficiency of the device's DC-DC converter needs to be improved. A version of the energy chip is expected to be at 65% to 85% efficient for this summer, whereas the goal for commercial applications is 90%.

Resources:

IEEE Spectrum – Circuit Could Swap Ultracapacitors for Batteries

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#1

Re: Ultracapacitors for Medical Implants

07/20/2010 6:33 PM

Interesting approach to power, Jaxy. I can't help wondering if the complexity of the system would make it prone to failure. What happens if one of the ultracapacitors fails - or is there another circuit option to keep it running in that case.

Would the ultracapacitor approach be feasible to power anything bigger than a medical device?

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#2
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Re: Ultracapacitors for Medical Implants

07/20/2010 9:35 PM

In my experiences with battery powered strobe lights dependent on capacitors I'd say there was evidence that the ultracapacitor approach was entirely possible.

It is amazing what can be done with off the shelf mechanisms, if you limit what the machine is actually supposed to do as a primary mission.

Remember I mostly used hotlights, but have worked with strobes. It is a drag that it has been a decade now since I seriously worked with lights. In consideration of what was available two decades ago, dependent on capacitors and batteries I'd recommend anyone seriously working on problems related to the technology involved, look at photographic strobe lights for platform integration of the components.

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#3

Re: Ultracapacitors for Medical Implants

07/20/2010 10:12 PM

There was much keenness to do this in the automotive application, but the stumbling block is 3 in series is about all you can do without greatly complex charge and discharge control. But for a small device/load - nice application - though 90% is a bit optimistic, given their low working voltages and the minimums required for the control smarts.

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#4

Re: Ultracapacitors for Medical Implants

07/20/2010 10:47 PM

This is not the "last" problem to overcome. Supercaps have relatively high leakage currents that cause them to self discharge way faster than say a lithium cell that has a shelf life in excess of 10 years.

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#5
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Re: Ultracapacitors for Medical Implants

07/20/2010 11:27 PM

That is why I said "one of the last." For as long as technology exists, there will always be obstacles. Solutions become obsolete as technology advances to address new problems.

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Re: Ultracapacitors for Medical Implants

07/21/2010 5:38 AM

Indeed you did. I guess I sort of interpreted it differently based on it following in the same paragraph starting: "may have found a feasible way for ultracapacitors to be used in place of batteries" which seemed to cast it in a bit more certain light. The time reference as to when or whether the other of the last obstacles were solved seemed to leave open the possibility that also they may have been resolved.

Thanks for clarifying.

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Re: Ultracapacitors for Medical Implants

07/21/2010 1:28 PM

I can't help wondering how the capacitor's tendency to leak current or self-discharge would affect a sensitive biological system.

The type of devices they are intending to power require very little to have a biological effect. Any discharge of power in the area of the device would presumably cause adverse effects.

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Re: Ultracapacitors for Medical Implants

07/21/2010 3:26 PM

No that's not the problem. The leakage is simply electrons flowing from one side of the cap. to the other: what rcapper is pointing out is that the caps. won't hold their charge for more than a few minutes hours.

Curiously some one has marked his comment off topic: cancelling my good answer vote. Hope some of you can correct that.

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Re: Ultracapacitors for Medical Implants

07/21/2010 3:31 PM

I stand corrected.

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#7

Re: Ultracapacitors for Medical Implants

07/21/2010 12:02 PM

After doing some math, I don't get it. If the energy stored in the capacitor is E=1/2CV^2, then:

4 capacitors in parallel (1000 Millifarads) at 2.5v stores 3.13 Mili-Joules

4 capacitors in series parallel (250 Milifarads) at 2.5v stores 0.781 Mili-Joules

4 capacitors in series (62.5 Milifarads) at 2.5v stores 0.195 Mili-Joules

If the energy drain is constant and we were to arbitrarily say that 4 capacitors in parallel would power the device for a year (365 days), then the other two cases provide an additional 91.25 days and 22.8 days.

Is all the complexity and inherent unreliability worth the risk? Why not just put 6 capacitors in series to add 50% more energy storage and life (182 days) between charges?

If I were a patient receiving a capacitor powered device, I know which system I would prefer as I would not want to risk an unscheduled emergency surgery or worse.

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#11
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Re: Ultracapacitors for Medical Implants

07/21/2010 5:35 PM

I think the notion is that you start out with all caps at 2.5v in which case, by your first calculation the total energy would be 3.13mJ.

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Re: Ultracapacitors for Medical Implants

07/21/2010 8:07 PM

rcapper,

Your are correct. I should have been more precise with my analysis. We start off with 3.13 m-J and we don't add more. Since, the usable energy is when the voltage is between 2.5v and discharged down to 1.25v. That means that usable energy for each configuration is as follows:

4 capacitors in parallel the energy difference between 2.5v & 1.25v is 2.344 m-J leaving .081 m-J unused

4 capacitors in series parallel the the energy difference between 2.5v & 1.25v is .586 m-J leaving .195 m-J unused

4 capacitors in series the the energy difference between 2.5v & 1.25v is .146 m-J leaving .049 m-J unused

The scaling of the relative time for each of the cases is still the same.

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#13

Re: Ultracapacitors for Medical Implants

07/23/2010 9:24 PM

Where are we on this?

I don't fully grasp the Energy Chip.

It is supposed to power the ultracapacitors right? If it works why push it over what it can do? If it is to only monitor events, which may or may not be critical, then I suppose the "Test" is okay. However if the goal is for the combinations to run your heart like a pacemaker, then I wonder if that is practical, or more practical?

What is the real place from which the power for the capacitors, ultra or not, comes from? I can see why you would need more than one to fire your heart like a pacemaker does.

This is not an area of great expertise on my part. I was only impressed with what strobe lights for photographic applications could do.

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