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Help replicating a medical device

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jonchoo:
(purely for non-profit, personal entertainment purposes of course).
The device in question is called a non-invasive vagus nerve stimulator (nVNS). Commercial models cost a ridiculous amount for what they do (electrically speaking) and some even require the patient to buy "top up cards" every month, or the device bricks itself. Draw your own conclusions about whether that is ethical or not.
Fortunately, as I mentioned, these devices are electronically simple. They are more or less TENS units operating with specific pulse patterns. I have already recreated one such pattern as an audio file using information gathered here : https://www.accessdata.fda.gov/cdrh_docs/reviews/DEN150048.pdf?fbclid=IwAR3moTZ3iikhwRm_k5_MGb8QdiEBnFo5l5zvTXi1KhRzGG6RdcrtQcc1Too
I'd appreciate it if somebody could double check it for me. The wav file is here : http://www.filedropper.com/gammacorewaveformreconstruction
My idea is to play files like this from a smartphone into a device which would recreate the electrical pulses at the correct voltage and current.
My ideas are
1) a simple mono audio amp circuit from ebay. I need help with how to limit/boost the output to the required specs (24v 60mA biphasic), and how I could measure this without a scope.
2) hack the signal into an existing TENS unit (can be bought on ebay for <$30).
3) ??? your idea here ???
What load/instrument configuration could be used to replicate/test the action of such a device on skin (again, I don't have a scope)?
Thanks for reading

Psi:
I'm not suggesting you should try what i say below, or that it would be safe to do so.
The TENS hardware might be physically capable of delivering more power than what's safe. So messing around with it might not end well.
It's possible to get nerve damage if you inject the wrong current/pulse patterns.
Also I'm not a doctor.

But.. here's how I might go about doing this, if it was me.

Buy a TENS unit.
Take it apart.
Locate the microcontroller (assuming it has one)
Probe/Monitor how the microcontroller is working to drive the rest of the circuitry.
Remove the micro controller and solder in a new (blank) micro controller of the same type.
Write some code for the microcontroller to drive the TENS unit hardware, except using the new pulse pattern.
This of course assumes that TENS units can output in the right voltage and current ranges, and in the right polarity to match a nVNS.
If not it would need to be further modified.

Ian.M:
I'm echoing Psi's IaNaD disclaimer and safety concerns, however:

24V, 60mA biphasic is well within the capabilities of a HV OPAMP, with a simple complimentary emitter follower current booster output stage.   If the supply rails for the OPAMP and output stage are set at +/-26V, and are strictly current limited to 65mA, it would be physically incapable of exceeding the max voltage or current by more than 10%.

Assuming that the output specs are set conservatively, 10% over is unlikely to be significantly more hazardous than the max. specified output.

I hope I don't have to tell you that such experiments should be powered from a fully floating limited energy battery pack, not from any sort of mains adapter!


Doing this sort of development work without a scope will be at *LEAST* ten times harder than with one.  As you are interested in relatively low frequencies even a crappy scope will be better than none at all.  However you *MAY* be able to get by with your PC soundcard line in and Christian Zeitnitz's Soundcard Oscilloscope which, with an appropriate probe, attenuuator and buffer, will at least let you probe around a circuit and check waveforms.  However it CAN'T show you DC offsets as the soundcard input is AC coupled, and if you get the soundcard scope's input circuit design wrong there is a significant risk of blowing up your PC at which point you will wish you'd bought a cheap standalone real scope! 

Also to reduce the risk of becoming the 'scratch monkey'use either a battery powered scope or a laptop running on battery with no other wired connections, so its fully floating if you are performing tests with a live patient.  *DO* *NOT* *EVER* attempt to 'float' (disconnect the ground from) a mains powered scope or use one for tests with a live patient.

jhpadjustable:

--- Code: ---#include <disclaimer.h>
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--- Quote from: Psi on October 14, 2019, 11:52:45 am ---Locate the microcontroller (assuming it has one)
Probe/Monitor how the microcontroller is working to drive the rest of the circuitry.

--- End quote ---
In the cheapest TENS units, the micro is an STM8S003 with debug port enabled. The output driver is a small laminated transformer driven by a few transistors. All is powered from a 9V "transistor" battery. Happy SWIMming!

mzzj:
Tiny audio transformer  and power opamp or PWM modulated H-bridge would handle the power stage easily.  Almost any microcontroller can generate the pulse pattern shown in the patent application.

https://www.ajmc.com/journals/supplement/2017/economic-considerations-in-cluster-headache-and-non-invasive-vagus-nerve-stimulation/costeffectiveness-of-gammacore--noninvasive-vagus-nerve-stimulation--for-acute-treatment-of-episodic-cluster-headache

10000 usd per year?  :-DD

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