Help designing RF interference shielding with Spinal Cord Stimulator

[andymat]

Hello All,

I am hoping that you guys can help me. I am getting a Boston Scientific Precision Series Spinal Cord Stimulator. This device generates a programmed series of impulses and discharges them at a designated point along the spinal cord. It is intended for chronic intractable pain - and works wonders for patients. The manufacturer states that the device is likely to be "adversely affected" by devices such as security scanners, arc and mig welders, even the possible automatic door. When quizzing the factory rep, the answer I get is that it is possible to induce currents into the leads that can lead to abnormal sensations or shock sensations. They advise turning the device off around these devices - especially welders. I weld on a "recreational" level and I am not too willing to turn the device off and do this in pain now if I don't have to. The last education I had on this subject was Physics 208 - so I am aware that signal blocking is completely possible. I am thinking I could essentially wear a faraday cage while around these devices. Without getting into specific frequencies, could I get away with wearing a copper or aluminum-lined vest? Would the coverage really be required to be continuous (all the way around) or could it be more like a body armor vest where there are plates covering the pulse generator, the leads, and the contacts? I imagine this is a relatively easy topic for you guys. My friends and I have struggled - but we're all structural engineers. I really appreciate any information you can offer.

 

[mtwieg]

First let me disclose that I'm not a medical professional, but I do research on medical tech (MRI hardware) for a living. I'm somewhat surprised that a device which is prone to interference is actually allowed to be sold...normally its an issue that must be solved by the manufacturer (for example, pacemakers are incredibly robust against interference). But if it did clear the regulatory hurdles, then it is likely that this problem is not actually a threat to health (you should confirm this with a rep from the company) besides some discomfort. Interestingly, I found a document that says the these devices are MRI conditional, meaning they're allowed to be worn inside MRI scanner (head scans only). Modern MRI machines generate a tremendous amount of interference (much more than a normal welder), but apparently they thought it was fine...

As far as shielding goes, just having a vest may be effective enough. A true faraday shield needs complete coverage, but hopefully you don't need such a thing (I can speak from experience that a faraday suit is not fun to wear). At the very least, you want the leads and the device to be covered by a nearby continuous conductive plane. Doing so will attenuate any electric fields parallel to the plane's surface nearby. You might try using a fencing lame as a first try. They're made of stainless steel, so they're not the most conductive material, but they are at least very robust and can be purchased online easily enough. If that doesn't cut it, then a mesh or linked aluminum vest would be better, though much harder to find.

Good luck.

 

[biff44]

Uhhh...you could have some problems there!

A mig/tig system often has a RF component to initiate the arc, and that could make the device go haywire.
A buz box, especially one running in DC mode, might be less likely to generate interference since it is mostly low frequency noise.

In either case, a really good earth ground, grounding of metal work surfaces, etc, might help a lot. Keep the welding leads draped as far from your body as you can...keep the welding unit as far away as you can. Magnetic fields fall off rapidly with distance.

Of course, any sort of induction heater would be right out!

Emi filter on the mains might be wise:
https://www.edaboard.com/threads/198697/

**broken link removed**

I know that some broadcast engineers wear Radiofrequency Protective clothing when working near hot antennas, such as this:


https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21095
https://www.lessemf.com/personal.html

https://www.norad4u.com/emr-protection/rf-protection#TOC-RF-protection-clothes

- - - Updated - - -

also, look starting on page 34:
**broken link removed**


And of course, if you are wearing a metal RFI shirt....you have to be really careful to no electrocute yourself!!!! NO touching bare electrodes, etc. Wear rubber soled boots. Try very hard to not use both hands....you do not want to be holding electrode in one hand and resting other hand on workpiece or grounded equipment/table.

 

[jiripolivka]

Uhhh...you could have some problems there!

A mig/tig system often has a RF component to initiate the arc, and that could make the device go haywire.
A buz box, especially one running in DC mode, might be less likely to generate interference since it is mostly low frequency noise.

In either case, a really good earth ground, grounding of metal work surfaces, etc, might help a lot. Keep the welding leads draped as far from your body as you can...keep the welding unit as far away as you can. Magnetic fields fall off rapidly with distance.

Of course, any sort of induction heater would be right out!

Emi filter on the mains might be wise:
https://www.edaboard.com/threads/198697/

**broken link removed**

I know that some broadcast engineers wear Radiofrequency Protective clothing when working near hot antennas, such as this:


https://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=21095
https://www.lessemf.com/personal.html

https://www.norad4u.com/emr-protection/rf-protection#TOC-RF-protection-clothes

- - - Updated - - -

also, look starting on page 34:
**broken link removed**


And of course, if you are wearing a metal RFI shirt....you have to be really careful to no electrocute yourself!!!! NO touching bare electrodes, etc. Wear rubber soled boots. Try very hard to not use both hands....you do not want to be holding electrode in one hand and resting other hand on workpiece or grounded equipment/table.

I am afraid no advice is available for using in-body electronic devices connected to nerve tissue.
Cardiac stimulators and defibrillators are easier in that they generate relatively high voltage pulses applied to muscles, not nerves.
Nerve stimulation uses voltages < 100 mV which makes the devices and nerves vulnerable by external electrical and RF fields. No models can replace real human body.
Such vulnerable installations call for a complete protection and prohibition from getting the human with such devices from any potentially dangerous situation. Pacemaker owners are excluded from airport gates, the more vulnerable nerve-stimulator users may need to avoid even common domestic appliances, Wifi and other RF electronics. At least till it becomes known on human volunteers what is safe. Getting close to an AC or DC welder circuit looks an absolute no-no due to high field intensity.

I hope nobody gets in any dangerous situation before we learn more. By using more vulnerable devices and human nerves, we enter a highly dangerous zone. Everybody wanting to zap others please start with your own body first.

 

[andymat]

Thanks to all of you for your suggestions. You guys have been a tremendous help to me. As mtwieg alluded - it is entirely possible that the manufacturer has tested and has provided protection for this device (and me) in these situations. It is likely that they throw this out as a general disclaimer of liability. However, I am not too interested in finding out if I can help it. On the other hand, I have no intentions of giving up welding.
I hadn't thought of the fencing gear. I was thinking of using copper or brass mesh and basically making a jacket lining with it - soldering all of the edges and attaching it to the inside of a military jacket/button up shirt. That way I have a lightweight outer jacket that's non-conductive and flame-resistant. The inner screen would be grounded to an outlet by a long wire. However, the fencing thing looks a good bit easier.
I have long since learned the lessons of welding - never touch the work while holding the lead - that hurts when you're all sweaty. Same thing with thin shoes.
The mig lead likes to be straight - not coiled. What would be the best way to shield that (hypothetically)?

Thanks guys - I'm off to fence.

Andy

 

[biff44]

What you have to realize is that no matter what you do some RF or magnetic field will still leak through. In engineering terms, some ways of shielding the person may be more effective than others. But even if you welded yourself inside of a steel drum, there will still be some leakage.

What you need to research, probably with the device maker's help, is how much stray field you would need to restrict your exposure too. And remember, if there was an incident while you were working on hot metal with explosive gasses nearby, it could be very bad.
But, I know if someone told me I could not pursue a lifelong hobby anymore...I would be looking for ways around it. Oxy/acetelyne welding/brazing/cutting is, of course, perfectly RF-free. How about moving over to that type of welding?