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November 15, 2010
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If you put electrodes on a body, you should assume they are placed across the heart. -Or someone will put them there. You must protect against all foreseeable modes of misuse, and failures, all happening at once.
I’m sure your design works fine, and is safe under normal conditions, but the safety issue is about what happens in a fault condition.
I don’t mean to be discouraging, but I would be advise caution before anything for sale. Ethics aside, this might take you into a position of legal responsibility. Maybe that’s why you dont see many kits of this nature.
And I’d also second noworries’s comment about applauding you doing this type of project. Industry tends to neglect these low profit areas, and seemingly small things here can be life changing for people - unlike the latest remote controlled touch screen widget fad.
I really recommend reading the iso standards for medical equipment. They are incredibly long, dry and boring - but every rule is there for a reason, and I certainly learnt a few things from them. Sometimes they seem prohibitive, but its more about producing a carefully considered design rather than something with multiple cages around it.
The purpose of isolation is to float the electrode wires with respect to mains ground.
As such, it not so important where the isolation occurs, so long as the patient side part of the circuit is isolated. Many commercial biopotential measuring systems isolate immediately after an ADC. Analogue amps (‘modular’ research lab type gear) tend to isolate as soon as a low enough impedance signal is available, usually very close to the input as you say. It’s about what makes sense in the design.
Granted, electrical faults within the device itself must be considered, but the bottom line is when electrodes are connected to a subject, you must assume any and every component can have a catastrophic failure, and protect against all combinations of 2 (I think..) concurrent failures which could deliver unintentional current to the patient. - so having a single stage preamp or a multi stage amp with ADC present a similar hazard. Although isolating earlier does reduce the chance of 2 concurrent failures, it may not change the protection scheme.
Agreed on the impedance(not resistance) of electrodes going quite low, and some faradaic behaviour occurs in any electrode, but a 9V DC battery directly across these electrodes –could it kill? Assuming 50 ohm resistance per electrode, 2 electrodes, 9v/100 = … it might hurt, but..
Having said all that you do have a point, these designs must be well thought out, and it safety goes beyond just isolation.
Without a schematic and pcb layout, it’s anyone’s guess about the safety issue.
Such a device MUST be isolated, even for the diy market.
Would be pretty easy for diy if it were battery powered - just add optoisolators for comms and keep the two sections well separated on the pcb.
Luis, if you read this I think you’d be wise to read ISO 60601 before offering anything for sale.
Safety issues aside, gotta say I think this is an awesome project.
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