MOPPs for PPG Sensor

ktsourapas · Mar 10, 2024

Hi all,
It is my first post and my first medical device as well. As a result, I would like to apologize in advance in case my questions are simple or already discussed.

I am designing a wristband that includes a PPG sensor (HRV and SPO2). The whole device will be considered as a single BF applied part.
I understand that the accessible parts touching the patient shall have two MEANS OF PROTECTION and that's not negotiable.
I have read past discussions that the designer had to implement several glass layer in order to fulfill the required 2 MOPPs via the creepage distance.

Does this mean that a 3.4mm creepage distance is required?
Other wearables definitely have a smaller creepage distance.

If this is not possible, as the sensor's manufacturer guidelines instruct a thinner glass, what other type of MOPP could I implement?

Can we use solid insulation to form SUPPLEMENTARY INSULATION or REINFORCED INSULATION for the PPG sensor?

Loekje · Mar 19, 2024

Hi ktsourapas,

Welcome in the wonderful world of medical device development.

Everyone was a starter once.
These forums are a great source of information.

Looking at other devices on the market might be useful but is not definitive: other companies may have found different solutions to comply to 60601, but more more probable is that they have it wrong, even with CExxxx mark.

First: accessible parts do not touch the patient or operator for it to function, but may be touched.
Applied parts do touch a patient, they need that to function. See the definitions 3.8 and 3.2.
And risk management may reveal that certain accessible parts are to be treated as applied parts, see 4.6 (this one creates a lot of discussion).

You solid insulation must comply to clause 8.8.3 and thus table 6. If you read this through you will find that this is not so hard to accomplish this with "thinner glass", even for 2 MOPPs.
On creepage and clearance you must assume an air gap along seams of the glass, unless you can proof you have a verified process of gluing the glass in its place that provides 2 MOPPs insulation on glue material. But that is very hard to proof.

What a lot of people forget is that for a voltage to source more than 10 uA (over the measuring device figure 12) it must somewhere close a loop over different body parts. So even if one point does have enough creepage/clearance, if it cannot close a loop it is safe.

ktsourapas · Mar 20, 2024

Loekje said:
Hi ktsourapas,

Welcome in the wonderful world of medical device development. Everyone was a starter once.
These forums are a great source of information.

Looking at other devices on the market might be useful but is not definitive: other companies may have found different solutions to comply to 60601, but more more probable is that they have it wrong, even with CExxxx mark.

First: accessible parts do not touch the patient or operator for it to function, but may be touched.
Applied parts do touch a patient, they need that to function. See the definitions 3.8 and 3.2.
And risk management may reveal that certain accessible parts are to be treated as applied parts, see 4.6 (this one creates a lot of discussion).

You solid insulation must comply to clause 8.8.3 and thus table 6. If you read this through you will find that this is not so hard to accomplish this with "thinner glass", even for 2 MOPPs.
On creepage and clearance you must assume an air gap along seams of the glass, unless you can proof you have a verified process of gluing the glass in its place that provides 2 MOPPs insulation on glue material. But that is very hard to proof.

What a lot of people forget is that for a voltage to source more than 10 uA (over the measuring device figure 12) it must somewhere close a loop over different body parts. So even if one point does have enough creepage/clearance, if it cannot close a loop it is safe.

Hi Loekje,

Thanks a lot for the reply and the solid insulation guidelines.

I have one last question. I was not able to find the requirements, e.g. thickness of the material, for the solid insulation in the IEC60601-1 document.
However, I have found elsewhere that the only requirement is to have a 0.4mm thick insulating material. Is this valid?

Loekje · Mar 24, 2024

Hi ktsourapas,

That requirement of minimal 0,4mm thickness is what you have found in clause 8.8.2, but as stated before that line: that is a requirement for supplementary or reinforced insulation for a peak working voltage greater than 71V. I doubt that you reach that peak working voltage in your PPG application. So no minimal thickness, but...

What is applicable to your device is
8.8.1 * General
~ Only insulation that is relied upon as a MEANS OF PROTECTION, including REINFORCED INSULATION, shall be subject to testing.

So your MOPP's (and MOOP's) will definitely be tested at a test house according to table 6.

These are considered type tests, not production line end tests (which gave a lot of discussion with our Notified Body auditor).
In our case we, very reluctantly, had to agree that the (manual) potting of a Infrared LED with a clear epoxy resin was forming an insulation relied upon as a MOPP. And validating our manual potting process to be 100% compliant with the requirements of table 6 turned out to be even more costly than installing a production line end test with the same test.
So now we are putting our finished devices through a HiPot test of 1000V AC for 60s where the LED working voltage is a whopping 2V max (or test with 1200V AC for 1s, or with equivalent peak DC values).

So be careful with your design to put in only MOPP's with a-priori proof and/or validated processes.
FWIW: companies that gas up a very thin layer of parylene claim to have verified the process to comply to these 60601-1 insulation requirements.

ktsourapas · Mar 28, 2024

Hi Loekje,

Thanks for the information and the warning.
I have already contacted the lab regarding the test requirements.
In our case, the insulation will be formed by the casing and the co-molded lens cover.
As a result, the result will be consistent, expecting to make the validation process easier.

MOPPs for PPG Sensor

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