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How to determine part that is most difficult to sterilize

Soog1459

Starting to get Involved
#1
Hi, cove member.

Question is relating to section 8.6 c) of ISO 11135-1 (in addition, maybe 7.1.3 is too)
The device to be sterilized is kind of medical tube, and it is EtO sterilization. The dimension of products is different, such as outer&inner diameter size, different length, whether narrowed part is or not, the size of the narrowed part.

I have to make a PCD after identifying the most difficult part. Some pre-evaluation will be necessary.
I'm thinking 2 step approach of desk-study and actual experiment. The 1st is to study that some products are obviously easier to sterilize than others. The 2nd step is to confirm which PCD is equivalent to (or more difficult than) the product past the 1st step.

What kind of basics should be considered to know easiness/difficulty of sterilization?
In other words, which is more easy/difficult, thicker & shorter tube, or thinner & longer tube??


Any clues will be helpful for me and appreciated.
 
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John Broomfield

Staff member
Super Moderator
#2
Hi, cove member.

Question is relating to section 8.6 c) of ISO 11135-1 (in addition, maybe 7.1.3 is too)
The device to be sterilized is kind of medical tube, and it is EtO sterilization. The dimension of products is different, such as outer&inner diameter size, different length, whether narrowed part is or not, the size of the narrowed part.

I have to make a PCD after identifying the most difficult part. Some pre-evaluation will be necessary.
I'm thinking 2 step approach of desk-study and actual experiment. The 1st is to study that some products are obviously easier to sterilize than others. The 2nd step is to confirm which PCD is equivalent to (or more difficult than) the product past the 1st step.

What kind of basics should be considered to know easiness/difficulty of sterilization?
In other words, which is more easy/difficult, thicker & shorter tube, or thinner & longer tube??


Any clues will be helpful for me and appreciated.
Soog,

Longer and thinner would be more difficult to clean.

I would think these were product design inputs.

Sorry, I am not into ISO 11135-1.

John
 
M

MIREGMGR

#3
The comparison will be lumen diameter vs. lumen length. It won't be obvious, and in fact will be very difficult to determine by analysis, whether a small diameter shorter lumen or a larger diameter longer lumen is harder. Passive gas diffusion-flow with small lumen diameters is hard to quantitatively analyze, because the complicated multi-variable boundary layer effects typically extend entirely across the lumen diameter.
 
#4
Soog
There are a few more factors to consider,
Are the materials of composition the same between products e.g. some materials are more permeable to EO, also are they packaged & presented for sterilization identically?
Does the lumen have more than one opening, the most difficult part to sterilize could be midway along the length of the lumen or is the lumen closed or sealed on one end with only one point of entry for gas?
Is it a single, dual or multiple lumen product?
Starting point is calculating the product with the smallest cross sectional area for the gas pathway and longest lumen. If such a combination doesn't exist on the product matrix you could build a 'challenge' device for the purposes of the validation.
You may also on need to assess which part of the product has most bioburden.
Hope this helps your assessment.

AAMI TIR 28 may be of interest

Ewan
 

Soog1459

Starting to get Involved
#5
Thank you for the advises.

The product consists of outer and inner plastic tubes, those are single lumen and only both ends are open, and individually packed to sterile package. So there are 2 space where gas should go in, one is inner space of the inner tube, and the other is in-between space of these tubes.

Headache is the in-between space of the tubes. About 5mm at one end of inner tube is thicker than the other part of inner tube due to connection with a pipe. The minimum gap of this part is 0.01mm in radius. One of my current question is if gas can go through this narrowed part.

Someone in my company said that pressure drop should be calculated. But I think that dominant principle is diffusion rather than fluid mechanics.
I don't know also that below spaces, for example, can be handled equivalently if these length is same.
1) cross-section is circle of dia.2mm -> area = 3.14mm2
2) cross-section is ring, outer is dia 3mm and inner is dia 1.118mm -> are a is 3.14mm2

Permeability of material is one of concern. Does anyone know PFA, PEEK, PTFE has perneability? For the moment, I'm not regarding them perneable.

The advised TIR, I will buy and check it.
 
#7
Soog
I gather you want to
1) establish if product is sterilizable; this can be done by testing in a trial half or full cycle. Place a biological indicator in a sealed connection attached to restricted end with last mm spliced open to allow gas path up normally open end of lumen, through restriction, in to connection to sterilize BI. If there is some kill there is potential to develop a cycle. If there is none the lethality of cycle can be increased, e.g. increasing concentration of EO or lengthening cycle time etc. From rough calculation a 0.1mm clearance of a tube 3mm ID is 0.09mm2. If it helps, have experience sterilizing product with 0.1mm2 restriction ....similar to your case lumen with restriction was harder to sterilize than a lumen significantly longer in length.
2) you are trying to come to engineering rationale for which code is harder to sterilize. I would say do the maths and as you indicated confirm which is harder to sterilize in a trial fractional cycle. Run cycle with 2 more PCDs to show relationship between the products.

Correction in maths
2mm ID 》3.14 mm2 cross section area

Outer; 3 mm ID 》7.06 mm2 - minus
Inner; 1.118 mm OD 》 0.98 mm2
Difference is 6.08 mm2?

Cannot subtract diameters first and use result to calculate area.
 

Soog1459

Starting to get Involved
#8
BradM,
It is EtO sterilization.

ewan,
Thank you for sharing your experience and correcting me.

2) you are trying to come to engineering rationale for which code is harder to sterilize. I would say do the maths and as you indicated confirm which is harder to sterilize in a trial fractional cycle. Run cycle with 2 more PCDs to show relationship between the products.
I didn't get what you mean.

My plan is to compare some kinds of PCDs, maybe I should say candidates of PCD, in order to know which PCD is worst. I have to check the past test results which cycle should be used for this comparison, half cycle or 1/6 cycle or something.

Regarding area calculation, let me correct my stupid mistake!
I uploaded a jpeg of cross sections of tubes. This picture is in my mind.
The areas of light blue part are same, but perimeter is much different. So boundary layer effect is different as MIREGMGR commented. Can we consider these shape equivalent??
 

Attachments

#9
Hi Soog

Apologies for not being clear.
I gather your products have a tube inside a tube, various lengths, diameters, cross sectional areas and narrrowed sections etc.

So yes (a) determine the products that are physically hardest to sterilize e.g. have longest / narrowest gas path from drawings. As there are many variables you may narrow the number of codes down to 2 or 3 being potentially the hardest to sterilize. e.g a code with narrowest & longest inner tube and the code with the smallest volume between inner & outer tube.

It is probable the code with the longest & smallest area between inner & outer will be the hardest to sterilize; as you already stated there is also a narrowing of this space for 5mm in length reducing the gap to 0.1mm, this is a significant narrowing of the gas path.

(b) you could confirm the code which is hardest to sterilize by inoculating these 2 or 3 codes with Biological Indicators (several replicates) and place these potential internal PCDs in a trial sub lethal or fractional cycle in order to determine the code with the least kill i.e. the code which is greatest challenge to the sterilization process. This is what I meant by showing relationship between products.

RE question about equivalency of areas; I would factor-in the narrowing of the space between tubes (for 5 mm) as making space between tubes harder to sterilize.

Happy Validation!
 

Soog1459

Starting to get Involved
#10
ewan,
Thank you for the explanation and your helpful advises.
Now I understand what you said, and am thinking my test plan.


Regarding question about equivalency of areas, it is just a question for me to understand basics of analysis.

Different question that I'm worndering now.
There is a single lumen tube, and its both ends are open. Only 5mm from the left end is narrowed (smaller I.D.).
We decided to regard only one-end open as worst case. Will test of below PCDs be same result?
- B.I. is attached to the left end (close to the narrowed part)
- B.I. is attached to the right end (far from the narrowed part)

Best regards,
Satoru Oogami
 
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