Accelerated Aging of Medical Devices | Part 1: Aging Temperature and Duration

Accelerated aging is probably not everyone's favorite med device topic, but it's one I enjoy.  Accelerated aging allows medical devices to enter the market before real-time data is available.  Accelerated aging is also known as shelf life, stability, or expiry dating testing.  ASTM F1980 is often the reference that guides this type of testing.

Accelerated aging is an area that each person involved in the product development process should have at least a high level understanding of because it is often on the critical path for new products and design changes.  

The importance of accelerated aging

• Overall, choosing the right aging temperature will avoid the timeline killer of redoing aging 
• Improper aging temperatures can cause changes that would not be seen under normal storage conditions.  
• Having a proper understanding of how temperature can affect materials and products will ensure the fastest time to market.   
• Alternatively, aging can be performed at multiple temperatures if data supporting the thermal thresholds of the relevant materials is not readily available or is not known. 

Selecting an Aging Temperature
Below are the main guidelines that should be used when selecting an aging temperature.  Consult your internal (or external) materials expert if you're unsure if your materials will not undergo abnormal material transitions during aging.  

• Ensure material thermal transitions are not reached.  This includes:
• Glass transition temperature
• Melting temperature
• Heat distortion temperature

•  Evaluate all the materials as a finished product:
• Processing (molding, extruding, thermoforming, etc) could have an effect on thermal transitions.  If you need to your material test it after processing.

• The material in your product with the lowest thermal transition should govern your accelerated aging temperature (T_AA) 

Calculating Aging Duration
In general, the Arrhenius Equation is basis for calculating aging duration.  While the details of activation energy that equation can be discussed, those details are not necessary for the majority of people.  So, the equation below is how we typically calculated accelerated aging durations.  

where:
- T_AA is the accelerated aging temperature
- T_S is the standard temperature 
- Q₁₀ is the aging rate per 10°C.  

• T_AA is governed by material properties and is typically chosen as high as possible to minimize aging time. 
• Typical temperatures range from 40°C to 60°C. 
• Increase aging temperature and you're increasing the power to which the Q₁₀ factor is taken, drastically reducing aging time. 
• T_AA = 55°C is often used because it minimizes aging time while ensuring the 60°C threshold called out in ASTM F9180 is not crossed given ± 2°C in allowable fluctuations. 

• T_S is the mean kinetic temperature (more info here) of the product storage environment (sometimes called standard temperature).  
• Typical values range from 20-25°C. 
• 25°C is usually the most conservative value.

• Q₁₀ is typically 2, which means the your aging rate is doubled for every 10°C increase above T_S.
• Increasing this factor will quickly decrease your aging time, but requires objective evidence the materials being tested follow the higher Q₁₀ factor.  
• Proving this is requires aging studies which take longer than using going with the Q₁₀= 2.
• The safe bet is to always use 2, unless you have knowledge that is doesn't follow Arrhenius or has a higher Q₁₀ factor 
• See Eastman Tritan, which has a Q₁₀ =8.  
• See ASTM D7161, which provides specific guidelines for aging outside of ASTM F1980 for medical gloves.  

Example Calculations
Given T_S = 25°C and Q₁₀ = 2 are the conservative standard, and T_AA = 55°C is the most common aging temperature, look what happens to the overall equation:

Knowing that for most aging you need to divide real time by 8 can help with quick calculations.  Below is a table detailing the number of days needed in a chamber (rounded up of course) at various conditions. 

Looking at the 1 Year RT (Real Time) row you can see that increasing from T_AA 45°C to 55°C halves the aging time required. 

The Impact of 2°C
In general, 2°C sounds like a minor temperature fluctuation, but because temperature differences influence the power to which the Q₁₀ factor is taken, it has a big impact.  

Look at the two calculations below.  One assumes T_S = 23°C and the other T_S = 25°C.  For 1 year of aging this results in 40 and 46 days, respectively.  That's a difference of 13%.  What's 13% of a year?  Over 47 days (~1.6 months), which would have a significant impact on labeling.  So, it can make a big difference if you're storage conditions are truly closer to 25°C. 

Closing
Since you made it to the end here is a link to the Excel file for the aging calculator I made (with bonus finish dates added!).

I hope this has added to your understanding of accelerated aging of medical devices.  Feel free to comment and provide feedback below.  Part 2 will focus on the role of humidity in aging.