11 Apr 2025
Question
“I completed testing recently for cytotoxicity, intracutaneous irritation and Kligman maximization sensitization. My device is a limited-contact surgical tool primarily made of stainless steel. In the saline extracts, red/brown discoloration on the device and the same colored particulates in the extract were seen. I presume this is rust but want your input on what we can do next to determine the source of the rust and why it would happen if the device is made of stainless steel.”
Answer
It’s not uncommon for us to find red/brown discoloration and particulates with stainless steel devices. While it’s likely this is rust, we would need to perform Inductively Coupled Plasma (ICP) to look for the presence of iron or other metals in order to confirm. We must consider a few factors when determining the reason for the discoloration/particulates.
- Is the device made of 304 or 316 stainless steel? 304 is generally resistant to corrosion, but in the presence of chloride, it can show signs of corrosion. 316 has additives in it that make it more resistant to corrosion in the presence of chloride. Seeing the corrosion in the sodium chloride (NaCl) extract in this case would make some sense if it’s made of 304.
- Even if you’ve used 316, which should be resistant to corrosion in chloride environments, the quality of that steel and any coatings or treatments could have an effect. We’ve seen examples where a device was made of 316, but unbeknownst to the manufacturer, the steel wasn’t of a good quality and included other additives. The lesson here? Always know your materials and vendors.
- Soldering on a stainless device can serve as the source of corrosion, so knowing what part of the device is discolored is key.
- Finally, we have to determine if the tool is or will be used in a way clinically that would cause the corrosion. If the corrosion can be determined to be purely an in vitro phenomenon, this can be used in your biological risk assessment. If it’s possible for chloride to contact the device clinically, a shorter, more clinically relevant exposure duration extraction study can be performed. Remember that biocompatibility extractions are typically anywhere from 24 or 72 hours, so if your tool is a single-use disposable only used for 30 minutes, keep this in mind.
In general, when changes happen to a device or the extract thereof, it is important to determine the source and cause, but also the clinical relevance. With in-house biocompatibility and analytical chemistry labs along with toxicologists, Labcorp is uniquely suited to help solve complex situations like these all in one place.
Christopher Parker, Associate Director and SME Medical Devices
And
Stephen Doherty, Analytical Chemistry Head