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How to Reduce Foreign Material Rejects in Silicone Medical Device Manufacturing

by | Nov 16, 2022

Platinum-cured silicones are widely employed in implantable medical devices because of their biocompatibility, chemical inertness, high-performance physical properties and stability across a wide range of environmental conditions. 

A significant challenge when processing platinum-cured silicone is contamination control. Silicone is a magnet for foreign material (FM). Silicone device fabricators often budget more than 10% for contamination-related reject rates. It is considered a victory if manufacturing lines can stay within this budget. 

But then there are those weeks where despite airborne counts being within specification, reject rates spike leading to production shortfalls, extensive reinspection and CAPAs.

While there are many categories of FM, the experience we have gained from carrying out particle identification investigations at many facilities indicates the most common sources are fibers and dark particulate. Moreover, the studies also reveal that FM (especially fibers) builds up on equipment and workstation surfaces. Hence, airborne counts within spec are insufficient to prevent spikes in rework and rejects.

The remainder of this blog will discuss fiber-based FM, and we will address sources of dark particle FM in a future blog.

The obvious first step to reducing particle contamination is identifying the foreign material’s source. However, current analytical techniques do not provide sufficient information to take root cause preventive actions. In most cases, even after costly outside laboratory fees, device engineers only understand the elemental makeup of contamination in the device. 

This is especially problematic in the case of organic fiber-based FM, which is commonly reported as “carbon-containing contamination.” 

Now what?

Without definitive analytical information, stop-gap measures such as temporarily ceasing cleanroom operations to perform a deep clean are the only remedy. Even worse is the implementation of longer-term, expensive but ineffective stop-gap measures such as increased inspections, static blow-off equipment and upgrades to the clean room and garments. 

To remedy the inability to identify root sources of contamination, Foamtec has pioneered the PolyCK particle identification process. The PolyCHECK FM service enables quality engineers to definitively identify the process and material source of contamination so that engineering can undertake targeted process improvements.

Foamtec has partnered with leading manufacturers of cosmetic, neurological, wound care, drug delivery, pacemaker and neuromodulation leads to identify and reduce primary sources of particle contamination that negatively impact quality and reject rates.

The case studies below will highlight how the PolyCHECK FM service can significantly reduce rework and reject rates while simplifying device inspections.

Liquid Silicone Rubber Injection Molding Case Study

Liquid silicone rubber (LSR) injection molding requires that the barrel and molds be cleaned in-between production runs. The device below in picture 1a was rejected for fiber-based FM. Because it was dark in color, it was thought to be related to garments (cleanroom gowns were blue in color). Over and over, we have found that color is not a definitive indicator of the source of the FM, given that many fibers undergo color changes in curing, molding or sterilization.  After routine cleaning with lint-free polyester cleanroom wipers, the barrel was sampled with the PolyCHECK black inspection wiper. Sample collection is achieved by dry wiping the surface with the PolyCHECK wiper. The results of the sample collection are captured in the picture below. PolyCHECK static charge attracts and holds particles for further analysis.

As the picture above illustrates, illuminating the face of PolyCHECK with a UV light demonstrates the number of particles left on the surface after cleaning. The combination of the black fabric and the UV light allows operators and technicians to see particles that are difficult to observe against stainless or aluminum equipment surfaces.

Furthermore, since organic FM fluoresce under UV wavelengths, the large amount of white-colored particulates suggests that the polyester wipers used to clean the barrel are the primary source. Most importantly, with the amount of FM collected, ample sample is available to allow FTIR to confirm the exact organic makeup and match the contamination to the source material.

Using optical, SEM EDX and FTIR techniques, we confirmed that the fibers pictured in 1B and 1C were polycellulose, while the fibers observed in 2B and 2C were polyester. In 1D, the red spectrograph is the contamination found in the device, whereas the blue spectrograph is the contamination collected from the barrel. To understand the material source of the contamination, we collected polymer-based consumables such as head covers, shoe covers, garments, gloves and packaging materials used in the change room and clean room. The analysis disclosed that the source of the polyester and polycellulose fibers were from polycellulose nonwoven and polyester knitted “lint free” clean room wipers.

Polycellulose wiper analyses.

As can be observed in picture 2C, the diameter of the fiber from the polycellulose wiper matches the cellulose fibers found in the failed device. In 2D, the spectrograph of the polycellulose wiper also matches the cellulose contamination found in the silicone device and the liquid injection molding tool. While polycellulose wipers are not used in the cleanroom, they are used in the change room and tool maintenance rooms, and as can be seen in the picture below, are present on the cleanroom garments. 

Polyester wiper analyses

Similarly, the fiber diameter and spectrograph in pictures 4C and 4D confirm that the source of the polyester fibers found in the rejected device as pictured in 3B originate with the knitted cleanroom polyester wipers.

Armed with forensic analysis that definitively identifies fiber-based FM as the primary source of device rejects, several silicone device manufacturers have replaced fabric-based cleanroom wipers with polyurethane foam cleanroom wipers.

Because foam wipers are constructed from polyurethane with an 8-sided pore that flexes in the face of abrasion, far fewer particles and no fibers greater than 20 microns are shed during use. In contrast, “lint-free” fabric wipers are constructed from short fibers that shed massive quantities of fibers and particles greater than 20 microns during cleaning. The fibers observed in 1B and 3B above are more than three hundred microns long. Given the quantities of these long fibers that are shed from fabric wipers during cleaning, it is no wonder that fiber-based FM is often ranked one on the FM reject Pareto chart. 

In summary, cleanroom foam wipers have been production-proven to dramatically improve quality and yield for facilities that manufacture silicone-based medical devices. 

For more information about foam wipers and particle identification services, contact us at or visit our website at