Assessment of Silicone Oil Cleaning on Indirect Surfaces in Aseptic Manufacturing

Peer-Reviewed

Submitted: July 13, 2026

Accepted: July 21, 2026

Abstract

The European Union Volume 4, Good Manufacturing Practice for Medicinal Products for Human and Veterinary Use, Annex 1 and Annex 15 provide guidance that must be followed in the design, qualification, and monitoring of cleaning processes to mitigate cross-contamination risks, which includes cleaning direct, indirect, and nondirect product contact surfaces as detailed in the site’s contamination control strategy (CCS). Understanding the critical cleaning parameters (CCPs) for removal of silicone oil from direct and indirect product contact surfaces, qualifying an analytical test method, and setting practical, achievable, and justifiable limits are important to the CCS. This article explores the historical use of silicone oil in the performance of primary drug product packaging; design of experiment test results for selection of the cleaning agent and CCPs; assessment of visible residue limits and Fourier-transform infrared spectroscopy surface analysis; and setting health-based exposure limits for silicone oil using a non–uniform distribution model, and a previously published permitted daily exposure.

Siliconization of parenteral drug packaging has been in use as a lubricant for the past 50 years.¹ The primary functions of the lubricant are to ensure machinability of the closure with metallic machinery, reduce insertion force as the closure is fitted in the parenteral packaging, and maintain integrity of the seal between the closure and primary packaging.² Siliconization can refer to the use of silicone oil, silicone emulsion, and air-curable silicone. This article focuses on silicone oil that includes silicone fluid, siloxane, dimethicone, polydimethylsiloxane, and other terms that represent the base formula structure of the liquid silicone lubricant of (CH₃)₃SiO[CH₃SiO CH₃]_nSi(CH₃)₃.¹

There is a requirement to develop and maintain a contamination control strategy (CCS) to minimize risks of microbial, particulate, and endotoxin/pyrogen contamination of sterile drug product manufacturing.³ The CCS must include a rationale for the selection, application, and removal of any process aids, such as silicone oil, from direct, indirect, and nondirect product contact surfaces. Residual silicone oil on indirect product contact surfaces, such as the stopper bowl, transfer chutes, star wheels, and other equipment parts, can result in visual residue and sticky surfaces. This could reduce the efficacy of the cleaning and sterilizing process, which can impact the functionality of the siliconization process (machinability, insertion, and seal integrity) and increase particulate and microbial risk of the primary packaging system.

A validated sterilization process with steam or vaporized hydrogen peroxide (VHP) bio-decontamination process begins with a clean, dry surface. Laboratory testing and/or field trials are the preferred cleaning development tools. The effects of residual lubricant on the sterilization process have been investigated with health care no-rinse lubricants.⁴ The historical approach to cleaning silicone oil is to use a formulated alkaline detergent containing surfactants at elevated temperature in an automated parts washer or clean-in-place system; however, there are several manufactures utilizing a manual, water-only, sodium hydroxide (NaOH), or alcohol-only cleaning process at ambient to warm temperatures that can be a concern due to the low solubility of silicone oil.^5-7

This article explores the development of a cleaning process through laboratory testing, ease of surface detection of silicone oil and cleaning agent residue, and application of practical, achievable, and scientifically justified limits based on health-based exposure limits (HBEL).

Methods

Standard cleanability practices were used, including a variety of cleaning methods such as agitated immersion (AI), spray wash (SW), cascading flow (CF), and manual cleaning (MC). In addition, visual residue limit (VRL) testing was used to determine an acceptable VRL for specified silicone oils. Lastly, benchtop Fourier-transform infrared spectroscopy (FTIR) was used to determine the presence of silicone oil on stainless steel surfaces.⁵ Representative 304 stainless steel coupons (3 × 6 in) were used throughout the study.

Critical cleaning parameters

The critical cleaning parameters have been well defined in literature; they include time, action, cleaning chemistry, concentration, temperature, water quality, surface, soil load and condition, and environmental factors.^8-10 Laboratory testing was performed to investigate changes in the critical parameters listed in Table 1 using 3 different silicone oils widely used in the pharmaceutical industry. Environmental factors such as room temperature, humidity, and particulate levels, which primarily affect dirty and clean hold times, were not investigated in this study.

The following procedure was used throughout the study unless specifically noted:

1. Weighed dry, clean stainless steel coupons on an analytical balance (±0.1 mg) to obtain the precoating weight.
2. Coated coupons with 1 to 3 mL of the sample.
3. Conditioned the sample on the coupon as indicated in each section.
4. Weighed the conditioned coupon on an analytical balance to obtain the precleaning weight.
5. Cleaned each coupon by AI, SW, CF, or MC with a wipe as indicated in Tables 2, 3, and 4.
6. At specified time intervals, removed and visually observed each coupon for cleanliness.
7. Rinsed each side of coupon with tap water for 10 seconds at a flow rate of 0.5 gal/min.
8. Rinsed each side of coupon with deionized water and examined for a water break-free surface.
9. Dried, visually inspected, and then weighed coupons on an analytical balance to obtain the postcleaning weight.
10. Confirmed coupons were clean (refer to Table 5 for additional details).

Visual residue limit testing

Two silicone oils (Ompi EZ Fill and 360 Dow Corning [DC] Medical Fluid) were evaluated in the laboratory tests. The following procedure was used throughout the study unless otherwise noted:

1. Diluted the silicone oils to 1, 2, 3, and 5 mg/mL using laboratory-grade, purified water (Milli-Q; MilliporeSigma).
2. Applied 1 µL of the diluted detergent and 20 µl of purified water onto a stainless-steel coupon over a 1-cm² area.
3. Air-dried the spiked coupons at ambient temperature for at least 16 hours; visually observed coupons as dry prior to testing.
4. Observed the coupons under a variety of conditions as described in Figure 1 and Table 6.
   1. Two analysts observed the visual residue on the coupons for each test.
   2. The following measurement tools were used:
      1. Lux meter (Cal-light 400; Cooke Corporation) to confirm light intensity.
      2. 12-ft tape measure to measure distance.
      3. Standard manual protractor to measure the viewing angle.
5. Recorded results as either visible (V) or not visible (NV) for each analyst.

FTIR

Test coupons were prepared and cleaned using a formulated alkaline detergent containing potassium hydroxide (KOH), NaOH, and deionized (DI) water. After cleaning, the coupon surfaces were dried and subsequently sampled by swabbing. The collected samples were analyzed by benchtop FTIR (Cary 660 FTIR spectrometer; Agilent) equipped with an attenuated total reflectance (ATR) crystal to evaluate the presence of silicone oil residues, allowing for direct analysis without additional sample preparation.

Results

Critical cleaning parameters

Effect of cleaning agent, concentration, time, and temperature in automated cleaning applications. To evaluate the effects on cleaning efficacy, the following cleaning agents were tested at different concentrations and temperatures:

1. 1% to 5% v/v formulated alkaline detergent containing NaOH
2. 1% to 5% v/v formulated alkaline detergent containing KOH
3. 1% to 5% v/v formulated neutral detergent containing surfactants
4. Formulated 1:64 quaternary ammonium-based disinfectant
5. 0.5 M NaOH
6. 70% isopropyl alcohol (IPA)
7. DI water

Formulated acidic detergents containing citric acid and oxalic acid were also tested, but were not effective in initial cleanings and therefore not tested further. Lower temperatures, such as 25 ºC and 45 ºC, either did not clean the residue or did not do so in sufficient time (less than 30 minutes).

When changing the critical parameters, the following general trends were seen to shorten cleaning time (see Table 2 and Figures 2 and 3 for more details):

• Increasing the wash solution temperature
• Increasing the cleaning solution concentration
• Using formulated detergents
• Using SW rather than other methods due to impingement.

Increased dirty hold times did not affect the cleanability when using formulated alkaline detergents, whereas neutral detergents required increased concentration or temperature to achieve passing results.

Effects of soil conditioning. Soil conditioning of the silicone oils onto the coupons was conducted in 2 different ways:

• Air-drying at ambient temperature for a specified amount of time.
• Baking at 121 °C for 30 minutes

The baking at 121 °C significantly increased the conditions required to clean the surface.

Effects of soil load. The lab also investigated soil load, attempting to simulate campaigning in industry. Coupons were coated with either 1 or 10 layers of designated silicone oil and air-dried for 120 hours. The 10-layer coupons required increased concentration, temperature, and cleaning time to achieve the same results as the 1-layer coupons (Table 3).

Manual Cleaning

Many applications in the industry include manual cleaning of silicone oil. A common example is the cleaning of silicone oil from stopper bowls.¹¹

A variety of cleaning agents were used for manual cleaning, with a focus on operator safety when using specified cleaning agents. Formulated alkaline detergents outperformed formulated neutral detergents, as well as formulated quaternary ammonium disinfectant, IPA, NaOH, and DI water. All tests were performed with a polyester wipe rather than a nylon brush, as the brush would smear the residue. Prepared residue had a 24-hour dirty hold time with 1 layer application.

Visual Residue Limit Testing

The VRL is determined by the point at which 2 analysts can no longer see the residue at any given condition with the unaided eye. Silicone oil was visible by 2 separate analysts under all conditions when applied at 1 µg/cm² on stainless steel coupons (Table 6). No further concentrations were tested as this was the lowest concentration.

FTIR

The FTIR‑ATR spectra in Figures 4 and 5 indicate the following:

• Coated coupon: Absorbance bands characteristic of silicone oil residue, most notably in the ~1000 to 1100 cm⁻¹ region, which are absent in the control coupon.
• Coupon cleaned with hot water (80 °C, SW): Although less intensity than the coated coupon, distinct silicone‑related peaks remained.
• Coupon cleaned with 0.5 M NaOH (80 °C, AI): A pronounced absorbance band persisted in the ~1000 to 1100 cm⁻¹ region.
• Coupon cleaned with 1% formulated alkaline detergent (80°C, AI): A profile comparable with that of the control coupon, with no distinct residual silicone oil signatures.

These results demonstrate that water alone and 0.5 M NaOH were insufficient to remove the coating residue, whereas the formulated alkaline detergent achieved effective cleaning under the conditions evaluated.

Case Study: Identifying Practical Cleaning Conditions for DC360 Residue

A pharmaceutical manufacturer was interested in determining the cleaning parameters needed to remove silicone/dimethicone (DC360) residue from stainless steel surfaces. Controlled coupon testing was used to identify these parameters using AI, SW, and CF. Representative stainless steel coupons were soiled with 1-2 g of sample, air‑dried at ambient temperature for 120 hours (Figure 6), and then cleaned by AI, SW, or CF. Performance was judged using defined acceptance criteria as per Table 5.

Across all 3 cleaning methods, a consistent set of conditions achieved removal on stainless steel:

• Baseline condition: CIP 100 detergent at 4% v/v, 75 °C, for 15 minutes.
• Addendum testing with high-pressure spray at 44 psi (3.0 bar), CIP 100 detergent at 3% v/v, 75 °C, for 5 minutes.
• Addendum testing with hot water pre-rinse at 75 °C for 1 minute, followed by CIP 100 detergent at 3% v/v, 75 °C, for 3 minutes with SW and CF.

The pharmaceutical manufacturer successfully reproduced and validated the recommended cleaning approach, as in the addendum testing with hot water pre-rinse.

Establishing residue limits for silicone oil residues

In this example, a residue limit for silicone oil on stopper bowls was established using the VRL and permitted daily exposure (PDE) value to determine the potential silicon remaining on the stainless steel surface after visual inspection. When establishing residue limits, the risk of leaving those residues on the surfaces should be assessed and understood. For direct product contact surfaces, the risk is associated with cross-contamination of product A to a subsequently produced product B. In the case of silicone oil on indirect surfaces within a filling line, those risks are mostly related to:

• Visually clean criteria not being met.
• An increase in the risk of particle contamination due to sticky surfaces, which may increase the risk of microbial contamination.

For direct product contact surfaces, residue limit calculation includes a PDE value, which is an exposure or dose that is unlikely to cause an adverse health effect if an individual were to be exposed by any route at or below this amount daily for a lifetime.¹² For indirect surfaces, the residue limits may include a calculation with a PDE value and confirmation via:

• VRL
• An analytical method such as FTIR.

For this example, the following information was used:

• PDE value for silicon elements¹³:
  • 0.093 mg/d (inorganic)
  • 0.100 mg/d (organic)
• Stopper-bowl surface area: ~1745 cm²
• Number of stoppers in the bowl (first batch): 900 units
• Maximum amount of residue based on a visually clean (ie, VRL): 1 mg/cm²

The following assumptions were made:

• The contact of the stopper to the opening of the vial is less than a third of the area of the stopper.
• The bowl was visually clean, which is reasonable because the bowl is easily visually inspected during preparation for use.
• The residue is uniformly distributed.
• The residue is 100% transferred from the surface of the bowl to the first batch of stoppers.

Equation 1 was used:

An estimate of 0.000582 mg per vial is well below the PDE value of 0.093 mg/d. Therefore, the estimated silicon residue remaining on the surface under a worse-case scenario is unlikely to cause an adverse health effect.

Conclusion

This study utilized laboratory testing to evaluate critical cleaning parameters to clean silicone oil. The dirty hold conditions, temperature, cleaning method, time, soil load, and cleaning agent influenced the cleaning recommendations, as follows:

• Different types of silicone oil were tested as part of this study, although they cleaned very similarly, showing no significant impact on the cleaning parameters.
• Alkaline-formulated detergents outperformed neutral-pH–formulated detergents.
• Higher temperatures showed a clear improvement in the cleaning performance, meaning shorter times were needed, whereas low temperatures (below 40 °C) were not sufficient to achieve a successful cleaning.
• A second detergent or additive was not needed to achieve a successful cleaning.
• Commodities such as NaOH, water, or IPA were considered not successful or required long cleaning times (more than 60 minutes with AI) to achieve passing results.

This article explores the use of analytical methods such as VRL and FTIR to detect silicone oil present on the surface after cleaning. The cleaning limit can be calculated by using the PDE value of the silicone oil and can be correlated to the determined VRL to confirm if visual inspection is enough to validate the cleaning of silicone oil. FTIR was used to determine the differences in the cleaning performance of different chemistries, such as formulated alkaline chemistries and commodities, by showing a signal from silicone oil residue present on the surfaces after cleaning. FTIR can be used to quantify silicone oil and correlate it to a calculated cleaning limit¹⁴; this was out of scope of the present article.

Disclosure of Interest

All acknowledged contributors are employees of either STERIS or Sanofi and may hold shares and/or stock options in their company.

Acknowledgements

The authors would like to thank Dayna Turner of STERIS for assistance with FTIR testing and Jeff Felker of Sanofi for arranging test samples and a review of the article.

References

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About the Authors

Sarah Riley is a technical services lab associate for Life Sciences at STERIS with 3 years of experience in the pharmaceutical and biotech industries. She provides lab testing to support cleaning validation via techniques including SDS-Page, TOC analysis, stainless-steel maintenance, and material compatibility. She has a bachelor’s degree in biochemical engineering from the Missouri University of Science and Technology in Rolla.

Dijana Hadziselimovic is a manager, laboratory technical services for the Life Sciences Division of STERIS in Mentor, Ohio. She provides technical support in the area of process and research cleaners and manages laboratory experiments to recommend cleaning procedures. Dijana has over 18 years of laboratory experience in the pharmaceutical and biotech industries. She holds a bachelor’s degree in chemistry from the University of Missouri–St Louis.

Cecilia Pierobon brings over 8 years of experience in pharmaceutical equipment qualification and GMP compliance. At STERIS Life Sciences, she provides global technical support on cleaning validation, contamination control, and sterility assurance. She also delivers technical presentations at industry events and develops technical literature.

Paul Lopolito is a technical services director for the Life Sciences Division of STERIS in Mentor, Ohio. Paul oversees the technical services Process and Cleaner Evaluation and analytical testing laboratories as well as providing global technical support related to process cleaning, cleaning validation, and contamination control. Paul has over 25 years of industry experience and has held positions as a technical services manager, manufacturing manager, and laboratory manager.