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Seal integrity is a major factor in pharmaceutical and medical packaging quality. Sealing processes must create consistent bonds between packaging materials to maintain the required barrier against external contaminants and product leakage. However, product residue, particles, fibers, and other foreign materials trapped within the seal area can affect seal formation and create potential pathways for leakage.

Airborne ultrasound provides a non-destructive approach to evaluating seal quality by analyzing ultrasonic signals transmitted through sealed packaging. When contamination interferes with seal formation, the resulting changes in seal structure may influence test measurements and inspection results. Understanding the relationship between seal contamination and airborne ultrasound results helps packaging manufacturers establish appropriate testing parameters, identify potential defects, and improve the consistency of pharmaceutical package testing.

What is Airborne Ultrasound in CCIT?

Airborne ultrasound is a non-destructive inspection technology used in container closure integrity testing (CCIT) to examine the quality and consistency of package seals. The method uses high-frequency sound waves transmitted through the sealed region to identify variations associated with seal properties. Unlike methods that require package immersion or the introduction of a tracer gas, airborne ultrasound evaluates the seal without requiring direct contact with the inspection area. This makes it suitable for applications where preserving the package and its contents is a priority. PTI's Airborne Ultrasound technology includes systems such as Seal-Scan for offline seal inspection and Seal-Sensor for inline inspection. These systems analyze ultrasonic signal characteristics to detect variations in seal properties, including defects that may not be readily visible during visual inspection.</ahref="https:>

The results provide information about seal uniformity and potential discontinuities, helping manufacturers investigate packaging process variation and determine whether additional integrity testing is appropriate.

What is Seal Contamination?

Seal contamination occurs when unwanted substances become trapped between packaging layers during the sealing process. These substances can interfere with the contact between materials and influence the formation of the sealed region.

Common sources of seal contamination include:

• Product residue: Powders, liquids, oils, or formulation residue entering the sealing area during filling.
• Particulate matter: Dust, environmental particles, and fragments generated during packaging operations.
• Fibers and foreign materials: Fibers from packaging components, wipes, or handling materials becoming trapped between sealing surfaces.
• Packaging material variation: Surface irregularities, wrinkles, or material inconsistencies that affect contact between layers.
• Process-related contamination: Residue or deposits accumulating on sealing equipment and transferring to packaging surfaces.

The effect of contamination depends on its composition, size, location, and distribution within the seal. Some contaminants create localized interruptions in material contact, while others influence a wider section of the seal. Not every contaminated seal develops a through-leak. However, contamination can alter the seal structure and increase the possibility of defects that compromise package integrity.

How Contamination Can Affect Leakage?

Seal contamination can interfere with bonding between packaging layers, creating weak regions, gaps, or channels that may allow gas or liquid to leak. Its impact depends on the contaminant type, packaging material, sealing conditions, and defect characteristics.

Airborne ultrasound evaluates seal quality by analyzing ultrasonic signal transmission through the sealed area. Changes in material contact, seal thickness, bonding, and structure can influence signal measurements. Contamination may produce detectable variations in signal amplitude or other inspection parameters. However, a signal variation does not automatically confirm a leak, as intact seals may also exhibit differences.

Contamination can create localized defects and increase seal variability across production batches. Airborne ultrasound can help identify these variations, while method development and validation establish their relationship with package integrity.

False accepts and false rejects are additional concerns. A false accept occurs when a leaking package passes inspection because the defect falls outside the method's detection capability or acceptance criteria. A false reject occurs when an intact package fails due to contamination or other seal variations.

Representative defect testing, appropriate acceptance criteria, and correlation with a suitable package integrity reference method help manufacturers assess inspection performance and improve result reliability.

Packaging Formats Where Seal Contamination Matters

• Flexible Pouches and Sachets

Pharmaceutical powders, granules, and liquids can enter the sealing area during filling. Product residue between the sealing layers may interfere with bonding and produce localized seal discontinuities. Airborne ultrasound can help identify changes in seal properties across the inspected region.

• Blister Packs

Blister packaging typically combines a formed cavity with a lidding material. Particles, product residue, or inconsistencies at the sealing interface may affect the seal between the lidding material and the blister structure. Inspection can help identify variations in seal quality, supporting the investigation of potential integrity concerns.

• Medical Device Packaging

Medical device packages may use porous medical-grade paper, films, Tyvek, and other materials joined through heat sealing. Fibers, particles, folds, and inconsistent material contact can influence seal formation. Because package integrity is relevant to maintaining the sterile barrier, detecting seal anomalies can contribute to packaging process monitoring and quality control.

• Flexible Pharmaceutical and Biopharmaceutical Packaging

Bags, liners, and other flexible packaging assemblies can be affected by residue, wrinkles, and contamination along the sealing interface. The consequences depend on the package construction, intended use, and required barrier performance. The applicability of airborne ultrasound should be confirmed for each material combination and package configuration.

Developing a Robust Airborne-Ultrasound Method

A reliable inspection method requires an understanding of how seal contamination affects both actual package integrity and measured ultrasonic signals. Method development should reflect the packaging materials, production conditions, and types of defects that may occur during routine operations.

1. Identify Potential Contamination Sources

Review filling, handling, sealing, and packaging operations to identify where product residue, particles, fibers, or other foreign materials could enter the seal area. This information helps establish realistic contamination scenarios for method development and process improvement.

2. Characterize Packaging Materials and Seal Conditions

Evaluate the materials used in the package, including their thickness, structure, surface properties, and sealing characteristics. Document relevant process parameters such as temperature, pressure, and dwell time. These variables can influence seal formation and ultrasonic measurements, so they should be considered when establishing inspection settings.

3. Prepare Representative Test Samples

Develop samples that represent both acceptable seals and relevant defect conditions. These may include seals containing controlled amounts of representative contamination, incomplete bonding, wrinkles, or other realistic discontinuities. Where possible, distinguish between contaminated seals that remain intact and contaminated seals that contain a confirmed leakage pathway. This distinction is necessary to understand whether the method identifies seal-quality variation, actual leakage, or both.

4. Establish Suitable Acceptance Criteria

Determine how ultrasonic measurements correlate with seal quality and package integrity. Acceptance criteria should reflect representative samples, expected process variation, and the intended application. Avoid treating every signal variation as proof of a leak. Instead, establish documented decision criteria supported by test data and suitable reference methods.

5. Correlate Results with an Appropriate Integrity Test

Use an appropriate reference method to investigate whether detected seal anomalies correspond to actual leaks. The selection of the reference method should reflect the package format, likely defect type, and required detection capability. This correlation helps define the limitations of airborne ultrasound and clarifies which types of contamination-related defects the method can reliably identify.

6. Assess Repeatability and Reproducibility

Evaluate whether the inspection method produces consistent results across repeated measurements, operators, equipment settings, and relevant environmental conditions. For inline inspection, assess performance under representative production speeds and normal process variation. For offline systems, examine the influence of package positioning and measurement setup.

7. Monitor Seal Quality During Routine Production

After method qualification, monitor inspection trends to identify recurring anomalies or changes in seal performance. Where contamination-related defects are observed, investigate the source and implement appropriate corrective actions. Combining inspection data with process monitoring can help manufacturers address recurring seal defects and reduce avoidable packaging rejects.

Conclusion

Seal contamination can interfere with bonding between packaging layers, creating weak areas or channels that may cause leakage. It can also alter ultrasonic signal transmission, revealing variations in seal properties. However, an ultrasonic anomaly does not automatically confirm a leak, as results depend on packaging materials, contamination characteristics, defect geometry, inspection parameters, and acceptance criteria.

PTI’s Airborne Ultrasound technologies, including Seal-Scan and Seal-Sensor, offer non-destructive options for offline and inline seal inspection. Identifying contamination sources, testing representative defects, and correlating ultrasonic measurements with appropriate integrity reference methods help establish reliable, package-specific inspection methods through development and validation.

Frequently Asked Questions

1. How does seal contamination affect airborne ultrasound results?

Seal contamination can alter ultrasonic signal transmission by affecting material contact, bonding, and seal structure. These variations may indicate potential seal defects, but they do not automatically confirm a leak.

2. Which packaging formats can be inspected using airborne ultrasound?

Airborne ultrasound can be used for suitable flexible pouches, sachets, blister packs, medical device packaging, and pharmaceutical or biopharmaceutical flexible packaging. Applicability depends on the materials, package configuration, and validated inspection method.

3. How can manufacturers validate airborne ultrasound for contamination-related defects?

Manufacturers can test representative contaminated and uncontaminated seals, establish appropriate acceptance criteria, and correlate ultrasonic measurements with a suitable package integrity reference method. Repeatability and reproducibility testing also help establish inspection performance.

Container closure integrity testing (CCIT) helps pharmaceutical manufacturers determine whether packaging can prevent product leakage and the ingress of contaminants. Two parameters frequently discussed in package integrity testing are defect size and leak rate. Although these parameters are related, they describe different characteristics of a package defect and should not be treated as interchangeable measurements.

Defect size refers to the physical dimensions of a defect or leak pathway, while leak rate describes the amount of gas or liquid passing through that pathway over a specified period under defined conditions. The relationship between these parameters helps manufacturers interpret test results, establish acceptance criteria, and select suitable deterministic CCIT methods for pharmaceutical packaging.

A small defect does not always produce the same leak rate across different packaging systems. Factors such as defect geometry, package materials, pressure differential, gas properties, and test conditions influence the rate of leakage. These variables should be considered when correlating physical defect dimensions with measured leak rates.

What is Defect Size?

Defect size refers to the physical dimensions of a discontinuity within a package or its sealing system. Depending on the defect, this may include the diameter of a hole, the width of a crack, the dimensions of a channel, or other geometric characteristics of a potential leak pathway.

Defects can occur in container walls, stopper interfaces, syringe closures, bottle seals, flexible packaging, and other areas of a container closure system. They may be intentionally introduced during method development and validation or develop during manufacturing, handling, transportation, or storage.

Defect geometry can vary considerably. For example, a circular hole through a rigid material differs from a narrow, elongated channel along a package seal, even when both have similar opening dimensions.

Defect dimensions are commonly expressed in micrometers (µm). However, a single dimensional measurement does not necessarily describe how a defect behaves as a leak pathway. Factors such as pathway length, geometry, tortuosity, and connectivity can influence the movement of gases or liquids through a defect.

Consequently, physical defect size is only one consideration when characterizing package integrity and evaluating leak detection method performance.

What is Leak Rate?

Leak rate is the quantity of gas or liquid passing through a leak pathway per unit of time under specified conditions. In pharmaceutical CCIT, gas leak rates are commonly expressed in units such as mbar·L/s or atm·cc/s, depending on the measurement method and reporting convention.

The measured leak rate indicates the amount of gas or liquid passing through a leak pathway under specific test conditions. Unlike defect dimensions, which describe physical geometry, leak rate quantifies the movement of a substance through that pathway.

Different CCIT methods measure leakage using different physical principles. Vacuum Decay detects pressure changes associated with gas escaping from a package, Helium Leak Detection measures tracer gas using mass spectrometry, and High Voltage Leak Detection (HVLD) identifies conductive leak pathways in compatible liquid-filled packages. The resulting measurements and detection capabilities depend on the method, package configuration, and test parameters.

A leak rate should therefore be interpreted alongside the test conditions, measurement units, method limitations, and predefined acceptance criteria. Values obtained using different methods or operating conditions may not be directly comparable without appropriate correlation.

How Defect Size Influences Leak Rate?

In general, a larger, unobstructed leak pathway can allow more gas to pass through than a smaller pathway under identical conditions. However, the relationship between defect size and leak rate is not always linear. Changes in the dimensions or geometry of a leak pathway can affect gas flow depending on the physical conditions and package properties.

For example, a circular hole through a thin material may produce a different leak rate than a narrow channel along a sealing interface. Even when their opening dimensions are similar, differences in pathway length, shape, and internal restrictions can result in different leak rates. The pressure differential across the defect and the properties of the gas also influence the amount of gas passing through the pathway.

The flow regime is another factor. Gas movement through a leak can involve viscous flow, molecular flow, or transitional flow, depending on the pathway dimensions, gas pressure, and other conditions. Consequently, a simple relationship between defect diameter and leak rate cannot be applied universally to every pharmaceutical package.

For pharmaceutical manufacturers, defect dimensions alone cannot establish the leak rate of a package. Physical defect dimensions and measured leak-rate data provide different information and can be used together during method development and package integrity studies.

Why the Same Defect can Produce Different Results?

A defect with identical nominal dimensions can produce different measured leak rates under different testing conditions. The pressure differential across the defect influences gas flow, while the pathway's shape, length, surface characteristics, and internal restrictions affect the amount of gas passing through it. Package materials also influence results, as rigid glass containers and flexible polymer seals may respond differently to pressure and temperature changes. In addition, helium, air, and other gases have different physical properties that influence their movement through a leak pathway.

Test method and configuration also influence the reported results. Equipment sensitivity, test duration, package volume, sample preparation, temperature, and changes in the sealing interface can affect measured leakage. Therefore, defects with similar dimensions may produce different leak rates across packaging formats and test conditions. Reliable correlation requires consistent testing parameters, suitable reference defects, and documented measurement procedures.

Relationship Across Common CCIT Technologies

1. Vacuum Decay: Vacuum Decay is a non-destructive CCIT method that detects leaks by measuring pressure changes within a test chamber when gas escapes from a package. The measured response depends on factors such as defect dimensions, leak pathway geometry, package volume, and applied vacuum conditions. Although larger leak pathways may produce greater pressure changes, the relationship is not universal. Physical defect size cannot be determined directly from the pressure response without method-specific correlation.

2. Helium Leak Detection: Helium Leak Detection uses helium as a tracer gas and a mass spectrometer to measure helium passing through a leak pathway. The method provides quantitative leak-rate data and is suitable for highly sensitive testing of compatible pharmaceutical packages. The measured leak rate depends on factors such as defect geometry, helium concentration, pressure differential, and test configuration. Therefore, defects with similar dimensions may produce different leak rates under different testing conditions.

3. High Voltage Leak Detection (HVLD): HVLD evaluates liquid-filled pharmaceutical packages by applying an electrical potential across the package and detecting changes in electrical current associated with conductive leak pathways. Its response depends on defect characteristics, product conductivity, package construction, and applied voltage. Unlike Vacuum Decay and Helium Leak Detection, HVLD does not directly measure gas leak rate or physical defect dimensions. Its results indicate package integrity based on the electrical response under defined test conditions.

4. Airborne Ultrasound: Airborne Ultrasound evaluates package seal integrity by analyzing ultrasonic signals transmitted through or reflected from the sealing area. It can identify seal discontinuities and defects that affect acoustic transmission in compatible packaging materials. The detected signal depends on defect characteristics, seal structure, material properties, and instrument configuration. Although the method can identify seal defects, its measurements do not directly establish a gas leak rate or a universal defect size in micrometres.

Why "Micron Size" Should Not Be Treated as a Universal Leak Specification?

Micron-based descriptions are frequently used when discussing package defects, but a defect diameter expressed in micrometres does not automatically define a specific leak rate. The term may refer to the diameter of a circular hole, the width of a channel, or another physical dimension. These geometries do not necessarily produce equivalent leakage.

For instance, two defects described as 10 µm may have different pathway lengths, shapes, and flow characteristics. Their measured leak rates can also differ when the pressure differential, gas type, or testing method changes. Therefore, a stated micron value should not be interpreted as a universal indicator of package leakage or a direct substitute for a quantitative leak-rate specification.

A reliable specification should distinguish between the physical dimensions of a reference defect and the functional leak-rate limit established for the package. Manufacturers should define the applicable test method, test conditions, measurement units, and acceptance criteria for the intended container closure system. Where a correlation between defect size and leak rate is required, it should be established using appropriate reference defects and validated test procedures.

This distinction is particularly relevant when comparing test technologies, interpreting supplier specifications, or establishing package integrity requirements for pharmaceutical products. A dimensional defect specification and a leak-rate acceptance limit address related but different characteristics.

Conclusion

Defect size and leak rate are related parameters in pharmaceutical container closure integrity testing, but they are not interchangeable. Defect size describes the physical dimensions of a leak pathway, whereas leak rate quantifies the amount of gas or liquid passing through it under defined conditions. Their relationship depends on pathway geometry, pressure differential, gas properties, package materials, and the selected test method.

Vacuum Decay, Helium Leak Detection, HVLD, and Airborne Ultrasound each provide information based on different measurement principles. Understanding these differences helps pharmaceutical manufacturers interpret results and establish suitable package integrity specifications.

Rather than relying on micron size alone, manufacturers should define acceptance criteria based on the intended packaging system, product requirements, and selected CCIT method. Where defect dimensions and leak rates need to be correlated, method-specific data and validated test procedures provide a more reliable basis for interpreting package integrity results.

Frequently Asked Questions

1. What is the difference between defect size and leak rate?

Defect size describes the physical dimensions of a leak pathway, usually expressed in micrometres (µm), while leak rate measures the amount of gas or liquid passing through the pathway per unit of time under specified conditions.

2. Does a larger defect always produce a higher leak rate?

Not always. Leak rate also depends on pathway geometry, pressure differential, package materials, gas properties, and test conditions. Therefore, defects with similar dimensions may produce different leak rates.

3. Can micron size be used as a universal leak specification?

No. A micron measurement describes a physical dimension but does not directly establish a leak rate. Pharmaceutical manufacturers should define acceptance criteria based on the package configuration, selected CCIT method, and validated test conditions.