ISO 10993-12: Key Considerations for Effective Biocompatibility Testing

Why ISO 10993-12 Matters for Biocompatibility Testing

When discussing biocompatibility, most attention is focused on biological endpoints such as cytotoxicity, sensitisation and irritation. However, the reliability of those test results depends heavily on the quality of the sample preparation process. This is where the guidance from ISO 10993-12:2021 – Biological evaluation of medical devices – Part 12: Sample preparation and reference materials, plays a critical role. 

ISO 10993-12 provides the framework for selecting appropriate test samples, preparing extracts and choosing control materials. ISO 10993-12 is not a supporting standard – it is a necessary guidance document that underpins almost every biological evaluation conducted under the ISO 10993 series. A poorly designed extraction strategy can result in data that is either too conservative or not representative of clinical use. 

Use the Finished Medical Device Where Possible

One of the fundamental principles of ISO 10993-12 is that testing should be performed, where possible, on the final finished medical device. Where not possible, representative materials of the final device must be chosen and justified. This means considering the impact of manufacturing processes, cleaning procedures, coatings, sterilisation and packaging where relevant. This can also mean considering representing the device at its end of life-cycle.  

While individual material data can provide supporting information, the patient is exposed to the finished device, not the raw materials. As a result, biological testing should reflect the device in its final form wherever possible.

Justify Medical Device Extraction Conditions

A common misconception is that there is a single “correct” extraction method for all medical devices. In reality, ISO 10993-12 takes a risk-based approach and expects manufacturers to justify their choice of extraction conditions. 

Factors that should be considered include: 

• Device material and construction – alterations to devices or selection of components for testing may be necessary to ensure compatibility with extraction procedures in a laboratory setting.  

• Nature and duration of patient contact – there may be a need to apply different extraction conditions to components of a device that have differing contact types and/or durations.  

• Physical and chemical properties of the material – compatibility with the extraction vehicle and temperature is necessary to ensure the extraction does not artificially degrade the device materials.  

• Potential extractables and leachables – identifying known interfering compounds that may be extracted or leached during testing could alter the testing strategy.  

• The biological endpoint being evaluated – the extraction conditions should be appropriate for the test method.  

The objective is to create an extract that provides a meaningful representation of potential patient exposure rather than simply applying a standard set of conditions to every device.

Choose the Appropriate Extraction Ratio

For many finished devices, surface area is the most relevant measure because patient exposure occurs at the device surface. Consequently, surface area-to-volume extraction ratios are commonly used for devices such as catheters, implants, surgical meshes and tubing. 

Mass-based extraction ratios are generally more appropriate for materials where surface area cannot be reliably determined, including powders, granules and highly porous materials. 

The key consideration is not necessarily which extraction ratio is used, but whether the chosen approach accurately reflects how the device interacts with the patient.

Why More Aggressive Extraction Is Not Always Better

It is sometimes assumed that the most conservative approach is to use the highest temperatures and longest extraction times available. However, this can lead to extracts that are no longer representative of clinical use. 

Excessive extraction conditions may: 

• Alter material properties 
• Generate degradation products 
• Increase extractable concentrations beyond clinically relevant levels 
• Lead to false-positive responses during biological testing 

A robust extraction strategy should be challenging enough to identify potential risks while remaining scientifically relevant to the intended use of the device. 

Consider Extraction Temperature and Time Together

Extraction temperature and duration should never be selected independently. Both parameters influence the amount and type of substances that can be released from a material. 

For example: 

• 37°C may be appropriate when physiological relevance is the priority. 
• 50°C is often used to facilitate extraction from polymeric materials without significantly altering material characteristics. 
• 70°C may be suitable for more stable materials where a more aggressive extraction is justified. 

Similarly, extraction durations can range from 24 hours to several days depending on the extraction objectives and material characteristics. The chosen conditions should always be supported by a clear scientific justification.  

Use Representative Samples for Large or Complex Devices

For large or complex devices, testing the entire product may not be practical. ISO 10993-12 therefore allows the use of representative portions, provided they adequately reflect the biological risks associated with the finished device.  

When selecting representative samples, consideration should be given to: 

• Patient-contacting materials 
• Coatings and surface treatments 
• Adhesives and joining materials 
• Components with the greatest potential biological risk 

Documenting the rationale for the sample selection is essential, particularly when the tested article differs from the marketed device configuration. 

Document Your ISO 10993-12 Sample Preparation Rationale

Many regulatory questions relating to biocompatibility are not because the testing itself was inappropriate, but because the rationale behind decisions was poorly documented. 

Manufacturers should ensure they can clearly explain: 

• Why a particular sample or representative sample was selected 
• How extraction ratios were determined 
• Why specific temperatures and durations were chosen 
• Why the extraction vehicle was considered appropriate 

A well-documented justification often provides regulators and notified bodies with greater confidence than simply following a predefined protocol.

Applying ISO 10993-12 to Biocompatibility Testing

ISO 10993-12 is sometimes viewed as a technical detail within the broader biological evaluation process. In reality, it forms the foundation upon which biocompatibility testing is built. The standard encourages manufacturers to move beyond a checklist approach and instead focus on developing extraction strategies that realistically represent patient exposure. 

Ultimately, successful application of ISO 10993-12 is not about selecting the most severe extraction conditions or following historical protocols. It is about applying scientific judgement to ensure that the data generated are relevant, defensible and representative of the device’s intended clinical use.