January 15, 2021

Proper Sample Preparation for Biocompatibility Testing

Sample Preparation

Sample preparation is one of the most critical steps in a preclinical testing program for a medical device. This initial step is often overlooked or unclear to sponsors, and if not thoroughly strategized up-front, can lead to unnecessary delays or unfavorable results in testing programs.

1. Guidelines

The FDA expects sponsors and device manufacturers to take complete ownership of the sample preparation process as part of the overall biological safety evaluation plan. ISO 10993-12 provides a default set of parameters to consider when developing a sample preparation plan for your device.

Know that in most cases, the entirety of your device will be cut into pieces and included for testing. If there are specific ways your device needs to be cut, this needs to be relayed to the lab before the process starts.

If you have a complex device, cutting it may expose electronics or other components that are not patient contacting. If you want to exclude any of these non-patient contacting parts from testing, make sure to provide clear sample preparation instructions to the testing lab and have a justification ready for the regulatory agency.

2. Surface Area

The surface area of the device or the portions to be tested is required to determine the number of samples needed to perform testing. Each biocompatibility test has its own surface area requirements. Providing an accurate surface area is crucial for accurate testing and reporting.

When calculating the surface area of your device or any object, Toxikon encourages the use of CAD-based drawings or software for accuracy.  When calculating surface area, careful considerations should be made on what exact surfaces are intended for testing.

Are you testing the entire device, or are you only testing the patient contacting parts? This difference could lead to two very different surface areas, which could lead to an improper extraction ratio.

The exception to using surface area is for devices that contain either low-density porous materials or are irregularly shaped; in these cases, a weight to volume ratio is an option. The testing lab should be made aware if a test article will swell if placed in media, as the volume of the vehicle may need to be adjusted to account for the volume retained by the test article.

3. Extraction Ratio

The ratio of material to solvent depends on the thinnest portion of the device that is being tested. This is a critical step that must be calculated correctly to ensure that the device is properly extracted.

Material ThicknessRatio
If all materials have thickness ≥ 0.5 mm in thickness3 cm² per 1 ml of extract solution
If there is any material that is < 0.5mm in thickness6 cm² per 1 ml of extract solution

4. Extraction Conditions

It is important to select a temperature that exaggerates the conditions the device will experience but will not deform or change the device.

As a general rule, the testing temperature drops as the duration of time goes up. ISO-10993-12 provides the following 4 standard extraction times.

  • 121° C for 1 hour
  • 50° C for 72 hours
  • 70° C for 24 hours
  • 37° C for 72 hours

Agitation is often introduced for the duration of extraction.

There should be no change in color or opacity of the extract solution. Any solids should be removed prior to dosing of an extract; however, manipulation of the extract with sterile filtering or centrifugation is not recommended.

Note: Extraction conditions are not applicable when performing Direct Contact testing

5. Vehicles

When selecting a vehicle for extraction, it is recommended that both polar & non-polar vehicles should be evaluated.

While several options are available for both types of vehicles, the standard for polar is 0.9% NaCl while cottonseed oil or sesame seed oil are both frequently used for non-polar and often come down to laboratory preference.

Proper sample preparation helps ensure accurate and efficient testing while minimizing unnecessary testing delays. While sample preparation may seem scary, it is a relatively simple process. Doing a bit of planning will go a long way to streamlining your preclinical testing program helping your medical device get to market quicker.