Histological evaluation is the preferred method to examine the in situ interaction between the implanted device and surrounding tissues. With paraffin histology, it is difficult to maintain the device/tissue interaction when examining the interface between two materials. This difficulty arises as a result of the differences in density and material properties between the tissue and device.
Because of the separation and tissue damage caused by the device material, paraffin sections fail to represent the device/tissue interaction, normal tissue morphology, and the true nature of the tissue integration with the implanted device. One solution to this problem is processing and embedding the device and surrounding tissues in a resin material. This method will present similar density properties, allowing for sectioning through the device/tissue, while maintaining interactions.
When performing biocompatibility testing/histology for implantable medical devices, it is important to choose the appropriate type of resin to ensure a quality representation of the device/tissue interface. Some of the more common types of medical device materials and the associated type of resin used during histology/biocompatibility testing are as follows:
- Soft metals, such as stents, may require a soft Methyl Methacrylate (MMA) or Glycol Methacrylate (GMA).
- Soft plastics and absorbable plastics, such as catheters or sutures, may require an Epoxy.
- Fabrics, such as scaffolds, work well with MMA.
- Hard metals, such as joint replacements, may require a hard MMA (Technovit 7200)
Commonly Used Low Viscosity Plastics
Methyl Methacrylate (MMA)
This type of plastic is useful for harder materials that cannot be sectioned with paraffin. Methyl Methacrylate can be sectioned by either microtomy or the saw and grind technique. This type of plastic allows the sections to be deplastinated if the chemicals don't compromise the implant material. However, this type of plastic can be expensive and cannot be used for implants made of Methacrylate.
Glycol Methacrylate (GMA)
This type of resin is less expensive than MMA and some formulas lend better to Enzyme Histochemistry. However, Glycol Methacrylate makes cutting and grinding difficult, as the sections get sticky during the water cooling process. This type of resin does not allow for deplastination of sections for IHC or special stains. The plastic continues to polymerize for weeks, getting harder with time and absorbing water from the surrounding air. The blocks need to be stored in a desiccator away from moisture to avoid damage. This type of plastic also cannot be used for implants made of Methacrylate.
Epoxy (Spurr's)
This type of resin can be used for materials that are dissolved or damaged by Methacrylates, for hard materials that require cutting and grinding, and for both Electron Microscopy and Light Microscopy. Spurr's Resin is often overlooked because of the toxic nature of its components.