ASTM F1609-23 Standard Specification for Calcium Phosphate Coatings for Implantable Materials

The standard “ASTM F1609-23 Standard specification for calcium phosphate coatings for implantable materials” published by the American Society for Testing and Materials (ASTM) describes the material requirements for calcium phosphate coatings in the context of surgical implant applications.

This standard covers hydroxylapatite coatings, other calcium phosphate (e.g. octacalcium calcium phosphate, amorphous calcium phosphate, dicalcium phosphate dihydrate) coatings, or a coating containing a combination of two or more calcium phosphate phases, with or without intentional minor additions of other elements or compounds (e.g. fluorine, manganese, magnesium, carbonate), and is applied by methods including, but not limited to:

• Plasma sputter deposition
• Solution precipitation
• Dipping or sintering
• Electrophoretic deposition
• Spraying

For a coating containing two or more calcium phosphate phases, one or more of which will be the major phase or major phases in the coating, while other phases may occur as secondary or minor phases, the phase compositions of the coating should be determined with respect to each corresponding crystalline phase respectively.

Substrates may include smooth, porous, textured, and other implantable topographic forms.

This standard excludes organic coatings that may contain calcium and phosphate ionic species.

For the purposes of this standard, the following definitions apply:

• Amorphous calcium phosphate is a non-crystalline type of calcium phosphate.
• Beta tricalcium phosphate is a calcium phosphate substance (the standard “ASTM F1088-18 Standard specification for beta-tricalcium phosphate for surgical implantation” applies to this matter.
• Calcium phosphate is any of a number of inorganic chemical compounds containing calcium and phosphate ions as their principal components.
• A coating is a mechanically or chemically attached layer of material that covers a substrate material.
• Hydroxylapatite is a calcium phosphate crystalline compound (the standard “ASTM F1185-03 Standard specification for the composition of hydroxylapatite for surgical implants” applies to this subject.

Elemental analysis and intentional additions (excluding elemental impurities) for calcium and phosphorus should be consistent with the expected succiometry of the particular calcium phosphate compound. Calcium and phosphorus content is determined based on the appropriate method:

• USP <191> Identification tests for calcium and phosphate
• USP <232> Elemental impurities – Limits
• USP <233> Elemental impurities – Procedure
• X-ray fluorescence

For crystallographic characterization, the standard “ASTM F2024-10 Standard procedure for X-ray diffraction determination of phase content of plasma sputtered hydroxyapatite coatings” is used. The test includes quantitative phase analysis and amorphous calcium phosphate content. FTIR (Fourier transform infrared spectroscopy) is performed to identify functional groups.

When determining physical characterization, microscopic examination of the surface is performed at 10x magnification. Bare areas, pinholes, cracks, foreign inclusions, non-melts, chips, delamination, and appearance at the coating or substrate interface are reported.

When determining mechanical characterization, the following mechanical characterizations can be applied to a coating, depending on the substrate material or geometry, coating thickness or location, or coating methods:

• The tensile bond strength of the coating to the substrate is determined according to the standard “ASTM F1147-05 Standard test method for tensile testing of calcium phosphate and metallic coatings”.
• Shear strength is determined according to the standard “ASTM F1044-05 Standard test method for shear testing of calcium phosphate coatings and metallic coatings”.
• Fatigue strength is determined according to the standard “ASTM F1160-14 Standard test method for shear and bending fatigue testing of calcium phosphate and metallic medical and composite calcium phosphate/metallic coatings”.

Both the coating or substrate interface and the effect on the substrate must be evaluated. The effect of the coating on the resulting fatigue strength of an actual device must also be considered.

Characterization reports should include sufficient information on the testing techniques, procedures, and standards used and details such as sample orientation and proportional thickness depth to accurately represent the analysis.

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