Kokubo SBF In-Vitro Bioactivity Protocol (ISO 23317)

1. Which edition of ISO 23317 should you cite?

ISO 23317 has been revised, and the change is not cosmetic. The 2007, 2012 and 2014 editions are withdrawn; the current text is ISO 23317:2025, retitled Materials - Simulated body fluid (SBF) preparation procedure and test method to detect apatite formation in SBF for initial screening of bone-contacting implant materials.

The words initial screening are now in the title of the standard. That is the committee stating the method's standing in a conformity file, and it is the single most useful thing to know before you build a study around it. A protocol that still cites ISO 23317:2014 refers to a withdrawn document, which an auditor will raise even when the bench work was faultless.

2. What does a positive SBF result actually prove?

Apatite forming on a surface in SBF is a screening signal, not a demonstration of bone bonding. The published assessment of the method is explicit: the SBF test alone does not establish bone-bonding ability, and is meant to be read alongside other in vitro and in vivo work. It also remains unclear whether apatite formation under any SBF formulation is representative of an in vivo biological response.

Used for the left-hand column the test is efficient and cheap. Used for the right-hand column it is the kind of overreach that costs a submission a review cycle.

3. Why is the fluid the hard part of an SBF test?

SBF reproduces the inorganic ion concentrations of human blood plasma: sodium, potassium, magnesium, calcium, chloride, hydrogen carbonate, hydrogen phosphate and sulfate. It is supersaturated with respect to apatite by design, because that is what makes the test work: the surface under study is meant to be what tips a metastable solution into nucleating.

The consequence is that the fluid is only ever metastable. It can nucleate on a vessel wall, on a dust particle, or on nothing in particular, and once calcium phosphate has begun to precipitate in the bulk the experiment is over: the solution has spent the supersaturation your specimen was supposed to consume. This is the failure mode behind most inconclusive SBF campaigns, and it is silent, because a faint haze in a clear bottle is easy to miss.

The order of addition during preparation is what governs this, which is why the standard prescribes it rather than leaving it to the operator. Adding the calcium salt before the buffer is set is the classic way to lose a batch.

4. Why must the buffer and the temperature be set together?

SBF is buffered with tris(hydroxymethyl)aminomethane and hydrochloric acid, and the pH target is stated at the test temperature of 36.5 °C, not at ambient. TRIS has a strongly temperature-dependent dissociation, so a solution titrated on a benchtop at 22 °C and then warmed to 36.5 °C is no longer at the pH written on the label. If a value is quoted without its temperature, the specification is incomplete.

Our SBF is supplied with its pH certified at the test temperature. It is worth confirming the same on any certificate you receive, whoever supplies it.

5. Why is Kokubo SBF supplied as two stock solutions?

Because the metastability problem is intrinsic, a revised preparation route was proposed in the literature and is now widely used: hold the incompatible ions in two separate stock solutions and mix them immediately before use. The calcium and the hydrogen carbonate or phosphate never sit together in a bottle for weeks, so the shelf life of the components is no longer the shelf life of a supersaturated solution.

This is why it ships as a two-part kit rather than ready-mixed. It means one extra step at the bench, and in exchange the fluid is at a defined age when it meets your specimen, which is the variable that decides whether a 28-day immersion is interpretable.

This guide covers the decisions and the failure modes. It does not reproduce the preparation procedure or the ion concentrations of ISO 23317:2025, which you should obtain from ISO or your national body and follow as written.