Materials & compliance · 9 min read
Body-safe silicone: what to require
Most suppliers say "high-grade silicone". Almost none can tell you what that means. Here is what to specify, and what to ask to find out.
"Body-safe" is not a grade
No standards body defines body-safe as a material grade. It is an adjective doing the work of a specification, and it is effective precisely because nobody can be proven wrong. What actually protects a buyer is specifying the things that are measurable: the curing chemistry, the filler loading, the hardness, and the substances that must be absent.
Our own range uses medical-grade silicone over a food-grade ABS chassis, and every skin-contact component carries a material declaration. That is stated as a specific claim rather than a general one because the general claim is unusable in a compliance file.
The curing chemistry — ask this first
Silicone is cross-linked from a polymer into an elastomer, and there are two common routes. Platinum addition curing uses a platinum catalyst, produces no reaction by-products, and needs no extended baking step to remove them. Peroxide curing is cheaper and tolerates a wider range of compounds, but it produces volatile by-products that require a post-production baking step; shorten that step and residues remain.
| Ask | A real answer | What it rules out |
|---|---|---|
| Is the silicone platinum-cured or peroxide-cured? | Names one, and explains why | A supplier who does not know the difference |
| Who supplies the raw compound? | A named manufacturer with a data sheet | Untraceable material bought on spot |
| Is it mixed in house or bought pre-compounded? | One or the other, with records | Bought pre-compounded with no visibility |
| What is the filler loading? | A stated ratio or "filler-free" | Heavy extension with silica or carbonate |
Platinum addition curing leaves no peroxide by-products to extract, which is why it is the route specified for skin-contact parts. [Noll, 1968] covers both chemistries and their residues in the detail a material engineer would want, and the practical difference shows up in the extraction test rather than on the datasheet.
Fillers explain most quality complaints
Industrial compounds are extended with fillers to reduce raw material cost and improve processing. Heavily filled compounds tear more readily at the surface, leave a chalky residue when handled, and vary in colour between batches because the filler distribution varies. They also complicate testing, because the filler chemistry has to be assessed alongside the base polymer rather than assumed inert.
You can detect heavy filling without a laboratory. Stretch a sample slowly and release it: nothing should whiten, tear, or stay deformed. Press a fingernail gently into the surface — it should recover cleanly. These are field tests, not certification, but they catch the majority of substitutions before you pay for anything.
What each certificate actually tests
- FDA 21 CFR 177.2600 sets composition requirements and extractables limits for rubber articles intended for repeated use. It is a material benchmark in this category, not an approval of a finished device — see the chemical standards guide.
- REACH restricts substances of very high concern in articles sold in the EU. Evidence is a third-party report against the current candidate list.
- RoHS restricts lead, mercury, cadmium, hexavalent chromium and certain flame retardants, and covers the electronic subassemblies rather than the silicone.
- MSDS gives the exact chemistry of the material supplied. It is the document most often promised and least often delivered.
The set we issue per skin-contact component, with the issuing laboratory named, is listed in the document pack for every shipment. The measurement conditions behind every published figure are on the technology page, and the material question is answered model by model in the wholesale FAQ.
References
- Brydson, J. A. (1999). Plastics Materials, 7th ed. Oxford: Butterworth-Heinemann.
- Noll, W. (1968). Chemistry and Technology of Silicones. New York: Academic Press.
- U.S. Food and Drug Administration (2024). 21 CFR 177.2600. Code of Federal Regulations, Title 21.
Questions on this guide
Is "medical-grade silicone" a regulated term?
No. It is widely used and has no single agreed definition, which is why the specific properties — curing chemistry, filler content, extractables — matter more than the adjective.
Can I test this myself without a laboratory?
Partly. Stretch recovery, surface residue and odour are useful field indicators. Certification requires a laboratory either way.
Why does colour affect compliance testing?
Pigments are themselves substances that have to be assessed. A new colourway therefore regularly requires a new restricted-substance report rather than a note on an existing one.
Is platinum-cured the same as medical grade?
No. Platinum-cured describes the curing chemistry; medical grade describes a biocompatibility test result against a specific standard. A material can be platinum-cured and untested, and the first does not imply the second.
What should I ask a supplier about the material, in one question?
Ask for the curing route, the filler loading, and the test report behind the claim — in that order. A supplier that answers with a certificate but not with the first two has answered a different question than the one you asked.
Does a softer silicone mean more filler?
Often, but not always. Softness can come from the base polymer or from the compound formulation, and only the supplier knows which. That is precisely why the number is worth asking for rather than inferring from how a sample feels.
Why does surface tackiness develop on some parts?
Usually incomplete curing or a migrating plasticiser, and both point back to the compound rather than to the moulding. It is a material question, not a finish question, which is why a surface fix does not hold.
Still open: what a silicone declaration should actually state — or see the documents that ship with an order.