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Titanium for Medical Devices: What Changes Compared With Industrial Work

Machining titanium for a medical device is not technically different from machining it for a bracket. The cutting data is the same, the tooling is the same, the fixturing problems are the same.

What changes is everything around the cut: which heat the bar came from, who is allowed to touch it, what the surface must be free of and how all of it is recorded.

Grades and the standards that govern them

Application	Grade	Standard
Load bearing implants	Grade 23, Ti-6Al-4V ELI	ASTM F136, ISO 5832-3
Dental implants and some hardware	Grade 2 or Grade 4 CP	ASTM F67, ISO 5832-2
Surgical instruments	Grade 5	ASTM B348 is often adequate
Instrument handles and non contact parts	Grade 5	General industrial standards

The line that matters: an implant standard is not the same as an industrial standard. ASTM F136 material is tested more thoroughly and controlled more tightly than ASTM B348 material of nominally the same alloy. They are not interchangeable and no shop should offer to substitute.

Instruments are the grey area. A retractor that touches tissue for an hour in theatre is not an implant and often does not need F136 material. Plenty of drawings call it anyway, because the template said so. That is a real cost with no clinical benefit.

Why Grade 23 and not Grade 5

Grade 23 caps the interstitial elements, mainly oxygen and iron. That gives up a little strength and gains fracture toughness and ductility.

For a part inside a body, carrying load, for twenty years, with no inspection access, fracture toughness is the property that matters. That is the whole argument, and it is a good one.

On the machine the two behave identically. The extra cost is in the material, the certificate and the traceability process, not in the cutting.

Traceability, medical style

The chain is the same shape as aerospace but the consequences of breaking it are different.

1. Mill certificate to EN 10204 3.1, material to F136 or F67, heat number recorded.
2. Segregated storage, specifically keeping Grade 23 apart from Grade 5. They look identical.
3. Transfer marking of the heat number before the original marking is machined away, witnessed.
4. Lot control through every operation, including outside processes.
5. A device history record or certificate of conformity that ties finished parts to the heat.

Ask a shop to walk you through step two. Grade 5 and Grade 23 bar are visually indistinguishable, and a shop without physical segregation and a labelling discipline is a shop that will eventually mix them.

ISO 13485, and when you actually need it

ISO 13485 is a quality management standard for medical device manufacture. It covers documented processes, traceability, change control, process validation, complaint handling and records retention.

It is not a machining capability. A shop can hold ISO 13485 and be mediocre at Ti-6Al-4V, and a shop can be excellent at titanium and hold ISO 9001 only.

Require it when:

• The shop supplies a finished device or a component you do not further process.
• The shop performs a process you cannot verify by inspection: cleaning, passivation, marking, packaging.
• Your own notified body or regulator expects it in your supplier file.

It may not be needed when:

• You receive a rough machined component and complete it under your own system.
• The part is an instrument component in a low risk class and your own procedures cover the supplier.

Ask your own quality function before you write the requirement. It changes the supplier pool dramatically.

Cleanliness, passivation and surface

This is where medical parts separate from industrial ones most sharply.

Embedded iron. Steel tooling, fixtures and handling leave free iron on a titanium surface. It corrodes, and it can compromise biocompatibility. Passivation removes it. A specification should be named rather than left as “passivate”.

Particulate and residue. Coolant, cutting oil and swarf have to be removed by a validated cleaning process with recorded parameters, not a wash in the sink.

Surface finish. Implant surfaces are frequently specified in Ra, and sometimes in more than Ra. Some implant surfaces are deliberately roughened or blasted for osseointegration, which is a specified process in its own right.

Electropolishing is common on instruments and on parts where cleanability matters. It removes a few microns, which has to be planned into the machining tolerance rather than discovered afterwards.

Packaging. Double bagged, cleanroom packed, labelled with lot and part number. Say what you need, because the default is a cardboard box.

Marking

Medical parts often carry a permanent identification: part number, lot, sometimes a UDI. Laser marking is the usual route on titanium.

Two things to specify: where the mark goes, and whether it is applied before or after passivation and final cleaning. Marking after cleaning reintroduces contamination that then has to be removed again.

The RFQ checklist

Material: Ti-6Al-4V ELI per ASTM F136, annealed. Certificate: EN 10204 3.1, heat number on the certificate of conformity. Quality system: ISO 13485 required. Cleaning and passivation: to our process specification, attached. Surface: Ra 0.8 on the articulating face, as machined elsewhere. Marking: laser mark part number and lot on the flat marked M, before final clean. Packaging: double bagged, lot labelled. Quantity and date.

Every one of those lines changes the price. Which is exactly why they should be on the RFQ rather than in an email three weeks later.

Frequently asked questions

Keep reading

• Titanium Aerospace Parts and the Requirements That Come With Them
• Drilling Titanium: Speeds, Heat and Why Holes Fail First