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Medical devices

Materials that must survive a body, and that a body must survive — where the qualification cost exceeds the material cost by orders of magnitude.

Making implants, instruments, diagnostics and disposables. The governing constraint is biocompatibility, which is not a property of a material alone but of a material in a particular tissue for a particular duration: something inert in contact with skin for an hour may be entirely unsuitable inside bone for thirty years.

The environment is more aggressive than it looks. Body fluid is a warm, oxygenated chloride solution at constant temperature, which is close to the worst case for the passive film that protects stainless steel — and it is why implant alloys are titanium and cobalt-chromium rather than the steel that would otherwise do.

History

Metal implants from the nineteenth century, mostly unsuccessful until the materials improved: early plates and screws corroded and provoked infection. Stainless steel made internal fixation practical from the 1920s and 1930s.

Two discoveries define the modern field. Titanium's osseointegration — bone bonding directly to the oxide surface — was found accidentally by Per-Ingvar Brånemark in the 1950s and is the basis of dental implantology. And the intraocular lens follows from acrylic canopy fragments being tolerated in airmen's eyes.

Economic significance

Regulatory approval, not material cost, dominates. Changing a material in an approved device can mean re-running a clinical evaluation, which is why device materials change slowly and why an obsolete polymer grade being discontinued is a genuine crisis for the manufacturers using it.

The consequence is a strong conservatism: the alloys used in joint replacement have been substantially unchanged for decades, not because nothing better exists but because the evidence base is what is actually being bought.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

uses

  • Ti-6Al-4V — alloy · hip and knee stems, bone plates and dental implants, and bone bonds directly to the oxide surface
  • Cobalt-chromium alloy — alloy · the bearing surfaces of joint replacements and the frameworks of dental prosthetics
  • Nitinol — alloy · self-expanding stents and guidewires, which the superelasticity rather than the shape memory makes possible
  • PEEK — material · spinal cages, where the stiffness is close enough to bone not to shield it from load, and which are radiolucent
  • Silicone rubber — material · tubing, catheters and implants, on inertness rather than on any mechanical property
  • PMMA — material · bone cement and intraocular lenses, the latter discovered because acrylic canopy splinters were tolerated in airmen's eyes
  • PLA — material · resorbable sutures, pins and screws, where degrading is the requirement rather than the drawback
  • Stainless steel — alloy · surgical instruments and temporary fixation, and specific low-nickel grades exist because nickel sensitisation is a real clinical constraint
  • Joint replacement — object · around two million hip and knee replacements a year
  • Zirconia — compound · dental crowns and bridges, and hip joint heads

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Medical devices back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Medical devices → uses (surgical instruments and temporary fixation, and specific low-nickel grades exist because nickel sensitisation is a real clinical constraint) → Stainless steel → is composed of (the balance) → Iron → is produced by (as pig iron, high in carbon and brittle until refined) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Medical devices → uses (spinal cages, where the stiffness is close enough to bone not to shield it from load, and which are radiolucent) → PEEK → is produced by (a step-growth polymerisation between aromatic monomers at high temperature in a polar solvent) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Medical devices → uses (tubing, catheters and implants, on inertness rather than on any mechanical property) → Silicone rubber → is produced by (and it is not organic chemistry at all: the backbone is silicon and oxygen, which is why the temperature range is what it is) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Medical devices → uses (resorbable sutures, pins and screws, where degrading is the requirement rather than the drawback) → PLA → is produced by (ring-opening polymerisation of lactide, itself made by fermenting plant sugar to lactic acid) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Medical devices → uses (bone cement and intraocular lenses, the latter discovered because acrylic canopy splinters were tolerated in airmen's eyes) → PMMA → is produced by (free-radical polymerisation of methyl methacrylate, cast between glass for optical sheet or in bulk for moulding granules) → Polymerisation → takes as input (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Medical devices → uses (hip and knee stems, bone plates and dental implants, and bone bonds directly to the oxide surface) → Ti-6Al-4V → is produced by (melted under vacuum or inert gas, because molten titanium reacts with essentially every crucible material and with air) → Alloying and melting → takes as input (the base metal of both brass and bronze) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (roasted to drive off sulfur, then reduced to copper) → Chalcopyrite

These are the most distinct paths back. Medical devices can be traced through others besides.