Material selection for medical devices goes far beyond strength and cost: biocompatibility, sterilisation, chemical resistance and regulatory change control all constrain the choice. The questions to answer before you commit.
A handheld medical device housing was moulded in a polycarbonate grade with excellent impact strength and clarity. In hospital use, it began to crack around screw bosses within months. The cause was a combination of moulded-in stress, assembly stress and daily wiping with aggressive disinfectants, which the material was not resistant to. Changing the material at that stage meant new testing, new documentation and regulatory notification.
Material selection for medical devices is constrained in ways that most consumer products are not. Getting it right early saves time, cost and regulatory pain later.
Biocompatibility
Materials that contact the body, directly or indirectly, must be biologically safe for the type and duration of contact. Biological evaluation follows the ISO 10993 series, assessed on the finished device including processing, additives, colourants and sterilisation effects. Medical-grade resins come with supporting data and change-control commitments that general-purpose grades do not.
Sterilisation compatibility
The sterilisation method can rule materials in or out:
- Gamma and electron beam radiation can embrittle and discolour some materials, such as standard polypropylene and acetal, while radiation-stabilised grades exist.
- Ethylene oxide is compatible with most plastics but requires adequate degassing.
- Steam autoclaving at 121–134°C needs high heat resistance; materials such as PEEK, PPSU and some PEIs handle repeated cycles, many others do not.
- Repeated cleaning and disinfection, rather than sterilisation, is often the real challenge for reusable equipment.
Chemical resistance
Hospital cleaning agents, including alcohols, quaternary ammonium compounds, hydrogen peroxide and bleach, can cause environmental stress cracking in many amorphous plastics, particularly where stress is present. Test with the actual chemicals and protocols, on moulded parts with realistic stress, as we discussed in choosing between ABS, PC and PC/ABS.
Mechanical and dimensional requirements
Consider impact at low temperatures, fatigue in snap fits and living hinges, creep under sustained load, dimensional stability for precise fits and fluid paths, and clarity if visibility is needed.
Regulatory and supply considerations
- Material changes after approval can require new testing and regulatory notification. Choose suppliers with formal change-notification agreements.
- Prefer materials with a medical-grade history and documentation, such as master files.
- Consider long-term availability. Discontinued grades can force costly requalification.
- Document the material selection rationale in the design history file.
Processing matters
Moulding conditions affect biocompatibility, mechanical properties and stress. Over-dried, degraded or highly stressed parts can behave differently from test samples. Specify drying, processing windows and annealing where needed, and validate them, using the same capability thinking applied to dimensions.
A practical selection sequence
- Define use conditions: contact type and duration, sterilisation, cleaning chemicals, temperatures, loads and life.
- Shortlist medical-grade materials meeting those conditions.
- Evaluate with moulded samples under realistic conditions.
- Confirm biological evaluation on the finished device.
- Lock the material and supplier under change control.
Material selection in medical devices is a regulatory decision as much as an engineering one. Treat it that way from the first concept.
