Medical device manufacturers are constantly seeking innovative materials that can withstand sterilization, resist wear and corrosion, and remain biocompatible for human use. Polyetherimide (PEI) has emerged as a high-performance thermoplastic with properties that make it well-suited for the demands of the medical industry.
In this blog post, we’ll explore the key characteristics, benefits, and potential drawbacks of polyetherimide as a polymer for medical applications. Understanding PEI’s properties allows medical OEMs to evaluate whether it is an ideal material choice for their device designs and requirements.
An Introduction to Polyetherimide (PEI)
Polyetherimide, commonly abbreviated as PEI, is an amorphous, transparent engineering thermoplastic. It offers an exceptional combination of high heat resistance, intrinsic flame retardancy, and chemical resistance. PEI maintains its mechanical strength across a wide range of temperatures, demonstrating excellent dimensional stability.
PEI is typically used in applications requiring:
- High heat capability and sterilizability
- Resistance to harsh chemical environments
- Durability and mechanical strength
- Dimensional stability
Its unique properties have allowed PEI to gain use in medical devices as well as aerospace, automotive, and electronics applications. The biocompatibility and sterilization compatibility of PEI makes it well-suited for use in implants and other healthcare applications.
Key Properties of Polyetherimide for Medical Devices
Polyetherimide has several properties that make it advantageous for medical device designs:
High Heat Resistance and Sterilizability
PEI has high heat resistance, with a glass transition temperature of 217°C and melting point of 230°C. It maintains strength and rigidity up to 170°C. This high temperature capability means PEI can withstand the temperatures required for autoclave sterilization and other sterilization methods used on medical hardware and components.
Chemical and Solvent Resistance
PEI offers broad chemical resistance, resisting attack from most dilute acids, alkalis, oils, greases, and solvents at room temperature. This allows PEI medical components to avoid damage from cleaning agents and harsh chemicals. The polymer also has excellent radiation resistance, a crucial property for medical tubing and devices requiring repeated sterilization by radiation.
Mechanical Strength
With a tensile strength of 170 MPa, PEI has higher mechanical properties than many other clear thermoplastics like ABS, PU, PS and PVC. It retains good impact strength and ductility at low temperatures. PEI compares favorably to PEEK polymer in terms of mechanical performance for implants and medical tools.
Biocompatibility
Testing has shown that PEI demonstrates good biocompatibility, meaning it does not produce significant cytotoxic or allergic reactions in human tissue. This makes it suitable for use in medical devices that have direct tissue contact like implants.
Dimensional Stability
PEI maintains its shape, size, and mechanical properties through temperature fluctuations and chemical exposures. This dimensional stability gives medical device manufacturers reliability in their finished components.
Low Moisture Absorption
With a moisture absorption rate of just 0.3-0.4%, PEI resists taking on water better than most thermoplastics. It does not require drying before processing and will not degrade or warp due to moisture absorption. This gives PEI an advantage in medical assemblies exposed to liquids and steam sterilization.
Benefits of Using PEI in Medical Applications
The unique benefits of polyetherimide make it an increasingly popular polymer for medical parts and device housings:
- Withstands repeated sterilization through autoclaving, radiation, chemical treatment, and other methods
- ** compatible for direct tissue contact** like in implants
- Resists degradation from exposure to bodily fluids and cleaning agents
- Maintains mechanical integrity and dimensional stability through repeated sterilization and use
- Clear aesthetics allow visualization of internal components
- Heat resistance allows use in higher temperature medical devices
- Does not require predrying like nylon or PET
PEI is commonly used for injection molded components like precision gears, bushings, frames, and trays. It is also extruded into multi-lumen medical tubing for catheters, endoscopes, cardio and IV equipment. These applications take advantage of PEI’s unique set of chemical, thermal, and mechanical capabilities.
How Does PEI Compare to Other Medical Polymers?
Polyetherimide has attracted attention due to its balanced performance versus other popular polymers:
- Stronger and lower cost alternative to PEEK: PEI approaches PEEK in heat resistance but has higher strength than PEEK. It offers a more economical choice.
- Superior chemical resistance to ABS or PU: PEI withstands acids, bases, and solvents better than other clear resins.
- Heat and chemically resistant like PPSU: PEI and PPSU both withstand sterilization and caustic chemicals. PEI is clearer optically.
- Clear replacement for acetal: For medical devices like hand tools and endoscopy parts than need clarity, PEI can replace easily damaged acetal parts.
- Alternative to nylons in gears and bearings: PEI has higher continuous-use temperature capabilities than nylon resins.
Engineering thermoplastics like PEEK, TPU, ABS, nylon, and acetal each have advantages in medical applications. But PEI offers a balanced set of properties that makes it competitive for more demanding applications.
Potential Drawbacks of Polyetherimide
While having many positive attributes, polyetherimide also has some potential disadvantages:
- Relatively high material costs compared to commodities like ABS and nylon
- Limited chemical resistance to strong alkalis and some solvents like chlorinated hydrocarbons
- Processing challenges like shear sensitivity can impact dimensional tolerances
- Hydrolytic sensitivity makes it important to properly dry PEI prior to processing
Manufacturers should take care to properly mold and machine PEI parts to achieve expected tolerances and avoid shear or thermal degradation. Despite these cautions, PEI remains an excellent material choice for the right medical applications.
Conclusion: When to Consider Polyetherimide
Polyetherimide will continue growing in popularity for medical devices requiring its unique set of properties. It should be considered:
- In applications needing repeated sterilization capability
- When high heat resistance, chemical resistance, and strength are critical
- For transparent medical assemblies and components
- To replace PEEK, acetal, or nylons in demanding environments
- If dimensional stability through temperature fluctuations is required
While not suited for all medical uses, polyetherimide’s advantages make it a top choice for products from surgical tools to complex medical equipment. Engineers and product designers specify PEI when they need a material that can withstand the intense demands of the medical industry.