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Sterilization Technologies For Medical Plastic Products And Devices

All materials used in medical devices, including the plastics used in them, are essential to be capable of being sterilized without loss of performance.

Sterilization can be defined as the removal or destruction of all living organisms, including resistant forms such as bacterial or fungal spores. Bacterial spores are most resistant to destruction, and if the sterilization is effective in eliminating them then it can generally be assumed that all other pathogenic and nonpathogenic organisms have been destroyed. It is important to reduce or eliminate the bioburden of certain medical devices and the materials used in their manufacture before end use.

Bio-burden is the concentration or the number of microorganisms like pyrogens, viruses, molds, and fungi present in or on a material. Pyrogens are substances that can cause a fever. There are various tests that can detect the number and kinds of micro-organisms including pyrogens and remnants of bacteria. A product may be sterile but it still may contain pyrogens. Use of high temperatures or radiation will typically kill pyrogens. It is important to realize that production in a ‘‘clean room’’ does not make a device sterile; it simply reduces the initial bioburden and concentration of foreign particles in or on the material to make sterilization more effective. Cleanliness does not mean sterile.

A comparative analysis for various sterilizations methods is given in the following table :

Comparison of Common Sterilization Methods

 

Sterilization Characteristic	Steam	Dry heat	Ethylene oxide (EtO)	Gamma radiation	Electron beam (e-beam)
Process type	Batch	Batch	Batch	Batch	Continuous
Post-sterilization testing for Sterility Assurance Level (SAL)	Parametric release; biological indicators	Parametric release; biological indicators	Parametric release; biological indicators	Dosimetric release	Dosimetric release
Post-sterilization treatment	Need to dry the product	None	Need to aerate product to remove residues	None	None
Penetration	Requires vapor permeable packaging. Surface penetration	Good penetration	Requires gas-permeable packaging; high pressure, temperatures for improved penetration	Excellent penetration	Near complete penetration, need dosimeters; low penetration in high- density materials
Safety	Almost no safety concerns	Almost no safety concerns	Considered a mutagen/ carcinogen; need to remove residual absorbed EtO	Minimal concern; environmentally safe; non-toxic (need protection from radiation)	Almost no safety Concerns
Reliability	Excellent	Good	Good	Excellent	Excellent
Turnaround time	Slow	Slow	Slow	Fast	Fast
Process Parameter Controls	Temperature, pressure, vacuum, relative humidity, time	Temperature, pressure, vacuum, time	Temperature, pressure, vacuum, relative humidity, gas conc., time	Time	Time
Material constraints	Heat resistant and hydrolysis resistant materials only	Heat-resistant materials only	Polymers that do not absorb or degrade with EtO	Radiation stable polymers; complex parts and kits not effectively sterilized	e-Beam stable polymers; low- density materials only
Relative cost	Inexpensive	Relatively inexpensive	High capital investment	High capital investment	High capital investment
Advantages	Simple process, widely used, excellent for reusable devices, excellent for heat- stable liquids	Relatively simple process	Well characterized, good for kits, combination products, parametric release	Simple, fast, excellent penetration, dose uniformity	Simple, fast, less material degradation
Disadvantages	Comparatively high temperatures, generally not appropriate for single-use devices and large lots	High temperatures, limited use	Relatively complex process; some limits to penetration; need to remove EtO residuals	Limited applicability to kits and complex designs/ products; no drug/ combination products; material degradation	Limited penetration, poor on high-density products, dosimetric release is not very uniform, affected by part configuration

Sterilization procedure includes always some disadvantages due to the simple fact that there is a certain amount of energy necessary to destroy living germs, even those, which are specialized on harsh living conditions. On the other hand, a sterilization procedure should be as fast and reliable as possible and has no impact on plastic materials as well. The various sterilization technologies in use by industry includes:

• Steam sterilization
• Dry Heat Sterilization
• Ethylene Oxide ( EtO ) Steriization
• Gamma Radiation Sterilization
• Electron-beam sterilization

A vast majority of plastic medical devices, which are applied sterile, are sterilized by γ-radiation or ethylene-oxide, especially almost all single-use devices. γ-radiation may be preferred due to its faster and less restrictive sterilization procedure regarding the actual geometry and shape of the device, however, it may cause more aging damage to the chemical structure of the plastics.

The following table gives overview on radiation stability of various Plastics.

Radiation Stability of Various Plastics

 

Polymer	Comment
Polyolefins	Polyethylene can cross-link. Polypropylene is especially susceptible to degradation and discoloration. Stabilized polypropylene and polypropylene copolymer grades are good
PVC	Susceptible to degradation and color change. Tint- based, stabilizers are incorporated into PVC to prevent discoloration and degradation
Acrylics	Must be stabilized to prevent degradation and color change
Polycarbonates	Must be stabilized to prevent degradation and color change
Polyurethanes	Some discoloration that reverses over time
Acetals	Typically not used for gamma sterilization
Polyamides	Polyamides containing aromatic rings are good Polyamides 10, 12, 6/10, and 6/12 are good
Polyester	Aromatic polyesters are radiation stable
High-temperature thermoplastics	PEK, PEEK, PEI, Polysulfones – Good
Fluoropolymers	Teflon embrittles when exposed to gamma radiation. Teflon and FEP must be stabilized. All other fluoropolymers are stable to gamma radiation
Elastomers	Generally stable to gamma and e-beam radiation
Thermosets	Thermosets typically are radiation stable

References :
01 - “Emerging Trends in Medical Plastic Engineering and Manufacturing” By : Markus Schonberger and Marc Hoffstetter
02 - “Plastics In Medical Devices” By : Vinny R Sastri