Ethylene oxide, gamma or superheated steam?

The vast majority of medical products are supplied in a sterile state. This applies both for single-use medical items sterilised once — such as respiratory tubes, connectors, syringes — and for device components and instruments which are reused for cost reasons. 

As the packaging for sterile medical products is also exposed to potential risks on its way to the patient — during the production process in the factory, the packaging process, and transport — it is necessary to consider packaging, sterilisation, and transport at the start of new product development in an effort to eliminate any possible hazards in terms regarding sterility. This also applies for the material used. 

Sterilisation can take the form of a physical (thermal, radiation) or chemical process. The following processes have proven to be the most popular:

Sterilisation with superheated steam is the standard method in most laboratories and hospitals and occurs at 121 °C or 134 °C and an overpressure of up to three bar in the autoclave. When the steam condenses on the sterilisation material, energy is released which is harmful to the microorganisms. This method is only suitable for materials that are stable in terms of temperature and hydrolysis. 

Actega's thermoplastic elastomer (TPE) formulation (ProvaMed 6145 TL), for example, has been optimised with regard to its temperature resistance. Following autoclaving at 121 °C (15 min.), no mechanical changes could be detected. A comparison of the mechanical values of an unsterilised specimen with those of a sterilised specimen (at 121 °C and for 15 min.) shows that there are only minimal changes which do not affect the final application.

In the case of ionising radiation, a distinction must be made between natural forms of radiation (alpha, beta, and gamma rays) and those which are generated artificially (electron beams, e-beam). The electrons are generated in an accelerator. Using a so-called scanhorn, the electron beam is then fanned out as a type of “electron shower” under which the products pass through on a transport system. The sterilisation process only takes a few seconds and is less energy-intensive than gamma radiation.

High-energy, ionising gamma radiation deactivates microorganisms. When this low-temperature method is applied, the minimum radiation dose must not be exceeded. The materials are sterilised with 25 kGy and 50 kGy doses and must not display any essential mechanical changes afterwards. Not all plastics are suitable for multiple sterilisation using gamma rays. This method is only applied industrially and almost exclusively for single-use items. 

In the form of ProvaMed 4085 TP, which is particularly suitable for the extrusion of medical tubes required to display transparency and buckling stability, a TPE material is available that has proven in comparative tests that neither yellowing nor any impairment of the mechanical properties occur, not even after high irradiation with 50 kGy.

Ethylene oxide (EtO) sterilisation is a low-temperature method which kills microorganisms even at 10 °C by forming a bond with the protein molecules and destroying them. As the sterilisation time is dependent on the temperature — the higher the temperature, the shorter the sterilisation time — a temperature range of 37–60 °C is usually applied. Many plastics display good chemical resistance to ethylene oxide. In combination with the low process temperature, this method is suitable for a wide variety of thermoplastic materials. Actega’s ProvaMed D1341 TP TPE formulation has proven to be particularly suitable in this regard: ultra-transparent, solvent-bondable, and without any impairment of the mechanical properties during the comparative test.

Each of these methods can prevent biological contamination. But they also display both advantages and disadvantages as well as considerable impacts on the material used. To avoid negative impacts, the material formulae need to be compiled carefully with stabilisers and other supporting ingredients — as is the case with the ProvaMed portfolio. Comprehensive tests on the variants during which gamma irradiation, autoclaving, and gassing with ethylene oxide are compared, show that these materials are resistant to signs of wear such as swift ageing, brittleness, discolouration, or changes in mechanical properties. 

EtO sterilisation has recently become a topic of discussion, whereby Eucomed, the umbrella organisation of companies in the medical device industry, has established that “When the existing regulations and protective measures for the various products and materials are complied with, EtO sterilization is an indispensable, reliable, and validated method of sterilization of medical products.”

As EtO sterilisation is unsuitable for materials and products with complex geometries, which are sensitive to temperature or moisture, sterilisation with beta or gamma rays is recommended here which radiate through the material, thereby reliably destroying pathogenic germs, mould, and spores. For both types of sterilisation, the germ load of the products to be sterilised is determined on the basis of the DIN EN ISO 11737 standard.

Radiation sterilisation is suitable for a wide range of medical products. Here, too, suitability of the materials should be examined within the framework of validation. Unlike EtO sterilisation, radiation sterilisation only requires physical performance evaluation (dose mapping) once per product as part of the validation process; thus saving time and money. This environmentally friendly and residue-free method enables use of the products immediately after approval. Unlike EtO sterilisation, no time-intensive desorption phase is required.