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Silicone is used in a variety of medical instruments and equipment which must be sterilized before use. Three main methods of sterilization can be considered: steam sterilization (autoclave), irradiation and ethylene oxide.

Steam Sterilization by Autoclave

Steam sterilization is typically carried out in an autoclave at 121°c (250°F) for 15 minutes, although other conditions are often used (Rogers, W., 2005). Silicone tubing may start to become gummy after having being steam sterilized several times and should then be replaced.

Irradiation:

Gamma Iradiation

Gamma irradiation is widely used for sterilization of silicone tubing. However, some changes are produced in the silicone, principally an increase in cross-linking, causing an increased hardness and shape memory (Rogers, W., 2005). The latter effect may make kinking of tubing more likely. The tensile elongation of platinum and peroxide cured silicones has been shown to decrease after gamma sterilization, while the tensile strength of platinum cured silicone remains nearly the same.

Electron Beam Irradiation

Electron Beam Irradiation is an alternative to gamma rays. The physical effects are similar, but somewhat less, to those found with gamma irradiation. Again greater degradation was noted with peroxide-cured silicone than with platinum-cured silicone (Gautriaud, E.).

Ethylene Oxide

Ethylene oxide (EO) is a very effective sterilizing method for most silicone materials (Rogers, W., 2005). The ethylene oxide is adsorbed by the silicone and must be removed by post-cycle aeration before the equipment is used. Appropriate testing is required to ensure that removal has occurred. A study (McGunnigle, R.G., 1975) showed that silicone tubing adsorbed about 85% less ethylene oxide than PVC tubing or polyester / polyurethane tubing. Also, desorption of the ethylene oxide was much faster for the silicone tubing than for the other two polymers. Ethylene oxide sterilization was found to have no significant adverse effects on platinum or peroxide cured silicone (Gautriaud, E.), so it is recommended in most cases for these materials. Since ethylene oxide is a toxic, carcinogenic gas, appropriate safety measures should always be in place.

Other Methods

Liquid sterilizing chemicals such as glutaraldehyde are sometime used. It is not clear if these are suitable in general for silicone medical equipment. Also, ozone is a highly toxic gas that can be used for silicone sterilization, but it can be less penetrating than ethylene oxide, only sterilizing surfaces.

Conclusions

Ethylene oxide is widely recommended to sterilize platinum and peroxide cured silicone. Irradiation or steam are also commonly used, but these methods should be considered on a case by case basis in order to not risk compromising critical material properties which ensure capabilities such as critical dosing in peristaltic pumps. Platinum-cured silicone is widely preferred to peroxide cured silicone where purity is a concern. However, peroxide cured silicones tend to have longer life in certain peristaltic pump applications. From the most exacting critical dosing to not so critical applications there are several types of silicones availabe to meet your specific needs. Contact your TBL Plastics representative for tailored recommendations about your process and technical information about our platinum cured silicone tubing or platinum cured silicone gaskets.

References

McGunnigle, R.G. et al (1975), “Residual ethylene oxide: levels in medical grade tubing and effects in an in-vitro biologic system”, Journal of Biomedical Materials Research, 9 (3), p.273-283. Palsule, A.S., Clarson,

S.J. & Widehouse C.W., (2008), “Gamma Irradiation of Silicones”, Journal of Inorganic and Organometallic Polymers and Materials, 18 (2), p.207-221. Rogers, W. (2005), Sterilisation of Polymer Healthcare Products, Shrewsbury: Rapra Technology.

Gautriad, E. et al. “Effect of Sterilization on the Mechanical Properties of Silicone Rubbers”

What is the Difference Between LDPE and LLDPE?

Low Density Polyethylene (LDPE) and Linear Low Density Polyethylene (LLDPE) are both inexpensive polymers with widely favorable mechanical and chemical resistance properties. Tubing made from both polymers is broadly used, particularly for water, chemicals and gases. Unlike with many other plastics, plasticizers are seldom necessary to obtain flexible products, such as tubing. Both plastics are highly stable with low toxicity. In fact, many grades can even be used for food-contact and medical applications.

LDPE is a homopolymer constituted by ethylene monomers. LLDPE is a copolymer of ethylene and another longer olefin, which is incorporated to improve properties such as tensile strength or resistance to harsh environments. One of four α-olefins (1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene) is commonly polymerized with ethylene to make LLDPE. The amount of the α-olefin is typically low compared to the amount of ethylene.

Which Makes Better Tubing?

LLDPE tends to have greater environmental stress-crack resistance ESCR than LDPE. It has been reported (Wypych, G., 2003) that outdoor LDPE pipes are readily affected by environmental stress cracking. It is stated that the resistance of LDPE can be “improved by a substantial addition of LLDPE (30-40%).” LLDPE also has a higher tensile strength than LDPE and greater puncture resistance (Robertson, G.L., 2006). Since LDPE is a weaker tubing than LLDPE a thicker wall grade can be chosen to compensate, but this has cost implications if a large amount of tubing is required. Flexibility is also affected negatively by a greater wall thickness.

LDPE also has advantages as it is more transparent than LLDPE (Robertson, G.L., 2006), which may be advantageous if visualization of the conveyed fluid is important. It also tends to be more flexible. The performance of LDPE can be greatly improved by incorporating it into a two-layer tube. A flexible polymer such as EVA can be used as the outer layer, while the chemically inert LDPE makes up the inner layer. We have taken advantage of such a “co-extrusion” in our Pharm-A-Line VI & Pharm-A-Line XL Polyethylene-Lined EVA tubing.

Read more about our LDPE and LLDPE tubing.

Conclusions

For most applications LLDPE tubing is preferred, as it is stronger than LDPE. LDPE may often be chosen where flexibility is a factor or if a more transparent tube is needed. LDPE performance is greatly improved when it is used as an inner layer with a more flexible polymer as the outer layer.

References

Robertson, G.L (2006). “Food Packaging, Principles and Practice”, Boca Raton: CRC Press.

Wypych, G. (2003). “Handbook of Material Weathering” 3rd ed. Toronto: ChemTec Publishing.