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Sterilization study of Santoprene TPV.

Santoprene TPV is a fully crosslinked thermoplastic elastomer (TPE) which has many unique properties over thermoset rubbers and other thermoplastic elastomers. Examples of these advantages include:

* Low compression set;

* thermoplastic processing; and

* wide hardness range.

The current medical grades available are included in table 1. Product data sheets for these grades can be found at www. santoprene.com.

From these unique properties, product designers often incorporate Santoprene TPV in medical devices such as syringe plungers, gaskets, grips and bumpers. When in such devices, these materials will likely be exposed to one of the following sterilization processes:

* Ethylene oxide (EtO);

* gamma radiation; and

* autoclave.

A key consideration in a medical device is to understand the effects sterilization has on the physical properties of Santoprene TPV.

Scope

To better understand how sterilization affects the properties of Santoprene TPV, several grades that span the available hardness were submitted to EtO, gamma radiation and autoclave sterilization, and tested for relevant properties. This technical document reviews the results from this testing.

For reference, the retention of a property is calculated by the following equation:

% retention = (property after aging)/(property unaged) * 100

(Additional note: This product, including the product name, shall not be used or tested in any medical application without the prior written acknowledgement of ExxonMobil Chemical as to the intended use.)

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Ethylene oxide sterilization

EtO sterilization is a popular method for "one time use" medical disposables that are also only sterilized once. The key property to monitor for this type of sterilization is the retention of residuals (measured in mgs/day) for EtO and ethylene chlorohydrins (ECHs). For this study:

* The grades evaluated were 181-57W180, 281-45MED, 281-64MED and 281-87MED.

* Plaques of each grade were submitted to two EtO sterilization cycles. Two cycles were used to represent the maximum exposure incurred in typical medical device manufacturing. Exposing a device to a second cycle arises when the first cycle prematurely ends. Exposure to the second cycle ensures the device was exposed to at least one complete cycle. Cycles beyond two are not expected.

* Test procedures and allowable residual limits are specified per ISO 10993-7 October 1995.

* The procedure for the residual analysis was per ISO 17025. The following additional steps were employed:

1. Test specimens were frozen after sterilization and thawed prior to testing to accurately control aeration time.

2. Residuals were extracted in an eluate by placing the plaques in 20 ml of purified water at 37[degrees]C for 24 hours.

3. Residual levels in the eluate were determined by gas chromatography.

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* Residual levels were measured at one and four days of aeration.

Results can be found in table 2 (a, b). Results were:

* EtO: After one day post-sterilization, EtO residuals for all grades were well below the maximum allowable limit of 20 mgs/day (per ISO 10993-7) and not detectable after four days.

* ECH: After one day post sterilization, ECH residuals for all grades were less than half the maximum allowable limit of 12 mgs/day (per ISO 10993-7) and below 1.8 mgs/day after four days.

In summary, all grades in this testing exhausted both EtO and ECH residuals well below the maximum allowable limits after only one day of aeration.

Autoclave

Autoclaving is a common technique used for sterilizing devices by the individual medical care professional (e.g., doctor's office, hospital, etc.) for durable medical instruments (scalpels, tongs, pliers, etc.). Autoclaving sterilizes a device by exposing it to super-heated steam (steam heated to temperatures above 100[degrees]C and pressures above one atmosphere) for a specific duration (usually a minimum of five minutes at peak temperature and pressure). To evaluate the effect autoclave sterilization has on Santoprene TPV, plaques were submitted to two different sets of autoclave exposures. The first set was for five autoclave exposures. The second set was for 250 autoclave exposures, with samples incrementally pulled and tested at 50, 100 and 250 cycles.

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Autoclave--five cycles

The first study (five autoclave exposures) exposed plaques of 281-45MED, 281-64MED, 281-87MED, 8281-45MED, 8281-65MED and 8281-90MED.

Autoclave parameters for each of these exposures were:

* Exposure temperature--132[degrees]C (270[degrees]F);

* exposure time--30 minutes;

* exposure pressure--28.6 psig; and

* containment--plaques were placed in plastic bags.

Parameters measured in the testing were:

* % retention of hardness--ISO 868;

* % retention of ultimate tensile strength--ISO 37;

* % retention of tensile stress at 100% elongation--ISO 37; and

* % retention of ultimate elongation--ISO 37. Results can be found in figure 1 (a-d).

Five cycles of autoclaving showed less than 20% effect on ultimate tensile strength and tensile stress at 100% elongation and less than 30% reduction of % elongation for all grades.

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Autoclave--250 cycles

The second study exposed plaques of 281-64MED, 281-73MED and 281-87MED to 250 autoclave cycles.

Autoclave parameters in this testing were:

* Exposure temperature--132[degrees]C (270[degrees]F);

* time at temperature--six minutes;

* time between exposures--30 minutes; and

* containment--plaques were directly exposed to the steam (no bags)

Results can be found in figure 2 (a-d). Properties measured in this testing were:

* % retention of ultimate tensile strength--ASTM D 412;

* % retention of tensile stress at 100% elongation--ASTM D412;

* % retention of ultimate elongation--ASTM D 412; and

* % retention of hardness--ASTM D 2240.

Two hundred-fifty cycles of autoclaving had little effect on hardness and ultimate tensile strength. The tensile stress at 100% elongation could only be measured for 281-87MED. This is due to the elongation of 281-73MED and 281-64MED going below 100% after 50 cycles. For the 250 cycles of autoclaving, the majority of changes in physical properties occurred in the first 50 cycles.

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Gamma sterilization

Gamma sterilization is a method that sterilizes devices by exposing them to controlled dosages of gamma radiation. Gamma sterilization has the advantage of short turnaround times, and low cost when performed in volume. From these advantages, this method is gaining popularity for many one time use medical disposables.

Historically, much of the medical industry expressed the amount a device was gamma irradiated in the units of megarads (Mrads). Today, the amount a device is exposed to gamma irradiation is expressed in the international standard of KiloGray (KGy). One KGy = ten Mrads (example: 25 KGy = 2.5 Mrads).

A typical irradiation for a medical device for one cycle is 10 to 25 KGy. Should a problem arise during sterilization, the devices may be exposed to a second cycle of up to an additional 25 KGy. Such a second cycle ensures the device was exposed to at least one full cycle. It should be noted the effects of gamma irradiation are cumulative. This means that if the first gamma cycle is 25 KGy and a second cycle of 25 KGy is performed, then the total effect on the device and its materials is the same as the device being exposed to one dose of 50 KGy. For this testing, dosages as high as 70 KGy were evaluated, which is an excessive overexposure for any medical device. From this, all data above 50 KGy should be viewed only for reference.

The Santoprene TPV grades selected for this testing were 281-45MED, 281-64MED, 281-87MED, 8281-45MED, 8281-65MED and 8281-90MED.

Parameters measured in testing were:

* % retention of ultimate tensile strength--ISO 37;

* % retention of tensile stress at 100% elongation--ISO 37;

* % retention of ultimate elongation--ISO 37;

* changes in color--TPE-0135; and

* changes in compression set--ISO 815.

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(Note: Because of limited samples, compression set testing was only conducted on the following grades: 281-45MED, 281-64MED and 8281-65MED). Results can be found in figure 3(a-f).

All grades showed a gradual change in tensile properties and color for gamma sterilization under 50 KGy. In compression set testing, the 281-45MED and 281-64MED showed little effect from gamma irradiation. The 8281-65MED showed a significant loss (or increase) in compression set. From this, it is suggested that the 8281-XXMED grades may possibly not be suitable for sealing applications that are exposed to 25 KGys or more gamma irradiation.

Summary

The Santoprene TPV grades in the above testing showed excellent performance when sterilized with EtO. Santoprene TPVs released all measured residuals quickly and to levels well below requirements.

From five cycles of autoclaving in the above testing, Santoprene TPV showed less than 30% change in properties. From 250 cycles of autoclaving in the above testing, most change in physical properties occurred in the first 50 cycles and was under 30% for the entire test.

For gamma sterilization, all grades in this study showed a gradual change in properties and color when irradiated to 50 KGys. 281-XXMED grades showed better property retention in compression set than 8281-XXMED. 8281-XXMED may not be suitable grades for sealing applications requiring 25 KGys or more irradiation.

by George Anthony, ExxonMobil Chemical (www.santoprene.com)
Table 1--medical grades

Colorable Black

281-55MED 8281-35MED 181-57W180
281-45MED 8281-45MED 181-55MED
281-64MED 8281-55MED 181-64MED
281-73MED 8281-65MED
281-87MED 8281-75MED
283-40MED 8281-90MED

Table 2a--EtO residuals

 Mgs/day
Materials One day Four days

Allowable limit per ISO 10993-7 20 20
Santoprene TPV 181-57W180 <0.18 <0.18
Santoprene TPV 281-45MED 0.32 <0.18
Santoprene TPV 281-64MED 0.40 <0.18
Santoprene TPV 281-87MED 0.35 <0.18

Note: <0.18 means actual values are below detectable levels.

Table 2b--ECH residuals

 Mgs/day
Materials One day Four days

Allowable limit per ISO 10993-7 12 12
Santoprene TPV 181-57W180 1.554 1.26
Santoprene TPV 281-45MED 3.91 1.48
Santoprene TPV 281-64MED 4.17 1.78
Santoprene TPV 281-87MED 4.13 0.96
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Title Annotation:Tech Service
Author:Anthony, George
Publication:Rubber World
Date:Mar 1, 2009
Words:1585
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