Features

Background

Single- and multiple-use medical devices typically require sterilization prior to their use in order to provide for patient safety. Medical equipment manufacturers and molders need to adhere to this protocol when providing devices to medical facilities.

Typically what happens is that after a medical device is produced, it is packaged, sterilized, and then shipped to its end user. There is often a lag time between manufacturing and receipt of the product by the facility however, not only because of the addition of this sterilization step to the process, but in the case of plastics — polycarbonate, in particular — the material color needs to “adjust” following sterilization.

Polycarbonate & Medical Devices

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Fig. 1 – Radiation-stabilized polycarbonate is typically used in blood filter dialysis equipment, similar to that pictured, where irradiation of parts is necessary. Other potential applications for this type of material include surgical and respiratory devices, syringes, stopcocks, and luers.

The properties of polycarbonate make it an ideal material for many medical applications. (Fig. 1) Strength, clarity, and dimensional stability along with extensive biocompatibility data make it the thermoplastic of choice for many single- and multiple-use medical devices.

Despite the excellent properties of polycarbonate, when sterilized by irradiation, it can exhibit a significant discoloration and a resulting reduction in light transmittance. The physical properties of the polycarbonate can be impacted as well.

These effects present a challenge to manufacturers and health care professionals who are concerned about the aesthetics and long-term functionality of their medical devices. An item that is discolored appears unsanitary, and a lack of transparency can impact the usefulness. For example, if a device is imprinted with instructions or a scale, it may become unreadable.

Therefore, it is challenging to identify the correct material for medical applications – a material that will withstand the sterilization process that will be required and maintain its properties and functionality. In order to do so, one must understand the sterilization regimen that a device will be subjected to since each sterilization method differs in its aggressiveness, radiation dosage, recommended cycles of radiation, and environmental conditions.

Radiation-Stabilized Solution

Styron, a global materials company and former part of The Dow Chemical Company, developed a solution to the challenge of polycarbonate discoloration. This year marks the company’s 25th anniversary of an innovative color compensation technology that allows polycarbonate color to quickly equilibrate following sterilization by irradiation to an acceptable threshold level. This technology has been the enabler for success addressing this phenomenon.

Up until that point, there were few solutions to color stability and recovery. The most common solution called for manufacturers to add a special tint to compensate for, or mask, the color shift that would occur. This resulted in a more aesthetically pleasing initial color, however it was temporary and manufacturers needed more stable appearance over a longer period of time. Other solutions called for the addition of antioxidants or other additives to control the color shift that would occur.

This article focuses on irradiation sterilization methods and the degree to which polycarbonate resins will maintain color integrity and physical properties over time. High energy irradiation is one of the most effective and useful types of sterilization methods used for disposable medical devices. Irradiation can be done by two methods: gamma radiation (cobalt-60) or by electron-beam (ebeam), which are explained below.

Gamma Radiation

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Fig. 2 – Discoloration resulting from irradiation and color recovery is shown. This figure indicates initial part color, color immediately after gamma sterilization and the final part color to a threshold level that takes place over time.

Gamma radiation is a form of ionizing radiation sterilization widely used to sterilize medical devices. It involves exposure to photons from a cobalt-60 source. The rays have no mass and are able to penetrate deeply into the material, easily penetrating most packaging materials, and enabling sterilization of devices in assembled and packaged form.

A common gamma dosage for sterilizing is 2.5 megarads (25 kGy), although higher doses of 5.0 and 7.5 are used depending on the application. The process temperature is low, usually between 30 °C and 40 °C. The sterilization cycle time is dependent on the level of gamma radiation required.

Gamma radiation is known to cause discoloration of polycarbonate resin. Color stabilized polycarbonate resins are designed to reduce post-sterilization color shift. To demonstrate what happens when polycarbonate is exposed to this type of radiation, polycarbonate resin was exposed to 2.5 Mrad of gamma radiation, in three different tints, and the color change was studied. In each case, the yellowness prior to sterilization was at zero. The color was then measured after one week and eight weeks later. Initially, an increase in Yellowness Index is observed due to the inherent tendency of polycarbonate to discolor. As expected, the yellowness of the material diminished over time. (Fig. 2) This phenomenon is known as photobleaching. After exposure to gamma radiation, the transparent polycarbonate samples equilibrate back toward their original color prior to irradiation. The color and radiation dosage must be coordinated or the resulting resin will be purple (too much tint for irradiation level) or yellow (too much irradiation for the tint package).

Since storage conditions can also play a role in photobleaching, a study was performed to determine the effect of storage lighting conditions. Samples were stored in fluorescent light and dark conditions and the Yellowness Index was measured every week for six weeks. It was observed that the color recovery was accelerated if the samples were stored in lighted conditions. No differences in physical properties were observed based on storage conditions.

Depending on the needs of the application, manufacturers can also apply advanced color compensation technology. The impact of this is that the product typically returns to the desired color faster. This would enable a manufacturer to reduce inventory hold time and introduce products for functional use sooner than with standard polycarbonate resins or those with a minimal color compensation package. This advanced additive technology may allow customers to consider oxygen-free irradiation, for example, which can potentially offer other performance benefits.

One of the advantages of gamma sterilization is that it has almost no effect on the physical performance of polycarbonate. To demonstrate this point, studies have been done where polycarbonate resin with color compensation technology was exposed to gamma sterilization up to 10 Mrad, which would simulate “worst case” exposure. The retention of physical properties is excellent, even at radiation doses up to 10 Mrad (100 kGy). All physical properties are nearly identical to the control prior to sterilization. (Fig. 3)

Product shelf life is another important consideration. The material must maintain its physical integrity time because of the delay between the time of manufacture/ sterilization and the time the device is actually used. When a study was performed to determine the physical properties after sterilization and storage up to one year it was noted that sterilization does not significantly affect the material performance in key properties areas, i.e., tensile strength, tensile elongation, flex modulus, notched Izod, and Vicat. In fact, all properties were retained at more than 95 percent of their pre-sterilization values.

Electron Beam

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Fig. 3 – Radiation-stabilized polycarbonate is commonly tinted purple to “mask” the color shift that takes place and provide for a more aesthetically-pleasing appearance following irradiation. Pictured is water white polycarbonate and polycarbonate with color compensation technology.

E-beam irradiation is another form of ionizing radiation sterilization and involves the penetration of high-energy electrons throughout the material. Typical doses are in the 1-6 Mrad range (10 – 60 kGy). E-beam sterilization is known to be significantly faster compared to gamma sterilization; parts need only be exposed for minutes rather than hours. However, the penetrating capability of e-beam radiation is not as good and articles may need to be treated from multiple directions to achieve complete sterilization.

A study was performed to determine the effects of e-beam sterilization on a sample of radiation-stabilized polycarbonate resin. It was noted that the color change caused by sterilization by e-beam and gamma is similar and the initial color change for e-beam was slightly lower compared to gamma, but over time the resulting color is the same.

The retention of physical properties for e-beam sterilized samples is also excellent. These results are similar to those for gamma sterilization.

Summary

There are many performance requirements to be considered when evaluating materials for a medical application. One of the most important considerations is the effect of the intended sterilization process on the appearance and performance. Polycarbonate is a choice that meets these needs for many applications. When exposed to gamma and ebeam irradiation, it maintains its physical properties and the appearance can be managed through color compensation technology.

Material innovation is needed to meet new challenges as sterilization technology continues to evolve. Styron recently introduced enhanced color compensation technology providing accelerated color recovery. CALIBRE™ MEGARAD™ 2091 Polycarbonate Resin offers an improvement in color recovery time, allowing manufacturers to introduce products for functional use 10 to 21 days sooner than with previously available polycarbonate resins. The material also allows sterilization facilities to consider an oxygen-free environment, which creates unique product differentiation advantages while managing final color expectations.

This article was written by Cheryl Weckle, Development Scientist for Styron LLC, Berwyn, PA. For more information, visit http://info.hotims.com/40437-164.