Originally Published MPMN June 2001
Conference Summary: Radiation Processing Professionals Go Back to the Future at Avignon Meeting
Ionization equipment manufacturers and subcontractors joined researchers from around the world at the majestic Palais des Papes in Avignon, France, in March for the 12th International Meeting on Radiation Processing. The biannual meeting provides industry professionals and members of academe with an opportunity to share news on scientific advances, new applications, and evolving standards. The unusual conference venue prompted chairman Théo Sadat from Thomson CSF Linac, the French manufacturer of accelerators, to welcome delegates to industry's "first high-tech meeting in the new millennium . . . in a historic 700-year-old building." Following a dramatic opening ceremony with a medieval accent, delegates rolled up their sleeves and got down to the business of the twenty-first century. In the Conclave room, where cardinals once sat to elect a pope, and elsewhere within the palace, discussions turned to topics such as polymeric grafting, e-calibration of dosimeters, and substantiating a 25-kGy sterilization dose.
Better polymers through grafting
|A palace guard leads the opening ceremony of the International Meeting on Radiation Processing, held this year at the Palais des Papes in Avignon, France.|
Grafting polymers by means of a radiation or UV source holds enormous potential for a range of industries including the biomedical sector, according to Adolphe Chapiro, former president of the French Association of Polymer Research and Applications (Strasbourg, France) and noted scientific author. Calling it a powerful means to alter a polymer's properties and to create new materials, Chapiro noted that the technology permits bulk or targeted modifications. "In grafting polyethylene to acrylic acids, for example, you retain all of the properties of polyethylene while fostering strong adhesion to metals," he said. "For implant applications, a biocompatible polymer can be grafted onto the surface of another polymer that is not suited for invasive use [but which may be desirable for other properties]," he added. Although there have been few industrial applications of this technology thus far, Chapiro noted that animal trials have shown great promise.
By combining grafting with the charge-transfer (CT) complex engendered by ionized curing, a much stronger bond between materials can be created, noted Jack Garnett from the department of materials and chemistry at the University of Western Sydney (Australia). Chairing a session devoted to curing processes, Garnett told attendees that the use of photoinitiators (PIs) to promote UV-initiated and E-beam curing, as is common today, limits the effectiveness of the process. "These additives increase the cost of UV-curing materials and may lead to contamination of the cured film with PI fragments," he said. Recently discovered UV- or gamma-initiated CT complexes--defined as an aggregate of two or more molecules in which the charge is transferred from a donor to an acceptor--eliminate the need to add PIs. However, he added, this may adversely affect a material's adhesion to a substrate. To remedy this, Garnett recommended concurrent grafting and curing, which can be accomplished by using either a UV or radiation source.
The combination of grafting, which produces a chemical bond, and curing, which creates a physical bond, results in strong adhesion, he said. Radiation processing has an advantage over UV in this regard, he added. "With ionizing radiation systems, you produce radicals as well as positive and negative ions," Garnett explained. "This provides additional mechanistic pathways to achieve grafting."
Compact radiation units introduced
A compact irradiator designed for low-volume applications was presented by MDS Nordion (Kanata, ON, Canada; www.mdsnordion.com) at the Avignon meeting. By rethinking the structure and operation of conventional gamma sterilizers, MDS was able to substantially reduce the cost of the unit (which is briefly described in the Equipment News section of the April 2001 issue of MPMN). The Brevion is affordable for a category of customers that, until now, may not have considered investing in an on-site gamma unit, according to Daniel Levesque, director of sales, Europe, industrial irradiation, at MDS Nordion.
Unlike traditional gamma systems, the Brevion transports totes to the irradiator source via tracks. To reduce downtime during batch changes, MDS developed a cartridge concept that enables a single unit to process two source passes. As one batch of irradiated products slides out from the sterilizer, the track system automatically positions the second batch for processing. Changeovers are performed in less than 5 minutes, and typical processing time is 3 to 5 hours. Because there is no maze or storage conveyor and the unit is shipped largely preassembled, installation costs have been slashed by 75% compared with conventional units. Setup time is less than 4 weeks. The primary markets for the Brevion, according to Levesque, are medical device manufacturers processing small product volumes and contract service providers in developing countries with a small but growing device industry.
A compact E-beam system developed by Mitsubishi Heavy Industries Ltd. (Nagoya, Japan; www.mhi.co.jp) was also presented at the meeting. The C-band 5712-MHz accelerator is reportedly the first of its kind to achieve 10 MeV acceleration energy. It measures 60 cm in length and weighs 20 kg. The machine is suited for sterilization and nondestructive inspection applications.
On-line dosimetry calibration coming soon
"Industry wants high-quality calibration services, rapid turnaround, and, everyone wants it as cheaply as possible," said Marc Desrosiers, research chemist at the Institute of Standards and Technology (NIST; Gaithersburg, MD) at a session devoted to dosimetry.
The organization is currently testing a system that would allow companies to calibrate dosimeters via the Internet. "The company would have to be equipped with an Electron Paramagnetic Resonance (EPR) spectrometer connected to a PC and a batch of presupplied alanine dosimeters," he explained. The Internet calibration program on the NIST server will control the evaluation and guide the technician through the various steps. A provisional certificate will be issued within a matter of minutes; after appropriate quality checks have been conducted at NIST, an official certificate will be delivered.
An obvious advantage, noted Desrosiers, is the elimination of the current 3- to 5-day turnaround time. "Calibration services will be offered on demand 24 hours a day, seven days a week, and the results will be available in minutes." Because the process requires no oversight from NIST staff, fees will be drastically reduced, he added.
On-line calibration against the U.S. national standard gamma-radiation source is currently undergoing testing. NIST expects to make the subscription-based service available to customers worldwide starting in 2002. For more information, go to the NIST Web site at www.nist.gov.
Substantiating the 25-kGy sterilization dose
Both ISO 11137 and EN 552, standards related to the validation and routine control of radiation sterilization, permit the use of 25 kGy as a sterilization dose provided that it can be substantiated that it achieves a sterility assurance level of 10-6. Unfortunately, neither document provides guidance on how to show substantiation. Craig Herring, senior fellow, sterilization science, at Ethicon Endo-Surgery Inc. (Somerville, NJ), a Johnson & Johnson company, presented a paper in which he described a process, called Method VDmax, for the substantiation of 25 kGy.
"The process is markedly similar to Method 1 in ISO 11137, which was done deliberately to avoid confusion," said Herring. "If you test 0/10 or 1/10 positive, then the dose is substantiated. If you show 2/10 positives, a confirmatory test is required; a 3/10 positive rate means that the dose is not substantiated," he explained. The method was tested in a parallel study against Method 1 at three different J&J facilities, said Herring, using three different categories of products: disposables, implants, and sutures. A total of 421 dose audits were conducted over a three-year period without any failures, reported Herring. "Only a handful of products required confirmatory tests, all of which ultimately proved to be positive," he added. The procedure will be described in AAMI/FDSB-1 TIR 27, which will be available this summer from the Association for the Advancement of Medical Instrumentation (www.aami.org).