Medical Device Link.

FEATURE

Advancing technology

Figure 1. (click to enlarge) The OTR and WVTR values of two-layer and three-layer structures before and after sterilisation.

Thermal sterilisation is used widely in medical packaging to obtain sterile products. Two methods exist, dry heat or steam autoclave, the latter includes water bath and/or water spray, as required. During autoclave sterilisation, temperatures above 121 °C are normally applied for at least 20 minutes. These conditions place high demands on the packaging material. Absolute tightness of the seams must be guaranteed as well as 100% integrity of the barrier layer after sterilisation. Needless to say, shrinkage, reduction in bond strength and changes to the printing inks must not take place.

Historically, aluminium foil was the material of choice for reaching high barrier values in terms of water vapour transmission rate (WVTR) and oxygen transmission rate (OTR). In recent times new materials have come onto the market. Polyester (PET) film and oriented polyamide (OPA) film coated with ceramic barrier layers such as aluminium oxide (AlOx) or silicon oxide (SiOx) have already proved their suitability for use in the food market. These materials offer a solution to medical device packagers seeking barrier materials because they have one significant advantage over aluminium: transparency. Furthermore, thanks to major improvements in coating technology in recent years increased barrier properties and stability of the coating under thermal
exposure are now possible.

When used in a three layer structure such as polyester (PET)-AlOx 12 µm/OPA15 µm/polypropylene (PP) 70 µm, the latest PET-AlOx films exhibit typical OTR and WVTR values of 0.3 after sterilisation. Before sterilisation, the barrier values are below 0.1 (see Figure 1).

Two-layer systems now possible

The biggest development in this area is that because of the higher thermal stability of the barrier, cost-effective sterilisable two-layer structures such as PET-AlOx/PP are now feasible. Nearly the same barrier properties as a standard three-layer structure can be achieved. This has always been a challenging target. There are two critical points in the sterilisation process for two-layer structures.

The first is the change of the crystallinity of the PP in the heating up and cooling down period. PP tends to expand and shrink during this process. Because the PP is only separated from the AlOx layer by a thin adhesive layer, it induces a high mechanical stress on the ceramic barrier. In standard three-layer structures, the OPA film acts as a buffer between the AlOx coating and the PP.

The second critical point is the control of the counter pressure in the sterilisation process itself. If the packed product contains water, there will be an overpressure built up in the pack. This overpressure has to be balanced with a counter pressure controlled by the autoclave. The counter pressure must be sufficient to prevent the overpressure causing pack expansion, which would lead to stretching of the material and consequent breakages in the barrier coating.

Beneficial features

This new generation of transparent, high barrier films has become an interesting option in medical device packaging.

• The films are suitable for thermal, gamma ray and beta ray sterilisation, as well as for aspetic filling.
• Having a clear view of the packed product and the option of using high quality reverse rotogravure printing with up to 12 colours is an obvious marketing advantage.
• Product safety is an important issue. Being able to see the packed product implies much better control of the product inside. Unlike aluminium laminates, these novel PET-AlOx or SiOx films allow installation of inline metal detectors directly on packaging lines after the sealing station.
• This transparency and added safety allows the possibility of inline control of the seams immediately after sealing. Sealing failures can be detected inline by optical measurement units.

Easy opening options

Figure 2. (click to enlarge) Tear propagation strength test set up of DIN 53363.

The convenience of easy opening is a requirement for nearly all packaging. Novel PP based materials that can be sterilised by thermal methods give new design choices to the medical sector. One PP based film provides perfectly linear and easy tear behaviour in the machine direction before and after sterilisation. In addition to these properties, this easy tear PP has the advantage that there is no mechanical pre-damage involved and it has increased shelf life compared with traditional easy tear systems. The tear propagation strength test (DIN 53363, Determining the Tear Resistance of Plastic Film and Sheeting by the Trouser Tear Method) is the main test for judging the easy tear behaviour of laminates and Figure 2 illustrates the test set up.

Figure 3. (click to enlarge) Typical tear behaviour of easy tear PP and other easy tear materials.

Figure 3 shows the typical tear behaviour of the material compared with other easy tear solutions. The low tear resistance of 1–2 N at a constant level is particularly significant. In terms of linearity, the specification for the easy tear PP in a PET 12 µm/Alu 8µm/easy tear PP 70µm laminate is set to a vertical deviation of 3 mm over a linear tear length of 95 mm.

Sealing forces for the above mentioned structure are specified at >35 N/15 mm with typical values of approximately 50–60/15 mm, both measured after sterilisation. Machine speeds at sealing temperatures of 210–240 °C are approximately 100 cycles per minute. Currently, there are a lot of development projects underway by various companies with regard to easy tear polyethylenes for nonsterile products, which will be commercialised within the next 12 to 18 months.

The second design choice in terms of easy opening is the peel option. For thermal sterilised medical products such as contact lenses and lid films for laboratory equipment, the main challenge lies in guaranteeing a tight package that endures the heat sterilisation process in combination with smooth peel properties.

Figure 4 shows several PP peel films tested in a PET/Alu/PP-peel laminate as lidding material on a solid PP tray. The requirements were: an easy constant peel after sterilisation and a shelf life of up to five years. In this case, PP-Peel 2 turned out to be the most favourable film. This was because of its lower sealing activation temperature and its constant sealing force plateau of approximately 16 N/15 mm in the temperature range of 140–210 °C. Both other peel films have their benefits in other applications where higher peel values are required.

Replacing paper with transparent cold seal packs

Figure 4. (click to enlarge) Several PP peel films tested in a PET/Alu/PP-peel laminate as lidding material on a solid PP tray.

Cold seal adhesives are widely used for wound dressing applications. The standard packaging in this sector is a paper film of 40–50 g/m² with cold seal used in a four-side-seal pack. There has been a recent change in that tradition. The increased need for consumers to see what they are using has led to transparent structures based on plastic films replacing the top layer of modern wound dressings such as hydrocolloids and pre-cut dressings. In addition, marketing aspects favour the replacement of nontranslucent materials, for example, showing colourful comic prints on children’s wound dressings. The requirements for this type of transparent lidding film on the standard paper/cold seal are: transparency, gamma sterilisablility up to 50 kGy, easy peel against paper cold seal, good machinability and a shelf life of five years. The main issue is to find a cold seal that achieves the required 0.8 N/15 mm opening peel strength, in combination with the cold seal on the paper structure. In addition, this cold seal has to be modified with antioxidants to withstand the gamma sterilisation and five years of shelf life. The transparent carrier film can be a biaxial orientated poly-propylene (OPP). These films combine good mechanical stability with high transparency, features that are important for applying the cold seal and for printability on high-speed rotogravure.

Although PP is known to lose mechanical properties during the gamma sterilisation process, tests have shown that the mechanical properties of the PP are still more than sufficient for this kind of packaging application. The combination of paper/cold seal on the bottom and the transparent OPP cold seal on top is also suitable for ethylene oxide sterilisation.

Because various applications need different peel forces, converters offer different cold seals to achieve sealing forces ranging from 0.5 N/15 mm to 4 N/15 mm. The low peel forces of 0.5 N/15 mm are required for specially designed wound dressing packs where the peeling angle is close to 180°. Higher peel forces would make it uncomfortable to open the pack for children and elderly patients.

Tobias Fackler is Research and Development Manager at Huhtamaki Deutschland GmbH & Co. KG, Huhtamaki Ronsberg, Heinrich Nicolaus-Strasse 6, D-87671 Ronsberg, Germany, tel. + 49 8306 77 128, e-mail: tobias.fackler@de.huhtamaki.com, www.huhtamaki.com.