Peelable Materials for Flexible Packaging

Access without contamination
Designers and developers of flexible packaging for medical devices must always have as their first consideration the establishment and maintenance of the sterile barrier for the device, from the point of manufacture and packaging of the product to point of use. However, another important consideration is package access to give the end user the ability to effectively remove the device from its package. The design of this feature should take into account the way the user will be employing the device, for example, whether the user is

• a paramedic utilising an airway management device at the scene of a road traffic accident
• an anaesthetist administering an epidural prior to childbirth
• a paraplegic performing self-administration of a urinary catheter
• circulating nurse transferring items to a sterile nurse whilst preparing an operating theatre or during a hip replacement when the transfer may take place during the procedure once the surgeon has confirmed the size of product to be used.

These end user situations are different, but they all require easy and consistent delivery without contamination. Peelability is a logical access feature to avoid concerns with particulates that occur when tearing open the package or using other destructive opening methods. This article provides an overview of the peelable flexible packaging materials that are available to the package designer. It is worth bearing in mind that the traditional inertia in changing packaging formats within the medical device industry, which is driven by the regulatory requirements of validation, means that older material technologies are still being used when more effective new formulations are available.

Coatings
The pace of development of disposable medical devices in the late 1970s and early 1980s led to increased demand for packaging alternatives. Packaging material suppliers, together with base material suppliers, endeavoured to develop structures to meet that burgeoning demand:

• A breathable substrate was needed to allow ingress and egress of the sterilant for devices sterilised by ethylene oxide (EtO) gas. A range of solvent-based heat seal coatings was developed for application to porous substrates, including paper and Tyvek, the spun-bonded polyolefin from Dupont de Nemours (www.medicalpackaging.dupont.com [6]). These coatings are breathable and allow the passage of the gas through the entire structure while permitting a wide seal temperature window for ease of heat sealing to materials such as polyethylene terephthalate (PET) and polyethylene (PE) for pouches or formable films used for three dimensional packages. They also provide a bright white seal transfer when peeled, which indicates a complete seal.
• For devices sterilised by gamma radiation, solvent based heat seal coatings were developed for application onto films such as two-layer PET–PE structures or high density PE for pouches or formable films. The coatings developed were generally ethylene vinyl acetate (EVA) dispersions applied by gravure cylinders. These coatings had a number of variations, which allowed sealing to a variety of materials, including PE, polypropylene (PP), polyamide (PA) polycarbonate (PC), PET and metal. In many cases the coating also enabled the user to change the seal characteristics by manipulating the sealing conditions. For example, an increase of seal temperature or seal pressure could yield an increase in seal strength, as shown in Figure I.

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Figure I: Typical peel strength, coated paper to film.

As versatile as these coatings were, there were inherent problems that drove the development of the next generation of peelable technology. These problems included:

• the retention of solvents in the coating
• variability in quality and coating adhesion
• greatly reduced porosity for breathable substrates
• difficulty in maintaining coating adhesion to film
• cost.

The next generation of coatings for porous substrates were aqueous-based EVA dispersions mainly applied by air knife and these addressed many of the concerns associated with the solvent based coatings. In addition, new polymer technology enabled increased porosity of the applied coating, and advances in coating control improved consistency and quality. Other coatings utilising a hot melt process also increased porosity while maintaining wide seal windows. Today, adhesive coated substrates are versatile options for the packaging designer to consider. They provide a wide seal window of approximately 30 °C, consistent visible seal transfer (turns bright white when peeled), and in some cases the possibility of constructing packages capable of being sterilised by EtO and gamma. However, recent testing in the company’s laboratories has shown that these coatings may not remain intact over time and could potentially lead to sterility breaches, although none have been widely reported to date.

Films
The explosive growth of disposable medical devices attracted the attention of increasing numbers of flexible packaging material suppliers, many with a significant base of business in other markets such as food and consumer packaging. This meant that many advances in flexible materials such as film coextrusion and lamination were being marketed to the medical device industry. The new options now available to the packaging designer include:

• Peelable coextruded films that can potentially be cost-effective options for radiation sterilised medical device packages, because of advantageous economies in the scale of production. Equipment size and output have increased over recent years making these materials more affordable to device manufacturers.
• Laminations, which allow reverse (trap) printing of high quality graphics, incorporation of additional substrates for better characteristics such as including foil for a barrier or a white background for graphic appeal. Laminating also enables the construction of peelable materials with wider operating windows of approximately 20 °C, because a heat stable outer layer such as PET or nylon (PA) contributes to greater machinability compared with using PE coextruded peelable film, which does not have this type of outer layer. The introduction of these films to medical packaging provided new options for cost savings and increased machine speeds. However, challenges soon became evident:
• Seal strength. The medical device industry was conditioned to using heat seal coated substrates with wide operating windows and a certain threshold of seal strength. The adoption of peelable films involved a significant learning curve. A film made on a large scale, for example, for the food industry, may have a favourable cost profile, but cannot readily be modified to increase or decrease seal strength unless the volume usage is attractive enough to sustain new product development activity specifically for medical requirements.
• Handling. Heat seal coated materials tend to be stiff and comparatively easy to handle. Films, especially lower gauge coextrusions and laminates, may exhibit more curl, have more static electricity retention, and require packaging machinery modifications in cutting and sealing.
• Versatility. Although heat seal coatings of different types can provide a consistent seal and peel to different flexible or rigid materials, most peelable coextruded films are designed to seal to only one type of material. Many are available for sealing to PE, with fewer options available for other materials such as PP, glycol modified polyethylene terephthalate (PETG) and high impact polystyrene (HIPS).

Latest advances
There have been a considerable number of advancements in flexible packaging in the past 10 years, including the development of coextrusion technology; new polymers such as ultralow density PE, elastomers and plastomers with greatly enhanced performance characteristics; and improvements in packaging machinery design. These advances, combined with a greater focus on the specific needs of the medical device industry, have expanded the options available to the packaging designer as follows.

• Peelable film lidstock for use when sealing to flexible forming webs for high volume medical devices that are radiation sterilised offers a cost-effective film or film package option. The peelable sealants used today have improved significantly and now provide a consistent seal strength that can be easily validated in accordance with ISO 11607: 2006, Packaging for Terminally Sterilised Medical Devices. The problem of “shredding” or “angel hair” observed in earlier films, which are unacceptable because of the particulate generated, has been largely eliminated.
• Peelable films are available for use when sealing to uncoated Tyvek or medical grade papers for the fabrication of flat pouches and bags. Although standard PET–PE has been used for many years in this way, new film formulations provide better consistency and performance. They prevent tearing or “shattering” of the breathable substrate when it is peeled (Tyvek has a “skin” effect and can delaminate if sealed right up a cut edge). The porosity of the package is also increased, which allows more aggressive EtO sterilisation in which faster cycles require faster pressure changes within the EtO chamber.
• Peelable, formable webs intended for use on horizontal thermoform-fill-seal machines have been developed and provide consistent seals to uncoated Tyvek and medical grade papers. This packaging platform can be a cost-effective alternative to using a coated breathable substrate for two reasons. It enhances the porosity of the package to allow faster EtO sterilisation cycles without increasing stress on seals; and the additional cost of coating the lidding material is no longer required.
• Treated or “sized” medical grade papers are recent developments that do not require an additional heat seal coating to provide a peelable seal. These papers, some of which also have a synthetic reinforcement added for strength, can be cost-effective for the reasons cited above while maintaining good package integrity and consistent peelable seals with an evident seal transfer.
• New versions of heat seal coatings have been developed with enhanced porosity, wider seal windows and better consistency, which yields better seals. These coatings offer advantages when high performance levels are required, particularly when sealing to rigid trays.

Naturally, the use of an uncoated top web, whether paper or Tyvek, requires additional heat to be transferred through the lidding material and into the specialised sealant layer within the film. For this reason, sealing temperatures are usually higher than with a coated material, as shown in Figure II.

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Figure II: Peel strength, uncoated paper to film.

Help with validation
Ever increasing demands are being placed on medical device manufacturers. These include packaging operations located at diverse global locations, which require the same specification of product to be achieved by the different lines in each of those different regions; the increased need for speed to market; high manufacturing output rates; more scrutiny regarding validation; packaging consistency; and more aggressive sterilisation methods. The new developments in flexible packaging now offer the package designer a much wider selection of options to meet those challenges.
The arrival of new materials provides choice and requires packaging manufacturers to be much more thorough in their approach to materials and validation. Device validation according to ISO 11607 for terminally sterilised packaging, including sterile barrier system (SBS) packaging, can take from 6 to 36 months, depending on available resources, which is clearly a disincentive to changing packaging format and/or materials.

One way to address this issue and to benefit from a new material technology is to obtain data from the packaging supplier. For example, in the case of a film sealed to uncoated paper or Tyvek, the packaging manufacturer may use the ISO 11607 protocols not only for the film itself, but also for the SBS combinations of film with uncoated Tyvek or paper lidding. This allows data to become available for five year accelerated and real-time ageing after sterilisation. Device manufacturers can then use these data as part of their validation protocol and thereby save considerable amounts of time and money. As film technology advances further and the additional costs of material coating become unnecessary with the advent of more peelable films, as described above, the provision of additional data such as these will allow greater packaging choice and faster
packaging changes to become a reality.

Chris Heezen
is Executive Director, Global
Medical Flexibles, Sealed Air Medical Applications, 200 Riverfront Boulevard, Elmwood Park 07407, New Jersey, USA.

Nicholas Berendt*
is Marketing Manager, Sealed Air Medical  Applications, Fleets Lane, Poole BH15 3BT, UK  tel. +44 1202 781 241
e-mail: nicholas.berendt@sealedair.com [9]
www.nexcelmedical.com [10]

* To whom all correspondence should be directed.