Medical Device Link.

MATERIALS

Winning combinations

A successful medical packaging line is the result of balancing the limitations, possibilities and needs of the packaging materials with those of the packaging machinery on which it is to run. Horizontal-form-fill-seal (HFFS) machinery is commonly used for packaging medical devices, particularly for products with a relatively high profile, and it can produce a variety of packaging styles, including

• flexible breathable pouches
• flexible nonbreathable pouches
• rigid trays with breathable lids
• rigid trays with nonbreathable lids.

Of course, the design of the machine and the materials selected are dictated by the nature of the product and the sterilisation process; however, to a large degree, the material used to make these various packages determines the machine design, and the details of design can dramatically drive up cost. The final cost of the machine is a function of the complexity of design and construction (largely determined by material selection) and ranges from
€100 000 to €500 000 or more.

Basic elements of HFFS machinery

Flat products such as surgeon’s gloves and sterile sponges are best handled by packaging machinery that simply brings material together around the product, executes the seal, performs the cut and discharges individual packages. However, it is advisable that high profile medical products such as syringes and disposable instruments are packaged on HFFS machinery.¹ This machinery is an intermittent-motion system with inline thermoforming and sealing capabilities, typically completing one or more packages per cycle. There are three basic elements of the HFFS machine that can be affected by material selection.

Forming. The forming station is the first major station of HFFS machinery. Here, a formable bottom-web is moved into the forming station and there, using one of a variety of forming methods, a number of prescribed cavities are formed that accommodate the shape of the product to be packaged. The pocket can be formed using vacuum, positive air pressure, male plugs or a combination of all three. The method chosen depends on the complexity of the package and the packaging materials selected. The cost for forming tools can vary widely.

Sealing. One of the strengths of the HFFS method is its ability to generate consistent, reliable seals with a variety of materials. The dual requirements of a robust hermetic seal, as well as the necessity of providing a package that can be opened consistently without tearing, differentiate medical packaging from other consumer packaging produced on HFFS machinery.

Figure 1: Heat seal profiles. (click image to enlarge)

Cutting. The cutting method chosen depends on the materials being used. Rigid materials such as polyethylene terephthalate glycol (PETG) and high impact polystyrene require more sophisticated and more costly cutting systems than those used for flexible packaging. “With cutting systems, especially those for rigid film, there are always balances between flexibility and/or investment cost and cutting quality,” remarked Stefan Krakow of Technical Packaging Applications at CFS (Tiromat) Germany GmbH (www.tiromat.com).

Packages produced on HFFS mach-inery fall into two categories: flexible packaging and rigid packaging. In general, flexible packaging systems are less complex and result in lower material cost per package; the machinery used to produce flexible packaging is also lower in cost.

Flexible packaging systems

Flexible packaging systems have flexible top webs as well as flexible bottom forming webs. Sponges, dressings, unfilled syringes, tubing and intravenous sets typically use flexible packaging. Polyolefin or polyamide are the most commonly used materials for forming web structures and thicknesses typically range from 60 to 300 µm. The thickness used is determined by the depth of the formed cavity, the cavity design and the nature of the product.

A variety of top webs are used for flexible packaging, including breathable and nonbreathable materials. Packages sterilised with nonparticulate forms of sterilisation such as gamma sterilisation need not be porous. Packages sterilised with ethylene oxide or steam must allow the sterilising agent to penetrate the package and to also be extracted from the package, but at the same time the substrate must also be an effective microbial barrier. The porosity of the package is critical to optimising the sterilisation cycle. More porous structures allow for better gas penetration and shorter venting cycles, which are required to minimise ethylene oxide residuals.

Figure 2: Forming station for simple flexible package.

In general, sealing and peeling characteristics are a function of the top web, and the bottom web is formulated with formability and performance in the distribution process in mind. For breathable applications, a variety of substrates are used for the top web, including papers and Tyvek, a spun bonded polyolefin from DuPont (www.dupont.com). Most of these substrates in uncoated forms have narrow operating windows, which means that the range of parameters that provide consistent seal strength with consistent peel characteristics is narrow. For this reason, heat seal coatings are widely used for medical packaging (Figure 1). Heat seal coatings, whose seal strengths vary less as the sealing temperature changes, will perform more consistently over a wider range of sealing conditions. For example, product C would be much more tolerant of fluctuations in sealing temperatures than product A or product B. In fact, as the slope of the curve for the heat seal profile approaches zero, the greater the process capability (Cpk).

Tyvek is the preeminent lidding stock for breathable top webs; it provides maximum barrier properties for superior tear and puncture resistance as well as maximum porosity, which allows for improved sterilisation cycles. Tyvek can be used in uncoated forms on occasion, but it is more often used in coated forms. This is because Tyvek has a much greater tendency to tear on opening in uncoated forms. Uncoated systems, whether paper or Tyvek, typically do not provide the optimal balance of seal strength and consistent peelability that can be achieved with coated systems. Coated and uncoated papers are also used in applications where there is less concern for tearing on opening. Tearing on opening poses a threat to the sterility of the device being used, thus paper, particularly in uncoated forms, has its limitations. Today, there are reinforced paper alternatives to paper or Tyvek (www.olivermedical.com) and there are also nonbreathable top webs used for gamma sterilisation including coated high density polyethylene film, coated polyethylene terephthalate (PET) and polyethylene (PE) foil laminations.

Rigid packaging systems

Rigid packaging systems are used where there is a need to keep the product secure in a fixed position, where the product presents serious challenges in terms of puncture resistance, and where the product cannot be effectively presented to the sterile field when packaged with flexible materials. Procedural kits, joint implants, prefilled syringes and instruments fall into this category. Rigid packaging is typically more expensive per square metre than flexible packaging and also poses more serious challenges to the machine design.

The most commonly used materials for rigid forming bottom webs in medical packaging include polystyrene, PETG and poly(vinyl chloride). Less often, polycarbonate (PC) and a few other materials are considered because they have unique qualities such as resistance to high heat, which allows them to be steam sterilised. However, this resistance to high temperatures means that the material must have high forming temperatures and this requirement presents major implications for machine design. High capacity heaters are required to provide consistent forming temperatures and more extensive insulation is needed to isolate and dissipate these high levels of heat. In rigid applications, material thickness can range from 300 to 750 µm.

Just as with flexible packaging, the top web used with rigid bottom webs may be breathable or nonbreathable and in both cases the same considerations would apply to lidding used for flexible packages, but with the addition of one other consideration: paper systems are less desirable as a lidding stock for rigid trays because they have a greater tendency to tear. Coated Tyvek is the preferred lidding stock for rigid tray applications.

Important implications of flexible packaging material

Flexible packages are generally cheaper than rigid trays and the machinery required to produce rigid trays is more expensive than the machinery required for flexible packages. With reference to the three basic elements of inline HFFS machinery described earlier, the following points should be considered.

Figure 3: Forming station with plug assist. (click image to enlarge)

Figure 4: Comparative performance of various lidding stock. (click image to enlarge)

Forming. Tools for forming flexible packages are relatively simple (Figure 2). After the material has been heated to the appropriate temperature, it is formed into the desired shape using vacuum or positive air pressure or a combination of both. In more difficult applications plug assist may also be used to help distribute material more evenly (Figure 3). This is the use of a mechanical assist to move the molten film into an optimal position for forming before the vacuum or positive air forming is applied.

Sealing. Sealing is always a critical area in medical packaging systems because the final package must provide a bacterial barrier, protect the product, allow sterilisation and survive processing and distribution. It must also open without tearing and allow the product to be easily accessed and extracted by the practitioner. Figure 4 shows the comparative performance of various lidding stocks.²

The Tyvek substrate is extremely puncture and tear resistant and the coating ensures consistent seal strength and peel characteristics, which minimises the impact of deviation in sealing parameters. Uncoated papers are lower in cost but they have narrow heat seal profiles. This means that to ensure consistent seals and peel strengths, the machine must provide consistent heat, pressure and dwell time with minimal variation. Papers in preprinted applications also create a challenge to print registration.³ With extensible materials such as Tyvek, print registration is more easily achieved by stretching the material into registration.

Cutting. Most HFFS machinery has two cutting systems, a cross cut system and a longitudinal cutting system, and in both cases there are a variety of alternatives for cutting.

• Cross cutting generally takes place after sealing. The most common cross cut systems for flexible materials are guillotines and flying knives. The choice of the system depends on the nature of the materials. “Guillotine systems most often with serrated blades are reliable and long lasting cutting systems,” according to Luc Van de Vel, Business Director for Medical, Consumer and Industrial Products at Multivac (www.multivac.com). He adds, “The knife blades in flying knife systems wear faster, and because they wear, machine stoppage due to incomplete cutting is more frequent.”
• Longitudinal cutting is typically the final step in the process before finished packages are discharged and there are two common types of cutting systems used on HFFS machinery: shear cut and squeeze cut systems. The system chosen will generally depend on the elasticity of the material being cut and its inherent abrasiveness. Squeezing knives are used more often for thinner materials, but they wear at a faster rate and as they dull they can generate incomplete cuts, which can result in more frequent machine stoppages and generate more particulates. Shear cut knives produce little particulate matter and last much longer, which results in cleaner cuts and better package separation; however, they are slightly more difficult to position properly.

Important implications of rigid packaging material

As mentioned, machinery required for rigid packaging is generally more expensive than machinery built for flexible packaging. Rigid materials are much more demanding in terms of forming and cutting and can also present challenges in sealing. In contrast to flexible forming webs, which often incorporate a PE outer layer that improves the consistency of the seal strength, rigid forming webs and rigid films have less receptive surfaces for sealing. Referring to the three basic elements of inline HFFS machinery, the following need to be considered.

Forming. The complexity of the tray to be formed and the product characteristics dictate forming tool design and the thickness of the material to be used. Commonly used rigid materials can range in thickness from 300 to 600 µm and in some cases up to 1500 µm. It is more difficult to heat material evenly as the gauge increases and the consistency with which the film is heated has a major affect on the quality for forming. As Luc Van de Vel, Business Director for Medical, Consumer and Industrial Products at Multivac, points out, “The amount of energy required to heat rigid material is much higher than the amount of energy required to heat flexible materials. Depending on the output requirements, it may even be necessary to use sandwich heating systems or even multiple sandwich heating stations.” Furthermore, the choice of material can affect the quality of forming. For consistent forming, some materials such as PETG require a tighter operating range.

In the case of thin gauge shallow draw applications with generous radii, tooling may not be much more sophisticated than the forming tools used for flexible films. For heavy gauge films and package designs with a deep draw, it can be difficult to pull heavy material into cavities of small radii (to ensure good material distribution and avoid thin spot). As a result, a mechanical plug assist may be used with thick film applications or with difficult cavity designs. The plug will mechanically push the material into place before the vacuum is actuated, thereby pulling the material into the final package shape.

In extremely difficult rigid forming applications, a combination of vacuum and positive air pressure and plug assist may be required. Depending on the size of the package and the complexity of the package to be formed and its depth, forming tools for rigid applications “can cost two to three times as much as tools for forming flexible films,” asserts Van de Vel.

Material selection has significant implications on the heating of the film prior to forming. This is particularly true when considering materials with high melt points such as PC. As the melt point of the material and as the size of the forming area increase, it becomes more difficult to heat the film evenly across the entire web. To maintain a tight temperature range, more heating capacity may be required and subsequent to forming, consideration may need to be given to then cooling the formed film as well as to dissipating heat transferred to the greater forming station.

Cutting. For rigid films, significant consideration needs to be given to cutting, particularly to the method of cross cutting. According to Van de Vel, “the cost of a matched male–female cross cutting system for rigid packages can be between a factor 15 and 20 more expensive than cutting tools for flexible materials.”

Figure 5: Strip removal to eliminate sharp corners. (click image to enlarge)

There are two conceptual means of cutting rigid film. The lowest cost method is to use replaceable steel rule dies. Here, a hardened strip of steel is applied against an anvil and the cut is made under high pressure. Although this system has the lowest cost and can be adjusted more readily to accommodate changes in film thickness, it is more prone to inconsistent performance such as incomplete cuts or requires more attention and frequent replacement.

The most effective alternative, but also the most expensive approach, involves the use of a matched male–female tool and die. This system operates under a different principle than the steel rule die in that the cut is actually made by sheering the material at the interface between the tool and the die. A properly designed tool and die can have a long life with consistent cutting and little maintenance. However, the tool and die are built to specific clearances dictated by film thickness and composition. This is a major consideration because changes in material thickness and even changes in composition after tools have been built can result in incomplete cutting or premature wear. The fact that large punch and die cutting systems can easily exceed €50 000, and in extreme cases can approach €100 000 or more, means this is a critical area in the selection of material and the design of the machine.

When running rigid materials fundamental questions must be asked at the start. To avoid producing trays with sharp corners special measures must be taken (Figure 5). The only way to ensure there are no sharp corners is to remove a small width of material between each lane when making the longitudinal cut in conjunction with a contoured cross cut knife.

Finding the right balance

Successful medical packaging is achieved with a balanced system. It is the result of taking full advantage of the possibilities of material and machinery within their limitations. The greatest advantages of HFFS machinery are that it can handle a variety of materials and package designs. This flexibility in material selection, package design concepts and machine design possibilities can result in a number of scenarios. As the packaging design is conceived and refined, any decision made relative to packaging selection or machine design must be reconciled against the limitations and possibilities of the other.

John P. Merritt, BA, MSME, MBA, CPP, is International Managing Director, Oliver Medical Packaging, 445 Sixth Street Grand Rapids, Michigan, 49504 5298, USA, tel. +1 616 456 7711, e-mail: jmerritt@olivermedical.com, www.olivermedical.com