Polypropylene, because of its exceptional overall properties increasingly finds applications growing in the medical sector, specifically in healthcare.
The growth of polypropylene (PP) in medical device applications progressed unabated throughout the last few decades of the 20th century and continues strongly into the future. The material possesses a combination of properties that makes it very useful for a wide variety of medical uses. It does not have the extreme clarity, stiffness and heat resistance of polycarbonate (PC), radiation resistance of polystyrene (PS) and polyethylene (PE), impact resistance of polyester (PET) or the flexibility of plasticized polyvinyl chloride (PVC), but it possesses high levels of all of these properties plus superior chemical resistance and a resultant utility that is unique among plastics and offers a great advantage of moderate cost. This combination of characteristics is expected to power the growth of PP in healthcare. The current worldwide consumption of PP in healthcare is estimated at around 350KT and is poised to grow at about 8.7% in the next few years.

With each advance in the clarity and impact resistance of PP, its ability to capture volume from competing polymers increases. Metallocene catalyzed homo polymer high-performance polypropylene (HPP) treated with the highest-performance 4th generation clarifiers and modified with an ethylene-based plastomer exemplify the type of stiff, impact-resistant, cost-effective PP-based compositions that can replace PC, PS, polymethylmethacrylate (PMMA) and other competitors in many medical applications.

Medical applications consume the second-largest volume of non woven fabrics in North America , of which the PP has a major share. Almost all of the PP used in medical non-wovens now is homopolymer, much of it still produced by the conventional Z-N catalysis. Non-woven textiles produced from Z-N HPP require relatively high temperatures for bonding of the mat in the strength-producing calendaring process and are relatively stiff fabrics with a somewhat unnatural feel. Z-N polyropylene random copolymers (RCPs) bond at lower temperatures and provide softer fabrics due to the lower modulus of the copolymer.

Metallocene-catalyzed PPs with narrower MWD than Z-N-produced polymers can yield finer fibers that provide better strength, improved barrier, better web uniformity and softer feel. Structural parts made from PP for medical applications fall into three categories: medical devices, medical device packaging, and packaging and delivery systems for solid and liquid pharmaceuticals and other medical liquids. The devices can be defined more broadly to also include medical and diagnostic labware, and packaging can be expanded to include nutritional supplement containers. In general, medical devices are injection moulded. PP usage in the injection-moulded device area totals 70% of all the polymer used in this application segment.

The single largest-volume PP medical device application is disposable hypodermic syringes. The largest fraction of the polymer used is clear, radiation-resistant HPP. Natural-colour, radiation-resistant HPP and clear, radiation-resistant RCP are also widely used. There is also a smaller volume of all configurations of HPP and RCP formulated for ethylene oxide and/or autoclave sterilization in smaller markets where the high volumes that make sterilization by radiation economical are not consumed. Closely related devices are needle shields, connectors and phlebotomy needle holders.

New developments in PP syringes and related devices are led by safety versions of the products. Several designs of these parts have seen relentless improvement over the years due to better designing. An example is the sliding sleeve modifications of syringes and phlebotomy needle holders into manually activated designs in which the needle was withdrawn into the barrel, now replaced by fully automatic, spring-loaded mechanisms by which the needle is withdrawn into the barrel directly out of the patient's body with no exposure after use. Success of these designs depends, to a great extent, on the properties of special PPs. Though radiation tolerance is required for these sterilized devices, transparency of clarified PPs in order to see and measure their contents easily is also very important to their functionality.

Development of automated, high-volume diagnostic tests and quantum leaps in biomedical research have led to expansion of labware applications of PP. Important labware devices are macro and microcentrifuge tubes, pipe tte tips, multi-well plates, diagnostic cuvettes, urine and sample cups. Newer device applications for PP include contact lens casting cup for lenses designed for a short-use life (one day to a few weeks) made from nucleated, higher-MFR, metallocene-catalysed HPPs. The PP is valued in this application for its resistance to the aggressive monomeric chemicals used to make the lenses, with accurate, precise and stable dimensions.

The lower MW and narrow MWD of these polymers make them very resistant to orientation and subsequent warpage, while the nucleation produces a polymer with a fast and highly reproducible crystallization rate. A major fraction of the polymer crystallizes while still fixed in the mould where the part takes on the dimensions of the tool better than one that experiences more of its crystallization and shrinkage after ejection from the mould. The higher resistance to softening at elevated temperatures of nucleated PPs makes cups moulded from these polymers better for thermally cured lenses than cups made from the traditional polymers. PP medical device packaging consists of both rigid containers and flexible pouches and lidstocks used to protect the sterility and properties of medical devices between manufacture and use. A variety of processes are used to produce the different packaging articles, including film casting, injection moulding and thermoforming.

The degradation of PP after sterilisation by high-energy radiation is due to oxidation of the radicals formed in the material during radiation. Therefore, stabilisation of PP to high-energy sterilizing radiation becomes increasingly more difficult with the increase in the ratio of the surface area of the material to its mass, or, in other words, with the increase in its relative exposure to oxygen. In general, medical device packages are very thin gauge with a high surface-area-to-mass ratio. For this reason, PP has not been widely used for the packaging of radiation- sterilized medical devices. With the advent of newer, more radiation-resistant formulations of PP, the potential for the packaging of radiation-sterilized devices in PP containers has grown considerably. Blends of PP and metallocene-catalyzed, ethylene- based plastomers are particularly suited to the construction of highly radiation-resistant, thin-gauge medical device packages. These could be used to produce thermoformed trays and blisters for packaging syringes, intravenous (IV) sets and many other common, radiation-sterilized medical device types.

Other common sterilization methods pose no difficulty, however, for the packaging of medical devices in containers made from PP. Most PP polymers especially HPPs, are suitably resistant to both ethylene oxide and autoclave sterilization conditions. One particularly successful example of medical device packaging is the primary packaging of short- use contact or disposable lenses. Injection-moulded blisters containing the lens in saline solution are sealed with moisture-impervious, multilayer lidstock, and the whole is sterilized with heat. The excellent moisture barrier of the monolayer PP blister provides long shelf life to the lens/saline product. PP is used for the packaging of medical liquids over a wide variety of application types from enterallyadministered nutritional supplements to parenterally administer red injectable drugs. All varieties of PP are used: HPP, RCP and injection-moulded copolymer polypropylene (ICP). Generally, the products are sterilized by heat, so the high resistance of PP to softening and deformation at elevated temperatures is a distinct advantage in this area, especially at the extreme upper end of the temperature range 120-140C Growth of this use of PP comes at the expense of glass, metal and other commodity thermoplastics like PVC. To maintain low extractables that are important in drug packaging, and excellent organoleptics that are so important in the nutritional packaging, very little use is made of CR PPs in medical liquids packaging, even in the minority of cases where injection moulding is used to produce the package. In these few situations, the need for high MFR and narrow MWD in an ultra-clean formulation not produced by CR makes metallocene -catalyzed PP an ideal choice. In cases like nutritional supplements, where an oxygen barrier is required to protect the freshness of the food product, multilayer structures are employed. Polyethylene-co-vinylalcohol, polyvinylidene chloride or nylon provides the required barrier, and adhesive layers tie the incompatible layers together. The range of converting processes used in this field is extremely broad, including extrusion (EBM), injection (IBM) and injection-stretch blow moulding (ISBM), sheet casting and sealing, thermoforming (TF) and injection moulding (IM).
Some specific examples of PP medical liquids packaging (and the conversion method by which they are made) are as follows:
• Inject able drug vials (IBM)
• IV drug bags (sheet casting and sealing)
• Irrigation solution bottles (EBM and ISBM)
• Fluid replacement bottles (EBM and ISBM)
• Nutritional supplement single serving containers (TF)
• Refillable syringes (IM)
• Bottles for over-the-counter liquid medications Like cough syrups and infant fever medication (EBM and IBM)
• Nose spray bottles (EBM and IM)

The recently introduced commercial syndiotactic (metallocene-catalyzed) PP is most likely to affect the properties of plastic medical goods in the rigid and flexible medical liquids packaging described previously. It is publicized as a toughening agent in blends with isotactic PP used in film, sheet and moulded containers and a highly effective, neat, low-temperature seal layer in multilayer film and sheet structures. The packaging of solid prescription and over-the- counter pharmaceuticals is a rapidly growing application for PP. As in liquid packaging, a broad range of conversion techniques is used to produce the packages for the solid drugs. Most prominent among these are all forms of blow moulding (including ISBM) ofbottles, thermoforming of individual pill blisters, and injection moulding of prescription pill vials and inhaler parts. These containers are not, in general, sterilized, so the choice of the type of PP used in their construction is controlled by the stiffness, impact resistance and clarity desired for the container.

Since it is so broadly applied in the medical field – including many product types and conversion and sterilization techniques – long-term expansion of the medical uses of PP is essentially assured. Posing neither environmental nor toxicological concerns, PP is well suited to displace less attractive materials in existing Applications and to compete strongly for inclusion in new areas. While the attractive projections for growth of PP medical volume undoubtedly consider these aspects of the material's utility, real breakthroughs in moulding techniques, sterilization, clarification, etc., could boost the use of this material even further.

High-power, ultraviolet light sterilization of PP medical devices and packaging, for example, is a development that is just now beginning to show promise. The eventual impact of this and other advances in PP technology on the medical usage of the material will surely be positive.