Materials

The Use of PVC for Critical Medical Applications

Dr. CSB Nair, Director, Research & Development, TERUMO PENPOL Ltd, Trivandrum, Kerala State

The Versatility of Polyvinyl Chloride

Vinyls are characterised by their toughness, inherent strength, abrasion and chemical resistance. Poly vinyl chloride can be compounded with plasticisers, stabilisers, lubricants and other additives to achieve a wide range of flexibility, transparency and resistance to high temperature (upto 120oC) and low temperatures (down -80oC). A distinct advantage of PVC materials is the wide range of sterilisation options available - it cn be autoclaved with steam or sterilised by gamma ray irradiation or by ethylene oxide. Yet another important feature is its ability to be welded by high frequency which enables the production of leak-free products and offers infinite design possibilities.

Because of the unique range of characteristics, PVC is the favoured material for use in food, beverages and medical applications.

Typical Medical Applications of PVC

The wide range of applications of PVC is evident from the following:

1. Containers for intravenous/dialysis fluids: PVC is the only suitable transparent flexible container material when solutions containing dextrose have to be sterilised. Steam sterilisation alone is applicable in such cases. Gamma ray irradiation would cause radiation induced degradation of dextrose. Ethylene oxide is absorbed by aqueous solutions followed by generation of toxic hydrolysis products.

2. Containers for the collection and storage of blood and blood products.

3. Soft shell oxygenator

4. Cardiotomy reservoir

5. Catheters

6. Infusion sets for blood and iv fluids

7. Endo tracheal tubing

8. Heart lung bypass sets

9. Wound drainage tubes

10. Chest drainage tubes

11. Colostomy bags

12. Oxygen therapy equipment

13. Drip chambers

14. Respirator masks

15. Rigid extruded luers and connectors

16. Ventilator fittings

17. Extruded profiles

18. Resilient vinyl tubing with low surface friction

19. Clear, semi rigid injection moulded components

20. Cardio vascular tubing

21. Heat exchanger coils

22. Surgical drapes

23. Examination gloves

24. Beverage cap liners

25. Beverage containers

Compounding is the Art which brings out the wide range of Characteristics of PVC

The desirable characteristics are built into PVC by the process of compounding which blends PVC with plasticisers, stabilisers, lubricants, viscosity modifiers and other additives. The selection of stabilisers is particularly critical. Powerful stabilisers based on tin, lead, cadmium and barium can not be used for food or medical applications. For such pruposes, comparatively milder stabilisers based on calcium, zinc and magnesium have to be used, preferably blended synergistically with epoxidised soya bean and/or linseed oils. Non toxic organic phosphites and other additives are added to improve stabilisation and colour. Lubricants help to improve flow characteristics of the PVC melt and also prevent metal to PVC-melt adhesion in the extruder. It will be advantageous to incorporate viscosity improves to optimise melt flow.

The plastic material must be so designed that it does not contribute substances to the preparations stored within or extract

Table-1 : Plasticisers for PVC in Medical and Food Contact Applications

material from it in quantities sufficient to affect the stability of the preparation or presnet risk of toxicity to the user. Product manufacturers should ensure that there is no change in composition or manufacturing process so that the containers from bulk production conform to the type sample in every respect.

Finding the right type of additives to achieve optimum operating characteristics still remains an art to a very great extent.

Choice of Plasticiser

A wide range of plasticisers are available for food contact and medical applications (Table-1). When the PVC is designed to contact blood and blood products, 

however, the choice is extremely limited. The principal plasticiser used for such applications is di, (2-ethyl hexyl) phthalate (DEHP which is often called DOP) which offers the benefits of overall performance, ready availability at high purity and cost effectiveness. Other plasticisers often favoured for selective applications such as for the storage of platelets are tri, (2-ethyl hexyl) mellitate (TEHM, also called TOTM), n-butyrul, tri n-hexyl citrate (BTHC) and di (n-decyl), phthalate (DnDP). PVC material used for contact with blood and blood products, must in addition to meeting physical, chemical and biological requirements have sufficient permeability to oxygen and carbon dioxide for the specific application envisaged.

DEHP - The Preferred Plasticisers

The plasiciser DEHP was under a cloud of suspicion regarding various deleterious effects ever since Jaegar and Rubins of John Hopkinds University reported in 1970 that blood collected in DEHP plasticised bags contained leached plasticizer. Since then numerous studies were conducted on the possible harmful effects of leached DEHP and there are over 3000 research publications on the subject. Subsequent assessment of these and other studies by the European Pharmacopoeia Committee (1989), the BUA - the West German advisory board of environmentally relevant substances, the CEFIC Review (1995), the expert panel convened by the American Council on Science and Health (1999) have all absolved DEHP of the various harmful results reported earlier. DEHP has in fact beneficial effects and improves the survival time of erythrocytes both in vitro and in vivo. DEHP plasticised PVC has also been shown to have very low adhesion to platelets. [ Next Page ]

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