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Medical Device Design using Polymer Compounds

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Plastics—more specifically, specially formulated polymer compounds—make possible a wide range of components used in medical device design, from syringe bulbs and blood-pressure cuffs to catheters and bags for intravenous (IV) fluids. Medical tubing includes peristaltic tubing used to pump fluids and multi-lumen catheters that can be used, for example, to administer multiple IV solutions or drugs at once. Medical compounds are essential for items used in surgery, including Yankauer suctioning tools, oxygen masks, and drip chambers. These plastic compounds are needed for gaskets, cushioning, insulation, bag spouts, caps, fittings, and luers.

How can Polymers be validated for design inputs?

Because medical compounds are used in these applications that are critical to life and health, they have specific requirements for biocompatibility, sterilization, and material physical performance; these qualifications help the devices in which they are used to gain regulatory approval.  The following article describes some of these characteristics, which can be used to validate whether a material will be fit for a specific use.

What is biocompatibility?

To be biocompatible, a material or device must not produce any adverse response in the human body. Compounds used in medical devices must meet the biocompatibility requirements of the application for which they are used. Regulatory agencies typically require the final medical device, which might include several different types of materials, to be tested for biocompatibility for a specific application. Although agencies approve the device, not the materials, materials can be tested ahead of time to validate that they are generally biocompatible.

Standards for measuring biocompatibility include those in the United States Pharmacopeia (USP), published by the non-governmental organization with the same name; the European Pharmacopoeia (Ph. Eur.), published by the European Directorate for the Quality of Medicines and Healthcare (EDQM); the Japanese Pharmacopoeia (JP), published by the Japanese Pharmaceuticals and Medical Devices Agency, and others. The globally accepted standard, however, is the International Organization for Standardization (ISO)’s ISO 10993, “Biological evaluation of medical devices—Part 1: Evaluation within a risk management process.”

USP General Chapter <87> “Biological Reactivity Tests, In Vitro,” describes three tests (agar diffusion, direct contact, and elution) used to determine biological reactivity of mammalian cell cultures in contact with polymer materials. The tests look at cytotoxicity (i.e., if and to what extent a material is toxic to cells).

USP General Chapter <88> “Biological Reactivity Tests, In Vivo,” details tests that can be used to classify materials into one of six classes (I to VI), based on biological reactivity under certain conditions related to the intended end use. USP Class VI requires the most comprehensive testing. According to US Pharmacopeia, if the material is to be exposed to a sterilization process prior to end-use, the tests should be conducted on specimens preconditioned under the relevant process. PVC Compound 2222C-85 is an extrusion-grade PVC that is widely used for urological tubing. It meets all requirements for biomedical Class VI devices.

Are medical compounds latex-free?  

Some people may be severely allergic to the proteins found in natural latex. For these patients, even a trace amount of latex or latex in even a minor component of a device can produce a life-threatening reaction. Mexichem Specialty Compounds medical compounds do not contain latex and are not processed on compounding lines with any latex materials, so they are latex free.

Do medical compounds contain phthalates?  

Flexible PVC has historically used phthalate-based plasticizers. Due to some potential health and safety concerns, some ortho-phthalate-based plasticizers have been eliminated or have had their use restricted in some countries. There are many plasticizers, however, that are not restricted and can be used safely in PVC. Some non-phthalate plasticizers are considered acceptable and there is a range of phthalate-free plasticizers being used in medical PVC compounds. TPEs are an alternative to PVC that does not use plasticizers at all.

Do medical compounds contain animal by-products?

In the past there was some concern that ingredients based on animal by-products (e.g., tallow, glycerol, casein) could have a risk of containing an infectious prion protein that causes TSE (Transmissible Spongiform Encephalopathy). Although animal-based tallow has been confirmed to be safe against the spread of TSE when processed properly, some customers wish to eliminate all animal by-products. Animal-based ingredients can often be replaced with vegetable-sourced or petroleum-based products to make compounds without any animal by-products.

For more information, Read a Case Study about PVC Compound ‘Animal By-Product Free’ solutions for Medical and Dental Products.

What are the physical performance requirements for medical compounds?

In addition to being non-toxic and sterilizable (as discussed previously in this article), compounds need to have the strength and flexibility appropriate for their intended use. For example, some uses may require flexible materials (with durometers in the 35-95 Shore A range), while others may require the material to be stiff, but still pliable (i.e., semi-rigid, with durometers in the 55-70 Shore D range). Other uses may require a rigid material (typically over 75 Shore D hardness) with good impact resistance. Compounds used in tubing should have kink resistance, so that the tube won’t collapse when it is bent. For some uses, water-blush resistance is important, meaning that there will be no migration of ingredients to the surface of the device upon contact with water or immersion in water. Applications such as drip chambers need a compound with high clarity. Compound formulators design materials to have the best balance of properties for a given application.

What does it mean to be sterilizable?

Medical devices are typically sterilized after they are manufactured to ensure that no pathogenic organisms are present on their surface. In most cases, compounds used in these medical devices must be sterilizable. In other words, they must be stable under sterilization conditions, and the material must not break down, degrade, or suffer a major loss of physical properties and/or aesthetics.  

Methods of sterilization include autoclaving, which uses saturated steam (i.e., moist heat sterilization), and gaseous chemical sterilization. Sterilizing by irradiation with high energy (e.g., electron beam or gamma rays) requires specially designed compounds to withstand these conditions.

What regulatory standards must medical compounds meet?

Many countries have a regulatory body that monitors and approves medical devices according to that country’s specific requirements. In the US, devices are regulated by the Food and Drug Administration (FDA)’s Center for Devices and Radiological Health (CDRH). Health Canada’s Therapeutic Products Directorate (TPD) authorizes devices in Canada under the Canadian Medical Device Regulations (SOR/98-282). In Japan, the Ministry of Health, Labor and Welfare (MHLW) administers Ministerial Ordinance 169 for medical devices. In Europe, devices must have a CE mark, which indicates compliance with the appropriate directives (e.g., the Medical Device Directive). Most regulations cover the finished medical device, not the materials going into those devices. However, device makers should consider the characteristics that will be needed, such as whether the materials are biocompatible and sterilizable.

Several chemical regulations also affect medical compounds. The Registration, Evaluation, Authorisation, and Restriction of Chemicals (REACH) is a comprehensive regulation for the use of chemicals and substances in the European Union. Additives used in medical compounds in Europe must comply.  

The European Union’s Restriction of Hazardous Substances (RoHS) directive was originally designed for reducing hazardous materials (e.g., heavy metals such as lead and mercury) in electrical and electronic equipment.  An updated version (RoHS 2) expanded the scope to include medical devices and additional restricted substances, including the phthalate plasticizer DEHP, which is widely used in medical plastics. To comply with RoHS by 2021, medical plastics containing DEHP will need to be reformulated with alternative plasticizers. 

Other common concerns for medical compounds

What are FDA sanctioned ingredients? The US Food and Drug Administration (FDA) regulates materials that come in contact with food. The components of a compound can be approved by FDA for direct or indirect food contact. This approval is not a requirement for medical devices, but it is often considered an indication of product purity. 

Drip  chambers, tubing caps and connectors benefit from the sparkling clarity, high thermal stability, and easy mold release of PVC Compound 3006-90.

Drip chambers, tubing caps and connectors benefit from the sparkling clarity, high thermal stability, and easy mold release of PVC Compound 3006-90.

 

What are biobased materials? Traditionally, plastic compounds have been made from petroleum-derived products. Today, some plastic compounds are made instead with biobased materials. These can include polymers made from renewable resources as well as biobased additives, including plasticizers, stabilizers, and lubricants. Biobased materials are considered more sustainable and environmentally friendly. 

What polymers are used in medical product design

Different types of polymers have been used for many years in medical devices. The most-used polymer for medical device applications has been PVC, because of its properties such as versatility, availability, clarity, kink-resistance, and low price. For enhanced properties such as resilience, abrasion resistance, and chemical resistance, PVC alloys (e.g., with polyurethane, ethylene vinyl acetate [EVA], and polyacrylonitrile [pAN]) have been developed.

Thermoplastic elastomer (TPE) compounds have been developed as an alternative to PVC. TPEs are flexible without containing plasticizers and are used for demanding medical applications requiring phthalate-free, soft touch, and high elasticity.  

 

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