Find out about.......Plastics, Polymer Engineering and Leadership: My Top 5 Commodity Plastics for Medical Device Applications

In this blog post series I will present to you my top 5 commodity plastics for medical device applications.

Just before we start, we should answer the following question: why are plastics so successful in healthcare applications nowadays?

I think there are easily over 20 reasons why the use of plastics is absolutely beneficial. Just to name the most important ones:

A wide range of tailor-made materials is possible.

Plastics processing technologies allow great freedom when designing parts.

It is possible to produce micro component parts as well.

Plastic parts allow the assembling of light weight structures.

Part’s transparency and impact resistance are simultaneously possible.

Chemical resistance including lipids resistance is possible.

Mass production can be easily accomplished enabling economies of scale.

Plastic materials are commonly divided into 3 major categories, i.e., commodity plastics, engineering thermoplastics and high performance plastics.

We will start from the bottom with the commodities. In terms of use, 70% of the medical device applications use commodity plastics [1]. In my opinion the following are the most important ones:

Polyvinylchloride (PVC)

Polyethylene (PE)

Polypropylene (PP)

Cycloolefincopolymers (COC)

Polystyrene (PS)

In this blog series, I will discuss each of these 5 materials in terms of medical device related design questions, sterilization capability and biocompatibility. In addition, I will show application examples as well as material suppliers.

Nr. 1 – Polyvinyl Chloride (PVC)

PVC-based materials are usually characterized by their K and Shore hardness values [2].

A common K-value for PVC is between 50 and 80. Higher K-values indicate superior mechanical properties as well as high processing temperatures. For injection moulding operations a K-value around 57 is suitable, whereas for rigid extrusion the K-value should be around 67. K-values above 70 are better suitable for calendaring operations.

PVC hardness can be easily adjusted by the addition of a plasticizer or by blending with other polymers (see below). Consequently, PVC-based materials may range from very soft and flexible to very hard and rigid.

Plasticization of PVC

Plasticized PVC (PVC-P) will have improved flexibility as well as reduced hardness. To plasticize PVC the addition of 40% to 65% plasticizer is usually necessary. The most used plasticizer is di-(2-ethyl hexyl phthalate) (DEHP). In the past years, the utilization of phthalate-based plasticizers such as DEHP has been associated with potential carcinogenic effects. Nevertheless, the studies conducted so far have failed to substantiate the risk associated to the utilization of DEHP in medical devices. The European view on DEHP in PVC was published by Eucomed (European medical technology industry cooperative body) via a position paper. They conclude that the many benefits of DEHP plasticized PVC in medical products offset any perceived risks [3, 4]. Current potential replacements to DEHP in this context are e.g. epoxidized soybean oil/linseed oil and Acetyl n-tributyl citrate.

Blending of PVC

A major motivation for blending PVC with other polymers is to obtain a material which shows similar properties to plasticized PVC in terms of toughness, flexibility and processability without the risk of plasticizer leaching. Furthermore, this can be achieved at a reasonable cost. Polyolefins and polystyrene are, due to their non-polarity, not miscible with PVC. Following are some possible blend combinations:

PVC/ABS: improved impact resistance without losing tensile strength and high heat resistance.
PVC/PMMA: good balance between toughness and impact resistance over a wide range of temperatures.
PVC/EVA: flexibility, toughness and clarity.
PVC/EVA-CO: clarity and permanent plasticization
PVC/NBR: permanent plasticization

How does PVC perform in terms of sterilization?

Generally, steam sterilization is no problem for plasticized PVC. Conversely, unplasticized PVC will start degrading. Therefore, it is better to use ethylene oxide (EtO) sterilization. EtO as well as low-temperature steam sterilization can be used for rigid and plasticized PVC. Sterilization by high-energy radiation will lead to chain scission degradation unless free radical scavengers and antioxidant stabilizers are applied [5].

What about biocompatibility?

Yes, PVC is biocompatible and hemocompatible. The latter can be further enhanced by coating devices with heparin (blood thinning medication).

Where is PVC used in medical device applications?

If we can believe most market estimates, approximately 25% of all polymer-based medical applications are made of PVC [1]. A major motivation to use PVC is that PVC is in use for 50 years without leading to any toxicological effects to the end-user. Consequently, healthcare authorities have classified PVC as safe.

For example, alongside flexibility PVC can exhibit good transparency, which is perfect for making flexible tubing such as infusers and catheters where to visually monitor contained fluids is desirable. In this context, PVC is also utilized for making containers such as flexible bags for intravenous fluids as well as storage bags for blood, plasma and urine. Furthermore, PVC is known for its toughness and strength (also at low temperatures). For this reason, PVC is used to make protective gloves which must have high resistance to tear propagation. Table 1 below shows a sum up of example applications of PVC in healthcare.

Table 1: Examples of PVC-based medical device applications.