Find out about.......Plastics, Polymer Engineering and Leadership: March 2017

Friday, 24 March 2017

My Top 5 Commodity Plastics for Medical Device Applications – Part 2: PE

Welcome back to this blog series about my top 5 commodity plastics used for medical device applications.

Here the link to part 1 - PVC.

Let’s jump right to it:

Nr. 2 – Polyethylene (PE)

The second polymer of this commodity series is Polyethylene (PE).

PE is available in 4 different forms:

Low Density Polyethylene (LDPE): constituted of long-chain polymer branches which prevent packing (crystallization) leading to a low density material.
Linear Low Density Polyethylene (LLDPE): contains 10 to 35 low molecular weight (MW) side chains per 1000 carbon atoms constituting a main polymer chain. This allows intermediate packing.
High Density Polyethylene (HDPE): contains 4 to 10 low MW side chains per 1000 carbon atoms of a main polymer chain. This leads to excellent packing.
Ultrahigh Molecular Weight Polyethylene (UHMWPE): it is a linear very high MW PE with lowest amount of short chains. This leads to superior strength and stiffness.

A comparison of properties, advantages and disadvantages is shown in the Table 1 [1].

Table 1: Comparison between the different PE grades.

How does PE perform in terms of sterilization?

EtO sterilization is usually suitable for PE [1]. Steam and autoclave sterilization are not an option due the low heat deflection temperatures (30-50°C) of PE. High-energy radiation sterilization methods such as gamma radiation and e-beam maybe used on stabilized PE (using radiation sterilization on non-stabilized PE will lead to oxidation and cross-linking). In case PE contains phosphite-based stabilizers yellowing may occur upon sterilization.

What about biocompatibility?

In general, polyolefins are inert, non-polar and possess biocompatibility. Surface oxidation during radiation sterilization procedures lead to a reduction of PE’s biocompatibility. Consequently, radiation sterilization must be performed under inert atmosphere. Furthermore, the Cosmetic Ingredient Review Expert Panel concluded in their safety assessment of PE that this is non-toxic and shows no threat when used in cosmetics and medical applications [2].

Where is PE used in medical device applications?

Starting with LDPE, it has good flexibility, strength, and barrier properties at low costs. Furthermore, it has high clarity together with good tear and stress crack resistance. For example, it finds application in sterile blister packs for drugs. LLDPE has a superior flexibility and toughness and is, therefore, used for films and packaging. HDPE has a much higher crystallinity, improved chemical resistance and stiffness compared to LDPE and LLDPE. As a result, it is used in surgical and medical instruments. In addition, its high energy absorption, considerable impact strength and low wear makes it the ideal candidate for artificial hip, knee and shoulder joint replacement implants. LDPE, LLDPE and HDPE are also used in flexible tubing, where they strongly compete with PVC. UHMWPE is mostly used in arthroplasty implants. Herein, vitamin E is a key additive that improves wear and long term stability of UHMWPE [3]. In Table 2, below, you can find further details regarding the application of PE in healthcare.

Table 2: Examples for PE based medical device applications.

Where to get PE for your medical device applications?

HC grade certified thermoplastics suppliers of PE [1]:

Thanks for reading! Have a beautiful day & till next time!

Greetings,

Herwig

Literature:

[1] Vinny R. Sastri: Plastics in Medical Devices, 2014
[2] Cosmetic Ingredient Review Expert Panel. Int J Toxicol 2007
[3] Wolf, C.; Krivec, T.; Lederer, K.; Schneider, W. Examination of the suitability of alpha-tocopherol as a stabilizer for ultra-high molecular weight polyethylene used for articulating surfaces in joint endoprostheses. J. Mater. Sci. Mater. Med. 2002, 13, 185–189.

Thursday, 23 March 2017

My Top 5 Commodity Plastics for Medical Device Applications – Part 1: PVC

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.