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.













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