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)
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
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.
Where
to get PVC for your medical device applications?
HC grade certified
thermoplastics suppliers of PVC [1]:
Thanks for reading! Have a
beautiful day & till next time!
Greetings,
Herwig
Literature:
[1] Vinny R. Sastri: Plastics
in Medical Devices, 2014
[2] EN ISO 1628-1: Fikentscher
K value
[3] Joel AT, Ted S, et al.
Health risks posed by use of DEHP in PVC medical devices: a critical review,
Am. J. Indus. Med. 2001
[4] EUCOMED Position on the
Use of Phthalate Plasticized PVC in Medical Products: www.medicalplast.com/upload/documents/document4.pdf
[5] Clough RL and Gillen KT:
complex radiation degradation behavior of PVC material, Radiat. Phys. Chem.
1983
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