Page 215 - Biomedical Engineering and Design Handbook Volume 2, Applications

194 MEDICAL DEVICE DESIGN

acrylic-based plastic has very low structural flexibility, low impact resistance, poor clarity, but has a
very high radiation resistance. The polyethylene terephthalate (PET) plastics (polyesters) have excellent
clarity, structural flexibility, impact resistance, sealability, and radiation resistance, but only marginal
water vapor barrier and heat resistance. So each material has its favorable and unfavorable properties,
and the material that most closely fits the desired packaging application must be selected. The most com-
mon packaging materials for thermoform tray applications are discussed in some detail.
Polyethylene Terephthalate (PET). The generic material called PET, or polyethylene terephtha-
late, is probably the most widely used material for medical packaging applications due to its favor-
able characteristics as mentioned previously. This material forms easily in thermoforming operations
and provides good barrier performance and sealability with various lidding materials. The material
imparts excellent clarity, flexibility, and radiation resistance—all important characteristics for pack-
aging medical devices. It is produced in forms for injection or blow molding of rigid containers like
bottles and jars, and in sheet form for thermoforming trays, and blisters. When PET is coextruded with
other materials such as glycol to make PETG, the barrier performance characteristics of the material
are improved. PETG is not heat sealable, so the lidding stock must be adhesive coated to facilitate a
functional seal for the package. Table 7.1 provides some specific physical properties for PET materials.
Polycarbonate (PC). Polycarbonate is used for high-performance package applications where
high strength and toughness are required due to the size, density, or shape of the product. In some appli-
cations PC is used because of its superior clarity and the aesthetic appeal of the product. PC is the most
impact resistant of all the plastics but has only average moisture- and gas-barrier properties (Table 7.2).
The cost of PC is somewhat prohibitive in a high-volume product application. However, for low-volume,
high-priced devices, such as pacemakers, defibrillators, and other implantable devices, it is an excellent
material for thermoform trays. Most of the common sterilization methods, such as autoclave, steam,
ethylene oxide, gamma, and e-beam, can be used on packages made from polycarbonate. Typically, PC
film for thermoform applications is coextruded with a polyolefin heat-seal layer.
Polyvinyl Chloride (PVC) and Polyvinylidene Chloride (PVdC). The material known as PVC
polyvinyl chloride is one vinyl-based polymer used commonly in packaging applications. Another
TM
material in the same family is PVdC, also known as polyvinylidene chloride (SARAN ). These
materials differ from polyethylene in having a chlorine atom that replaces one hydrogen atom in its
chemical structure. This is important, since it is this chlorine atom that has caused the material to lose
favor for packaging applications due to environmental concerns. The environmental concern is that
when incinerated, the material generates a hydrogen chloride gas. Several European countries have
banned the use of vinyl-based materials. The criticism is controversial. The perceived environmental
threat has caused many PVC applications to be replaced by PET. PVC is used most frequently in

TABLE 7.1 Specific Physical Properties for PET Materials

PET
Molecular formula (C H O )
10 8 4 n
Density 1370 kg/m 3
Young’s modulus (E) 2800–3100 MPa
Tensile strength (σ ) 55–75 MPa
t
Elastic limit 50–150%
Notch test 3.6 kJ/m 2
Glass temperature 75°C
Melting point 260°C
Vicat B 170°C
Thermal conductivity 0.24 W/(m · K)
–5
Linear expansion coefficient (α) 7 × 10 /K
Specific heat (c) 1.0 kJ/(kg · K)
Water absorption (ASTM) 0.16
Refractive index 1.5750
Source: A.K. van der Vegt and L.E. Govaert, Polymeren, van keten tot kunstof,
ISBN 90-407-2388–5.