High Altitude as a Distribution Hazard
The majority of sterile medical devices are transported by air or sea or rail; all will travel by road for at least part of their journey from manufacturer to the eventual user.
Sea transport is safe from high altitude effects, but in various parts of the world, the other transport modes are not. According to the car altimeter, I have been driven at above 4,500 metres on Tibetan roads. Medical devices may require to be used in an emergency at a mountain rescue site. Hospitals exist at high altitudes such as prevail in the Bolivian mountains. The Chinese have built a passenger railroad at Tanggula Pass (elevation: 16,640 ft.; 5072m) and a railroad tunnel at Fenghuoshan (elevation: 16,093 ft.; 4905m); this is the only railway into Lhasa from outside Tibet. Packages may be stressed during an emergency ascent to avoid bad weather when being conveyed in an unpressurised aircraft. Cabin pressure within an aircraft will drop dramatically as it is equalised with external pressure during landing approach at high altitude airports.
IATA Dangerous Goods "Regulations" advise those shipping dangerous goods by air that "due to altitude, pressure reductions will be experienced under flight conditions which may in extreme conditions be in the order of 68kPa (0.68 bar, 10lb/sq. in.)" Such pressure reductions may lead to "bursting of the receptacles or packages during flight."
We may not be transporting dangerous goods, but it is clear that if our medical device packaging is to tolerate such conditions, our package designs must either allow for pressure equalisation, or seal strength must be such that the closure will remain in place despite the pressure differential. When products are to be transported or stored under high altitude conditions, the packaging designer must also consider the potential for seal creep or even package explosion.
The properties of peel seals on pouches and thermoformed tray lids, produced from impervious materials, can only be specified after consideration of the potential bursting force that will be encountered. Multiplying the area of a blister opening by the expected pressure reduction enables a rough estimation of the total force that the package seals must resist to prevent failure. See The Engineering Toolbox for a table of altitude and mean atmospheric pressure.
The seal strength chosen should not exceed the opening force that it is reasonable to expect a nurse to apply when wearing two pairs of surgical gloves with the added impediment of body fluid contamination. Generally the strength of a peel seal falls in the range of 400 mg to 1500 mg per 25mm width (measured at 180° peel angle and 300mm crosshead speed).
There are two approaches to avoid unnecessary seal stress when a non-breathable package will be subjected to major changes in altitude:
- Minimise the presence of air within the package. In the food industry, when packaging a liquid into a plastic pot with an impermeable lid, a common approach to prevent lids blowing is to totally fill the pot so that there isn't any air to expand.
- Design the package so that its capacity can increase to accommodate expansion of its contained air. An example of such a package is a loose-fitting pouch from which the air is partially evacuated as it is closed and sealed. With this type of flexible package, when the internal pressure exceeds the external atmospheric pressure, the small amount of air contained will inflate but with little force applied to the seals.
Testing finished packages in a vacuum chamber will determine if the design is compatible with air pressure changes.
Rolande Hall, FIMMM Pkg Prof