Have you ever thought of the journey that a medical device goes through before it reaches the OR? Who ensures that the device is sterile when it arrives in the hands of the surgeon? One major thing to consider, that most do not, is the multiple different types of vibrations that a device might encounter during transportation. In the medical-device industry, vibration testing is critical because unforeseen damage from vibration can cause significant repercussions.

“The problem with vibration is that most of its damage is unseen until the product is delivered and about to be used,” says DDL Packaging Engineer Scott Levy. “Therefore, it's necessary to conduct package testing to ensure the device can withstand the vibration hazards that it might encounter during transit.”

Unavoidable oscillations

Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic, such as the motion of a pendulum, or they may be random such as the movement of truck on a highway or an aircraft at takeoff. Vibration frequencies from ground transport vary depending on continuing changes in surface condition, grade, joints in concrete, and rails, and from other similar recurring discontinuities in road beds. For this reason, most testing is done with random vibrations.

A survey conducted by our company found that only about half the engineers considered vibration testing an important factor when evaluating the worthiness of products and packages during distribution. The repercussions are significant because vibration can be the source of failure modes which include contact damage, breakage or abrasion to components, abrasion of sterile barrier systems, and fatigue damage. It is possible to jeopardize a whole product launch if products and packaging are not designed with these effects in mind.

Failure modes are characterized by contact damage which results when a component of the product yields enough to strike another component resulting in either breakage of the yielding component, breakage of the struck component, or abrading or chipping of either component. Critical damage to the sterile barrier results when packages constantly rub against one another inside the shelf box or shipping container enough to wear holes in the materials. Fatigue damage comes from constant yielding, such as the bending of flexible materials, and leads to cumulative damage over time. The end result generates pinholes in the material causing a product's loss of sterile integrity.

It is not difficult to imagine significant consequences arising from using a damaged product. In lesser cases, the damage is recognized, and the product is rejected and returned to the manufacturer. Or if the sterile barrier (the primary package) is picked from the shelf for a surgical procedure and has sustained visible damage, a new product will have to be retrieved, perhaps at a critical time in the surgery. In the worst case, damage to the sterile barrier is not recognized (because pinholes are not readily visible) rendering the product non-sterile. That puts the health and safety of a patient at risk.

Although it is difficult to determine the appropriate amount of testing time to accurately simulate miles of road travel, identifying critical frequencies and the nature of package stresses can help minimize the effect of these occurrences.

Frequency ranges associated with truck transport can be in the range of 3 to 100Hz and can reach sustained amplitudes of ½ g. Standard tests have ‘profiles’ or PSD's produced from statistical field measurements. These let labs reproduce similar forces to those found in shipping.

Several options can satisfy the requirements for package validation. First, the package could be tested by simply shipping it to a destination using the anticipated shipping mode. Although economical, the method does not lend itself to a high degree of control and repeatability. Nor does it provide a statistically significant sampling of the product under evaluation.

Laboratory simulations, however, provide a way to subject packages to the distribution hazards of shock, vibration, and dynamic compression in a controlled and repeatable manner. Observations of the package's performance, as it is subjected to various hazards, can be accomplished in the laboratory. Corrective action can then be taken in a timely fashion to alleviate problems that reveal themselves.

Laboratory methods can be performed using standardized laboratory simulations such as ASTM D4169 “Performance Testing of Shipping Containers and Systems” or International Safe Transit (ISTA) procedures, such as Procedure 1A, 2A, or 3A.

Standardized laboratory procedures describe several dynamic tests that use realistic intensity levels. The ASTM method also lets users with significant knowledge of their distribution system design a test sequence which more closely matches specific shipping conditions. This may let a laboratory base the simulations on actual field measurements and include tests for vibration, dropped containers, temperature and humidity, and atmospheric pressure.

The most commonly used standardized-distribution-simulation test for medical device package validation is ASTM D4169, Distribution Cycle #13. This method is intended for packages weighing less than 100 lb and transported by air and ground vehicles. The ASTM says the test “provides a uniform basis of evaluating in the laboratory, the ability of shipping units to withstand the distribution environment. This is accomplished by subjecting the packages to a test plan consisting of a sequence of anticipated hazard elements encountered in the chosen distribution environment.”

When considering which standard to use for compliance, factor in whether the standard is approved as a consensus standard. ASTM standards are consensus documents, so called because a large group of experts develop and democratically approve the documents for publication. The group imposes a strict voting regimen. All issues, concerns, and comments must be addressed before final approval as an ASTM standard. All ASTM standards have gone through this process. This is important from the regulatory standpoint because the FDA reviews consensus standards and adds standards to the approved list of consensus standards found in FDA's guidance document “Recognized Consensus Standards”. Following these test standards can provide evidence of product efficacy, and compliance to vertical standards, such as ISO 11607 in the case of package systems.

Difficult but controllable

Vibration is a force in nature that is difficult to control and reduce. Packages and their contents will be exposed to these forces, which may be the cause of damage to products. It is important for the medical device manufacturer to assess the ability of both the product and the packaged product to withstand these forces. In most package performance protocols, these vibration forces are incorporated into a regime of other transport hazards such as handling (shock), compression and environmental extremes. Often times these forces are cumulative and cause damage that might never occur with only one ‘hazard’ assessed.

There are several published simulation standards available for compliance to ISO 11607. Test and package engineers should choose the best simulation test for the risk level they are willing to sustain. Vibration forces do cause damage to sterile barrier systems and is often the hidden origin. Don't ignore this nebulous transport threat.