By: Buffalo Manufacturing Works Team | September 9th, 2020

Choosing materials for a new medical device is not a trivial
task; without proper care, costly late-stage issues can derail an entire
program. Material failures for medical devices are common and costly, 30-40% of
FDA recalls are due to material issues.¹ These issues may include compatibility
of the fluid formulation with contacting materials, biocompatibility of the
materials with the patient, reduced fracture resistance, mechanical properties
degradation, undesired aesthetic changes, and the effects of long term stress
or stain. These failures and others are presented in Figure 1.
To avoid these, you should incorporate a comprehensive materials selection and
testing protocol early into the product design cycle.

Materials selection begins with writing a requirements
document that incorporates the input from both the engineering team and the
human factors team to ensure that the device will function and meet the user’s
expectations. The engineering team must consider all conditions that the
materials will experience from resin compounding, shipping, assembly/joining,
storage, use, and disposal, while the human factors team must consider how
people will interact with the device. These include considerations of human,
mechanical, chemical, and thermal stresses exerted on the device, such as
gripping the device, cleaning it with harsh sanitizing chemicals, or leaving it
in a hot car.

Sometimes device manufactures, design firms, and injection
molders have preferred polymer vendors. While many times their vendor has the
best material for the application, sometimes they do not. This is where an unbiased
third-party review of device materials can give a broader perspective of which
polymers are better suited to meet all the requirements for the device. 

The requirements document and the Failure Modes and Effects
Analysis (FMEA) document are then used along with material datasheets to select
materials that will be suitable for all conditions the product will encounter.
There is always a gap between the requirements and the data sheet information
because resin manufacturers cannot test their polymers under all conditions.
This is where additional testing under real-world conditions must be performed
to challenge the viability of the top material candidates.

All concerns for product viability must be tested to best
mitigate failure risk. Polymer testing requires considerations under actual use
conditions. Examples of this include:

• Measuring fluid absorption into the polymer and
  its corresponding effect on mechanical properties by Instron testing
• Determining the efficacy of the fluid in the
  device after contact with materials by measuring its remaining activity
• Assessing biocompatibility via ISO 10993 testing
• Measuring mechanical properties (Young’s
  modulus, ultimate tensile strength, strain at break, storage/loss modulus, tan
  delta etc.) at the highest and lowest temperatures the device will experience
  by Instron and dynamic mechanical analysis (DMA) testing
• Consideration of joining applications during
  assembly and the material related properties necessary to their success, such
  as melt flow index of dissimilar polymers joining by ultrasonic welding or
  energy absorption of the materials for laser transmission through welding
• Monitoring environmental stress-cracking while
  exposed to liquid formulation or cleaners by a static bend and soak test
• Estimating long term stress relaxation or creep
  using time-temperature superposition (TTS) from data obtained on a dynamic
  mechanical analyzer (DMA)
• Implementing accelerated aging of whole devices,
  followed by inspection and testing, to understand system level issues

These tests will reduce the risk for both product failures
in late stage development and costly recalls after product launch. Despite the
added time and cost, imposing the discipline of these analyses will help avoid
embarrassing and costly recalls later. To realize the full cost savings
potential this material selection protocol must be implemented early in the product
development.  

For more information, contact Jeff Ellis at [email protected]

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1. Ellis, J., Material Evidence: Examining Mysterious Design Failures, Med Device Online, May 15, 2017  https://www.meddeviceonline.com/doc/material-evidence-examining-mysterious-design-failures-0001

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