So you’ve determined that your medical device needs silicone, perhaps as a lubricant or a molded part or a length of tubing.  How do you choose the right material?   The best answer is carefully.  What’s the big deal you might ask, after all it’s just a material.  Yes, but when the material is molded it becomes a component and when the components are assembled they become the device.  That’s Device with a capital “D”.  Try this, tell your Regulatory Affairs specialist that you want to change the silicone used to make the molded valve in your device.  You get the idea.  Get it right the first time.  Consider the following examples. 

A designer chooses a high tensile strength silicone for a “high strength” balloon.  In actual use however, the maximum expansion of the balloon is well below the maximum elongation of the material. Comparing the modulus of candidate materials would have been a much better indicator of balloon strength.  Maximum tensile as measured at maximum elongation is seldom an adequate predictor of a component’s performance.

A device is designed that includes a belt-like component that can be cinched and locked during surgery.  A molded hub on one end of a strap will pass through a slot on the other.  A silicone material is chosen such that the force-to-lock is within the design specification.  Months later it’s discovered that the force increases after sterilization.  The choice of material must take into consideration all downstream processes including sterilization.

Explant studies showed that a cardiac lead was prone to longitudinal cracking.  A team was formed to develop a next-generation device that would be resistant to this problem.  The team identified a silicone with superior tear strength.  Ten months later animal studies showed that the cracking was even more prevalent.  Don’t mistake cracking with tearing as different forces are at work.  In cases such as this, cyclic compression tests may be a better indicator of performance.

A large syringe manufacturer wants to offer a line of siliconized needles.  A project is started in November and several silicone oils at various viscosities are evaluated.  When coated on the needle and tested, one particular viscosity gives superior penetration and drag results.  The product line is launched in April.  Performance issues began to surface in mid-summer and an investigation was initiated.  It was found that only needles transported during the hotter summer months were affected.   Never underestimate the tendency of temperature changes during transportation or storage to affect your product.

A device manufacturer wants to mold a spring-like component and surveys silicones of various durometers.  A material is chosen that provides the appropriate compression under a given load and expansion when the load is removed.  The component is packaged preloaded in a delivery device.   Later it’s found that devices older than 3 months fail to deploy properly.   Durometer is related to the compressiblity of a material but it is not necessarily predictive of compression set.  Compression set is an often overlooked variable.

A device manufacturer wants to expand his product line to include facial implants.  A material is chosen for  cheek and chin implants based on durometer.  Because of the type of material selected, the part can be produced only by compression molding.   The manufacturer later signs a distribution agreement with a large Asian distributor and orders increase tenfold.   The molding process cannot be scaled up and orders go unfilled.   All else being equal, choose a material that allows efficient and scaleable processing.

A device manufacturer hires an outside contractor to extrude tubing for an implantable drug pump.  A material is selected and 10 pounds are ordered.  The contractor extrudes 5 pounds of the material and then submits the small sample run to the customer who puts the tubing through a battery of tests.  Two months later all testing is complete and more tubing is ordered.  The contractor takes the remaining 5 pounds of stored material and begins extruding.  This time the contractor discovers that it is much more difficult to maintain concentricity of the tubing.  Small voids are also observed in the vulcanized tubing.   Silicones can and do change over time. Changes to material properties such as plasticity, viscosity, and cure rate can have a dramatic affect on the processabilty of the material. Also, different types of silicones will age differently.  It is important to understand and anticipate these age-related material changes.  Process procedures should be written broadly enough to mitigate the affects of material change. 

Selecting the appropriate silicone material involves numerous considerations, many of them not obvious and some just downright counterintuitive.   Get it right the first time!