Silicone tubing for medical devices: What to consider

Dan Sanchez, product manager for Trelleborg Healthcare & Medical, outlines important factors to evaluate when specifying silicone tubing for medical devices.

The global medical tubing segment continues to rise at a significant rate and is expected to reach USD 9.38 billion by 2022, according to MarketsandMarkets Research. Therefore, silicone is identified, in terms of volume, as the largest and fastest growing material within the global medical tubing segment.

What to look for

One of the key mechanical considerations for silicone tubing is size, especially as medical device developers seek to produce ever-smaller devices and drug delivery systems. It is currently possible to produce implant-quality silicone tubing at the scale of 0.007 inch x 0.014 inch. However, not all suppliers are capable of doing so reliably and at the speed necessary to deliver products to market in a timely manner. 

Another consideration is hardness. Generally speaking, extruded silicone is available in durometers from Shore A 20 to Shore A 80. As the durometer increases, tensile strength decreases. This is therefore an important factor to evaluate for devices such as wound drains, which are put under significant pressure when removed from the body.  

Two other mechanical factors to consider are tear strength and elongation. Tear strength influences, for example, how difficult it is for a nick in the tube to grow. This is something to consider in situations where the tube might be exposed to a sharp edge. Elongation properties relate to how stretchy the tube is, which is a significant factor for balloon devices.

Finally, the surface finish of the tube must be carefully evaluated. Quite often, when it’s important for a tube to have minimal surface friction, the tube can be coated to decrease the silicone’s inherent “stickiness.” However, the coating must be biocompatible if it is likely to come into contact with the body.

Tool design and processing parameters, such as extrusion pressure, can also positively influence the surface texture of the final tube.

Managing volatiles and transparency  

While highly biocompatible, silicone grades and additives must be considered based on the application. In cases where there’s a high degree of concern, leachable testing and post-cure processing, in which tubing is heated to +350 ºF, may be undertaken.

Visibility is important for certain types of devices, such as those carrying fluids into or out of the body. Although silicone is naturally translucent, certain silica fillers that increase durometer can reduce or eliminate transparency. A stripe of barium added to the tube makes it visible within soft tissue, while still enabling the fluid to be viewed to verify flow.

Innovative extruded shapes and antimicrobials

Kink-resistant and multilumen silicone tubing is invaluable in certain medical device designs. Several types of reinforcements, including some that are biocompatible, reduce a tube’s tendency to kink and stop the flow of fluid. This is especially important in implantable devices when fluid transport is critical, and the tube must remain open during bodily movement.

Similarly, when wires are insulated within a silicone tube and are subjected to repeat dynamic loading, such as where a cardiac pacing lead connects to the pacemaker housing, designers can consider multilumen tubing produced with a continuous twist to minimise strain hardening, reducing the probability of or eliminating wire breakage.

Finally, silicone’s inherent permeability can be leveraged to control the release of additives. For example, an antimicrobial ingredient added to a catheter reduces the risk of infection, and an anti-inflammatory ingredient in a pacemaker lead aids healing after insertion.

Summary

As the medical tubing market continues to grow, designers should consider mechanical properties, surface finish, biocompatibility, permeability, and advanced extrusion capabilities when assessing product requirements and supplier partners. Innovation, engineering and manufacturing advice provided by a well-chosen supplier can help achieve a competitive edge by significantly improving the total cost of ownership, quality,  and overall product performance.