When it comes to silicone tubing, medical device designer need to consider a wide variety of mechanical properties.
Dan Sanchez, Product Manager, Trelleborg Healthcare & Medical
The global medical tubing segment continues to rise at a significant rate and is expected to reach $9.4 billion by 2022, according to MarketsandMarkets Research. Within this forecast, silicone is identified, in terms of volume, as the largest and fastest-growing material within the global medical tubing segment.
Specifying silicone tubing can be complex, requiring a medical device designer to consider mechanical properties such as size and hardness, visible features such as transparency or the need for radiopaque stripes and advanced design aspects such as kink resistance.
Additionally, when selecting silicone tubing that may be implanted within a patient or entered into the body, material purity, chemical and biocompatibility, size and durability need consideration, as they play key roles in product performance.
When developing successful solutions to medical device challenges, designers must understand not only the mechanical aspects of silicone, but also biocompatibility, permeability and advanced extrusion processes. In this article, we’ll outline all the factors that should be evaluated when specifying silicone tubing for medical devices, both external and implantable.
Understanding mechanical properties
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 a scale of 0.007 in. by 0.014 in. However, not all suppliers are capable of doing so reliably and at the speed necessary to deliver products to the market in a timely manner. This should be kept in mind during the supplier selection process.
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 in to or out of the body. Although silicone is naturally translucent, certain silica fillers that increase durometer, as well as barium sulfate (BaS04) and titanium dioxide (TiO2), can reduce or eliminate transparency, affecting the ability of the clinician to see whether fluids are flowing as expected.
In some cases, designers wish to make a tube more visible within the body, for example, to aid in guiding a device through the body when viewing it using an X-ray or fluoroscope. 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. A higher percentage of barium additive can be used to ensure a tube can be seen through bone to help a surgeon place a catheter in the spine.
Advanced extrusion processes
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 minimize 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.
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. Enlisting suppliers to act as an extension of the design team can simplify the development process and speed the time to market for inventive medical devices. 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.
Dan Sanchez is a product manager at Trelleborg Healthcare & Medical