LCHT material selection and ASTM requirements

While catheter systems built on braided shaft technology are the standard for catheter development, Laser Cut Hypotubes (LCHT) have emerged as a unique technology which allows for the creation of devices with improved performance characteristics. With the ability to precisely control shaft behavior such as flexibility and bend radius at any given point along the catheter shaft with infinitely customizable laser cut patterns, LCHT can be optimized to meet the exact functional needs of any device. The improved performance of LCHT comes from both the laser cut pattern, as well as the inherent characteristics of hypotubes themselves. 

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Let's take a look at the hypotube and why it is such a powerful component in medical device manufacturing.

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Material selection for hypotubes

Firstly, let's consider the inputs for hypotubes and how they impact the laser cutting process. As with all components in the medical device industry, material selection has a large impact on the performance of the part. In the world of hypotubes for medical applications, the most commonly used metal is 304 and 316 Stainless Steel due to its plentiful availability, affordability, and impressive tensile strength. For most catheter applications, 304 stainless is suitable as it is slightly cheaper than 316, and has excellent welding qualities. 316 stainless is often used for more challenging environments where superior tensile strength is critical, and corrosion resistance is required. Because of the addition of molybdenum, 316 stainless performs better in acidic environments, and is preferable for re-usable devices as it is more resistant to rust. Another commonly used metal for hypotubes is nitinol due to it's super elasticity and shape memory, which allow catheters to navigate challenging tortuous anatomy with tight bends and return back to its original shape without kinking.

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ASMT requirements for medical hypotubes

It is important to note that not all metals are created equal. Careful attention to detail is required when manufacturing the base material for hypotubes, and all of the hypotubes that we use at Symmetry Laser meet stringent ASTM requirements. Specifically, ASTM269 details a variety of requirements including chemical composition, heat treatment, analysis, hardness, and more that hypotubes must meet. These requirements ensure that engineers can count on repeatable results and performance characteristics from hypotube components across lots and manufacturers which is critical for creating safe and effective devices.

Stainless steel 304 & 316

For the purposes of this discussion, we will take a look at the chemical composition requirements for 304 and 316 stainless steel in order to understand why these materials specifically are used for catheter systems.

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(Maybe add section about why both have good performance characteristics compared to other metals generally)

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The first distinction that engineers might come across is the difference between 304 stainless and 304L stainless (similarly 316 stainless and 316L stainless). The L here indicates extra-low carbon. ASTM269 requirements details a maximum carbon contents of .035% for both 304L and 316L compared to the standard .08% max of 304 and 316. This slight decrease in carbon, paired with a slightly higher Nickel composition (up to 2% higher for 304 and 1% higher for 316) result in small but noticeable material characteristic differences. 304L and 316L exhibit a reduction in both yield strength (maximum stress that can be applied before material permanently changes shape) and tensile strength (maximum stress that material can withstand while being stretched before it breaks). So why then would an engineer choose to use the low carbon option for their catheter? The answer lies in the welding process where the decreased carbon content helps minimize and even eliminate carbide precipitation (carbide precipitation can lead to a loss of corrosion resistance), resulting in stronger weld joints and eliminating the need to anneal weld joints which saves time and effort.

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What about the difference between 304 and 316 stainless? As mentioned previously, the addition of molybdenum to 316 stainless gives the material superior corrosion resistance. 316 also has a slightly higher amount of nickel than 304 which also contributes to its increased corrosion resistance, as well as its increased cost. 316 stainless also has slightly less chromium than 304 stainless. These three chemical differences give 316 stainless better corrosion resistance as well as increased tensile strength, which can be critical for catheter systems that need to withstand high forces.

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The best material for laser cutting

When it comes to laser cutting, there are a couple of key similarities and differences between the two materials. Both 304 and 316 stainless make for great laser cutting candidates due to their relatively low thermal conductivity. Both metals have a similar melting point and reflectivity, and produce clean edges when cut properly. Where the two metals differ is the addition of molybdenum in 316 stainless, where heat absorption is slightly affected. The presence of molybdenum increases the thermal resistance and can influence cutting efficiency, requiring fine-tuning of laser parameters. Because of this, 304 stainless can be cut at slightly faster speeds. This is especially true for hypotubes with thicker walls.

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