Laser-cut hypotubes: the pattern is only half the design
The sample looked perfect. A clean spiral, no burrs visible at 50x, a smooth curve on the three-point bending fixture. Then it went into a tortuous path model, got pushed past the second bend, and folded.
Not somewhere in the cut zone. Exactly at the last millimeter of the spiral, where the pattern stops and the full-wall tube begins.
If you have worked with laser-cut hypotubes for a while, you have probably seen this failure yourself. In most cases it is not the laser's fault. It is the drawing. Most of what decides whether a cut hypotube works is decided before the tube ever goes into the machine, and a good part of the rest is decided after it comes out. Here is what we look at.
Why so many shafts start as a metal tube
A hypotube is simply a thin-walled metal tube, usually 304 or 316L stainless steel, sometimes nitinol, occasionally cobalt-chromium. You find it in the proximal shaft of rapid exchange balloon catheters, in stent and valve delivery systems, in aspiration catheters, in neurovascular microcatheters and in steerable sheaths.
The reason is always the same. At the outer diameters these devices allow, a polymer shaft cannot push or transmit torque the way a metal tube does. A coronary hypotube often sits somewhere around 0.6 to 0.7 mm OD with a wall of 0.07 to 0.1 mm. That is very little material, and it still has to carry the whole push from the physician's hand.
The catch is that a plain steel tube is stiff along its entire length, and the anatomy is not. Laser cutting is how you remove material selectively, so the shaft stays stiff where you need pushability and becomes flexible where it has to follow a curve. In other words, the cut pattern is the stiffness profile of your device.
The pattern: pitch gets the attention, the ends cause the trouble
There are a handful of basic patterns, and most designs are variations of them:
Teams usually spend weeks on the pitch in the flexible zone. Far less time goes into the place where the cut starts and the place where it ends, and that is exactly where the fold from the opening happened. An abrupt jump from full wall to cut tube is a stiffness step, and a stiffness step under compression is where a shaft kinks.
Two rules we would push for on almost every drawing. First, ramp the pitch in and out over a real transition length, often 10 to 20 mm, not over two turns. Second, never let a spiral end in a sharp kerf tip. Finish it with a small radius or a stop hole. Under cyclic bending, a sharp tip is a ready-made crack starter.
And look at the raw tube, not only the pattern. A kerf from a fiber laser is typically in the range of 15 to 30 µm, but if the wall thickness varies by 10% around the circumference, your bridges will not be equally strong no matter how precise the cut is. Concentricity and straightness belong in the tubing spec.
Fiber laser or ultrashort pulse?
This question comes up in almost every project, usually framed as "which laser is better." That is the wrong framing.
Fiber lasers have been the workhorse of stent and hypotube cutting for decades. They are fast and well understood, but they melt material. That leaves a recast layer, a heat-affected zone and dross that has to be removed afterward. Picosecond and femtosecond lasers ablate with very little heat input, so edges come out cleaner, with little dross and almost no recast. They are also slower and more expensive per part.
So the real question is how much post-processing you are prepared to develop and validate. Our view, which not everyone shares: for 304 or 316L stainless with an accessible inner diameter and an electropolishing step already in the process, a well-tuned fiber laser is enough for most hypotube work. For nitinol, go ultrashort from the start. Heat input changes the material's behavior locally near the cut edge and leaves oxide that is hard to remove cleanly. Switching lasers after design freeze costs more than the laser itself, because half your verification has to be repeated.
The side of the tube nobody looks at
When the beam breaks through the wall, it does not stop. It keeps going and hits the opposite inner wall. On a tube with a 0.5 mm ID, that happens very close to the focus, and the result is back-wall damage: small marks, molten spatter and dross stuck to the inside of the lumen.
From the outside the part looks fine. But a guidewire runs against that surface, or a liner gets reflowed onto it, or inflation fluid flows through it. The usual countermeasures are water flowing through the tube during cutting (it also flushes out dross), a sacrificial wire or mandrel inside larger tubes, careful assist gas pressure and parameters tuned so the beam has as little energy left as possible after breakthrough.
One habit we recommend for every first article: cut a few tubes lengthwise and look at the inner surface under the microscope. It takes an hour and it regularly surprises people.
After the cut: finishing, sealing and joining
Finishing. Stainless parts typically go through deburring or microblasting, then electropolishing and passivation. Electropolishing rounds the edges, removes the recast layer and improves fatigue and corrosion behavior. It also removes material, a few microns per surface. That means bridges get thinner and kerfs get wider. If the drawing does not include an allowance for this, the finished part will be softer than the part that was simulated.
Sealing. In a rapid exchange balloon catheter, the hypotube is often the inflation lumen. A cut section leaks. So the flexible zone needs a polymer jacket, a liner or a coating that keeps the lumen tight at rated pressure. If you reflow polymer over the cut zone, expect it to flow into the kerfs and lock the bridges. Your shaft gets noticeably stiffer, and only bench testing tells you by how much. A thin liner or heat shrink under the jacket helps keep the cuts open.
PTFE-coated hypotubes are common in proximal shafts. Cutting through the coating burns it back at the kerf edges, so where possible, cut first and coat afterward, or mask the zone.
Joining. The hypotube has to connect to the rest of the device: a core wire, a distal polymer shaft, a hub. Metal to metal joints are usually laser welded. Metal to polymer joints need surface preparation, and in some designs the hypotube end gets a small laser-cut pattern purely as a mechanical anchor for the polymer. Every one of these joints needs a tensile test, typically following ISO 10555-1.
Cut a ladder, not a single design
FEA is useful for narrowing down options. It is not a reason to freeze a cut pattern. Our approach is to cut a small ladder of five or six variants that differ in pitch, bridge size and transition length, and then put them all through the same bench tests:
A ladder like this costs a few days of cutting and testing. It usually saves weeks, because it shows you which parameter actually moves your device and which one you have been tuning for nothing.
If you are about to send a cut hypotube drawing to a cutting house, we are happy to look at it first. The transition zones and spiral ends are usually where our first comments land. You can find more about our prototyping and design services or simply get in touch.
Not somewhere in the cut zone. Exactly at the last millimeter of the spiral, where the pattern stops and the full-wall tube begins.
If you have worked with laser-cut hypotubes for a while, you have probably seen this failure yourself. In most cases it is not the laser's fault. It is the drawing. Most of what decides whether a cut hypotube works is decided before the tube ever goes into the machine, and a good part of the rest is decided after it comes out. Here is what we look at.
Why so many shafts start as a metal tube
A hypotube is simply a thin-walled metal tube, usually 304 or 316L stainless steel, sometimes nitinol, occasionally cobalt-chromium. You find it in the proximal shaft of rapid exchange balloon catheters, in stent and valve delivery systems, in aspiration catheters, in neurovascular microcatheters and in steerable sheaths.
The reason is always the same. At the outer diameters these devices allow, a polymer shaft cannot push or transmit torque the way a metal tube does. A coronary hypotube often sits somewhere around 0.6 to 0.7 mm OD with a wall of 0.07 to 0.1 mm. That is very little material, and it still has to carry the whole push from the physician's hand.
The catch is that a plain steel tube is stiff along its entire length, and the anatomy is not. Laser cutting is how you remove material selectively, so the shaft stays stiff where you need pushability and becomes flexible where it has to follow a curve. In other words, the cut pattern is the stiffness profile of your device.
The pattern: pitch gets the attention, the ends cause the trouble
There are a handful of basic patterns, and most designs are variations of them:
- Continuous spiral. Maximum flexibility for a given pitch. The downside: it loses tensile strength and behaves differently depending on which way you twist it, because one direction tightens the coil and the other opens it.
- Interrupted spiral. Each turn is cut in segments with small uncut bridges between them. The bridges carry tension and torque. In our experience, bridge size changes stiffness more than pitch does, so it deserves at least as much attention on the drawing.
- Variable pitch. The pitch shrinks gradually toward the distal end, so stiffness drops in steps you control instead of all at once.
Teams usually spend weeks on the pitch in the flexible zone. Far less time goes into the place where the cut starts and the place where it ends, and that is exactly where the fold from the opening happened. An abrupt jump from full wall to cut tube is a stiffness step, and a stiffness step under compression is where a shaft kinks.
Two rules we would push for on almost every drawing. First, ramp the pitch in and out over a real transition length, often 10 to 20 mm, not over two turns. Second, never let a spiral end in a sharp kerf tip. Finish it with a small radius or a stop hole. Under cyclic bending, a sharp tip is a ready-made crack starter.
And look at the raw tube, not only the pattern. A kerf from a fiber laser is typically in the range of 15 to 30 µm, but if the wall thickness varies by 10% around the circumference, your bridges will not be equally strong no matter how precise the cut is. Concentricity and straightness belong in the tubing spec.
Fiber laser or ultrashort pulse?
This question comes up in almost every project, usually framed as "which laser is better." That is the wrong framing.
Fiber lasers have been the workhorse of stent and hypotube cutting for decades. They are fast and well understood, but they melt material. That leaves a recast layer, a heat-affected zone and dross that has to be removed afterward. Picosecond and femtosecond lasers ablate with very little heat input, so edges come out cleaner, with little dross and almost no recast. They are also slower and more expensive per part.
So the real question is how much post-processing you are prepared to develop and validate. Our view, which not everyone shares: for 304 or 316L stainless with an accessible inner diameter and an electropolishing step already in the process, a well-tuned fiber laser is enough for most hypotube work. For nitinol, go ultrashort from the start. Heat input changes the material's behavior locally near the cut edge and leaves oxide that is hard to remove cleanly. Switching lasers after design freeze costs more than the laser itself, because half your verification has to be repeated.
The side of the tube nobody looks at
When the beam breaks through the wall, it does not stop. It keeps going and hits the opposite inner wall. On a tube with a 0.5 mm ID, that happens very close to the focus, and the result is back-wall damage: small marks, molten spatter and dross stuck to the inside of the lumen.
From the outside the part looks fine. But a guidewire runs against that surface, or a liner gets reflowed onto it, or inflation fluid flows through it. The usual countermeasures are water flowing through the tube during cutting (it also flushes out dross), a sacrificial wire or mandrel inside larger tubes, careful assist gas pressure and parameters tuned so the beam has as little energy left as possible after breakthrough.
One habit we recommend for every first article: cut a few tubes lengthwise and look at the inner surface under the microscope. It takes an hour and it regularly surprises people.
After the cut: finishing, sealing and joining
Finishing. Stainless parts typically go through deburring or microblasting, then electropolishing and passivation. Electropolishing rounds the edges, removes the recast layer and improves fatigue and corrosion behavior. It also removes material, a few microns per surface. That means bridges get thinner and kerfs get wider. If the drawing does not include an allowance for this, the finished part will be softer than the part that was simulated.
Sealing. In a rapid exchange balloon catheter, the hypotube is often the inflation lumen. A cut section leaks. So the flexible zone needs a polymer jacket, a liner or a coating that keeps the lumen tight at rated pressure. If you reflow polymer over the cut zone, expect it to flow into the kerfs and lock the bridges. Your shaft gets noticeably stiffer, and only bench testing tells you by how much. A thin liner or heat shrink under the jacket helps keep the cuts open.
PTFE-coated hypotubes are common in proximal shafts. Cutting through the coating burns it back at the kerf edges, so where possible, cut first and coat afterward, or mask the zone.
Joining. The hypotube has to connect to the rest of the device: a core wire, a distal polymer shaft, a hub. Metal to metal joints are usually laser welded. Metal to polymer joints need surface preparation, and in some designs the hypotube end gets a small laser-cut pattern purely as a mechanical anchor for the polymer. Every one of these joints needs a tensile test, typically following ISO 10555-1.
Cut a ladder, not a single design
FEA is useful for narrowing down options. It is not a reason to freeze a cut pattern. Our approach is to cut a small ladder of five or six variants that differ in pitch, bridge size and transition length, and then put them all through the same bench tests:
- Three-point bending stiffness along the shaft, to see whether the stiffness profile matches the intent.
- Torque response in a tortuous path model, including the lag between proximal and distal rotation.
- Kink resistance at the transition zones, not only in the middle of the flexible section.
- Tensile strength of the cut section and of every joint.
- Rotational fatigue in a curved path, followed by a close look at the bridges and the spiral ends.
A ladder like this costs a few days of cutting and testing. It usually saves weeks, because it shows you which parameter actually moves your device and which one you have been tuning for nothing.
If you are about to send a cut hypotube drawing to a cutting house, we are happy to look at it first. The transition zones and spiral ends are usually where our first comments land. You can find more about our prototyping and design services or simply get in touch.