Tech Talk – Guide Wires

I sometimes enjoy The Oil Drum’s tech talks, so I thought I’d give one a try.

Guide wires, or guidewires, are used in the vasculature to act as a guide for other devices, an example video shows guide wire and stent.  Guide wires are generally more flexible and steerable than devices used to treat or diagnose patients.  Typically, once access to an artery is gained, the guide wire is inserted and steered under fluoroscopy to the location of interest.  Then one or more devices (usually catheters) are delivered over the guide wire to diagnose and treat the condition.

Guide wires usually come in diameters of 0.010” to 0.035” with 0.014” being the most common (0.013” is equivalent to one french or 1/3 of a mm – so the typical guide wire is slightly over 1 french in size).  Guide wire lengths vary up to 400 cm, depending on the anatomy you want to reach and the work flow.  The flexible distal tip portion is usually 3 cm long, the slightly less flexible portion is usually 30 to 50 cm long, the less flexible proximal portion makes up the balance.

The user requirements of a guide wire are can it quickly get to where it needs to go and then can I reliably deliver devices over it without patient safety issues.

A typical guide wire construction is shown below:

In this diagram, the blue proximal section is the hypotube, similar to a syringe needle (minus the sharp end).  The hypotube is generally coated in PTFE- but other coatings are also used.  The PTFE coating allows devices to more easily slide over the guide wire and is probably the major component in device “deliverability” on the guide wire side.  It is easier to slide a catheter over a guide wire with PTFE than it is to slide a catheter over a guide wire without PTFE.  The PTFE coating of the hypotube can be a tricky part of manufacturer and is usually outsourced.  Just as your pans vary in quality of the non stick surface, so do guide wires and different variations in processing and materials can affect how well your guide wire delivers devices.

The core wire material is usually nitinol or stainless steel and tapers from the proximal end to the tip.  At the tip it is generally flattened.  The core wire affects the torquability of the device; you want the tip of your guide wire to turn in a 1:1 ratio with the proximal end.  The torquability affects the steerability of the device, can it get to where you want to go.  Nitinol core wires are harder to kink, but can lose some torquability.  The core wire is attached to the proximal end of the hypotube using solder or adhesive.

Alternative guide wire construction can look like this:

In this design there is no hypotube, the core wire is the proximal portion of the device.  Coating is applied directly to the core wire and you have better torquability because you are turning the core wire directly, instead of turning the hypotube, which turns the core wire.

In both designs, the next section is a more flexible distal section, usually consisting of a wire coil.  The connection is made to the hypotube or core wire by solder or adhesive at this point.  The coils are more flexible than the hypotube and the distal portion of the device is suited to navigate to the desired portion of the anatomy.  There may be several types of coils attached together or one long coil where the coil spacing changes.  The more proximal section of the coil is generally made of stainless steel for cost savings.  The distal tip coil of the guide wire is made of radioopaque metal, generally a platinum alloy, but a palladium alloy may also be used.  Some hospitals recycle these religiously for the $4 in platinum they can get in the tips.

At the very distal tip, the wire coil is soldered to the core wire using tin - silver solder, although in some older wires the core wire may be attached using adhesive.

The tip itself is rounded into a ball shape, which the solder or adhesive naturally forms when applied to the tip.  This allows the guide wire to follow the curve of the vessel and is generally called an atraumatic tip.  From a manufacturing point of view, manufacturers are very careful to not allow any burrs in the tip assembly for fear of vessel damage, but I have not seen any direct reports of vessel damage caused by badly made tips.

The tip itself is always shaped by the user to aid in steerability, usually to around 45 degrees.  Usually the tip is shaped by using an introducer or needle.  Here is a video of a shaped tip, although this method isn’t encouraged.  Some manufacturers offer tips pre-shaped.  One notable alternative tip construction is shown below:

In this case an extra wire has been attached to the core wire and the tip to allow better tip shapeability and can give a softer tip.  Depending on where you want to go in the vasculature different tip softness is desired, for example in the cerebral vasculature usually the softer the better.  Tip softness can be measured as tip load, the amount of force (or load) it requires to bend the tip a certain amount.  Obviously you want the tip to flex instead of damage the vessel wall.

The distal portion of the wire is coated with a lubricious coating, generally a hydrophilic coating, older wires used hydrophobic coatings, but they are becoming rarer.  The hydrophilic coating is a proprietary coating that manufacturers generally outsource and pay a considerable amount of money for.  There is some skill in developing a coating that does not come off of the guide wire, and retains its lubricity over time.

When building a guide wire you want to start at one end and work your way down, starting at the tip and working to the proximal end.  Making guide wires is not that labor intensive with the most skill you need is to be decent at solder and soldering cleaning.  You need to work at keeping them undamaged in manufacturing due to their small size and the less flexing of the wire you do the better.

There are other variations on guide wires, such as plastic coatings and different combinations of coils and hypotubes, but I’ve covered the major ones.  Guide wires are basically a commodity at this point and only the regulatory barriers to entry and the fact that physicians aren’t typically price sensitive are keeping it as a reasonably attractive market to pursue.

When performing performance verification testing on a guide wire, some common tests are turns to failure (you clamp the distal tip and turn the device until it breaks), particulate / adhesion tests (of the coating), tip tensile strength, torquability (which you turn the proximal end of the guide wire a set amount and measure how much the distal end turns), tip load, radioopacity, and deliverability (where you measure the force it takes to move a catheter along the guide wire).  DDL has uploaded this video on testing a guide wire to ISO 11070, but I’ve never used that particular test.

For validation testing you generally request feedback on navigating a guide wire to the desired location in a bench or animal model and delivering a device over the guide wire.

I was unable to find the size of the guide wire market, but every endovasculature procedure uses at least one guide wire, many times more.  Many companies outsource the manufacture of guide wire to a contract manufacturer; it is difficult to enter the market fresh at this point.  However, as a company, you want to sell a guide wire so you can get your 40% margin on an outsourced guide wire instead of the competitor getting 60%.  Also, it is generally beneficial to sell a whole suite of devices when selling to a hospital, if you sell only catheters you will have a harder time getting shelf space unless you can bundle it with other products.

More information is available at:  PCI equipment: Guidewire selection.  And the FDA chimes in with helpful guide wire tips on device safety-  always read those IFUs.