Machining micron-sized components

As medical devices and their internal components continue to decrease in overall size, diameter, and wall thickness, medical device manufacturers are increasingly turning to custom-built laser systems designed to cut, weld, drill, ablate (remove material), and engrave micron-sized metal and polymer parts. The industry trend toward smaller micro-machined products is well documented.

Medical stainless steel tubing, for example, is used to create everything from hypodermic needles to implantable stents and catheters. This tubing can measure as little as 0.008" to 0.060" in diameter with extremely thin walls.

Extremely fine wire made of stainless, titanium, Nitinol, and other exotic metals is also used to manufacture everything from guidewires, leads, and electrodes.

In another trend, advanced polymers are now increasingly being utilized to manufacture miniaturized parts as well.

However, as medical devices and component parts move further down the micron scale, conventional machining techniques are no longer sufficient. Instead, medical device manufacturers are opting for the precision, non-contact processing and higher speeds of laser-based systems complete with optical verification systems.

Available as stand-alone units or sub-systems integrated into a larger manufacturing line, these systems are capable of cutting, welding, and ablating in an extremely precise, controlled manner.

“As components are becoming more and more miniaturized, lasers are becoming the only truly viable option,” says Jason Eddy of Vascular Solutions, a company that develops hemostat, catheter, and vein devices used in coronary and peripheral vascular procedures.
 

Laser cutting

Vascular Solutions began investigating the use of lasers to automate the cutting of 9.0" long segments of an extremely small, thin (0.0015") stainless steel wire approximately 0.0080" in width. The highly flexible wire is a component in the company’s GuideLiner catheter product line.

The GuideLiner catheter is a coaxial mother-and-child guide extension designed to facilitate placement and exchange of guidewires and other interventional devices.

Previously, cutting to length was done by hand using expensive, specialized side cutters under magnification. Although the side cutters delivered precise cuts, they were expensive and quickly became dull, so they needed to be replaced often.

By automating the process, Vascular Solutions sought to increase throughput while also eliminating the potential for repetitive motion injuries.

Based on a referral, Eddy contacted Custom Laser Systems LLC of Savage, Minn. Custom Laser is an established laser technology firm that provides engineering design and manufacturing of laser-based systems. It also provides contract manufacturing services on a wide array of in-house laser equipment.

Although Custom Laser routinely designs and operates equipment with kerf cutting widths in the range of 25µm to 50µm, its staff has the knowledge and capability to design laser systems that can cut down to 5µm.

For Vascular Solutions, the primary concern using lasers was whether it could cut such small, thin-walled stainless steel wire in a way that mimicked the perpendicular cut of side cutters. According to Eddy, a sharp point could inadvertently catch on, or cut, the coil’s liners.

“Most people will tell you that this is kind of the Holy Grail with respect to coil processing,” he says.

Vascular Solutions ultimately purchased a standalone, pulsed diode laser system from Custom Laser. Workers now load the stainless steel coil using a fixture and place it in the machine. Using a sophisticated vision system, the laser cycles to the precise location and makes the cut. The equipment is designed to make seven to nine cuts per minute, speeding production time. The equipment is already through the acceptance-testing phase and has been qualified for use in production.

Although a few tweaks are still required, Eddy says, “The system is performing as promised.”
 

Welding Nitinol to stainless steel

In the medical device field, the majority of work has been with stainless steel, and titanium to a lesser degree. In the past 10 years, however, there have been increasing requests for parts made of Nitinol.

Nitinol is an alloy of nickel and titanium with shape memory capability. It is easily fabricated, like stainless steel, resistant to most chemicals, and therefore is ideal for implantation into the body due to its unique properties. Unfortunately, Nitinol does not weld easily to stainless steel due to incompatibilities in the chemical composition of both alloys.

“Medical device manufacturers would love to bond these materials together,” says Kent Ramthun, president of Custom Laser Systems.

Through experimentation, Custom Laser has discovered how to weld Nitinol to specific alloys of stainless steel successfully.

Ramthun expects further refinements of the process to open up laser welding as a viable alternative for joining Nitinol to all types of stainless.
 

Welding polymers

The use of polymers is also increasing in medical devices. Lasers can be used to weld, cut, or ablate polymers. In the case of polymers, this could include removing very thin layers of coated material in various areas of a part.

“In the past 5 years, we’ve had quite a few requests related to using lasers on polymers,” Ramthun says. “Many requests are for welding small components, but include welding layers of thin plastic films [25µm to 50µm thick].”

Ramthun cites the example of a medical device development project with Breast-Med, a company currently developing a new generation of breast biopsy site markers using the polymer PEEK.

For every biopsy, a small, implantable tissue marker is placed at the lesion site.

These radiographic markers are designed to remain well-visualized under all key imaging modalities, including conventional mammography. Given the quantity of breast biopsies globally, millions of these markers are required annually.

According to Michael T. Nelson, MD, a practicing board-certified radiologist and professor of radiology at the University of Minnesota, radiographic markers traditionally have one major drawback: there are no markers compatible with and well-visualized under all key imaging modalities.

That includes everything from X-ray based (conventional mammography), computed tomography (CT), and fluoroscopy, to other imaging approaches that do not involve X-rays, such as ultrasound, magnetic resonance imaging (MRI), MR spectroscopy, and nuclear medicine SPECT and positron emission tomography (PET).

“There are 45 [FDA approved] markers and none of them have this capability,” Nelson says.

According to Nelson, early breast biopsy site markers interfered with imaging procedures and generally were not permanently visible.

“Stainless steel and titanium markers created a blooming affect on an MRI that creates a void that obscures the surrounding tissue where the biopsy was taken,” Nelson explains. “That isn’t acceptable because we are doing molecular imaging, including spectroscopy, over the area where the markers are located.”

This spurred Nelson to form his own medical device company, Breast-Med, to create next-generation markers. As CEO of Breast-Med, Dr. Nelson personally assisted in the development of a new generation of implantable soft tissue markers made of carbon-coated ceramic and the polymer PEEK. To date, the company has been involved in the design and FDA approval of five separate markers. The new marker by Breast-Med, currently pending FDA approval, consists of a capsule made of PEEK, a medically approved polymer.

During the molding process, barium sulfite is added to the PEEK as a contrast medium to enhance X-ray based imaging methods. The capsule is then filled with a diluted tincture of gadolinium, a contrast medium for MRI.

Lasers are then used to weld the cap to the body of the vial, to create a hermetic seal. The entire capsule, including the cap, measures 4mm long and 2mm in diameter.

The integrity of the laser weld is critical, as gadolinium is known to cause problems with the kidneys. Although Breast-Med is using a diluted form of gadolinium, if the capsule leaks, then it will not remain visible over a long period of time.

Breast-Med briefly considered utilizing a coating that would keep the capsule from leaking. However, it was difficult to find a coating that adhered to PEEK and met all FDA requirements. Early research into welding options, including lasers, proved unsuccessful as well.

“There are very limited welders that can do this type of custom work on such small parts,” Nelson explains.

Nelson ultimately turned to Custom Laser to weld the part on a contract manufacturing basis. As part of the project, Custom Laser developed several leak tests to ensure integrity of the laser welds. A non-destructive test assures each vial is hermetically sealed. Once the prototypes are FDA approved, Breast-Med expects to manufacture 800 to 900 markers for human testing.

According to Dr. Nelson, Breast-Med intends to continue working with Custom Laser to develop different geometrically designed soft tissue markers in the future.

Custom Laser Systems
www.customlsr.com

Elizabeth Engler Modic is the editor of TMD and can be contacted at emodic@gie.net or 330.523.5344.