Laser Micromachining Technology for Device Manufacturing

Medical Device & Diagnostic Industry Magazine
MDDI Article Index

An MD&DI November 1998 Column

COVER STORY

Reaping the benefits of laser processing comes through understanding the most appropriate applications and knowing what to expect from contract service providers.

Since the invention of lasers in 1960, many laser applications in the medical field have been developed. Today, in fact, it is difficult to name an area of medicine in which lasers have not been employed. Clinical use began as early as 1961, when ophthalmologists discovered that they could use the ruby laser to treat tears or holes in the retina in a procedure called retinal photocoagulation, in which the laser beam was focused—through the cornea—onto precise spots on the retina. The light was absorbed and converted to heat in the retinal tissues, leading to coagulation and formation of a localized scar. Many other medical applications for lasers stemmed from this first clinical procedure.

At present, laser technology has been successfully applied to processes like angioplasty and corneal sculpting, as well as to potential clinical applications—such as treating cartilage and nerves—that would not be possible with conventional techniques. However, beyond the widely publicized clinical applications, laser technology has made major contributions to medicine in the industrial and scientific arenas. In fact, many clinical procedures are possible because of the precision and the reliability of the medical devices and instrumentation that are processed and manufactured by laser processing.

There are two basic reasons for the generally small size of medical devices. They often must fit into small spaces in the body, and they are often made of expensive materials. Lasers, which can work within tolerances of a few microns, can provide ideal solutions for manufacturing cost-effective, miniaturized devices.

When confronting a challenging production problem, manufacturing engineers will evaluate a variety of available processing solutions, including lasers. They will be faced not only with the issue of technical feasibility, but often with the question of whether to incorporate the technology in-house or to contract out the job—a decision based on a number of factors. In this article, we address both technical questions and some of the issues associated with contract laser manufacturing in an attempt to assist production managers and manufacturing engineers in their decision-making process.

LASER MICROMACHINING BASICS

Lasers can generate very short pulses of light of a single wavelength, a characteristic that allows for the deposition of a great amount of energy onto a selected region of material. Among the many types of medical lasers now in use (CO₂, YAG, excimer, dye, argon-ion, diode, etc.), each has its own unique properties and capabilities suited to particular applications. Factors that determine the type of laser to use for a particular application include laser wavelength, energy, power, and temporal and spatial modes; material type; feature sizes and tolerances; processing speed; and cost.

Figure 1. The excimer laser beam delivery travels through a mask and skives its pattern on a part (see Figure 2, below) after being deflected off the mirror. Photo courtesy of Resonetics (Nashua, NH).

The action of CO₂ and Nd:YAG lasers is essentially a thermal process, whereby focusing optics are used to direct a predetermined energy/power density to a well-defined location on the work piece to melt or vaporize the material. Another mechanism, which is nonthermal and referred to as photoablation, occurs when organic materials are exposed to ultraviolet radiation generated from excimer, harmonic YAG, or other UV sources (Figure 1). In medical device manufacturing, both types of laser processes can be employed.

TECHNOLOGY MATCH

When one thinks of contract manufacturing in the medical device industry, lasers and laser micromachining may not immediately come to mind. Instead, manufacturing engineers tend to look toward contract houses to outsource medical microelectronics, plastic extrusions, specialty coating services, precision machining of surgical tools, or offshore turnkey assembly. To understand the relation between laser technology and medical contract manufacturing, one needs to consider how laser micromachining can be used for device manufacturing and, more importantly, why one would use such technology in place of more traditional methods.

Initially, laser micromachining applications are developed because traditional manufacturing technologies cannot meet the requirements or specifications for a particular product. For example, if one needs to microdrill an F2 catheter for a specific cardiovascular or neurological application with a side hole that has a diameter of less than 0.004 in., there are not very many options available. Laser micromachining—which can achieve features 100 times smaller—is a natural fit (Figure 2).

However, the real growth of laser technology for production outsourcing has very little to do with simply exceeding the limits of the traditional technology spectrum. Instead, the real advantage lies in attaining the original goal of outsourcing—finding a way to manufacture a product at an acceptable target cost, quality, and yield, without having to invest in capital equipment. If these goals can be met, then the operations manager, while having a tacit interest in the technology, will be more concerned with measuring the performance of the outsource partner as a function of delivery, quality, and the overall business relationship.

APPLICATIONS

Today, a wide range of medical device manufacturing applications can be found for which laser micromachining technology offers advantages. These benefits include the ability to treat a range of materials with extremely high precision and definition; superior repeatability and process control; cost-effectiveness with miniaturized devices; ease of automation; and a methodology that is noncontact, dry, and clean.

Processing of Catheters. Catheters are some of the most commonly used medical devices. As catheter designs become more complex to meet the needs of new clinical procedures and applications, laser applications are used in catheter welding, cutting, and drilling. Because most catheters are disposable, single-use products, this area represents a huge volume of ongoing work, with single part volumes reaching into the millions.

One classical laser welding application involves fixing a hollow tip to the spiral-wound coil at the end of a catheter. The end of the spiral must be precisely cut to prepare it for spot welding, as the welds must provide a strong connection without penetrating into the bore of the tip. The spot size required is often in the range of 100–115 µm.

Among potential cutting applications is the manufacture of catheter stents. These are made primarily from three materials that satisfy biocompatibility requirements: stainless steel, titanium, and tantalum. The diameters range from several millimeters down to less than one millimeter. Processing these stents calls for a cut width (kerf) of approximately 35 µm in the 100- to 150-µm-thick material, with a kerf of 15 µm in thinner sections.

The skiving of slots or holes with dimensions between 0.001 and 0.020 in. is also a natural fit for lasers. Traditionally, the medical device industry has employed techniques such as manual cutting with razor blades, in which the skill of the operator will often determine the yield. Alternatively, if a laser beam is used, tubes can be skived without worrying about repeated tool replacement, unacceptable yield losses, or positioning inaccuracies (Figure 2). In some cases, manufacturers can choose to free up valuable cleanroom space by contracting out the skiving process, while others will prefer to bring the technology to their own facilities.

Yet another catheter-processing procedure is drilling the very precise holes—less than 0.004 in. diam—that are often required in F1–F4 catheters. The primary purpose of such holes is venting of an IV delivery system, whereby, for example, air is expelled by squeezing an attached saline bag while viscosity and surface tension prevent the saline from escaping. Precise microholes also allow for accurately metered and distributed drug delivery to locations within the body. Another example is a catheter with electrical sensors that can be used to monitor blood oxygenation in premature babies: the blood is sucked into the catheter through a 700-µm-diam hole machined by an excimer laser. A different laser process is employed in assembling the electrical sensors in these catheters, which involves stripping the plastic insulating sleeve from the very small sensor wires before soldering.

Finally, lasers can be used to machine plastic catheter profiles. The precision and control of laser machining produces an IV catheter of superior quality. Liquid flow is precise and insertion is smooth and easy (minimum insertion force).

Plastic Film Skiving/Excising. Plastic films from polymers such as polyimide, polyester, polyurethane, PTFE (Teflon), parylene, and PVC are some of the more common materials used to insulate metal or glass substrates, especially in medical device electronics, where they often serve as electrical insulators. In fluidic applications, such film overlays help direct drug flow more easily. When manual film-excision techniques such as the use of small blades are employed, the production yield becomes a function of the skill of the operator, since any tiny nicks into the underlying glass or metal layer are unacceptable. Hence, the laser beam, with its very precise depth control and discrimination for the underlying substrate, presents an attractive processing option for applications such as the stripping of insulating layers from electrical leads used in heart defibrillators and pacemakers. The process is similar to that used for wire stripping for computer disk drive heads (Figure 3).

Figure 3. Parylene removed from tapered precious metal pin using 248-nm excimer laser.

Laser cutting, skiving, or barring of polyimide flex circuits is common in the industry. The inherent miniaturization of medical devices intensifies the need for this process in medical electronics. Flex circuits for pacemakers, hearing aids, and ultrasonic transducers are often etched and cut by using lasers.

Microdrilling. In addition to catheters, many other medical devices require the drilling of very small and precise holes. The purpose of the holes can range from a mechanism for time-release flow from capsules, to interconnect conduits for electronic components such as coil actuators. As the holes get extremely small (<50 µm), laser drilling may be the only practicable technology.

Precision microhole drilling can be applied to latex condoms where dimensionally controlled holes are used to calibrate condom leak-testing equipment. The production process for condoms has the potential to leave minute voids that need to be detected in an electronic test process. The test devices are continuously recalibrated by examining condoms with predrilled holes of known sizes (normally 10 µm in diam). These holes are typically drilled using an excimer laser.

Another promising application is the fabrication of biomedical meshes and artificial skin—novel materials used for guided tissue regeneration that have appeared on the market in the last several years. These membrane products are positioned as a support for in vitro growth of skin cells, notably keratinocytes. These cells proliferate on the membrane, creating a uniform cellular network and ultimately a cell sheet that is then applied to a burn or wound site, accelerating the healing process.

A technique currently under study involves laser microfenestration of contact lenses. Contact lens discomfort is caused primarily by a lack of oxygen at the corneal surface, and microfenestration of the lenses can increase oxygen permeability. Because of its short wavelength and nonthermal interaction with the substrate material, an excimer laser allows one to precisely control the process.

Microchannel Machining. Researchers in biomedical fields often need to sort molecules in solution according to their sizes. Electrophoresis is a favored separation method for large biomolecules such as proteins and DNA fragments. In traditional electrophoresis, a small amount of solution is dropped onto one end of a glass plate that is covered with a layer of fine silica gel. As electrodes on the ends of the plate create an electrical field, most biomolecules acquire a net charge in solution and thus are drawn along the gel by the field. Larger molecules travel slower than smaller ones, so simple size separation is possible. Recently developed techniques have addressed problems related to speed and sample size, since large biomolecules like DNA can take many hours to separate by gel electrophoresis, and researchers often have only microscopic amounts of samples with which to work. One of the newest methods is microchannel plate electrophoresis, which enables separations in minutes instead of hours. In this technique, tiny channels (grooves) are created in plastic plates with an excimer laser. Typical groove depths are in the range of 5 to 50 µm, with widths from 10 to 200 µm. Similar microchannels can be generated in glass for biomedical cell counting (Figure 4).

Figure 4. Precision grid (100-µm squares, 5-µm linewidths) on glass for biomedical counting, generated by a 248-nm excimer laser.

Microsensor Processing. A popular application employs lasers to ablate the dielectric film, such as parylene, from the electrodes of microdisk-array sensors used for processes such as anode stripping voltametry. With an excimer laser, an array of holes can be drilled through the parylene, with each array containing several thousand holes. The hole pattern and the number of holes can be optimized for particular detection requirements. The low production costs allow the microdisk array to be offered as a disposable cell.

Device Marking. Devices such as surgical tools, pacemakers, and various other implants can be permanently laser engraved for ultimate product traceability, as can a wide range of plastics. For complex multilumen catheters, lasers can be used to mark or notch along the tube, either to help orient the part for further manufacturing procedures or for lot-control identification. Although other technologies can be used for marking or notching, the ability to carry out processes like marking, skiving, stripping, and drilling with a single process is an attractive option.

CONTRACT MANUFACTURING

There are four essential keys to successful contract manufacturing using laser micromachining technology:

Capability and Capacity. For most medical device companies, achieving product repeatability within a given specification is an absolute necessity. When products are contracted out, incoming material inspection alone does not ensure a reliable flow of material conforming to product specifications. It is incumbent upon the contract manufacturer to ensure this flow of material by rigorously controlling any material or process variability.

The contract manufacturer's operations should also be of sufficient size to allow for redundancy of laser processing systems, operators, in-process inspectors, and manufacturing and maintenance engineers. The reason for this is that reliance on a single engineer's skill or job-specific experience to set up and complete the production of complex parts—or, similarly, reliance on a single piece of equipment—is highly risky. Cross-training programs and redundancy of equipment are essential in mitigating this risk.

Another concern often overlooked is the level of cleanroom capacity. The use of cleanroom space for manufacturing continues to increase as product complexity rises, product feature sizes decrease, and environmental controls become more stringent. Many medical devices must be processed entirely within a certified cleanroom environment (Figure 5). Manufacturers should insist on performing vendor audits or certifications if their products require a high degree of cleanliness.

Figure 5. Class 10,000 cleanroom for laser micromachining medical devices in contract manufacturing environment.

Control of the Process. Manufacturing quality is based on defining a process, monitoring it, and controlling it. Laser micromachining comprises many variables, among them the laser source, wavelength, optical-beam delivery, power and energy levels, optimum focus, in situ environment, flow-gas requirements, positioning, and part-handling equipment. Statistical process control (SPC) is an excellent tool for monitoring these variables. In essence, SPC can detect when a process begins to deviate from specifications and can prevent material and resources from being wasted. Device manufacturers should require vendors to provide them with a process control specification for all products (Figure 6).

Figure 6. Sample process control specification.

Understanding FDA Requirements. Regardless of the level of FDA scrutiny of a particular product, a contract manufacturer must be able to identify and control the flow of raw material or parts as they enter the processing facility and make their way through various operational work centers before being packaged and shipped to the client. Product security, hygiene, and lot control are essential. Often, achieving this level of control can be as simple as assigning responsibility to an individual for a specific project. Other times, it can be complex enough to require a procedural system that guarantees that a process cannot be compromised by bad or rejected material or parts. This quarantine type of system is commonly found in certified quality systems and is typical in most medical device manufacturing facilities. In order for a manufacturer to understand a supplier's methods for material control, the supplier's flow diagram should be examined (Figure 7).

Figure 7. Process flow diagram.

Focus on Manufacturability. Laser micromachining technology was derived from the scientific community. As a result, several small job shops have emerged in which the emphasis is placed on pure design and innovation. However, as manufacturing engineers realize too well, a product design may be wonderful and innovative, but the next step of implementing or outsourcing the manufacturing of that product requires a completely different level of discipline. Many times, a product that works wonderfully in a laboratory environment cannot be reproducibly manufactured, or manufacturing it proves to be prohibitively expensive.

Therefore, the keyword of successful contract manufacturing using laser micromachining technology is manufacturing, not technology. Ideally, a contract manufacturer should be headed by an operations group that has a strong manufacturing background. In turn, this group can be supported by technical specialists in their respective laser application fields. Such a team approach brings the technology into a realistic, operator-driven environment, facilitating control of quality, cost, and timeliness.

CONCLUSION

Laser micromachining is an established technology that is becoming both a necessity and a valuable asset in the fabrication of medical devices. With medical devices becoming smaller yet more complex, reliable lasers and laser systems offer manufacturers the opportunity to design medical devices that are very difficult, if not impossible, to produce using traditional fabrication tools. As competition in the industry escalates, companies are turning to contract manufacturers with long-established laser micromachining experience who can help them remove costly manual operations from existing production lines. These same contract manufacturers are increasingly helping client companies automate their production processes with laser technology, further strengthening the fit between laser micromachining and the medical device industry.

ACKNOWLEDGMENTS

The authors would like to thank Glenn Ogura and Cliff Gabay of Resonetics, Inc. (Nashua, NH), for their helpful input during the preparation of this article. Bo Gu would like to extend his thanks to his former colleagues at Lumonics, Inc. (Kanata, ON, Canada) for some useful discussions.

The authors are employees of Resonetics, Inc. (Nashua, NH). Bo Gu, PhD, is director of marketing; Rich Hunter is vice president of operations; David Wall is director of sales; and Mike Frechette is manager of quality affairs/quality control.

Lead photo by Coherent Inc., Laser Group

Copyright ©1998 Medical Device & Diagnostic Industry