When most people think of a laser, a continuous beam of light is what comes to mind. While this is one way in which the technology is used, in some aspects of manufacturing, the beam is delivered in pulses (i.e., short, rapid bursts of light with energy in each pulse ablating the target material). Control over each pulse—from pulse repetition frequency to the pulse energy and focused intensity—enables accurate control over the process of material removal. 

Conventional lasers, with pulse duration measured in nanoseconds, can ablate material efficiently. However, nanosecond pulses typically also create rapid melting and ejection of the target material. This, in turn, can result in a heat-affected zone (HAZ) with collateral slag and rough edges. In comparison, femtosecond lasers produce ultra-short pulses of light that can be as brief as 290 femtoseconds (10^-15 seconds) long. When an ultra-short pulse of light interacts with a surface, the burst of energy transmitted is so short that material is removed with little or no heat transferred to the surrounding area. The material is removed by the high peak power of the femtosecond laser breaking the molecular bonds of the material, as opposed to rapid melting and ejection. The result is sharp, clean ablation with high accuracy and no HAZ.

Adding to the advancement of laser technology is the ability to electronically fine-tune the pulse duration from a few hundred femtoseconds to picoseconds and nanoseconds, as well as the ability to program collections of pulses, allowing users to optimize the final results and capitalize upon unique designs.

Machining Versus Surface Structuring

Generally, there are two approaches in pulsed laser-based processes: machining and surface structuring. Machining is defined as creating a final shape through the removal of material (i.e., subtractive manufacturing), as would be accomplished with a milling machine. While the material removal rates are relatively low, the laser adds accuracy and micron-level precision, greater than that possible with traditional milling or other technologies. 

Laser surface structuring involves treating the external layer of a product through the use of a laser to change the surface through either a random or a strictly defined pattern of photon energy blasts. While the goal of this process may be for aesthetics (e.g., to create a matte effect), it can also be leveraged to generate functional surfaces, such as to enable osseointegration. While the laser source is the same, it is useful to note the laser optics for other processes, such as micromachining or milling, are different than those for surface structuring.

Surface Structuring

Rough surfaces are often produced using sand or grit blasting, which may be accompanied by a secondary treatment with acid. Originally patented in 1870, sandblasting has not evolved substantially since. It still involves the use of air to propel small, hard particles onto a surface. However, the blasting process is not always reliable or repeatable. Variations in the size of the blast material (often aluminum oxide) and changes in air pressure, humidity level, and temperature can result in differences in the surface produced. Grit blasting remains difficult to repeat and is an energy-intensive and dirty process that can leave contaminants on the product. 

For over a decade, it has been understood lasers can also create a surface structure very similar to grit blasting. However, the laser-based process is superior since it is a digital, repeatable technology that is fundamentally clean. A laser machine can also blast only where required, thus eliminating the need for masking/de-masking. 

Previously, the challenge involved the speed of lasers—too slow and, thus, not economically viable. That obstacle has been resolved. New technologies have been designed for better precision and high speed, offering new and efficient possibilities in medical device manufacturing. Benefitting from thermally stabilized linear drives, a polymer concrete base, unique and highly efficient software, and a digital Z-axis that shortens processing times to just a few minutes, this new class of lasers allows many implant types and even many surgical instruments to be laser-treated at a cost below traditional sandblasting. 

These lasers have also proven capable of producing surfaces for bone growth that have outperformed traditional processes in short-term stability. In a small animal study, implant retention after four weeks was over two times higher when a bio-inspired, femtosecond laser-structured surface was employed as compared to a surface established with a state-of-the-art sandblasting and acid-etching device. Ongoing research is investigating laser-structured surfaces that have antibacterial, hydrophobic, hydrophilic, and soft-tissue integrative properties. 

Since the laser can fabricate a nearly infinite number of textures—each producing slightly different results in terms of topography and material interaction—it should often be considered the optimal choice for this type of application. Clean and repeatable, the laser can simplify and reduce risk in manufacturing, while potentially providing superior results for the patient. 

Micromachining

Medical devices, including implants (e.g., spine), are becoming smaller and more complex. Lasers are enabling the machining of smaller features and providing better precision, opening design options that were unthinkable only a few years ago. With a spot size as small as 13 microns in diameter—less than the most intricate cutting tool—lasers enable miniature, accurate features to be machined in even the hardest of materials. Capable of machining a cutting edge with a radius of approximately 15 microns (more than twice as sharp as the edge of a razor blade), lasers can create unique features. 

Achieving this precision requires both accurate five-axis movement of the workpiece and a software/CAM program that is specific to the laser process and the physical characteristics of the beam. 

Conclusion

Lasers are a relatively recent arrival in the manufacturing toolbox; however, they can enable orthopedic implant designers and manufacturers to create devices with functional surfaces that often outperform older state-of-the-art technology or benefit from a cleaner, more repeatable process than sandblasting. The technology is evolving quickly and can help accelerate the product and production innovation process.

Reference Sources/For More Information

1. tinyurl.com/mpo251161
2. tinyurl.com/mpo251162
3. tinyurl.com/mpo251163
4. tinyurl.com/mpo251164
5. tinyurl.com/mpo251165

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Erik Poulsen is a mechanical engineer with decades of experience in the design and build of complex medical device production equipment. He has held various roles in engineering, project management, and sales management at companies in both Europe and North America. Poulsen has been working at UNITED MACHINING since 2018 as the global medical market manager. He is based in Biel, Switzerland.

Mark Keirstead has worked in various roles within the laser industry, including R&D, product management, marketing, and international sales management across Asia and Latin America. He holds 22 U.S. patents for the development of solid-state laser technologies. Keirstead is currently the North America sales manager for UNITED MACHINING’s Advanced Solutions Group. He is based in Silicon Valley, Calif.