A Laser Focus on Implant Surfaces

During the 1980s, the medical community embraced titanium as the material of choice for implantable devices destined to be attached to bone. Numerous studies presented titanium’s osseointegration characteristics and further research documented the positive impact of textured, functional surfaces on osseointegration. Compared to smooth “as-machined” surfaces, texturing not only improves bone integration—and thus, implant stability—it also allows for the growth of supportive tissue and may even provide antibacterial advantages.

Today, the gold standard for titanium implants features a textured surface on all areas where integration with bone needs to take place. These functional textured surfaces are found on diverse devices such as bone plates, hip joints, and cervical and dental implants (Figure 1).

Additional research has been conducted in recent years to examine the relationship between the roughness of the surface and both osseointegration and vascularization rates. Determining the optimum values for surface roughness for a given set of conditions is expected to remain an area of focus for several years to come. Although in the past, roughness was mostly described by the two-dimensional Ra value (a measure of the variation in height), 3D measurements, including the arithmetical mean height of the surface (Sa), texture aspect ratio (STr), interfacial area ratio (Sdr), core void volume (Vvc) and valley void volume (Vvv) are all now commonly used to describe a desired end result.

Production Challenges
Textured surfaces have traditionally been executed through the use of grit blasting (also known as sandblasting) alone or in combination with chemical etching. Grit blasting in combination with acid etching is perhaps the most widely used approach today. The etching process involves using a strong acid—often hydrochloric, nitric, or sulfuric acid—to erode the surface after blasting, resulting in microstructures from one to several microns in diameter. A neutralization phase, in which the acid is reduced by the addition of a base or multiple washing cycles with de-ionized water, completes the process. Etching is highly sensitive to both time and temperature and requires appropriate infrastructure to handle both dangerous chemicals and waste by-products.

Blasting is a mechanical process by which a hard particulate (sand, salt, or ceramic) is projected at high velocity against the surface to be treated. Particle size, velocity, and impact angle all influence final surface roughness. In medical device manufacturing, the blasting process is most often “semi-automated,” whereby process time and blast angle are controlled via a robotic system.

Both blasting and etching result in a random distribution of surface features. Chemical changes to the outer layer of titanium can also occur. The choice of blast material as well as process parameters (time, angle, size, and blast velocity) need to be developed based on the desired end result. Most often, the blast material is a single-use consumable and must be disposed of after part processing.

Secondary Processes Introduce Risk
Both blasting and chemical etching usually require manual handling by operators in order to mask surfaces that are to remain untextured. Both also require the component to be cleaned after the texturing treatment; residue from blasting is difficult, if not impossible, to eliminate entirely. Proper washing after blasting can be a multistep process that adds time and requires specialized equipment, such as tanks or ultrasonic wave technology.

An error in the washing process (e.g., wrong or incorrectly dosed chemicals, incorrect cycle) or handling can result in serious product quality issues or lead to product failure and costly recalls. Due to these issues, orthopedic device manufacturers are looking for alternative ways to manufacture textured functional surfaces without secondary washing and gain design flexibility in terms of surface properties.

Laser Texturing Offers Alternative
Laser texturing replaces a random process (e.g., blasting, etching) with a digital one (Figure 2). Pulses of laser light, often delivered in a nitrogen or argon shield gas environment, are directed at the material’s surface. The laser heats and modifies the metal, creating local surface deformation. Pulse duration—measured in nano-, pico-, or femtoseconds—is very short and the impact location and pattern are precisely controlled with an end result that:

• Is repeatable (not dependent on operator judgement but programmed into the controller).
• Is clean (no particulate generated, little or no chemical change to the metal).
• Does not use consumables such as blasting materials or acids, bases, or washing systems.
• Does not require secondary handling, part masking, or (in most cases) any secondary cleaning.
• Does not require 100 percent final product inspection.

The use of laser texturing allows a surface to be structured with a precise, repeatable pattern and enables both product designers and manufacturers to design in and meet more exacting specifications for roughness. Multiple different textures can be applied on the same device without any need for masking. Further, elements such as 2D bar codes or other unique device identification (UDI) features can be easily integrated. Manufacturers concerned with counterfeiting can use proprietary textures, or add logos or hidden patterns confirming the device origin (detectable even after many years of use). Recent research has also indicated that precisely controlled, laser-generated nanostructures may yield functional surfaces with antibacterial properties.

The choice of laser texturing technology (i.e., the type of system required based upon pulse duration, power, and texturing pattern) will be driven by the desired surface the manufacturer wants to produce. Lasers that operate with a nanopulse duration will both ionize metal and locally heat the surface being treated, resulting in a surface with an increased Sdr, Vvc, and Vvv as compared to femtosecond lasers. Femtosecond pulses are much shorter and essentially eliminate the heat-affected zone, resulting in a surface with lower variation (Figure 3). Both technologies are useful; again, the choice depends on the desired end result for the surface in question.


Most textured surfaces are not flat, but have complex, curved geometries. A key criterion in the quality of a laser texturing solution is the ability of the system to correctly compensate for these curved surfaces. Most commercially available systems introduce errors when applying a texture to a curved geometry. There are laser texturing solutions, however, that make use of advanced software to allow the texturing of even the most complex surfaces.

Conclusion
The use of texturing on functional surfaces in order to speed bone growth and provide other patient benefits is well documented. Texturing can be achieved through various processes, including blasting, chemical etching, and laser texturing. Blasting and etching, however, create a random surface and increase both cost and risk by requiring multiple part handling and cleaning operations, as well as use of consumables (e.g., blasting materials, acids) and infrastructure.

A functional surface produced using laser light and an appropriate shield gas under a digital process results in a repeatable, structured functional surface that is essentially identical for all parts being produced. In addition, laser texturing significantly reduces, or even eliminates, the risk associated with masking non-textured surfaces and secondary cleaning. The ability of a laser texturing solution to correctly adapt textures to curved surfaces is a key for overall product quality. Improved part quality, increased product differentiation, the ability to customize patterns (and more easily identify counterfeit products), a reduced risk of contamination, and reduced production costs and time are all benefits that manufacturers can derive from laser texturing. 

---

Characteristics of the GF Machining Solutions’ Laser P 400 U The Laser P 400 U is a one-of-a-kind laser engraving, texturing, and marking solution system that combines efficiency with femtosecond laser technology in an extremely small footprint. It is specifically designed for aesthetic and functional texturing of precision parts. The system offers clean, sharp results, as well as burr-free texturing and engraving with no post-treatment steps required. The machine employs an ytterbium-pulsed fiber laser in combination with an ultrashort-pulsed laser (femtosecond) to achieve high quality. The unit offers: Burr-free laser marking, engraving, and texturing Available as a three- or full five-axis tool X, Y, Z travel of 400 x 600 x 300 mm; 4th and 5th axis capability Configurable with single or twin laser heads Scalable from nano- to femto-pulse configurations Complete software package, including LaserDesign texturing software Ready for fully automated manufacturing