Creating a new generation of medical implants

Christoph Zwahr and Frederic Schell describe how direct laser interference patterning can augment additively manufactured medical implants

This article was co-authored by Phil Goldberg, Avinash Hariharan and Annett Gebert, of the Leibniz Institute for Solid State and Materials Research Dresden

With the increasing average lifespan of populations in developed countries, skeletal and bone diseases such as osteoporosis are expected to increase significantly.

Making progress in implant technology is therefore becoming more vital than ever.

Additive manufacturing (AM) offers high flexibility in fabricating novel implants that can be individually tailored to the patients’ needs with minimal effort compared to classical manufacturing processes.

The layer-based construction of the implants allows for cavities and scaffold geometries that would otherwise not be possible. 

Another way to optimise implants is to modify their surface, which is known to influence adhesion and proliferation of bone cells^1,2. An established method of altering the implant topography and enhance osseointegration is grain blasting and subsequent etching of the surface³. However, these processes can introduce silica or aluminium impurities as well as residues of acid to the implant surface, which can have a negative impact on osseointegration.

New possibilities using ultrashort pulse lasers

Laser-based surface texturing techniques offer efficiency and scalability and are an excellent choice for fabricating periodic textures on the implant surface. Especially with the advent of ultrashort pulse (USP) lasers, surface texturing is on the way to becoming a competitive technology for industrial applications. USP lasers emit pulses of radiation that are typically only picoseconds (10^-12s) down to femtoseconds (10^-15s) long. Compared to the more established nanosecond-pulsed lasers, this technology offers multiple advantages. During laser material interaction, surface material is vaporised, which can shield the workpiece from further incoming radiation.

However, ultrashort pulses interact with the material on such a short time scale that vaporisation takes place mostly after the energy of the pulse is already transferred to the surface. Furthermore, ultrashort pulses enable so-called ‘cold ablation’, where thermal influence on the material is reduced significantly compared to longer pulse durations due to the highly localised energy density, allowing for extremely precise texturing of the surface while avoiding almost any melting. At the ultrashort timescale, nonlinear effects such as multi-photon absorption become relevant, allowing the processing of materials that are otherwise transparent to laser radiation.

Microtextured implant surfaces for enhanced osseointegration

The joint project ‘OsteoLas’, set to conclude at the end of March, is being carried out by the Fraunhofer Institute for Material and Beam Technology IWS together with Leibniz Institute for Solid State and Materials Research Dresden. Through it we have been investigating how laser-based surface textures can be applied to additively manufactured near-beta titanium implants to form a new generation of medical implants. 

Using laser powder bed fusion we have fabricated implant samples from the novel near-beta Ti-13Nb-13Zr alloy, which offers high yield-strength and a Young’s modulus close to that of bone. The implant surfaces are then textured using direct laser interference patterning (DLIP), where the laser beam of a USP laser is split into two sub-beams using a diffractive optical element (see Figure 1a). The beams are superimposed on the material’s surface to form a line-like interference pattern that locally ablates the material at the interference maxima positions, while not exposing it to radiation in the interference minima positions. 

Figure 1: a) Experimental DLIP setup, b) pulse-wise texturing method using pulse overlap and c) DLIPflex module developed by Fraunhofer IWS with indicated raw beam and sub-beams

This technique offers the advantage of producing periodic micro- and nanotextures inside a single laser pulse, and the ability to produce feature sizes not restricted by the diffraction limit, while also being able to reach high throughputs compared to conventional laser texturing methods such as direct laser writing. 

Throughout the course of the OsteoLas project, textures have been fabricated using ultraviolet (UV, 355nm), visible (VIS, 532nm) and near-infrared (NIR, 1,064nm) laser wavelengths by converting the fundamental harmonic of a 12ps laser source using a second and third harmonic generator. The line-like interference pattern emerging in the overlapping volume of the beams is applied in a pulse-wise texturing strategy to the material surface, where pulses are overlapped by translating the sample by the pulse distance and hatching lines at an integer multiple of the pattern period (see Figure 1b). 

At Fraunhofer IWS we have developed a compact DLIP module based on the principle in Figure 1a that can be modified to work with UV, VIS and NIR radiation emitted by USP laser systems (see Figure 1c).

Figure 2 shows scanning electron microscope images of an exemplary texture fabricated with each wavelength, showing the main DLIP pattern and so-called laser-induced periodic surface structures (LIPSS) that arise from the light-material interaction, especially when using USP lasers. Among these, the grooves possess periods greater than the wavelength and can be controlled by the total laser fluence per unit area. On the other hand, the size of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) is determined mostly by the laser wavelength itself, where larger wavelengths produce larger LSFL and HSFL periods. Our results show that USP-DLIP is a versatile tool to fabricate complex hierarchical structures that can be fine-tuned for achieving precise geometries regarding pattern period, depth as well as multiscale features.

Figure 2: Examples of hierarchical periodic surface textures achieved with three different laser wavelengths. Different scale features are marked in the images⁴

In the project we have been evaluating these textures in terms of their chemical, mechanical and corrosive properties, as well as their performance regarding the adhesion and proliferation of human osteoblast cells. Ongoing cell-tests have already shown promising in-vitro performance regarding multiple investigated osteoblast parameters. 

Process monitoring for quality assurance

The production of DLIP surface patterns with micro- and sub-micron resolution requires advanced monitoring strategies to ensure ablation quality and repeatability of DLIP patterns. In addition, the monitoring systems employed require inline capabilities to enable closed-loop processing. One possible approach that has been used to identify the working position for DLIP is to use the acoustic emission generated by laser interaction with the surface⁵. It has been shown that the interference working position can be identified and monitored by analysing the change in sound level during processing. In the future, this approach will be used to automatically maintain the working distance at a specific position when machining non-planar surfaces, such as additively manufactured implants.

Preparing for industrialisation

In order for the laser-based texturing methods to become viable for industrial applications, high throughput and process reliability are critical factors. The design of classical DLIP heads offers flexibility in texture periods, however only provides a relatively small volume of interference, which can be detrimental to the process stability when working with complex 3D-shapes. These challenges can be tackled by using high-speed DLIP concepts, such as that developed by Fraunhofer IWS⁶ which creates elongated interference areas up to several millimetres in length compared to tens to hundreds of micrometres for classical DLIP and reaching throughputs up to 1m²/min⁷. When the beams are also collimated in the axis perpendicular to the interference lines, a very large vertical processing window of up to a centimetre can be obtained, over the length of which the pattern period remains virtually stable. 

Using these high-speed and high-tolerance DLIP solutions, the industrial scale texturing of implant surfaces comes within reach of becoming the next step in battling the growing prevalence of bone diseases.

Christoph Zwahr leads the direct laser interference patterning working group at the Fraunhofer Institute for Material and Beam Technology IWS

Frederic Schell is a PhD candidate at the Fraunhofer Institute for Material and Beam Technology IWS

^{This work was partially financed by the European Regional Development Fund (EFRE) and by tax revenues on the basis of the budget adopted by the Members of the Parliament of Saxony (funding reference 100382988 / 100382989).}

References

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