LightStem

Motivation

Metallic implants such as endoprostheses in the area of bone and especially as joint replacements are part of modern medical care. In the contact area between the metallic implant surface and the patient's own body tissue, a biomechanical interface is formed with almost abrupt changes in the mechanical properties. 

This often leads to tissue irritation with bone resorption and subsequent loosening of the implant. Poorly ingrown implant surfaces also allow colonization by bacteria, which can lead to inflammation and even rejection, as well as sepsis. In the case of interaction with blood, the implant surface can cause further complications such as thrombosis or abnormal tissue proliferation, which are in direct opposition to the aim of the implant application. These negative influencing factors caused by technical material surfaces in contact with body media have a negative effect on healing in the best case and a critical effect on the patient's condition in the worst case. 

One way of counteracting this is the targeted modification of material surfaces, which in the case of endoprostheses is already done by sandblasting or etching processes, for example. Contrary to the resulting stochastic topography, the generation of deterministic periodic structures using Direct Laser Interference Patterning (DLIP) makes it possible to optimize the interaction between body tissue and the technical material surface to achieve the desired interaction.

Aims and Procedure

Using DLIP, customized periodic surface topographies in the micro- and nanometer scale range reduce the adhesion and interaction possibilities e.g. for bacteria and thrombocytes, or additionally enhance inherent antimicrobial effects. On the other hand, similar microtopographies for endothelial and fibroblast cells can create favorable conditions for implant colonization and body integration, which promote healing and suppress further inflammatory processes.

The challenge is to functionalize the usually complex geometry of the implants with the help of DLIP technology. Until now, direct laser interference structuring has been used for flat component surfaces. With the development of a system with 5-axis workpiece guidance, Fraunhofer IWS has addressed this technology gap and will functionalize titanium hip joints with DLIP for the first time in the LightStem project. Fraunhofer IWS is supported by its partner SurFunction GmbH, which will develop a new type of DLIP optics with a large inference volume and tophat profile during the course of the project. The 5-axis workpiece guide with implemented SurFunction optics module will enable homogeneous DLIP structures on free-form surfaces.

Innovations and Perspectives

By combining the technological know-how of both partners, a process technology can be developed for the first time that enables the realization of extended surface functionalizations to increase biocompatibility and improve the ingrowth behavior of patient tissue and to advance the transfer of the technology into industrial practice.
 

Project Results

As part of the LightStem project, a 3D structuring system for direct laser interference structuring (DLIP) of complex implant geometries was successfully built and commissioned at Fraunhofer IWS. In addition, SurFunction GmbH developed an ELIPSYS optical system specifically designed for 5-axis machining, featuring an elongated laser focus and adjustable ellipticity, optimized for a structural period of 5 µm. In joint experiments, suitable process parameters were determined for generating homogeneous surface structures with varying depths and aspect ratios of up to 1:1.

Through the development of a proprietary script-based path control system, the team ultimately succeeded in achieving full-surface DLIP structuring of a titanium hip socket. This marked the first successful demonstration of the functionalization of complex three-dimensional implant geometries using DLIP.

The results establish an important technological foundation for the targeted functionalization of implant surfaces. Looking ahead, the technology opens up new possibilities for improving the biocompatibility and osseointegration of implants and for reducing undesirable biological interactions.