By Christian Nölke, Matthias Gieseke, Ronny Hagemann and Stefan Kaierle
Using two step laser additive manufacturing (LAM), commonly known as Selective Laser Melting, offers the opportunity to manufacture three dimensional (3D) parts. This manufacturing technique has gained a lot of attention and interest during the recent years and is of particular interest because prior computer tomography investigations allow an easy providing of the required patient individual dataset. This way offers a completely digital process chain until the required implant is produced. Manufacturing of steel and titanium implants with adapted surface structures and adapted mechanical properties is already within the scope of industrial research.
Using additive manufactured products for surgery is nothing completely new within the medical area. Currently there are already a number of applications where such products are used. One option is the use of additive manufactured parts for the patient individual pre-operative planning of complex surgeries. This enables the visualization and planning of critical steps. Second there is a use of AM products for patient individual manufactured surgical guides that can simplify the correct placement and increase the accuracy of the required cuttings and drillings. And finally there is the manufacturing of inert implants itself; prominent examples are tooth implants and hip cups.
Although AM products are already in use, this technique is still in the very beginning stages, surgeons are just recognizing its potential and, consequently, they are continuously developing new applications and demands for medical AM components. So, when we are looking towards the future of medical implants, there is an increasing request for special properties of the implants like individuality, “intelligence” and biodegradability. Consequently, one approach in the research line of LZH e.V. is the SLM-based processing of special materials to realize individual implants with tailored properties regarding their intelligent functionality and biodegradability.
Two alloys that are currently under investigation are suitable to fulfill these requests: First, shape memory alloys like Nitinol can be used to realize actuating properties within micro actuators that can be enabled on demand. And second, adequate biodegradable magnesium alloys can offer a tailored degradation behavior for osteosynthesis or vascular application. Due to their specific material properties, successful processing using SLM is challenging for each of these alloys. Special machine systems have been developed to overcome restrictions regarding the environmental conditions.
Processing of Nitinol requires a special inert process atmosphere, free of any impurities due to the sensitive behavior of the material regarding changes in its chemical composition. Slight changes already show a significant impact on the grade of functionality and the activation temperature of the one way shape memory effect. Due to this, a special laboratory SLM machine system has been developed for processing of Nitinol (Fig. 1A). Up to now it has been shown that it is possible to realize fully functional NiTi-parts using a Selective Laser Melting process (Fig. 1B). Nevertheless there is a significant dependency regarding the functional properties between each step of the production line that has to be taken into account to finally achieve the requested material characteristics. One possible medical application is the use of Nitinol for cochlea implants to improve the insertion behavior and the custom fit of these implants.
Figure 1A. Laboratory machine platform for SLM of Nitinol
Figure 1B. Samples made by SLM of Nitinol
Investigations on SLM of magnesium are carried out at Laser Zentrum Hannover e.V. using an industrial SLM machine system with an overpressure building chamber in order to overcome existing difficulties in processing magnesium. The new machine platform allows two bar overpressure within the processing chamber and is used to investigate the processing behavior of magnesium powders at elevated pressure conditions. Since magnesium alloys show a non-standard processing behavior, manufacturing of non-porous and three dimensional parts from magnesium was not possible yet. Currently, the first promising result could be achieved, following a new strategy (Fig. 2).
Figure 2. Samples made by SLM of Magnesium
Finally, the SLM process of Mg-alloys shall be used for the production of bioresorbable implants for the individual replacement of cranial defects. This means, that directly after the surgery the implant is the load bearing part. Through the healing procedure, the bone is regenerating and the implant is slowly disappearing during this time. Consequently, there will be a successive change of the load bearing from the implant towards the regenerated bone. If necessary, there is the option foreseen to integrate a titanium mesh to increase the mechanical stability of the hybrid implant (Fig. 3). Using an additional P(3HB) polymer coating shall realize a controlled degradation behavior of the implant during the healing procedure. In combination with a special pre–vitalizing, an improved cell growth shall be achieved.
Figure 3. Schematic sketch of the hybrid implant structure
In conclusion, it can be said that AM products are not only interesting for industrial applications coming from the automotive and aircraft companies. There is a strong demand existing in the medical area as well, and there are new demands growing continuously for future implants and procedures. Using SLM manufactured Niti-parts could be one way to increase the functionality of implants and biodegradable magnesium alloys can be a way to provide metallic bioresorbable implants.
The presented results are partially funded within the national projects: BMBF “Remedis” (FKZ: 03IS2081), “Gentle CI” (FKZ: 16SV3944) and the DFG project HA 1213/77-1.