As part of the BioStruct project, RWTH Aachen University Digital Additive Production (DAP), Germany, is developing a process for a novel zinc-magnesium alloy combination with lattice structures which can only be produced via Laser Beam Powder Bed Fusion (PBF-LB) Additive Manufacturing. The project results will be transferred to the reACT Alliance, which began its interdisciplinary research on the subject in the second half of 2022.
Permanent implants made from titanium or surgical steels and autologous bone grafts (transplants made from the patient’s own bones) are among the most commonly used solutions for treating bone defects. However, they can only partially meet the complex requirements for a patient-friendly healing process. The mechanical properties of permanent implants lead to reduced stress on surrounding bone tissue, weakening it and increasing the risk of refracture, and the bone tissue itself does not heal. Additionally, prolonged retention of implants in the body increases the risk of further surgical intervention, especially in an ageing population.
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Although autologous bone transplants promote self-healing of surrounding bone tissue and therefore lead to optimal treatment results, they can only be used for a certain volume of defects. To this day, ‘critical-size’ bone defects pose a complex medical problem: due to the large missing bone volume and the distance between any free bone ends, the bone cannot heal on its own in these defects. With that in mind, the partners of the BioStruct consortium are developing a bioresorbable implant concept whose material properties and geometric design meet the complex requirements of patient-friendly bone healing.
The challenge here lies in the selection of suitable and processable materials and geometries, both for the body and for processing using PBF-LB. Zinc and magnesium alloys in particular have shown promising results in the field of resorbable bone implants.
In the context of alloy development of bioresorbable metallic alloys, pure zinc (Zn) is characterised by good degradation properties in the human body. However, its mechanical strength is not sufficient for use as an implant. Magnesium (Mg), on the other hand, is already used as a material for the manufacture of implants, such as in foot surgery, due to its bone-like mechanical properties. However, it degrades too quickly in the body in special applications, and gas formation can occur in the moist environment of the tissue. For this reason, the research is investigating different alloy compositions of these pure metals to effectively combine the properties of both for use in the body as well as suitability for PBF-LB.
Laser Beam Powder Bed Fusion provides new design possibilities for implants to best meet patient-specific requirements, such as mechanical stress and corrosion behaviour at the site of application. The approach lies in an algorithmic lattice structure design: based on the defined requirements, geometry and arrangement of the individual struts or lattice cells are generated parametrically, the resulting lattice structure is adapted to the bone defect site and prepared for production using PBF-LB.
The strut diameter is an important parameter in this context. Adjustments to the lattice structure design allow for uniform corrosion throughout the entire component, as well as the flushing out of degradation products and the simultaneous ingrowth of tissue as the implant is absorbed by the body.
In their research, the scientists were able to achieve grain refinement and targeted microstructure adjustment by adding small amounts of magnesium to zinc. In a wide-ranging alloy screening of different compositions, from pure zinc to a Zn8Mg alloy, the ZnMg alloy with ≤ 1 wt-% magnesium showed the best properties for use as a bone replacement product. A first demonstrator in the form of a lattice-structured jawbone implant was successfully and reproducibly manufactured from this ZnMg alloy. The strut diameter of the lattice structure used in the demonstrator is 200 μm.
Additional structures were manufactured via PBF-LB for investigations of the biocompatibility of ZnMg. In the future, these structures are intended to form a stable framework for the infiltration of materials such as collagen or fibroin, enabling directed bone growth.
The results from the BioStruct project are to be further developed in another interdisciplinary initiative: the reACT alliance. Based on the knowledge gained from the production and biocompatibility of the ZnMg demonstrator implants, ready-to-use demonstrator implants will now be created. In addition, the design process will be optimised and automated.
To automatically incorporate patient- and production-related requirements into the design process, members of the DAP chair are developing a material- and post-processing-specific database, as well as an application-specific database. The former includes input variables such as mechanical properties and PBF-LB design restrictions and process parameters. The latter database contains information such as the patient’s age, gender, defect size, and attachment geometry.
The prototypical realisation of a novel material and design concept, along with the exploration of design assistants, completes the sub-project of the reACT alliance. The overarching goal of the alliance is to develop customised, bioresorbable implants that meet the individual needs of patients and enable gentler therapy. By systematically researching design, material, and process optimisation, the game-changing potential of the developed implant solutions should be sustainably increased.