Laser additive manufacturing of biodegradable magnesium alloy WE43: A detailed microstructure analysis.

WE43, a magnesium alloy containing yttrium and neodymium as main alloying elements, has become a well-established bioresorbable implant material. Implants made of WE43 are often fabricated by powder extrusion and subsequent machining, but for more complex geometries laser powder bed fusion (LPBF) appears to be a promising alternative. However, the extremely high cooling rates and subsequent heat treatment after solidification of the melt pool involved in this process induce a drastic change in microstructure, which governs mechanical properties and degradation behaviour in a way that is still unclear. In this study we investigated the changes in the microstructure of WE43 induced by LPBF in comparison to that of cast WE43. We did this mainly by electron microscopy imaging, and chemical mapping based on energy-dispersive X-ray spectroscopy in conjunction with electron diffraction for the identification of the various phases. We identified different types of microstructure: an equiaxed grain zone in the center of the laser-induced melt pool, and a lamellar zone and a partially melted zone at its border. The lamellar zone presents dendritic lamellae lying on the Mg basal plane and separated by aligned Nd-rich nanometric intermetallic phases. They appear as globular particles made of Mg3Nd and as platelets made of Mg41Nd5 occurring on Mg prismatic planes. Yttrium is found in solid solution and in oxide particles stemming from the powder particles' shell. Due to the heat influence on the lamellar zone during subsequent laser passes, a strong texture developed in the bulk material after substantial grain growth. STATEMENT OF SIGNIFICANCE: Additively manufactured magnesium alloys have the potential of providing a major breakthrough in bone-reconstruction surgery by serving as biodegradable porous scaffold material. This study is the first to report in detail on the microstructure development of the established magnesium alloy WE43 fabricated by the additive manufacturing process of Laser Powder Bed Fusion (LPBF). It presents unique microstructural features which originate from the laser-melting process. An in situ transmission electron microscopy heating experiment further demonstrates the development of two distinct intermetallic phases in additively manufactured WE43 alloys. While one forms already during solidification, the other precipitates due to the ongoing heat treatment during LPBF processing.

Influence of design and postprocessing parameters on the degradation behavior and mechanical properties of additively manufactured magnesium scaffolds.

Magnesium shows promising properties concerning its use in absorbable implant applications such as biodegradability, improved mechanical strength and plastic deformability. Following extensive research, the first fixation and compression screws composed of magnesium rare earth alloys were commercialised, notably in the field of orthopaedic surgery. Preclinical and clinical follow-up studies showed that the rapid degradation of unprotected metallic Magnesium surfaces and concomitant hydrogen gas bursts still raise concern regarding certain surgical indications and need to be further improved. In order to enlarge the scope of further applications, the development of future magnesium implants must aim at freedom of design and reduction of volume, hereby enabling higher functionalised implants, as e.g. plate systems or scaffold grafts for bone replacement therapy. In order to overcome the boundaries of conventional manufacturing methods such as turning or milling, the process of Laser Powder Bed Fusion (LPBF) for magnesium alloys was recently introduced. It enables the production of lattice structures, therefore allowing for reduction of implant material volume. Nevertheless, the concomitant increase of free surface of such magnesium scaffolds further stresses the aforementioned disadvantages of vast degradation and early loss of mechanical stability if not prevented by suitable postprocessing methods. Magnesium scaffold structures with different pore sizes were therefore manufactured by LPBF and consequently further modified either by thermal heat treatment or Plasma Electrolytic Oxidation (PEO). Implant performance was assessed by conducting degradation studies and mechanical testing. PEO modified scaffolds with small pore sizes exhibited improved long-term stability, while heat treated specimens showed impaired performance regarding degradation and mechanical stability. STATEMENT OF SIGNIFICANCE: Magnesium based scaffold structures offer wide possibilities for advanced functionalized bioabsorbable implants. By implementing lattice structures, big implant sizes and mechanically optimized implant geometries can be achieved enabling full bone replacement or large-scale plate systems, e.g. for orthopedic applications. As shape optimization and lattice structuring of such scaffolds consequently lead to enlarged surface, suitable design and postprocessing routines come into focus. The presented study addresses these new and relevant topics for the first time by evaluating geometry as well as heat and surface treatment options as input parameters for improved chemical and mechanical stability. The outcome of these variations is measured by degradation tests and mechanical analysis. Evaluating these methods, a significant contribution to the development of absorbable magnesium scaffolds is made. The findings can help to better understand the interdependence of high surface to volume ratio Magnesium implants and to deliver methods to incorporate such lattice structures into future large-scale implant applications manufactured from bioabsorbable Magnesium alloys.

Development of a solvent-free polylactide/calcium carbonate composite for selective laser sintering of bone tissue engineering scaffolds.

Since large bone defects cannot be healed by the body itself, continuous effort is put into the development of 3D scaffolds for bone tissue engineering. One method to fabricate such scaffolds is selective laser sintering (SLS). However, there is a lack of solvent-free prepared microparticles suitable for SLS. Hence, the aim of this study was to develop a solvent-free polylactide/calcium carbonate composite powder with tailored material properties for SLS. Four composite powders with a composition of approximately 75 wt% polylactide (PLLA as well as PDLLA) and 25 wt% calcium carbonate (calcite) were prepared by a milling process based on GMP standards. Four different grades of polylactide were chosen to cover a broad inherent viscosity range of 1.0-3.6 dl/g. The composite material with the lowest inherent viscosity (1.0 dl/g) showed the best processability by SLS. This was caused by the small polymer particle diameter (50 µm) and the small zero-shear melt viscosity (400 Pa·s), which led to fast sintering. The SLS process parameters were developed to achieve low micro-porosity (approx. 2%) and low polymer degradation (no measurable decrease of the inherent viscosity). A biaxial bending strength of up to 75 MPa was achieved. Cell culture assays indicated good viability of MG-63 osteoblast-like cells on the SLS specimens. Finally, the manufacture of 3D scaffolds with interconnected pore structure was demonstrated. After proving the biocompatibility of the material, the developed scaffolds could have great potential to be used as patient-specific bone replacement implants.

Additive manufacturing of biodegradable metals: Current research status and future perspectives.

The combination of biodegradable metals and additive manufacturing (AM) leads to a revolutionary change of metal implants in many aspects including materials, design, manufacturing, and clinical applications. The AM of nondegradable metals such as titanium and CoCr alloys has proven to be a tremendous success in clinical applications. The AM of biodegradable metals including magnesium (Mg), iron (Fe), and zinc (Zn) is still in its infancy, although much progress has been made in the research field. Element loss and porosity are common processing problems for AM of biodegradable metals like Zn and Mg, which are mainly caused by evaporation during melting under a high-energy beam. The resulting formation quality and properties are closely related to material, design, and processing, making AM of biodegradable metals a typical interdisciplinary subject involving biomaterials, mechanical engineering, and medicine. This work reviews the state of research and future perspective on AM of biodegradable metals from extensive viewpoints such as material, processing, formation quality, design, microstructure, and properties. Effects of powder properties and processing parameters on formation quality are characterized in detail. The microstructure and metallurgical defects encountered in the AM parts are described. Mechanical and biodegradable properties of AM samples are introduced. Design principles and potential applications of biodegradable metal implants produced by AM are discussed. Finally, current research status is summarized together with some proposed future perspectives for advancing knowledge about AM of biodegradable metals. STATEMENT OF SIGNIFICANCE: Rapid development of research and applications on biodegradable metals and additive manufacturing (AM) has been made in recent years. Customized geometric shapes of medical metals with porous structure can be realized accurately and efficiently by laser powder bed fusion (L-PBF), which is beneficial to achieve reliable stress conduction and balanced properties. This review introduces the development history and current status of AM of biodegradable metals and then critically surveys L-PBF of Mg-, Fe-, and Zn-based metals from multiple viewpoints including materials, processing, formation quality, structural design, microstructure, and mechanical and biological properties. The present findings are summarized together with some proposed future challenges for advancing AM of biodegradable metals into real clinical applications.

Influence of the material properties of a poly(D,L-lactide)/ß-tricalcium phosphate composite on the processability by selective laser sintering.

Complex 3D scaffolds with interconnected pores are a promising tool for bone regeneration. Such 3D scaffolds can be manufactured by selective laser sintering (SLS) from biodegradable composite powders. However, the mechanical strength of these scaffolds is often too low for medical application. We propose that the mechanical strength of laser-sintered scaffolds can be improved through composite powders with tailored properties (e.g., suitable powder particle size and melt viscosity for SLS). To prove this, two batches of a poly(D,L-lactide) (PDLLA)/ß-tricalcium phosphate (ß-TCP) composite powder with 50 wt% PDLLA and 50 wt% ß-TCP were synthesized. The two batches differed in polymer particle size, filler particle size, and polymer molecular weight. Both batches were processed with identical SLS process parameters to study the extent to which the material properties influence how well a PDLLA/ß-TCP (50/50) composite can be processed with SLS. In the SLS process, batch 2 showed improved melting behavior due to its smaller polymer particle size (approx. 35 µm vs. 50 µm) and its lower zero-shear melt viscosity (5800 Pa∙s vs. 17,900 Pa∙s). The better melting behavior of batch 2 led to SLS test specimens with lower porosity compared to batch 1. In consequence, the batch 2 specimens exhibited a larger biaxial bending strength (62 MPa) than the batch 1 specimens did (23 MPa). We conclude that a tailored composite powder with optimized polymer particle size, filler particle size, and polymer molecular weight can increase the achievable mechanical strength of laser-sintered scaffolds.

Development and characterization of a coronary polylactic acid stent prototype generated by selective laser melting.

In-stent restenosis is still an important issue and stent thrombosis is an unresolved risk after coronary intervention. Biodegradable stents would provide initial scaffolding of the stenosed segment and disappear subsequently. The additive manufacturing technology Selective Laser Melting (SLM) enables rapid, parallel, and raw material saving generation of complex 3- dimensional structures with extensive geometric freedom and is currently in use in orthopedic or dental applications. Here, SLM process parameters were adapted for poly-L-lactid acid (PLLA) and PLLA-co-poly-ε-caprolactone (PCL) powders to generate degradable coronary stent prototypes. Biocompatibility of both polymers was evidenced by assessment of cell morphology and of metabolic and adhesive activity at direct and indirect contact with human coronary artery smooth muscle cells, umbilical vein endothelial cells, and endothelial progenitor cells. γ-sterilization was demonstrated to guarantee safety of SLM-processed parts. From PLLA and PCL, stent prototypes were successfully generated and post-processing by spray- and dip-coating proved to thoroughly smoothen stent surfaces. In conclusion, for the first time, biodegradable polymers and the SLM technique were combined for the manufacturing of customized biodegradable coronary artery stent prototypes. SLM is advocated for the development of biodegradable coronary PLLA and PCL stents, potentially optimized for future bifurcation applications.