3D Printing of Salt as a Template for Magnesium with Structured Porosity

Abstract

and chemical stability, and applicability to a vast range of scaffold materials. Indeed, salt templates have been used to achieve porosity in a wide variety of scaffold materials, including natural polymers such as silk fibroin, [7] synthetic polymers such as poly(l-lactic acid), [8] bulk metallic glasses, [9] crystalline metals such as aluminum, [10] and even magnesium, which is known for its high chemical reactivity. [11] In these examples, the pore size of the final scaffold is defined by the size of the original salt particles or the salt aggregates used as template.For all of these porous scaffolds, the salttemplating approach has led to random porosity with broad pore-size distributions. [6,8,11] This reflects the polydisperse nature of the templating salt particles and limits our ability to control the porous architecture of the final scaffold. By contrast, recent advances in additive manufacturing (AM) have added freedom of design to the manufacturing of porous materials, opening the possibility to create architectured gridlike structures with well-controlled porosity and pore sizes at the macroscale. [5,[12][13][14][15] While the pool of materials printable by AM is extending rapidly, [16,17] materials that possess a high chemical reactivity remain a challenge to shape using additive technologies. Among such reactive materials, magnesium (Mg) is receiving increasing attention as a metallic biodegradable implant material for temporary bone replacement or osteosynthesis. [18][19][20] This stems from its similarity in mechanical properties to bone, and its ability to induce new bone formation [21,22] while also being bioresorbable. [23] It is widely accepted that pore size, [24][25][26] shape, [27][28][29] directionality, [30,31] and degree of porosity [24,32,33] strongly influence cell viability and growth. To guide bone-tissue growth, large open porosity with pore sizes >300 µm in combination with surface roughness appears to be most successful. [24,34] Thus, the ability to shape Mg into structures with controlled porosity and pore size in a patient-specific geometry is highly desired.Staiger et al. [35] and Nguyen et al. [36] previously reported a three-step process for indirect AM of Mg by printing first a polymer that was then infiltrated with an NaCl paste. The latter served, upon removal of the polymer, as a template for Mg infiltration. While being an important first approach to structuring Mg using AM, the additional processing step required to generate first the polymer template resulted in imperfect structure replication and was limited to geometries that allow NaCl infiltration into the template.Porosity is an essential feature in a wide range of applications that combine light weight with high surface area and tunable density. Porous materials can be easily prepared with a vast variety of chemistries using the salt-leaching technique. However, this templating approach has so far been limited to the fabrication of structures with random porosity and relatively simple macroscopic shapes. Here, a ...

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