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Advanced additive manufacturing techniques have been recently used to tackle the two fundamental challenges of biodegradable Fe-based bone-substituting materials, namely low rate of biodegradation and insufficient bioactivity. While additively manufactured porous iron has been somewhat successful in addressing the first challenge, the limited bioactivity of these biomaterials hinder their progress towards clinical application. Herein, we used extrusion-based 3D printing for additive manufacturing of iron-matrix composites containing silicate-based bioceramic particles (akermanite), thereby addressing both of the abovementioned challenges. We developed inks that carried iron and 5, 10, 15, or 20 vol% of akermanite powder mixtures for the 3D printing process and optimized the debinding and sintering steps to produce geometrically-ordered iron-akermanite composites with an open porosity of 69–71%. The composite scaffolds preserved the designed geometry and the original α-Fe and akermanite phases. The in vitro biodegradation rates of the composites were improved as much as 2.6 times the biodegradation rate of geometrically identical pure iron. The yield strengths and elastic moduli of the scaffolds remained within the range of the mechanical properties of the cancellous bone, even after 28 days of biodegradation. The composite scaffolds (10–20 vol% akermanite) demonstrated improved MC3T3-E1 cell adhesion and higher levels of cell proliferation. The cellular secretion of collagen type-1 and the alkaline phosphatase activity on the composite scaffolds (10–20 vol% akermanite) were, respectively higher than and comparable to Ti6Al4V in osteogenic medium. Taken together, these results clearly show the potential of 3D printed porous iron-akermanite composites for further development as promising bone substitutes. Statement of significance: Porous iron matrix composites containing akermanite particles were produced by means of multi-material additive manufacturing to address the two fundamental challenges associated with biodegradable iron-based biomaterials, namely very low rate of biodegradation and insufficient bioactivity. Our porous iron-akermanite composites exhibited enhanced biodegradability and superior bioactivity compared to porous monolithic iron scaffolds. The murine bone cells proliferated on the composite scaffolds, and secreted the collagen type-1 matrix that stimulated bony-like mineralization. The results show the exceptional potential of the developed porous iron-based composite scaffolds for application as bone substitutes.

The development of biodegradable Fe-based bone implants has rapidly progressed in recent years. Most of the challenges encountered in developing such implants have been tackled individually or in combination using additive manufacturing technologies. Yet not all the challenges have been overcome. Herein, we present porous FeMn-akermanite composite scaffolds fabricated by extrusion-based 3D printing to address the unmet clinical needs associated with Fe-based biomaterials for bone regeneration, including low biodegradation rate, MRI-incompatibility, mechanical properties, and limited bioactivity. In this research, we developed inks containing Fe, 35 wt% Mn, and 20 or 30 vol% akermanite powder mixtures. 3D printing was optimized together with the debinding and sintering steps to obtain scaffolds with interconnected porosity of 69%. The Fe-matrix in the composites contained the γ-FeMn phase as well as nesosilicate phases. The former made the composites paramagnetic and, thus, MRI-friendly. The in vitro biodegradation rates of the composites with 20 and 30 vol% akermanite were respectively 0.24 and 0.27 mm/y, falling within the ideal range of biodegradation rates for bone substitution. The yield strengths of the porous composites stayed within the range of the values of the trabecular bone, despite in vitro biodegradation for 28 d. All the composite scaffolds favored the adhesion, proliferation, and osteogenic differentiation of preosteoblasts, as revealed by Runx2 assay. Moreover, osteopontin was detected in the extracellular matrix of cells on the scaffolds. Altogether, these results demonstrate the remarkable potential of these composites in fulfilling the requirements of porous biodegradable bone substitutes, motivating future in vivo research. Statement of significance: We developed FeMn-akermanite composite scaffolds by taking advantage of the multi-material capacity of extrusion-based 3D printing. Our results demonstrated that the FeMn-akermanite scaffolds showed an exceptional performance in fulfilling all the requirements for bone substitution in vitro, i.e., a sufficient biodegradation rate, having mechanical properties in the range of trabecular bone even after 4 weeks biodegradation, paramagnetic, cytocompatible and most importantly osteogenic. Our results encourage further research on Fe-based bone implants in in vivo.

Lithium salts have been proposed as promising environmentally friendly alternatives to carcinogenic hexavalent chromium-based inhibitors for the corrosion protection of aerospace aluminium alloys (AAs). Incorporated into organic coatings, lithium salts are released at damaged locations to establish a conversion layer in which distinct sublayers have different barrier characteristics. Thus, detailed knowledge on the sequence of formation events from the early stages of nucleation towards the final multi-layered arrangement is essential for developing and optimising lithium-leaching technology for protective coatings. Here, liquid-phase-transmission electron microscopy (LP-TEM) is employed to observe nanoscopic morphological evolutions in situ during the lithium-based conversion process of AA2024-T3. Thanks to dedicated preparation of delicate sandwiched TEM specimens allowing us to explore the events cross-sectionally, we provide real-time direct mechanistic information on the conversion process from the initiation to an advanced growth stage. In parallel, we perform supplementary ex situ SEM and TEM investigations to support and validate the LP-TEM findings. The unprecedented experimental approach developed and executed in this study provides an inspiring base for studying also other complicated surface conversion processes in situ and at the nanoscopic scale.

Additively manufacturing (AM) opens up the possibility for biodegradable metals to possess uniquely combined characteristics that are desired for bone substitution, including bone-mimicking mechanical properties, topologically ordered porous structure, pore interconnectivity and biodegradability. Zinc is considered to be one of the promising biomaterials with respect to biodegradation rate and biocompatibility. However, no information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is yet available. Here, we applied powder bed fusion to fabricate porous zinc with a topologically ordered diamond structure. An integrative study was conducted on the static and dynamic biodegradation behavior (in vitro, up to 4 weeks), evolution of mechanical properties with increasing immersion time, electrochemical performance, and biocompatibility of the AM porous zinc. The specimens lost 7.8% of their weight after 4 weeks of dynamic immersion in a revised simulated body fluid. The mechanisms of biodegradation were site-dependent and differed from the top of the specimens to the bottom. During the whole in vitro immersion time of 4 weeks, the elastic modulus values of the AM porous zinc (E = 700–1000 MPa) even increased and remained within the scope of those of cancellous bone. Indirect cytotoxicity revealed good cellular activity up to 72 h according to ISO 10,993–5 and -12. Live-dead staining confirmed good viability of MG-63 cells cultured on the surface of the AM porous zinc. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that will enable reconstruction and regeneration of critical-size load-bearing bone defects. Statement of significance: No information regarding the biodegradability and biocompatibility of topologically ordered AM porous zinc is available. We applied selective laser melting to fabricate topologically ordered porous zinc and conducted a comprehensive study on the biodegradation behavior, electrochemical performance, time-dependent mechanical properties, and biocompatibility of the scaffolds. The specimens lost 7.8% of their weight after4 weeks dynamic biodegradation while their mechanical properties surprisingly increased after 4 weeks. Indirect cytotoxicity revealed good cellular activity up to 72 h. Intimate contact between MG-63 cells and the scaffolds was also observed. These important findings could open up unprecedented opportunities for the development of multifunctional bone substituting materials that mimic bone properties and enable full regeneration of critical-size load-bearing bony defects.

The ability of an active protective organic coating to restore its protection functionality in case of a coating defect is of pivotal importance to ensure durable performance under demanding corrosive conditions. In this paper, a self-healing epoxy system is fabricated by separate encapsulation of epoxy and polyamine in carbon hollow spheres (CHSs) and the autonomous healing performance of the system applied on mild steel is investigated. CHSs were synthesized via a silica templating method using carbonization of polysaccharide shells formed on the surface of silica templates. Consequently, epoxy and polyamine were loaded in separate capsules by dispersion of CHSs into the dilute solutions of epoxy/acetone and polyamine/acetone respectively, under vacuum conditions. The synthesized CHSs were characterized before and after the silica removal using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The CHSs loaded with the film forming agents were assessed using thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. Furthermore, the protective and self-healing properties of the coatings fabricated were studied using electrochemical impedance spectroscopy (EIS), scanning vibrating electrode technique (SVET) and salt spray testing. The results showed that scribe defects in epoxy coatings with 10 wt % epoxy and polyamine capsules were healed effectively upon efficient release and subsequent recombination of the epoxy and polyamine agents reforming a protective layer at the damaged region.

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