The treatment of articular cartilage defects remains a significant clinical challenge. This is partially due to current tissue engineering strategies failing to recapitulate native organization. Articular cartilage is a graded tissue with three layers exhibiting different cell densities: the superficial zone having the highest density and the deep zone having the lowest density. However, the introduction of cell gradients for cartilage tissue engineering, which could promote a more biomimetic environment, has not been widely explored. Here, we aimed to bioprint a scaffold with different zonal cell densities to mimic the organization of articular cartilage. The scaffold was bioprinted using an alginate-based bioink containing human articular chondrocytes. The scaffold design included three cell densities, one per zone: 20 × 106 (superficial), 10 × 106 (middle), and 5 × 106 (deep) cells/mL. The scaffold was cultured in a chondrogenic medium for 25 days and analyzed by live/dead assay and histology. The live/dead analysis showed the ability to generate a zonal cell density with high viability. Histological analysis revealed a smooth transition between the zones in terms of cell distribution and a higher sulphated glycosaminoglycan deposition in the highest cell density zone. These findings pave the way toward bioprinting complex zonal cartilage scaffolds as single units, thereby advancing the translation of cartilage tissue engineering into clinical practice.@en

Despite the potential of small-scale pillars of black titanium (bTi) for killing the bacteria and directing the fate of stem cells, not much is known about the effects of the pillars’ design parameters on their biological properties. Here, three distinct bTi surfaces are designed and fabricated through dry etching of the titanium, each featuring different pillar designs. The interactions of the surfaces with MC3T3-E1 preosteoblast cells and Staphylococcus aureus bacteria are then investigated. Pillars with different heights and spatial organizations differently influence the morphological characteristics of the cells, including their spreading area, aspect ratio, nucleus area, and cytoskeletal organization. The preferential formation of focal adhesions (FAs) and their size variations also depend on the type of topography. When the pillars are neither fully separated nor extremely tall, the colocalization of actin fibers and FAs as well as an enhanced matrix mineralization are observed. However, the killing efficiency of these pillars against the bacteria is not as high as that of fully separated and tall pillars. This study provides a new perspective on the dual-functionality of bTi surfaces and elucidates how the surface design and fabrication parameters can be used to achieve a surface topography with balanced bactericidal and osteogenic properties.@en

The recently developed additively manufacturing techniques have enabled the fabrication of porous biomaterials that mimic the characteristics of the native bone, thereby avoiding stress shielding and facilitating bony ingrowth. However, aseptic loosening and bacterial infection, as the leading causes of implant failure, need to be further addressed through surface biofunctionalization. Here, we used a combination of (1) plasma electrolytic oxidation (PEO) using Ca-, P-, and silver nanoparticle-rich electrolytes and (2) post-PEO hydrothermal treatments (HT) to furnish additively manufactured Ti-6Al-4V porous implants with a multi-functional surface. The applied HT led to the formation of hydroxyapatite (HA) nanocrystals throughout the oxide layer. This process was controlled by the supersaturation of Ca2+ and PO43− during the hydrothermal process. Initially, the high local supersaturation resulted in homogenous nucleation of spindle-like nanocrystals throughout the surface. As the process continued, the depletion of reactant ions in the outermost surface layer led to a remarkable decrease in the supersaturation degrees. High aspect-ratio nanorods and hexagonal nanopillars were, therefore, created. The unique hierarchical structure of the microporous PEO layer (pore size < 3 μm) and spindle-like HA nanocrystals (<150 nm) on the surface of macro-porous additively manufactured Ti-6Al-4V implants provided a favorable substrate for the anchorage of cytoplasmic extensions assisting cell attachment and migration on the surface. The results of our in vitro assays clearly showed the important benefits of the HT and the spindle-like HA nanocrystals including a significantly stronger and much more sustained antibacterial activity, significantly higher levels of pre-osteoblasts metabolic activity, and significantly higher levels of alkaline phosphatase activity as compared to similar PEO-treated implants lacking the HT.@en

We designed and fabricated a simple setup for the controlled crumpling of nanopatterned, surface-porous flat metallic sheets for the fabrication of volume-porous biomaterials and showed that crumpling can be considered as an efficient alternative to origami-inspired folding. Before crumpling, laser cutting was used to introduce pores to the sheets. We then fabricated titanium (Ti) nanopatterns through reactive ion etching on the polished Ti sheets. Thereafter, nanopatterned porous Ti sheets were crumpled at two deformation velocities (i.e., 2 and 100 mm/min). The compression tests of the scaffolds indicated that the elastic modulus of the specimens vary in the range of 11.8–13.9 MPa. Micro-computed tomography scans and computational simulations of crumpled scaffolds were performed to study the morphological properties of the resulting meta-biomaterials. The porosity and pore size of the scaffolds remained within the range of those reported for trabecular bone. Finally, the in vitro cell preosteoblasts culture demonstrated the cytocompatibility of the nanopatterned scaffolds. Moreover, the aspect ratio of the cells residing on the nanopatterned surfaces was found to be significantly higher than those cultured on the control scaffolds, indicating that the nanopatterned surface may have a higher potential for inducing the osteogenic differentiation of the preosteoblasts.@en

Physical patterns represent potential surface cues for promoting osteogenic differentiation of stem cells and improving osseointegration of orthopedic implants. Understanding the early cell–surface interactions and their effects on late cellular functions is essential for a rational design of such topographies, yet still elusive. In this work, fluidic force microscopy (FluidFM) and atomic force microscopy (AFM) combined with optical and electron microscopy are used to quantitatively investigate the interaction of preosteoblasts with 3D-printed patterns after 4 and 24 h of culture. The patterns consist of pillars with the same diameter (200 nm) and interspace (700 nm) but distinct heights (500 and 1000 nm) and osteogenic properties. FluidFM reveals a higher cell adhesion strength after 24 h of culture on the taller pillars (32 ± 7 kPa versus 21.5 ± 12.5 kPa). This is associated with attachment of cells partly on the sidewalls of these pillars, thus requiring larger normal forces for detachment. Furthermore, the higher resistance to shear forces observed for these cells indicates an enhanced anchorage and can be related to the persistence and stability of lamellipodia. The study explains the differential cell adhesion behavior induced by different pillar heights, enabling advancements in the rational design of osteogenic patterns.@en

Extrusion-based 3D printing followed by debinding and sintering is a powerful approach that allows for the fabrication of porous scaffolds from materials (or material combinations) that are otherwise very challenging to process using other additive manufacturing techniques. Iron is one of the materials that have been recently shown to be amenable to processing using this approach. Indeed, a fully interconnected porous design has the potential of resolving the fundamental issue regarding bulk iron, namely a very low rate of biodegradation. However, no extensive evaluation of the biodegradation behavior and properties of porous iron scaffolds made by extrusion-based 3D printing has been reported. Therefore, the in vitro biodegradation behavior, electrochemical response, evolution of mechanical properties along with biodegradation, and responses of an osteoblastic cell line to the 3D printed iron scaffolds were studied. An ink formulation, as well as matching 3D printing, debinding and sintering conditions, was developed to create iron scaffolds with a porosity of 67%, a pore interconnectivity of 96%, and a strut density of 89% after sintering. X-ray diffracometry confirmed the presence of the α-iron phase in the scaffolds without any residuals from the rest of the ink. Owing to the presence of geometrically designed macropores and random micropores in the struts, the in vitro corrosion rate of the scaffolds was much improved as compared to the bulk counterpart, with 7% mass loss after 28 days. The mechanical properties of the scaffolds remained in the range of those of trabecular bone despite 28 days of in vitro biodegradation. The direct culture of MC3T3-E1 preosteoblasts on the scaffolds led to a substantial reduction in living cell count, caused by a high concentration of iron ions, as revealed by the indirect assays. On the other hand, the ability of the cells to spread and form filopodia indicated the cytocompatibility of the corrosion products. Taken together, this study shows the great potential of extrusion-based 3D printed porous iron to be further developed as a biodegradable bone substituting biomaterial.@en

Surface biofunctionalization is frequently applied to enhance the functionality and longevity of orthopedic implants. Here, we investigated the osteogenic effects of additively manufactured porous Ti6Al4V implants whose surfaces were biofunctionalized using plasma electrolytic oxidation (PEO) in Ca/P-based electrolytes with or without strontium. Various levels of Sr and Ca were incorporated in the oxide layers by using different current densities and oxidation times. Increasing the current density and oxidation time resulted in thicker titanium oxide layers and enhanced the release of Ca2+ and Sr2+. Biofunctionalization with strontium resulted in enhanced pore density, a thinner TiO2 layer, four-fold reduced release of Ca2+, and mainly anatase phases as compared to implants biofunctionalized in electrolytes containing solely Ca/P species under otherwise similar conditions. Different current densities and oxidation times significantly increased the osteogenic differentiation of MC3T3-E1 cells on implants biofunctionalized with strontium, when the PEO treatment was performed with a current density of 20 A/dm2 for 5 and 10 min as well as for a current density of 40 A/dm2 for 5 min. Therefore, addition of Sr in the PEO electrolyte and control of the PEO processing parameters represent a promising way to optimize the surface morphology and osteogenic activity of future porous AM implants.@en

Modulation of the immune response following the implantation of biomaterials can have beneficial effects on bone regeneration. This involves complex interactions between the inflammatory and osteogenic cells. Therefore, the study of cell-cell interactions using direct co-culture models integrated with biomaterials is of great interest. This research aimed to study the viability, morphology, and osteogenic activity of preosteoblasts (OBs) co-cultured with pro-inflammatory macrophages (M1s) on the 3D printed (non)patterned surfaces. OBs and M1s remained alive and proliferated actively for 14 days in the mixture of Dulbecco's Modified Eagle's Medium (DMEM) and alpha Minimum Essential Medium (α-MEM) (1:1), regardless of the cell ratio in the co-cultures. The spatial organization of the two types of cells changed with the time of culture from an initially uniform cell distribution to the formation of a thick layer of OBs covered by clusters of M1s. On day 7, the expression of PGE2 and TNF-α were upregulated in the co-culture relative to the mono-culture of OBs and M1s. The inflammation decreased differentiation and matrix mineralization of OBs after 28 days of culture. Interestingly, the incorporation of 3D printed submicron pillars into the direct co-culture model enhanced the differentiation of preosteoblasts, as shown by relatively higher RUNX2 expression, thereby revealing the osteoimmunomodulatory potential of such surface patterns.@en

Individual cells and multicellular systems respond to cell-scale curvatures in their environments, guiding migration, orientation, and tissue formation. However, it remains largely unclear how cells collectively explore and pattern complex landscapes with curvature gradients across the Euclidean and non-Euclidean spectra. Here, we show that mathematically designed substrates with controlled curvature variations induce multicellular spatiotemporal organization of preosteoblasts. We quantify curvature-induced patterning and find that cells generally prefer regions with at least one negative principal curvature. However, we also show that the developing tissue can eventually cover unfavorably curved territories, can bridge large portions of the substrates, and is often characterized by collectively aligned stress fibers. We demonstrate that this is partly regulated by cellular contractility and extracellular matrix development, underscoring the mechanical nature of curvature guidance. Our findings offer a geometric perspective on cell-environment interactions that could be harnessed in tissue engineering and regenerative medicine applications.@en

The surface topography of implantable devices is of crucial importance for guiding the cascade of events that starts from the initial contact of the cells with the surface and continues until the complete integration of the device in its immediate environment. There is, however, limited quantitative information available regarding the relationships between the different stages of such cascade(s) and how the design of surface topography influences them. We, therefore, used direct laser writing to 3D-print submicron pillars with precisely controlled dimensions and spatial arrangements to perform a systematic study of such relationships. Using single-cell force spectroscopy, we measured the adhesion force and the work of adhesion of the preosteoblast cells residing on the different types of surfaces. Not only the adhesion parameters (after 2-60 s) but also the formation of focal adhesions was strongly dependent on the geometry and arrangement of the pillars: sufficiently tall and dense pillars enhanced both adhesion parameters and the formation of focal adhesions. Our morphological study of the cells (after 24 h) showed that those enhancements were associated with a specific way of cell settlement onto the surface (i.e., "top state"). The cells interacting with tall and dense pillars were also characterized by numerous thick actin stress fibers in the perinuclear region and possibly high internal stresses. Furthermore, living cells with highly organized cytoskeletal networks exhibited greater values of the elastic modulus. The early responses of the cells predicted their late response including matrix mineralization: tall and dense submicron pillars significantly upregulated the expression of osteopontin after 21 days of culture under both osteogenic and nonosteogenic conditions. Our findings paint a detailed picture of at least one possible cascade of events that starts from initial cell adhesion and continues to subsequent cellular functions and eventual matrix mineralization. These observations could inform the future developments of instructive surfaces for medical devices based on physical surface cues and early markers. @en

The holy grail of orthopedic implant design is to ward off both aseptic and septic loosening for long enough that the implant outlives the patient. Questing this holy grail is feasible only if orthopedic biomaterials possess a long list of functionalities that enable them to discharge the onerous task of permanently replacing the native bone tissue. Here, we present a rationally designed and additive manufacturing (AM) topologically ordered porous metallic biomaterial that is made from Ti-6Al-4V using selective laser melting and packs most (if not all) of the required functionalities into a single implant. In addition to presenting a fully interconnected porous structure and form-freedom that enables realization of patient-specific implants, the biomaterials developed here were biofunctionalized using plasma electrolytic oxidation to locally release both osteogenic (i.e. strontium) and antibacterial (i.e. silver ions) agents. The same single-step biofunctionalization process also incorporated hydroxyapatite into the surface of the implants. Our measurements verified the continued release of both types of active agents up to 28 days. Assessment of the antibacterial activity in vitro and in an ex vivo murine model demonstrated extraordinarily high levels of bactericidal effects against a highly virulent and multidrug-resistant Staphylococcus aureus strain (i.e. USA300) with total eradication of both planktonic and adherent bacteria. This strong antibacterial behavior was combined with a significantly enhanced osteogenic behavior, as evidenced by significantly higher levels of alkaline phosphatase (ALP) activity compared with non-biofunctionalized implants. Finally, we discovered synergistic antibacterial behavior between strontium and silver ions, meaning that 4–32 folds lower concentrations of silver ions were required to achieve growth inhibition and total killing of bacteria. The functionality-packed biomaterial presented here demonstrates a unique combination of functionalities that make it an advanced prototype of future orthopedic biomaterials where implants will outlive patients.@en

Antibiotic-resistant bacteria are frequently involved in implant-associated infections (IAIs), making the treatment of these infections even more challenging. Therefore, multifunctional implant surfaces that simultaneously possess antibacterial activity and induce osseointegration are highly desired in order to prevent IAIs. The incorporation of multiple inorganic antibacterial agents onto the implant surface may aid in generating synergistic antibacterial behavior against a wide microbial spectrum while reducing the occurrence of bacterial resistance. In this study, porous titanium implants synthesized by selective laser melting (SLM) were biofunctionalized with plasma electrolytic oxidation (PEO) using electrolytes based on Ca/P species as well as silver and zinc nanoparticles in ratios from 0 to 100% that were tightly embedded into the growing titanium oxide layer. After the surface bio-functionalization process, silver and zinc ions were released from the implant surfaces for at least 28 days resulting in antibacterial leaching activity against methicillin-resistant Staphylococcus aureus (MRSA). Furthermore, the biofunctionalized implants generated reactive oxygen species, thereby contributing to antibacterial contact-killing. While implant surfaces containing up to 75% silver and 25% zinc nanoparticles fully eradicated both adherent and planktonic bacteria in vitro as well as in an ex vivo experiment performed using murine femora, solely zinc-bearing surfaces did not. The minimum inhibitory and bactericidal concentrations determined for different combinations of both types of ions confirmed the presence of a strong synergistic antibacterial behavior, which could be exploited to reduce the amount of required silver ions by two orders of magnitude (i.e., 120 folds). At the same time, the zinc bearing surfaces enhanced the metabolic activity of pre-osteoblasts after 3, 7, and 11 days. Altogether, implant biofunctionalization by PEO with silver and zinc nanoparticles is a fruitful strategy for the synthesis of multifunctional surfaces on orthopedic implants and the prevention of IAIs caused by antibiotic-resistant bacteria. Statement of Significance: Implant-associated infections are becoming increasingly challenging to treat due to growing antibiotic resistance against antibiotics. Here, we propose an alternative approach where silver and zinc nanoparticles are simultaneously used for the biofunctionalization of rationally designed additively manufactured porous titanium. This combination of porous design and tailored surface treatment allows us to reduce the amount of required silver nanoparticles by two orders of magnitude, fully eradicate antibiotic-resistant bacteria, and enhance the osteogenic behavior of pre-osteoblasts. We demonstrate that the resulting implants display antibacterial activity in vitro and ex vivo against methicillin-resistant Staphylococcus aureus.@en

Nanoparticles (NPs) have high multifunctional potential to simultaneously enhance implant osseointegration and prevent infections caused by antibiotic-resistant bacteria. Here, we present the first report on using plasma electrolytic oxidation (PEO) to incorporate different combinations of reduced graphene oxide (rGO) and silver (Ag) NPs on additively manufactured geometrically ordered volume-porous titanium implants. The rGO nanosheets were mainly embedded parallel with the PEO surfaces. However, the formation of ‘nano-knife’ structures (particles embedded perpendicularly to the implant surfaces) was also found around the pores of the PEO layers. Enhanced in vitro antibacterial activity against methicillin-resistant Staphylococcus aureus was observed for the rGO+Ag-containing surfaces compared to the PEO surfaces prepared only with AgNPs. This was caused by a significant improvement in the generation of reactive oxygen species, higher levels of Ag+ release, and the presence of rGO ‘nano-knife’ structures. In addition, the implants developed in this study stimulated the metabolic activity and osteogenic differentiation of MC3T3-E1 preosteoblast cells compared to the PEO surfaces without nanoparticles. Therefore, the PEO titanium surfaces incorporating controlled levels of rGO+Ag nanoparticles have high clinical potential as multifunctional surfaces for 3D-printed orthopaedic implants.@en

Fabricating large areas of geometrically complex and precisely controlled topographies is required for the studies of cell behavior on patterned surfaces. Direct laser writing (DLW) is an advanced 3D-fabrication technique, which facilitates the manufacturing of structures within various scales (from a few hundred nanometers to millimeters). However, this method requires improvements in the accuracy and reproducibility of the submicron and nanoscale features that are printed over a large area. Here, we present a scheme to both improve the uniformity of the printed submicron patterns and decrease the printing time. The effects of various processing parameters (e.g., laser power and writing field) on the dimensions and uniformity of submicron pillars as well as on their Young’s modulus and surface wettability were assessed. Decreasing the writing field to 33 × 33 μm2 significantly improved the uniformity of submicron pillars that were printed over an area of 4 mm2 in a single-step process. Preosteoblast cells (MC3T3-E1) were used to assess the cytocompatibility of the used material (IP-L780 resin) with a focus on cell morphology, cell proliferation, cytoskeletal organization, and the elastic modulus of the cells. The cells cultured for 2 days on the submicron pillars showed a polarized shape and a higher Young’s modulus of the area corresponding to the nucleus relative to those cultured on flat surfaces. Taken together, the results of the current study clearly show that the submicron patterns created using DLW are both cytocompatible and could modulate the morphology and mechanical properties of cells. This work paves the way for direct printing of submicron features with controlled Young’s moduli over large areas in a single-step process, which is necessary for systematically studying how such patterns modulate cellular functions.@en

The surface topography of engineered extracellular matrices is one of the most important physical cues regulating the phenotypic polarization of macrophages. However, not much is known about the ways through which submicron (i.e., 100-1000 nm) topographies modulate the polarization of macrophages. In the context of bone tissue regeneration, it is well established that this range of topographies stimulates the osteogenic differentiation of stem cells. Since the immune response affects the bone tissue regeneration process, the immunomodulatory consequences of submicron patterns should be studied prior to their clinical application. Here, we 3D printed submicron pillars (using two-photon polymerization technique) with different heights and interspacings to perform the first ever systematic study of such effects. Among the studied patterns, the highest degree of elongation was observed for the cells cultured on those with the tallest and densest pillars. After 3 days of culture with inflammatory stimuli (LPS/IFN-γ), sparsely decorated surfaces inhibited the expression of the pro-inflammatory cellular marker CCR7 as compared to day 1 and to the other patterns. Furthermore, sufficiently tall pillars polarized the M1 macrophages towards a pro-healing (M2) phenotype, as suggested by the expression of CD206 within the first 3 days. As some of the studied patterns are known to be osteogenic, the osteoimmunomodulatory capacity of the patterns should be further studied to optimize their bone tissue regeneration performance.@en

Effective preventive measures against implant-associated infection (IAI) are desperately needed. Therefore, the development of self-defending implants with intrinsic antibacterial properties has gained significant momentum. Biomaterials biofunctionalized with silver (Ag) have resulted in effective antibacterial biomaterials, yet regularly induce cytotoxicity. In this study, the use of both Ag and copper (Cu) nanoparticles (NPs) on TiO2 surfaces was investigated to generate antibacterial and osteoconductive biomaterials. Hence, additively manufactured Ti-6Al-4V volume-porous implants were biofunctionalized with plasma electrolytic oxidation (PEO) through the incorporation of varying ratios of Ag and/or Cu NPs in the TiO2 layer covering the implant surface. For all experimental groups, the surface morphology, chemical composition, ion release profile, generation of reactive ion species, antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA) in vitro and ex vivo, as well as the response of pre-osteoblastic MC3T3-E1 cells in metabolic activity and differentiation assays were determined. PEO biofunctionalization resulted in rough and highly porous surfaces that released Ag and Cu ions for 28 days and generated hydroxyl as well as methyl radicals. A strong synergistic bactericidal behavior between Ag and Cu ions was detected, which allowed to decrease the concentration of Ag ions by 10-fold, while maintaining the same level of antibacterial activity. Antibacterial agar diffusion and quantitative assays indicated strong antibacterial activity in vitro for the implants containing Ag and Ag/Cu, while no antibacterial activity was observed for implants bearing only Cu NPs. Moreover, the biofunctionalized implants with ratios of up to 75% Ag and 25% Cu NP totally eradicated all bacteria in an ex vivo model using murine femora. Meanwhile, the biofunctionalized implants did not show any signs of cytotoxicity, while implants bearing only Cu NPs improved the metabolic activity after 7 and 11 days. The biomaterials developed here, therefore, exploit the synergistic behavior of Ag and Cu to simultaneously offer strong antibacterial behavior while fully mitigating the cytotoxicity of Ag against mammalian cells.@en