3D Printed Templating of Extrinsic Freeze-Casting for Macro–Microporous Biomaterials

In-Bath 3D Printing of Anisotropic Shape-Memory Cryogels Functionalized with Bone-Bioactive Nanoparticles

Cryogels exhibit unique shape memory with full recovery and structural stability features after multiple injections. These constructs also possess enhanced cell permeability and nutrient diffusion when compared to typical bulk hydrogels. Volumetric processing of cryogels functionalized with nanosized units has potential to widen their biomedical applications, however this has remained challenging and relatively underexplored. In this study, we report a novel methodology that combines suspension 3D printing with directional freezing for the fabrication of nanocomposite cryogels with configurable anisotropy. When compared to conventional bulk or freeze-dried hydrogels, nanocomposite cryogel formulations exhibit excellent shape recovery (>95%) and higher pore connectivity. Suspension printing, assisted with a prechilled metal grid, was optimized to induce anisotropy. The addition of calcium- and phosphate-doped mesoporous silica nanoparticles into the cryogel matrix enhanced bioactivity toward orthopedic applications without hindering the printing process. Notably, the nanocomposite 3D printed cryogels exhibit injectable shape memory while also featuring a lamellar topography. The fabrication of these constructs was highly reproducible and exhibited potential for a cell-delivery injectable cryogel with no cytotoxicity to human-derived adipose stem cells. Hence, in this work, it was possible to combine a gravity defying 3D printed methodology with injectable and controlled anisotropic macroporous structures containing bioactive nanoparticles. This methodology ameliorates highly tunable injectable 3D printed anisotropic nanocomposite cryogels with a user-programmable degree of structural complexity.

Morphological Integrated Preparation Method and Implementation of Inorganic/Organic Dual-Phase Composite Gradient Bionic Bone Scaffold

Large bone defects caused by congenital deformities and acquired accidents are increasing day by day. A large number of patients mainly rely on artificial bone for repair. However, artificial bone cannot fully imitate the structure and composition of human bone, resulting in a large gap with autologous bone function. Therefore, this article proposes a continuous preparation method for inorganic/organic biphasic composite gradient biomimetic bulk bone scaffolds. First, a controllable gradient hybrid forming platform for inorganic/organic dual-phase biomaterials was constructed, and the feeding control strategy was studied to achieve precise control of the feeding of sodium alginate/gelatin composite organic materials and hydroxyapatite inorganic materials. The speed is, respectively, sent from the corresponding feeding nozzle to the mixing chamber to realize the uniform mixing of the biphasic material and the extrusion of the composite material, and the inorganic/organic biphasic composite gradient biomimetic bone scaffold with gradual structure and composition is prepared. Second, to prove the superiority of the preparation method, the physicochemical and biological properties of the prepared scaffolds were evaluated. The test results showed that the morphological characteristics of the biphasic composite gradient bone scaffold showed good microscopic porosity and the structure and composition showed gradients. The mechanical properties are close to that of human bone tissue and in vitro cell experiments show that the scaffold has good biocompatibility and bioactivity. In conclusion, this article provides a new type of bone scaffold preparation technology and equipment for the field of tissue engineering, which has research value and application prospects.

Freeze casting of hydroxyapatite-titania composites for bone substitutes

Hydroxyapatite (HA) is commonly used as a bone substitute material, but it lacks mechanical strength when compared to native bone tissues. To improve the efficacy of HA as a bone substitute by improving the mechanical strength and cell growth attributes, porous composite scaffolds of HA and titania (HA-TiO2 ) were fabricated through a freeze-casting process. Three different compositions by weight percent, 25-75 HA-TiO2 , 50-50 HA-TiO2 , and 75-25 HA-TiO2 , were custom-made for testing. After sintering at 1250°C, these composite scaffolds exhibited improved mechanical properties compared to porous HA scaffolds. Substrate mixing was observed, which helped reduce crystal size and introduced new phases such as β-TCP and CaTiO3 , which also led to improved mechanical properties. The composition of 50-50 HA-TiO2 had the highest ultimate compressive strength of 3.12 ± 0.36 MPa and elastic modulus 63.29 ± 28.75 MPa. Human osteoblast cell proliferation assay also increased on all three different compositions when compared to porous HA at 14 days. These results highlight the potential of freeze casting composites for the fabrication of bone substitutes, which provide enhanced mechanical strength and biocompatibility while maintaining porosity.

Reprogramming mitochondrial metabolism of macrophages by miRNA-released microporous coatings to prevent peri-implantitis

Although various new biomaterials have enriched the methods for peri-implant inflammation treatment, their efficacy is still debated, and secondary operations on the implant area have also caused pain for patients. Recently, strategies that regulate macrophage polarization to prevent or even treat peri-implantitis have attracted increasing attention. Here, we prepared a laser-drilled and covered with metal organic framework-miR-27a agomir nanomembrane (L-MOF-agomir) implant, which could load and sustain the release of miR-27a agomir. In vitro, the L-MOF-agomir titanium plate promoted the repolarization of LPS-stimulated macrophages from M1 to M2, and the macrophage culture supernatant promoted BMSCs osteogenesis. In a ligation-induced rat peri-implantitis model, the L-MOF-agomir implants featured strong immunomodulatory activity of macrophage polarization and alleviated ligation-induced bone resorption. The mechanism of repolarization function may be that the L-MOF-agomir implants promote the macrophage mitochondrial function and metabolism reprogramming from glycolysis to oxidative phosphorylation. Our study demonstrates the feasibility of targeting cell metabolism to regulate macrophage immunity for peri-implantitis inhibition and provides a new perspective for the development of novel multifunctional implants.

Bone Tissue Engineering Scaffolds: Function of Multi-Material Hierarchically Structured Scaffolds

Bone tissue engineering (BTE) is a topic of interest for the last decade, and advances in materials, processing techniques, and the understanding of bone healing pathways have opened new avenues of research. The dual responsibility of BTE scaffolds in providing load-bearing capability and interaction with the local extracellular matrix to promote bone healing is a challenge in synthetic scaffolds. This article describes the usage and processing of multi-materials and hierarchical structures to mimic the structure of natural bone tissues to function as bioactive and load-bearing synthetic scaffolds. The first part of this literature review describes the physiology of bone healing responses and the interactions at different stages of bone repair. The following section reviews the available literature on biomaterials used for BTE scaffolds followed by some multi-material approaches. The next section discusses the impact of the scaffold's structural features on bone healing and the necessity of a hierarchical distribution in the scaffold structure. Finally, the last section of this review highlights the emerging trends in BTE scaffold developments that can inspire new tissue engineering strategies and truly develop the next generation of synthetic scaffolds.

Tuning the biomimetic behavior of hybrid scaffolds for bone tissue engineering through surface modifications and drug immobilization

Bone defects arising from injury and/or disease are a common and debilitating clinical lesion. While the development of tissue microenvironments utilizing biomimetic constructs is an emerging approach for bone tissue engineering. In this context, bioactive glass nanoparticles (BGNPs) were embedded within polycaprolactone (PCL) scaffolds. The scaffolds exhibit an engineered unidirectional pore structure which are surface activated via oxygen plasma to allow immobilization of simvastatin (SIM) on the pore surface. Microscopic observation indicated the surface modification did not disturb the lamellar orientation of the pores improving the biomimetic formation of hydroxyapatite. Mathematically modelled release profiles reveal that the oxygen plasma pre-treatment can be utilized to modulate the release profile of SIM from the scaffolds. With the release mechanism controlled by the balance between the diffusion and erosion mechanisms. Computational modelling shows that Human Serum Albumin and Human α₂-macroglobulin can be utilized to increase SIM bioavailability for cells via a molecular docking mechanism. Cellular studies show positive MG-63 cell attachment and viability on optimized scaffolds with alkaline phosphatase activity enhanced along with enhanced expression of osteocalcoin biomarker.

Multidimensional Mechanics of Three-Dimensional Printed and Micro-Architectured Scaffolds

Mechanical properties of porous materials depend on their micro-architectural characteristics. Freeze casting is an effective method to fabricate micro-architectured porous scaffolds. Three key characteristics generated during freeze casting are wall thickness, number of domains at the cross-section, and transverse bridges connecting adjacent walls. To specifically study the effect of these structural characteristics on the mechanics and anisotropic compressive properties of scaffolds, we utilize additive manufacturing, i.e., 3D printing, to fabricate strictly designed cubic scaffolds with varying one characteristic at a time. We then compare strength, toughness, resilience, stiffness, and strain to failure in three orthogonal directions of the scaffolds, including longitudinal and transverse directions. To compare these multidimensional mechanics in a single diagram, we use a previously developed radar chart method to evaluate different scaffolds and unravel the effect of the structural characteristics. We find that the multidimensional mechanics can be effectively tuned by the micro-architectural characteristics. Notably, the buckling resistance of the scaffolds depends on all three structural characteristics. Our results show that an increased number of domains leads to enhanced toughness in all three directions. Increasing wall thickness leads to enhanced mechanical properties but comes at the price of losing small-sized pores, which is not favored for certain applications. In addition, adding transverse bridges increase not only the transverse strength of the scaffolds but also the longitudinal strength as they also enhance the buckling resistance. Our study provides important insights into the structure-property relationships of 3D-printed micro-architectured porous scaffolds.

Recent advances in ice templating: from biomimetic composites to cell culture scaffolds and tissue engineering

We review the evolution of ice-templating process from initial inorganic materials to recent developments in shaping increasingly labile biological matter.

A review of multifunctional nacre-mimetic materials based on bidirectional freeze casting

Nacre has achieved an excellent combination of strength and toughness through its unique brick-and-mortar structure of layered aragonite platelets bonded with biopolymers. Mimicking nacre has been considered as a practical way for the development of high-performance structural composites. Over the past years, many techniques have been developed to fabricate multifunctional nacre-mimetic materials, including freeze casting, layer-by-layer assembly, vacuum filtration, 3D printing and so on. Among them, freeze casting, especially bidirectional freeze casting, as an environmentally friendly and scalable method, has attracted extensive attention recently. In this review, we begin with the introduction and discussion of various fabrication techniques comparing their advantages and disadvantages, focusing on the most recent advances of the bidirectional freeze casting technique. Then, we summarize representative examples of applying the bidirectional freeze casting technique to assemble various building blocks into multifunctional nacre-mimetic materials and their wide applications. At the end, we discuss the future direction of using bidirectional freeze casting to make nacre-mimetic materials.