In VitroThree-Dimensional Bone Tissue Models: From Cells to Controlled and Dynamic Environment

Internal flow field analysis of a dendritic pore scaffold for bone tissue engineering

The effective reconstruction of osteochondral biomimetic structures is a key factor in guiding the regeneration of full-thickness osteochondral defects. Due to the avascular nature of hyaline cartilage, the greatest challenge in constructing this scaffold lies in both utilizing the biomimetic structure to promote vascular differentiation for nutrient delivery to hyaline cartilage, thereby enhancing the efficiency of osteochondral reconstruction, and effectively blocking vascular ingrowth into the cartilage layer to prevent cartilage mineralization. However, the intrinsic relationship between the planning of the microporous pipe network and the flow resistance in the biomimetic structure, and the mechanism of promoting cell adhesion to achieve vascular differentiation and inhibiting cell adhesion to block the growth of blood vessels are still unclear. Inspired by the structure of tree trunks, this study designed a biomimetic tree-like tubular network structure for osteochondral scaffolds based on Murray's law. Utilizing computational fluid dynamics, the study investigated the influence of the branching angle of micro-pores on the flow velocity, pressure distribution, and scaffold permeability within the scaffold. The results indicate that when the differentiation angle exceeds 50 degrees, the highest flow velocity occurs at the confluence of tributaries at the ninth fractal position, forming a barrier layer. This structure effectively guides vascular growth, enhances nutrient transport capacity, increases flow velocity to promote cell adhesion, and inhibits cell infiltration into the cartilage layer.

EnduroBone: A 3D printed bioreactor for extended bone tissue culture

Studies of the effects of external stimuli on bone tissue, disease transmission mechanisms, and potential medication discoveries benefit from long-term tissue viability ex vivo. By simulating the in-vivo environment, bioreactors are essential for studying bone cellular activity throughout biological processes. We present the development of an automated 3D-printed bioreactor EnduroBone designed to sustain the ex-vivo viability of 10 mm diameter cancellous bone cores for an extended period. The device is supplied with two critical parameters for maintaining bone tissue viability: closed-loop continuous flow perfusion of 1 mL/min for nutrient diffusion and waste removal and direct mechanical stimulation with cyclic compression at 13.2 RPM (revolutions per minute) to promote cell viability which can lead to improved tissue stability during ex vivo culturing. The bioreactor addresses several limitations of existing systems and provides a versatile open-source platform for bone cancer research, orthopedic device testing, and other related applications. To validate the bioreactor, fresh swine samples were cultured ex-vivo, and their cell viability was determined to be maintained for up to 28 days. Periodic cell viability assessment through live/dead cell staining and confocal imaging at the start (0 days) and at several time points throughout the culture period (7, 14, 21, and 28 days) was used to demonstrate EnduroBone effectiveness in sustaining bone cell health for the extended period tested.

Advanced Immunomodulatory Biomaterials for Therapeutic Applications

The multifaceted biological defense system modulating complex immune responses against pathogens and foreign materials plays a critical role in tissue homeostasis and disease progression. Recently developed biomaterials that can specifically regulate immune responses, nanoparticles, graphene, and functional hydrogels have contributed to the advancement of tissue engineering as well as disease treatment. The interaction between innate and adaptive immunity, collectively determining immune responses, can be regulated by mechanobiological recognition and adaptation of immune cells to the extracellular microenvironment. Therefore, applying immunomodulation to tissue regeneration and cancer therapy involves manipulating the properties of biomaterials by tailoring their composition in the context of the immune system. This review provides a comprehensive overview of how the physicochemical attributes of biomaterials determine immune responses, focusing on the physical properties that influence innate and adaptive immunity. This review also underscores the critical aspect of biomaterial-based immune engineering for the development of novel therapeutics and emphasizes the importance of understanding the biomaterials-mediated immunological mechanisms and their role in modulating the immune system.

3D printing nacre powder/sodium alginate scaffold loaded with PRF promotes bone tissue repair and regeneration

Bone defects are a common complication of bone diseases, which often affect the quality of life and mental health of patients. The use of biomimetic bone scaffolds loaded with bioactive substances has become a focal point in the research on bone defect repair. In this study, composite scaffolds resembling bone tissue were created using nacre powder (NP) and sodium alginate (SA) through 3D printing. These scaffolds exhibit several physiological structural and mechanical characteristics of bone tissue, such as suitable porosity, an appropriate pore size, applicable degradation performance and satisfying the mechanical requirements of cancellous bone, etc. Then, platelet-rich fibrin (PRF), containing a mass of growth factors, was loaded on the NP/SA scaffolds. This was aimed to fully maximize the synergistic effect with NP, thereby accelerating bone tissue regeneration. Overall, this study marks the first instance of preparing a bionic bone structure scaffold containing NP by 3D printing technology, which is combined with PRF to further accelerate bone regeneration. These findings offer a new treatment strategy for bone tissue regeneration in clinical applications.

Micro-CT imaging and finite element models reveal how sintering temperature affects the microstructure and strength of bioactive glass-derived scaffolds

This study focuses on the finite element simulation and micromechanical characterization of bioactive glass-ceramic scaffolds using Computed micro Tomography ([Formula: see text]CT) imaging. The main purpose of this work is to quantify the effect of sintering temperature on the morphometry and mechanical performance of the scaffolds. In particular, the scaffolds were produced using a novel bioactive glass material (47.5B) through foam replication, applying six different sintering temperatures. Through [Formula: see text]CT imaging, detailed three-dimensional images of the scaffold's internal structure are obtained, enabling the extraction of important geometric features and how these features change with sintering temperature. A finite element model is then developed based on the [Formula: see text]CT images to simulate the fracture process under uniaxial compression loading. The model incorporates scaffold heterogeneity and material properties-also depending on sintering temperature-to capture the mechanical response, including crack initiation, propagation, and failure. Scaffolds sintered at temperatures equal to or higher than 700 [Formula: see text]C exhibit two-scale porosity, with micro and macro pores. Finite element analyses revealed that the dual porosity significantly affects fracture mechanisms, as micro-pores attract cracks and weaken strength. Interestingly, scaffolds sintered at high temperatures, the overall strength of which is higher due to greater intrinsic strength, showed lower normalized strength compared to low-temperature scaffolds. By using a combined strategy of finite element simulation and [Formula: see text]CT-based characterization, bioactive glass-ceramic scaffolds can be optimized for bone tissue engineering applications by learning more about their micromechanical characteristics and fracture response.

Additive manufacture of PLLA scaffolds reinforced with graphene oxide nano-particles via digital light processing (DLP)

In this study, 3D printing of poly-l-lactic acid (PLLA) scaffolds reinforced with graphene oxide (GO) nanoparticles via Digital Light Processing (DLP) was investigated to mimic bone tissue. Stereolithography is one of the most accurate additive manufacturing methods, but the dominant available materials used in this method are toxic. In this research, a biocompatible resin (PLLA) was synthetized and functionalized to serve the purpose. Due to the low mechanical properties of the printed product with the neat resin, graphene oxide nanoparticles in three levels (0.5, 1, and 1.5 wt%) were added with the aim of enhancing the mechanical properties. At first, the optimum post cure time of the neat resin was investigated. Consequently, all the parts were post-cured for 3 h after printing. Due to the temperature-dependent structure of GO, all samples were placed in an oven at 85°C for different time periods of 0, 6, 12, and 18 h to increase mechanical properties. The compression test of heat-treated samples reveals that the compressive strength of the printed parts containing 0.5,1, and 1.5% of GO increased by 151,162 ad 235%, respectively. Scaffolds with the designed pore sizes of 750 microns and a porosity of 40% were printed. Surface hydrophilicity test was performed for all samples showing that the hydrophilicity of the samples increased with increasing GO percentage. The degradation behavior of the samples was evaluated in a PBS environment, and it revealed that by increasing GO, the rate of component degradation increased, but the heat treatment had the opposite effect and decreased the degradation rate. Finally, besides improving biological properties, a significant increase in mechanical properties under compression can introduce the printed scaffolds as a suitable option for bone implants.

Perfusion of MC3T3E1 Preosteoblast Spheroids within Polysaccharide-Based Hydrogel Scaffolds: An Experimental and Numerical Study at the Bioreactor Scale

The traditional 3D culture systems in vitro lack the biological and mechanical spatiotemporal stimuli characteristic to native tissue development. In our study, we combined porous polysaccharide-based hydrogel scaffolds with a bioreactor-type perfusion device that generates favorable mechanical stresses while enhancing nutrient transfers. MC3T3E1 mouse osteoblasts were seeded in the scaffolds and cultivated for 3 weeks under dynamic conditions at a perfusion rate of 10 mL min^-1. The spatial distribution of the cells labeled with superparamagnetic iron oxide nanoparticles was visualized by MRI. Confocal microscopy was used to assess cell numbers, their distribution inside the scaffolds, cell viability, and proliferation. The oxygen diffusion coefficient in the hydrogel was measured experimentally. Numerical simulations of the flow and oxygen transport within the bioreactor were performed using a lattice Boltzmann method with a two-relaxation time scheme. Last, the influence of cell density and spheroid size on cell oxygenation was investigated. The cells spontaneously organized into spheroids with a diameter of 30-100 μm. Cell viability remained unchanged under dynamic conditions but decreased under static culture. The cell proliferation (Ki67 expression) in spheroids was not observed. The flow simulation showed that the local fluid velocity reached 27 mm s^-1 at the height where the cross-sectional area of the flow was the smallest. The shear stress exerted by the fluid on the scaffolds may locally rise to 100 mPa, compared with the average value of 25 mPa. The oxygen diffusion coefficient in the hydrogel was 1.6×10-9 m² s^-1. The simulation of oxygen transport and consumption confirmed that the cells in spheroids did not suffer from hypoxia when the bioreactor was perfused at 10 mL min^-1, and suggested the existence of optimal spheroid size and spacing for appropriate oxygenation. Collectively, these findings enabled us to define the optimal conditions inside the bioreactor for an efficient in vitro cell organization and survival in spheroids, which are paramount to future applications with organoids.

A Review of 3D Polymeric Scaffolds for Bone Tissue Engineering: Principles, Fabrication Techniques, Immunomodulatory Roles, and Challenges

Over the last few years, biopolymers have attracted great interest in tissue engineering and regenerative medicine due to the great diversity of their chemical, mechanical, and physical properties for the fabrication of 3D scaffolds. This review is devoted to recent advances in synthetic and natural polymeric 3D scaffolds for bone tissue engineering (BTE) and regenerative therapies. The review comprehensively discusses the implications of biological macromolecules, structure, and composition of polymeric scaffolds used in BTE. Various approaches to fabricating 3D BTE scaffolds are discussed, including solvent casting and particle leaching, freeze-drying, thermally induced phase separation, gas foaming, electrospinning, and sol-gel techniques. Rapid prototyping technologies such as stereolithography, fused deposition modeling, selective laser sintering, and 3D bioprinting are also covered. The immunomodulatory roles of polymeric scaffolds utilized for BTE applications are discussed. In addition, the features and challenges of 3D polymer scaffolds fabricated using advanced additive manufacturing technologies (rapid prototyping) are addressed and compared to conventional subtractive manufacturing techniques. Finally, the challenges of applying scaffold-based BTE treatments in practice are discussed in-depth.

Improved Bone Regeneration Using Biodegradable Polybutylene Succinate Artificial Scaffold in a Rabbit Model

The treatment of extensive bone loss represents a great challenge for orthopaedic and reconstructive surgery. Most of the time, those treatments consist of multiple-stage surgeries over a prolonged period, pose significant infectious risks and carry the possibility of rejection. In this study, we investigated if the use of a polybutylene succinate (PBS) micro-fibrillar scaffold may improve bone regeneration in these procedures. In an in vivo rabbit model, the healing of two calvarial bone defects was studied. One defect was left to heal spontaneously while the other was treated with a PBS scaffold. Computed tomography (CT) scans, histological and immunohistochemical analyses were performed at 4, 12 and 24 weeks. CT examination showed a significantly larger area of mineralised tissue in the treated defect. Histological examination confirmed a greater presence of active osteoblasts and mineralised tissue in the scaffold-treated defect, with no evidence of inflammatory infiltrates around it. Immunohistochemical analysis was positive for CD56 at the transition point between healthy bone and the fracture zone. This study demonstrates that the use of a PBS microfibrillar scaffold in critical bone defects on a rabbit model is a potentially effective technique to improve bone regeneration.

Chitosan-based scaffolds as drug delivery systems in bone tissue engineering

The bone tissue engineering approach for treating large bone defects becomes necessary when the tissue damage surpasses the threshold of the inherent regenerative ability of the human body. A myriad of natural biodegradable polymers and scaffold fabrication techniques have emerged in the last decade. Chitosan (CS) is especially attractive as a bone scaffold material to support cell attachment and proliferation and mineralization of the bone matrix. The primary amino groups in CS are responsible for properties such as controlled drug release, mucoadhesion, in situ gelation, and transfection. CS-based smart drug delivery scaffolds that respond to environmental stimuli have been reported to have a localized sustained delivery of drugs in the large bone defect area. This review outlines the recent advances in the fabrication of CS-based scaffolds as a pharmaceutical carrier to deliver drugs such as antibiotics, growth factors, nucleic acids, and phenolic compounds for bone tissue regeneration.

Application of Nano-Inspired Scaffolds-Based Biopolymer Hydrogel for Bone and Periodontal Tissue Regeneration

This review’s objectives are to provide an overview of the various kinds of biopolymer hydrogels that are currently used for bone tissue and periodontal tissue regeneration, to list the advantages and disadvantages of using them, to assess how well they might be used for nanoscale fabrication and biofunctionalization, and to describe their production processes and processes for functionalization with active biomolecules. They are applied in conjunction with other materials (such as microparticles (MPs) and nanoparticles (NPs)) and other novel techniques to replicate physiological bone generation more faithfully. Enhancing the biocompatibility of hydrogels created from blends of natural and synthetic biopolymers can result in the creation of the best scaffold match to the extracellular matrix (ECM) for bone and periodontal tissue regeneration. Additionally, adding various nanoparticles can increase the scaffold hydrogel stability and provide a number of biological effects. In this review, the research study of polysaccharide hydrogel as a scaffold will be critical in creating valuable materials for effective bone tissue regeneration, with a future impact predicted in repairing bone defects.

Locally Applied Repositioned Hormones for Oral Bone and Periodontal Tissue Engineering: A Narrative Review

Bone and periodontium are tissues that have a unique capacity to repair from harm. However, replacing or regrowing missing tissues is not always effective, and it becomes more difficult as the defect grows larger. Because of aging and the increased prevalence of debilitating disorders such as diabetes, there is a considerable increase in demand for orthopedic and periodontal surgical operations, and successful techniques for tissue regeneration are still required. Even with significant limitations, such as quantity and the need for a donor area, autogenous bone grafts remain the best solution. Topical administration methods integrate osteoconductive biomaterial and osteoinductive chemicals as hormones as alternative options. This is a promising method for removing the need for autogenous bone transplantation. Furthermore, despite enormous investigation, there is currently no single approach that can reproduce all the physiologic activities of autogenous bone transplants. The localized bioengineering technique uses biomaterials to administer different hormones to capitalize on the host’s regeneration capacity and capability, as well as resemble intrinsic therapy. The current study adds to the comprehension of the principle of hormone redirection and its local administration in both bone and periodontal tissue engineering.

Internal flow field analysis of heterogeneous porous scaffold for bone tissue engineering

The internal pore structure of the porous scaffold for bone tissue engineering and the pressure and velocity distributions of its flow field affect the attachment, proliferation and differentiation of osteoblasts. The permeability of the porous scaffold determines its ability to transport cellular nutrients and metabolites. Therefore, studying the fluid flow characteristics of the porous scaffold plays a vital role in its biological applications. Heterogeneous porous scaffolds (HPS) with irregular internal pore structure have more bionic characteristics of natural structure than uniform porous scaffolds with regular internal pore structure. In order to comprehensively grasp the biological properties of HPS, this article designed HPS with different porosities based on the Voronoi generation method and random theory, and then used computational fluid dynamics (CFD)software to conduct fluid flow simulations. The velocity and pressure distribution rules of the internal flow field of HPS with different porosities were obtained by CFD simulation analysis, and the relationship between the porosity and the distribution rules was studied. Furthermore, the permeabilities of HPS with different porosities were calculated based on Darcy's law, and the influence rule of porosity on the permeability was obtained.

Potential bioactive coating system for high-performance absorbable magnesium bone implants

Magnesium alloys are considered the most suitable absorbable metals for bone fracture fixation implants. The main challenge in absorbable magnesium alloys is their high corrosion/degradation rate that needs to be controlled. Various coatings have been applied to magnesium alloys to slow down their corrosion rates to match their corrosion rate to the regeneration rate of the bone fracture. In this review, a bioactive coating is proposed to slow down the corrosion rate of magnesium alloys and accelerate the bone fracture healing process. The main aim of the bioactive coatings is to enhance the direct attachment of living tissues and thereby facilitate osteoconduction. Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are six bioactive agents that show high potential for developing a bioactive coating system for high-performance absorbable magnesium bone implants. In addition to coating, the substrate itself can be made bioactive by alloying magnesium with calcium, zinc, copper, and manganese that were found to promote bone regeneration.

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Bioactive-coated magnesium implant could accelerate bone fracture healing time to match with magnesium degradation.

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Hydroxyapatite, collagen type I, recombinant human bone morphogenetic proteins 2, simvastatin, zoledronate, and strontium are high potential bioactive coating materials.

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The incorporation of Ca, Zn, Cu, Sr, and Mn in Mg base-metal could further enhance bone formation.

Conductive Scaffolds for Bone Tissue Engineering: Current State and Future Outlook

Bone tissue engineering strategies attempt to regenerate bone tissue lost due to injury or disease. Three-dimensional (3D) scaffolds maintain structural integrity and provide support, while improving tissue regeneration through amplified cellular responses between implanted materials and native tissues. Through this, scaffolds that show great osteoinductive abilities as well as desirable mechanical properties have been studied. Recently, scaffolding for engineered bone-like tissues have evolved with the use of conductive materials for increased scaffold bioactivity. These materials make use of several characteristics that have been shown to be useful in tissue engineering applications and combine them in the hope of improved cellular responses through stimulation (i.e., mechanical or electrical). With the addition of conductive materials, these bioactive synthetic bone substitutes could result in improved regeneration outcomes by reducing current factors limiting the effectiveness of existing scaffolding materials. This review seeks to overview the challenges associated with the current state of bone tissue engineering, the need to produce new grafting substitutes, and the promising future that conductive materials present towards alleviating the issues associated with bone repair and regeneration.

Alginate bone scaffolds coated with a bioactive lactose modified chitosan for human dental pulp stem cells proliferation and differentiation

Bioactive and biodegradable porous scaffolds can hasten the healing of bone defects; moreover, patient stem cells seeded onto scaffolds can enhance the osteoinductive and osteoconductive properties of these biomaterials. In this work, porous alginate/hydroxyapatite scaffolds were functionalized with a bioactive coating of a lactose-modified chitosan (CTL). The highly interconnected porous structure of the scaffold was homogeneously coated with CTL. The scaffolds showed remarkable stability up to 60 days of aging. Human Dental Pulp Stem Cells (hDPSCs) cultured in the presence of CTL diluted in culture medium, showed a slight and negligible increase in terms of proliferation rate; on the contrary, an effect on osteogenic differentiation of the cells was observed as a significant increase in alkaline phosphatase activity. hDPSCs showed higher cell adhesion on CTL-coated scaffolds than on uncoated ones. CTL coating did not affect cell proliferation, but stimulated cell differentiation as shown by alkaline phosphatase activity analysis.

Tantalum as a Novel Biomaterial for Bone Implant: A Literature Review

Titanium (Ti) has been used in metallic implants since the 1950s due to various biocompatible and mechanical properties. However, due to its high Young’s modulus, it has been modified over the years in order to produce a better biomaterial. Tantalum (Ta) has recently emerged as a new potential biomaterial for bone and dental implants. It has been reported to have better corrosion resistance and osteo-regenerative properties as compared to Ti alloys which are most widely used in the bone-implant industry. Currently, Tantalum cannot be widely used yet due to its limited availability, high melting point, and high-cost production. This review paper discusses various manufacturing methods of Tantalum alloys, including conventional and additive manufacturing and also discusses their drawbacks and shortcomings. Recent research includes surface modification of various metals using Tantalum coatings in order to combine bulk material properties of different materials and the porous surface properties of Tantalum. Design modification also plays a crucial role in controlling bulk properties. The porous design does provide a lower density, wider surface area, and more immense specific strength. In addition to improved mechanical properties, a porous design could also escalate the material's biological and permeability properties. With current advancement in additive manufacturing technology, difficulties in processing Tantalum could be resolved. Therefore, Tantalum should be considered as a serious candidate material for future bone and dental implants.

Using 3D in vitro cell culture models in anti-cancer drug discovery

INTRODUCTION

The high failure rate in drug discovery remains a costly and time-consuming challenge. Improving the odds of success in the early steps of drug development requires disease models with high biological relevance for biomarker discovery and drug development. The adoption of three-dimensional (3D) cell culture systems over traditional monolayers in cell-based assays is considered a promising step toward improving the success rate in drug discovery.

AREAS COVERED

In this article, the author focuses on new technologies for 3D cell culture and their applications in cancer drug discovery. Besides the most common 3D cell-culture systems for tumor cells, the article emphasizes the need for 3D cell culture technologies that can mimic the complex tumor microenvironment and cancer stem cell niche.

EXPERT OPINION

There has been a rapid increase in 3D cell culture technologies in recent years in an effort to more closely mimic in vivo physiology. Each 3D cell culture system has its own strengths and weaknesses with regard to in vivo tumor growth and the tumor microenvironment. This requires careful consideration of which 3D cell culture system is chosen for drug discovery and should be based on factors like drug target and tumor origin.

Advances in the Fabrication of Scaffold and 3D Printing of Biomimetic Bone Graft

The need for bone grafts is tremendous, and that leads to the use of autograft, allograft, and bone graft substitutes. The biology of the bone is quite complex regarding cellular composition and architecture, hence developing a mineralized connective tissue graft is challenging. Traditionally used bone graft substitutes including metals, biomaterial coated metals and biodegradable scaffolds, suffer from persistent limitations. With the advent and rise of additive manufacturing technologies, the future of repairing bone trauma and defects seems to be optimistic. 3D printing has significant advantages, the foremost of all being faster manipulation of various biocompatible materials and live cells or tissues into the complex natural geometries necessary to mimic and stimulate cellular bone growth. The advent of new-generation bioprinters working with high-precision, micro-dispensing and direct digital manufacturing is aiding in ground-breaking organ and tissue printing, including the bone. The future bone replacement for patients holds excellent promise as scientists are moving closer to the generation of better 3D printed bio-bone grafts that will be safer and more effective. This review aims to summarize the advances in scaffold fabrication techniques, emphasizing 3D printing of biomimetic bone grafts.

A Biodegradable Chitosan-Polyurethane Cryogel with Switchable Shape Memory

Cryogels are matrices that are formed in moderately frozen solutions of monomeric or polymeric precursors. They have the advantages of interconnected macropores, structural stability, and compressibility. Meanwhile, thermally induced shape memory is an attractive feature of certain functional materials. Although there have been several studies concerning shape-memory cryogels, little work has been conducted on shape-memory cryogels with biodegradability. In this study, a water-based biodegradable difunctional polyurethane with a shape-memory property was synthesized and used as the nanoparticulate crosslinker to react with chitosan to form a shape-memory cryogel. The thermally induced shape-memory mechanism was clarified using in situ wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) during the shape-memory process. The in situ WAXS showed the changes of crystallinity in the crosslinker and the cryogel during the shape fixation and recovery processes. The in situ SAXS revealed the orientation of crystallinity of the crosslinker and the cryogel as the mechanism for shape memory. The strip-shape cryogel was deformed at 50 °C to U-shape and fixed at - 20 °C, which was squeezable at 25 °C and returned to the strip-shape at 50 °C in air. The shape recovery was further tested in water at two different temperatures. The injected cryogel recovered the U-shape in 4 °C water, representing elastic recovery, and transformed to a long strip in 37 °C water, representing the switchable shape memory. Moreover, the shape-memory cryogel sheet with a large dimension (10 mm × 10 mm × 1.1 mm cryogel sheet) or with complex structures (N, T, and U shapes) could be fixed as a rod, injected through a 16 G needle, and return to its original shape in 37 °C water, all of which could not be achieved by the conventional cryogel. Human mesenchymal stem cells grown in the shape-memory cryogel scaffolds displayed long-term proliferation and chondrogenic potential. Their unique injectability and cytocompatibility suggested potential applications of shape-memory cryogels as injectable and expandable templates for tissue engineering and minimally invasive surgery.

Molecular Imprinting Strategies for Tissue Engineering Applications: A Review

Tissue Engineering (TE) represents a promising solution to fabricate engineered constructs able to restore tissue damage after implantation. In the classic TE approach, biomaterials are used alongside growth factors to create a scaffolding structure that supports cells during the construct maturation. A current challenge in TE is the creation of engineered constructs able to mimic the complex microenvironment found in the natural tissue, so as to promote and guide cell migration, proliferation, and differentiation. In this context, the introduction inside the scaffold of molecularly imprinted polymers (MIPs)—synthetic receptors able to reversibly bind to biomolecules—holds great promise to enhance the scaffold-cell interaction. In this review, we analyze the main strategies that have been used for MIP design and fabrication with a particular focus on biomedical research. Furthermore, to highlight the potential of MIPs for scaffold-based TE, we present recent examples on how MIPs have been used in TE to introduce biophysical cues as well as for drug delivery and sequestering.

Cell Sources for Human In vitro Bone Models

PURPOSE OF REVIEW

One aim in bone tissue engineering is to develop human cell-based, 3D in vitro bone models to study bone physiology and pathology. Due to the heterogeneity of cells among patients, patient's own cells are needed to be obtained, ideally, from one single cell source. This review attempts to identify the appropriate cell sources for development of such models.

RECENT FINDINGS

Bone marrow and peripheral blood are considered as suitable sources for extraction of osteoblast/osteocyte and osteoclast progenitor cells. Recent studies on these cell sources have shown no significant differences between isolated progenitor cells. However, various parameters such as medium composition affect the cell's proliferation and differentiation potential which could make the peripheral blood-derived stem cells superior to the ones from bone marrow. Peripheral blood can be considered a suitable source for osteoblast/osteocyte and osteoclast progenitor cells, being less invasive for the patient. However, more investigations are needed focusing on extraction and differentiation of both cell types from the same donor sample of peripheral blood.

3D printed Ti6Al4V bone scaffolds with different pore structure effects on bone ingrowth

The microstructure of porous scaffolds plays a vital role in bone regeneration, but its optimal shape is still unclear. In this study, four kinds of porous titanium alloy scaffolds with similar porosities (65%) and pore sizes (650 μm) and different structures were prepared by selective laser melting. Four scaffolds were implanted into the distal femur of rabbits to evaluate bone tissue growth in vivo. Micro-CT and hard tissue section analyses were performed 6 and 12 weeks after the operation to reveal the bone growth of the porous scaffold. The results show that diamond lattice unit (DIA) bone growth is the best of the four topological scaffolds. Through computational fluid dynamics (CFD) analysis, the permeability, velocity and flow trajectory inside the scaffold structure were calculated. The internal fluid velocity difference of the DIA structure is the smallest, and the trajectory of fluid flow inside the scaffold is the longest, which is beneficial for blood vessel growth, nutrient transport and bone formation. In this study, the mechanism of bone growth in different structures was revealed by in vivo experiments combined with CFD, providing a new theoretical basis for the design of bone scaffolds in the future.

Berberine-releasing electrospun scaffold induces osteogenic differentiation of DPSCs and accelerates bone repair

The repair of skeletal defects in maxillofacial region remains an intractable problem, the rising technology of bone tissue engineering provides a new strategy to solve it. Scaffolds, a crucial element of tissue engineering, must have favorable biocompatibility as well as osteoinductivity. In this study, we prepared berberine/polycaprolactone/collagen (BBR/PCL/COL) scaffolds with different concentrations of berberine (BBR) (25, 50, 75 and 100 μg/mL) through electrospinning. The influence of dosage on scaffold morphology, cell behavior and in vivo bone defect repair were systematically studied. The results indicated that scaffolds could release BBR stably for up to 27 days. Experiments in vitro showed that BBR/PCL/COL scaffolds had appropriate biocompatibility in the concentration of 25-75 μg/mL, and 50 and 75 μg/mL scaffolds could significantly promote osteogenic differentiation of dental pulp stem cells. Scaffold with 50 μg/mL BBR was implanted into the critical bone defect of rats to evaluate the ability of bone repair in vivo. It was found that BBR/PCL/COL scaffold performed more favorable than polycaprolactone/collagen (PCL/COL) scaffold. Overall, our study is the first to evaluate the capability of in vivo bone repair of BBR/PCL/COL electrospun scaffold. The results indicate that BBR/PCL/COL scaffold has prospective potential for tissue engineering applications in bone regeneration therapy.

Surface-treated 3D printed Ti-6Al-4V scaffolds with enhanced bone regeneration performance: an in vivo study

Background

Given their highly adjustable and predictable properties, three-dimensional(3D) printed geometrically ordered porous biomaterials offer unique opportunities as orthopedic implants. The performance of such biomaterials is, however, as much a result of the surface properties of the struts as it is of the 3D porous structure. In our previous study, we have investigated the in vitro performances of selective laser melted (SLM) Ti-6Al-4V scaffolds which are surface modified by the bioactive glass (BG) and mesoporous bioactive glass (MBG), respectively. The results demonstrated that such modification enhanced the attachment, proliferation, and differentiation of human bone marrow stromal cells (hBMSC). Here, we take the next step by assessing the therapeutic potential of 3D printed Ti-6Al-4V scaffolds with BG and MBG surface modifications for bone regeneration in a rabbit bone defect model.

Methods

3D printed Ti-6Al-4V scaffolds with BG and MBG surface modifications were implanted into the femoral condyle of the rabbits, the Ti-6Al-4V scaffolds without surface modification were used as the control. At week 3, 6, and 9 after the implantation, micro-computed tomography (micro-CT) imaging, fluorescence double-labeling to determine the mineral apposition rate (MAR), and histological analysis of non-decalcified sections were performed.

Results

We found significantly higher volumes of regenerated bone, significantly higher values of the relevant bone morphometric parameters, clear signs of bone matrix apposition and maturation, and the evidence of progressed angiogenesis and blood vessel formation in the groups where the bioactive glass was added as a coating, particularly the MGB group.

Conclusions

The MBG coating resulted in enhanced osteoconduction and vascularization in bone defect healing, which was attributed to the release of silicon and calcium ions and the presence of a nano-mesoporous structure on the surface of the MBG specimens.

Towards 3D Multi-Layer Scaffolds for Periodontal Tissue Engineering Applications: Addressing Manufacturing and Architectural Challenges

Reduced periodontal support, deriving from chronic inflammatory conditions, such as periodontitis, is one of the main causes of tooth loss. The use of dental implants for the replacement of missing teeth has attracted growing interest as a standard procedure in clinical practice. However, adequate bone volume and soft tissue augmentation at the site of the implant are important prerequisites for successful implant positioning as well as proper functional and aesthetic reconstruction of patients. Three-dimensional (3D) scaffolds have greatly contributed to solve most of the challenges that traditional solutions (i.e., autografts, allografts and xenografts) posed. Nevertheless, mimicking the complex architecture and functionality of the periodontal tissue represents still a great challenge. In this study, a porous poly(ε-caprolactone) (PCL) and Sr-doped nano hydroxyapatite (Sr-nHA) with a multi-layer structure was produced via a single-step additive manufacturing (AM) process, as a potential strategy for hard periodontal tissue regeneration. Physicochemical characterization was conducted in order to evaluate the overall scaffold architecture, topography, as well as porosity with respect to the original CAD model. Furthermore, compressive tests were performed to assess the mechanical properties of the resulting multi-layer structure. Finally, in vitro biological performance, in terms of biocompatibility and osteogenic potential, was evaluated by using human osteosarcoma cells. The manufacturing route used in this work revealed a highly versatile method to fabricate 3D multi-layer scaffolds with porosity levels as well as mechanical properties within the range of dentoalveolar bone tissue. Moreover, the single step process allowed the achievement of an excellent integrity among the different layers of the scaffold. In vitro tests suggested the promising role of the ceramic phase within the polymeric matrix towards bone mineralization processes. Overall, the results of this study demonstrate that the approach undertaken may serve as a platform for future advances in 3D multi-layer and patient-specific strategies that may better address complex periodontal tissue defects.

Bone 'spackling' paste: Mechanical properties and in vitro response of a porous ceramic composite bone tissue scaffold

Bone defects are treated with bone grafts, replacing damaged or diseased bone tissue with either natural bone or bone substitutes. This study investigates the structure, mechanical properties, and in vitro response of an all-ceramic composite designed for use as a bone 'spackling' paste: a formable synthetic bone graft paste used to repair bone defects in place, reacting with CO₂ gas in ambient conditions to become a void-filling, rigid scaffold. The composite is comprised of bioactive glass frit and a soluble liquid silicate precursor combined to form an air-setting, open porous scaffold with compressive strength within the low range for trabecular bone (1.3-4.4 MPa). Characterization of scaffolds, with varying amounts of binder, was executed in accordance with established design criteria of porosity, load-bearing capacity, and bioactivity. Bioactivity was assessed via morphological, structural, and chemical changes in surface mineralization that occurred during in vitro immersion in simulated body fluid. All phases of composite specimens were observed to form calcium phosphate minerals, indicating that a chemical change occurred between the bioactive glass and sodium silicate binder phase. Ion exchange between the two phases was likely, as sodium silicate (control) was not found to produce calcium phosphate in the absence of bioactive glass. Of the selected compositions, composites with 7.4 vol% sodium silicate binder were observed to possess the highest open porosity (44 vol%), highest rate of calcium phosphate mineralization, most uniform surface mineral distribution, and largest amount of hydroxycarbonate apatite formation. The structure, mechanical properties, and in vitro response of the composite scaffolds analyzed in this research signify their potential success as bone tissue scaffolds.

Recent Advances in Mechanically Loaded Human Mesenchymal Stem Cells for Bone Tissue Engineering

Large bone defects are a major health concern worldwide. The conventional bone repair techniques (e.g., bone-grafting and Masquelet techniques) have numerous drawbacks, which negatively impact their therapeutic outcomes. Therefore, there is a demand to develop an alternative bone repair approach that can address the existing drawbacks. Bone tissue engineering involving the utilization of human mesenchymal stem cells (hMSCs) has recently emerged as a key strategy for the regeneration of damaged bone tissues. However, the use of tissue-engineered bone graft for the clinical treatment of bone defects remains challenging. While the role of mechanical loading in creating a bone graft has been well explored, the effects of mechanical loading factors (e.g., loading types and regime) on clinical outcomes are poorly understood. This review summarizes the effects of mechanical loading on hMSCs for bone tissue engineering applications. First, we discuss the key assays for assessing the quality of tissue-engineered bone grafts, including specific staining, as well as gene and protein expression of osteogenic markers. Recent studies of the impact of mechanical loading on hMSCs, including compression, perfusion, vibration and stretching, along with the potential mechanotransduction signalling pathways, are subsequently reviewed. Lastly, we discuss the challenges and prospects of bone tissue engineering applications.

Biomimetic Aspects of Restorative Dentistry Biomaterials

Biomimetic has emerged as a multi-disciplinary science in several biomedical subjects in recent decades, including biomaterials and dentistry. In restorative dentistry, biomimetic approaches have been applied for a range of applications, such as restoring tooth defects using bioinspired peptides to achieve remineralization, bioactive and biomimetic biomaterials, and tissue engineering for regeneration. Advancements in the modern adhesive restorative materials, understanding of biomaterial–tissue interaction at the nano and microscale further enhanced the restorative materials’ properties (such as color, morphology, and strength) to mimic natural teeth. In addition, the tissue-engineering approaches resulted in regeneration of lost or damaged dental tissues mimicking their natural counterpart. The aim of the present article is to review various biomimetic approaches used to replace lost or damaged dental tissues using restorative biomaterials and tissue-engineering techniques. In addition, tooth structure, and various biomimetic properties of dental restorative materials and tissue-engineering scaffold materials, are discussed.

Key components of engineering vascularized 3-dimensional bioprinted bone constructs

Vascularization has a pivotal role in engineering successful tissue constructs. However, it remains a major hurdle of bone tissue engineering, especially in clinical applications for the treatment of large bone defects. Development of vascularized and clinically-relevant engineered bone substitutes with sufficient blood supply capable of maintaining implant viability and supporting subsequent host tissue integration remains a major challenge. Since only cells that are 100-200 µm from blood vessels can receive oxygen through diffusion, engineered constructs that are thicker than 400 µm face a challenging oxygenation problem. Following implantation in vivo, spontaneous ingrowth of capillaries in thick engineered constructs is too slow. Thus, it is critical to provide optimal conditions to support vascularization in engineered bone constructs. To achieve this, an in-depth understanding of the mechanisms of angiogenesis and bone development is required. In addition, it is also important to mimic the physiological milieu of native bone to fabricate more successful vascularized bone constructs. Numerous applications of engineered vascularization with cell-and/or microfabrication-based approaches seek to meet these aims. Three-dimensional (3D) printing promises to create patient-specific bone constructs in the future. In this review, we discuss the major components of fabricating vascularized 3D bioprinted bone constructs, analyze their related challenges, and highlight promising future trends.

A 3D computational model of perfusion seeding for investigating cell transport and adhesion within a porous scaffold

The process of cell seeding within a porous scaffold is an essential first step in the development of tissue-engineered bone grafts. Understanding the underlying mechanisms of cell distribution and adhesion is fundamental for the design and optimization of the seeding process. To that end, we present a numerical model to investigate the perfusion cell seeding process that incorporates cell mechanics, cell-fluid interaction, and cell-scaffold adhesion. The individual cells are modeled as deformable spherical capsules capable of adhering to the scaffold surface as well as to other cells with probabilistic bond formation and rupture. The mechanical deformation of the cell is calibrated with the stretching of mice mesenchymal stem cells induced by optical tweezers, while the predicted adhesive forces are consistent with the experimental data reported in the literature. A sub-domain is numerically reconstructed as the region of interest (ROI) which is representative of an actual scaffold. Through the simulations, the perfusion seeding kinetics within the ROI involving detailed transport and adhesion of cells over time is analyzed. The effects of the perfusion pressure and initial cell concentration on the seeding kinetics are studied in terms of adhesion rates, cell cluster formation, seeding uniformity, and efficiency, as well as scaffold permeability. The results highlight the importance of cell-fluid interaction and adhesion dynamics in modeling the dynamic seeding process. This bottom-up model provides a way to bridge detailed behaviors of individual cells to the seeding outcomes at the macroscopic scale, allowing for finding the best configuration to enhance cell seeding.

The effect of pore size within fibrous scaffolds fabricated using melt electrowriting on human bone marrow stem cell osteogenesis

Limitations associated with current bone grafting materials has necessitated the development of synthetic scaffolds that mimic the native tissue for bone repair. Scaffold parameters such as pore size, pore interconnectivity, fibre diameter, and fibre stiffness are crucial parameters of fibrous bone tissue engineering (BTE) scaffolds required to replicate the native environment. Optimum values vary with material, fabrication method and cell type. Melt electrowriting (MEW) provides precise control over extracellular matrix (ECM)-like fibrous scaffold architecture. The goal of this study was to fabricate and characterise poly-ε-caprolactone (PCL) fibrous scaffolds with 100, 200, and 300 μm pore sizes using MEW and determine the influence of pore size on human bone marrow stem cell (hMSC) adhesion, morphology, proliferation, mechanosignalling and osteogenesis. Each scaffold was fabricated with a fibre diameter of 4.01 ± 0.06 μm. The findings from this study highlight the enhanced osteogenic effects of controlled micro-scale fibre deposition using MEW, where the benefits of 100 μm square pores in comparison with larger pore sizes are illustrated, a pore size traditionally reported as a lower limit for osteogenesis. This suggests a lower pore size is optimal when hMSCs are seeded in a 3D ECM-like fibrous structure, with the 100 μm pore size optimal as it demonstrates the highest global stiffness, local fibre stiffness, highest seeding efficiency, maintains a spread cellular morphology, and significantly enhances hMSC collagen and mineral deposition. Similarly, this platform represents an effective in vitro model for the study of hMSC behaviour to determine the significant osteogenic benefits of controlling ECM-like fibrous BTE scaffold pore size using MEW.

Strategy for achieving standardized bone models

Reliably producing functional in vitro organ models, such as organ-on-chip systems, has the potential to considerably advance biology research, drug development time, and resource efficiency. However, despite the ongoing major progress in the field, three-dimensional bone tissue models remain elusive. In this review, we specifically investigate the control of perfusion flow effects as the missing link between isolated culture systems and scientifically exploitable bone models and propose a roadmap toward this goal.

Rational design of gelatin/nanohydroxyapatite cryogel scaffolds for bone regeneration by introducing chemical and physical cues to enhance osteogenesis of bone marrow mesenchymal stem cells

Identification of key components in the chemical and physical milieu for directing osteogenesis is a requirement in the investigation of tissue engineering scaffolds for advancement of bone regeneration. In this study, we engineered different gelatin-based cryogels and studied the effect of nanohydroxyapatite (nHAP) and crosslinking agents on scaffold properties and its osteogenic response towards bone marrow stem cells (BMSCs). The cryogels examined are 5% gelatin and 5% gelatin/2.5% nHAP, crosslinked either with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) or glutaraldehyde (GA). We confirmed that nHAP or the crosslinking agent has no effects on scaffold pore size and porosity. Nonetheless, incorporation of nHAP increased mechanical strength, swelling ratio and degree of crosslinking, but decreased degradation rate. Cryogels crosslinked with EDC showed faster degradation and promoted osteogenic differentiation of BMSCs while those prepared from GA crosslinking promoted proliferation of BMSCs. Furthermore, osteogenic differentiation was always enhanced in the presence of nHAP irrespective of the culture medium (normal or osteogenic) used but osteogenic medium always provide a higher extent of osteogenic differentiation. Employing gelatin/nHAP cryogel crosslinked by EDC in a bioreactor for dynamic culture of BMSCs, cyclic compressive mechanical simulation was found to be beneficial for both cell proliferation and osteogenic differentiation. However, the optimum conditions for osteogenic differentiation and cell proliferation were found at 30% and 60% strain, respectively. We thus demonstrated that osteogenic differentiation of BMSCs could be tuned by taking advantages of chemical cues generated from scaffold chemistry or physical cues generated from dynamic cell culture in vitro. Furthermore, by combining the best cryogel preparation and in vitro cell culture condition for osteogenesis, we successfully employed in vitro cultured cryogel/BMSCs constructs for repair of rabbit critical-sized cranial bone defects.

POLYMERIC BIOMATERIALS FOR SCAFFOLD-BASED BONE REGENERATIVE ENGINEERING

Reconstruction of large bone defects resulting from trauma, neoplasm, or infection is a challenging problem in reconstructive surgery. The need for bone grafting has been increasing steadily partly because of our enhanced capability to salvage limbs after major bone loss. Engineered bone graft substitutes can have advantages such as lack of antigenicity, high availability, and varying properties depending on the applications chosen for use. These favorable attributes have contributed to the rise of scaffold-based polymeric tissue regeneration. Critical components in the scaffold-based polymeric regenerative engineering approach often include 1. The existence of biodegradable polymeric porous structures with properties selected to promote tissue regeneration and while providing appropriate mechanical support during tissue regeneration. 2. Cellular populations that can influence and enhance regeneration. 3. The use of growth and morphogenetic factors which can influence cellular migration, differentiation and tissue regeneration in vivo. Biodegradable polymers constitute an attractive class of biomaterials for the development of scaffolds due to their flexibility in chemistry and their ability to produce biocompatible degradation products. This paper presents an overview of polymeric scaffold-based bone tissue regeneration and reviews approaches as well as the particular roles of biodegradable polymers currently in use.

Biocompatibility and osteogenic activity of guided bone regeneration membrane based on chitosan-coated magnesium alloy

Ideally, a guided bone regeneration membrane (GBRM) should possess high strength, as for titanium membranes, along with excellent biocompatibility and osteoconductivity, as for natural absorbable collagen membranes. Besides titanium, magnesium (Mg) is another metal widely used in the biomedical field, which also exhibits biodegradability. In this study, a composite chitosan‑magnesium (CS-Mg) membrane was fabricated by dip-coating Mg alloy into chitosan solution. In vitro and in vivo tests were performed to investigate whether this membrane could be used as biodegradable GBRM, and the test results were compared with those obtained for a commercial GBRM (Heal-All). The microstructure was analyzed by scanning electron microscopy-electron dispersive spectroscopy. The degradation behavior was investigated by immersing the membranes into Dulbecco's modified Eagle medium (DMEM). The in vitro biocompatibility was evaluated by cell adhesion, cytotoxicity and alkaline phosphatase (ALP) assays using MG63 cells. The cytotoxicity and ALP assays were performed with diluted extracts of Mg, CS-Mg and Heal-All. The results show that CS-Mg has a suitable degradation rate, as well as similar cell adhesion and cytocompatibility to Heal-All. However, the 10% CS-Mg extracts exhibited higher ALP activity at 3 and 5 days (p < 0.05) compared with the medium control and the Heal-All extracts, but no differences with 10% Mg extracts (p > 0.05). Rabbit calvarial defects were used for testing the osteogenic activity in vivo. Three groups of samples were examined: CS-Mg, Heal-All, and a blank control. Higher amounts of new bone were formed for the CS-Mg and Heal-All groups (p < 0.05) compared with the blank control, whereas no significant differences between the CS-Mg and Heal-All groups were observed (p > 0.1). In conclusion, the CS-Mg membrane shows great potential for application as a biodegradable metallic GBRM with excellent osteogenic activity.

A Three-Dimensional Dense Collagen Hydrogel to Model Cancer Cell/Osteoblast Interactions

No curative treatment options exist once breast cancer metastasizes to bone. This is due, in part, to an incomplete understanding of how osteolytic cancers interact with bone. Presented here is a novel approach to study the interactions between triple negative breast cancer cells and osteoblasts within a 3D collagenous environment. More specifically, a dense collagen hydrogel was employed to model interactions between MDA-MB-231 breast cancer cells and MC3T3-E1 pre-osteoblasts. Co-cultures with these two cell types, or MDA-MB-231-derived conditioned medium applied to MC3T3-E1 cells, were established in the context of plastically compressed dense collagen gel matrices. Importantly, breast cancer-derived conditioned medium or the establishment of breast cancer/osteoblast co-cultures did not negatively influence MC3T3-E1 cell viability. The inclusion of either conditioned medium or the presence of MDA-MB-231 cells resulted in impaired MC3T3-E1 differentiation into osteoblasts, which coincided with reduced osteoblast-mediated mineralization. The results presented here demonstrate that dense collagen gels provide a model environment to examine the effect of osteolytic breast cancer cells on osteoblast differentiation and subsequent mineralization of the collagen scaffold.

Additive Manufacturing for Guided Bone Regeneration: A Perspective for Alveolar Ridge Augmentation

Three-dimensional (3D) printing has become an important tool in the field of tissue engineering and its further development will lead to completely new clinical possibilities. The ability to create tissue scaffolds with controllable characteristics, such as internal architecture, porosity, and interconnectivity make it highly desirable in comparison to conventional techniques, which lack a defined structure and repeatability between scaffolds. Furthermore, 3D printing allows for the production of scaffolds with patient-specific dimensions using computer-aided design. The availability of commercially available 3D printed permanent implants is on the rise; however, there are yet to be any commercially available biodegradable/bioresorbable devices. This review will compare the main 3D printing techniques of: stereolithography; selective laser sintering; powder bed inkjet printing and extrusion printing; for the fabrication of biodegradable/bioresorbable bone tissue scaffolds; and, discuss their potential for dental applications, specifically augmentation of the alveolar ridge.

Relevance of bioreactors and whole tissue cultures for the translation of new therapies to humans

The purpose of this review is to provide a brief overview of bioreactor-based culture systems as alternatives to conventional two- and three-dimensional counterparts. The role, challenges, and future aspirations of bioreactors in the musculoskeletal field (e.g., cartilage, intervertebral disc, tendon, and bone) are discussed. Bioreactors, by recapitulating physiological processes, can be used effectively as part of the initial in vitro screening, reducing that way the number of animal required for preclinical assessment, complying with the 3R principles and, in most cases, allowing working with human tissues. The clinical significance of bioreactors is that, by providing more physiologically relevant conditions to customarily used two- and three-dimensional cultures, they hold the potential to provide a testing platform that is more predictable of a whole tissue response, thereby facilitating the screening of treatments before the initiation of clinical trials. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:10-21, 2018.

Tissue Engineering Approaches in the Design of Healthy and Pathological In Vitro Tissue Models

In the tissue engineering (TE) paradigm, engineering and life sciences tools are combined to develop bioartificial substitutes for organs and tissues, which can in turn be applied in regenerative medicine, pharmaceutical, diagnostic, and basic research to elucidate fundamental aspects of cell functions in vivo or to identify mechanisms involved in aging processes and disease onset and progression. The complex three-dimensional (3D) microenvironment in which cells are organized in vivo allows the interaction between different cell types and between cells and the extracellular matrix, the composition of which varies as a function of the tissue, the degree of maturation, and health conditions. In this context, 3D in vitro models can more realistically reproduce a tissue or organ than two-dimensional (2D) models. Moreover, they can overcome the limitations of animal models and reduce the need for in vivo tests, according to the "3Rs" guiding principles for a more ethical research. The design of 3D engineered tissue models is currently in its development stage, showing high potential in overcoming the limitations of already available models. However, many issues are still opened, concerning the identification of the optimal scaffold-forming materials, cell source and biofabrication technology, and the best cell culture conditions (biochemical and physical cues) to finely replicate the native tissue and the surrounding environment. In the near future, 3D tissue-engineered models are expected to become useful tools in the preliminary testing and screening of drugs and therapies and in the investigation of the molecular mechanisms underpinning disease onset and progression. In this review, the application of TE principles to the design of in vitro 3D models will be surveyed, with a focus on the strengths and weaknesses of this emerging approach. In addition, a brief overview on the development of in vitro models of healthy and pathological bone, heart, pancreas, and liver will be presented.

Scaffolds for Bone Tissue Engineering: State of the art and new perspectives

This review is intended to give a state of the art description of scaffold-based strategies utilized in Bone Tissue Engineering. Numerous scaffolds have been tested in the orthopedic field with the aim of improving cell viability, attachment, proliferation and homing, osteogenic differentiation, vascularization, host integration and load bearing. The main traits that characterize a scaffold suitable for bone regeneration concerning its biological requirements, structural features, composition, and types of fabrication are described in detail. Attention is then focused on conventional and Rapid Prototyping scaffold manufacturing techniques. Conventional manufacturing approaches are subtractive methods where parts of the material are removed from an initial block to achieve the desired shape. Rapid Prototyping techniques, introduced to overcome standard techniques limitations, are additive fabrication processes that manufacture the final three-dimensional object via deposition of overlying layers. An important improvement is the possibility to create custom-made products by means of computer assisted technologies, starting from patient's medical images. As a conclusion, it is highlighted that, despite its encouraging results, the clinical approach of Bone Tissue Engineering has not taken place on a large scale yet, due to the need of more in depth studies, its high manufacturing costs and the difficulty to obtain regulatory approval. PUBMED search terms utilized to write this review were: "Bone Tissue Engineering", "regenerative medicine", "bioactive scaffolds", "biomimetic scaffolds", "3D printing", "3D bioprinting", "vascularization" and "dentistry".

Fabrication of Extracellular Matrix-derived Foams and Microcarriers as Tissue-specific Cell Culture and Delivery Platforms

Cell function is mediated by interactions with the extracellular matrix (ECM), which has complex tissue-specific composition and architecture. The focus of this article is on the methods for fabricating ECM-derived porous foams and microcarriers for use as biologically-relevant substrates in advanced 3D in vitro cell culture models or as pro-regenerative scaffolds and cell delivery systems for tissue engineering and regenerative medicine. Using decellularized tissues or purified insoluble collagen as a starting material, the techniques can be applied to synthesize a broad array of tissue-specific bioscaffolds with customizable geometries. The approach involves mechanical processing and mild enzymatic digestion to yield an ECM suspension that is used to fabricate the three-dimensional foams or microcarriers through controlled freezing and lyophilization procedures. These pure ECM-derived scaffolds are highly porous, yet stable without the need for chemical crosslinking agents or other additives that may negatively impact cell function. The scaffold properties can be tuned to some extent by varying factors such as the ECM suspension concentration, mechanical processing methods, or synthesis conditions. In general, the scaffolds are robust and easy to handle, and can be processed as tissues for most standard biological assays, providing a versatile and user-friendly 3D cell culture platform that mimics the native ECM composition. Overall, these straightforward methods for fabricating customized ECM-derived foams and microcarriers may be of interest to both biologists and biomedical engineers as tissue-specific cell-instructive platforms for in vitro and in vivo applications.