Microsphere-based selective laser sintering for building macroporous bone scaffolds with controlled microstructure and excellent biocompatibility

Selective Laser Melting and Spark Plasma Sintering: A Perspective on Functional Biomaterials

Achieving lightweight, high-strength, and biocompatible composites is a crucial objective in the field of tissue engineering. Intricate porous metallic structures, such as lattices, scaffolds, or triply periodic minimal surfaces (TPMSs), created via the selective laser melting (SLM) technique, are utilized as load-bearing matrices for filled ceramics. The primary metal alloys in this category are titanium-based Ti6Al4V and iron-based 316L, which can have either a uniform cell or a gradient structure. Well-known ceramics used in biomaterial applications include titanium dioxide (TiO2), zirconium dioxide (ZrO2), aluminum oxide (Al2O3), hydroxyapatite (HA), wollastonite (W), and tricalcium phosphate (TCP). To fill the structures fabricated by SLM, an appropriate ceramic is employed through the spark plasma sintering (SPS) method, making them suitable for in vitro or in vivo applications following minor post-processing. The combined SLM-SPS approach offers advantages, such as rapid design and prototyping, as well as assured densification and consolidation, although challenges persist in terms of large-scale structure and molding design. The individual or combined application of SLM and SPS processes can be implemented based on the specific requirements for fabricated sample size, shape complexity, densification, and mass productivity. This flexibility is a notable advantage offered by the combined processes of SLM and SPS. The present article provides an overview of metal-ceramic composites produced through SLM-SPS techniques. Mg-W-HA demonstrates promise for load-bearing biomedical applications, while Cu-TiO2-Ag exhibits potential for virucidal activities. Moreover, a functionally graded lattice (FGL) structure, either in radial or longitudinal directions, offers enhanced advantages by allowing adjustability and control over porosity, roughness, strength, and material proportions within the composite.

Bioactive Materials for Bone Regeneration: Biomolecules and Delivery Systems

Novel tissue regeneration strategies are constantly being developed worldwide. Research on bone regeneration is noteworthy, as many promising new approaches have been documented with novel strategies currently under investigation. Innovative biomaterials that allow the coordinated and well-controlled repair of bone fractures and bone loss are being designed to reduce the need for autologous or allogeneic bone grafts eventually. The current engineering technologies permit the construction of synthetic, complex, biomimetic biomaterials with properties nearly as good as those of natural bone with good biocompatibility. To ensure that all these requirements meet, bioactive molecules are coupled to structural scaffolding constituents to form a final product with the desired physical, chemical, and biological properties. Bioactive molecules that have been used to promote bone regeneration include protein growth factors, peptides, amino acids, hormones, lipids, and flavonoids. Various strategies have been adapted to investigate the coupling of bioactive molecules with scaffolding materials to sustain activity and allow controlled release. The current manuscript is a thorough survey of the strategies that have been exploited for the delivery of biomolecules for bone regeneration purposes, from choosing the bioactive molecule to selecting the optimal strategy to synthesize the scaffold and assessing the advantages and disadvantages of various delivery strategies.

Additive manufacturing of sustainable biomaterials for biomedical applications

Biopolymers are promising environmentally benign materials applicable in multifarious applications. They are especially favorable in implantable biomedical devices thanks to their excellent unique properties, including bioactivity, renewability, bioresorbability, biocompatibility, biodegradability and hydrophilicity. Additive manufacturing (AM) is a flexible and intricate manufacturing technology, which is widely used to fabricate biopolymer-based customized products and structures for advanced healthcare systems. Three-dimensional (3D) printing of these sustainable materials is applied in functional clinical settings including wound dressing, drug delivery systems, medical implants and tissue engineering. The present review highlights recent advancements in different types of biopolymers, such as proteins and polysaccharides, which are employed to develop different biomedical products by using extrusion, vat polymerization, laser and inkjet 3D printing techniques in addition to normal bioprinting and four-dimensional (4D) bioprinting techniques. This review also incorporates the influence of nanoparticles on the biological and mechanical performances of 3D-printed tissue scaffolds. This work also addresses current challenges as well as future developments of environmentally friendly polymeric materials manufactured through the AM techniques. Ideally, there is a need for more focused research on the adequate blending of these biodegradable biopolymers for achieving useful results in targeted biomedical areas. We envision that biopolymer-based 3D-printed composites have the potential to revolutionize the biomedical sector in the near future.

Review on Porous Scaffolds Generation Process: A Tissue Engineering Approach

Porous scaffolds have been widely explored for tissue regeneration and engineering in vitro three-dimensional models. In this review, a comprehensive literature analysis is conducted to identify the steps involved in their generation. The advantages and disadvantages of the available techniques are discussed, highlighting the importance of considering pore geometrical parameters such as curvature and size, and summarizing the requirements to generate the porous scaffold according to the desired application. This paper considers the available design tools, mathematical models, materials, fabrication techniques, cell seeding methodologies, assessment methods, and the status of pore scaffolds in clinical applications. This review compiles the relevant research in the field in the past years. The trends, challenges, and future research directions are discussed in the search for the generation of a porous scaffold with improved mechanical and biological properties that can be reproducible, viable for long-term studies, and closer to being used in the clinical field.

Scaffold Fabrication Techniques of Biomaterials for Bone Tissue Engineering: A Critical Review

Bone tissue engineering (BTE) is a promising alternative to repair bone defects using biomaterial scaffolds, cells, and growth factors to attain satisfactory outcomes. This review targets the fabrication of bone scaffolds, such as the conventional and electrohydrodynamic techniques, for the treatment of bone defects as an alternative to autograft, allograft, and xenograft sources. Additionally, the modern approaches to fabricating bone constructs by additive manufacturing, injection molding, microsphere-based sintering, and 4D printing techniques, providing a favorable environment for bone regeneration, function, and viability, are thoroughly discussed. The polymers used, fabrication methods, advantages, and limitations in bone tissue engineering application are also emphasized. This review also provides a future outlook regarding the potential of BTE as well as its possibilities in clinical trials.

Scaffold-based bone tissue engineering in microgravity: potential, concerns and implications

One of humanity's greatest challenges is space exploration, which requires an in-depth analysis of the data continuously collected as a necessary input to fill technological gaps and move forward in several research sectors. Focusing on space crew healthcare, a critical issue to be addressed is tissue regeneration in extreme conditions. In general, it represents one of the hottest and most compelling goals of the scientific community and the development of suitable therapeutic strategies for the space environment is an urgent need for the safe planning of future long-term manned space missions. Osteopenia is a commonly diagnosed disease in astronauts due to the physiological adaptation to altered gravity conditions. In order to find specific solutions to bone damage in a reduced gravity environment, bone tissue engineering is gaining a growing interest. With the aim to critically investigate this topic, the here presented review reports and discusses bone tissue engineering scenarios in microgravity, from scaffolding to bioreactors. The literature analysis allowed to underline several key points, such as the need for (i) biomimetic composite scaffolds to better mimic the natural microarchitecture of bone tissue, (ii) uniform simulated microgravity levels for standardized experimental protocols to expose biological materials to the same testing conditions, and (iii) improved access to real microgravity for scientific research projects, supported by the so-called democratization of space.

In vitro evaluation of porous poly(hydroxybutyrate-co-hydroxyvalerate)/akermanite composite scaffolds manufactured using selective laser sintering

Incorporation of a bioactive mineral filler in a biodegradable polyester scaffold is a promising strategy for scaffold assisted bone tissue engineering (TE). The current study evaluates the in vitro behavior of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)/Akermanite (AKM) composite scaffolds manufactured using selective laser sintering (SLS). Exposure of the mineral filler on the surface of the scaffold skeleton was evident from in vitro mineralization in PBS. PHBV scaffolds and solvent cast films served as control samples and all materials showed preferential adsorption of fibronectin compared to serum albumin as well as non-cytotoxic response in human osteoblasts (hOB) at 24 h. hOB culture for up to 21 days revealed that the metabolic activity in PHBV films and scaffolds was significantly higher than that of PHBV/AKM scaffolds within the first two weeks of incubation. Afterwards, the metabolic activity in PHBV/AKM scaffolds exceeded that of the control samples. Confocal imaging showed cell penetration into the porous scaffolds. Significantly higher ALP activity was observed in PHBV/AKM scaffolds at all time points in both basal and osteogenic media. Mineralization during cell culture was observed on all samples with PHBV/AKM scaffolds exhibiting distinctly different mineral morphology. This study has demonstrated that the bioactivity of PHBV SLS scaffolds can be enhanced by incorporating AKM, making this an attractive candidate for bone TE application.

Integrated polycaprolactone microsphere-based scaffolds with biomimetic hierarchy and tunable vascularization for osteochondral repair

Osteochondral lesion potentially causes a variety of joint degenerative diseases if it cannot be treated effectively and timely. Microfracture as the conservative surgical choice achieves limited results for the larger defect whereas cartilage patches trigger integrated instability and cartilage fibrosis. To tackle aforementioned issues, here we explore to fabricate an integrated osteochondral scaffold for synergetic regeneration of cartilage and subchondral bone in one system. On the macro level, we fabricated three integrated scaffolds with distinct channel patterns of Non-channel, Consecutive-channel and Inconsecutive-channel via Selective Laser Sintering (SLS). On the micro level, both cartilage zone and subchondral bone zone of integrated scaffold were made of small polycaprolactone (PCL) microspheres and large PCL microspheres, respectively. Our findings showed that Inconsecutive-channel scaffolds possessed integrated hierarchical structure, adaptable compression strength, gradient interconnected porosity. Cartilage zone presented a dense phase for the inhibition of vessel invasion while subchondral bone zone generated a porous phase for the ingrowth of bone and vessel. Both cartilage regeneration and subchondral bone remodeling in the group of Inconsecutive-channel scaffolds have been demonstrated by histological evaluation and immunofluorescence staining in vivo. Consequently, our current work not only achieves an effective and regenerative microsphere scaffold for osteochondral reconstruction, but also provides a feasible methodology to recover injured joint through integrated design with diverse hierarchy. STATEMENT OF SIGNIFICANCE: Recovery of osteochondral lesion highly depends on hierarchical architecture and tunable vascularization in distinct zones. We therefore design a special integrated osteochondral scaffold with inconsecutive channel structure and vascularized modulation. The channel pattern impacts on mechanical strength and the infiltration of bone marrow, and eventually triggers synergetic repair of osteochondral defect. The cartilage zone of integrated scaffolds consisted of small PCL microspheres forms a dense phase for physical restriction of vascularized infiltration whereas the subchondral bone zone made of large PCL microspheres generates porous trabecula-like structure for promoting vascularization. Consequently, the current work indicates both mechanical adaptation and regional vascularized modulation play a pivotal role on osteochondral repair.

Enabling technologies towards personalization of scaffolds for large bone defect regeneration

Additive manufacturing (AM) can deliver personalized scaffolds to support large volume defect tissue regeneration - a major clinical challenge in many medical disciplines. The freedom in scaffold design and composition (biomaterials and biologics) offered by AM yields a plethora of possibilities but is confronted with a heterogenous biological regeneration potential across individuals. A key challenge is to make the right choice for individualized scaffolds that match biology, anatomy, and mechanics of patients. This review provides an overview of state-of-the-art technologies, that is, in silico modelling for scaffold design, omics and bioinformatics to capture patient biology and information technology for data management, that, when combined in a synergistic way with AM, have great potential to make personalized tissue regeneration strategies available to all patients, empowering precision medicine.

Process Phenomena and Material Properties in Selective Laser Sintering of Polymers: A Review

Selective laser sintering (SLS) is a powder bed fusion technology that uses a laser source to melt selected regions of a polymer powder bed based on 3D model data. Components with complex geometry are then obtained using a layer-by-layer strategy. This additive manufacturing technology is a very complex process in which various multiphysical phenomena and different mechanisms occur and greatly influence both the quality and performance of printed parts. This review describes the physical phenomena involved in the SLS process such as powder spreading, the interaction between laser beam and powder bed, polymer melting, coalescence of fused powder and its densification, and polymer crystallization. Moreover, the main characterization approaches that can be useful to investigate the starting material properties are reported and discussed.

Large-sized bone defect repair by combining a decalcified bone matrix framework and bone regeneration units based on photo-crosslinkable osteogenic microgels

Physiological repair of large-sized bone defects is great challenging in clinic due to a lack of ideal grafts suitable for bone regeneration. Decalcified bone matrix (DBM) is considered as an ideal bone regeneration scaffold, but low cell seeding efficiency and a poor osteoinductive microenvironment greatly restrict its application in large-sized bone regeneration. To address these problems, we proposed a novel strategy of bone regeneration units (BRUs) based on microgels produced by photo-crosslinkable and microfluidic techniques, containing both the osteogenic ingredient DBM and vascular endothelial growth factor (VEGF) for accurate biomimic of an osteoinductive microenvironment. The physicochemical properties of microgels could be precisely controlled and the microgels effectively promoted adhesion, proliferation, and osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) in vitro. BRUs were successfully constructed by seeding BMSCs onto microgels, which achieved reliable bone regeneration in vivo. Finally, by integrating the advantages of BRUs in bone regeneration and the advantages of DBM scaffolds in 3D morphology and mechanical strength, a BRU-loaded DBM framework successfully regenerated bone tissue with the desired 3D morphology and effectively repaired a large-sized bone defect of rabbit tibia. The current study developed an ideal bone biomimetic microcarrier and provided a novel strategy for bone regeneration and large-sized bone defect repair.

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The photo-crosslinkable microgels contained both osteogenic ingredient DBM powders and angiogenic growth factor VEGF.

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The photo-crosslinkable microgels effectively promote adhesion, proliferation, and osteogenic differentiation of BMSCs in vitro.

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Bone regeneration units (BRUs) successfully achieve reliable bone regeneration in vivo.

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The combination of DBM scaffold and BRUs successfully regenerate bone tissue with the desired 3D morphology and repair large-sized bone defect of rabbit tibia.

Regenerative Medicine Technologies to Treat Dental, Oral, and Craniofacial Defects

Additive manufacturing (AM) is the automated production of three-dimensional (3D) structures through successive layer-by-layer deposition of materials directed by computer-aided-design (CAD) software. While current clinical procedures that aim to reconstruct hard and soft tissue defects resulting from periodontal disease, congenital or acquired pathology, and maxillofacial trauma often utilize mass-produced biomaterials created for a variety of surgical indications, AM represents a paradigm shift in manufacturing at the individual patient level. Computer-aided systems employ algorithms to design customized, image-based scaffolds with high external shape complexity and spatial patterning of internal architecture guided by topology optimization. 3D bioprinting and surface modification techniques further enhance scaffold functionalization and osteogenic potential through the incorporation of viable cells, bioactive molecules, biomimetic materials and vectors for transgene expression within the layered architecture. These computational design features enable fabrication of tissue engineering constructs with highly tailored mechanical, structural, and biochemical properties for bone. This review examines key properties of scaffold design, bioresorbable bone scaffolds produced by AM processes, and clinical applications of these regenerative technologies. AM is transforming the field of personalized dental medicine and has great potential to improve regenerative outcomes in patient care.

Selective Laser Melting of PA 2200 for Hip Implant Applications: Finite Element Analysis, Process Optimization, and Morphological and Mechanical Characterization

Polyamide 12 (PA 22000) is a well-known material and one of the most biocompatible materials tested and used to manufacture customized medical implants by selective laser sintering technology. To optimize the implants, several research activities were considered, starting with the design and manufacture of test samples made of PA 2200 by selective laser sintering (SLS) technology, with different processing parameters and part orientations. The obtained samples were subjected to compression tests and later to SEM analyses of the fractured zones, in which we determined the microstructural properties of the analyzed samples. Finally, an evaluation of the surface roughness of the material and the possibility of improving the surface roughness of the realized parts using finite element analysis to determine the optimum contact pressure between the component made of PA 2200 by SLS and the component made of TiAl6V4 by SLM was performed.

Leveraging advances in chemistry to design biodegradable polymeric implants using chitosan and other biomaterials

The metamorphosis of biodegradable polymers in biomedical applications is an auspicious myriad of indagation. The utmost challenge in clinical conditions includes trauma, organs failure, soft and hard tissues, infection, cancer and inflammation, congenital disorders which are still not medicated efficiently. To overcome this bone of contention, proliferation in the concatenation of biodegradable materials for clinical applications has emerged as a silver bullet owing to eco-friendly, nontoxicity, exorbitant mechanical properties, cost efficiency, and degradability. Several bioimplants are designed and fabricated in a way to reabsorb or degrade inside the body after performing the specific function rather than eliminating the bioimplants. The objective of this comprehensive is to unfurl the anecdote of emerging biological polymers derived implants including silk, lignin, soy, collagen, gelatin, chitosan, alginate, starch, etc. by explicating the selection, fabrication, properties, and applications. Into the bargain, emphasis on the significant characteristics of current discernment and purview of nanotechnology integrated biopolymeric implants has also been expounded. This robust contrivance shed light on recent inclinations and evolution in tissue regeneration and targeting organs followed by precedency and fly in the ointment concerning biodegradable implants evolved by employing fringe benefits provided by 3D printing technology for building tissues or organs construct for implantation.

Delivering Proangiogenic Factors from 3D-Printed Polycaprolactone Scaffolds for Vascularized Bone Regeneration

Natural bone is a highly vascularized tissue that relies on the vasculature for blood and nutrients supply to maintain skeletal integrity. Inadequacy of neovascularization may compromise the tissue ingrowth to the implanted scaffolds, and eventually results in failure for the repair. To tackle this issue and enhance self-vascularized bone regeneration, herein a 3D biomimetic selective lasersintering (SLS) derived scaffold, with an angiogenic growth factor immobilized on its surface, that can be released in a controlled manner is proposed. To this end, a porous polycaprolactone/hydroxyapatite (PCL/HA) scaffold is prepared via the SLS technique, which is further modified with vascular endothelial growth factor (VEGF) by coprecipitation with apatite. The resultant scaffold (PCL/HA/VEGF) has an excellent cytocompatibility, and subcutaneous implantation experiment shows that the VEGF-loaded scaffold significantly enhances the blood vessel formation compared with the VEGF-free control. It is further demonstrated that the PCL/HA/VEGF scaffold is able to enhance the in vivo bone regeneration in a rat cranial defect model. Taken together, the current study provides not only a feasible and promising scaffold candidate to enhance the vascularized bone regeneration, but also a general strategy to overcome the inadequate vascularization issue for the repair of other tissue and organs.

Polysiloxane Derived Macroporous Silicon Oxycarbide Microspheroidal Particles and Their Decoration with 1D Structures

A simple method of synthesis for the macroporous SiOC spheroidal microparticles was developed. Preceramic polysiloxane macroporous microbeads were obtained by a one-step aqueous emulsion process involving poly(hydromethylsiloxane) (PHMS) with the addition of FeCl3·6H2O. Thermal treatment of these particles gave various SiOC materials depending on the pyrolysis temperature. Hierarchically porous spheroidal mezo-macroporous ceramers were obtained at 600 °C. Heating at 850–1000 °C gave SiOC ceramic spheroidal particles with about 60% open porosity. Ceramization at 1200–1400 °C led to the particles decorated with ceramic whiskers and wires. Heating at 1600 °C gave wires and spilled ceramic particles. These 1D ceramic structures were formed by a Vapor–Liquid–Solid (VLS) mechanism.

Functionalized Polycaprolactone/Hydroxyapatite Composite Microspheres for Promoting Bone Consolidation in a Rat Distraction Osteogenesis Model

Distraction osteogenesis (DO) is an ideal model to study bone regeneration. The major limitation is the relatively long period required for new bone consolidation. Here, we investigated whether the application of polycaprolactone (PCL) and hydroxyapatite (HA) composite microspheres could enhance bone formation in DO. Pure PCL microspheres and composite PCL and 10% HA microspheres were synthesized. Bone mesenchymal stem cells isolated from green fluorescent protein rats (GFP-rBMSCs) were cultured with microspheres in a rotary bioreactor system. Scanning electron microscopy was used to examine the microstructures. Osteogenic differentiation of rBMSCs was confirmed. Moreover, PCL/HA (20 mg) and PCL (20 mg) were locally administered into the distraction gap in the rat DO model toward the end of the distraction period. Imaging detection, mechanical and histological examinations were performed to assess the quality of the 4-week regenerates. Results showed that the microspheres were of uniform size and monodisperse. After incubation with rBMSCs in culture, PCL/HA microspheres showed a better ability for cell adhesion and osteogenic differentiation compared with PCL microspheres. In vivo, bone volume/total tissue volume, bone mineral density, and mechanical properties of the new callus were significantly higher in the PCL/HA group compared with the PCL group. Histological analyses confirmed improved bone formation and vascularization in PCL/HA group. We presented an effective protocol for the generation of functionalized microspheres and demonstrated implantation of PCL/HA microspheres into the distraction regenerate could significantly enhance bone consolidation. Thus, the application of PCL/HA composite microspheres may be a novel approach for promoting bone regeneration. This article is protected by copyright. All rights reserved © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:961-971, 2020.

Solvent-Free Approaches for the Processing of Scaffolds in Regenerative Medicine

The regenerative medicine field is seeking novel strategies for the production of synthetic scaffolds that are able to promote the in vivo regeneration of a fully functional tissue. The choices of the scaffold formulation and the manufacturing method are crucial to determine the rate of success of the graft for the intended tissue regeneration process. On one hand, the incorporation of bioactive compounds such as growth factors and drugs in the scaffolds can efficiently guide and promote the spreading, differentiation, growth, and proliferation of cells as well as alleviate post-surgical complications such as foreign body responses and infections. On the other hand, the manufacturing method will determine the feasible morphological properties of the scaffolds and, in certain cases, it can compromise their biocompatibility. In the case of medicated scaffolds, the manufacturing method has also a key effect in the incorporation yield and retained activity of the loaded bioactive agents. In this work, solvent-free methods for scaffolds production, i.e., technological approaches leading to the processing of the porous material with no use of solvents, are presented as advantageous solutions for the processing of medicated scaffolds in terms of efficiency and versatility. The principles of these solvent-free technologies (melt molding, 3D printing by fused deposition modeling, sintering of solid microspheres, gas foaming, and compressed CO2 and supercritical CO2-assisted foaming), a critical discussion of advantages and limitations, as well as selected examples for regenerative medicine purposes are herein presented.

Preparation and properties of a biodegradable poly(lactide-co-glycolide)/poly(trimethylene carbonate) porous composite scaffold for bone tissue engineering

New biodegradable PLGA/PTMC composite porous scaffold with high porosity, mechanical properties, significant homogeneous, interconnected pore network and good biocompatibility.

Modular Strategies to Build Cell-Free and Cell-Laden Scaffolds towards Bioengineered Tissues and Organs

Engineering three-dimensional (3D) scaffolds for functional tissue and organ regeneration is a major challenge of the tissue engineering (TE) community. Great progress has been made in developing scaffolds to support cells in 3D, and to date, several implantable scaffolds are available for treating damaged and dysfunctional tissues, such as bone, osteochondral, cardiac and nerve. However, recapitulating the complex extracellular matrix (ECM) functions of native tissues is far from being achieved in synthetic scaffolds. Modular TE is an intriguing approach that aims to design and fabricate ECM-mimicking scaffolds by the bottom-up assembly of building blocks with specific composition, morphology and structural properties. This review provides an overview of the main strategies to build synthetic TE scaffolds through bioactive modules assembly and classifies them into two distinct schemes based on microparticles (µPs) or patterned layers. The µPs-based processes section starts describing novel techniques for creating polymeric µPs with desired composition, morphology, size and shape. Later, the discussion focuses on µPs-based scaffolds design principles and processes. In particular, starting from random µPs assembly, we will move to advanced µPs structuring processes, focusing our attention on technological and engineering aspects related to cell-free and cell-laden strategies. The second part of this review article illustrates layer-by-layer modular scaffolds fabrication based on discontinuous, where layers' fabrication and assembly are split, and continuous processes.

Hierarchically designed bone scaffolds: From internal cues to external stimuli

Bone tissue engineering utilizes three critical elements - cells, scaffolds, and bioactive factors - to recapitulate the bone tissue microenvironment, inducing the formation of new bone. Recent advances in materials development have enabled the production of scaffolds that more effectively mimic the hierarchical features of bone matrix, ranging from molecular composition to nano/micro-scale biochemical and physical features. This review summarizes recent advances within the field in utilizing these features of native bone to guide the hierarchical design of materials and scaffolds. Biomimetic strategies discussed in this review cover several levels of hierarchical design, including the development of element-doped compositions of bioceramics, the usage of molecular templates for in vitro biomineralization at the nanoscale, the fabrication of biomimetic scaffold architecture at the micro- and nanoscale, and the application of external physical stimuli at the macroscale to regulate bone growth. Developments at each level are discussed with an emphasis on their in vitro and in vivo outcomes in promoting osteogenic tissue development. Ultimately, these hierarchically designed scaffolds can complement or even replace the usage of cells and biological elements, which present clinical and regulatory barriers to translation. As the field progresses ever closer to clinical translation, the creation of viable therapies will thus benefit from further development of hierarchically designed materials and scaffolds.

Biopolymers as bone substitutes: a review

Human bones have unique structures and characteristics, and replacing a natural bone in the case of bone fracture or bone diseases is a very complicated problem. The main goal of this paper was to summarize the recent research on polymer materials as bone substitutes and for bone repair. Bone treatment methods, bone substitute materials as well as their advantages and drawbacks, and manufacturing methods were reviewed. Biopolymers are the most promising materials in the field of artificial bones and using biopolymers with the shape memory effect can improve the integration of an artificial bone into the human body by better mimicking the structure and properties of natural bones, decreasing the invasiveness of surgical procedures by producing deployable implants. It has been shown that the application of the rapid prototyping technology for artificial bones allows the customization of bone substitutes for a patient and the creation of artificial bones with a complex structure.

Additive manufacturing of bone scaffolds

Additive manufacturing (AM) can obtain not only customized external shape but also porous internal structure for scaffolds, both of which are of great importance for repairing large segmental bone defects. The scaffold fabrication process generally involves scaffold design, AM, and post-treatments. Thus, this article firstly reviews the state-of-the-art of scaffold design, including computer-aided design, reverse modeling, topology optimization, and mathematical modeling. In addition, the current characteristics of several typical AM techniques, including selective laser sintering, fused deposition modeling (FDM), and electron beam melting (EBM), especially their advantages and limitations are presented. In particular, selective laser sintering is able to obtain scaffolds with nanoscale grains, due to its high heating rate and a short holding time. However, this character usually results in insufficient densification. FDM can fabricate scaffolds with a relative high accuracy of pore structure but with a relative low mechanical strength. EBM with a high beam-material coupling efficiency can process high melting point metals, but it exhibits a low-resolution and poor surface quality. Furthermore, the common post-treatments, with main focus on heat and surface treatments, which are applied to improve the comprehensive performance are also discussed. Finally, this review also discusses the future directions for AM scaffolds for bone tissue engineering.

An Overview of 3D Printing Technologies for Soft Materials and Potential Opportunities for Lipid-based Drug Delivery Systems

PURPOSE

Three-dimensional printing (3DP) is a rapidly growing additive manufacturing process and it is predicted that the technology will transform the production of goods across numerous fields. In the pharmaceutical sector, 3DP has been used to develop complex dosage forms of different sizes and structures, dose variations, dose combinations and release characteristics, not possible to produce using traditional manufacturing methods. However, the technology has mainly been focused on polymer-based systems and currently, limited information is available about the potential opportunities for the 3DP of soft materials such as lipids.

METHODS

This review paper emphasises the most commonly used 3DP technologies for soft materials such as inkjet printing, binder jetting, selective laser sintering (SLS), stereolithography (SLA), fused deposition modeling (FDM) and semi-solid extrusion, with the current status of these technologies for soft materials in biological, food and pharmaceutical applications.

RESULT

The advantages of 3DP, particularly in the pharmaceutical field, are highlighted and an insight is provided about the current studies for lipid-based drug delivery systems evaluating the potential of 3DP to fabricate innovative products. Additionally, the challenges of the 3DP technologies associated with technical processing, regulatory and material issues of lipids are discussed in detail.

CONCLUSION

The future utility of 3DP for printing soft materials, particularly for lipid-based drug delivery systems, offers great advantages and the technology will potentially support patient compliance and drug effectiveness via a personalised medicine approach.

Current state of fabrication technologies and materials for bone tissue engineering

A range of traditional and free-form fabrication technologies have been investigated and, in numerous occasions, commercialized for use in the field of regenerative tissue engineering (TE). The demand for technologies capable of treating bone defects inherently difficult to repair has been on the rise. This quest, accompanied by the advent of functionally tailored, biocompatible, and biodegradable materials, has garnered an enormous research interest in bone TE. As a result, different materials and fabrication methods have been investigated towards this end, leading to a deeper understanding of the geometrical, mechanical and biological requirements associated with bone scaffolds. As our understanding of the scaffold requirements expands, so do the capability requirements of the fabrication processes. The goal of this review is to provide a broad examination of existing scaffold fabrication processes and highlight future trends in their development. To appreciate the clinical requirements of bone scaffolds, a brief review of the biological process by which bone regenerates itself is presented first. This is followed by a summary and comparisons of commonly used implant techniques to highlight the advantages of TE-based approaches over traditional grafting methods. A detailed discussion on the clinical and mechanical requirements of bone scaffolds then follows. The remainder of the manuscript is dedicated to current scaffold fabrication methods, their unique capabilities and perceived shortcomings. The range of biomaterials employed in each fabrication method is summarized. Selected traditional and non-traditional fabrication methods are discussed with a highlight on their future potential from the authors' perspective. This study is motivated by the rapidly growing demand for effective scaffold fabrication processes capable of economically producing constructs with intricate and precisely controlled internal and external architectures. STATEMENT OF SIGNIFICANCE: The manuscript summarizes the current state of fabrication technologies and materials used for creating scaffolds in bone tissue engineering applications. A comprehensive analysis of different fabrication methods (traditional and free-form) were summarized in this review paper, with emphasis on recent developments in the field. The fabrication techniques suitable for creating scaffolds for tissue engineering was particularly targeted and their use in bone tissue engineering were articulated. Along with the fabrication techniques, we emphasized the choice of materials in these processes. Considering the limitations of each process, we highlighted the materials and the material properties critical in that particular process and provided a brief rational for the choice of the materials. The functional performance for bone tissue engineering are summarized for different fabrication processes and the choice of biomaterials. Finally, we provide a perspective on the future of the field, highlighting the knowledge gaps and promising avenues in pursuit of effective scaffolds for bone tissue engineering. This extensive review of the field will provide research community with a reference source for current approaches to scaffold preparation. We hope to encourage the researchers to generate next generation biomaterials to be used in these fabrication processes. By providing both advantages and disadvantage of each fabrication method in detail, new fabrication techniques might be devised that will overcome the limitations of the current approaches. These studies should facilitate the efforts of researchers interested in generating ideal scaffolds, and should have applications beyond the repair of bone tissue.

Additive manufacturing: current concepts, future trends

Increasing innovation in rapid prototyping (RP) and additive manufacturing (AM), also known as 3D printing, is bringing about major changes in translational surgical research. This review describes the current position in the use of additive manufacturing in orthopaedic surgery. Cite this article: Bone Joint J 2018;100-B:455-60.

Bone biomaterials and interactions with stem cells

Bone biomaterials play a vital role in bone repair by providing the necessary substrate for cell adhesion, proliferation, and differentiation and by modulating cell activity and function. In past decades, extensive efforts have been devoted to developing bone biomaterials with a focus on the following issues: (1) developing ideal biomaterials with a combination of suitable biological and mechanical properties; (2) constructing a cell microenvironment with pores ranging in size from nanoscale to submicro- and microscale; and (3) inducing the oriented differentiation of stem cells for artificial-to-biological transformation. Here we present a comprehensive review of the state of the art of bone biomaterials and their interactions with stem cells. Typical bone biomaterials that have been developed, including bioactive ceramics, biodegradable polymers, and biodegradable metals, are reviewed, with an emphasis on their characteristics and applications. The necessary porous structure of bone biomaterials for the cell microenvironment is discussed, along with the corresponding fabrication methods. Additionally, the promising seed stem cells for bone repair are summarized, and their interaction mechanisms with bone biomaterials are discussed in detail. Special attention has been paid to the signaling pathways involved in the focal adhesion and osteogenic differentiation of stem cells on bone biomaterials. Finally, achievements regarding bone biomaterials are summarized, and future research directions are proposed.

Biomimetic Materials and Fabrication Approaches for Bone Tissue Engineering

Various strategies have been explored to overcome critically sized bone defects via bone tissue engineering approaches that incorporate biomimetic scaffolds. Biomimetic scaffolds may provide a novel platform for phenotypically stable tissue formation and stem cell differentiation. In recent years, osteoinductive and inorganic biomimetic scaffold materials have been optimized to offer an osteo-friendly microenvironment for the osteogenic commitment of stem cells. Furthermore, scaffold structures with a microarchitecture design similar to native bone tissue are necessary for successful bone tissue regeneration. For this reason, various methods for fabricating 3D porous structures have been developed. Innovative techniques, such as 3D printing methods, are currently being utilized for optimal host stem cell infiltration, vascularization, nutrient transfer, and stem cell differentiation. In this progress report, biomimetic materials and fabrication approaches that are currently being utilized for biomimetic scaffold design are reviewed.

Selective laser sintering scaffold with hierarchical architecture and gradient composition for osteochondral repair in rabbits

Osteochondral defects cannot be adequately self-repaired due to the presence of the sophisticated hierarchical structure and the lack of blood supply in cartilage. Thus, one of the major challenges remaining in this field is the structural design of a biomimetic scaffold that satisfies the specific requirements for osteochondral repair. To address this hurdle, a bio-inspired multilayer osteochondral scaffold that consisted of the poly(ε-caprolactone) (PCL) and the hydroxyapatite (HA)/PCL microspheres, was constructed via selective laser sintering (SLS) technique. The SLS-derived scaffolds exhibited an excellent biocompatibility to support cell adhesion and proliferation in vitro. The repair effect was evaluated by implanting the acellular multilayer scaffolds into osteochondral defects of a rabbit model. Our findings demonstrated that the multilayer scaffolds were able to induce articular cartilage formation by accelerating the early subchondral bone regeneration, and the newly formed tissues could well integrate with the native tissues. Consequently, the current study not only achieves osteochondral repair, but also suggests a promising strategy for the fabrication of bio-inspired multilayer scaffolds with well-designed architecture and gradient composition via SLS technique.

In silico modeling of structural and porosity properties of additive manufactured implants for regenerative medicine

Additive manufacturing technologies are a promising technology towards patient-specific implants for applications in regenerative medicine. The Net-Shape-Nonwoven technology is used to manufacture structures from short fibers with interconnected pores and large functional surfaces that are predestined for cell adhesion and growth. The present study reports on a modeling approach with a particular focus on the specific structural properties. The overall porosities and mean pore-sizes of the digital models are simulated according to liquid-displacement porosity in a tool implemented in the modeling software. This allows adjusting the process parameters fiber length and fiber diameter to generate biomimetic structures with pore-sizes adapted to the requirements of the tissue that is to be replaced. Modeling the structural and porosity properties of scaffolds and implants leads to an efficient use of the processed biomaterials as the trial-and-error method is avoided.

Editor’s Pick: 3D Printing for Biomedical Applications: Where Are We Now?

Three-dimensional (3D) printing is an additive manufacturing process. This technology provides us with the opportunity to create 3D structures by adding material on a layer-by-layer basis, using different kinds of materials such as ceramics, metals, plastics, and polymers. Nowadays, tissue engineering investigations are taking place on a widespread basis in the fields of regeneration, restoration, or replacement of defective or injured functional living organs and tissues. For this reason, it is important to understand the basic concept of 3D bioprinting as a tool for producing a 3D structure combining living cells and biomaterials and controlling cell proliferation, attachment, and migration within 3D structures. There are a variety of applications for additive manufacturing printing technology available to surgeons at this moment, like scaled models for preoperative planning based prosthetics or custom implants and biocompatible scaffolds. Moreover, this technology can be used as a tool to improve surgical and medical education, by using simulation models and utilising its potential to replicate complex anatomy by employing distinct materials that mimic the characteristics of the native tissue in an effort to increase patient safety through repetition of common procedures.

3-dimensional bioprinting for tissue engineering applications

The 3-dimensional (3D) printing technologies, referred to as additive manufacturing (AM) or rapid prototyping (RP), have acquired reputation over the past few years for art, architectural modeling, lightweight machines, and tissue engineering applications. Among these applications, tissue engineering field using 3D printing has attracted the attention from many researchers. 3D bioprinting has an advantage in the manufacture of a scaffold for tissue engineering applications, because of rapid-fabrication, high-precision, and customized-production, etc. In this review, we will introduce the principles and the current state of the 3D bioprinting methods. Focusing on some of studies that are being current application for biomedical and tissue engineering fields using printed 3D scaffolds.