Development of melt electrohydrodynamic 3D printing for complex microscale poly ( ε -caprolactone) scaffolds

Review on Engineering of Bone Scaffolds Using Conventional and Additive Manufacturing Technologies

Bone is a complex connective tissue that serves as mechanical and structural support for the human body. Bones' fractures are common, and the healing process is physiologically complex and involves both mechanical and biological aspects. Tissue engineering of bone scaffolds holds great promise for the future treatment of bone injuries. However, conventional technologies to prepare bone scaffolds cannot provide the required properties of human bones. Over the past decade, three-dimensional (3D) printing or additive manufacturing technologies have enabled control over the creation of bone scaffolds with personalized geometries, appropriate materials, and tailored pores. This article aims to review recent advances in the fabrication of bone scaffolds for bone repair and regeneration. A detailed review of bone fracture repair and an in-depth discussion on conventional manufacturing and 3D printing techniques are introduced with an emphasis on novel studies concepts, potentials, and limitations.

Antibacterial and Osteogenic Dual-Functional Micronano Composite Scaffold Fabricated via Melt Electrowriting and Solution Electrospinning for Bone Tissue Engineering

The utilization of micronano composite scaffolds has been extensively demonstrated to confer the superior advantages in bone repair compared to single nano- or micron-sized scaffolds. Nevertheless, the enhancement of bioactivities within these composite scaffolds remains challenging. In this study, we propose a novel approach to combine melt electrowriting (MEW) and solution electrospinning (SES) techniques for the fabrication of a composite scaffold incorporating hydroxyapatite (HAP), an osteogenic component, and roxithromycin (ROX), an antibacterial active component. Scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FTIR) confirmed the hierarchical architecture of the nanofiber-microgrid within the scaffold, as well as the successful loading of HAP and ROX. The incorporation of HAP enhanced the water absorption capacity of the composite scaffold, thus promoting cell adhesion and proliferation, as well as osteogenic differentiation. Furthermore, ROX resulted in effective antibacterial capability without any observable cytotoxicity. Finally, the scaffolds were applied to a rat calvarial defect model, and the results demonstrated that the 20% HAP group exhibited superior new bone formation without causing adverse reactions. Therefore, our findings present a promising strategy for designing and fabricating bioactive scaffolds for bone regeneration.

Electrohydrodynamic Direct-Writing Micro/Nanofibrous Architectures: Principle, Materials, and Biomedical Applications

Electrohydrodynamic (EHD) direct-writing has recently gained attention as a highly promising additive manufacturing strategy for fabricating intricate micro/nanoscale architectures. This technique is particularly well-suited for mimicking the extracellular matrix (ECM) present in biological tissue, which serves a vital function in facilitating cell colonization, migration, and growth. The integration of EHD direct-writing with other techniques has been employed to enhance the biological performance of scaffolds, and significant advancements have been made in the development of tailored scaffold architectures and constituents to meet the specific requirements of various biomedical applications. Here, a comprehensive overview of EHD direct-writing is provided, including its underlying principles, demonstrated materials systems, and biomedical applications. A brief chronology of EHD direct-writing is provided, along with an examination of the observed phenomena that occur during the printing process. The impact of biomaterial selection and architectural topographic cues on biological performance is also highlighted. Finally, the major limitations associated with EHD direct-writing are discussed.

Melt electrowritten scaffold architectures to mimic tissue mechanics and guide neo-tissue orientation

All human tissues present with unique mechanical properties critical to their function. This is achieved in part through the specific architecture of the extracellular matrix (ECM) fibres within each tissue. An example of this is seen in the walls of the vasculature where each layer presents with a unique ECM orientation critical to its functions. Current adopted vascular grafts to bypass a stenosed/damaged vessel fail to recapitulate this unique mechanical behaviour, particularly in the case of small diameter vessels (<6 mm), leading to failure. Therefore, in this study, melt-electrowriting (MEW) was adopted to produce a range of fibrous scaffolds to mimic the extracellular matrix (ECM) architecture of the tunica media of the vasculature, in an attempt to match the mechanical and biological behaviour of the native porcine tissue. Initially, the range of collagen architectures within the native vessel was determined, and subsequently replicated using MEW (winding angles (WA) 45°, 26.5°, 18.4°, 11.3°). These scaffolds recapitulated the anisotropic, non-linear mechanical behaviour of native carotid blood vessels. Moreover, these grafts facilitated human mesenchymal stem cell (hMSC) infiltration, differentiation, and ECM deposition that was independent of WA. The bioinspired MEW fibre architecture promoted cell alignment and preferential neo-tissue orientation in a manner similar to that seen in native tissue, particularly for WA 18.4° and 11.3°, which is a mandatory requirement for long-term survival of the regenerated tissue post-scaffold degradation. Lastly, the WA 18.4° was translated to a tubular graft and was shown to mirror the mechanical behaviour of small diameter vessels within physiological strain. Taken together, this study demonstrates the capacity to use MEW to fabricate bioinspired scaffolds to mimic the tunica media of vessels and recapitulate vascular mechanics which could act as a framework for small diameter graft development to guide tissue regeneration and orientation.

A comprehensive review on nanocomposite biomaterials based on gelatin for bone tissue engineering

The creation of a suitable scaffold is a crucial step in the process of bone tissue engineering (BTE). The scaffold, acting as an artificial extracellular matrix, plays a significant role in determining the fate of cells by affecting their proliferation and differentiation in BTE. Therefore, careful consideration should be given to the fabrication approach and materials used for scaffold preparation. Natural polypeptides such as gelatin and collagen have been widely used for this purpose. The unique properties of nanoparticles, which vary depending on their size, charge, and physicochemical properties, have demonstrated potential in solving various challenges encountered in BTE. Therefore, nanocomposite biomaterials consisting of polymers and nanoparticles have been extensively used for BTE. Gelatin has also been utilized in combination with other nanomaterials to apply for this purpose. Composites of gelatin with various types of nanoparticles are particularly promising for creating scaffolds with superior biological and physicochemical properties. This review explores the use of nanocomposite biomaterials based on gelatin and various types of nanoparticles together for applications in bone tissue engineering.

Tailoring the pore design of embroidered structures by melt electrowriting to enhance the cell alignment in scaffold-based tendon reconstruction

Tissue engineering of ligaments and tendons aims to reproduce the complex and hierarchical tissue structure while meeting the biomechanical and biological requirements. For the first time, the additive manufacturing methods of embroidery technology and melt electrowriting (MEW) were combined to mimic these properties closely. The mechanical benefits of embroidered structures were paired with a superficial micro-scale structure to provide a guide pattern for directional cell growth. An evaluation of several previously reported MEW fiber architectures was performed. The designs with the highest cell orientation of primary dermal fibroblasts were then applied to embroidery structures and subsequently evaluated using human adipose-derived stem cells (AT-MSC). The addition of MEW fibers resulted in the formation of a mechanically robust layer on the embroidered scaffolds, leading to composite structures with mechanical properties comparable to those of the anterior cruciate ligament. Furthermore, the combination of embroidered and MEW structures supports a higher cell orientation of AT-MSC compared to embroidered structures alone. Collagen coating further promoted cell attachment. Thus, these investigations provide a sound basis for the fabrication of heterogeneous and hierarchical synthetic tendon and ligament substitutes.

Conductive fibers for biomedical applications

Bioelectricity has been stated as a key factor in regulating cell activity and tissue function in electroactive tissues. Thus, various biomedical electronic constructs have been developed to interfere with cell behaviors to promote tissue regeneration, or to interface with cells or tissue/organ surfaces to acquire physiological status via electrical signals. Benefiting from the outstanding advantages of flexibility, structural diversity, customizable mechanical properties, and tunable distribution of conductive components, conductive fibers are able to avoid the damage-inducing mechanical mismatch between the construct and the biological environment, in return to ensure stable functioning of such constructs during physiological deformation. Herein, this review starts by presenting current fabrication technologies of conductive fibers including wet spinning, microfluidic spinning, electrospinning and 3D printing as well as surface modification on fibers and fiber assemblies. To provide an update on the biomedical applications of conductive fibers and fiber assemblies, we further elaborate conductive fibrous constructs utilized in tissue engineering and regeneration, implantable healthcare bioelectronics, and wearable healthcare bioelectronics. To conclude, current challenges and future perspectives of biomedical electronic constructs built by conductive fibers are discussed.

Electric-Field-Driven Printed 3D Highly Ordered Microstructure with Cell Feature Size Promotes the Maturation of Engineered Cardiac Tissues

Engineered cardiac tissues (ECTs) derived from human induced pluripotent stem cells (hiPSCs) are viable alternatives for cardiac repair, patient-specific disease modeling, and drug discovery. However, the immature state of ECTs limits their clinical utility. The microenvironment fabricated using 3D scaffolds can affect cell fate, and is crucial for the maturation of ECTs. Herein, the authors demonstrate an electric-field-driven (EFD) printed 3D highly ordered microstructure with cell feature size to promote the maturation of ECTs. The simulation and experimental results demonstrate that the EFD jet microscale 3D printing overcomes the jet repulsion without any prior requirements for both conductive and insulating substrates. Furthermore, the 3D highly ordered microstructures with a fiber diameter of 10-20 µm and spacing of 60-80 µm have been fabricated by maintaining a vertical jet, achieving the largest ratio of fiber diameter/spacing of 0.29. The hiPSCs-derived cardiomyocytes formed ordered ECTs with their sarcomere growth along the fiber and developed synchronous functional ECTs inside the 3D-printed scaffold with matured calcium handling compared to the 2D coverslip. Therefore, the EFD jet 3D microscale printing process facilitates the fabrication of scaffolds providing a suitable microenvironment to promote the maturation of ECTs, thereby showing great potential for cardiac tissue engineering.

Engineering a dynamic three-dimensional cell culturing microenvironment using a "sandwich" structure-liked microfluidic device with 3D printing scaffold

Most of in vivo tissue cells reside in 3D extracellular matrix (ECM) with fluid flow. To better study cell physiology and pathophysiology, there has been an increasing need in the development of methods for culturing cells in in vivo like microenvironments with a number of strategies currently being investigated including hydrogels, spheroids, tissue scaffolds and very promising microfluidic systems. In this paper, a "sandwich" structure-liked microfluidic device integrated with a 3D printing scaffold is proposed for three-dimensional and dynamic cell culture. The device consists of three layers, i.e. upper layer, scaffold layer and bottom layer. The upper layer is used for introducing cells and fixing scaffold, the scaffold layer mimicking ECM is used for providing 3D attachment areas, and the bottom layer mimicking blood vessels is used for supplying dynamic medium for cells. Thermally assisted electrohydrodynamic jet (TAEJ) printing technology and microfabrication technology are combined to fabricate the device. The flow field in the chamber of device is evaluated by numerical simulation and particle tracking technology to investigate the effects of scaffold on fluid microenvironment. The cell culturing processes are presented by the flow behaviours of inks with different colors. The densities and viabilities of HeLa cells are evaluated and compared after 72 h of culturing in the microfluidic devices and 48-well plate. The dose-dependent cell responses to doxorubicin hydrochloride (DOX) are observed after 24 h treatment at different concentrations. These experimental results, including the evaluation of cell proliferation and in vitro cytotoxicity assessment of DOX in the devices and plate, demonstrate that the presented microfluidic device has good biocompatibility and feasibility, which have great potential in providing native microenvironments for in vitro cell studies, tissue engineering and drug screening for tumor therapy.

Experimental Analysis of Wax Micro-Droplet 3D Printing Based on a High-Voltage Electric Field-Driven Jet Deposition Technology

High-voltage electric field-driven jet deposition technology is a novel high resolution micro scale 3D printing method. In this paper, a novel micro 3D printing method is proposed to fabricate wax micro-structures. The mechanism of the Taylor cone generation and droplet eject deposition was analyzed, and a high-voltage electric field-driven jet printing experimental system was developed based on the principle of forming. The effects of process parameters, such as pulse voltages, gas pressures, pulse width, pulse frequency, and movement velocity, on wax printing were investigated. The experimental results show that the increasing of pulse width and duration of pulse high voltage increased at the same pulse frequency, resulting in the micro-droplet diameter being increased. The deposited droplet underwent a process of spreading, shrinking, and solidifying. The local remelting and bonding were acquired between the contact surfaces of the adjacent deposited droplets. According to the experiment results, a horizontal line and a vertical micro-column were fabricated by adjusting the process parameters; their size deviation was controlled within 2%. This research shows that it is feasible to fabricate the micro-scale wax structure using high-voltage electric field-driven jet deposition technology.

Tip-Viscid Electrohydrodynamic Jet 3D Printing of Composite Osteochondral Scaffold

A novel method called tip-viscid electrohydrodynamic jet printing (TVEJ), which produces a viscous needle tip jet, was presented to fabricate a 3D composite osteochondral scaffold with controllability of fiber size and space to promote cartilage regeneration. The tip-viscid process, by harnessing the combined effects of thermal, flow, and electric fields, was first systematically investigated by simulation analysis. The influences of process parameters on printing modes and resolutions were investigated to quantitatively guide the fabrication of various structures. 3D architectures with high aspect ratio and good interlaminar bonding were printed, thanks to the stable fine jet and its predictable viscosity. 3D composite osteochondral scaffolds with controllability of architectural features were fabricated, facilitating ingrowth of cells, and eventually inducing homogeneous cell proliferation. The scaffold’s properties, which included chemical composition, wettability, and durability, were also investigated. Feasibility of the 3D scaffold for cartilage tissue regeneration was also proven by in vitro cellular activities.

Highly Ordered 3D Tissue Engineering Scaffolds as a Versatile Culture Platform for Nerve Cells Growth

Tissue engineering scaffolds provide an encouraging alternative for nerve injuries due to their biological support for nerve cell growth, which can be used for neuronal repair. Nerve cells have been reported to be mostly cultured on 2D scaffolds that cannot mimic the native extracellular matrix. Herein, highly ordered 3D scaffolds are fabricated for nerve cell culture by melt electrospinning writing, the microstructures and geometries of the scaffolds could be well modulated. An effective strategy for scaffold surface modification to promote nerve cell growth is proposed. The effects of scaffolds with different surface modifications, viz., plasma treatment, single poly-D-lysine (PDL) coating after plasma treatment, single laminin (LM) coating after plasma treatment, double PDL and LM coatings after plasma treatment, on PC12 cell growth are evaluated. Experiments show the scaffold modified with double PDL and LM coatings after plasma treatment facilitated the growth of PC12 cells most effectively, indicating the synergistic effect of PDL and LM on the growth of nerve cells. This is the first systematic and quantitative study of the effects of different scaffold surface modifications on nerve cell growth. The above results provide a versatile culture platform for growing nerve cells, and for recovery from peripheral nerve injury.

Development of Cryogenic Electrohydrodynamic Jet Printing for Fabrication of Fine Scaffolds with Extra Filament Surface Topography

Electrohydrodynamic jet printing (EJP) is a developing additive manufacture technology that enables the fabrication of fine scaffolds directly from polymer solutions or melt. Timely solidification of the polymer jet is the key factor for the success of EJP process. In conventional solution-based EJP methods, it is usually achieved by rapid solvent evaporation and producing a scaffold with smooth filaments. In current study, by combining solution-based EJP with a cryogenic workbench, a cryogenic electrohydrodynamic jet printing (CEJP) system was developed, in which the polymer jet was frozen and solidified quickly at the freezing temperature rather than solvent evaporation. The feasibility and versatility of the CEJP system were verified by successful printing of scaffolds with different hole shapes and pore sizes. Meanwhile, the resulting scaffolds not only had a resolution in the range of 50-80 μm but also possessed oriented "ridges" and "valleys" on surface of the filaments, which was conductive to cell orientation. Therefore, this work provides a novel method to print fine scaffolds with extra surface topography.

Electrohydrodynamic jet 3D printing of PCL/PVP composite scaffold for cell culture

Controlled printing of biodegradable and bioresorbable polymers at desired 3D scaffold is of great importance for cell growth and tissue regeneration. In this work, a novel electrohydrodynamic jet 3D printing technology with the resultant effect of electrohydrodynamic force and thermal convection was developed, and its feasibility to fabricate controllable filament composite scaffolds was verified. This method introduces an effective thermal field under the needle to simultaneously enhance the ink viscosity, jetting morphology controllability and printing structure solidify. The fabrication mechanisms of thermal convection on jetting morphology and printed structures feature were investigated through theoretical analysis and experimental characterization. Under optimized conditions, a stable and finer jet was formed; then with the use of this jet, various 3D structures were directly printed at a high aspect ratio ~30. Furthermore, the PCL/PVP composite scaffolds with the controllable filament diameter (~10 μm) which is closed to living cells were printed. Cell culture experiments showed that the printed scaffolds had excellent cell biocompatibility and facilitated cellular proliferation in vitro. It is a great potential that the developed electrohydrodynamic jet 3D printing technology might provide a novel approach to directly print composite synthetic biopolymers into flexibly scale structures for tissue engineering applications.

Near-Field Electrospinning for Three-Dimensional Stacked Nanoarchitectures with High Aspect Ratios

Near-field electrospinning (NFES) was developed to overcome the intrinsic instability of traditional electrospinning processes and to facilitate the controllable deposition of nanofibers under a reduced electric field. This technique offers a straightforward and versatile method for the precision patterning of two-dimensional (2D) nanofibers. However, three-dimensional (3D) stacked structures built by NFES have been limited to either micron-scale sizes or special shapes. Herein, we report on a direct-write 3D NFES technique to construct self-aligned, template-free, 3D stacked nanoarchitectures by simply adding salt to the polymer solution. Numerical simulations suggested that the electric field could be tuned to achieve self-aligned nanofibers by adjusting the conductivity of the polymer solution. This was confirmed experimentally by using poly(ethylene oxide) (PEO) solutions containing 0.1-1.0 wt% NaCl. Using 0.1 wt% NaCl, nanowalls with a maximum of 80 layers could be built with a width of 92 ± 3 nm, height of 6.6 ± 0.1 μm, and aspect ratio (height/width) of 72. We demonstrate the 3D printing of nanoskyscrapers with various designs, such as curved "nanowall arrays", nano "jungle gyms," and "nanobridges". Further, we present an application of the 3D stacked nanofiber arrays by preparing transparent and flexible polydimethylsiloxane films embedded with Ag-sputtered nanowalls as 3D nanoelectrodes. The conductivity of the nanoelectrodes can be precisely tuned by adjusting the number of 3D printed layers, without sacrificing transmittance (98.5%). The current NFES approach provides a simple, reliable route to build 3D stacked nanoarchitectures with high-aspect ratios for potential application in smart materials, energy devices, and biomedical applications.

Melt electrohydrodynamic 3D printed poly (ε-caprolactone)/polyethylene glycol/roxithromycin scaffold as a potential anti-infective implant in bone repair

Implanted scaffold or bone substitute is a common method to treat bone defects. However, the possible bone infection caused by orthopaedic surgery has created a challenging clinical problem and generally invalidate bone repair and regeneration. In this study, a poly (ε-caprolactone) (PCL)/polyethylene glycol (PEG)/roxithromycin (ROX) composite scaffold was prepared via melt electrohydrodynamic (EHD) 3D printing. Fourier transform infrared spectroscopy (FTIR) spectroscopy was performed to verify the existence of PEG and ROX in the scaffolds. By water contact angle measurement, the addition of both PEG and ROX was found to improve the hydrophilicity of the scaffolds. By in vitro drug release assay, the PCL/PEG/ROX scaffolds showed an initial burst drug release and subsequent long-term sustained release behaviour, which is favourable for the prevention and treatment of bone infections. The antibacterial assays against E. coli and S. aureus demonstrated that the composite scaffold with ROX possessed effective antibacterial activity, especially for S. aureus, the main cause of bone infection. The immunostaining and MTT assay with human osteoblast-like cells (MG63) indicated that cells showed good viability and growth on the scaffolds. Therefore, the melt EHD 3D printed PCL/PEG/ROX scaffold could be a promising anti-infective implant for bone tissue engineering.

Biomimicry in Bio-Manufacturing: Developments in Melt Electrospinning Writing Technology Towards Hybrid Biomanufacturing

Melt electrospinning writing has been emerged as a promising technique in the field of tissue engineering, with the capability of fabricating controllable and highly ordered complex three-dimensional geometries from a wide range of polymers. This three-dimensional (3D) printing method can be used to fabricate scaffolds biomimicking extracellular matrix of replaced tissue with the required mechanical properties. However, controlled and homogeneous cell attachment on melt electrospun fibers is a challenge. The combination of melt electrospinning writing with other tissue engineering approaches, called hybrid biomanufacturing, has introduced new perspectives and increased its potential applications in tissue engineering. In this review, principles and key parameters, challenges, and opportunities of melt electrospinning writing, and particularly, recent approaches and materials in this field are introduced. Subsequently, hybrid biomanufacturing strategies are presented for improved biological and mechanical properties of the manufactured porous structures. An overview of the possible hybrid setups and applications, future perspective of hybrid processes, guidelines, and opportunities in different areas of tissue/organ engineering are also highlighted.

Application of Three-Dimensional Printing Technology for Improved Orbital-Maxillary-Zygomatic Reconstruction

The reconstruction of orbital-maxillary-zygomatic complex (OMZC) on patients suffering from trauma and space-occupying lesions is challenging due to the irregularity of craniomaxillofacial bones. To overcome the challenge in precise OMZC reconstruction, individual three-dimensional (3D) disease models and mirror-imaged 3D reconstruction models were printed on the basis of the computer tomography. Preoperative planning by rehearsing surgical procedures was made on the 3D disease models and the scaffolds including titanium and absorbable meshes or plates were anatomically premolded using the mirror-imaged 3D models as guide. Many benefits were achieved including more precise OMZC reconstruction, fluent and smooth procedures of surgeries, shorter operation time, less blood loss, and improved cosmetic outcomes of craniomaxillofacial shapes. There were no complications such as diplopia, infection, foreign body reaction, exophthalmos, enophthalmos, disordered occlusal relationship, and hematoma. And patients were satisfied with the functional and esthetic outcome during the following-up time. Therefore, OMZC reconstruction can be optimized and successful through preoperative planning and premolded scaffolds with 3D printing bone model by computer-aid design and manufacturing.

Electrohydrodynamic 3D printing of layer-specifically oriented, multiscale conductive scaffolds for cardiac tissue engineering

Mimicking the hierarchical microarchitecture of native myocardium in vitro plays an important role in cardiac tissue engineering. Here we present a novel strategy to produce multiscale conductive scaffolds with layer-specific fiber orientations for cardiac regeneration by combining solution-based and melt-based electrohydrodynamic (EHD) printing techniques. Polycaprolactone (PCL) microfibers were printed by melt-based EHD printing and the fiber orientation was flexibly controlled in a layer-by-layer manner according to user-specific design. The as-printed microfibrous scaffolds can provide the seeded cells necessary contact cues to guide layer-specific cellular alignments. Sub-microscale conductive fibers were simultaneously incorporated inside the well-organized PCL scaffolds by solution-based EHD printing, which significantly improved the conductivity as well as the cellular adhesion and proliferation capacity. The multiscale conductive scaffolds can further direct the multiple-layer alignments of primary cardiomyocytes and facilitate cardiomyocyte-specific gene expressions, which exhibited enhanced synchronous beating behavior compared with pure microfibrous scaffolds. It is envisioned that the proposed hybrid EHD printing technique might provide a promising strategy to fabricate multifunctional micro/nanofibrous scaffolds with biomimetic architectures, electrical conductivity and even biosensing properties for the regeneration of electroactive tissues.

Role of Polymers in 3D Printing Technology for Drug Delivery - An Overview

Background:

3D printing (3DP) is an emerging technique for fabrication of a variety of structures and complex geometries using 3D model data. In 1986, Charles Hull introduced stereolithography technique that took advances to beget new methods of 3D printing such as powder bed fusion, fused deposition modeling (FDM), inkjet printing, and contour crafting (CC). Being advantageous in terms of less waste, freedom of design and automation, 3DP has been evolved to minimize incurred cost for bulk production of customized products at the industrial outset. Due to these reasons, 3DP technology has acquired a significant position in pharmaceutical industries. Numerous polymers have been explored for manufacturing of 3DP based drug delivery systems for patient-customized medication with miniaturized dosage forms.

Method:

Published research articles on 3D printed based drug delivery have been thoroughly studied and the polymers used in those studies are summarized in this article.

Results:

We have discussed the polymers utilized to fabricate 3DP systems including their processing considerations, and challenges in fabrication of high throughput 3DP based drug delivery systems.

Conclusion:

Despite several advantages of 3DP in drug delivery, there are still a few issues that need to be addressed such as lower mechanical properties and anisotropic behavior, which are obstacles to scale up the technology. Polymers as a building material certainly plays crucial role in the final property of the dosage form. It is an effort to bring an assemblage of critical aspects for scientists engaged in 3DP technology to create flexible, complex and personalized dosage forms.

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.

Mediating human stem cell behaviour via defined fibrous architectures by melt electrospinning writing

The architecture within which cells reside is key to mediating their specific functions within the body. In this study, we use melt electrospinning writing (MEW) to fabricate cell micro-environments with various fibrous architectures to study their effect on human stem cell behaviour. We designed, built and optimised a MEW apparatus and used it to fabricate four different platform designs of 10.4 ± 2 μm fibre diameter, with angles between fibres on adjacent layers of 90°, 45°, 10° and R (random). Mechanical characterisation was conducted via tensile testing, and human skeletal stem cells (hSSCs) were seeded to scaffolds to study the effect of architecture on cell morphology and mechanosensing (nuclear YAP). Cell morphology was significantly altered between groups, with cells on 90° scaffolds having a lower aspect ratio, greater spreading, greater cytoskeletal tension and nuclear YAP expression. Long term cell culture studies were then conducted to determine the differentiation potential of scaffolds in terms of alkaline phosphatase activity, collagen and mineral production. Across these studies, an increased cell spreading in 3-dimensions is seen with decreasing alignment of architecture correlated with enhanced osteogenesis. This study therefore highlights the critical role of fibrous architecture in regulating stem cell behaviour with implications for tissue engineering and disease progression.

STATEMENT OF SIGNIFICANCE

This is the first study which has investigated the effect of controlled fibrous architectures fabricated via melt electrospinning writing on stem cell behaviour and differentiation. After optimising the fabrication process and characterising scaffolds via SEM and mechanical testing, skeletal stem cells were seeded onto fibrous scaffolds with various micro-architectures. These architectures drove cell shape changes resulting in architecture dependent nuclear YAP localisation, suggesting altered mechanosensing at early time points. In agreement with these early markers, long term cell culture studies revealed for the first time that a 90° fibrous architecture is optimal for the osteogenic differentiation of skeletal stem cells.

Dimension-Based Design of Melt Electrowritten Scaffolds

The electrohydrodynamic stabilization of direct-written fluid jets is explored to design and manufacture tissue engineering scaffolds based on their desired fiber dimensions. It is demonstrated that melt electrowriting can fabricate a full spectrum of various fibers with discrete diameters (2-50 µm) using a single nozzle. This change in fiber diameter is digitally controlled by combining the mass flow rate to the nozzle with collector speed variations without changing the applied voltage. The greatest spectrum of fiber diameters was achieved by the simultaneous alteration of those parameters during printing. The highest placement accuracy could be achieved when maintaining the collector speed slightly above the critical translation speed. This permits the fabrication of medical-grade poly(ε-caprolactone) into complex multimodal and multiphasic scaffolds, using a single nozzle in a single print. This ability to control fiber diameter during printing opens new design opportunities for accurate scaffold fabrication for biomedical applications.

Electrospun Fibrous Scaffolds for Tissue Engineering: Viewpoints on Architecture and Fabrication

Electrospinning has been used for the fabrication of extracellular matrix (ECM)-mimicking fibrous scaffolds for several decades. Electrospun fibrous scaffolds provide nanoscale/microscale fibrous structures with interconnecting pores, resembling natural ECM in tissues, and showing a high potential to facilitate the formation of artificial functional tissues. In this review, we summarize the fundamental principles of electrospinning processes for generating complex fibrous scaffold geometries that are similar in structural complexity to the ECM of living tissues. Moreover, several approaches for the formation of three-dimensional fibrous scaffolds arranged in hierarchical structures for tissue engineering are also presented.

Advanced Material Strategies for Next-Generation Additive Manufacturing

Additive manufacturing (AM) has drawn tremendous attention in various fields. In recent years, great efforts have been made to develop novel additive manufacturing processes such as micro-/nano-scale 3D printing, bioprinting, and 4D printing for the fabrication of complex 3D structures with high resolution, living components, and multimaterials. The development of advanced functional materials is important for the implementation of these novel additive manufacturing processes. Here, a state-of-the-art review on advanced material strategies for novel additive manufacturing processes is provided, mainly including conductive materials, biomaterials, and smart materials. The advantages, limitations, and future perspectives of these materials for additive manufacturing are discussed. It is believed that the innovations of material strategies in parallel with the evolution of additive manufacturing processes will provide numerous possibilities for the fabrication of complex smart constructs with multiple functions, which will significantly widen the application fields of next-generation additive manufacturing.

Microscale Electro-Hydrodynamic Cell Printing with High Viability

Cell printing has gained extensive attentions for the controlled fabrication of living cellular constructs in vitro. Various cell printing techniques are now being explored and developed for improved cell viability and printing resolution. Here an electro-hydrodynamic cell printing strategy is developed with microscale resolution (<100 µm) and high cellular viability (>95%). Unlike the existing electro-hydrodynamic cell jetting or printing explorations, insulating substrate is used to replace conventional semiconductive substrate as the collecting surface which significantly reduces the electrical current in the electro-hydrodynamic printing process from milliamperes (>0.5 mA) to microamperes (<10 µA). Additionally, the nozzle-to-collector distance is fixed as small as 100 µm for better control over filament deposition. These features ensure high cellular viability and normal postproliferative capability of the electro-hydrodynamically printed cells. The smallest width of the electro-hydrodynamically printed hydrogel filament is 82.4 ± 14.3 µm by optimizing process parameters. Multiple hydrogels or multilayer cell-laden constructs can be flexibly printed under cell-friendly conditions. The printed cells in multilayer hydrogels kept alive and gradually spread during 7-days culture in vitro. This exploration offers a novel and promising cell printing strategy which might benefit future biomedical innovations such as microscale tissue engineering, organ-on-a-chip systems, and nanomedicine.

Additive manufacturing with polypropylene microfibers

The additive manufacturing of small diameter polypropylene microfibers is described, achieved using a technique termed melt electrospinning writing. Sequential fiber layering, which is important for accurate three-dimensional fabrication, was achieved with the smallest fiber diameter of 16.4±0.2μm obtained. The collector speed, temperature and melt flow rate to the nozzle were optimized for quality and minimal fiber pulsing. Of particular importance to the success of this method is appropriate heating of the collector plate, so that the electrostatically drawn filament adheres during the direct-writing process. By demonstrating the direct-writing of polypropylene, new applications exploiting the favorable mechanical, stability and biocompatible properties of this polymer are envisaged.