Combined additive manufacturing approaches in tissue engineering

Recent Advances in 3D Printing of Smart Scaffolds for Bone Tissue Engineering and Regeneration

The repair and functional reconstruction of bone defects resulting from severe trauma, surgical resection, degenerative disease, and congenital malformation pose significant clinical challenges. Bone tissue engineering (BTE) holds immense potential in treating these severe bone defects, without incurring prevalent complications associated with conventional autologous or allogeneic bone grafts. 3D printing technology enables control over architectural structures at multiple length scales and has been extensively employed to process biomimetic scaffolds for BTE. In contrast to inert and functional bone grafts, next-generation smart scaffolds possess a remarkable ability to mimic the dynamic nature of native extracellular matrix (ECM), thereby facilitating bone repair and regeneration. Additionally, they can generate tailored and controllable therapeutic effects, such as antibacterial or antitumor properties, in response to exogenous and/or endogenous stimuli. This review provides a comprehensive assessment of the progress of 3D-printed smart scaffolds for BTE applications. It begins with an introduction to bone physiology, followed by an overview of 3D printing technologies utilized for smart scaffolds. Notable advances in various stimuli-responsive strategies, therapeutic efficacy, and applications of 3D-printed smart scaffolds are discussed. Finally, the review highlights the existing challenges in the development and clinical implementation of smart scaffolds, as well as emerging technologies in this field.

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.

Future prospects in the tissue engineering of heart valves: a focus on the role of stem cells

INTRODUCTION

Heart valve disease is a growing burden on the healthcare system. Current solutions are insufficient for young patients and do not offer relief from reintervention. Tissue engineered heart valves (TEHVs) offer a solution that grows and responds to the native environment in a similar way to a healthy valve. Stem cells hold potential to populate these valves as a malleable source that can adapt to environmental cues.

AREAS COVERED

This review covers current methods of recapitulating features of native heart valves with tissue engineering through use of stem cell populations with in situ and in vitro methods.

EXPERT OPINION

In the field of TEHVs, we see a variety of approaches in cell source, biomaterial, and maturation methods. Choosing appropriate cell populations may be very patient specific; consistency and predictability will be key to long-term success. In situ methods are closer to translation but struggle with consistent cellularization. In vitro culture requires specialized methods but may recapitulate native valve cell populations with higher fidelity. Understanding how cell populations react to valve conditions and immune response is vital for success. Detrimental valve pathologies have proven to be difficult to avoid in early translation attempts.

Recent advances in 3D-printed polylactide and polycaprolactone-based biomaterials for tissue engineering applications

The three-dimensional printing (3DP) also known as the additive manufacturing (AM), a novel and futuristic technology that facilitates the printing of multiscale, biomimetic, intricate cytoarchitecture, function-structure hierarchy, multi-cellular tissues in the complicated micro-environment, patient-specific scaffolds, and medical devices. There is an increasing demand for developing 3D-printed products that can be utilized for organ transplantations due to the organ shortage. Nowadays, the 3DP has gained considerable interest in the tissue engineering (TE) field. Polylactide (PLA) and polycaprolactone (PCL) are exemplary biomaterials with excellent physicochemical properties and biocompatibility, which have drawn notable attraction in tissue regeneration. Herein, the recent advancements in the PLA and PCL biodegradable polymer-based composites as well as their reinforcement with hydrogels and bio-ceramics scaffolds manufactured through 3DP are systematically summarized and the applications of bone, cardiac, neural, vascularized and skin tissue regeneration are thoroughly elucidated. The interaction between implanted biodegradable polymers, in-vivo and in-vitro testing models for possible evaluation of degradation and biological properties are also illustrated. The final section of this review incorporates the current challenges and future opportunities in the 3DP of PCL- and PLA-based composites that will prove helpful for biomedical engineers to fulfill the demands of the clinical field.

3D Plotting of Calcium Phosphate Cement and Melt Electrowriting of Polycaprolactone Microfibers in One Scaffold: A Hybrid Additive Manufacturing Process

The fabrication of patient-specific scaffolds for bone substitutes is possible through extrusion-based 3D printing of calcium phosphate cements (CPC) which allows the generation of structures with a high degree of customization and interconnected porosity. Given the brittleness of this clinically approved material, the stability of open-porous scaffolds cannot always be secured. Herein, a multi-technological approach allowed the simultaneous combination of CPC printing with melt electrowriting (MEW) of polycaprolactone (PCL) microfibers in an alternating, tunable design in one automated fabrication process. The hybrid CPC+PCL scaffolds with varying CPC strand distance (800-2000 µm) and integrated PCL fibers featured a strong CPC to PCL interface. While no adverse effect on mechanical stiffness was detected by the PCL-supported scaffold design; the microfiber integration led to an improved integrity. The pore distance between CPC strands was gradually increased to identify at which critical CPC porosity the microfibers would have a significant impact on pore bridging behavior and growth of seeded cells. At a CPC strand distance of 1600 µm, after 2 weeks of cultivation, the incorporation of PCL fibers led to pore coverage by a human mesenchymal stem cell line and an elevated proliferation level of murine pre-osteoblasts. The integrated fabrication approach allows versatile design adjustments on different levels.

Novel Approaches Guiding the Future of Spinal Biologics for Bone Regeneration

Purpose of Review

Despite the continued growth of spine fusion procedures, the ideal material for bone regeneration remains unclear. Current bone graft substitutes and extenders in use such as exogenous BMP-2 or demineralized bone matrix and hydroxyapatite either have serious complications associated with use or lead to clinically significant rates of non-union. The introduction of nanotechnology and 3D printing to regenerative medicine facilitates the development of safer and more efficacious bone regenerative scaffolds that present solutions to these problems. Many researchers in orthopedics recognize the importance of lowering the dose of recombinant growth factors like BMP-2 to avoid the complications associated with its normal required supraphysiologic dosing to achieve high rates of fusion in spine surgery.

Recent Findings

Recent iterations of bioactive scaffolds have moved towards peptide amphiphiles that bind endogenous osteoinductive growth factor sources at the site of implantation. These molecules have been shown to provide a highly fluid, natural mimetic of natural extracellular matrix to achieve 100% fusion rates at 10–100 times lower doses of BMP-2 relative to controls in pre-clinical animal posterolateral fusion models. Alternative approaches to bone regeneration include the combination of existing natural growth factor sources like human bone combined with bioactive, biocompatible components like hydroxyapatite using 3D-printing technologies. Their elastomeric, 3D-printed scaffolds demonstrate an optimal safety profile and high rates of fusion (~92%) in the rat posterolateral fusion model.

Summary

Bioactive peptide amphiphiles and developments in 3D printing offer the promising future of a recombinant growth factor- free bone graft substitute with similar efficacy but improved safety profiles compared to existing bone graft substitutes.

Polymeric Biomaterials for the Treatment of Cardiac Post-Infarction Injuries

Cardiac regeneration aims to reconstruct the heart contractile mass, preventing the organ from a progressive functional deterioration, by delivering pro-regenerative cells, drugs, or growth factors to the site of injury. In recent years, scientific research focused the attention on tissue engineering for the regeneration of cardiac infarct tissue, and biomaterials able to anatomically and physiologically adapt to the heart muscle have been proposed as valuable tools for this purpose, providing the cells with the stimuli necessary to initiate a complete regenerative process. An ideal biomaterial for cardiac tissue regeneration should have a positive influence on the biomechanical, biochemical, and biological properties of tissues and cells; perfectly reflect the morphology and functionality of the native myocardium; and be mechanically stable, with a suitable thickness. Among others, engineered hydrogels, three-dimensional polymeric systems made from synthetic and natural biomaterials, have attracted much interest for cardiac post-infarction therapy. In addition, biocompatible nanosystems, and polymeric nanoparticles in particular, have been explored in preclinical studies as drug delivery and tissue engineering platforms for the treatment of cardiovascular diseases. This review focused on the most employed natural and synthetic biomaterials in cardiac regeneration, paying particular attention to the contribution of Italian research groups in this field, the fabrication techniques, and the current status of the clinical trials.

Structural optimization of 3D-printed patient-specific ceramic scaffolds for in vivo bone regeneration in load-bearing defects

Tissue engineering has recently gained popularity as an alternative to autografts to stimulate bone tissue regeneration through structures called scaffolds. Most of the in vivo experiments on long-bony defects use internally-stabilized generic scaffolds. Despite the wide variety of computational methods, a standardized protocol is required to optimize ceramic scaffolds for load-bearing bony defects stabilized with flexible fixations. An optimization problem was defined for applications to sheep metatarsus defects. It covers biological parameters (porosity, pore size, and the specific surface area) and mechanical constraints based on in vivo and in vitro results reported in the literature. The optimized parameters (59.30% of porosity, 5768.91 m^-1 of specific surface area, and 360.80 μm of pore size) and the compressive strength of the selected structure were validated in vitro by means of tomographic images and compression tests of six 3D-printed samples. Divergences between the design and measured values of the optimized parameters, mainly due to manufacturing defects, are consistent with the previous studies. Using the mixed experimental-mathematical scaffold-design procedure described, they could be implanted in vivo with instrumented external fixators, therefore facilitating biomechanical monitoring of the regeneration process.

Additive manufacturing of polyhydroxyalkanoates (PHAs) biopolymers: Materials, printing techniques, and applications

Additive manufacturing (AM) is recently imposing as a fast, reliable, and highly flexible solution to process various materials, that range from metals to polymers, to achieve a broad variety of customized end-goods without involving the injection molding process. The employment of biomaterials is of utmost relevance as the environmental footprint of the process and, consequently, of the end-goods is significantly decreased. Additive manufacturing can provide, in particular, an all-in-one platform to fabricate complex-shaped biobased items such as bone implants or biomedical devices, that would be, otherwise, extremely troublesome and costly to achieve. Polyhydroxyalkanoates (PHAs) is an emerging class of biobased and biodegradable polymeric materials achievable by fermentation from bacteria. There are some promising scientific and technical reports on the manufacturing of several commodities in PHAs by additive manufacturing. However, many challenges must still be faced in order to expand further the use of PHAs. In this framework, the present work reviews and classifies the relevant papers focused on the design and development of PHAs for different 3D printing techniques and overviews the most recent applications of this approach.

3D Printed Poly(ε-Caprolactone)/Meniscus Extracellular Matrix Composite Scaffold Functionalized With Kartogenin-Releasing PLGA Microspheres for Meniscus Tissue Engineering

Meniscus tissue engineering (MTE) aims to fabricate ideal scaffolds to stimulate the microenvironment for recreating the damaged meniscal tissue. Indeed, favorable mechanical properties, suitable biocompatibility, and inherent chondrogenic capability are crucial in MTE. In this study, we present a composite scaffold by 3D printing a poly(ε-caprolactone) (PCL) scaffold as backbone, followed by injection with the meniscus extracellular matrix (MECM), and modification with kartogenin (KGN)-loaded poly(lactic-co-glycolic) acid (PLGA) microsphere (μS), which serves as a drug delivery system. Therefore, we propose a plan to improve meniscus regeneration via KGN released from the 3D porous PCL/MECM scaffold. The final results showed that the hydrophilicity and bioactivity of the resulting PCL/MECM scaffold were remarkably enhanced. In vitro synovium-derived mesenchymal stem cells (SMSCs) experiments suggested that introducing MECM components helped cell adhesion and proliferation and maintained promising ability to induce cell migration. Moreover, KGN-incorporating PLGA microspheres, which were loaded on scaffolds, showed a prolonged release profile and improved the chondrogenic differentiation of SMSCs during the 14-day culture. Particularly, the PCL/MECM-KGN μS seeded by SMSCs showed the highest secretion of total collagen and aggrecan. More importantly, the synergistic effect of the MECM and sustained release of KGN can endow the PCL/MECM-KGN μS scaffolds with not only excellent cell affinity and cell vitality preservation but also chondrogenic activity. Thus, the PCL/MECM-KGN μS scaffolds are expected to have good application prospects in the field of MTE.

The Use of Bioactive Polymers for Intervention and Tissue Engineering: The New Frontier for Cardiovascular Therapy

Coronary heart disease remains one of the leading causes of death in most countries. Healthcare improvements have seen a shift in the presentation of disease with a reducing number of ST-segment elevation myocardial infarctions (STEMIs), largely due to earlier reperfusion strategies such as percutaneous coronary intervention (PCI). Stents have revolutionized the care of these patients, but the long-term effects of these devices have been brought to the fore. The conceptual and technologic evolution of these devices from bare-metal stents led to the creation and wide application of drug-eluting stents; further research introduced the idea of polymer-based resorbable stents. We look at the evolution of stents and the multiple advantages and disadvantages offered by each of the different polymers used to make stents in order to identify what the stent of the future may consist of whilst highlighting properties that are beneficial to the patient alongside the role of the surgeon, the cardiologist, engineers, chemists, and biophysicists in creating the ideal stent.

Next-generation surgical meshes for drug delivery and tissue engineering applications: materials, design and emerging manufacturing technologies

Abstract

Surgical meshes have been employed in the management of a variety of pathological conditions including hernia, pelvic floor dysfunctions, periodontal guided bone regeneration, wound healing and more recently for breast plastic surgery after mastectomy. These common pathologies affect a wide portion of the worldwide population; therefore, an effective and enhanced treatment is crucial to ameliorate patients’ living conditions both from medical and aesthetic points of view. At present, non-absorbable synthetic polymers are the most widely used class of biomaterials for the manufacturing of mesh implants for hernia, pelvic floor dysfunctions and guided bone regeneration, with polypropylene and poly tetrafluoroethylene being the most common. Biological prostheses, such as surgical grafts, have been employed mainly for breast plastic surgery and wound healing applications. Despite the advantages of mesh implants to the treatment of these conditions, there are still many drawbacks, mainly related to the arising of a huge number of post-operative complications, among which infections are the most common. Developing a mesh that could appropriately integrate with the native tissue, promote its healing and constructive remodelling, is the key aim of ongoing research in the area of surgical mesh implants. To this end, the adoption of new biomaterials including absorbable and natural polymers, the use of drugs and advanced manufacturing technologies, such as 3D printing and electrospinning, are under investigation to address the previously mentioned challenges and improve the outcomes of future clinical practice. The aim of this work is to review the key advantages and disadvantages related to the use of surgical meshes, the main issues characterizing each clinical procedure and the future directions in terms of both novel manufacturing technologies and latest regulatory considerations.

Graphic abstract

Computer-Aided Wet-Spinning

Computer-aided wet-spinning (CAWS) has emerged in the past few years as a hybrid fabrication technique coupling the advantages of additive manufacturing in controlling the external shape and macroporous structure of biomedical polymeric scaffold with those of wet-spinning in endowing the polymeric matrix with a spread microporosity. This book chapter is aimed at providing a detailed description of the experimental methods developed to fabricate by CAWS polymeric scaffolds with a predefined external shape and size as well as a controlled internal porous structure. The protocol for the preparation of poly(ε-caprolactone)-based scaffolds with a predefined pore size and geometry will be reported in detail as a reference example that can be followed and simply adapted to fabricate other kinds of scaffold, with a different porous structure or based on different biodegradable polymers, by applying the processing parameters reported in relevant tables included in the text.

Challenges on optimization of 3D-printed bone scaffolds

Advances in biomaterials and the need for patient-specific bone scaffolds require modern manufacturing approaches in addition to a design strategy. Hybrid materials such as those with functionally graded properties are highly needed in tissue replacement and repair. However, their constituents, proportions, sizes, configurations and their connection to each other are a challenge to manufacturing. On the other hand, various bone defect sizes and sites require a cost-effective readily adaptive manufacturing technique to provide components (scaffolds) matching with the anatomical shape of the bone defect. Additive manufacturing or three-dimensional (3D) printing is capable of fabricating functional physical components with or without porosity by depositing the materials layer-by-layer using 3D computer models. Therefore, it facilitates the production of advanced bone scaffolds with the feasibility of making changes to the model. This review paper first discusses the development of a computer-aided-design (CAD) approach for the manufacture of bone scaffolds, from the anatomical data acquisition to the final model. It also provides information on the optimization of scaffold's internal architecture, advanced materials, and process parameters to achieve the best biomimetic performance. Furthermore, the review paper describes the advantages and limitations of 3D printing technologies applied to the production of bone tissue scaffolds.

3D Bioprinting in Tissue Engineering for Medical Applications: The Classic and the Hybrid

Three-dimensional (3D) printing, as one of the most popular recent additive manufacturing processes, has shown strong potential for the fabrication of biostructures in the field of tissue engineering, most notably for bones, orthopedic tissues, and associated organs. Desirable biological, structural, and mechanical properties can be achieved for 3D-printed constructs with a proper selection of biomaterials and compatible bioprinting methods, possibly even while combining additive and conventional manufacturing (AM and CM) procedures. However, challenges remain in the need for improved printing resolution (especially at the nanometer level), speed, and biomaterial compatibilities, and a broader range of suitable 3D-printed materials. This review provides an overview of recent advances in the development of 3D bioprinting techniques, particularly new hybrid 3D bioprinting technologies for combining the strengths of both AM and CM, along with a comprehensive set of material selection principles, promising medical applications, and limitations and future prospects.

Multi-layer Scaffolds of Poly(caprolactone), Poly(glycerol sebacate) and Bioactive Glasses Manufactured by Combined 3D Printing and Electrospinning

Three-dimensional (3D) printing has been combined with electrospinning to manufacture multi-layered polymer/glass scaffolds that possess multi-scale porosity, are mechanically robust, release bioactive compounds, degrade at a controlled rate and are biocompatible. Fibrous mats of poly (caprolactone) (PCL) and poly (glycerol sebacate) (PGS) have been directly electrospun on one side of 3D-printed grids of PCL-PGS blends containing bioactive glasses (BGs). The excellent adhesion between layers has resulted in composite scaffolds with a Young's modulus of 240-310 MPa, higher than that of 3D-printed grids (125-280 MPa, without the electrospun layer). The scaffolds degraded in vitro by releasing PGS and BGs, reaching a weight loss of ~14% after 56 days of incubation. Although the hydrolysis of PGS resulted in the acidification of the buffer medium (to a pH of 5.3-5.4), the release of alkaline ions from the BGs balanced that out and brought the pH back to 6.0. Cytotoxicity tests performed on fibroblasts showed that the PCL-PGS-BGs constructs were biocompatible, with cell viability of above 125% at day 2. This study demonstrates the fabrication of systems with engineered properties by the synergy of diverse technologies and materials (organic and inorganic) for potential applications in tendon and ligament tissue engineering.

Engineering Tissue Fabrication With Machine Intelligence: Generating a Blueprint for Regeneration

Regenerating lost or damaged tissue is the primary goal of Tissue Engineering. 3D bioprinting technologies have been widely applied in many research areas of tissue regeneration and disease modeling with unprecedented spatial resolution and tissue-like complexity. However, the extraction of tissue architecture and the generation of high-resolution blueprints are challenging tasks for tissue regeneration. Traditionally, such spatial information is obtained from a collection of microscopic images and then combined together to visualize regions of interest. To fabricate such engineered tissues, rendered microscopic images are transformed to code to inform a 3D bioprinting process. If this process is augmented with data-driven approaches and streamlined with machine intelligence, identification of an optimal blueprint can become an achievable task for functional tissue regeneration. In this review, our perspective is guided by an emerging paradigm to generate a blueprint for regeneration with machine intelligence. First, we reviewed recent articles with respect to our perspective for machine intelligence-driven information retrieval and fabrication. After briefly introducing recent trends in information retrieval methods from publicly available data, our discussion is focused on recent works that use machine intelligence to discover tissue architectures from imaging and spectral data. Then, our focus is on utilizing optimization approaches to increase print fidelity and enhance biomimicry with machine learning (ML) strategies to acquire a blueprint ready for 3D bioprinting.

Advances in bioprinting using additive manufacturing

Since its conception in the 1980's, several advances in the field of additive manufacturing have led to exploration of alternate as well as combination biomaterials. These progresses have directed the use of 3D printing in wider applications such as printing of dermal layers, cartilage, bone defects, and surgical implants. Furthermore, the incorporation of live and functional cells with or atop biomaterials has laid the foundation for its use in tissue engineering. The purpose of this review is to summarize the advances in 3D printing and bioprinting of several types of tissues such as skin, cartilage, bones, and cardiac valves. This review will address the current 3D technologies used in tissue construction and study the biomaterials being investigated. There are several requirements that need to be addressed, in order to reconstruct functional tissue such as mechanical strength, porosity of the replicate and cellular incorporation. Researchers have focused their studies to answer questions regarding these requirements.

Enhanced bioactivity of titanium-coated polyetheretherketone implants created by a high-temperature 3D printing process

Polyetheretherketone (PEEK), one of the potential alternatives to metallic materials for implants, necessarily involves high temperature process conditions to be three-dimensionally (3D) printed. We developed a 3D printing setup equipped with thermally stabilized modules of the printing nozzle and building chamber, by which the PEEK implants could be successfully manufactured. Under optimized printing conditions, the maximal mechanical strength of the 3D printed sample attained over 80% of the original bulk property of PEEK. To enhance the interfacial biocompatibility, the as-printed implants were postprocessed with titanium (Ti) sputtering. The Ti-coated surfaces were evaluated through characterization studies of x-ray diffraction spectra, microscopic topographies, and wetting properties. For the in vitro tests, preosteoblasts were cultured on the developed PEEK-Ti structures and evaluated in terms of cell adhesion, proliferation, and osteogenic differentiation. In addition, the bone regeneration capability of the PEEK-Ti implants was confirmed by animal experiments using a rabbit tibia defect model for a period of 12 weeks. In the overall in vitro and in vivo tests, we confirmed the superior bioactivities of the Ti-modified and 3D printed interface by comparisons between the samples of machined and printed samples with or without Ti coating. Taken together, the comprehensive manufacturing approaches that involve 3D printing and biocompatible postprocessing are expected to have universal applicability in a wide range of bone tissue engineering.

In Operando Monitoring of Dynamic Recovery in 3D-Printed Thermoset Nanocomposites by XPCS

Extrusion-based additive manufacturing methods, such as direct-write of carbon fiber-reinforced epoxy inks, have become an attractive route toward development of structural composites in recent years, because of emerging techniques such as big area additive manufacturing. The development of improved materials for these methods has been a major focus area; however, an understanding of the effects of the printing process on the structural and dynamic recovery in printed materials remains largely unexplored. The goal of this work is to capture multiscale and temporal morphology and dynamics within thermosetting composite inks to determine the parameters during the printing process that influence the recovery of the printed material. Herein, we use X-ray photon correlation spectroscopy in small-angle scattering geometry to reveal both morphology and recovery dynamics of a nanoparticle (layered-silicate Cloisite 30B) in a thermoset epoxy resin (EPON 826) during the printing process in real time. Our results show that the dynamics of the layered silicate particles during recovery are anisotropic and slow down to behavior which is characteristic of aging in colloidal clay suspensions around  t_age ≈ 12 s. The dynamics and alignment of the particles during recovery were tempo-spatially mapped, and the recovery post printing was shown to be strongly influenced by the deposition onto the build plate in addition to the extrusion through the print head. Our in operando results provide insight into the parameters that must be considered when optimizing materials and methods for precisely tailored local properties during 3D printing.

Fabrication and Evaluation of 3D Printed Poly(l-lactide) Scaffold Functionalized with Quercetin-Polydopamine for Bone Tissue Engineering

Quercetin (Qu), a bioflavonoid, has been reported to positively affect bone metabolism. For the first time, Qu with different concentrations was utilized to functionalize 3D-printed poly(l-lactide) (PLLA) scaffold with the aid of a polydopamine (PDA) layer through a convenient and effective way in this study. Results revealed that the coexistence of PDA and Qu can capacitate the 3D-printed PLLA scaffold to possess rougher surface, as well as better hydrophilicity and compressive properties. The resulting PDA- and Qu-modified PLLA scaffolds (Qu/PD-PLLA) can sustainably release Qu to some extent, which is more beneficial to the proliferation and attachment of MC3T3-E1 cells, upregulating ALP activity and calcium nodules as well as promoting the expression of related osteogenic genes and proteins. More significantly, such a positive impact of the Qu on the cell affinity and osteogenic activity played in a dose-dependent manner. This study revealed the potential of the 3D-printed Qu/PD-PLLA scaffolds with a certain amount of Qu as bone-repair materials.

FOUR-DIMENSIONAL BIOPRINTING FOR REGENERATIVE MEDICINE: MECHANISMS TO INDUCE SHAPE VARIATION AND POTENTIAL APPLICATIONS

Regenerative medicine is an exciting field of research, in which significant steps are being taken that are leading to the translation of the technique into clinical practice. In the near future, it is expected that clinicians will have the opportunity to bioprint tissues and organs that closely mimic native human tissues. To do so, imaging of patients must be translated to digital models and then fabricated in a layer-by-layer fashion. The main aim of this review is to elaborate on the possible mechanisms that support four-dimensional bioprinting, as well as provide examples of current and future applications of the technology. This technology, considering time as the fourth dimension, emerged with the aim to develop bioactive functional constructs with programmed stimuli responses. The main idea is to have three-dimensional-printed constructs that are responsive to preplanned stimuli. With this review, the authors aim to provoke creative thinking, highlighting several issues that need to be addressed when reproducing such a complex network as the human body. The authors envision that there are some key features that need to be studied in the near future: printed constructs should be able to respond to different types of stimuli in a timely manner, bioreactors must be developed combining different types of automated stimuli and aiming to replicate the in vivo ecology, and adequate testing procedures must be developed to obtain a proper assessment of the constructs. The effective development of a printed construct that supports tissue maturation according to the anticipated stimuli will significantly advance this promising approach to regenerative medicine.

Hybrid polycaprolactone/hydrogel scaffold fabrication and in-process plasma treatment using PABS

A challenge for tissue engineering is to produce synthetic scaffolds of adequate chemical, physical, and biological cues effectively. Due to the hydrophobicity of the commonly used synthetic polymers, the printed scaffolds are limited in cell-seeding and proliferation efficiency. Furthermore, non-uniform cell distribution along the scaffolds with rare cell attachment in the core region is a common problem. There are no available commercial systems able to produce multi-type material and gradient scaffolds which could mimic the nature tissues. This paper describes a plasma-assisted bio-extrusion system (PABS) to overcome the above limitations and capable of producing functional-gradient scaffolds; it comprises pressure-assisted and screw-assisted extruders and plasma jets. A hybrid scaffold consisting of synthetic biopolymer and natural hybrid hydrogel alginate-gelatin (Alg-Gel) methacrylate anhydride, and full-layer N₂ plasma modification scaffolds were produced using PABS. Water contact angle and in vitro biological tests confirm that the plasma modification alters the hydrophilicity properties of synthetic polymers and promotes proliferation of cells, leading to homogeneous cell colonization. The results confirm the printing capability for soft hard material integration of PABS and suggest that it is promising for producing functional gradient scaffolds of biomaterials.

Near-field electrospinning of a polymer/bioactive glass composite to fabricate 3D biomimetic structures

Bioactive glasses have recently gained attention in tissue engineering and three-dimensional (3D) bioprinting because of their ability to enhance angiogenesis. Some challenges for developing biological tissues with bioactive glasses include incorporation of glass particles and achieving a 3D architecture mimicking natural tissues. In this study, we investigate the fabrication of scaffolds with a polymer/bioactive glass composite using near-field electrospinning (NFES). An overall controlled 3D scaffold with pores, containing random fibers, is created and aimed to provide superior cell proliferation. Highly angiogenic borate bioactive glass (13-93B3) in 20 wt.% is added to polycaprolactone (PCL) to fabricate scaffolds using the NFES technique. Scaffolds measuring 5 mm × 5 mm × 0.2 mm3 in overall dimensions were seeded with human adipose-derived mesenchymal stem cells to investigate the cell viability. The cell viability on PCL and PCL+glass scaffolds fabricated using NFES technique and 3D printing is compared and discussed. The results indicated higher cell proliferation on 3D biomimetic scaffolds fabricated by NFES technique.

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.

Increased differentiation and production of extracellular matrix components of primary human osteoblasts after cocultivation with endothelial cells: A quantitative proteomics approach

Coculturing of bone-forming and blood vessel-forming cells is a strategy aimed at increasing vascularity of implanted bone constructs in tissue-engineering applications. We previously described that the coculture of primary human osteoblasts (hOBs) and human umbilical vein endothelial cells (HUVECs) improves the differentiation of both cell types, leading to the formation of functional blood vessels and enhanced bone regeneration. The objective of this study was to further delineate the multifaceted interactions between both cell types. To investigate the proteome of hOBs after cocultivation with HUVECs we used stable isotope labeling by amino acids in cell culture, revealing 49 significantly upregulated, and 54 significantly downregulated proteins. Amongst the highest regulated proteins, we found the proteins important for osteoblast differentiation, cellular adhesion, and extracellular matrix function, notably: connective tissue growth factor, desmoplakin, galectin-3, and cyclin-dependent kinase 6. The findings were confirmed by enzyme-linked immunosorbent assays. We also investigated whether the mRNA transcripts correlate with the changes in protein levels by quantitative real-time reverse transcription polymerase chain reaction. In addition, the data was compared to our previous microarray analysis of hOB transcriptome. Taken together, this in-depth analysis delivers reliable data suggesting the importance of coculturing of hOBs and HUVECs in tissue engineering.

Engineering in-vitro stem cell-based vascularized bone models for drug screening and predictive toxicology

The production of veritable in-vitro models of bone tissue is essential to understand the biology of bone and its surrounding environment, to analyze the pathogenesis of bone diseases (e.g., osteoporosis, osteoarthritis, osteomyelitis, etc.), to develop effective therapeutic drug screening, and to test potential therapeutic strategies. Dysregulated interactions between vasculature and bone cells are often related to the aforementioned pathologies, underscoring the need for a bone model that contains engineered vasculature. Due to ethical restraints and limited prediction power of animal models, human stem cell-based tissue engineering has gained increasing relevance as a candidate approach to overcome the limitations of animals and to serve as preclinical models for drug testing. Since bone is a highly vascularized tissue, the concomitant development of vasculature and mineralized matrix requires a synergistic interaction between osteogenic and endothelial precursors. A number of experimental approaches have been used to achieve this goal, such as the combination of angiogenic factors and three-dimensional scaffolds, prevascularization strategies, and coculture systems. In this review, we present an overview of the current models and approaches to generate in-vitro stem cell-based vascularized bone, with emphasis on the main challenges of vasculature engineering. These challenges are related to the choice of biomaterials, scaffold fabrication techniques, and cells, as well as the type of culturing conditions required, and specifically the application of dynamic culture systems using bioreactors.

Development of an omentum-cultured oesophageal scaffold reinforced by a 3D-printed ring: feasibility of an in vivo bioreactor

Current treatments of oesophageal diseases, such as carcinoma, congenital abnormality or trauma, require surgical intervention and oesophageal reconstruction with the stomach, jejunum or colon. However, serious side effects are possible with each treatment option. Despite tissue engineering promising to be an effective regenerative strategy, no functional solution currently exists for oesophageal reconstruction. Here, we developed an omentum-cultured oesophageal scaffold reinforced by a 3D-printed ring. The nano-structured scaffolds were wrapped into the omentum of rats and orthotopically transplanted for the repair of circumferential oesophageal defects two weeks later. The artificial oesophagus exhibited complete healing of the surgically created circumferential defects by the second week. The integration of the omentum-cultured oesophageal scaffold and the regenerative tissue remained intact. Macroscopically, there was no evidence of a fistula, perforation, abscess formation or surrounding soft-tissue necrosis. The omentum-cultured nano-structure scaffold reinforced by a 3D-printed ring is a more practical model with better vascularization for artificial neo-oesophagus reconstruction in a rat model.

The role of hierarchical design and morphology in the mechanical response of diatom-inspired structures via simulation

Novel path towards the design and fabrication of diatom-inspired hierarchical microstructures.

Influence of Controlled Cooling in Bimodal Scaffold Fabrication Using Polymers with Different Melting Temperatures

The combination of different materials and capabilities to manufacture at several scales open new possibilities in scaffold design for bone regeneration. This work is focused on bimodal scaffolds that combine polylactic acid (PLA) melt extruded strands with polycaprolactone (PCL) electrospun fibers. This type of bimodal scaffold offers better mechanical properties, compared to the use of PCL for the extruded strands, and provides potential a means for controlled drug and/or growth factor delivery through the electrospun fibers. The technologies of fused deposition modeling (FDM) and electrospinning were combined to create 3D bimodal constructs. The system uses a controlled cooling system allowing the combination of polymers with different melting temperatures to generate integrated scaffold architecture. The thermoplastic polymers used in the FDM process enhance the mechanical properties of the bimodal scaffold and control the pore structure. Integrated layers of electrospun microfibers induce an increase of the surface area for cell culture purposes, as well as potential in situ controlled drug and/or growth factor delivery. The proposed bimodal scaffolds (PLA extruded strands and PCL electrospun fibers) show appropriate morphology and better mechanical properties when compared to the use of PCL extruded strands. On average, bimodal scaffolds with overall dimensions of 30 × 30 × 2.4 mm³ (strand diameter of 0.5 mm, strand stepover of 2.5 mm, pore size of 2 mm, and layer height of 0.3 mm) showed scaffold stiffness of 23.73 MPa and compression strength of 3.85 MPa. A cytotoxicity assay based human fibroblasts showed viability of the scaffold materials.

Biofabrication and biomaterials for urinary tract reconstruction

Reconstructive urologists are constantly facing diverse and complex pathologies that require structural and functional restoration of urinary organs. There is always a demand for a biocompatible material to repair or substitute the urinary tract instead of using patient's autologous tissues with its associated morbidity. Biomimetic approaches are tissue-engineering tactics aiming to tailor the material physical and biological properties to behave physiologically similar to the urinary system. This review highlights the different strategies to mimic urinary tissues including modifications in structure, surface chemistry, and cellular response of a range of biological and synthetic materials. The article also outlines the measures to minimize infectious complications, which might lead to graft failure. Relevant experimental and preclinical studies are discussed, as well as promising biomimetic approaches such as three-dimensional bioprinting.

Bone Regenerative Medicine in Oral and Maxillofacial Region Using a Three-Dimensional Printer<sup/>

Bone grafts currently used for the treatment of large bone defect or asymmetry in oral and maxillofacial region include autologous, allogeneic, and artificial bones. Although artificial bone is free from the concerns of donor site morbidity, limitation of volume, disease transmission, and ethical issues, it lacks osteogenic and osteoinductive activities. In addition, molding of the artificial bone is an issue especially when it is used for the augmentation of bone as onlay grafts. To solve this problem, additive manufacturing techniques have been applied to fabricate bones which have outer shapes conformed to patients' bones. We developed a custom-made artificial bone called a computed tomography (CT)-bone. Efficacy of CT-bone was proven in a clinical research and clinical trial, showing good manipulability, stability, and patient satisfaction. However, low replacement rate of artificial bones by endogenous bones remain an unsolved issue. Loading of cells and growth factors will improve the bone replacement by inducing osteogenic and osteoinductive activities. In addition, the three-dimensional bioprinting technique will facilitate bone regeneration by placing cells and biological substances into appropriate sites.

Challenges in engineering large customized bone constructs

The ability to treat large tissue defects with customized, patient-specific scaffolds is one of the most exciting applications in the tissue engineering field. While an increasing number of modestly sized tissue engineering solutions are making the transition to clinical use, successfully scaling up to large scaffolds with customized geometry is proving to be a considerable challenge. Managing often conflicting requirements of cell placement, structural integrity, and a hydrodynamic environment supportive of cell culture throughout the entire thickness of the scaffold has driven the continued development of many techniques used in the production, culturing, and characterization of these scaffolds. This review explores a range of technologies and methods relevant to the design and manufacture of large, anatomically accurate tissue-engineered scaffolds with a focus on the interaction of manufactured scaffolds with the dynamic tissue culture fluid environment. Biotechnol. Bioeng. 2017;114: 1129-1139. © 2016 Wiley Periodicals, Inc.

The effects of different amounts of drug microspheres on the vivo and vitro performance of the PLGA/β-TCP scaffold

OIC-A006 (BMPs osteogenesis compounds), can stimulate bone marrow mesenchymal stem cells ALP, OPN, OC, Cbfal expression. To stimulate new bone formation in the body. We postulate different amounts of drug microspheres on the PLGA/β-CPT scaffold can produce the effects on performance and sustained release characteristics. In this paper, through adding different amount of carrier drug microsphere, three concentrations scaffolds which are 12.5, 18.75 and 25 μmol/L are prepared by adding different amounts of drug-loaded microspheres. Hereafter called OICM/CPT-200, OICM/CPT-300, OICM/CPT-400. We implant them in rat femur diameter 3 mm depth of 3 mm hole for eight weeks. The degradation, microsphere, delivery properties, with X-ray, micro-CT and histology are tested. Results show that the contain carrier drug microsphere scaffolds become radiopaque, and the gaps between the scaffold and radial cut ends are often invisible. This preliminary study reveals that different carrier drug microsphere has a corresponding effect the performance of stent body, OICM/CPT – 200 scaffolds induction effect is best. Illustrates that the low concentration load OIC-A006 microspheres can promote bone healing, and high concentration of OIC-A006 micro ball is played a inhibitory effect on bone healing process.

Surface delivery of tunable doses of BMP-2 from an adaptable polymeric scaffold induces volumetric bone regeneration

The rapid and effective bone regeneration of large non-healing defects remains challenging. Bioactive proteins, such as bone morphogenetic protein (BMP)-2, are proved their osteoinductivity, but their clinical use is currently limited to collagen as biomaterial. Being able to deliver BMP-2 from any other biomaterial would broaden its clinical use. This work presents a novel means for repairing a critical size volumetric bone femoral defect in the rat by combining a osteoinductive surface coating (2D) to a polymeric scaffold (3D hollow tube) made of commercially-available PLGA. Using a polyelectrolyte film as BMP-2 carrier, we tune the amount of BMP-2 loaded in and released from the polyelectrolyte film coating over a large extent by controlling the film crosslinking level and initial concentration of BMP-2 in solution. Using microcomputed tomography and quantitative analysis of the regenerated bone growth kinetics, we show that the amount of newly formed bone and kinetics can be modulated: an effective and fast repair was obtained in 1-2 weeks in the best conditions, including complete defect bridging, formation of vascularized and mineralized bone tissue. Histological staining and high-resolution computed tomography revealed the presence of bone regeneration inside and around the tube with spatially distinct organization for trabecular-like and cortical bones. The amount of cortical bone and its thickness increased with the BMP-2 dose. In view of the recent developments in additive manufacturing techniques, this surface-coating technology may be applied in combination with various types of polymeric or metallic scaffolds to offer new perspectives of bone regeneration in personalized medicine.