In vitro bioactivity and degradability of β-tricalcium phosphate porous scaffold fabricated via selective laser sintering

Biomimetic Hydrogel Applications and Challenges in Bone, Cartilage, and Nerve Repair

Tissue engineering and regenerative medicine is a highly sought-after field for researchers aiming to compensate and repair defective tissues. However, the design and development of suitable scaffold materials with bioactivity for application in tissue repair and regeneration has been a great challenge. In recent years, biomimetic hydrogels have shown great possibilities for use in tissue engineering, where they can tune mechanical properties and biological properties through functional chemical modifications. Also, biomimetic hydrogels provide three-dimensional (3D) network spatial structures that can imitate normal tissue microenvironments and integrate cells, scaffolds, and bioactive substances for tissue repair and regeneration. Despite the growing interest in various hydrogels for biomedical use in previous decades, there are still many aspects of biomimetic hydrogels that need to be understood for biomedical and clinical trial applications. This review systematically describes the preparation of biomimetic hydrogels and their characteristics, and it details the use of biomimetic hydrogels in bone, cartilage, and nerve tissue repair. In addition, this review outlines the application of biomimetic hydrogels in bone, cartilage, and neural tissues regarding drug delivery. In particular, the advantages and shortcomings of biomimetic hydrogels in biomaterial tissue engineering are highlighted, and future research directions are proposed.

Advances in additive manufacturing processes and their use for the fabrication of lower limb prosthetic devices

INTRODUCTION

Traditional methods of prosthesis fabrication are not efficient and user centric and are made for common purposes without focusing on individual demands of user which leads to rejection of prosthesis for long-term use. Utilizing advanced additive manufacturing techniques for fabrication of prosthesis makes the development process user centric and covers all the user demands thus providing better fit, comfort, and more stable gait rehabilitation for the user.

AREAS COVERED

The articles reporting fabrication of lower limb prosthesis and its socket are included in the study. Standard fabrication and additive manufacturing method are both systematically assessed by the reviewers. The review also covers the advanced methods of additive manufacturing that are presently being used for fabrication of rehabilitation devices.

EXPERT OPINION

Additive manufacturing method of fabrication of prosthesis provides more flexibility for manufacturing prosthesis parts as per demand of the user. The fabrication method takes into account the residual limb and thus makes the prosthesis user-specific providing better comfort and fit.

Review of Physical, Mechanical, and Biological Characteristics of 3D-Printed Bioceramic Scaffolds for Bone Tissue Engineering Applications

This review focuses on the advancements in additive manufacturing techniques that are utilized for fabricating bioceramic scaffolds and their characterizations leading to bone tissue regeneration. Bioscaffolds are made by mimicking the human bone structure, material composition, and properties. Calcium phosphate apatite materials are the most commonly used scaffold materials as they closely resemble live bone in their inorganic composition. The functionally graded scaffolds are fabricated by utilizing the right choice of the 3D printing method and material combinations to achieve the requirement of the bioscaffold. To tailor the physical, mechanical, and biological properties of the scaffold, certain materials are reinforced, doped, or coated to incorporate the functionality. The biomechanical loading conditions that involve flexion, torsion, and tension exerted on the implanted scaffold are discussed. The finite element analysis (FEA) technique is used to investigate the mechanical property of the scaffold before fabrication. This helps in reducing the actual number of samples used for testing. The FEA simulated results and the experimental result are compared. This review also highlights some of the challenges associated while processing the scaffold such as shrinkage, mechanical instability, cytotoxicity, and printability. In the end, the new materials that are evolved for tissue engineering applications are compiled and discussed.

Calcium Orthophosphate (CaPO4)-Based Bioceramics: Preparation, Properties, and Applications

Various types of materials have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A short time later, such synthetic biomaterials were called bioceramics. Bioceramics can be prepared from diverse inorganic substances, but this review is limited to calcium orthophosphate (CaPO4)-based formulations only, due to its chemical similarity to mammalian bones and teeth. During the past 50 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the CaPO4-based implants would remain biologically stable once incorporated into the skeletal structure or whether they would be resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed, and such formulations became an integrated part of the tissue engineering approach. Now, CaPO4-based scaffolds are designed to induce bone formation and vascularization. These scaffolds are usually porous and harbor various biomolecules and/or cells. Therefore, current biomedical applications of CaPO4-based bioceramics include artificial bone grafts, bone augmentations, maxillofacial reconstruction, spinal fusion, and periodontal disease repairs, as well as bone fillers after tumor surgery. Prospective future applications comprise drug delivery and tissue engineering purposes because CaPO4 appear to be promising carriers of growth factors, bioactive peptides, and various types of cells.

Laser Sintering Approaches for Bone Tissue Engineering

The adoption of additive manufacturing (AM) techniques into the medical space has revolutionised tissue engineering. Depending upon the tissue type, specific AM approaches are capable of closely matching the physical and biological tissue attributes, to guide tissue regeneration. For hard tissue such as bone, powder bed fusion (PBF) techniques have significant potential, as they are capable of fabricating materials that can match the mechanical requirements necessary to maintain bone functionality and support regeneration. This review focuses on the PBF techniques that utilize laser sintering for creating scaffolds for bone tissue engineering (BTE) applications. Optimal scaffold requirements are explained, ranging from material biocompatibility and bioactivity, to generating specific architectures to recapitulate the porosity, interconnectivity, and mechanical properties of native human bone. The main objective of the review is to outline the most common materials processed using PBF in the context of BTE; initially outlining the most common polymers, including polyamide, polycaprolactone, polyethylene, and polyetheretherketone. Subsequent sections investigate the use of metals and ceramics in similar systems for BTE applications. The last section explores how composite materials can be used. Within each material section, the benefits and shortcomings are outlined, including their mechanical and biological performance, as well as associated printing parameters. The framework provided can be applied to the development of new, novel materials or laser-based approaches to ultimately generate bone tissue analogues or for guiding bone regeneration.

Biodegradable materials for bone defect repair

Compared with non-degradable materials, biodegradable biomaterials play an increasingly important role in the repairing of severe bone defects, and have attracted extensive attention from researchers. In the treatment of bone defects, scaffolds made of biodegradable materials can provide a crawling bridge for new bone tissue in the gap and a platform for cells and growth factors to play a physiological role, which will eventually be degraded and absorbed in the body and be replaced by the new bone tissue. Traditional biodegradable materials include polymers, ceramics and metals, which have been used in bone defect repairing for many years. Although these materials have more or fewer shortcomings, they are still the cornerstone of our development of a new generation of degradable materials. With the rapid development of modern science and technology, in the twenty-first century, more and more kinds of new biodegradable materials emerge in endlessly, such as new intelligent micro-nano materials and cell-based products. At the same time, there are many new fabrication technologies of improving biodegradable materials, such as modular fabrication, 3D and 4D printing, interface reinforcement and nanotechnology. This review will introduce various kinds of biodegradable materials commonly used in bone defect repairing, especially the newly emerging materials and their fabrication technology in recent years, and look forward to the future research direction, hoping to provide researchers in the field with some inspiration and reference.

Mussel-inspired polydopamine induced the osteoinductivity to ice-templating PLGA-gelatin matrix for bone tissue engineering application

In this study, poly(lactic-co-glycolic acid) (PLGA)-gelatin scaffolds were fabricated using the freeze-casting technique. Polydopamine (PDA) coating was applied on the surface of scaffolds to enhance the hydrophilicity, bioactivity, and cellular behavior of the composite constructs. Further, the synergistic effect of PDA coating and lamellar microstructure of scaffolds was evaluated on the promotion of properties. Based on morphological observations, freeze-casting constructs showed lamellar pore channels while the uniformity and pore size were slightly affected by deposition of PDA. The hydrophilicity and swelling capacity of the scaffolds were assessed using contact angle measurement and phosphate buffered saline absorption ratio. The results indicated a significant increment in water-matrix interactions following surface modification. The evaluation of the biodegradation ratio revealed the higher degree of degradation in PDA-coated samples owing to the presence of hydrophilic functional groups in the chemical structure of PDA. On the other hand, the bioactivity potential of PDA in the simulated body fluid solution confirmed the possibility of using coated constructs as a bone reconstructive substitute. The improvement of cellular attachment and filopodia formation in PDA-contained matrixes was the other benefit of the coating process. Furthermore, cellular proliferation and ALP activity were enhanced after PDA coating. The suggested PDA-coated PLGA-gelatin scaffolds can be applied in bone tissue regeneration.

Indirect selective laser sintering-printed microporous biphasic calcium phosphate scaffold promotes endogenous bone regeneration via activation of ERK1/2 signaling

The fabrication technique determines the physicochemical and biological properties of scaffolds, including the porosity, mechanical strength, osteoconductivity, and bone regenerative potential. Biphasic calcium phosphate (BCP)-based scaffolds are superior in bone tissue engineering due to their suitable physicochemical and biological properties. We developed an indirect selective laser sintering (SLS) printing strategy to fabricate 3D microporous BCP scaffolds for bone tissue engineering purposes. The green part of the BCP scaffold was fabricated by SLS at a relevant low temperature in the presence of epoxy resin (EP), and the remaining EP was decomposed and eliminated by a subsequent sintering process to obtain the microporous BCP scaffolds. Physicochemical properties, cell adhesion, biocompatibility, in vitro osteogenic potential, and rabbit critical-size cranial bone defect healing potential of the scaffolds were extensively evaluated. This indirect SLS printing eliminated the drawbacks of conventional direct SLS printing at high working temperatures, i.e. wavy deformation of the scaffold, hydroxyapatite decomposition, and conversion of β-tricalcium phosphate (TCP) to α-TCP. Among the scaffolds printed with various binder ratios (by weight) of BCP and EP, the scaffold with 50/50 binder ratio (S4) showed the highest mechanical strength and porosity with the smallest pore size. Scaffold S4 showed the highest effect on osteogenic differentiation of precursor cells in vitro, and this effect was ERK1/2 signaling-dependent. Scaffold S4 robustly promoted precursor cell homing, endogenous bone regeneration, and vascularization in rabbit critical-size cranial defects. In conclusion, BCP scaffolds fabricated by indirect SLS printing maintain the physicochemical properties of BCP and possess the capacity to recruit host precursor cells to the defect site and promote endogenous bone regeneration possibly via the activation of ERK1/2 signaling.

Three-dimensional (3D) printed scaffold and material selection for bone repair

Critical-sized bone defect repair remains a substantial challenge in clinical settings and requires bone grafts or bone substitute materials. However, existing biomaterials often do not meet the clinical requirements of structural support, osteoinductive property, and controllable biodegradability. To treat large-scale bone defects, the development of three-dimensional (3D) porous scaffolds has received considerable focus within bone engineering. A variety of biomaterials and manufacturing methods, including 3D printing, have emerged to fabricate patient-specific bioactive scaffolds that possess controlled micro-architectures for bridging bone defects in complex configurations. During the last decade, with the development of the 3D printing industry, a large number of tissue-engineered scaffolds have been created for preclinical and clinical applications using novel materials and innovative technologies. Thus, this review provides a brief overview of current progress in existing biomaterials and tissue engineering scaffolds prepared by 3D printing technologies, with an emphasis on the material selection, scaffold design optimization, and their preclinical and clinical applications in the repair of critical-sized bone defects. Furthermore, it will elaborate on the current limitations and potential future prospects of 3D printing technology. STATEMENT OF SIGNIFICANCE: 3D printing has emerged as a critical fabrication process for bone engineering due to its ability to control bulk geometry and internal structure of tissue scaffolds. The advancement of bioprinting methods and compatible ink materials for bone engineering have been a major focus to develop optimal 3D scaffolds for bone defect repair. Achieving a successful balance of cellular function, cellular viability, and mechanical integrity under load-bearing conditions is critical. Hybridization of natural and synthetic polymer-based materials is a promising approach to create novel tissue engineered scaffolds that combines the advantages of both materials and meets various requirements, including biological activity, mechanical strength, easy fabrication and controllable degradation. 3D printing is linked to the future of bone grafts to create on-demand patient-specific scaffolds.

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

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

Macro- and micro-designed chitosan-alginate scaffold architecture by three-dimensional printing and directional freezing

While many tissue-engineered constructs aim to treat cartilage defects, most involve chondrocyte or stem cell seeding on scaffolds. The clinical application of cell-based techniques is limited due to the cost of maintaining cellular constructs on the shelf, potential immune response to allogeneic cell lines, and autologous chondrocyte sources requiring biopsy from already diseased or injured, scarce tissue. An acellular scaffold that can induce endogenous influx and homogeneous distribution of native stem cells from bone marrow holds great promise for cartilage regeneration. This study aims to develop such an acellular scaffold using designed, channeled architecture that simultaneously models the native zones of articular cartilage and subchondral bone. Highly porous, hydrophilic chitosan-alginate (Ch-Al) scaffolds were fabricated in three-dimensionally printed (3DP) molds designed to create millimeter scale macro-channels. Different polymer preform casting techniques were employed to produce scaffolds from both negative and positive 3DP molds. Macro-channeled scaffolds improved cell suspension distribution and uptake overly randomly porous scaffolds, with a wicking volumetric flow rate of 445.6 ± 30.3 mm(3) s(-1) for aqueous solutions and 177 ± 16 mm(3) s(-1) for blood. Additionally, directional freezing was applied to Ch-Al scaffolds, resulting in lamellar pores measuring 300 μm and 50 μm on the long and short axes, thus creating micrometer scale micro-channels. After directionally freezing Ch-Al solution cast in 3DP molds, the combined macro- and micro-channeled scaffold architecture enhanced cell suspension uptake beyond either macro- or micro-channels alone, reaching a volumetric flow rate of 1782.1 ± 48 mm(3) s(-1) for aqueous solutions and 440.9 ± 0.5 mm(3) s(-1) for blood. By combining 3DP and directional freezing, we can control the micro- and macro-architecture of Ch-Al to drastically improve cell influx into and distribution within the scaffold, while achieving porous zones that mimic articular cartilage zonal architecture. In future applications, precisely controlled micro- and macro-channels have the potential to assist immediate endogenous bone marrow uptake, stimulate chondrogenesis, and encourage vascularization of bone in an osteochondral scaffold.

Preparation, characterization and in vitro dissolution behavior of porous biphasic α/β-tricalcium phosphate bioceramics

The ideal bone tissue engineering scaffolds are long-cherished with the properties of interconnected macroporous structures, adjustable degradation and excellent biocompatibility. Here, a series of porous α/β-tricalcium phosphate (α/β-TCP) biphasic bioceramics with different phase ratios of α-TCP and β-TCP were successfully synthesized by heating an amorphous calcium phosphate precursor. The chemical and morphological characterization showed that α- and β-TCP phases co-existed in the α/β-TCP bioceramics and they had interconnected pore structures with size between 200 and 500μm. The in vitro dissolution behavior and bioactivity of the dual α/β-TCP were also probed in static and dynamic SBF for the first time. The results revealed that α/β-TCP scaffolds had good in vitro bioactivity, as the formation of bone-like apatite layers was induced on the scaffolds after mineralization in SBF. Moreover, dissolution rate of α/β-TCP bioceramics in dynamic environment was higher than that under static condition. Compared with monophasic TCP ceramics, these porous α/β-TCP bioceramics displayed a tailored dissolution rate proportionate to the TCP content (α and β) in the materials. Further, the degradation profile of porous α/β-TCP was well-described by Avrami equation. The porous dual α/β-TCP bioceramics with controllable degradation behavior hold great potential to be applied in bone tissue engineering as bone substitutes.

Bone tissue engineering scaffolding: computer-aided scaffolding techniques

Tissue engineering is essentially a technique for imitating nature. Natural tissues consist of three components: cells, signalling systems (e.g. growth factors) and extracellular matrix (ECM). The ECM forms a scaffold for its cells. Hence, the engineered tissue construct is an artificial scaffold populated with living cells and signalling molecules. A huge effort has been invested in bone tissue engineering, in which a highly porous scaffold plays a critical role in guiding bone and vascular tissue growth and regeneration in three dimensions. In the last two decades, numerous scaffolding techniques have been developed to fabricate highly interconnective, porous scaffolds for bone tissue engineering applications. This review provides an update on the progress of foaming technology of biomaterials, with a special attention being focused on computer-aided manufacturing (Andrade et al. 2002) techniques. This article starts with a brief introduction of tissue engineering (Bone tissue engineering and scaffolds) and scaffolding materials (Biomaterials used in bone tissue engineering). After a brief reviews on conventional scaffolding techniques (Conventional scaffolding techniques), a number of CAM techniques are reviewed in great detail. For each technique, the structure and mechanical integrity of fabricated scaffolds are discussed in detail. Finally, the advantaged and disadvantage of these techniques are compared (Comparison of scaffolding techniques) and summarised (Summary).

Current progress in bioactive ceramic scaffolds for bone repair and regeneration

Bioactive ceramics have received great attention in the past decades owing to their success in stimulating cell proliferation, differentiation and bone tissue regeneration. They can react and form chemical bonds with cells and tissues in human body. This paper provides a comprehensive review of the application of bioactive ceramics for bone repair and regeneration. The review systematically summarizes the types and characters of bioactive ceramics, the fabrication methods for nanostructure and hierarchically porous structure, typical toughness methods for ceramic scaffold and corresponding mechanisms such as fiber toughness, whisker toughness and particle toughness. Moreover, greater insights into the mechanisms of interaction between ceramics and cells are provided, as well as the development of ceramic-based composite materials. The development and challenges of bioactive ceramics are also discussed from the perspective of bone repair and regeneration.

Calcium Orthophosphate-Based Bioceramics

Various types of grafts have been traditionally used to restore damaged bones. In the late 1960s, a strong interest was raised in studying ceramics as potential bone grafts due to their biomechanical properties. A bit later, such synthetic biomaterials were called bioceramics. In principle, bioceramics can be prepared from diverse materials but this review is limited to calcium orthophosphate-based formulations only, which possess the specific advantages due to the chemical similarity to mammalian bones and teeth. During the past 40 years, there have been a number of important achievements in this field. Namely, after the initial development of bioceramics that was just tolerated in the physiological environment, an emphasis was shifted towards the formulations able to form direct chemical bonds with the adjacent bones. Afterwards, by the structural and compositional controls, it became possible to choose whether the calcium orthophosphate-based implants remain biologically stable once incorporated into the skeletal structure or whether they were resorbed over time. At the turn of the millennium, a new concept of regenerative bioceramics was developed and such formulations became an integrated part of the tissue engineering approach. Now calcium orthophosphate scaffolds are designed to induce bone formation and vascularization. These scaffolds are often porous and harbor different biomolecules and/or cells. Therefore, current biomedical applications of calcium orthophosphate bioceramics include bone augmentations, artificial bone grafts, maxillofacial reconstruction, spinal fusion, periodontal disease repairs and bone fillers after tumor surgery. Perspective future applications comprise drug delivery and tissue engineering purposes because calcium orthophosphates appear to be promising carriers of growth factors, bioactive peptides and various types of cells.