Optimization of mechanical stiffness and cell density of 3D bioprinted cell-laden scaffolds improves extracellular matrix mineralization and cellular organization for bone tissue engineering

Rational integration of defense and repair synergy on PEEK osteoimplants via biomimetic peptide clicking strategy

Polyetheretherketone (PEEK) has been widely used as orthopedic and dental materials due to excellent mechanical and physicochemical tolerance. However, its biological inertness, poor osteoinduction, and weak antibacterial activity make the clinical applications in a dilemma. Inspired by the mussel adhesion mechanism, here we reported a biomimetic surface strategy for rational integration and optimization of anti-infectivity and osteo-inductivity onto PEEK surfaces using a mussel foot proteins (Mfps)-mimic peptide with clickable azido terminal. The peptide enables mussel-like adhesion on PEEK biomaterial surfaces, leaving azido groups for the further steps of biofunctionalizations. In this study, antimicrobial peptide (AMP) and osteogenic growth peptide (OGP) were bioorthogonally clicked on the azido-modified PEEK biomaterials to obtain a dual-effect of host defense and tissue repair. Since bioorthogonal clicking allows precise collocation between AMP and OGP through changing their feeding molar ratios, an optimal PEEK surface was finally obtained in this research, which could long-term inhibit bacterial growth, stabilize bone homeostasis and facilitate interfacial bone regeneration. In a word, this upgraded mussel surface strategy proposed in this study is promising for the surface bioengineering of inert medical implants, in particular, achieving rational integration of multiple biofunctions to match clinical requirements.

Effect on Rheological Properties and 3D Printability of Biphasic Calcium Phosphate Microporous Particles in Hydrocolloid-Based Hydrogels

The production of patient-specific bone substitutes with an exact fit through 3D printing is emerging as an alternative to autologous bone grafting. To the success of tissue regeneration, the material characteristics such as porosity, stiffness, and surface topography have a strong influence on the cell–material interaction and require significant attention. Printing a soft hydrocolloid-based hydrogel reinforced with irregularly-shaped microporous biphasic calcium phosphate (BCP) particles (150–500 µm) is an alternative strategy for the acquisition of a complex network with good mechanical properties that could fulfill the needs of cell proliferation and regeneration. Three well-known hydrocolloids (sodium alginate, xanthan gum, and gelatin) have been combined with BCP particles to generate stable, homogenous, and printable solid dispersions. Through rheological assessment, it was determined that the crosslinking time, printing process parameters (infill density percentage and infill pattern), as well as BCP particle size and concentration all influence the stiffness of the printed matrices. Additionally, the swelling behavior on fresh and dehydrated 3D-printed structures was investigated, where it was observed that the BCP particle characteristics influenced the constructs’ water absorption, particle diffusion out of the matrix and degradability.

Naturally prefabricated 3D chitinous skeletal scaffold of marine demosponge origin, biomineralized ex vivo as a functional biomaterial

Solutions developed by nature for structural and functional optimization of three-dimensional (3D) skeletal structures provide unique windows not only into the evolutionary pathways of organisms, but also into bioinspired materials science and biomimetics. Great examples are naturally formed 3D chitinous scaffolds of marine sponge remain a focus of modern biomedicine and tissue engineering. Due to its properties like renewability, bioactivity, and biodegradability such constructs became very interesting players as components of organic-inorganic biocomposites. Herein, we developed chitin-based biocomposites by biomimetic ex vivo deposition of calcium carbonate particles using hemolymph from the cultivated mollusk Cornu aspersum and chitinous matrix from the marine demosponge Aplysina fistularis. The biological potential of the developed biofunctionalized scaffolds for bone tissue engineering was evaluated by investigating the spreading and viability of a human fetal osteoblast cell line has been determined for the first time. Performed analyses like dynamic mechanical analysis and atomic force microscopy shown that biofunctionalized scaffold possess about 4 times higher mechanical resistance. Moreover, several topographical changes have been observed, as e.g., surface roughness (Rq) increased from 31.75 ± 2.7 nm to 120.7 ± 0.3 nm. The results are indicating its potential for use in the modification of cell delivery systems in future biomedical applications.

A Microfluidic Device to Fabricate One-Step Cell Bead-Laden Hydrogel Struts for Tissue Engineering

Cell-laden structures are widely applied for a variety of tissue engineering applications, including tissue restoration. Cell-to-cell interactions in bioprinted structures are important for successful tissue restoration, because cell-cell signaling pathways can regulate tissue development and stem cell fate. However, the low degree of cell-cell interaction in conventional cell-laden bioprinted structures is challenging for the therapeutic application of this modality. Herein, a microfluidic device with cell-laden methacrylated gelatin (GelMa) bioink and alginate as a matrix hydrogel is used to fabricate a functional hybrid structure laden with cell-aggregated microbeads. This approach effectively increases the degree of cell-to-cell interaction to a level comparable to cell spheroids. The hybrid structure is obtained using a one-step process without the exhausting procedure. It consists of cell bead fabrication and an extrusion process for the cell-bead laden structure. Different flow rates are appropriately selected to develop cell-laden struts with homogeneously distributed cell beads for each hydrogel in the process. The hybrid struts exhibit significantly higher cellular activities than those of conventional alginate/GelMa struts, which are bioprinted using similar cell densities and bioink formulations. Furthermore, hybrid struts with adipose stem cells are implanted into mice, resulting in significantly higher myogenesis in comparison to normally bioprinted struts.

Recent approaches towards bone tissue engineering

Bone tissue engineering approaches have evolved towards addressing the challenges of tissue mimetic requirements over the years. Different strategies have been combining scaffolds, cells, and biologically active cues using a wide range of fabrication techniques, envisioning the mimicry of bone tissue. On the one hand, biomimetic scaffold-based strategies have been pursuing different biomaterials to produce scaffolds, combining with diverse and innovative fabrication strategies to mimic bone tissue better, surpassing bone grafts. On the other hand, biomimetic scaffold-free approaches mainly foresee replicating endochondral ossification, replacing hyaline cartilage with new bone. Finally, since bone tissue is highly vascularized, new strategies focused on developing pre-vascularized scaffolds or pre-vascularized cellular aggregates have been a motif of study. The recent biomimetic scaffold-based and scaffold-free approaches in bone tissue engineering, focusing on materials and fabrication methods used, are overviewed herein. The biomimetic vascularized approaches are also discussed, namely the development of pre-vascularized scaffolds and pre-vascularized cellular aggregates.

Materials for Dentoalveolar Bioprinting: Current State of the Art

Although current treatments can successfully address a wide range of complications in the dentoalveolar region, they often still suffer from drawbacks and limitations, resulting in sub-optimal treatments for specific problems. In recent decades, significant progress has been made in the field of tissue engineering, aiming at restoring damaged tissues via a regenerative approach. Yet, the translation into a clinical product is still challenging. Novel technologies such as bioprinting have been developed to solve some of the shortcomings faced in traditional tissue engineering approaches. Using automated bioprinting techniques allows for precise placement of cells and biological molecules and for geometrical patient-specific design of produced biological scaffolds. Recently, bioprinting has also been introduced into the field of dentoalveolar tissue engineering. However, the choice of a suitable material to encapsulate cells in the development of so-called bioinks for bioprinting dentoalveolar tissues is still a challenge, considering the heterogeneity of these tissues and the range of properties they possess. This review, therefore, aims to provide an overview of the current state of the art by discussing the progress of the research on materials used for dentoalveolar bioprinting, highlighting the advantages and shortcomings of current approaches and considering opportunities for further research.

Advanced Strategies for 3D Bioprinting of Tissue and Organ Analogs Using Alginate Hydrogel Bioinks

Alginate is a natural polysaccharide that typically originates from various species of algae. Due to its low cost, good biocompatibility, and rapid ionic gelation, the alginate hydrogel has become a good option of bioink source for 3D bioprinting. However, the lack of cell adhesive moieties, erratic biodegradability, and poor printability are the critical limitations of alginate hydrogel bioink. This review discusses the pivotal properties of alginate hydrogel as a bioink for 3D bioprinting technologies. Afterward, a variety of advanced material formulations and biofabrication strategies that have recently been developed to overcome the drawbacks of alginate hydrogel bioink will be focused on. In addition, the applications of these advanced solutions for 3D bioprinting of tissue/organ mimicries such as regenerative implants and in vitro tissue models using alginate-based bioink will be systematically summarized.

Carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds for bone tissue engineering applications

In bone tissue engineering, the development of advanced biomimetic scaffolds has led to the quest for biomotifs in scaffold design that better recreate the bone matrix structure and composition and hierarchy at different length scales. In this study, an advanced hierarchical scaffold consisting of silk fibroin combined with a decellularized cell-derived extracellular matrix and reinforced with carbon nanotubes was developed. The goal of the carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds is to harvest the individual properties of their constituents to introduce hierarchical capacity in order to improve standard silk fibroin scaffolds. The scaffolds were fabricated using enzymatic cross-linking, freeze modeling, and decellularization methods. The developed scaffolds were assessed for the pore structure and mechanical properties showing satisfying results to be used in bone regeneration. The developed carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds were shown to be bioactive in vitro and expressed no hemolytic effect. Furthermore, cellular in vitro studies on human adipose-derived stem cells (hASCs) showed that scaffolds supported cell proliferation. The hASCs seeded onto these scaffolds evidenced similar metabolic activity to standard silk fibroin scaffolds but increased ALP activity. The histological staining showed cell infiltration into the scaffolds and visible collagen production. The expression of several osteogenic markers was investigated, further supporting the osteogenic potential of the developed carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds. The hemolytic assay demonstrated the hemocompatibility of the hierarchical scaffolds. Overall, the carbon nanotube-reinforced cell-derived matrix-silk fibroin hierarchical scaffolds presented the required architecture for bone tissue engineering applications.

Effect of Polymeric Matrix Stiffness on Osteogenic Differentiation of Mesenchymal Stem/Progenitor Cells: Concise Review

Mesenchymal stem/progenitor cells (MSCs) have a multi-differentiation potential into specialized cell types, with remarkable regenerative and therapeutic results. Several factors could trigger the differentiation of MSCs into specific lineages, among them the biophysical and chemical characteristics of the extracellular matrix (ECM), including its stiffness, composition, topography, and mechanical properties. MSCs can sense and assess the stiffness of extracellular substrates through the process of mechanotransduction. Through this process, the extracellular matrix can govern and direct MSCs' lineage commitment through complex intracellular pathways. Hence, various biomimetic natural and synthetic polymeric matrices of tunable stiffness were developed and further investigated to mimic the MSCs' native tissues. Customizing scaffold materials to mimic cells' natural environment is of utmost importance during the process of tissue engineering. This review aims to highlight the regulatory role of matrix stiffness in directing the osteogenic differentiation of MSCs, addressing how MSCs sense and respond to their ECM, in addition to listing different polymeric biomaterials and methods used to alter their stiffness to dictate MSCs' differentiation towards the osteogenic lineage.

Three-Dimensional Cell Printed Lock-Key Structure for Oral Soft and Hard Tissue Regeneration

Alveolar ridge absorbs rapidly following tooth extraction. To promote implant rehabilitation, an adequate bone and soft tissue volume are required. Three-dimensional (3D) cell printing technique provides the advantages of precise spatial distribution and personalization. In this study, 3D cell printing was used to establish a soft-hard construct that is composed of alginate/gelatin (AG)/gingival fibroblast cells (GFs) and alginate/gelatin/nano-hydroxyapatite (AGH)/bone marrow-derived mesenchymal stem cells (BMSCs). Physicochemical results showed that nano-hydroxyapatite (nHA) added in the bioink maintained its crystalline phase. In addition, an increase of viscosity, the improvement of compressive modulus (p < 0.01), and slow degradation rate (p < 0.01) were found after adding nHA. SEM showed cell stretched and attached well on the surface of the 3D printed construct. At day 7 after printing, the viability of GFs in AG was 94.80% ± 1.14%, while BMSC viability in AGH was 86.59% ± 0.75%. Polymerase chain reaction results indicated that the expression levels of ALP, RUNX-2, and OCN in BMSCs were higher in AGH than AG bioink (p < 0.01). After 8-week implantation into the dorsum of 6- to 8-week-old male athymic and inbred (BALB/c) nude mice, the cellular printed construct displayed a more integrated structure and better healing of subcutaneous tissue compared with the acellular printed construct. In conclusion, this 3D cell printed soft-hard construct exhibits favorable biocompatibility and has potential for alveolar ridge preservation. Impact statement Alveolar ridge resorption after tooth extraction has posed great difficulty in the subsequent restorative procedure. Clinically, to preserve the dimension of alveolar ridge, covering soft tissue healing and underlying bone formation is necessary after tooth extraction. Three-dimensional (3D) cell printing, which can distribute different biomaterials and cells with spatial control, provides a novel approach to develop a customized plug to put in the fresh socket to minimize bone resorption and improve gingiva growth. In this study, an integrated and heterogeneous soft-hard construct with lock-key structure was successfully developed using 3D cell printing. The physicochemical and biological properties were tested in vitro and in vivo. This 3D cell printed soft-hard construct will be a customized plug in alveolar ridge preservation in the future.

Three-dimensional bioprinting of artificial ovaries by an extrusion-based method using gelatin-methacryloyl bioink

PURPOSE

The aim of this study was to design and fabricate a three-dimensional (3D) printed artificial ovary.

METHODS

We first compared the printability of gelatin-methacryloyl (GelMA), alginate and GelMA-alginate bioinks, of which GelMA was selected for further investigation. The swelling properties, degradation kinetics and shape fidelity of GelMA scaffolds were characterized by equilibrium swelling/lyophilization, collagenase processing and micro-computed tomography evaluation. Commercial ovarian tumor cell lines (COV434, KGN, ID8) and primary culture ovarian somatic cells were utilized to perform cell-laden 3D printing, and the results were evaluated by live/dead assays and TUNEL detection. Murine ovarian follicles were seeded in the ovarian scaffold and their diameters were recorded every day. Finally, in vitro maturation was performed, and the ovulated oocytes were collected and observed.

RESULTS

Our results indicated that GelMA was suitable for 3D printing fabrication. Its scaffolds performed well in terms of hygroscopicity, degradation kinetics and shape fidelity. The viability of ovarian somatic cells was lower than that of commercial cell lines, suggesting that extrusion-based 3D culture fabrication is not suitable for primary ovarian cells. Nevertheless, the GelMA-based 3D printing system provided an appropriate microenvironment for ovarian follicles, which successfully grew and ovulated in the scaffolds. Metaphase II oocytes were also observed after in vitro maturation.

CONCLUSIONS

The GelMA-based 3D printing culture system is a viable alternative option for follicular growth, development and transfer. Accordingly, it shows promise for clinical application in the treatment of female endocrine and reproductive conditions.

3D Bioprinting of Human Tissues: Biofabrication, Bioinks, and Bioreactors

The field of tissue engineering has progressed tremendously over the past few decades in its ability to fabricate functional tissue substitutes for regenerative medicine and pharmaceutical research. Conventional scaffold-based approaches are limited in their capacity to produce constructs with the functionality and complexity of native tissue. Three-dimensional (3D) bioprinting offers exciting prospects for scaffolds fabrication, as it allows precise placement of cells, biochemical factors, and biomaterials in a layer-by-layer process. Compared with traditional scaffold fabrication approaches, 3D bioprinting is better to mimic the complex microstructures of biological tissues and accurately control the distribution of cells. Here, we describe recent technological advances in bio-fabrication focusing on 3D bioprinting processes for tissue engineering from data processing to bioprinting, mainly inkjet, laser, and extrusion-based technique. We then review the associated bioink formulation for 3D bioprinting of human tissues, including biomaterials, cells, and growth factors selection. The key bioink properties for successful bioprinting of human tissue were summarized. After bioprinting, the cells are generally devoid of any exposure to fluid mechanical cues, such as fluid shear stress, tension, and compression, which are crucial for tissue development and function in health and disease. The bioreactor can serve as a simulator to aid in the development of engineering human tissues from in vitro maturation of 3D cell-laden scaffolds. We then describe some of the most common bioreactors found in the engineering of several functional tissues, such as bone, cartilage, and cardiovascular applications. In the end, we conclude with a brief insight into present limitations and future developments on the application of 3D bioprinting and bioreactor systems for engineering human tissue.

Printability in extrusion bioprinting

Extrusion bioprinting has been widely used to extrude continuous filaments of bioink (or the mixture of biomaterial and living cells), layer-by-layer, to build three-dimensional (3D) constructs for biomedical applications. In extrusion bioprinting, printability is an important parameter used to measure the difference between the designed construct and the one actually printed. This difference could be caused by the extrudability of printed bioink and/or the structural formability and stability of printed constructs. Although studies have reported in characterizing printability based on the bioink properties and printing process, the concept of printability is often confusingly and, sometimes, conflictingly used in the literature. The objective of this perspective is to define the printability for extrusion bioprinting in terms of extrudability, filament fidelity, and structural integrity, as well as to review the effect of bioink properties, bioprinting process, and construct design on the printability. Challenges related to the printability of extrusion bioprinting are also discussed, along with recommendations for improvements.

3D bioprinting of graphene oxide-incorporated cell-laden bone mimicking scaffolds for promoting scaffold fidelity, osteogenic differentiation and mineralization

Bioprinting is a promising technique for facilitating the fabrication of engineered bone tissues for patient-specific defect repair and for developing in vitro tissue/organ models for ex vivo tests. However, polymer-based ink materials often result in insufficient mechanical strength, low scaffold fidelity and loss of osteogenesis induction because of the intrinsic swelling/shrinking and bioinert properties of most polymeric hydrogels. Here, we developed a human mesenchymal stem cells (hMSCs)-laden graphene oxide (GO)/alginate/gelatin composite bioink to form 3D bone-mimicking scaffolds using a 3D bioprinting technique. Our results showed that the GO composite bioinks (0.5GO, 1GO, 2GO) with higher GO concentrations (0.5, 1 and 2 mg/ml) improved the bioprintability, scaffold fidelity, compressive modulus and cell viability at day 1. The higher GO concentration increased the cell body size and DNA content, but the 2GO group swelled and had the lowest compressive modulus at day 42. The 1GO group had the highest osteogenic differentiation of hMSC with the upregulation of osteogenic-related gene (ALPL, BGLAP, PHEX) expression. To mimic critical-sized calvarial bone defects in mice and prove scaffold fidelity, 3D cell-laden GO defect scaffolds with complex geometries were successfully bioprinted. 1GO maintained the best scaffold fidelity and had the highest mineral volume after culturing in the bioreactor for 42 days. In conclusion, GO composite bioinks had better bioprintability, scaffold fidelity, cell proliferation, osteogenic differentiation and ECM mineralization than the pure alginate/gelatin system. The optimal GO group was 1GO, which demonstrated the potential for 3D bioprinting of bone tissue models and tissue engineering applications.

Advances in Engineering Human Tissue Models

Research in cell biology greatly relies on cell-based in vitro assays and models that facilitate the investigation and understanding of specific biological events and processes under different conditions. The quality of such experimental models and particularly the level at which they represent cell behavior in the native tissue, is of critical importance for our understanding of cell interactions within tissues and organs. Conventionally, in vitro models are based on experimental manipulation of mammalian cells, grown as monolayers on flat, two-dimensional (2D) substrates. Despite the amazing progress and discoveries achieved with flat biology models, our ability to translate biological insights has been limited, since the 2D environment does not reflect the physiological behavior of cells in real tissues. Advances in 3D cell biology and engineering have led to the development of a new generation of cell culture formats that can better recapitulate the in vivo microenvironment, allowing us to examine cells and their interactions in a more biomimetic context. Modern biomedical research has at its disposal novel technological approaches that promote development of more sophisticated and robust tissue engineering in vitro models, including scaffold- or hydrogel-based formats, organotypic cultures, and organs-on-chips. Even though such systems are necessarily simplified to capture a particular range of physiology, their ability to model specific processes of human biology is greatly valued for their potential to close the gap between conventional animal studies and human (patho-) physiology. Here, we review recent advances in 3D biomimetic cultures, focusing on the technological bricks available to develop more physiologically relevant in vitro models of human tissues. By highlighting applications and examples of several physiological and disease models, we identify the limitations and challenges which the field needs to address in order to more effectively incorporate synthetic biomimetic culture platforms into biomedical research.

Current Advances in 3D Bioprinting Technology and Its Applications for Tissue Engineering

Three-dimensional (3D) bioprinting technology has emerged as a powerful biofabrication platform for tissue engineering because of its ability to engineer living cells and biomaterial-based 3D objects. Over the last few decades, droplet-based, extrusion-based, and laser-assisted bioprinters have been developed to fulfill certain requirements in terms of resolution, cell viability, cell density, etc. Simultaneously, various bio-inks based on natural-synthetic biomaterials have been developed and applied for successful tissue regeneration. To engineer more realistic artificial tissues/organs, mixtures of bio-inks with various recipes have also been developed. Taken together, this review describes the fundamental characteristics of the existing bioprinters and bio-inks that have been currently developed, followed by their advantages and disadvantages. Finally, various tissue engineering applications using 3D bioprinting are briefly introduced.

3D bioprinting for orthopaedic applications: Current advances, challenges and regulatory considerations

In the era of personalised medicine, novel therapeutic approaches raise increasing hopes to address currently unmet medical needs by developing patient-customised treatments. Three-dimensional (3D) bioprinting is rapidly evolving and has the potential to obtain personalised tissue constructs and overcome some limitations of standard tissue engineering approaches. Bioprinting could support a wide range of biomedical applications, such as drug testing, tissue repair or organ transplantation. There is a growing interest for 3D bioprinting in the orthopaedic field, with remarkable scientific and technical advances. However, the full exploitation of 3D bioprinting in medical applications still requires efforts to anticipate the upcoming challenges in translating bioprinted products from bench to bedside. In this review we summarised current trends, advances and challenges in the application of 3D bioprinting for bone and cartilage tissue engineering. Moreover, we provided a detailed analysis of the applicable regulations through the 3D bioprinting process and an overview of available standards covering bioprinting and additive manufacturing.