A bioprintable form of chitosan hydrogel for bone tissue engineering

Glycerylphytate as an ionic crosslinker for 3D printing of multi-layered scaffolds with improved shape fidelity and biological features

The fabrication of intricate and long-term stable 3D polymeric scaffolds by a 3D printing technique is still a challenge. In the biomedical field, hydrogel materials are very frequently used because of their excellent biocompatibility and biodegradability, however the improvement of their processability and mechanical properties is still required. This paper reports the fabrication of dual crosslinked 3D scaffolds using a low concentrated (<10 wt%) ink of gelatin methacryloyl (GelMA)/chitosan and a novel crosslinking agent, glycerylphytate (G1Phy) to overcome the current limitations in the 3D printing field using hydrogels. The applied methodology consisted of a first ultraviolet light (UV) photopolymerization followed by a post-printing ionic crosslinking treatment with G1Phy. This crosslinker provides a robust framework and avoids the necessity of neutralization with strong bases. The blend ink showed shear-thinning behavior and excellent printability in the form of a straight and homogeneous filament. UV curing was undertaken simultaneously to 3D deposition, which enhanced precision and shape fidelity (resolution ≈150 μm), and prevented the collapse of the subsequent printed layers (up to 28 layers). In the second step, the novel G1Phy ionic crosslinker agent provided swelling and long term stability properties to the 3D scaffolds. The multi-layered printed scaffolds were mechanically stable under physiological conditions for at least one month. Preliminary in vitro assays using L929 fibroblasts showed very promising results in terms of adhesion, spreading, and proliferation in comparison to other phosphate-based traditional crosslinkers (i.e. TPP). We envision that the proposed combination of the blend ink and 3D printing approach can have widespread applications in the regeneration of soft tissues.

Natural and Synthetic Polymers for Bone Scaffolds Optimization

Bone tissue is the structural component of the body, which allows locomotion, protects vital internal organs, and provides the maintenance of mineral homeostasis. Several bone-related pathologies generate critical-size bone defects that our organism is not able to heal spontaneously and require a therapeutic action. Conventional therapies span from pharmacological to interventional methodologies, all of them characterized by several drawbacks. To circumvent these effects, tissue engineering and regenerative medicine are innovative and promising approaches that exploit the capability of bone progenitors, especially mesenchymal stem cells, to differentiate into functional bone cells. So far, several materials have been tested in order to guarantee the specific requirements for bone tissue regeneration, ranging from the material biocompatibility to the ideal 3D bone-like architectural structure. In this review, we analyse the state-of-the-art of the most widespread polymeric scaffold materials and their application in in vitro and in vivo models, in order to evaluate their usability in the field of bone tissue engineering. Here, we will present several adopted strategies in scaffold production, from the different combination of materials, to chemical factor inclusion, embedding of cells, and manufacturing technology improvement.

Cell-Laden Thermosensitive Chitosan Hydrogel Bioinks for 3D Bioprinting Applications

Three-dimensional (3D) bioprinting is a technology used to deposit cell-laden biomaterials for the construction of complex tissues. Thermosensitive hydrogels are physically cross-linked by non-covalent interaction without using crosslinkers, facilitating low cytotoxicity and cell viability. Chitosan, which is a non-toxic, biocompatible and biodegradable polysaccharide, can be used as a thermosensitive hydrogel. Therefore, chitosan hydrogel could be of potential use as a 3D bioprinting ink. The purpose of this study was to develop and compare the effectivity of different bioinks based on chitosan hydrogels for 3D bioprinting. The solvent type did not affect the gel shape and gelation time, whereas acetic acid exhibited better biocompatibility compared to lactic and hydrochloric acids. The nature of the gelling agent was found to have a stronger influence on these characteristics than that of the solvent. The NaHCO3 moiety exhibited a higher growth rate of the storage modulus (G′) and a more irregular porous structure than that of the β-glycerophosphate (β-GP) and K2HPO4 groups. Cell viability, and live and dead assays, showed that the NaHCO3 group was more efficient for cell adhesion. The type of gelling agent did not lead to appreciable differences in cell-laden constructs. The NaHCO3 group was more amenable to bioprinting, compared to the β-GP and K2HPO4 groups. The chitosan hydrogel bioinks could, therefore, be good candidates for 3D bioprinting and would pave the way for patient-specific regenerative medicines.

Polylactide/Hydroxyapatite Nonwovens Incorporated into Chitosan/Graphene Materials Hydrogels to Form Novel Hierarchical Scaffolds

In this study, hierarchical, cylindrical scaffolds based on polylactide (PLA) microfibers incorporated into chitosan (CS) hydrogel were prepared for potential use in bone tissue engineering. PLA nonwovens modified with hydroxyapatite particles (HAp) were obtained using the electrospinning method. Then, three-dimensional scaffolds were created by rolling up the nonwovens and immersing them in CS-based solutions with graphene oxide (GO) or reduced graphene oxide (rGO) dispersed in the polymer matrix. Hydrogels were cross-linked using a novel freezing-thawing-gelling method. A broad spectrum of research methods was applied in order to thoroughly characterize both the nanofillers and the composite systems: scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffractometry, attenuated total reflection Fourier transform infrared spectroscopy, rheological and mechanical testing, as well as the assessment of chemical stability, bioactivity and cytocompatibility.

3D printing of bone tissue engineering scaffolds

Tissue engineering is promising in realizing successful treatments of human body tissue loss that current methods cannot treat well or achieve satisfactory clinical outcomes. In scaffold-based bone tissue engineering, a high performance scaffold underpins the success of a bone tissue engineering strategy and a major direction in the field is to produce bone tissue engineering scaffolds with desirable shape, structural, physical, chemical and biological features for enhanced biological performance and for regenerating complex bone tissues. Three-dimensional (3D) printing can produce customized scaffolds that are highly desirable for bone tissue engineering. The enormous interest in 3D printing and 3D printed objects by the science, engineering and medical communities has led to various developments of the 3D printing technology and wide investigations of 3D printed products in many industries, including biomedical engineering, over the past decade. It is now possible to create novel bone tissue engineering scaffolds with customized shape, architecture, favorable macro-micro structure, wettability, mechanical strength and cellular responses. This article provides a concise review of recent advances in the R & D of 3D printing of bone tissue engineering scaffolds. It also presents our philosophy and research in the designing and fabrication of bone tissue engineering scaffolds through 3D printing.

Assessment methodologies for extrusion-based bioink printability

Extrusion-based bioprinting is one of the leading manufacturing techniques for tissue engineering and regenerative medicine. Its primary limitation is the lack of materials, known as bioinks, which are suitable for the bioprinting process. The degree to which a bioink is suitable for bioprinting has been described as its 'printability.' However, a lack of clarity surrounding the methodologies used to evaluate a bioink's printability, as well as the usage of the term itself, have hindered the field. This article presents a review of measures used to assess the printability of extrusion-based bioinks in an attempt to assist researchers during the bioink development process. Many different aspects of printability exist and many different measurements have been proposed as a consequence. Researchers often do not evaluate a new bioink's printability at all, while others simply do so qualitatively. Several quantitative measures have been presented for the extrudability, shape fidelity, and printing accuracy of bioinks. Different measures have been developed even within these aspects, each testing the bioink in a slightly different way. Additionally, other relevant measures which had little or no examples of quantifiable methods are also to be considered. Looking forward, further work is needed to improve upon current assessment methodologies, to move towards a more comprehensive view of printability, and to standardize these printability measurements between researchers. Better assessment techniques will naturally lead to a better understanding of the underlying mechanisms which affect printability and better comparisons between bioinks. This in turn will help improve upon the bioink development process and the bioinks available for use in bioprinting.

3D printing applications in bone tissue engineering

PURPOSE

3D printing technology provides an excellent capability to manufacture customised implants for patients. Now, its applications are also successful in bone tissue engineering. This paper tries to provide a review of the applications of 3D printing in bone tissue engineering.

METHODS

Searching by keywords, from the Scopus database, to identify relevant latest research articles on 3D printing in bone tissue engineering, through "3D printing" "bone tissue engineering". This study makes a bibliometric analysis of the identified research articles and identified major applications and steps.

RESULTS

3D printing technology creates innovative development in bone tissue engineering. It involves the manufacturing of a scaffold with the combination of cells and materials. We identified a total number of 257 research articles through bibliometric analysis by searching through keywords "3D printing" "bone tissue engineering". This paper studies 3D printing technology and its significant contributions, benefits and steps used for bone tissue engineering. Result discusses the essential elements of bone tissue engineering and identifies its five significant advancements when 3D printing is used. Finally, ten useful applications of 3D printing in bone tissue engineering are identified and studied with a brief description.

CONCLUSION

In orthopaedics, bone defects create a high impact on the quality of life of the patient. It leads to a higher demand for bone substitutes for replacement of bone defect. Bone tissue engineering can help to replace a critical defect bone. 3D printing is a useful technology for the fabrication of scaffolds critical in bone tissue engineering. There are different binders which can create bone scaffolds with requisite mechanical strength. These binders are used to create excellent osteoconductive, bioactive scaffolds. Computed tomography (CT) and Magnetic resonance imaging (MRI) help to provide images of specific defects of an individual patient, and these images can further be used for 3D printing the detective object. A bone defect caused by specific disease is sorted out by transplantation in clinical practice. Now a day bone tissue engineering opens a new option for this treatment of bone defects with the manufacturing of porous bone scaffold using 3D printing technology.

Four-dimensional bioprinting: Current developments and applications in bone tissue engineering

Four-dimensional (4D) bioprinting, in which the concept of time is integrated with three-dimensional (3D) bioprinting as the fourth dimension, has currently emerged as the next-generation solution of tissue engineering as it presents the possibility of constructing complex, functional structures. 4D bioprinting can be used to fabricate dynamic 3D-patterned biological architectures that will change their shapes under various stimuli by employing stimuli-responsive materials. The functional transformation and maturation of printed cell-laden constructs over time are also regarded as 4D bioprinting, providing unprecedented potential for bone tissue engineering. The shape memory properties of printed structures cater to the need for personalized bone defect repair and the functional maturation procedures promote the osteogenic differentiation of stem cells. In this review, we introduce the application of different stimuli-responsive biomaterials in tissue engineering and a series of 4D bioprinting strategies based on functional transformation of printed structures. Furthermore, we discuss the application of 4D bioprinting in bone tissue engineering, as well as the current challenges and future perspectives. STATEMENTS OF SIGNIFICANCE: In this review, we have demonstrated the 4D bioprinting technologies, which integrate the concept of time within the traditional 3D bioprinting technology as the fourth dimension and facilitate the fabrications of complex, functional biological architectures. These 4D bioprinting structures could go through shape or functional transformation over time via using different stimuli-responsive biomaterials and a series of 4D bioprinting strategies. Moreover, by summarizing potential applications of 4D bioprinting in the field of bone tissue engineering, these emerging technologies could fulfill unaddressed medical requirements. The further discussions about future challenges and perspectives will give us more inspirations about widespread applications of this emerging technology for tissue engineering in biomedical field.

An insight into cell-laden 3D-printed constructs for bone tissue engineering

In this review, we have spotlighted various combinations of bioinks to optimize the biofabrication of 3D bone constructs.

Hydrogel Bioink Reinforcement for Additive Manufacturing: A Focused Review of Emerging Strategies

Bioprinting is an emerging approach for fabricating cell-laden 3D scaffolds via robotic deposition of cells and biomaterials into custom shapes and patterns to replicate complex tissue architectures. Bioprinting uses hydrogel solutions called bioinks as both cell carriers and structural components, requiring bioinks to be highly printable while providing a robust and cell-friendly microenvironment. Unfortunately, conventional hydrogel bioinks have not been able to meet these requirements and are mechanically weak due to their heterogeneously crosslinked networks and lack of energy dissipation mechanisms. Advanced bioink designs using various methods of dissipating mechanical energy are aimed at developing next-generation cellularized 3D scaffolds to mimic anatomical size, tissue architecture, and tissue-specific functions. These next-generation bioinks need to have high print fidelity and should provide a biocompatible microenvironment along with improved mechanical properties. To design these advanced bioink formulations, it is important to understand the structure-property-function relationships of hydrogel networks. By specifically leveraging biophysical and biochemical characteristics of hydrogel networks, high performance bioinks can be designed to control and direct cell functions. In this review article, current and emerging approaches in hydrogel design and bioink reinforcement techniques are critically evaluated. This bottom-up perspective provides a materials-centric approach to bioink design for 3D bioprinting.

Synthesis and characterization of osteoinductive visible light-activated adhesive composites with antimicrobial properties

Orthopedic surgical procedures based on the use of conventional biological graft tissues are often associated with serious post-operative complications such as immune rejection, bacterial infection, and poor osseointegration. Bioresorbable bone graft substitutes have emerged as attractive alternatives to conventional strategies because they can mimic the composition and mechanical properties of the native bone. Among these, bioactive glasses (BGs) hold great potential to be used as biomaterials for bone tissue engineering owing to their biomimetic composition and high biocompatibility and osteoinductivity. Here, we report the development of a novel composite biomaterial for bone tissue engineering based on the incorporation of a modified strontium- and lithium-doped 58S BG (i.e., BG-5/5) into gelatin methacryloyl (GelMA) hydrogels. We characterized the physicochemical properties of the BG formulation via different analytical techniques. Composite hydrogels were then prepared by directly adding BG-5/5 to the GelMA hydrogel precursor, followed by photocrosslinking of the polymeric network via visible light. We characterized the physical, mechanical, and adhesive properties of GelMA/BG-5/5 composites, as well as their in vitro cytocompatibility and osteoinductivity. In addition, we evaluated the antimicrobial properties of these composites in vitro, using a strain of methicillin-resistant Staphylococcus Aureus. GelMA/BG-5/5 composites combined the functional characteristics of the inorganic BG component, with the biocompatibility, biodegradability, and biomimetic composition of the hydrogel network. This novel biomaterial could be used for developing osteoinductive scaffolds or implant surface coatings with intrinsic antimicrobial properties and higher therapeutic efficacy.

Graphene oxide/alginate composites as novel bioinks for three-dimensional mesenchymal stem cell printing and bone regeneration applications

Three-dimensional (3D) cell printing is a versatile technique enabling the creation of 3D constructs containing hydrogel and cells in the desired shape or pattern. Bioinks exhibiting appropriate mechanical properties and biological activities to support cell growth and/or differentiation toward a specific lineage play critical roles in 3D cell printing and tissue engineering applications. Herein, we explored alginate/graphene oxide (GO) composites as bioinks for their potential to improve printability, structural stability, and osteogenic activities for osteogenic tissue engineering applications. The addition of GO (0.05-1.0 mg mL^-1) to 3% alginate significantly enhanced the printing performances of the alginate bioink. In addition, mesenchymal stem cells (MSCs) printed with alginate/GO showed good proliferation and higher survival in an oxidative stress environment. The 3D scaffolds printed with MSCs and alginate/GO demonstrated significantly enhanced osteogenic differentiation compared with those printed with MSCs and alginate. Overall, a bioink of 3% alginate and 0.5 mg mL^-1 GO showed the most balanced characteristics in terms of printability, structural stability, and osteogenic induction of the printed MSCs. Alginate/GO composite bioinks will be useful for bioprinting research for various tissue engineering applications.

Chitosans for Tissue Repair and Organ Three-Dimensional (3D) Bioprinting

Chitosan is a unique natural resourced polysaccharide derived from chitin with special biocompatibility, biodegradability, and antimicrobial activity. During the past three decades, chitosan has gradually become an excellent candidate for various biomedical applications with prominent characteristics. Chitosan molecules can be chemically modified, adapting to all kinds of cells in the body, and endowed with specific biochemical and physiological functions. In this review, the intrinsic/extrinsic properties of chitosan molecules in skin, bone, cartilage, liver tissue repair, and organ three-dimensional (3D) bioprinting have been outlined. Several successful models for large scale-up vascularized and innervated organ 3D bioprinting have been demonstrated. Challenges and perspectives in future complex organ 3D bioprinting areas have been analyzed.

Adipose-Derived Stem Cells in Bone Tissue Engineering: Useful Tools with New Applications

Adipose stem cells (ASCs) are a crucial element in bone tissue engineering (BTE). They are easy to harvest and isolate, and they are available in significative quantities, thus offering a feasible and valid alternative to other sources of mesenchymal stem cells (MSCs), like bone marrow. Together with an advantageous proliferative and differentiative profile, they also offer a high paracrine activity through the secretion of several bioactive molecules (such as growth factors and miRNAs) via a sustained exosomal release which can exert efficient conditioning on the surrounding microenvironment. BTE relies on three key elements: (1) scaffold, (2) osteoprogenitor cells, and (3) bioactive factors. These elements have been thoroughly investigated over the years. The use of ASCs has offered significative new advancements in the efficacy of each of these elements. Notably, the phenotypic study of ASCs allowed discovering cell subpopulations, which have enhanced osteogenic and vasculogenic capacity. ASCs favored a better vascularization and integration of the scaffolds, while improvements in scaffolds' materials and design tried to exploit the osteogenic features of ASCs, thus reducing the need for external bioactive factors. At the same time, ASCs proved to be an incredible source of bioactive, proosteogenic factors that are released through their abundant exosome secretion. ASC exosomes can exert significant paracrine effects in the surroundings, even in the absence of the primary cells. These paracrine signals recruit progenitor cells from the host tissues and enhance regeneration. In this review, we will focus on the recent discoveries which have involved the use of ASCs in BTE. In particular, we are going to analyze the different ASCs' subpopulations, the interaction between ASCs and scaffolds, and the bioactive factors which are secreted by ASCs or can induce their osteogenic commitment. All these advancements are ultimately intended for a faster translational and clinical application of BTE.

Biomaterials Based on Marine Resources for 3D Bioprinting Applications

Three-dimensional (3D) bioprinting has become a flexible tool in regenerative medicine with potential for various applications. Further development of the new 3D bioprinting field lies in suitable bioink materials with satisfied printability, mechanical integrity, and biocompatibility. Natural polymers from marine resources have been attracting increasing attention in recent years, as they are biologically active and abundant when comparing to polymers from other resources. This review focuses on research and applications of marine biomaterials for 3D bioprinting. Special attention is paid to the mechanisms, material requirements, and applications of commonly used 3D bioprinting technologies based on marine-derived resources. Commonly used marine materials for 3D bioprinting including alginate, carrageenan, chitosan, hyaluronic acid, collagen, and gelatin are also discussed, especially in regards to their advantages and applications.

3D Bioprinting of the Sustained Drug Release Wound Dressing with Double-Crosslinked Hyaluronic-Acid-Based Hydrogels

Hyaluronic acid (HA)-based hydrogels are widely used in biomedical applications due to their excellent biocompatibility. HA can be Ultraviolet (UV)-crosslinked by modification with methacrylic anhydride (HA-MA) and crosslinked by modification with 3,3'-dithiobis(propionylhydrazide) (DTP) (HA-SH) via click reaction. In the study presented in this paper, a 3D-bioprinted, double-crosslinked, hyaluronic-acid-based hydrogel for wound dressing was proposed. The hydrogel was produced by mixing HA-MA and HA-SH at different weight ratios. The rheological test showed that the storage modulus (G') of the HA-SH/HA-MA hydrogel increased with the increase in the HA-MA content. The hydrogel had a high swelling ratio and a high controlled degradation rate. The in vitro degradation test showed that the hydrogel at the HA-SH/HA-MA ratio of 9:1 (S9M1) degraded by 89.91% ± 2.26% at 11 days. The rheological performance, drug release profile and the cytocompatibility of HA-SH/HA-MA hydrogels with loaded Nafcillin, which is an antibacterial drug, were evaluated. The wound dressing function of this hydrogel was evaluated by Live/Dead staining and CCK-8 assays. The foregoing results imply that the proposed HA-SH/HA-MA hydrogel has promise in wound repair applications.

Review of alginate-based hydrogel bioprinting for application in tissue engineering

The dawn of 3D printing in medicine has given the field the hope of vitality in many patients fighting a multitude of diseases. Also entitled bioprinting, this appertains to its sequential printing of precursor ink, embodying cells and polymer/composite in a predetermined trajectory. The precursor ink, in addition to cells, is predominantly constituted of hydrogels due to its biodegradability and ability to mimic the body's anatomy and mechanical features, e.g. bones, etc. This review paper is devoted to explicating the bioprinting (3D/4D) of alginate hydrogels, which are extracts from brown algae, through extrusion additive manufacturing. Alginates are salt derivatives of alginic acid and constitute long chains of polysaccharides, which provides pliability and gelling adeptness to their structure. Alginate hydrogel (employed for extrusion) can be pristine or composite relying on the requisite properties (target application controlled or in vivo environment), e.g. alginate-natural (gelatin/agarose/collagen/hyaluronic acid/etc) and alginate-synthetic (polyethylene glycol (PEG)/pluronic F-127/etc). Extrusion additive manufacturing of alginate is preponderate among others with its uncomplicated processing, material efficiency (cut down on wastage), and outspread adaptability for viscosities (0.03-6 * 10⁴ Pa.s), but the procedure is limited by resolution (200 μm) in addition to accuracy. However, 3D-fabricated biostructures display rigidness (unvarying with conditions) i.e. lacks a smart response, which is reassured by accounting time feature as a noteworthy accessory to printing, interpreted as 4D bioprinting. This review propounds the specific processing itinerary for alginate (meanwhile traversing across its composites/blends with natural and synthetic consideration) in extrusion along with its pre-/during/post-processing parameters intrinsic to the process. Furthermore, propensity is also presented in its (alginate extrusion processing) application for tissue engineering, i.e. bones, cartilage (joints), brain (neural), ear, heart (cardiac), eyes (corneal), etc, due to a worldwide quandary over accessibility to natural organs for diverse types of diseases. Additionally, the review contemplates recently invented advance printing, i.e. 4D printing for biotic species, with its challenges and future opportunities.

Progress in the Development of Chitosan-Based Biomaterials for Tissue Engineering and Regenerative Medicine

Over the last few decades, chitosan has become a good candidate for tissue engineering applications. Derived from chitin, chitosan is a unique natural polysaccharide with outstanding properties in line with excellent biodegradability, biocompatibility, and antimicrobial activity. Due to the presence of free amine groups in its backbone chain, chitosan could be further chemically modified to possess additional functional properties useful for the development of different biomaterials in regenerative medicine. In the current review, we will highlight the progress made in the development of chitosan-containing bioscaffolds, such as gels, sponges, films, and fibers, and their possible applications in tissue repair and regeneration, as well as the use of chitosan as a component for drug delivery applications.

Fabrication of biocompatible porous scaffolds based on hydroxyapatite/collagen/chitosan composite for restoration of defected maxillofacial mandible bone

Fabrication of scaffolds from biomaterials for restoration of defected mandible bone has attained increased attention due to limited accessibility of natural bone for grafting. Hydroxyapatite (Ha), collagen type 1 (Col1) and chitosan (Cs) are widely used biomaterials which could be fabricated as a scaffold to overcome the paucity of bone substitutes. Here, rabbit Col1, shrimp Cs and bovine Ha were extracted and characterized with respect to physicochemical properties. Following the biocompatibility, degradability and cytotoxicity tests for Ha, Col1 and Cs a hydroxyapatite/collagen/chitosan (Ha·Col1·Cs) scaffold was fabricated using thermally induced phase separation technique. This scaffold was cross-linked with (1) either glutaraldehyde (GTA), (2) de-hydrothermal treatment (DTH), (3) irradiation (IR) and (4) 2-hydroxyethyl methacrylate (HEMA), resulting in four independent types (Ha·Col1·Cs-GTA, Ha·Col1·Cs-IR, Ha·Col1·Cs-DTH and Ha·Col1·Cs-HEMA). The developed composite scaffolds were porous with 3D interconnected fiber microstructure. However, Ha·Col1·Cs-IR and Ha·Col1·Cs-GTA showed better hydrophilicity and biodegradability. All four scaffolds showed desirable blood biocompatibility without cytotoxicity for brine shrimp. In vitro studies in the presence of human amniotic fluid-derived mesenchymal stem cells revealed that Ha·Col1·Cs-IR and Ha·Col1·Cs-DHT scaffolds were non-cytotoxic and compatible for cell attachment, growth and mineralization. Further, grafting of Ha·Col1·Cs-IR and Ha·Col1·Cs-DHT was performed in a surgically created non-load-bearing rabbit maxillofacial mandible defect model. Histological and radiological observations indicated the restoration of defected bone. Ha·Col1·Cs-IR and Ha·Col1·Cs-DHT could be used as an alternative treatment in bone defects and may contribute to further development of scaffolds for bone tissue engineering.

Customized tracheal design using 3D printing of a polymer hydrogel: influence of UV laser cross-linking on mechanical properties

BACKGROUND

The use of 3D printing of hydrogels as a cell support in bio-printing of cartilage, organs and tissue has attracted much research interest. For cartilage applications, hydrogels as soft materials must show some degree of rigidity, which can be achieved by photo- or chemical polymerization. In this work, we combined chemical and UV laser polymeric cross-linkage to control the mechanical properties of 3D printed hydrogel blends. Since there are few studies on UV laser cross-linking combined with 3D printing of hydrogels, the work here reported offered many challenges.

METHODS

Polyethylene glycol diacrylate (PEGDA), sodium alginate (SA) and calcium sulphate (CaSO₄) polymer paste containing riboflavin (vitamin B2) and triethanolamine (TEOHA) as a biocompatible photoinitiator was printed in an extrusion 3D plotter using a coupled UV laser. The influence of the laser power on the mechanical properties of the printed samples was then examined in unconfined compression stress-strain tests of 1 × 1 × 1 cm³ sized samples. To evaluate the adhesion of the material between printed layers, compression measurements were performed along the parallel and perpendicular directions to the printing lines.

RESULTS

At a laser density of 70 mW/cm², Young's modulus was approximately 6 MPa up to a maximum compression of 20% in the elastic regime for both the parallel and perpendicular measurements. These values were within the range of biological cartilage values. Cytotoxicity tests performed with Vero cells confirmed the cytocompatibility.

CONCLUSIONS

We printed a partial tracheal model using optimized printing conditions and proved that the materials and methods developed may be useful for printing of organ models to support surgery or even to produce customized tracheal implants, after further optimization.

Biopolymers as bone substitutes: a review

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

3D bioprinted hydrogel model incorporating β-tricalcium phosphate for calcified cartilage tissue engineering

One promising strategy to reconstruct osteochondral defects relies on 3D bioprinted three-zonal structures comprised of hyaline cartilage, calcified cartilage, and subchondral bone. So far, several studies have pursued the regeneration of either hyaline cartilage or bone in vitro while-despite its key role in the osteochondral region-only few of them have targeted the calcified layer. In this work, we present a 3D biomimetic hydrogel scaffold containing β-tricalcium phosphate (TCP) for engineering calcified cartilage through a co-axial needle system implemented in extrusion-based bioprinting process. After a thorough bioink optimization, we showed that 0.5% w/v TCP is the optimal concentration forming stable scaffolds with high shape fidelity and endowed with biological properties relevant for the development of calcified cartilage. In particular, we investigate the effect induced by ceramic nano-particles over the differentiation capacity of bioprinted bone marrow-derived human mesenchymal stem cells in hydrogel scaffolds cultured up to 21 d in chondrogenic media. To confirm the potential of the presented approach to generate a functional in vitro model of calcified cartilage tissue, we evaluated quantitatively gene expression of relevant chondrogenic (COL1, COL2, COL10A1, ACAN) and osteogenic (ALPL, BGLAP) gene markers by means of RT-qPCR and qualitatively by means of fluorescence immunocytochemistry.

Advances in personalized treatment of metastatic spine disease

The spine is one of the most common sites of bony metastases, and its involvement leads to significant patient morbidity. Surgical management in these patients is aimed at improving quality of life and functional status throughout the course of the disease. Resection of metastases often leads to critical size bone defects, presenting a challenge to achieving adequate bone regeneration to fill the void. Current treatment options for repairing these defects are bone grafting and commercial bone cements; however, each has associated limitations. Additionally, tumor recurrence and tumor-induced bone loss make bone regeneration particularly difficult. Systemic therapeutic delivery, such as bisphosphonates, have become standard of care to combat bone loss despite unfavorable systemic side-effects and lack of local efficacy. Developments from tissue engineering have introduced novel materials with osteoinductive and osteoconductive properties which also act as structural support scaffolds for bone regeneration. These new materials can also act as a therapeutic reservoir to sustainably release drugs locally as an alternative to systemic therapy. In this review, we outline recent advancements in tissue engineering and the role of translational research in developing implants that can fully repair bone defects while also delivering local therapeutics to curb tumor recurrence and improve patient quality of life.

Three-Dimensional Bioprinting of Cell-Laden Constructs Using Polysaccharide-Based Self-Healing Hydrogels

Development of biomaterial-based bioinks is critical for replacement and/or regeneration of tissues and organs by three-dimensional (3D) printing techniques. However, the number of 3D-printable biomaterials in practical use remains limited despite the rapid development of 3D printing techniques. Controlling the flow properties of bioinks and mechanical properties of the resultant printed objects is key considerations in the design of biomaterial-based bioinks for practical applications. In this study, a printable hydrogel comprising biocompatible polysaccharides that has potential for cartilage regeneration via tissue engineering approaches was designed. Self-healing hydrogels were prepared from partially oxidized hyaluronate (OHA) and glycol chitosan (GC) in the presence of adipic acid dihydrazide (ADH). The self-healing ability of OHA/GC/ADH hydrogels was attributed to the combination of two dynamic bonds in the gels, including imine bonds obtained via a Schiff base reaction between OHA and GC, as well as acylhydrazone bonds formed by the reaction between OHA and ADH. The OHA/GC/ADH hydrogels did not require any postgelation or additional cross-linking processes for use in the fabrication of 3D constructs using an extrusion-based 3D printer. The concentrations and molecular weights of the constituent polymers were found to be critical parameters affecting the flow and mechanical properties of the self-healing hydrogels, which showed great potential as bioinks for fabricating cell-laden structures using a 3D printer. The expression of chondrogenic marker genes such as SOX-9 and collagen type II of ATDC5 cells encapsulated in the OHA/GC/ADH hydrogel was not significantly affected by the printing process. This self-healing hydrogel system may have the potential in tissue engineering applications, including cartilage regeneration.

In vitro evaluation of barium titanate nanoparticle/alginate 3D scaffold for osteogenic human stem cell differentiation

Nanomaterials can mimic properties of extracellular matrix molecules, promising great potential for scaffold composition in tissue engineering. In the present study, we investigated whether barium titanate nanoparticles (BT NP) combined with alginate polymer would provide a new cytocompatible three-dimensional (3D) scaffold to induce osteogenic stem cell differentiation. In vitro cytocompatibility and osteogenic differentiation potential were investigated using human mesenchymal stem cells (MSC). Firstly, we studied the cell viability and oxidative stress by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) thiazolyl blue tetrazolium bromide (MTT) and superoxide dismutase (SOD) assays. Overall, neither pure BT NP or BT NP/alginate 3D scaffold induced cytotoxicity. The scanning electron and atomic force microscopy revealed that BT NP/alginate 3D scaffold produced exhibited highly interconnected pores and surface nanotopography that were favorable for MSC differentiation. Von Kossa staining showed mineralization nodules and MSCs morphology changed from spindle to cuboid shape after 21 d. Finally, BMP-2 and ALP mRNA were significantly upregulated on cells grown into the BT NP/alginate 3D scaffold. Thus, the BT NP/alginate 3D scaffold showed an osteogenic differentiation induction potential, without the addition of osteogenic supplements. These results indicate that the BT NP/alginate 3D scaffold provides a cytocompatible and bioactive microenvironment for osteogenic human MSC differentiation.

Advancing Frontiers in Bone Bioprinting

Three-dimensional (3D) bioprinting of cell-laden biomaterials is used to fabricate constructs that can mimic the structure of native tissues. The main techniques used for 3D bioprinting include microextrusion, inkjet, and laser-assisted bioprinting. Bioinks used for bone bioprinting include hydrogels loaded with bioactive ceramics, cells, and growth factors. In this review, a critical overview of the recent literature on various types of bioinks used for bone bioprinting is presented. Major challenges, such as the vascularity, clinically relevant size, and mechanical properties of 3D printed structures, that need to be addressed to successfully use the technology in clinical settings, are discussed. Emerging approaches to solve these problems are reviewed, and future strategies to design customized 3D printed structures are proposed.

Three Dimensional Printed Bone Implants in the Clinic

Implants are being continuously developed to achieve personalized therapy. With the advent of 3-dimensional (3D) printing, it is becoming possible to produce customized precisely fitting implants that can be derived from 3D images fed into 3D printers. In addition, it is possible to combine various materials, such as ceramics, to render these constructs osteoconductive or growth factors to make them osteoinductive. Constructs can be seeded with cells to engineer bone tissue. Alternatively, it is possible to load cells into the biomaterial to form so called bioink and print them together to from 3D bioprinted constructs that are characterized by having more homogenous cell distribution in their matrix. To date, 3D printing was applied in the clinic mostly for surgical training and for planning of surgery, with limited use in producing 3D implants for clinical application. Few examples exist so far, which include mostly the 3D printed implants applied in maxillofacial surgery and in orthopedic surgery, which are discussed in this report. Wider clinical application of 3D printing will help the adoption of 3D printers as essential tools in the clinics in future and thus, contribute to realization of personalized medicine.

The summary of the most important cell-biomaterial interactions that need to be considered during in vitro biocompatibility testing of bone scaffolds for tissue engineering applications

Tissue engineered products (TEPs), which mean biomaterials containing either cells or growth factors or both cells and growth factors, may be used as an alternative to the autografts taken directly from the bone of the patients. Nevertheless, the use of TEPs needs much more understanding of biointeractions between biomaterials and eukaryotic cells. Despite the possibility of the use of in vitro cellular models for initial evaluation of the host response to the implanted biomaterial, it is observed that most researchers use cell cultures only for the evaluation of cytotoxicity and cell proliferation on the biomaterial surface, and then they proceed to animal models and in vivo testing of bone implants without fully utilizing the scientific potential of in vitro models. In this review, the most important biointeractions between eukaryotic cells and biomaterials were discussed, indicating molecular mechanisms of cell adhesion, proliferation, and biomaterial-induced activation of immune cells. The article also describes types of cellular models which are commonly used for biomaterial testing and highlights the possibilities and drawbacks of in vitro tests for biocompatibility evaluation of novel scaffolds. Finally, the review summarizes recent findings concerning the use of adult mesenchymal stem cells for TEP generation and compares the potential of bone marrow- and adipose tissue-derived stem cells in regenerative medicine applications.

Bioinks and bioprinting technologies to make heterogeneous and biomimetic tissue constructs

The native tissues are complex structures consisting of different cell types, extracellular matrix materials, and biomolecules. Traditional tissue engineering strategies have not been able to fully reproduce biomimetic and heterogeneous tissue constructs because of the lack of appropriate biomaterials and technologies. However, recently developed three-dimensional bioprinting techniques can be leveraged to produce biomimetic and complex tissue structures. To achieve this, multicomponent bioinks composed of multiple biomaterials (natural, synthetic, or hybrid natural-synthetic biomaterials), different types of cells, and soluble factors have been developed. In addition, advanced bioprinting technologies have enabled us to print multimaterial bioinks with spatial and microscale resolution in a rapid and continuous manner, aiming to reproduce the complex architecture of the native tissues. This review highlights important advances in heterogeneous bioinks and bioprinting technologies to fabricate biomimetic tissue constructs. Opportunities and challenges to further accelerate this research area are also described.

Incorporation of Conductive Materials into Hydrogels for Tissue Engineering Applications

In the field of tissue engineering, conductive hydrogels have been the most effective biomaterials to mimic the biological and electrical properties of tissues in the human body. The main advantages of conductive hydrogels include not only their physical properties but also their adequate electrical properties, which provide electrical signals to cells efficiently. However, when introducing a conductive material into a non-conductive hydrogel, a conflicting relationship between the electrical and mechanical properties may develop. This review examines the strengths and weaknesses of the generation of conductive hydrogels using various conductive materials such as metal nanoparticles, carbons, and conductive polymers. The fabrication method of blending, coating, and in situ polymerization is also added. Furthermore, the applications of conductive hydrogel in cardiac tissue engineering, nerve tissue engineering, and bone tissue engineering and skin regeneration are discussed in detail.

Integrating 3D-printed PHBV/Calcium sulfate hemihydrate scaffold and chitosan hydrogel for enhanced osteogenic property

We developed the 3D-printed poly(3-hydroxybutyrate-co-3-hydroxyvalerate)/calcium sulfate hemihydrate (PHBV/CaSH) scaffolds by using fused deposition modelling (FDM) technique and then coated the scaffolds with chitosan (CS) acetic acid solution. After drying and neutralization, CS hydrogel was formed on the surface of the scaffolds. The resultant PHBV/CaSH/CS scaffolds could promote the adhesion and proliferation of rat bone marrow stromal cells (rBMSCs) and enhance the osteogenesis of rBMSCs by up-regulating the expression level of osteogenic genes compared to the PHBV and PHBV/CaSH scaffolds. In vivo studies further demonstrated the PHBV/CaSH/CS scaffolds could effectively promote new bone formation. Therefore, integrating 3D-printed PHBV/CaSH scaffold and CS hrydogel represents a novel strategy to promote osteogensis property, showing full potential for bone defects repair.

[Research progress on chitosan composite scaffolds in bone tissue engineering]

Bone tissue engineering is a scientific field devoted to the development of materials that can repair or replace human bone tissue with biological and engineering methods. The stent, which provides structural support and adhesion sites for cell and tissue growth, is one of the key elements in tissue engineering. The scaffold may comprise metal, polymer, and ceramic biomaterial. The polymer scaffold is widely used due to its biocompatibility, biodegradability, and mechanical stability. Chitosan, as a natural polymer, is derived from chitin and has played a particularly important role in bone tissue engineering over the past two decades. In recent years, chitosan composites and their application in bone tissue engineering have received considerable attention due to their small foreign body reaction, excellent antibacterial properties, plasticity, suitability for inward cell growth, and bone conduction. This review will discuss the biocompatibility and osteogenesis research in vivo and in vitro of several common chitosan composites in bone tissue engineering.