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

3D Printing of Polysaccharide-Based Self-Healing Hydrogel Reinforced with Alginate for Secondary Cross-Linking

Three-dimensional (3D) bioprinting has been attractive for tissue and organ regeneration with the possibility of constructing biologically functional structures useful in many biomedical applications. Autonomous healing of hydrogels composed of oxidized hyaluronate (OHA), glycol chitosan (GC), and adipic acid dihydrazide (ADH) was achieved after damage. Interestingly, the addition of alginate (ALG) to the OHA/GC/ADH self-healing hydrogels was useful for the dual cross-linking system, which enhanced the structural stability of the gels without the loss of their self-healing capability. Various characteristics of OHA/GC/ADH/ALG hydrogels, including viscoelastic properties, cytotoxicity, and 3D printability, were investigated. Additionally, potential applications of 3D bioprinting of OHA/GC/ADH/ALG hydrogels for cartilage regeneration were investigated in vitro. This hydrogel system may have potential for bioprinting of a custom-made scaffold in various tissue engineering applications.

Hyaluronic Acid Hydrogels Crosslinked in Physiological Conditions: Synthesis and Biomedical Applications

Hyaluronic acid (HA) hydrogels display a wide variety of biomedical applications ranging from tissue engineering to drug vehiculization and controlled release. To date, most of the commercially available hyaluronic acid hydrogel formulations are produced under conditions that are not compatible with physiological ones. This review compiles the currently used approaches for the development of hyaluronic acid hydrogels under physiological/mild conditions. These methods include dynamic covalent processes such as boronic ester and Schiff-base formation and click chemistry mediated reactions such as thiol chemistry processes, azide-alkyne, or Diels Alder cycloaddition. Thermoreversible gelation of HA hydrogels at physiological temperature is also discussed. Finally, the most outstanding biomedical applications are indicated for each of the HA hydrogel generation approaches.

Recent Trends and Innovation in Additive Manufacturing of Soft Functional Materials

The growing demand for wearable devices, soft robotics, and tissue engineering in recent years has led to an increased effort in the field of soft materials. With the advent of personalized devices, the one-shape-fits-all manufacturing methods may soon no longer be the standard for the rapidly increasing market of soft devices. Recent findings have pushed technology and materials in the area of additive manufacturing (AM) as an alternative fabrication method for soft functional devices, taking geometrical designs and functionality to greater heights. For this reason, this review aims to highlights recent development and advances in AM processable soft materials with self-healing, shape memory, electronic, chromic or any combination of these functional properties. Furthermore, the influence of AM on the mechanical and physical properties on the functionality of these materials is expanded upon. Additionally, advances in soft devices in the fields of soft robotics, biomaterials, sensors, energy harvesters, and optoelectronics are discussed. Lastly, current challenges in AM for soft functional materials and future trends are discussed.

Hydrogel, a novel therapeutic and delivery strategy, in the treatment of intrauterine adhesions

Intrauterine adhesions (IUAs) are caused by damage to the underlying lining of the endometrium. They' re related to disorder of endometrial repair. In recent years, hydrogels with controllable biological activity have been widely used for treating IUAs. They encapsulate estrogen, cytokines, cells, or exosomes, forming a delivery system to release therapeutic components for the treatment of IUAs. In addition, the hydrogel acting as a barrier can be degraded in the body automatically, reducing the risk of infection caused by secondary surgeries. In this review, we summarize the recent progress of hydrogels and their application in IUAs as both a novel alternative therapeutic and an artificial delivery strategy.

Feasibility study of oxidized hyaluronic acid cross-linking acellular bovine pericardium with potential application for abdominal wall repair

Bovine pericardium(BP)is one of the biological membranes with extensive application in tissue engineering. To fully investigate the potential clinical applications of this natural biological material, a suitable cross-linking reagent is hopefully adopted for modification. Glutaraldehyde (GA) is a clinically most common synthetic cross-linking reagent. In the study, oxidized hyaluronic acid (AHA) was developed to substitute GA to fix acellular bovine pericardium (ABP) for lower cytotoxicity, aiming to evaluate the feasibility of AHA as a cross-linking reagent and develop AHA-fixed ABP as a biological patch for abdominal wall repair. The AHA with the feeding ratio (1.8:1.0) has an appropriate molecular weight and oxidation degree, almost no cytotoxicity and good cross-linking effect. The critical cross-linking characteristics and cytocompatibility of AHA-fixed ABP were also investigated. The results demonstrated that 2.0% AHA-fixed ABP had the most suitable mechanical properties, thermal stability, resistance to enzymatic degradation and hydrophilicity. Moreover, 2.0% AHA-fixed samples exhibited an excellent cytocompatibility with human peritoneal mesothelial cells (HPMC) and low antigenicity. It also showed a prominent anti-calcification ability required for abdominal wall repair. Our data provided experimental basis for future research on AHA as a new cross-linking reagent and AHA-fixed ABP for abdominal wall repair.

Review of Low-Cost 3D Bioprinters: State of the Market and Observed Future Trends

Three-dimensional (3D) bioprinting has become mainstream for precise and repeatable high-throughput fabrication of complex cell cultures and tissue constructs in drug testing and regenerative medicine, food products, dental and medical implants, biosensors, and so forth. Due to this tremendous growth in demand, an overwhelming amount of hardware manufacturers have recently flooded the market with different types of low-cost bioprinter models-a price segment that is most affordable to typical-sized laboratories. These machines range in sophistication, type of the underlying printing technology, and possible add-ons/features, which makes the selection process rather daunting (especially for a nonexpert customer). Yet, the review articles available in the literature mostly focus on the technical aspects of the printer technologies under development, as opposed to explaining the differences in what is already on the market. In contrast, this paper provides a snapshot of the fast-evolving low-cost bioprinter niche, as well as reputation profiles (relevant to delivery time, part quality, adherence to specifications, warranty, maintenance, etc.) of the companies selling these machines. Specifically, models spanning three dominant technologies-microextrusion, droplet-based/inkjet, and light-based/crosslinking-are reviewed. Additionally, representative examples of high-end competitors (including up-and-coming microfluidics-based bioprinters) are discussed to highlight their major differences and advantages relative to the low-cost models. Finally, forecasts are made based on the trends observed during this survey, as to the anticipated trickling down of the high-end technologies to the low-cost printers. Overall, this paper provides insight for guiding buyers on a limited budget toward making informed purchasing decisions in this fast-paced market.

Oxi-HA/ADH Hydrogels: A Novel Approach in Tissue Engineering and Regenerative Medicine

Hyaluronic acid (HA) is a natural polyelectrolyte abundant in mammalian connective tissues, such as cartilage and skin. Both endogenous and exogenous HA produced by fermentation have similar physicochemical, rheological, and biological properties, leading to medical and dermo-cosmetic products. Chemical modifications such as cross-linking or conjugation in target groups of the HA molecule improve its properties and in vivo stability, expanding its applications. Currently, HA-based scaffolds and matrices are of great interest in tissue engineering and regenerative medicine. However, the partial oxidation of the proximal hydroxyl groups in HA to electrophilic aldehydes mediated by periodate is still rarely investigated. The introduced aldehyde groups in the HA backbone allow spontaneous cross-linking with adipic dihydrazide (ADH), thermosensitivity, and noncytotoxicity to the hydrogels, which are advantageous for medical applications. This review provides an overview of the physicochemical properties of HA and its usual chemical modifications to better understand oxi-HA/ADH hydrogels, their functional properties modulated by the oxidation degree and ADH concentration, and the current clinical research. Finally, it discusses the development of biomaterials based on oxi-HA/ADH as a novel approach in tissue engineering and regenerative medicine.

Designing Hydrogels for 3D Cell Culture Using Dynamic Covalent Crosslinking

Designing simple biomaterials to replicate the biochemical and mechanical properties of tissues is an ongoing challenge in tissue engineering. For several decades, new biomaterials have been engineered using cytocompatible chemical reactions and spontaneous ligations via click chemistries to generate scaffolds and water swollen polymer networks, known as hydrogels, with tunable properties. However, most of these materials are static in nature, providing only macroscopic tunability of the scaffold mechanics, and do not reflect the dynamic environment of natural extracellular microenvironment. For more complex applications such as organoids or co-culture systems, there remain opportunities to investigate cells that locally remodel and change the physicochemical properties within the matrices. In this review, advanced biomaterials where dynamic covalent chemistry is used to produce stable 3D cell culture models and high-resolution constructs for both in vitro and in vivo applications, are discussed. The implications of dynamic covalent chemistry on viscoelastic properties of in vitro models are summarized, case studies in 3D cell culture are critically analyzed, and opportunities to further improve the performance of biomaterials for 3D tissue engineering are discussed.

Double network laminarin-boronic/alginate dynamic bioink for 3D bioprinting cell-laden constructs

The design of dynamically crosslinked hydrogel bioinks for three-dimensional (3D) bioprinting is emerging as a valuable strategy to advance the fabrication of mechanically tuneable cell-laden constructs for 3Din vitrodisease modelling and tissue engineering applications. Herein, a dynamic bioink comprising boronic acid-functionalised laminarin and alginate is explored for bioprinting 3D constructs under physiologically relevant conditions. The formulated bioink takes advantage of a double crosslinked network that combines covalent but reversible boronate ester bonds and ionic gelation via divalent cations. Moreover, it exhibits suitable rheological properties and improved mechanical features owing to its modular crosslinking chemistry, yielding stable constructs with user-programmable architecture. We explored such dynamic bioink as a supporting matrix for different cell classes, namely osteoblast precursors, fibroblasts and breast cancer cells. The resulting cell-laden bioprinted hydrogels display a homogeneous cell distribution post-printing and exceptional cell viability (>90%) that can be maintained for prolonged time periods in culture (14 days) for all cell lines. This simple and chemically versatile approach is envisaged to accelerate the development of multifunctional bioinks and contribute towards the fabrication of biomimetic 3D scaffolds with applicability in a wide range of predictive or exploratory biomedical platforms.

Advances in Injectable and Self-healing Polysaccharide Hydrogel Based on the Schiff Base Reaction

Injectable hydrogel possesses great application potential in disease treatment and tissue engineering, but damage to gel often occurs due to the squeezing pressure from injection devices and the mechanical forces from limb movement, and leads to the rapid degradation of gel matrix and the leakage of the load material. The self-healing injectable hydrogels can overcome these drawbacks via automatically repairing gel structural defects and restoring gel function. The polysaccharide hydrogels constructed through the Schiff base reaction own advantages including simple fabrication, injectability, and self-healing under physiological conditions, and therefore have drawn extensive attention and investigation recently. In this short review, the preparation and self-healing properties of the polysaccharide hydrogels that is established on the Schiff base reaction are focused on and their biological applications in drug delivery and cell therapy are discussed.

A structure-supporting, self-healing, and high permeating hydrogel bioink for establishment of diverse homogeneous tissue-like constructs

The ready-to-use, structure-supporting hydrogel bioink can shorten the time for ink preparation, ensure cell dispersion, and maintain the preset shape/microstructure without additional assistance during printing. Meanwhile, ink with high permeability might facilitate uniform cell growth in biological constructs, which is beneficial to homogeneous tissue repair. Unfortunately, current bioinks are hard to meet these requirements simultaneously in a simple way. Here, based on the fast dynamic crosslinking of aldehyde hyaluronic acid (AHA)/N-carboxymethyl chitosan (CMC) and the slow stable crosslinking of gelatin (GEL)/4-arm poly(ethylene glycol) succinimidyl glutarate (PEG-SG), we present a time-sharing structure-supporting (TSHSP) hydrogel bioink with high permeability, containing 1% AHA, 0.75% CMC, 1% GEL and 0.5% PEG-SG. The TSHSP hydrogel can facilitate printing with proper viscoelastic property and self-healing behavior. By crosslinking with 4% PEG-SG for only 3 min, the integrity of the cell-laden construct can last for 21 days due to the stable internal and external GEL/PEG-SG networks, and cells manifested long-term viability and spreading morphology. Nerve-like, muscle-like, and cartilage-like in vitro constructs exhibited homogeneous cell growth and remarkable biological specificities. This work provides not only a convenient and practical bioink for tissue engineering, targeted cell therapy, but also a new direction for hydrogel bioink development.

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A time-sharing structure-supporting (TSHSP) bioink based on gelation time difference between two gelling systems.

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The high permeability of TSHSP hydrogel is the basis for effective matter exchange.

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The TSHSP hydrogel facilitates room temperature printing with proper viscoelastic property and self-healing behavior.

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Cells manifest long-term viability and spreading morphology in bioprinted TSHSP constructs.

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In vitro tissue-like TSHSP constructs exhibit homogeneous cell growth and remarkable biological specificities.

Recent advances in bioprinting technologies for engineering different cartilage-based tissues

Inadequate self-repair and regenerative efficiency of the cartilage tissues has motivated the researchers to devise advanced and effective strategies to resolve this issue. Introduction of bioprinting to tissue engineering has paved the way for fabricating complex biomimetic engineered constructs. In this context, the current review gears off with the discussion of standard and advanced 3D/4D printing technologies and their implications for the repair of different cartilage tissues, namely, articular, meniscal, nasoseptal, auricular, costal, and tracheal cartilage. The review is then directed towards highlighting the current stem cell opportunities. On a concluding note, associated critical issues and prospects for future developments, particularly in this sphere of personalized medicines have been discussed.

Supramolecular Biopolymers for Tissue Engineering

Supramolecular biopolymers (SBPs) are those polymeric units derived from macromolecules that can assemble with each other by noncovalent interactions. Macromolecular structures are commonly found in living systems such as proteins, DNA/RNA, and polysaccharides. Bioorganic chemistry allows the generation of sequence-specific supramolecular units like SBPs that can be tailored for novel applications in tissue engineering (TE). SBPs hold advantages over other conventional polymers previously used for TE; these materials can be easily functionalized; they are self-healing, biodegradable, stimuli-responsive, and nonimmunogenic. These characteristics are vital for the further development of current trends in TE, such as the use of pluripotent cells for organoid generation, cell-free scaffolds for tissue regeneration, patient-derived organ models, and controlled delivery systems of small molecules. In this review, we will analyse the 3 subtypes of SBPs: peptide-, nucleic acid-, and oligosaccharide-derived. Then, we will discuss the role that SBPs will be playing in TE as dynamic scaffolds, therapeutic scaffolds, and bioinks. Finally, we will describe possible outlooks of SBPs for TE.

Stem Cells and Extrusion 3D Printing for Hyaline Cartilage Engineering

Hyaline cartilage is deficient in self-healing properties. The early treatment of focal cartilage lesions is a public health challenge to prevent long-term degradation and the occurrence of osteoarthritis. Cartilage tissue engineering represents a promising alternative to the current insufficient surgical solutions. 3D printing is a thriving technology and offers new possibilities for personalized regenerative medicine. Extrusion-based processes permit the deposition of cell-seeded bioinks, in a layer-by-layer manner, allowing mimicry of the native zonal organization of hyaline cartilage. Mesenchymal stem cells (MSCs) are a promising cell source for cartilage tissue engineering. Originally isolated from bone marrow, they can now be derived from many different cell sources (e.g., synovium, dental pulp, Wharton’s jelly). Their proliferation and differentiation potential are well characterized, and they possess good chondrogenic potential, making them appropriate candidates for cartilage reconstruction. This review summarizes the different sources, origins, and densities of MSCs used in extrusion-based bioprinting (EBB) processes, as alternatives to chondrocytes. The different bioink constituents and their advantages for producing substitutes mimicking healthy hyaline cartilage is also discussed.

Hydrogel Properties and Their Impact on Regenerative Medicine and Tissue Engineering

Hydrogels (HGs), as three-dimensional structures, are widely used in modern medicine, including regenerative medicine. The use of HGs in wound treatment and tissue engineering is a rapidly developing sector of medicine. The unique properties of HGs allow researchers to easily modify them to maximize their potential. Herein, we describe the physicochemical properties of HGs, which determine their subsequent applications in regenerative medicine and tissue engineering. Examples of chemical modifications of HGs and their applications are described based on the latest scientific reports.

A Versatile Open-Source Printhead for Low-Cost 3D Microextrusion-Based Bioprinting

Three-dimensional (3D) bioprinting promises to be essential in tissue engineering for solving the rising demand for organs and tissues. Some bioprinters are commercially available, but their impact on the field of Tissue engineering (TE) is still limited due to their cost or difficulty to tune. Herein, we present a low-cost easy-to-build printhead for microextrusion-based bioprinting (MEBB) that can be installed in many desktop 3D printers to transform them into 3D bioprinters. We can extrude bioinks with precise control of print temperature between 2-60 °C. We validated the versatility of the printhead, by assembling it in three low-cost open-source desktop 3D printers. Multiple units of the printhead can also be easily put together in a single printer carriage for building a multi-material 3D bioprinter. Print resolution was evaluated by creating representative calibration models at different temperatures using natural hydrogels such as gelatin and alginate, and synthetic ones like poloxamer. Using one of the three modified low-cost 3D printers, we successfully printed cell-laden lattice constructs with cell viabilities higher than 90% after 24-h post printing. Controlling temperature and pressure according to the rheological properties of the bioinks was essential in achieving optimal printability and great cell viability. The cost per unit of our device, which can be used with syringes of different volume, is less expensive than any other commercially available product. These data demonstrate an affordable open-source printhead with the potential to become a reliable alternative to commercial bioprinters for any laboratory.

Effects of Processing Parameters of 3D Bioprinting on the Cellular Activity of Bioinks

In this review, few established cell printing techniques along with their parameters that affect the cell viability during bioprinting are considered. 3D bioprinting is developed on the principle of additive manufacturing using biomaterial inks and bioinks. Different bioprinting methods impose few challenges on cell printing such as shear stress, mechanical impact, heat, laser radiation, etc., which eventually lead to cell death. These factors also cause alteration of cells phenotype, recoverable or irrecoverable damages to the cells. Such challenges are not addressed in detail in the literature and scientific reports. Hence, this review presents a detailed discussion of several cellular bioprinting methods and their process-related impacts on cell viability, followed by probable mitigation techniques. Most of the printable bioinks encompass cells within hydrogel as scaffold material to avoid the direct exposure of the harsh printing environment on cells. However, the advantages of printing with scaffold-free cellular aggregates over cell-laden hydrogels have emerged very recently. Henceforth, optimal and favorable crosslinking mechanisms providing structural rigidity to the cell-laden printed constructs with ideal cell differentiation and proliferation, are discussed for improved understanding of cell printing methods for the future of organ printing and transplantation.

Self-crosslinking hyaluronic acid-carboxymethylcellulose hydrogel enhances multilayered 3D-printed construct shape integrity and mechanical stability for soft tissue engineering

One of the primary challenges in extrusion-based 3D bioprinting is the ability to print self-supported multilayered constructs with biocompatible hydrogels. The bioinks should have sufficient post-printing mechanical stability for soft tissue and organ regeneration. Here, we report on the synthesis, characterization and 3D printability of hyaluronic acid (HA)-carboxymethylcellulose (CMC) hydrogels cross-linked through N-acyl-hydrazone bonding. The hydrogel's hydrolytic stability was acquired by the effects of both the prevention of the oxidation of the six-membered rings of HA, and the stabilization of acyl-hydrazone bonds. The shear-thinning and self-healing properties of the hydrogel allowed us to print different 3D constructs (lattice, cubic and tube) of up to 50 layers with superior precision and high post-printing stability without support materials or post-processing depending on their compositions (H7:C3, H5:C5 and H3:C7). Morphological analyses of different zones of the 3D-printed constructs were undertaken for verification of the interconnection of pores. Texture profile analysis (TPA) (hardness (strength), elastic recovery, etc) and cyclic compression studies of the 3D-printed constructs demonstrated exceptional elastic properties and fast recovery after 50% strain, respectively, which have been attributed to the addition of CMC into HA. A model drug quercetin was released in a sustained manner from hydrogels and 3D constructs. In vitro cytotoxicity studies confirmed the excellent cyto-compatibility of these gels. In vivo mice studies prove that these biocompatible hydrogels enhance angiogenesis. The results indicate that controlling the key properties (e.g. self-crosslinking capacity, composition) can lead to the generation of multilayered constructs from 3D-bioprintable HA-CMC hydrogels capable of being leveraged for soft tissue engineering applications.

Crosslinking Strategies for 3D Bioprinting of Polymeric Hydrogels

Three-dimensional (3D) bioprinting has recently advanced as an important tool to produce viable constructs that can be used for regenerative purposes or as tissue models. To develop biomimetic and sustainable 3D constructs, several important processing aspects need to be considered, among which crosslinking is most important for achieving desirable biomechanical stability of printed structures, which is reflected in subsequent behavior and use of these constructs. In this work, crosslinking methods used in 3D bioprinting studies are reviewed, parameters that affect bioink chemistry are discussed, and the potential toward improving crosslinking outcomes and construct performance is highlighted. Furthermore, current challenges and future prospects are discussed. Due to the direct connection between crosslinking methods and properties of 3D bioprinted structures, this Review can provide a basis for developing necessary modifications to the design and manufacturing process of advanced tissue-like constructs in future.

Hydrogels for Bioprinting: A Systematic Review of Hydrogels Synthesis, Bioprinting Parameters, and Bioprinted Structures Behavior

Nowadays, bioprinting is rapidly evolving and hydrogels are a key component for its success. In this sense, synthesis of hydrogels, as well as bioprinting process, and cross-linking of bioinks represent different challenges for the scientific community. A set of unified criteria and a common framework are missing, so multidisciplinary research teams might not efficiently share the advances and limitations of bioprinting. Although multiple combinations of materials and proportions have been used for several applications, it is still unclear the relationship between good printability of hydrogels and better medical/clinical behavior of bioprinted structures. For this reason, a PRISMA methodology was conducted in this review. Thus, 1,774 papers were retrieved from PUBMED, WOS, and SCOPUS databases. After selection, 118 papers were analyzed to extract information about materials, hydrogel synthesis, bioprinting process, and tests performed on bioprinted structures. The aim of this systematic review is to analyze materials used and their influence on the bioprinting parameters that ultimately generate tridimensional structures. Furthermore, a comparison of mechanical and cellular behavior of those bioprinted structures is presented. Finally, some conclusions and recommendations are exposed to improve reproducibility and facilitate a fair comparison of results.

Advances in Extrusion 3D Bioprinting: A Focus on Multicomponent Hydrogel-Based Bioinks

3D bioprinting involves the combination of 3D printing technologies with cells, growth factors and biomaterials, and has been considered as one of the most advanced tools for tissue engineering and regenerative medicine (TERM). However, despite multiple breakthroughs, it is evident that numerous challenges need to be overcome before 3D bioprinting will eventually become a clinical solution for a variety of TERM applications. To produce a 3D structure that is biologically functional, cell-laden bioinks must be optimized to meet certain key characteristics including rheological properties, physico-mechanical properties, and biofunctionality; a difficult task for a single component bioink especially for extrusion based bioprinting. As such, more recent research has been centred on multicomponent bioinks consisting of a combination of two or more biomaterials to improve printability, shape fidelity and biofunctionality. In this article, multicomponent hydrogel-based bioink systems are systemically reviewed based on the inherent nature of the bioink (natural or synthetic hydrogels), including the most current examples demonstrating properties and advances in application of multicomponent bioinks, specifically for extrusion based 3D bioprinting. This review article will assist researchers in the field in identifying the most suitable bioink based on their requirements, as well as pinpointing current unmet challenges in the field.

Composite Hydrogels in Three-Dimensional in vitro Models

3-dimensional (3D) in vitro models were developed in order to mimic the complexity of real organ/tissue in a dish. They offer new possibilities to model biological processes in more physiologically relevant ways which can be applied to a myriad of applications including drug development, toxicity screening and regenerative medicine. Hydrogels are the most relevant tissue-like matrices to support the development of 3D in vitro models since they are in many ways akin to the native extracellular matrix (ECM). For the purpose of further improving matrix relevance or to impart specific functionalities, composite hydrogels have attracted increasing attention. These could incorporate drugs to control cell fates, additional ECM elements to improve mechanical properties, biomolecules to improve biological activities or any combinations of the above. In this Review, recent developments in using composite hydrogels laden with cells as biomimetic tissue- or organ-like constructs, and as matrices for multi-cell type organoid cultures are highlighted. The latest composite hydrogel systems that contain nanomaterials, biological factors, and combinations of biopolymers (e.g., proteins and polysaccharide), such as Interpenetrating Networks (IPNs) and Soft Network Composites (SNCs) are also presented. While promising, challenges remain. These will be discussed in light of future perspectives toward encompassing diverse composite hydrogel platforms for an improved organ environment in vitro.

3D Bioprinting of Carbohydrazide-Modified Gelatin into Microparticle-Suspended Oxidized Alginate for the Fabrication of Complex-Shaped Tissue Constructs

Extrusion-based bioprinting of hydrogels in a granular secondary gel enables the fabrication of cell-laden three-dimensional (3D) constructs in an anatomically accurate manner, which is challenging using conventional extrusion-based bioprinting processes. In this study, carbohydrazide-modified gelatin (Gel-CDH) was synthesized and deposited into a new multifunctional support bath consisting of gelatin microparticles suspended in an oxidized alginate (OAlg) solution. During extrusion, Gel-CDH and OAlg were rapidly cross-linked because of the Schiff base formation between aldehyde groups of OAlg and amino groups of Gel-CDH, which has not been demonstrated in the domain of 3D bioprinting before. Rheological results indicated that hydrogels with lower OAlg to Gel-CDH ratios possessed superior mechanical rigidity. Different 3D geometrically intricate constructs were successfully created upon the determination of optimal bioprinting parameters. Human mesenchymal stem cells and human umbilical vein endothelial cells were also bioprinted at physiologically relevant cell densities. The presented study has offered a novel strategy for bioprinting of natural polymer-based hydrogels into 3D complex-shaped biomimetic constructs, which eliminated the need for cytotoxic supplements as external cross-linkers or additional cross-linking processes, therefore expanding the availability of bioinks.

Dynamic Covalent Polymers for Biomedical Applications

The rapid development of supramolecular polymer chemistry and constitutional dynamic chemistry over the last decades has made tremendous impact on the emergence of dynamic covalent polymers. These materials are formed through reversible covalent bonds, endowing them with adaptive and responsive features that have resulted in high interest throughout the community. Owing to their intriguing properties, such as self-healing, shape-memory effects, recyclability, degradability, stimuli-responsiveness, etc., the materials have found multiple uses in a wide range of areas. Of special interest is their increasing use for biomedical applications, and many examples have been demonstrated in recent years. These materials have thus been used for the recognition and sensing of biologically active compounds, for the modulation of enzyme activity, for gene delivery, and as materials for cell culture, delivery, and wound-dressing. In this review, some of these endeavors are discussed, highlighting the many advantages and unique properties of dynamic covalent polymers for use in biology and biomedicine.

Polysaccharide-based recoverable double-network hydrogel with high strength and self-healing properties

Polysaccharide-based hydrogels (PSBHs) have received significant attention for numerous bio-applications due to their biocompatibility and non-immunogenic performance. However, the construction of PSBH with superior mechanical properties by a simple method is rarely adequately researched. This study focuses on the construction of a novel PSBH with superior mechanical and recoverable properties by integrating the synergistic and complementary interactions of covalent bond-associated oxidized sodium alginate (SA-CHO) gel and hydrogen bond-associated agarose (Aga) gel. With the synergy and complementarity of the SA-CHO and Aga networks, the hydrogel exhibited 17 and 15 times (20 and 9 times) greater compressive stress and modulus, respectively, compared with the SA-CHO gel (Aga gel). The hydrogel also displayed excellent fatigue resistance, recurrent shapeability, acid resistance and recovery ability, as well as self-healing ability. This study provides a unique perspective for enhancing the mechanical properties of PSBH through the synergy and complementarity of different kinds of polysaccharides without sacrificing the functionality of the PSBH.

Self-Healing Polyurethane Elastomers Based on a Disulfide Bond by Digital Light Processing 3D Printing

A type of polyurethane elastomer with excellent self-healing ability has been fabricated through digital light processing 3D printing. First, a type of polyurethane acrylate containing disulfide bonds is synthesized and then compounded with reactive diluent and photoinitiators to get a photopolymer resin. Due to the good fluidity and high curing rate, the photopolymer resin can be applied in DLP 3D printing, and various 3D objects with complicated structures, high printing accuracy, and remarkable self-healing ability have been printed. The tensile strength and elongation at break of the polyurethane elastomer are 3.39 ± 0.09 MPa and 400.38 ± 14.26%, respectively, and the healing efficiency can get to 95% after healing at 80 °C for 12 h and can be healed for multiple times. With the ease of fabrication and excellent performance, the polyurethane elastomers from DLP 3D printing have great potential applications in flexible electronics, soft robotics, and sensors.

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.

Smart polymers for cell therapy and precision medicine

Soft materials have been developed very rapidly in the biomedical field over the past 10 years because of advances in medical devices, cell therapy, and 3D printing for precision medicine. Smart polymers are one category of soft materials that respond to environmental changes. One typical example is the thermally-responsive polymers, which are widely used as cell carriers and in 3D printing. Self-healing polymers are one type of smart polymers that have the capacity to recover the structure after repeated damages and are often injectable through needles. Shape memory polymers are another type with the ability to memorize their original shape. These smart polymers can be used as cell/drug/protein carriers. Their injectability and shape memory performance allow them to be applied in bioprinting, minimally invasive surgery, and precision medicine. This review will describe the general materials design, characterization, as well as the current progresses and challenges of these smart polymers.

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.

Supramolecular and dynamic covalent hydrogel scaffolds: from gelation chemistry to enhanced cell retention and cartilage regeneration

This review highlights the most recent progress in gelation strategies of biomedical supramolecular and dynamic covalent crosslinking hydrogels and their applications for enhancing cell retention and cartilage regeneration.