Nanoengineered Granular Hydrogel Bioinks with Preserved Interconnected Microporosity for Extrusion Bioprinting

4D Printing for Biomedical Applications

4D (bio-)printing endows 3D printed (bio-)materials with multiple functionalities and dynamic properties. 4D printed materials have been recently used in biomedical engineering for the design and fabrication of biomedical devices, such as stents, occluders, microneedles, smart 3D-cell engineered microenvironments, drug delivery systems, wound closures, and implantable medical devices. However, the success of 4D printing relies on the rational design of 4D printed objects, the selection of smart materials, and the availability of appropriate types of external (multi-)stimuli. Here, this work first highlights the different types of smart materials, external stimuli, and design strategies used in 4D (bio-)printing. Then, it presents a critical review of the biomedical applications of 4D printing and discusses the future directions of biomedical research in this exciting area, including in vivo tissue regeneration studies, the implementation of multiple materials with reversible shape memory behaviors, the creation of fast shape-transformation responses, the ability to operate at the microscale, untethered activation and control, and the application of (machine learning-based) modeling approaches to predict the structure-property and design-shape transformation relationships of 4D (bio)printed constructs.

New light-illuminated silk road: emerging silk fibroin-based optical biomedical sensors

Biomedical silk protein optics has become the subject of intensive research aimed at solving the challenges associated with traditional medical devices in terms of biocompatibility and performance balance. With its significant potential for biomedical applications in the field of drug storage and wound monitoring, it is dedicated to reducing the perturbation of neighbouring tissues. The transparency and biocompatibility of silk proteins make them ideal materials in the field of optical device fabrication, effectively overcoming the challenges posed by conventional materials. In this paper, we explore in detail the complex aspects of the design, synthesis and application related to biomedical silk protein optical devices and comprehensively analyse the potential use of silk protein-centric microstructures (e.g., micropillars, microneedles, and photonic crystals) in the development of optical devices. This review also offers insights into the challenges of applying silk protein optical devices in healthcare and their future trends, aiming to provide a comprehensive overview of the advances, potential impacts and emerging research directions in the field of biomedical silk protein optical devices.

Microgels for Cell Delivery in Tissue Engineering and Regenerative Medicine

Microgels prepared from natural or synthetic hydrogel materials have aroused extensive attention as multifunctional cells or drug carriers, that are promising for tissue engineering and regenerative medicine. Microgels can also be aggregated into microporous scaffolds, promoting cell infiltration and proliferation for tissue repair. This review gives an overview of recent developments in the fabrication techniques and applications of microgels. A series of conventional and novel strategies including emulsification, microfluidic, lithography, electrospray, centrifugation, gas-shearing, three-dimensional bioprinting, etc. are discussed in depth. The characteristics and applications of microgels and microgel-based scaffolds for cell culture and delivery are elaborated with an emphasis on the advantages of these carriers in cell therapy. Additionally, we expound on the ongoing and foreseeable applications and current limitations of microgels and their aggregate in the field of biomedical engineering. Through stimulating innovative ideas, the present review paves new avenues for expanding the application of microgels in cell delivery techniques.

Design approaches for 3D cell culture and 3D bioprinting platforms

The natural habitat of most cells consists of complex and disordered 3D microenvironments with spatiotemporally dynamic material properties. However, prevalent methods of in vitro culture study cells under poorly biomimetic 2D confinement or homogeneous conditions that often neglect critical topographical cues and mechanical stimuli. It has also become increasingly apparent that cells in a 3D conformation exhibit dramatically altered morphological and phenotypical states. In response, efforts toward designing biomaterial platforms for 3D cell culture have taken centerstage over the past few decades. Herein, we present a broad overview of biomaterials for 3D cell culture and 3D bioprinting, spanning both monolithic and granular systems. We first critically evaluate conventional monolithic hydrogel networks, with an emphasis on specific experimental requirements. Building on this, we document the recent emergence of microgel-based 3D growth media as a promising biomaterial platform enabling interrogation of cells within porous and granular scaffolds. We also explore how jammed microgel systems have been leveraged to spatially design and manipulate cellular structures using 3D bioprinting. The advent of these techniques heralds an unprecedented ability to experimentally model complex physiological niches, with important implications for tissue bioengineering and biomedical applications.

3D Bioprinting of Food Grade Hydrogel Infused with Living Pleurotus ostreatus Mycelium in Non-sterile Conditions

Mycelium is the root-like network of fungi. Mycelium biocomposites prepared by template replication (molding) can function as environmentally friendly alternatives to conventional polystyrene foams, which are energy- and carbon-intensive to manufacture. Recently, several studies have shown that 3D bioprinting technologies can be used to produce high value functional mycelium products with intricate geometries that are otherwise difficult or impossible to achieve via template replication. A diverse range of nutrients, thickeners, and gelling agents can be combined to produce hydrogels suitable for 3D bioprinting. 3D bioprinting with hydrogel formulations infused with living fungi produces engineered living materials that continue to grow after bioprinting is complete. However, a hydrogel formulation optimized for intricate 3D bioprinting of Pleurotus ostreatus mycelium, which is among the strains most commonly used in mycelium biocomposite fabrication, has yet to be described. Here, we design and evaluate a versatile hydrogel formulation consisting of malt extract (nutrient), carboxymethylcellulose and cornstarch (thickeners), and agar (gelling agent), all of which are easily sourced food grade reagents. We also outline a reproducible workflow to infuse this hydrogel with P. ostreatus liquid culture for 3D bioprinting of intricate structures comprised of living P. ostreatus mycelium and characterize the changes in height and mass as well as hardness of the prints during mycelium growth. Finally, we demonstrate that the workflow does not require a sterile bioprinting environment to achieve successful prints and that the same mycelium-infused hydrogel can be supplemented with additives such as sawdust to produce mycelium biocomposite objects. These findings demonstrate that 3D bioprinting using mycelium-based feedstocks could be a promising biofabrication technique to produce engineered living materials for applications such as mushroom cultivation, food preparation, or construction of the built environment.

Granular Biphasic Colloidal Hydrogels for 3D Bioprinting

Granular hydrogels composed of hydrogel microparticles are promising candidates for 3D bioprinting due to their ability to protect encapsulated cells. However, to achieve high print fidelity, hydrogel microparticles need to jam to exhibit shear-thinning characteristics, which is crucial for 3D printing. Unfortunately, this overpacking can significantly impact cell viability, thereby negating the primary advantage of using hydrogel microparticles to shield cells from shear forces. To overcome this challenge, a novel solution: a biphasic, granular colloidal bioink designed to optimize cell viability and printing fidelity is introduced. The biphasic ink consists of cell-laden polyethylene glycol (PEG) hydrogel microparticles embedded in a continuous gelatin methacryloyl (GelMA)-nanosilicate colloidal network. Here, it is demonstrated that this biphasic bioink offers outstanding rheological properties, print fidelity, and structural stability. Furthermore, its utility for engineering complex tissues with multiple cell types and heterogeneous microenvironments is demonstrated, by incorporating β-islet cells into the PEG microparticles and endothelial cells in the GelMA-nanosilicate colloidal network. Using this approach, it is possible to induce cell patterning, enhance vascularization, and direct cellular function. The proposed biphasic bioink holds significant potential for numerous emerging biomedical applications, including tissue engineering and disease modeling.

Gelatin methacryloyl granular scaffolds for localized mRNA delivery

mRNA therapy is the intracellular delivery of messenger RNA (mRNA) to produce desired therapeutic proteins. Developing strategies for local mRNA delivery is still required where direct intra-articular injections are inappropriate for targeting a specific tissue. The mRNA delivery efficiency depends on protecting nucleic acids against nuclease-mediated degradation and safe site-specific intracellular delivery. Herein, we report novel mRNA-releasing matrices based on RGD-moiety-rich gelatin methacryloyl (GelMA) microporous annealed particle (MAP) scaffolds. GelMA concentration in aerogel-based microgels (μgels) produced through a microfluidic process, MAP stiffnesses, and microporosity are crucial parameters for cell adhesion, spreading, and proliferation. After being loaded with mRNA complexes, MAP scaffolds composed of 10 % GelMA μgels display excellent cell viability with increasing cell infiltration, adhesion, proliferation, and gene transfer. The intracellular delivery is achieved by the sustained release of mRNA complexes from MAP scaffolds and cell adhesion on mRNA-releasing scaffolds. These findings highlight that hybrid systems can achieve efficient protein expression by delivering mRNA complexes, making them promising mRNA-releasing biomaterials for tissue engineering.

Reasoning on Pore Terminology in 3D Bioprinting

Terminology is pivotal for facilitating clear communication and minimizing ambiguity, especially in specialized fields such as chemistry. In materials science, a subset of chemistry, the term "pore" is traditionally linked to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature, which categorizes pores into "micro", "meso", and "macro" based on size. However, applying this terminology in closely-related areas, such as 3D bioprinting, often leads to confusion owing to the lack of consensus on specific definitions and classifications tailored to each field. This review article critically examines the current use of pore terminology in the context of 3D bioprinting, highlighting the need for reassessment to avoid potential misunderstandings. We propose an alternative classification that aligns more closely with the specific requirements of bioprinting, suggesting a tentative size-based division of interconnected pores into 'parvo'-(d < 25 µm), 'medio'-(25 < d < 100 µm), and 'magno'-(d > 100 µm) pores, relying on the current understanding of the pore size role in tissue formation. The introduction of field-specific terminology for pore sizes in 3D bioprinting is essential to enhance the clarity and precision of research communication. This represents a step toward a more cohesive and specialized lexicon that aligns with the unique aspects of bioprinting and tissue engineering.

Accelerating Patterned Vascularization Using Granular Hydrogel Scaffolds and Surgical Micropuncture

Bulk hydrogel scaffolds are common in reconstructive surgery. They allow for the staged repair of soft tissue loss by providing a base for revascularization. Unfortunately, they are limited by both slow and random vascularization, which may manifest as treatment failure or suboptimal repair. Rapidly inducing patterned vascularization within biomaterials has profound translational implications for current clinical treatment paradigms and the scaleup of regenerative engineering platforms. To address this long-standing challenge, a novel microsurgical approach and granular hydrogel scaffold (GHS) technology are co-developed to hasten and pattern microvascular network formation. In surgical micropuncture (MP), targeted recipient blood vessels are perforated using a microneedle to accelerate cell extravasation and angiogenic outgrowth. By combining MP with an adjacent GHS with precisely tailored void space architecture, microvascular pattern formation as assessed by density, diameter, length, and intercapillary distance is rapidly guided. This work opens new translational opportunities for microvascular engineering, advancing reconstructive surgery, and regenerative medicine.

Iron inhibits glioblastoma cell migration and polarization

Glioblastoma is one of the deadliest malignancies facing modern oncology today. The ability of glioblastoma cells to diffusely spread into neighboring healthy brain makes complete surgical resection nearly impossible and contributes to the recurrent disease faced by most patients. Although research into the impact of iron on glioblastoma has addressed proliferation, there has been little investigation into how cellular iron impacts the ability of glioblastoma cells to migrate-a key question, especially in the context of the diffuse spread observed in these tumors. Herein, we show that increasing cellular iron content results in decreased migratory capacity of human glioblastoma cells. The decrease in migratory capacity was accompanied by a decrease in cellular polarization in the direction of movement. Expression of CDC42, a Rho GTPase that is essential for both cellular migration and establishment of polarity in the direction of cell movement, was reduced upon iron treatment. We then analyzed a single-cell RNA-seq dataset of human glioblastoma samples and found that cells at the tumor periphery had a gene signature that is consistent with having lower levels of cellular iron. Altogether, our results suggest that cellular iron content is impacting glioblastoma cell migratory capacity and that cells with higher iron levels exhibit reduced motility.

Injectable Microporous Annealed Crescent-Shaped (MAC) Particle Hydrogel Scaffold for Enhanced Cell Infiltration

Hydrogels are widely used for tissue engineering applications to support cellular growth, yet the tightly woven structure often restricts cell infiltration and expansion. Consequently, granular hydrogels with microporous architectures have emerged as a new class of biomaterial. Particularly, the development of microporous annealed particle (MAP) hydrogel scaffolds has shown improved stability and integration with host tissue. However, the predominant use of spherically shaped particles limits scaffold porosity, potentially limiting the level of cell infiltration. Here, a novel microporous annealed crescent-shaped particle (MAC) scaffold that is predicted to have improved porosity and pore interconnectivity in silico is presented. With microfluidic fabrication, tunable cavity sizes that optimize interstitial void space features are achieved. In vitro, cells incorporated into MAC scaffolds form extensive 3D multicellular networks. In vivo, the injectable MAC scaffold significantly enhances cell infiltration compared to spherical MAP scaffolds, resulting in increased numbers of myofibroblasts and leukocytes present within the gel without relying on external biomolecular chemoattractants. The results shed light on the critical role of particle shape in cell recruitment, laying the foundation for MAC scaffolds as a next-generation granular hydrogel for diverse tissue engineering applications.

Cell Microencapsulation within Gelatin-PEG Microgels Using a Simple Pipet Tip-Based Device

Microgels are microscale particles of hydrogel that can be laden with cells and used to create macroporous tissue constructs. Their ability to support cell-ECM and cell-cell interactions, along with the high levels of nutrient and metabolite exchange facilitated by their high surface area-to-volume ratio, means that they are attracting increasing attention for a variety of tissue regeneration applications. Here, we present methods for fabricating and modifying the structure of microfluidic devices using commonly available laboratory consumables including pipet tips and PTFE and silicon tubing to produce microgels. Different microfluidic devices realized the controlled generation of a wide size range (130-800 μm) of microgels for cell encapsulation. Subsequently, we describe the process of encapsulating mesenchymal stromal cells in microgels formed by photo-cross-linking of gelatin-norbornene and PEG dithiol. The introduced pipet-based chip offers simplicity, tunability, and versatility, making it easily assembled in most laboratories to effectively produce cell-laden microgels for various applications in tissue engineering.

Organ bioprinting: progress, challenges and outlook

Bioprinting, as a groundbreaking technology, enables the fabrication of biomimetic tissues and organs with highly complex structures, multiple cell types, mechanical heterogeneity, and diverse functional gradients. With the growing demand for organ transplantation and the limited number of organ donors, bioprinting holds great promise for addressing the organ shortage by manufacturing completely functional organs. While the bioprinting of complete organs remains a distant goal, there has been considerable progress in the development of bioprinted transplantable tissues and organs for regenerative medicine. This review article recapitulates the current achievements of organ 3D bioprinting, primarily encompassing five important organs in the human body (i.e., the heart, kidneys, liver, pancreas, and lungs). Challenges from cellular techniques, biomanufacturing technologies, and organ maturation techniques are also deliberated for the broad application of organ bioprinting. In addition, the integration of bioprinting with other cutting-edge technologies including machine learning, organoids, and microfluidics is envisioned, which strives to offer the reader the prospect of bioprinting in constructing functional organs.

3D Printable Gelatin Methacryloyl (GelMA)-Dextran Aqueous Two-Phase System with Tunable Pores Structure and Size Enables Physiological Behavior of Embedded Cells In Vitro

The restricted porosity of most hydrogels established for in vitro 3D tissue engineering applications limits embedded cells with regard to their physiological spreading, proliferation, and migration behavior. To overcome these confines, porous hydrogels derived from aqueous two-phase systems (ATPS) are an interesting alternative. However, while developing hydrogels with trapped pores is widespread, the design of bicontinuous hydrogels is still challenging. Herein, an ATPS consisting of photo-crosslinkable gelatin methacryloyl (GelMA) and dextran is presented. The phase behavior, monophasic or biphasic, is tuned via the pH and dextran concentration. This, in turn, allows the formation of hydrogels with three distinct microstructures: homogenous nonporous, regular disconnected-pores, and bicontinuous with interconnected-pores. The pore size of the latter two hydrogels can be tuned from ≈4 to 100 µm. Cytocompatibility of the generated ATPS hydrogels is confirmed by testing the viability of stromal and tumor cells. Their distribution and growth pattern are cell-type specific but are also strongly defined by the microstructure of the hydrogel. Finally, it is demonstrated that the unique porous structure is sustained when processing the bicontinuous system by inkjet and microextrusion techniques. The proposed ATPS hydrogels hold great potential for 3D tissue engineering applications due to their unique tunable interconnected porosity.

Dynein-Powered Cell Locomotion Guides Metastasis of Breast Cancer

The principal cause of death in cancer patients is metastasis, which remains an unresolved problem. Conventionally, metastatic dissemination is linked to actomyosin-driven cell locomotion. However, the locomotion of cancer cells often does not strictly line up with the measured actomyosin forces. Here, a complementary mechanism of metastatic locomotion powered by dynein-generated forces is identified. These forces arise within a non-stretchable microtubule network and drive persistent contact guidance of migrating cancer cells along the biomimetic collagen fibers. It is also shown that the dynein-powered locomotion becomes indispensable during invasive 3D migration within a tissue-like luminal network formed by spatially confining granular hydrogel scaffolds (GHS) made up of microscale hydrogel particles (microgels). These results indicate that the complementary motricity mediated by dynein is always necessary and, in certain instances, sufficient for disseminating metastatic breast cancer cells. These findings advance the fundamental understanding of cell locomotion mechanisms and expand the spectrum of clinical targets against metastasis.

Biomimetic cell culture for cell adhesive propagation for tissue engineering strategies

Biomimetic cell culture, which involves creating a biomimetic microenvironment for cells in vitro by engineering approaches, has aroused increasing interest given that it maintains the normal cellular phenotype, genotype and functions displayed in vivo. Therefore, it can provide a more precise platform for disease modelling, drug development and regenerative medicine than the conventional plate cell culture. In this review, initially, we discuss the principle of biomimetic cell culture in terms of the spatial microenvironment, chemical microenvironment, and physical microenvironment. Then, the main strategies of biomimetic cell culture and their state-of-the-art progress are summarized. To create a biomimetic microenvironment for cells, a variety of strategies has been developed, ranging from conventional scaffold strategies, such as macroscopic scaffolds, microcarriers, and microgels, to emerging scaffold-free strategies, such as spheroids, organoids, and assembloids, to simulate the native cellular microenvironment. Recently, 3D bioprinting and microfluidic chip technology have been applied as integrative platforms to obtain more complex biomimetic structures. Finally, the challenges in this area are discussed and future directions are discussed to shed some light on the community.

Dynamically Cross-Linked Granular Hydrogels for 3D Printing and Therapeutic Delivery

Granular hydrogels have recently emerged as promising biomaterials for tissue engineering and 3D-printing applications, addressing the limitations of bulk hydrogels while exhibiting desirable properties such as injectability and high porosity. However, their structural stability can be improved with post-injection interparticle cross-linking. In this study, we developed granular hydrogels with interparticle cross-linking through reversible and dynamic covalent bonds. We fragmented photo-cross-linked bulk hydrogels to produce aldehyde or hydrazide-functionalized microgels using chondroitin sulfate. Mixing these microgels facilitated interparticle cross-linking through reversible hydrazone bonds, providing shear-thinning and self-healing properties for injectability and 3D printing. The resulting granular hydrogels displayed high mechanical stability without the need for secondary cross-linking. Furthermore, the porosity and sustained release of growth factors from these hydrogels synergistically enhanced cell recruitment. Our study highlights the potential of reversible interparticle cross-linking for designing injectable and 3D printable therapeutic delivery scaffolds using granular hydrogels. Overall, our study highlights the potential of reversible interparticle cross-linking to improve the structural stability of granular hydrogels, making them an effective biomaterial for use in tissue engineering and 3D-printing applications.

Advances in Gelatin Bioinks to Optimize Bioprinted Cell Functions

Gelatin is a widely utilized bioprinting biomaterial due to its cell-adhesive and enzymatically cleavable properties, which improve cell adhesion and growth. Gelatin is often covalently cross-linked to stabilize bioprinted structures, yet the covalently cross-linked matrix is unable to recapitulate the dynamic microenvironment of the natural extracellular matrix (ECM), thereby limiting the functions of bioprinted cells. To some extent, a double network bioink can provide a more ECM-mimetic, bioprinted niche for cell growth. More recently, gelatin matrices are being designed using reversible cross-linking methods that can emulate the dynamic mechanical properties of the ECM. This review analyzes the progress in developing gelatin bioink formulations for 3D cell culture, and critically analyzes the bioprinting and cross-linking techniques, with a focus on strategies to optimize the functions of bioprinted cells. This review discusses new cross-linking chemistries that recapitulate the viscoelastic, stress-relaxing microenvironment of the ECM, and enable advanced cell functions, yet are less explored in engineering the gelatin bioink. Finally, this work presents the perspective on the areas of future research and argues that the next generation of gelatin bioinks should be designed by considering cell-matrix interactions, and bioprinted constructs should be validated against currently established 3D cell culture standards to achieve improved therapeutic outcomes.

Granular Ionogel Particle Inks for 3D Printed Tough and Stretchable Ionotronics

Ionogels have garnered great attention as promising soft conducting materials for the fabrication of flexible energy storage devices, soft actuators, and ionotronics. However, the leakage of the ionic liquids, weak mechanical strength, and poor manufacturability have greatly limited their reliability and applications. Here, we propose a new ionogel synthesis strategy by utilizing granular zwitterionic microparticles to stabilize ionic liquids. The ionic liquids swell the microparticles and physically crosslink microparticles via either electronic interaction or hydrogen bonding. Further introducing a photocurable acrylic monomer enables the fabrication of double-network (DN) ionogels with high stretchability (>600%) and ultrahigh toughness (fracture energy > 10 kJ/m2). The synthesized ionogels exhibit a wide working temperature of -60 to 90 °C. By tuning the crosslinking density of microparticles and physical crosslinking strength of ionogels, we synthesize DN ionogel inks and print them into three-dimensional (3D) motifs. Several ionogel-based ionotronics are 3D printed as demonstrations, including strain gauges, humidity sensors, and ionic skins made of capacitive touch sensor arrays. Via covalently linking ionogels with silicone elastomers, we integrate the ionogel sensors onto pneumatic soft actuators and demonstrate their capacities in sensing large deformation. As our last demonstration, multimaterial direct ink writing is harnessed to fabricate highly stretchable and durable alternating-current electroluminescent devices with arbitrary structures. Our printable granular ionogel ink represents a versatile platform for the future manufacturing of ionotronics.

Dynein-Powered Cell Locomotion Guides Metastasis of Breast Cancer

Metastasis is a principal cause of death in cancer patients, which remains an unresolved fundamental and clinical problem. Conventionally, metastatic dissemination is linked to the actomyosin-driven cell locomotion. However, locomotion of cancer cells often does not strictly line up with the measured actomyosin forces. Here, we identify a complementary mechanism of metastatic locomotion powered by the dynein-generated forces. These forces that arise within a non-stretchable microtubule network drive persistent contact guidance of migrating cancer cells along the biomimetic collagen fibers. We also show that dynein-powered locomotion becomes indispensable during invasive 3D migration within a tissue-like luminal network between spatially confining hydrogel microspheres. Our results indicate that the complementary contractile system of dynein motors and microtubules is always necessary and in certain instances completely sufficient for dissemination of metastatic breast cancer cells. These findings advance fundamental understanding of cell locomotion mechanisms and expand the spectrum of clinical targets against metastasis.

3D Printing of Self-Assembling Nanofibrous Multidomain Peptide Hydrogels

3D printing has become one of the primary fabrication strategies used in biomedical research. Recent efforts have focused on the 3D printing of hydrogels to create structures that better replicate the mechanical properties of biological tissues. These pose a unique challenge, as soft materials are difficult to pattern in three dimensions with high fidelity. Currently, a small number of biologically derived polymers that form hydrogels are frequently reused for 3D printing applications. Thus, there exists a need for novel hydrogels with desirable biological properties that can be used as 3D printable inks. In this work, the printability of multidomain peptides (MDPs), a class of self-assembling peptides that form a nanofibrous hydrogel at low concentrations, is established. MDPs with different charge functionalities are optimized as distinct inks and are used to create complex 3D structures, including multi-MDP prints. Additionally, printed MDP constructs are used to demonstrate charge-dependent differences in cellular behavior in vitro. This work presents the first time that self-assembling peptides have been used to print layered structures with overhangs and internal porosity. Overall, MDPs are a promising new class of 3D printable inks that are uniquely peptide-based and rely solely on supramolecular mechanisms for assembly.

Application of Hydrogels as Three-Dimensional Bioprinting Ink for Tissue Engineering

The use of three-dimensional bioprinting technology combined with the principle of tissue engineering is important for the construction of tissue or organ regeneration microenvironments. As a three-dimensional bioprinting ink, hydrogels need to be highly printable and provide a stiff and cell-friendly microenvironment. At present, hydrogels are used as bioprinting inks in tissue engineering. However, there is still a lack of summary of the latest 3D printing technology and the properties of hydrogel materials. In this paper, the materials commonly used as hydrogel bioinks; the advanced technologies including inkjet bioprinting, extrusion bioprinting, laser-assisted bioprinting, stereolithography bioprinting, suspension bioprinting, and digital 3D bioprinting technologies; printing characterization including printability and fidelity; biological properties, and the application fields of bioprinting hydrogels in bone tissue engineering, skin tissue engineering, cardiovascular tissue engineering are reviewed, and the current problems and future directions are prospected.

The Role of Microsphere Structures in Bottom-Up Bone Tissue Engineering

Bone defects have caused immense healthcare concerns and economic burdens throughout the world. Traditional autologous allogeneic bone grafts have many drawbacks, so the emergence of bone tissue engineering brings new hope. Bone tissue engineering is an interdisciplinary biomedical engineering method that involves scaffold materials, seed cells, and "growth factors". However, the traditional construction approach is not flexible and is unable to adapt to the specific shape of the defect, causing the cells inside the bone to be unable to receive adequate nourishment. Therefore, a simple but effective solution using the "bottom-up" method is proposed. Microspheres are structures with diameters ranging from 1 to 1000 µm that can be used as supports for cell growth, either in the form of a scaffold or in the form of a drug delivery system. Herein, we address a variety of strategies for the production of microspheres, the classification of raw materials, and drug loading, as well as analyze new strategies for the use of microspheres in bone tissue engineering. We also consider new perspectives and possible directions for future development.

Recent Advances in Macroporous Hydrogels for Cell Behavior and Tissue Engineering

Hydrogels have been extensively used as scaffolds in tissue engineering for cell adhesion, proliferation, migration, and differentiation because of their high-water content and biocompatibility similarity to the extracellular matrix. However, submicron or nanosized pore networks within hydrogels severely limit cell survival and tissue regeneration. In recent years, the application of macroporous hydrogels in tissue engineering has received considerable attention. The macroporous structure not only facilitates nutrient transportation and metabolite discharge but also provides more space for cell behavior and tissue formation. Several strategies for creating and functionalizing macroporous hydrogels have been reported. This review began with an overview of the advantages and challenges of macroporous hydrogels in the regulation of cellular behavior. In addition, advanced methods for the preparation of macroporous hydrogels to modulate cellular behavior were discussed. Finally, future research in related fields was discussed.