A Biomimetic 3D-Self-Forming Approach for Microvascular Scaffolds

4D Printing: A Comprehensive Review of Technologies, Materials, Stimuli, Design, and Emerging Applications

4D printing is a groundbreaking technology that seamlessly integrates additive manufacturing with smart materials, enabling the creation of multiscale objects capable of changing shapes and/or functions in a controlled and programmed manner in response to applied energy inputs. Printing technologies, mathematical modeling, responsive materials, stimuli, and structural design constitute the blueprint of 4D printing, all of which have seen rapid advancement in the past decade. These advancements have opened up numerous possibilities for dynamic and adaptive structures, finding potential use in healthcare, textiles, construction, aerospace, robotics, photonics, and electronics. However, current 4D printing primarily focuses on proof-of-concept demonstrations. Further development is necessary to expand the range of accessible materials and address fabrication complexities for widespread adoption. In this paper, we aim to deliver a comprehensive review of the state-of-the-art in 4D printing, probing into shape programming, exploring key aspects of resulting constructs including printing technologies, materials, structural design, morphing mechanisms, and stimuli-responsiveness, and elaborating on prominent applications across various fields. Finally, we discuss perspectives on limitations, challenges, and future developments in the realm of 4D printing. While the potential of this technology is undoubtedly vast, continued research and innovation are essential to unlocking its full capabilities and maximizing its real-world impact.

Rapid angiogenic hydrogel nanofiber scaffold promotes random flap regeneration and modulates inflammation

Random skin flaps are commonly used to cover thick skin wounds. However, necrosis frequently occurs when the flap's aspect ratio exceeds 2:1. Promoting angiogenesis and regulating inflammation are essential for treating ischemia-reperfusion injury in random flaps. In this study, calcium-doped silica nanoparticles loaded with deferoxamine (CD) were created through physical adsorption by rapidly mixing biodegradable calcium-doped silica nanoparticles (CS) with deferoxamine (DFO). A gelatin methacryloyl (GM) hydrogel nanofiber scaffold containing 1% (w/v) CD (GM/CD-1) was subsequently produced using electrospinning technology. The GM/CD-1 scaffold showed excellent biocompatibility and significantly promoted flap regeneration in mice, achieving a 96.17 ± 3.17% flap survival rate at 14 days. Additionally, it effectively stimulated hair follicle growth and exhibited an inflammatory-modulating effect. These features suggest that the GM/CD-1 scaffold could be valuable for clinical applications in flap regeneration and other tissue engineering fields.

Recent trends in embedded 3D bioprinting of vascularized tissue constructs

3D bioprinting technology offers significant advantages in the fabrication of tissue and organ structures by allowing precise layer-by-layer patterning of cells and various biomaterials. However, conventional bioinks exhibit poor mechanical properties, which limit their use in the fabrication of large-scale vascularized tissue constructs. To address these limitations, recent studies have focused on the development of rapidly crosslinkable bioinks through chemical modification. These enable rapid crosslinking within minutes, offering substantial advantages for engineering large-scale tissue constructs. Nevertheless, challenges remain in the production of constructs that fully incorporate the complex vascular networks inherent to native tissues. Recently, embedded bioprinting technique, which involves the direct writing of bioink into a support bath that provides physical support, has gained significant attention for enabling the freeform fabrication of 3D structures. This method has been extensively studied and offers the advantage of fabricating structures ranging from tissue constructs with simple vascular channels to complex structures containing multiscale vascular networks. This review presents an overview of the various materials utilized in embedded bioprinting and elucidates the rheological properties of these materials. Furthermore, it examines the current research trends in the biofabrication of vascularized tissue constructs using embedded bioprinting techniques, along with their associated limitations. The review concludes by proposing areas for future improvement, specifically addressing material and biofabrication approaches as well as bioprinting systems.

Advances in 4D Bioprinting: The Next Frontier in Regenerative Medicine and Tissue Engineering Applications

4D bioprinting is a critical advancement in tissue engineering and regenerative medicine (TERM), enabling the creation of structures that dynamically respond to environmental stimuli over time. This review investigates various fabrication techniques and responsive materials that are central to these fields. It underscores the integration of multi-material and biocomposite approaches in 4D bioprinting, which is crucial for fabricating complex and functional constructs with heterogeneous properties. Using 4D bioprinting techniques enhances the mimicry of natural tissue characteristics, offering tailored responses and improved integration with biological systems. Furthermore, this study highlights the synergy between 4D bioprinting and tissue engineering and demonstrates the technology's potential for developing tissues and organs. In regenerative medicine, 4D bioprinting's applications extend to creating smart implants and advanced drug delivery systems that adapt to the body's changes, promoting healing and tissue regeneration. Finally, the challenges and future directions of 4D bioprinting are also explored and emphasize its transformative impact on biomedical engineering and the future of healthcare.

Spider-silk inspired ultrafast alkali-induced molecular aggregation for 3D printing arbitrary tubular hydrogels

Fabricating tubular hydrogel models with arbitrary structural complexity and controllable diameters using an ultrafast, facile yet universal method is desirable for vascular prototypes yet still a great challenge. Herein, inspired by the denaturing ability of spider silks, a novel strategy to induce complexation via applying highly concentrated alkali into a polyvinyl alcohol/ionic liquid (PVA/IL) solution, i.e., alkali-induced molecular aggregation (AMA), is proposed to achieve such purpose. This strategy enables the rapid and facile fabrication of tubular hydrogel architectures with tunable diameters, controllable thicknesses, and excellent mechanical performance with a tensile strength of up to 1.1 MPa and stretchability exceeding 600%. Importantly, this novel strategy combined with 3D printing facilitates the rapid fabrication of a variety of precise tubular hydrogel models with connected cavity structures which are difficult to achieve using current methods. This ultrafast solidification strategy could also be extended to various alkalis, cations and anions to build different hydrogels, showcasing its versatility and universality. Hence, this strategy can be pioneering to rapidly fabricate complex three-dimensional and hollow enclosed hydrogel models for simulating endovascular interventional therapy.

A 4D printed nanoengineered super bioactive hydrogel scaffold with programmable deformation for potential bifurcated vascular channel construction

Four-dimensional (4D) printing of hydrogels enabled the fabrication of complex scaffold geometries out of static parts. Although current 4D fabrication strategies are promising for creating vascular parts such as tubes, developing branched networks or tubular junctions is still challenging. Here, for the first time, a 4D printing approach is employed to fabricate T-shaped perfusable bifurcation using an extrusion-based multi-material 3D printing process. An alginate/methylcellulose-based dual-component hydrogel system (with defined swelling behavior) is nanoengineered with carbonized alginate (∼100 nm) to introduce anti-oxidative, anti-inflammatory, and anti-thrombotic properties and shape-shifting properties. A computational model to predict shape deformations in the printed hydrogels with defined infill angles was designed and further validated experimentally. Shape deformations of the 3D-printed flat sheets were achieved by ionic cross-linking. An undisrupted perfusion of a dye solution through a T-junction with minimal leakage mimicking blood flow through vessels is also demonstrated. Moreover, human umbilical vein endothelial and fibroblast cells seeded with printed constructs show intact morphology and excellent cell viability. Overall, the developed strategy paves the way for manufacturing self-actuated vascular bifurcations with remarkable anti-thrombotic properties to potentially treat coronary artery diseases.

Decellularized Tumor Tissues Integrated with Polydopamine for Wound Healing

Natural biomaterials have been showing extensive potential in wound healing; attempts therefore focus on productions achieving both antimicrobial and tissue regenerative abilities. Here, we construct a decellularized human colon tumor (DHCT)-derived scaffold for wound remolding via microfluidic bioprinting. The DHCT retains a series of growth factors, fibrin, and the collagen configuration, that favor tissue repair and reconstruction. Specifically, the scaffold shows superior abilities in cell migration and angiogenesis. The biocompatible scaffold is also imparted with tissue adhesion ability and photothermal effect due to the coating of biologically derived polydopamine on the surface. The strong photothermal effect under near-infrared irradiation also present the scaffold with an antibacterial rate exceeding 90%. Furthermore, in vivo experiments convinced that the polydopamine-integrated DHCT scaffold can markedly expedite the healing process of acute extensive wounds. These findings indicate that composite materials derived from natural tumors have substantial potential in pertinent clinical applications.

Functional hydrogels for hepatocellular carcinoma: therapy, imaging, and in vitro model

Hepatocellular carcinoma (HCC) is among the most common malignancies worldwide and is characterized by high rates of morbidity and mortality, posing a serious threat to human health. Interventional embolization therapy is the main treatment against middle- and late-stage liver cancer, but its efficacy is limited by the performance of embolism, hence the new embolic materials have provided hope to the inoperable patients. Especially, hydrogel materials with high embolization strength, appropriate viscosity, reliable security and multifunctionality are widely used as embolic materials, and can improve the efficacy of interventional therapy. In this review, we have described the status of research on hydrogels and challenges in the field of HCC therapy. First, various preparation methods of hydrogels through different cross-linking methods are introduced, then the functions of hydrogels related to HCC are summarized, including different HCC therapies, various imaging techniques, in vitro 3D models, and the shortcomings and prospects of the proposed applications are discussed in relation to HCC. We hope that this review is informative for readers interested in multifunctional hydrogels and will help researchers develop more novel embolic materials for interventional therapy of HCC.

4D bioprinting of programmed dynamic tissues

Setting time as the fourth dimension, 4D printing allows us to construct dynamic structures that can change their shape, property, or functionality over time under stimuli, leading to a wave of innovations in various fields. Recently, 4D printing of smart biomaterials, biological components, and living cells into dynamic living 3D constructs with 4D effects has led to an exciting field of 4D bioprinting. 4D bioprinting has gained increasing attention and is being applied to create programmed and dynamic cell-laden constructs such as bone, cartilage, and vasculature. This review presents an overview on 4D bioprinting for engineering dynamic tissues and organs, followed by a discussion on the approaches, bioprinting technologies, smart biomaterials and smart design, bioink requirements, and applications. While much progress has been achieved, 4D bioprinting as a complex process is facing challenges that need to be addressed by transdisciplinary strategies to unleash the full potential of this advanced biofabrication technology. Finally, we present future perspectives on the rapidly evolving field of 4D bioprinting, in view of its potential, increasingly important roles in the development of advanced dynamic tissues for basic research, pharmaceutics, and regenerative medicine.

4D hydrogels: fabrication strategies, stimulation mechanisms, and biomedical applications

Shape-morphing hydrogels have emerged as a promising biomaterial due to their ability to mimic the anisotropic tissue composition by creating a gradient in local swelling behavior. In this case, shape deformations occur due to the non-uniform distribution of internal stresses, asymmetrical swelling, and shrinking of different parts of the same hydrogel. Herein, we discuss the four-dimensional (4D) fabrication techniques (extrusion-based printing, dynamic light processing, and solvent casting) employed to prepare shape-shifting hydrogels. The important distinction between mono- and dual-component hydrogel systems, the capabilities of 3D constructs to undergo uni- and bi-directional shape changes, and the advantages of composite hydrogels compared to their pristine counterparts are presented. Subsequently, various types of actuators such as moisture, light, temperature, pH, and magnetic field and their role in achieving the desired and pre-determined shapes are discussed. These 4D gels have shown remarkable potential as programmable scaffolds for tissue regeneration and drug-delivery systems. Finally, we present futuristic insights into integrating piezoelectric biopolymers and sensors to harvest mechanical energy from motions during shape transformations to develop self-powered biodevices.

Development of a biofabricated 3Din vitrovessel model for investigating transendothelial migration in stem cell therapy

Systemic stem cell therapies hold promise for treating severe diseases, but their efficiency is hampered by limited migration of injected stem cells across vascular endothelium towards diseased tissues. Understanding transendothelial migration is crucial for improving therapy outcomes. We propose a novel 3Din vitrovessel model that aids to unravel these mechanisms and thereby facilitates stem cell therapy development. Our model simulates inflammation through cytokine diffusion from the tissue site into the vessel. It consists of a biofabricated vessel embedded in a fibrin hydrogel, mimicking arterial wall composition with smooth muscle cells and fibroblasts. The perfusable channel is lined with a functional endothelium which expresses vascular endothelial cadherin, provides an active barrier function, aligns with flow direction and is reconstructed byin situtwo-photon-microscopy. Inflammatory cytokine release (tumor necrosis factorα, stromal-derived factor (1) is demonstrated in both a transwell assay and the 3D model. In proof-of-principle experiments, mesoangioblasts, known as a promising candidate for a stem cell therapy against muscular dystrophies, are injected into the vessel model, showing shear-resistant endothelial adhesion under capillary-like flow conditions. Our 3Din vitromodel offers significant potential to study transendothelial migration mechanisms of stem cells, facilitating the development of improved stem cell therapies.

Interfacial Dehydration Strategy for Chitosan Film Shape Morphing and Its Application

Shape morphing of biopolymer materials, such as chitosan (CS) films, has great potential for applications in many fields. Traditionally, their responsive behavior has been induced by the differential water swelling through the preparation of multicomponent composites or cross-linking as deformation is not controllable in the absence of these processes. Here, we report an interfacial dehydration strategy to trigger the shape morphing of the monocomponent CS film without cross-linking. The release of water molecules is achieved by spraying the surface with a NaOH solution or organic solvents, which results in the interfacial shrinkage and deformation of the entire film. On the basis of this strategy, a range of CS actuators were developed, such as soft grippers, joint actuators, and a light switch. Combined with the geometry effect, edited deformation was also achieved from the planar CS film. This shape-morphing strategy is expected to enable the application of more biopolymers in a wide range of fields.

Developing organs-on-chips for biomedical applications

In recent years, organs-on-chips have been arousing great interest for their bionic and stable construction of crucial human organs in vitro. Compared with traditional animal models and two-dimensional cell models, organs-on-chips could not only overcome the limitations of species difference and poor predict ability but also be capable of reappearing the complex cell-cell interaction, tissue interface, biofluid and other physiological conditions of humans. Therefore, organs-on-chips have been regarded as promising and powerful tools in diverse fields such as biology, chemistry, medicine and so on. In this perspective, we present a review of organs-on-chips for biomedical applications. After introducing the key elements and manufacturing craft of organs-on-chips, we intend to review their cut-edging applications in biomedical fields, incorporating biological analysis, drug development, robotics and so on. Finally, the emphasis is focused on the perspectives of organs-on-chips.

Tissue-Penetrating Ultrasound-Triggered Hydrogel for Promoting Microvascular Network Reconstruction

The microvascular network plays an important role in providing nutrients to the injured tissue and exchanging various metabolites. However, how to achieve efficient penetration of the injured tissue is an important bottleneck restricting the reconstruction of microvascular network. Herein, the hydrogel precursor solution can efficiently penetrate the damaged tissue area, and ultrasound triggers the release of thrombin from liposomes in the solution to hydrolyze fibrinogen, forming a fibrin solid hydrogel network in situ with calcium ions and transglutaminase as catalysts, effectively solving the penetration impedance bottleneck of damaged tissues and ultimately significantly promoting the formation of microvascular networks within tissues. First, the fibrinogen complex solution is effectively permeated into the injured tissue. Second, ultrasound triggered the release of calcium ions and thrombin, activates transglutaminase, and hydrolyzes fibrinogen. Third, fibrin monomers are catalyzed to form fibrin hydrogels in situ in the damaged tissue area. In vitro studies have shown that the fibrinogen complex solution effectively penetrated the artificial bone tissue within 15 s after ultrasonic triggering, and formed a hydrogel after continuous triggering for 30 s. Overall, this innovative strategy effectively solved the problem of penetration resistance of ultrasound-triggered hydrogels in the injured tissues, and finally activates in situ microvascular networks regeneration.

Gelatin Methacryloyl (GelMA)-Based Biomaterial Inks: Process Science for 3D/4D Printing and Current Status

Tissue engineering for injured tissue replacement and regeneration has been a subject of investigation over the last 30 years, and there has been considerable interest in using additive manufacturing to achieve these goals. Despite such efforts, many key questions remain unanswered, particularly in the area of biomaterial selection for these applications as well as quantitative understanding of the process science. The strategic utilization of biological macromolecules provides a versatile approach to meet diverse requirements in 3D printing, such as printability, buildability, and biocompatibility. These molecules play a pivotal role in both physical and chemical cross-linking processes throughout the biofabrication, contributing significantly to the overall success of the 3D printing process. Among the several bioprintable materials, gelatin methacryloyl (GelMA) has been widely utilized for diverse tissue engineering applications, with some degree of success. In this context, this review will discuss the key bioengineering approaches to identify the gelation and cross-linking strategies that are appropriate to control the rheology, printability, and buildability of biomaterial inks. This review will focus on the GelMA as the structural (scaffold) biomaterial for different tissues and as a potential carrier vehicle for the transport of living cells as well as their maintenance and viability in the physiological system. Recognizing the importance of printability toward shape fidelity and biophysical properties, a major focus in this review has been to discuss the qualitative and quantitative impact of the key factors, including microrheological, viscoelastic, gelation, shear thinning properties of biomaterial inks, and printing parameters, in particular, reference to 3D extrusion printing of GelMA-based biomaterial inks. Specifically, we emphasize the different possibilities to regulate mechanical, swelling, biodegradation, and cellular functionalities of GelMA-based bio(material) inks, by hybridization techniques, including different synthetic and natural biopolymers, inorganic nanofillers, and microcarriers. At the close, the potential possibility of the integration of experimental data sets and artificial intelligence/machine learning approaches is emphasized to predict the printability, shape fidelity, or biophysical properties of GelMA bio(material) inks for clinically relevant tissues.

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

Stimuli-Responsive Hydrogels: The Dynamic Smart Biomaterials of Tomorrow

In the past decade, stimuli-responsive hydrogels are increasingly studied as biomaterials for tissue engineering and regenerative medicine purposes. Smart hydrogels can not only replicate the physicochemical properties of the extracellular matrix but also mimic dynamic processes that are crucial for the regulation of cell behavior. Dynamic changes can be influenced by the hydrogel itself (isotropic vs anisotropic) or guided by applying localized triggers. The resulting swelling-shrinking, shape-morphing, as well as patterns have been shown to influence cell function in a spatiotemporally controlled manner. Furthermore, the use of stimuli-responsive hydrogels as bioinks in 4D bioprinting is very promising as they allow the biofabrication of complex microstructures. This perspective discusses recent cutting-edge advances as well as current challenges in the field of smart biomaterials for tissue engineering. Additionally, emerging trends and potential future directions are addressed.

Rhabdomyosarcoma: Current Therapy, Challenges, and Future Approaches to Treatment Strategies

Rhabdomyosarcoma is a rare cancer arising in skeletal muscle that typically impacts children and young adults. It is a worldwide challenge in child health as treatment outcomes for metastatic and recurrent disease still pose a major concern for both basic and clinical scientists. The treatment strategies for rhabdomyosarcoma include multi-agent chemotherapies after surgical resection with or without ionization radiotherapy. In this comprehensive review, we first provide a detailed clinical understanding of rhabdomyosarcoma including its classification and subtypes, diagnosis, and treatment strategies. Later, we focus on chemotherapy strategies for this childhood sarcoma and discuss the impact of three mechanisms that are involved in the chemotherapy response including apoptosis, macro-autophagy, and the unfolded protein response. Finally, we discuss in vivo mouse and zebrafish models and in vitro three-dimensional bioengineering models of rhabdomyosarcoma to screen future therapeutic approaches and promote muscle regeneration.

4D Printed Programmable Shape-Morphing Hydrogels as Intraoperative Self-Folding Nerve Conduits for Sutureless Neurorrhaphy

There are only a few reports of implantable 4D printed biomaterials, most of which exhibit slow deformations rendering them unsuitable for in situ surgical deployment. In this study, a hydrogel system is engineered with defined swelling behaviors, which demonstrated excellent printability in extrusion-based 3D printing and programmed shape deformations post-printing. Shape deformations of the spatially patterned hydrogels with defined infill angles are computationally predicted for a variety of 3D printed structures, which are subsequently validated experimentally. The gels are coated with gelatin-rich nanofibers to augment cell growth. 3D-printed hydrogel sheets with pre-programmed infill patterns rapidly self-rolled into tubes in vivo to serve as nerve-guiding conduits for repairing sciatic nerve defects in a rat model. These 4D-printed hydrogels minimized the complexity of surgeries by tightly clamping the resected ends of the nerves to assist in the healing of peripheral nerve damage, as revealed by histological evaluation and functional assessments for up to 45 days. This work demonstrates that 3D-printed hydrogels can be designed for programmed shape changes by swelling in vivo to yield 4D-printed tissue constructs for the repair of peripheral nerve damage with the potential to be extended in other areas of regenerative medicine.

Bioinspired Adaptable Indwelling Microneedles for Treatment of Diabetic Ulcers

Microneedles provide an effective strategy for transdermal drug delivery. Many endeavors have been devoted to developing smart microneedles that can respond to and interact with pathophysiological environments. Here, novel bioinspired adaptable indwelling microneedles with therapeutic exosome encapsulation are presented for diabetic wound healing by a combined fabrication strategy of template replication and 3D transfer printing. Such microneedles are composed of mesenchymal stem cell (MSC)-exosomes-encapsulated adjustable poly(vinyl alcohol) (PVA) hydrogel needle tips and detachable 3M medical tape supporting substrate. As the mechanical strength of the PVA hydrogel is ionically responsive due to Hofmeister effects, the hardness of the resultant microneedle tips can be upregulated by sulfate ions to ensure skin penetration and be softened by nitrate ions after tip-substrate detachment to adapt to the surrounding tissue and release exosomes. Because the MSC-exosomes can effectively activate fibroblasts, vascular endothelial cells, and macrophages, the indwelling microneedles are demonstrated with the function of promoting tissue regeneration and diabetic wound healing in full-thickness cutaneous wounds of diabetic rat models. These features indicate that the bioinspired adaptable indwelling microneedles are with practical values and clinical prospects in tissue and wound regeneration.

Living microecological hydrogels for wound healing

Chronic hard-to-heal wounds draw great attention worldwide, as their treatments are limited by infections and hypoxia. Inspired by the natural oxygen production capacity of algae and the competitive advantage of beneficial bacteria over other microbes, we presented a living microecological hydrogel (LMH) with functionalized Chlorella and Bacillus subtilis encapsulation to realize continuous oxygen delivery and anti-infections for promoting chronic wound healing. As the hydrogel consisted of thermosensitive Pluronic F-127 and wet-adhesive polydopamine, the LMH could keep liquid at a low temperature while quickly solidifying and tightly adhering to the wound bed. It was demonstrated that by optimizing the proportion of the encapsulated microorganism, the Chlorella could continuously produce oxygen to relieve hypoxia and support the proliferation of B. subtilis, while B. subtilis could eliminate the colonized pathogenic bacteria. Thus, the LMH substantially promoted the healing of infected diabetic wounds. These features make the LMH valuable for practical clinical applications.

Properties and Printability of the Synthesized Hydrogel Based on GelMA

Gelatin methacryloyl (GelMA) has recently attracted increasing attention. Unlike other hydrogels, it allows for the adjustment of the mechanical properties using such factors as degree of functionalization, concentration, and photocrosslinking parameters. In this study, GelMA with a high degree of substitution (82.75 ± 7.09%) was synthesized, and its suitability for extrusion printing, cytocompatibility, and biocompatibility was studied. Satisfactory printing quality was demonstrated with the 15% concentration hydrogel. The high degree of functionalization led to a decrease in the ability of human adipose-derived stem cells (ADSCs) to adhere to the GelMA surface. During the first 3 days after sowing, proliferation was observed. Degradation in animals after subcutaneous implantation was slowed down.

4D Biofabrication of Mechanically Stable Tubular Constructs Using Shape Morphing Porous Bilayers for Vascularization Application

This study reports the fabrication of highly porous electrospun self-folding bilayers, which fold into tubular structures with excellent mechanical stability, allowing them to be easily manipulated and handled. Two kinds of bilayers based on biocompatible and biodegradable soft (PCL, polycaprolactone) and hard (PHB, poly-hydroxybutyrate) thermoplastic polymers have been fabricated and compared. Multi-scroll structures with tunable diameter are obtained after the shape transformation of the bilayer in aqueous media, where PCL-based bilayer rolled longitudinally and PHB-based one rolled transversely with respect to the fiber direction. A combination of higher elastic modulus and transverse orientation of fibers with respect to rolling direction allowed precise temporal control of shape transformation of PHB-bilayer - stress produced by swollen methacrylated hyaluronic acid (HA-MA) do not relax with time and folding is not affected by the fact that bilayer is fixed in unfolded state in cell culture medium for more than 1 h. This property of PHB-bilayer allowed cell culturing without a negative effect on its shape transformation ability. Moreover, PHB-based tubular structure demonstrated superior mechanical stability compared to PCL-based ones and do not collapse during manipulations that happened to PCL-based one. Additionally, PHB/HA-MA bilayers showed superior biocompatibility, degradability, and long-term stability compared to PCL/HA-MA.

Histidine-Triggered GO Hybrid Hydrogels for Microfluidic 3D Printing

Graphene oxide (GO) hydrogels have provided tremendous opportunities in designing and fabricating complex constructs for diverse applications, while their 3D printing without photocuring is still a challenging task due to their low viscosity, uncontrollable gelation, and low interfacial tension. Here, we report a histidine-assisted printing strategy to prepare GO hybrid hydrogels through the microfluidic 3D printing technique. We found that the GO additive could significantly hamper the Knoevenagel condensation (KC) reaction between benzaldehyde and cyanoacetate group-functionalized polymers to form a hydrogel, while these GO mixed solutions were rapidly solidified into a hydrogel when histidine was added. This fascinating phenomenon enabled us to prepare low-viscosity GO mixed polymer solutions as printable inks and generate hydrogel microfibers in histidine solutions. The hydrogel fibers could support cell survival and be further constructed into complex 3D structures through microfluidic 3D printing techniques. Moreover, due to the addition of GO, the microfibers exhibited excellent electrical conductivity and could sense the motion changes and convert these stimuli as electrical resistance signals. This strategy adds an option for the design and application of 3D printable aqueous GO inks in many fields.

4D Printing: A Cutting-edge Platform for Biomedical Applications

Nature's materials have evolved over time to be able to respond to environmental stimuli by generating complex structures that can change their functions in response to distance, time, and direction of stimuli. A number of technical efforts are currently being made to improve printing resolution, shape fidelity, and printing speed to mimic the structural design of natural materials with three-dimensional (3D) printing. Unfortunately, this technology is limited by the fact that printed objects are static and cannot be reshaped dynamically in response to stimuli. In recent years, several smart materials have been developed that can undergo dynamic morphing in response to a stimulus, thus resolving this issue. Four-dimensional (4D) printing refers to a manufacturing process involving additive manufacturing, smart materials, and specific geometries. It has become an essential technology for biomedical engineering and has the potential to create a wide range of useful biomedical products. This paper will discuss the concept of 4D bioprinting and the recent developments in smart matrials, which can be actuated by different stimuli and be exploited to develop biomimetic materials and structures, with significant implications for pharmaceutics and biomedical research, as well as prospects for the future.

Dynamically Responsive Scaffolds from Microfluidic 3D Printing for Skin Flap Regeneration

Biological scaffolds hold promising perspectives for random skin flap regeneration, while the practical application is greatly limited by their insufficient vascularization ability and the lack of responsiveness during the dynamical healing process. Herein, a novel MXene-incorporated hollow fibrous (MX-HF) scaffold with dynamically responsive channels is presented for promoting vascularization and skin flap regeneration by using a microfluidic-assisted 3D printing strategy. Benefiting from the photothermal conversion capacity of the MXene nanosheets and temperature-responsive ability of poly(NIPAM) hydrogels in the MX-HF scaffolds, they display a near-infrared (NIR)-responsive shrinkage/swelling behavior, which facilitates the cell penetration into the scaffold channels from the surrounding environment. Moreover, by incorporating vascular endothelial growth factor (VEGF) into the hydrogel matrix for controllable delivery, the MX-HF scaffolds can achieve promoted proliferation, migration, and proangiogenic effects of endothelial cells under NIR irradiation. It is further demonstrated in vivo that the NIR-responsive VEGF@MX-HF scaffolds can effectively improve skin flap survival by promoting angiogenesis, decreasing inflammation, and attenuating apoptosis in skin flaps. Thus, it is believed that such responsive MX-HF scaffolds are promising candidates for clinical random skin flap regeneration as well as other diverse tissue engineering applications.

Bioactive Fish Scale Scaffolds with MSCs-Loading for Skin Flap Regeneration

Skin flap transplantations are common methods for covering and repairing tissue defects in surgery, while the survival rates of these skin flaps are still low due to the vascular crisis and necrosis. To improve this situation, herein a novel biohybrid scaffold is proposed by integrating the advantages of anisotropic fish scales and mesenchymal stem cells (MSCs) for skin flap regeneration. The fish scale scaffold is obtained through its decellularization and decalcification processes, which reserved intact collagen, glycosaminoglycan, and other endogenous growth factors for MSCs and human vascular endothelial cells proliferation. As the scaffold maintains intrinsic anisotropic structures on both surfaces, the proliferative cells can be elongated along the aligned structures on the fish scale, which endow them the capacity to differentiate into multiple directions. Based on these features, it is demonstrated from an in vivo experiment that the MSCs-loading fish scale scaffolds can effectively convert the activated inflammatory macrophages into anti-inflammatory properties, reduce the inflammation around the flap, and improve the survival rate. These results indicate that the MSCs-loading fish scale scaffold is suitable and has the potential for skin flap regeneration and functional recovery.

Electroconductive and Anisotropic Structural Color Hydrogels for Visual Heart-on-a-Chip Construction

Heart-on-a-chip plays an important role in revealing the biological mechanism and developing new drugs for cardiomyopathy. Tremendous efforts have been devoted to developing heart-on-a-chip systems featuring simplified fabrication, accurate imitation and microphysiological visuality. In this paper, the authors present a novel electroconductive and anisotropic structural color hydrogel by simply polymerizing non-close-packed colloidal arrays on super aligned carbon nanotube sheets (SACNTs) for visualized and accurate heart-on-a-chip construction. The generated anisotropic hydrogel consists of a colloidal array-locked hydrogel layer with brilliant structural color on one surface and a conductive methacrylated gelatin (GelMA)/SACNTs film on the other surface. It is demonstrated that the anisotropic morphology of the SACNTs could effectively induce the alignment of cardiomyocytes, and the conductivity of SACNTs could contribute to the synchronous beating of cardiomyocytes. Such consistent beating rhythm caused the deformation of the hydrogel substrates and dynamic shifts in structural color and reflection spectra of the whole hybrid hydrogels. More attractively, with the integration of such cardiomyocyte-driven living structural color hydrogels and microfluidics, a visualized heart-on-a-chip system with more consistent beating frequency has been established for dynamic cardiomyocyte sensing and drug screening. The results indicate that the electroconductive and anisotropic structural color hydrogels are potential for various biomedical applications.

Bioinspired Perovskite Nanocrystals-Integrated Photonic Crystal Microsphere Arrays for Information Security

Information security occupies an important position in the era of big data. Attempts to improve the security performance tend to impart them with more additional encryption strategies. Herein, inspired by the wettability feature of Stenocara beetle elytra and signal model of traffic light, a novel array of perovskite nanocrystals (PNs)-integrated PhC microsphere for information security is presented. The photoluminescent PNs are encapsulated in angle-independent PhC microspheres to impart them with binary optical signals as coding information. Through the multimask superposition approach, PNs-integrated PhC microspheres with different codes are placed into fluorosilane-treated PDMS substrate to form different arrays. These arrays could converge moisture on PhC microspheres in wet environment, which avoids the ions loss of the PNs and effectively prevented mutual contamination. In addition, the fluorescence of the PNs inside PhC microspheres could reversibly quench or recover in response to the environmental moisture. Based on these features, it is demonstrated that the PNs-integrated PhC microsphere arrays could realize various information encryption modes, which indicate their excellent values in information security fields.

Honeycomb-Like Hydrogel Microspheres for 3D Bulk Construction of Tumor Models

A two-dimensional (2D) cell culture-based model is widely applied to study tumorigenic mechanisms and drug screening. However, it cannot authentically simulate the three-dimensional (3D) microenvironment of solid tumors and provide reliable and predictable data in response to in vivo, thus leading to the research illusions and failure of drug screening. In this study, honeycomb-like gelatin methacryloyl (GelMA) hydrogel microspheres are developed by synchronous photocrosslinking microfluidic technique to construct a 3D model of osteosarcoma. The in vitro study shows that osteosarcoma cells (K7M2) cultured in 3D GelMA microspheres have stronger tumorous stemness, proliferation and migration abilities, more osteoclastogenetic ability, and resistance to chemotherapeutic drugs (DOX) than that of cells in 2D cultures. More importantly, the 3D-cultured K7M2 cells show more tumorigenicity in immunologically sound mice, characterized by shorter tumorigenesis time, larger tumor volume, severe bone destruction, and higher mortality. In conclusion, honeycomb-like porous microsphere scaffolds are constructed with uniform structure by microfluidic technology to massively produce tumor cells with original phenotypes. Those microspheres could recapitulate the physiology microenvironment of tumors, maintain cell-cell and cell-extracellular matrix interactions, and thus provide an effective and convenient strategy for tumor pathogenesis and drug screening research.

Osteon-mimetic 3D nanofibrous scaffold enhances stem cell proliferation and osteogenic differentiation for bone regeneration

The scaffold microstructure is important for bone tissue engineering. Failure to synergistically imitate the hierarchical microstructure of the components of bone, such as an osteon with concentric multilayers assembled by nanofibers, hinders the performance for guiding bone regeneration. Here, a 2D bilayer nanofibrous membrane (BLM) containing poly(lactide-co-glycolide) (PLGA)/polycaprolactone (PCL) composite membranes in similar compositions (PCL15 and PCL20), but possessing different degrees of shrinkage, was fabricated via sequential electrospinning. Upon incubation in phosphate buffered saline (PBS) (37 °C), the 2D BLM spontaneously deformed into a 3D shape induced by PCL crystallization within the PLGA matrix, and the PCL15 and PCL20 layer formed a concave and convex surface, respectively. The 3D structure contained curved multilayers with an average diameter of 776 ± 169 μm, and on the concave and convex surface the nanofiber diameters were 792 ± 225 and 881 ± 259 nm, respectively. The initial 2D structure facilitated the even distribution of seeded cells. Adipose-derived stem cells from rats (rADSCs) proliferated faster on a concave surface than on a convex surface. For the 3D BLM, the osteogenic differentiation of rADSCs was significantly higher than that on 2D surfaces, even without osteogenic supplements, which resulted from the stretched cell morphology on the curved sublayer leading to increased expression of lamin-A. After being implanted into cranial defects in Sprague Dawley (SD) rats, 3D BLM significantly accelerated bone formation. In summary, 3D BLM with an osteon-like structure provides a potential strategy to repair bone defects.

Bioinspired Artificial Liver System with hiPSC-Derived Hepatocytes for Acute Liver Failure Treatment

Bioartificial liver (BAL) system has become a promising alternative to traditional liver transplantation in rescuing acute liver failure (ALF) patients. Herein, inspired by natural microstructure of hepatic lobules, a novel biomimetic bioartificial liver system (BBALS) is developed by integrating human induced pluripotent stem cell-derived hepatocytes (hiPSC-Heps) -laden microparticles and semipermeable microtubes into a microfluidic platform. As the working units are hepatic lobules-like semipermeable microtubes surrounding with serum-free suspension differentiated hiPSC-Heps microcarriers, the BBALS is endowed with functional cell aggregates and effective circulation system. Thus, the BBALS possesses high cell viability, favorable function regeneration, and effective substances exchange. Based on these features, a 3D liver chip with multiple parallel BBALS units is created for filtering the plasma of ALF rabbits, which validates the research significance and application potential of the proposed BBALS. Moreover, the novel integrated BBALS is applied to treat ALF rabbits and shows great advantages in increasing survival, generating serum proteins, and decreasing inflammation. These properties point to the broad prospects of BBALS in treating related diseases and improving traditional clinical methods.

Advances in tissue engineering of vasculature through three-dimensional bioprinting

BACKGROUND

A significant challenge facing tissue engineering is the fabrication of vasculature constructs which contains vascularized tissue constructs to recapitulate viable, complex and functional organs or tissues, and free-standing vascular structures potentially providing clinical applications in the future. Three-dimensional (3D) bioprinting has emerged as a promising technology, possessing a number of merits that other conventional biofabrication methods do not have. Over the last decade, 3D bioprinting has contributed a variety of techniques and strategies to generate both vascularized tissue constructs and free-standing vascular structures.

RESULTS

This review focuses on different strategies to print two kinds of vasculature constructs, namely vascularized tissue constructs and vessel-like tubular structures, highlighting the feasibility and shortcoming of the current methods for vasculature constructs fabrication. Generally, both direct printing and indirect printing can be employed in vascularized tissue engineering. Direct printing allows for structural fabrication with synchronous cell seeding, while indirect printing is more effective in generating complex architecture. During the fabrication process, 3D bioprinting techniques including extrusion bioprinting, inkjet bioprinting and light-assisted bioprinting should be selectively implemented to exert advantages and obtain the desirable tissue structure. Also, appropriate cells and biomaterials matter a lot to match various bioprinting techniques and thus achieve successful fabrication of specific vasculature constructs.

CONCLUSION

The 3D bioprinting has been developed to help provide various fabrication techniques, devoting to producing structurally stable, physiologically relevant, and biologically appealing constructs. However, although the optimization of biomaterials and innovation of printing strategies may improve the fabricated vessel-like structures, 3D bioprinting is still in the infant period and has a great gap between in vitro trials and in vivo applications. The article reviews the present achievement of 3D bioprinting in generating vasculature constructs and also provides perspectives on future directions of advanced vasculature constructs fabrication.

Capturing dynamic biological signals via bio-mimicking hydrogel for precise remodeling of soft tissue

Soft tissue remodeling is a sophisticated process that sequentially provides dynamic biological signals to guide cell behavior. However, capturing these signals within hydrogel and directing over time has still been unrealized owing to the poor comprehension of physiological processes. Here, a bio-mimicking hydrogel is designed via thiol-ene click reaction to capture the early physical signal triggered by inflammation, and the chemical signals provided with chemokine and natural adhesion sites, which guaranteed the precise soft tissue remodeling. This bio-mimicking hydrogel efficiently facilitated cell anchoring, migration, and invasion in the 3D matrix due to the permissive space and the interaction with integrin receptors. Besides, the covalently grafted chemokine-like peptide is optimal for colonization and functional differentiation of endothelial cells through a HIF-1α dependent signal pathway. Furthermore, the early polarization of macrophages, collagen deposition and angiogenesis in rat acute wound model, and the increased blood perfusion in mouse skin flap model have confirmed that the bio-mimicking hydrogel realized precise soft tissue remodeling and opens new avenues for the phased repair of different tissues such as nerve, myocardium, and even bone.

Cell-Laden Multiple-Step and Reversible 4D Hydrogel Actuators to Mimic Dynamic Tissue Morphogenesis

Shape-morphing hydrogels bear promising prospects as soft actuators and for robotics. However, they are mostly restricted to applications in the abiotic domain due to the harsh physicochemical conditions typically necessary to induce shape morphing. Here, multilayer hydrogel actuator systems are developed using biocompatible and photocrosslinkable oxidized, methacrylated alginate and methacrylated gelatin that permit encapsulation and maintenance of living cells within the hydrogel actuators and implement programmed and controlled actuations with multiple shape changes. The hydrogel actuators encapsulating cells enable defined self-folding and/or user-regulated, on-demand-folding into specific 3D architectures under physiological conditions, with the capability to partially bioemulate complex developmental processes such as branching morphogenesis. The hydrogel actuator systems can be utilized as novel platforms for investigating the effect of programmed multiple-step and reversible shape morphing on cellular behaviors in 3D extracellular matrix and the role of recapitulating developmental and healing morphogenic processes on promoting new complex tissue formation.

Pressure-driven spreadable deferoxamine-laden hydrogels for vascularized skin flaps

The development of hydrogels that support vascularization to improve the survival of skin flaps, yet establishing homogeneous angiogenic niches without compromising the ease of use in surgical settings remains a challenge. Here, pressure-driven spreadable hydrogels were developed utilizing beta-sheet rich silk nanofiber materials. These silk nanofiber-based hydrogels exhibited excellent spreading under mild pressure to form a thin coating to cover all the regions of the skin flaps. Deferoxamine (DFO) was loaded onto the silk nanofibers to support vascularization and these DFO-laden hydrogels were implanted under skin flaps in rats to fill the interface between the wound bed and the flap using the applied pressure. The thickness of the spread hydrogels was below 200 μm, minimizing the physical barrier effects from the hydrogels. The distribution of the hydrogels provided homogeneous angiogenic stimulation, accelerating rapid blood vessel network formation and significantly improving the survival of the skin flaps. The hydrogels also modulated the immune reactions, further facilitating the regeneration of the skin flaps. Considering the homogeneous distribution at the wound sites, improved vascularization, reduced barrier effects and low inflammation, these hydrogels appear to be promising candidates for use in tissue repair where a high blood supply is in demand. The pressure-driven spreading properties should simplify the use of the hydrogels in surgical settings to facilitate clinical translation.

4D Biofabrication Using a Combination of 3D Printing and Melt-Electrowriting of Shape-Morphing Polymers

We report the fabrication of scroll-like scaffolds with anisotropic topography using 4D printing based on a combination of 3D extrusion printing of methacrylated alginate, melt-electrowriting of polycaprolactone fibers, and shape-morphing of the fabricated object. A combination of 3D extrusion printing and melt-electrowriting allows programmed deposition of different materials and fabrication of structures with high resolution. Shape-morphing allows the transformation of a patterned surface of a printed structure in a pattern on inner surface of a folded object that is used to align cells. We demonstrate that the concentration of calcium ions, the environment media, and the geometrical shape of the scaffold influences shape-morphing that allows it to be efficiently programmed. Myoblasts cultured inside a scrolled bilayer scaffold demonstrate excellent viability and proliferation. Moreover, the patterned surface generated by PCL fibers allow a very high degree of orientation of cells, which cannot be achieved on the alginate layer without fibers.

Electric-Field-Induced Gradient Ionogels for Highly Sensitive, Broad-Range-Response, and Freeze/Heat-Resistant Ionic Fingers

Human fingers exhibit both high sensitivity and wide tactile range. The finger skin structures are designed to display gradient microstructures and compressibility. Inspired by the gradient mechanical Young's modulus distribution, an electric-field-induced cationic crosslinker migration strategy is demonstrated to prepare gradient ionogels. Due to the gradient of the crosslinkers, the ionogels exhibit more than four orders of magnitude difference between the anode and the cathode side, enabling gradient ionogel-based flexible iontronic sensors having high-sensitivity and broader-range detection (from 3 × 10² to 2.5 × 10⁶  Pa) simultaneously. Moreover, owing to the remarkable properties of the gradient ionogels, the flexible iontronic sensors also show good long-time stability (even after 10 000 cycles loadings) and excellent performance over a wide temperature range (from -108 to 300 °C). The flexible iontronic sensors are further integrated on soft grips, exhibiting remarkable performance under various conditions. These attractive features demonstrate that gradient ionogels will be promising candidates for smart sensor applications in complex and extreme conditions.

Biomedical application of photo-crosslinked gelatin hydrogels

Abstract

During the past decades, photo-crosslinked gelatin hydrogel (methacrylated gelatin, GelMA) has gained a lot of attention due to its remarkable application in the biomedical field. It has been widely used in cell transplantation, cell culture and drug delivery, based on its crosslinking to form hydrogels with tunable mechanical properties and excellent bio-compatibility when exposed to light irradiation to mimic the micro-environment of native extracellular matrix (ECM). Because of its unique biofunctionality and mechanical tenability, it has also been widely applied in the repair and regeneration of bone, heart, cornea, epidermal tissue, cartilage, vascular, peripheral nerve, oral mucosa, and skeletal muscle et al. The purpose of this review is to summarize the recent application of GelMA in drug delivery and tissue engineering field. Moreover, this review article will briefly introduce both the development of GelMA and the characterization of GelMA. Finally, we discuss the challenges and future development prospects of GelMA as a tissue engineering material and drug or gene delivery carrier, hoping to contribute to accelerating the development of GelMA in the biomedical field.

Graphical abstract