Handheld skin printer: in situ formation of planar biomaterials and tissues

In Situ Bioprinting: Process, Bioinks, and Applications

Traditional tissue engineering methods face challenges, such as fabrication, implantation of irregularly shaped scaffolds, and limited accessibility for immediate healthcare providers. In situ bioprinting, an alternate strategy, involves direct deposition of biomaterials, cells, and bioactive factors at the site, facilitating on-site fabrication of intricate tissue, which can offer a patient-specific personalized approach and align with the principles of precision medicine. It can be applied using a handled device and robotic arms to various tissues, including skin, bone, cartilage, muscle, and composite tissues. Bioinks, the critical components of bioprinting that support cell viability and tissue development, play a crucial role in the success of in situ bioprinting. This review discusses in situ bioprinting techniques, the materials used for bioinks, and their critical properties for successful applications. Finally, we discuss the challenges and future trends in accelerating in situ printing to translate this technology in a clinical settings for personalized regenerative medicine.

A sustainable strategy for generating highly stable human skin equivalents based on fish collagen

Tissue engineered skin equivalents are increasingly recognized as potential alternatives to traditional skin models such as human ex vivo skin or animal skin models. However, most of the currently investigated human skin equivalents (HSEs) are constructed using mammalian collagen which can be expensive and difficult to extract. Fish skin is a waste product produced by fish processing industries and identified as a cost-efficient and sustainable source of type I collagen. In this work, we describe a method for generating highly stable HSEs based on fibrin fortified tilapia fish collagen. The fortified fish collagen (FFC) formulation is optimized to enable reproducible fabrication of full-thickness HSEs that undergo limited contraction, facilitating the incorporation of human donor-derived skin cells and formation of biomimetic dermal and epidermal layers. The morphology and barrier function of the FFC HSEs are compared with a commercial skin model and validated with immunohistochemical staining and transepithelial electrical resistance testing. Finally, the potential of a high throughput screening platform with FFC HSE is explored by scaling down its fabrication to 96-well format.

Sheet-based extrusion bioprinting: a new multi-material paradigm providing mid-extrusion micropatterning control for microvascular applications

As bioprinting advances into clinical relevance with patient-specific tissue and organ constructs, it must be capable of multi-material fabrication at high resolutions to accurately mimick the complex tissue structures found in the body. One of the most fundamental structures to regenerative medicine is microvasculature. Its continuous hierarchical branching vessel networks bridge surgically manipulatable arteries (∼1-6 mm) to capillary beds (∼10µm). Microvascular perfusion must be established quickly for autologous, allogeneic, or tissue engineered grafts to survive implantation and heal in place. However, traditional syringe-based bioprinting techniques have struggled to produce perfusable constructs with hierarchical branching at the resolution of the arterioles (∼100-10µm) found in microvascular tissues. This study introduces the novel CEVIC bioprinting device (i.e.ContinuouslyExtrudedVariableInternalChanneling), a multi-material technology that breaks the current extrusion-based bioprinting paradigm of pushing cell-laden hydrogels through a nozzle as filaments, instead, in the version explored here, extruding thin, wide cell-laden hydrogel sheets. The CEVIC device adapts the chaotic printing approach to control the width and number of microchannels within the construct as it is extruded (i.e. on-the-fly). Utilizing novel flow valve designs, this strategy can produce continuous gradients varying geometry and materials across the construct and hierarchical branching channels with average widths ranging from 621.5 ± 42.92%µm to 11.67 ± 14.99%µm, respectively, encompassing the resolution range of microvascular vessels. These constructs can also include fugitive/sacrificial ink that vacates to leave demonstrably perfusable channels. In a proof-of-concept experiment, a co-culture of two microvascular cell types, endothelial cells and pericytes, sustained over 90% viability throughout 1 week in microchannels within CEVIC-produced gelatin methacryloyl-sodium alginate hydrogel constructs. These results justify further exploration of generating CEVIC-bioprinted microvasculature, such as pre-culturing and implantation studies.

Surface slicing and toolpath planning forin-situbioprinting of skin implants

Bioprinting has emerged as a successful method for fabricating engineered tissue implants, offering great potential for wound healing applications. This study focuses on an advanced surface-based slicing approach aimed at designing a skin implant specifically forin-situbioprinting. The slicing step plays a crucial role in determining the layering arrangement of the tissue during printing. By utilizing surface slicing, a significant shift from planar fabrication methods is achieved. The developed methodology involves the utilization of a customized robotic printer to deliver biomaterials. A multilayer slicing and toolpath generation procedure is presented, enabling the fabrication of skin implants that incorporate the epidermal, dermal, and hypodermal layers. One notable advantage of using the approximate representation of the native wound site surface as the slicing surface is the avoidance of planar printing effects such as staircasing. This surface slicing method allows for the design of non-planar and ultra-thin skin implants, ensuring a higher degree of geometric match between the implant and the wound interface. Furthermore, the proposed methodology demonstrates superior surface quality of thein-situbio-printed implant on a hand model, validating its ability to create toolpaths on implants with complex surfaces.

Long-term day-by-day tracking of microvascular networks sprouting in fibrin gels: From detailed morphological analyses to general growth rules

Understanding and controlling of the evolution of sprouting vascular networks remains one of the basic challenges in tissue engineering. Previous studies on the vascularization dynamics have typically focused only on the phase of intense growth and often lacked spatial control over the initial cell arrangement. Here, we perform long-term day-by-day analysis of tens of isolated microvasculatures sprouting from endothelial cell-coated spherical beads embedded in an external fibrin gel. We systematically study the topological evolution of the sprouting networks over their whole lifespan, i.e., for at least 14 days. We develop a custom image analysis toolkit and quantify (i) the overall length and area of the sprouts, (ii) the distributions of segment lengths and branching angles, and (iii) the average number of branch generations-a measure of network complexity. We show that higher concentrations of vascular endothelial growth factor (VEGF) lead to earlier sprouting and more branched networks, yet without significantly affecting the speed of growth of individual sprouts. We find that the mean branching angle is weakly dependent on VEGF and typically in the range of 60°-75°, suggesting that, by comparison with the available diffusion-limited growth models, the bifurcating tips tend to follow local VEGF gradients. At high VEGF concentrations, we observe exponential distributions of segment lengths, which signify purely stochastic branching. Our results-due to their high statistical relevance-may serve as a benchmark for predictive models, while our new image analysis toolkit, offering unique features and high speed of operation, could be exploited in future angiogenic drug tests.

Advancements of 3D bioprinting in regenerative medicine: Exploring cell sources for organ fabrication

3D bioprinting has unlocked new possibilities for generating complex and functional tissues and organs. However, one of the greatest challenges lies in selecting the appropriate seed cells for constructing fully functional 3D artificial organs. Currently, there are no cell sources available that can fulfill all requirements of 3D bioprinting technologies, and each cell source possesses unique characteristics suitable for specific applications. In this review, we explore the impact of different 3D bioprinting technologies and bioink materials on seed cells, providing a comprehensive overview of the current landscape of cell sources that have been used or hold potential in 3D bioprinting. We also summarized key points to guide the selection of seed cells for 3D bioprinting. Moreover, we offer insights into the prospects of seed cell sources in 3D bioprinted organs, highlighting their potential to revolutionize the fields of tissue engineering and regenerative medicine.

3D bioprinting bioglass to construct vascularized full-thickness skin substitutes for wound healing

Constructing three-dimensional (3D) bioprinted skin tissues that accurately replicate the mechanical properties of native skin and provide adequate oxygen and nutrient support remains a formidable challenge. In this study, we incorporated phosphosilicate calcium bioglasses (PSCs), a type of bioactive glass (BG), into the bioinks used for 3D bioprinting. The resulting bioink exhibited mechanical properties and biocompatibility that closely resembled those of natural skin. Utilizing 3D bioprinting technology, we successfully fabricated full-thickness skin substitutes, which underwent comprehensive evaluation to assess their regenerative potential in treating full-thickness skin injuries in rats. Remarkably, the skin substitutes loaded with PSCs exhibited exceptional angiogenic activity, as evidenced by the upregulation of angiogenesis-related genes in vitro and the observation of enhanced vascularization in wound tissue sections in vivo. These findings conclusively demonstrated the outstanding efficacy of PSCs in promoting angiogenesis and facilitating the repair of full-thickness skin wounds. The insights garnered from this study provide a valuable reference strategy for the development of skin tissue grafts with potent angiogenesis-inducing capabilities.

Advanced function, design and application of skin substitutes for skin regeneration

The development of skin substitutes aims to replace, mimic, or improve the functions of human skin, regenerate damaged skin tissue, and replace or enhance skin function. This includes artificial skin, scaffolds or devices designed for treatment, imitation, or improvement of skin function in wounds and injuries. Therefore, tremendous efforts have been made to develop functional skin substitutes. However, there is still few reports systematically discuss the relationship between the advanced function and design requirements. In this paper, we review the classification, functions, and design requirements of artificial skin or skin substitutes. Different manufacturing strategies for skin substitutes such as hydrogels, 3D/4D printing, electrospinning, microfluidics are summarized. This review also introduces currently available skin substitutes in clinical trials and on the market and the related regulatory requirements. Finally, the prospects and challenges of skin substitutes in the field of tissue engineering are discussed.

3D biofabrication and space: A 'far-fetched dream' or a 'forthcoming reality'?

The long duration space missions across the Low Earth Orbit (LEO) often expose the voyagers to an abrupt zero gravity influence. The severe extraterrestrial cosmic radiation directly causes a plethora of moderate to chronic healthcare crises. The only feasible solution to manage critical injuries on board is surgical interventions or immediate return to Earth. This led the group of space medicine practitioners to adopt principles from tissue engineering and develop human tissue equivalents as an immediate regenerative therapy on board. The current review explicitly demonstrates the constructive application of different tissue-engineered equivalents matured under the available ground-based microgravity simulation facilities. Further, it elucidates how augmenting the superiority of biomaterial-based 3D bioprinting technology can enhance their clinical applicability. Additionally, the regulatory role of weightlessness condition on the underlying cellular signaling pathways governing tissue morphogenesis has been critically discussed. This information will provide future directions on how 3D biofabrication can be used as a plausible tool for healing on-flight chronic health emergencies. Thus, in our review, we aimed to precisely debate whether 3D biofabrication is deployed to cater to on-flight healthcare anomalies or space-like conditions are being utilized for generating 3D bioprinted human tissue constructs for efficient drug screening and regenerative therapy.

An Adhesive Bioink toward Biofabrication under Wet Conditions

Three-dimensional (3D) bioprinting is driving significant innovations in biomedicine over recent years. Under certain scenarios such as in intraoperative bioprinting, the bioinks used should exhibit not only cyto/biocompatibility but also adhesiveness in wet conditions. Herein, an adhesive bioink composed of gelatin methacryloyl, gelatin, methacrylated hyaluronic acid, and skin secretion of Andrias davidianus is designed. The bioink exhibits favorable cohesion to allow faithful extrusion bioprinting in wet conditions, while simultaneously showing good adhesion to a variety of surfaces of different chemical properties, possibly achieved through the diverse bonds presented in the bioink formulation. As such, this bioink is able to fabricate sophisticated planar and volumetric constructs using extrusion bioprinting, where the dexterity is further enhanced using ergonomic handheld bioprinters to realize in situ bioprinting. In vitro experiments reveal that cells maintain high viability; further in vivo studies demonstrate good integration and immediate injury sealing. The characteristics of the bioink indicate its potential widespread utility in extrusion bioprinting and will likely broaden the applications of bioprinting toward situations such as in situ dressing and minimally invasive tissue regeneration.

Advances in volumetric bioprinting

The three-dimensional (3D) bioprinting technologies are suitable for biomedical applications owing to their ability to manufacture complex and high-precision tissue constructs. However, the slow printing speed of current layer-by-layer (bio)printing modality is the major limitation in biofabrication field. To overcome this issue, volumetric bioprinting (VBP) is developed. VBP changes the layer-wise operation of conventional devices, permitting the creation of geometrically complex, centimeter-scale constructs in tens of seconds. VBP is the next step onward from sequential biofabrication methods, opening new avenues for fast additive manufacturing in the fields of tissue engineering, regenerative medicine, personalized drug testing, and soft robotics, etc. Therefore, this review introduces the printing principles and hardware designs of VBP-based techniques; then focuses on the recent advances in VBP-based (bio)inks and their biomedical applications. Lastly, the current limitations of VBP are discussed together with future direction of research.

An Overview of Recent Developments in the Management of Burn Injuries

According to the World Health Organization (WHO), around 11 million people suffer from burns every year, and 180,000 die from them. A burn is a condition in which heat, chemical substances, an electrical current or other factors cause tissue damage. Burns mainly affect the skin, but can also affect deeper tissues such as bones or muscles. When burned, the skin loses its main functions, such as protection from the external environment, pathogens, evaporation and heat loss. Depending on the stage of the burn, the patient's condition and the cause of the burn, we need to choose the most appropriate treatment. Personalization and multidisciplinary collaboration are key to the successful management of burn patients. In this comprehensive review, we have collected and discussed the available treatment options, focusing on recent advances in topical treatments, wound cleansing, dressings, skin grafting, nutrition, pain and scar tissue management.

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.

Tissue engineered skin substitutes: A comprehensive review of basic design, fabrication using 3D printing, recent advances and challenges

The multi-layered skin structure includes the epidermis, dermis and hypodermis, which forms a sophisticated tissue composed of extracellular matrix (ECM). The wound repair is a well-orchestrated process when the skin is injured. However, this natural wound repair will be ineffective for large surface area wounds. Autografts-based treatment is efficient but, additional pain and secondary healing of the patient limits its successful application. Therefore, there is a substantial need for fabricating tissue-engineered skin constructs. The development of a successful skin graft requires a fundamental understanding of the natural skin and its healing process, as well as design criteria for selecting a biopolymer and an appropriate fabrication technique. Further, the fabrication of an appropriate skin graft needs to meet physicochemical, mechanical, and biological properties equivalent to the natural skin. Advanced 3D bioprinting provides spatial control of the placement of functional components, such as biopolymers with living cells, which can satisfy the prerequisites for the preparation of an ideal skin graft. In this view, here we elaborate on the basic design requirements, constraints involved in the fabrication of skin graft and choice of ink, the probable solution by 3D bioprinting technique, as well as their latest advancements, challenges, and prospects.

The third dimension of tumor microenvironment-The importance of tumor stroma in 3D cancer models

Recent advances in the three-dimensional (3D) cancer models give rise to a plethora of new possibilities in the development of anti-cancer drug therapies and bring us closer to personalized medicine. Three-dimensional models are undoubtedly more authentic than traditional two-dimensional (2D) cell cultures. Nowadays, they are becoming preferentially used in most cancer research fields due to their more accurate biomimetic characteristics. On the contrary, they still lack the cellular and matrix complexity of the native tumor microenvironment (TME). This review focuses on the description of existing 3D models, the incorporation of TME and fluidics into these models, and their perspective in the future research. It is clear that such an improvement would need not only biological but also technical progress. Therefore, the modern approach to anti-cancer drug discovery should involve various fields.

In vivo bioprinting: Broadening the therapeutic horizon for tissue injuries

Tissue injury is a collective term for various disorders associated with organs and tissues induced by extrinsic or intrinsic factors, which significantly concerns human health. In vivo bioprinting, an emerging tissue engineering approach, allows for the direct deposition of bioink into the defect sites inside the patient's body, effectively addressing the challenges associated with the fabrication and implantation of irregularly shaped scaffolds and enabling the rapid on-site management of tissue injuries. This strategy complements operative therapy as well as pharmacotherapy, and broadens the therapeutic horizon for tissue injuries. The implementation of in vivo bioprinting requires targeted investigations in printing modalities, bioinks, and devices to accommodate the unique intracorporal microenvironment, as well as effective integrations with intraoperative procedures to facilitate its clinical application. In this review, we summarize the developments of in vivo bioprinting from three perspectives: modalities and bioinks, devices, and clinical integrations, and further discuss the current challenges and potential improvements in the future.

3D bioprinting-a model for skin aging

Human lifespan continues to extend as an unprecedented number of people reach their seventh and eighth decades of life, unveiling chronic conditions that affect the older adult. Age-related skin conditions include senile purpura, seborrheic keratoses, pemphigus vulgaris, bullous pemphigoid, diabetic foot wounds and skin cancer. Current methods of drug testing prior to clinical trials require the use of pre-clinical animal models, which are often unable to adequately replicate human skin response. Therefore, a reliable model for aged human skin is needed. The current challenges in developing an aged human skin model include the intrinsic variability in skin architecture from person to person. An ideal skin model would incorporate innate functionality such as sensation, vascularization and regeneration. The advent of 3D bioprinting allows us to create human skin equivalent for use as clinical-grade surgical graft, for drug testing and other needs. In this review, we describe the process of human skin aging and outline the steps to create an aged skin model with 3D bioprinting using skin cells (i.e. keratinocytes, fibroblasts and melanocytes). We also provide an overview of current bioprinted skin models, associated limitations and direction for future research.

Portable Handheld "SkinPen" Loaded with Biomaterial Ink for In Situ Wound Healing

In situ bioprinting has emerged as an attractive tool for directly depositing therapy ink at the defective area to adapt to the irregular wound shape. However, traditional bioprinting exhibits an obvious limitation in terms of an unsatisfactory bioadhesive effect. Here, a portable handheld bioprinter loaded with biomaterial ink is designed and named "SkinPen". Gelatin methacrylate (GelMA) and Cu-containing bioactive glass nanoparticles (Cu-BGn) serve as the main components to form the hydrogel ink, which displays excellent biocompatibility and antibacterial and angiogenic properties. More importantly, by introducing ultrasound and ultraviolet in a sequential programmed manner, the SkinPen achieves in situ instant gelation and amplified (more than threefold) bioadhesive shear strength. It is suggested that ultrasound-induced cavitation and the resulting topological entanglement contribute to the enhanced bioadhesive performance together. Combining the ultrasound-enhanced bioadhesion with the curative role of the hydrogel, the SkinPen shows a satisfactory wound-healing effect in diabetic rats. Given the detachable property of the SkinPen, the whole device can be put in a first-aid kit. Therefore, the application scenarios can be expanded to many kinds of accidents. Overall, this work presents a portable handheld SkinPen that might provide a facile but effective approach for clinical wound management.

Hyaluronic Acid as Bioink and Hydrogel Scaffolds for Tissue Engineering Applications

Bioprinting is an additive manufacturing technique that focuses on developing living tissue constructs using bioinks. Bioink is crucial in determining the stability of printed patterns, which remains a major challenge in bioprinting. Thus, the choices of bioink composition, modifications, and cross-linking methods are being continuously researched to augment the clinical translation of bioprinted constructs. Hyaluronic acid (HA) is a naturally occurring polysaccharide with the repeating unit of N-acetyl-glucosamine and d-glucuronic acid disaccharides. It is present in the extracellular matrix (ECM) of tissues (skin, cartilage, nerve, muscle, etc.) with a wide range of molecular weights. Due to the nature of its chemical structure, HA could be easily subjected to chemical modifications and cross-linking that would enable better printability and stability. These interesting properties have made HA an ideal choice of bioinks for developing tissue constructs for regenerative medicine applications. In this Review, the physicochemical properties, reaction chemistry involved in various cross-linking strategies, and biomedical applications of HA have been elaborately discussed. Further, the features of HA bioinks, emerging strategies in HA bioink preparations, and their applications in 3D bioprinting have been highlighted. Finally, the current challenges and future perspectives in the clinical translation of HA-based bioinks are outlined.

Developing a bioink for single-step deposition and maturation of human epidermis

Patients with severe burns, which cause extensive damage to their skin, require rapid intervention to prevent life-threatening hypothermia, infection, and fluid loss. Current treatments typically involve surgical excision of the burned skin and reconstruction of the wound with the aid of skin autografts. However, there is a lack of donor site in the most severe cases. While alternative treatments such as cultured epithelial autografts and "spray-on" skin can allow much smaller donor tissues to be used (and hence reduce donor site morbidity), they present their own challenges in terms of fragility of the tissues and control of the cell deposition, respectively. Recent advances in bioprinting technology have led researchers to explore its use to fabricate skin grafts, which depend on several factors, including appropriate bioinks, cell types, and printability. In this work, we describe a collagen-based bioink that allows the deposition of a contiguous layer of the keratinocytes directly onto the wound. Special attention was given to the intended clinical workflow. For example, since media changes are not feasible once the bioink is deposited onto the patient, we first developed a media formulation designed to permit a single deposition step and promote self-organization of the cells into the epidermis. Using a collagen-based dermal template populated with dermal fibroblasts, we demonstrated by immunofluorescence staining that the resulting epidermis recapitulates the features of natural skin in expressing p63 (stem cell marker), Ki67 and keratin 14 (proliferation markers), filaggrin and keratin 10 (keratinocyte differentiation and barrier function markers), and collagen type IV (basement membrane protein involved in adherence of the epidermis to the dermis). While further tests are still required to verify its utility as a burn treatment, based on the results we have achieved thus far, we believe that our current protocol can already produce donor-specific model for testing purposes.

Bioinks adapted for in situ bioprinting scenarios of defect sites: a review

In situ bioprinting provides a reliable solution to the problem of in vitro tissue culture and vascularization by printing tissue directly at the site of injury or defect and maturing the printed tissue using the natural cell microenvironment in vivo. As an emerging field, in situ bioprinting is based on computer-assisted scanning results of the defect site and is able to print cells directly at this site with biomaterials, bioactive factors, and other materials without the need to transfer prefabricated grafts as with traditional in vitro 3D bioprinting methods, and the resulting grafts can accurately adapt to the target defect site. However, one of the important reasons hindering the development of in situ bioprinting is the absence of suitable bioinks. In this review, we will summarize bioinks developed in recent years that can adapt to in situ printing scenarios at the defect site, considering three aspects: the in situ design strategy of bioink, the selection of commonly used biomaterials, and the application of bioprinting to different treatment scenarios.

Robot-assisted in situ bioprinting of gelatin methacrylate hydrogels with stem cells induces hair follicle-inclusive skin regeneration

Large skin defects caused by accidents or disease can cause fluid loss, water and electrolyte disorders, hypoproteinemia and serious infection and remain a difficult problem in clinical practice. In situ bioprinting is a promising, recently developed technology that involves timely, customized, and morphologically adapted bioprinting of bioink into tissue defects to promote the recovery of human tissues or organs. During this process, bioink is a key factor. In this study, we synthesized a biocompatible, photosensitive hydrogel material comprising gelatin methacrylate (GelMA) for robot-assisted in situ bioprinting of skin wounds. The results showed that GelMA demonstrated good printability of that supported the proliferation of skin-derived precursors (SKPs) and maintained their properties. Furthermore, in situ bioprinting of GelMA hydrogels with epidermal stem cells (Epi-SCs) and SKPs onto skin wounds showed complete wound healing and functional tissue skin regeneration. The regenerated skin contains epidermis, dermis, blood vessels, hair follicles, and sebaceous glands and resembling native skin. These results provide an effective strategy for skin repair through the combined application of GelMA hydrogels, Epi-SCs, SKPs and in situ bioprinting and its promising clinical translational potential for further applications.

Deployable extrusion bioprinting of compartmental tumoroids with cancer associated fibroblasts for immune cell interactions

Realizing the translational impacts of three-dimensional (3D) bioprinting for cancer research necessitates innovation in bioprinting workflows which integrate affordability, user-friendliness, and biological relevance. Herein, we demonstrate 'BioArm', a simple, yet highly effective extrusion bioprinting platform, which can be folded into a carry-on pack, and rapidly deployed between bio-facilities. BioArm enabled the reconstruction of compartmental tumoroids with cancer-associated fibroblasts (CAFs), forming the shell of each tumoroid. The 3D printed core-shell tumoroids showedde novosynthesized extracellular matrices, and enhanced cellular proliferation compared to the tumour alone 3D printed spheroid culture. Further, thein vivophenotypes of CAFs normally lost after conventional 2D co-culture re-emerged in the bioprinted model. Embedding the 3D printed tumoroids in an immune cell-laden collagen matrix permitted tracking of the interaction between immune cells and tumoroids, and subsequent simulated immunotherapy treatments. Our deployable extrusion bioprinting workflow could significantly widen the accessibility of 3D bioprinting for replicating multi-compartmental architectures of tumour microenvironment, and for developing strategies in cancer drug testing in the future.

Skin Tissue Engineering Advances in Burns: A Brief Introduction to the Past, the Present, and the Future Potential

Burn injuries are a severe form of skin damage with a significant risk of scarring and systemic sequelae. Approximately 11 million individuals worldwide suffer burn injuries annually, with 180,000 people dying due to their injuries. Wound healing is considered the main determinant for the survival of severe burns and remains a challenge. The surgical treatment of burn wounds entails debridement of necrotic tissue, and the wound is covered with autologous skin substitutes taken from healthy donor areas. Autologous skin transplantation is still considered to be the gold standard for wound repair. However, autologous skin grafts are not always possible, especially in cases with extensive burns and limited donor sites. Allografts from human cadaver skin and xenografts from pig skin may be used in these situations to cover the wounds temporarily. Alternatively, dermal analogs are used until permanent coverage with autologous skin grafts or artificial skins can be achieved, requiring staged procedures to prolong the healing times with the associated risks of local and systemic infection. Over the last few decades, the wound healing process through tissue-engineered skin substitutes has significantly enhanced as the advances in intensive care ensuring early survival have led to the need to repair large skin defects. The focus has shifted from survival to the quality of survival, necessitating accelerated wound repair. This special volume of JBCR is dedicated to the discoveries, developments, and applications leading the reader into the past, present, and future perspectives of skin tissue engineering in burn injuries.

Structural and biological engineering of 3D hydrogels for wound healing

Chronic wounds have become one of the most important issues for healthcare systems and are a leading cause of death worldwide. Wound dressings are necessary to facilitate wound treatment. Engineering wound dressings may substantially reduce healing time, reduce the risk of recurrent infections, and reduce the disability and costs associated. In the path of engineering of an ideal wound dressing, hydrogels have played a leading role. Hydrogels are 3D hydrophilic polymeric structures that can provide a protective barrier, mimic the native extracellular matrix (ECM), and provide a humid environment. Due to their advantages, hydrogels (with different architectural, physical, mechanical, and biological properties) have been extensively explored as wound dressing platforms. Here we describe recent studies on hydrogels for wound healing applications with a strong focus on the interplay between the fabrication method used and the architectural, mechanical, and biological performance achieved. Moreover, we review different categories of additives which can enhance wound regeneration using 3D hydrogel dressings. Hydrogel engineering for wound healing applications promises the generation of smart solutions to solve this pressing problem, enabling key functionalities such as bacterial growth inhibition, enhanced re-epithelialization, vascularization, improved recovery of the tissue functionality, and overall, accelerated and effective wound healing.

Advances in 3D skin bioprinting for wound healing and disease modeling

Even with many advances in design strategies over the past three decades, an enormous gap remains between existing tissue engineering skin and natural skin. Currently available in vitro skin models still cannot replicate the three-dimensionality and heterogeneity of the dermal microenvironment sufficiently to recapitulate many of the known characteristics of skin disorder or disease in vivo. Three-dimensional (3D) bioprinting enables precise control over multiple compositions, spatial distributions and architectural complexity, therefore offering hope for filling the gap of structure and function between natural and artificial skin. Our understanding of wound healing process and skin disease would thus be boosted by the development of in vitro models that could more completely capture the heterogeneous features of skin biology. Here, we provide an overview of recent advances in 3D skin bioprinting, as well as design concepts of cells and bioinks suitable for the bioprinting process. We focus on the applications of this technology for engineering physiological or pathological skin model, focusing more specifically on the function of skin appendages and vasculature. We conclude with current challenges and the technical perspective for further development of 3D skin bioprinting.

Microfluidic Formulation of Topological Hydrogels for Microtissue Engineering

Microfluidics has recently emerged as a powerful tool in generation of submillimeter-sized cell aggregates capable of performing tissue-specific functions, so-called microtissues, for applications in drug testing, regenerative medicine, and cell therapies. In this work, we review the most recent advances in the field, with particular focus on the formulation of cell-encapsulating microgels of small "dimensionalities": "0D" (particles), "1D" (fibers), "2D" (sheets), etc., and with nontrivial internal topologies, typically consisting of multiple compartments loaded with different types of cells and/or biopolymers. Such structures, which we refer to as topological hydrogels or topological microgels (examples including core-shell or Janus microbeads and microfibers, hollow or porous microstructures, or granular hydrogels) can be precisely tailored with high reproducibility and throughput by using microfluidics and used to provide controlled "initial conditions" for cell proliferation and maturation into functional tissue-like microstructures. Microfluidic methods of formulation of topological biomaterials have enabled significant progress in engineering of miniature tissues and organs, such as pancreas, liver, muscle, bone, heart, neural tissue, or vasculature, as well as in fabrication of tailored microenvironments for stem-cell expansion and differentiation, or in cancer modeling, including generation of vascularized tumors for personalized drug testing. We review the available microfluidic fabrication methods by exploiting various cross-linking mechanisms and various routes toward compartmentalization and critically discuss the available tissue-specific applications. Finally, we list the remaining challenges such as simplification of the microfluidic workflow for its widespread use in biomedical research, bench-to-bedside transition including production upscaling, further in vivo validation, generation of more precise organ-like models, as well as incorporation of induced pluripotent stem cells as a step toward clinical applications.

In situ bioprinting: intraoperative implementation of regenerative medicine

Bioprinting has emerged as a strong tool for devising regenerative therapies to address unmet medical needs. However, the translation of conventional in vitro bioprinting approaches is partially hindered due to challenges associated with the fabrication and implantation of irregularly shaped scaffolds and their limited accessibility for immediate treatment by healthcare providers. An alternative strategy that has recently drawn significant attention is to directly print the bioink into the patient's body, so-called 'in situ bioprinting'. The bioprinting strategy and the associated bioink need to be specifically designed for in situ bioprinting to meet the particular requirements of direct deposition in vivo. In this review, we discuss the developed in situ bioprinting strategies, their advantages, challenges, and possible future improvements.

Innovative Treatment Strategies to Accelerate Wound Healing: Trajectory and Recent Advancements

Wound healing is highly specialized dynamic multiple phase process for the repair of damaged/injured tissues through an intricate mechanism. Any failure in the normal wound healing process results in abnormal scar formation, and chronic state which is more susceptible to infections. Chronic wounds affect patients’ quality of life along with increased morbidity and mortality and are huge financial burden to healthcare systems worldwide, and thus requires specialized biomedical intensive treatment for its management. The clinical assessment and management of chronic wounds remains challenging despite the development of various therapeutic regimens owing to its painstakingly long-term treatment requirement and complex wound healing mechanism. Various conventional approaches such as cell therapy, gene therapy, growth factor delivery, wound dressings, and skin grafts etc., are being utilized for promoting wound healing in different types of wounds. However, all these abovementioned therapies are not satisfactory for all wound types, therefore, there is an urgent demand for the development of competitive therapies. Therefore, there is a pertinent requirement to develop newer and innovative treatment modalities for multipart therapeutic regimens for chronic wounds. Recent developments in advanced wound care technology includes nanotherapeutics, stem cells therapy, bioengineered skin grafts, and 3D bioprinting-based strategies for improving therapeutic outcomes with a focus on skin regeneration with minimal side effects. The main objective of this review is to provide an updated overview of progress in therapeutic options in chronic wounds healing and management over the years using next generation innovative approaches. Herein, we have discussed the skin function and anatomy, wounds and wound healing processes, followed by conventional treatment modalities for wound healing and skin regeneration. Furthermore, various emerging and innovative strategies for promoting quality wound healing such as nanotherapeutics, stem cells therapy, 3D bioprinted skin, extracellular matrix-based approaches, platelet-rich plasma-based approaches, and cold plasma treatment therapy have been discussed with their benefits and shortcomings. Finally, challenges of these innovative strategies are reviewed with a note on future prospects.

Advances in 3D bioprinting of tissues/organs for regenerative medicine and in-vitro models

Tissue/organ shortage is a major medical challenge due to donor scarcity and patient immune rejections. Furthermore, it is difficult to predict or mimic the human disease condition in animal models during preclinical studies because disease phenotype differs between humans and animals. Three-dimensional bioprinting (3DBP) is evolving into an unparalleled multidisciplinary technology for engineering three-dimensional (3D) biological tissue with complex architecture and composition. The technology has emerged as a key driver by precise deposition and assembly of biomaterials with patient's/donor cells. This advancement has aided in the successful fabrication of in vitro models, preclinical implants, and tissue/organs-like structures. Here, we critically reviewed the current state of 3D-bioprinting strategies for regenerative therapy in eight organ systems, including nervous, cardiovascular, skeletal, integumentary, endocrine and exocrine, gastrointestinal, respiratory, and urinary systems. We also focus on the application of 3D bioprinting to fabricated in vitro models to study cancer, infection, drug testing, and safety assessment. The concept of in situ 3D bioprinting is discussed, which is the direct printing of tissues at the injury or defect site for reparative and regenerative therapy. Finally, issues such as scalability, immune response, and regulatory approval are discussed, as well as recently developed tools and technologies such as four-dimensional and convergence bioprinting. In addition, information about clinical trials using 3D printing has been included.

Development of in situ bioprinting: A mini review

Bioprinting has rapidly progressed over the past decade. One branch of bioprinting known as in situ bioprinting has benefitted considerably from innovations in biofabrication. Unlike ex situ bioprinting, in situ bioprinting allows for biomaterials to be printed directly into or onto the target tissue/organ, eliminating the need to transfer pre-made three-dimensional constructs. In this mini-review, recent progress on in situ bioprinting, including bioink composition, in situ crosslinking strategies, and bioprinter functionality are examined. Future directions of in situ bioprinting are also discussed including the use of minimally invasive bioprinters to print tissues within the body.

Artificial Intelligence-Empowered 3D and 4D Printing Technologies toward Smarter Biomedical Materials and Approaches

In the last decades, 3D printing has played a crucial role as an innovative technology for tissue and organ fabrication, patient-specific orthoses, drug delivery, and surgical planning. However, biomedical materials used for 3D printing are usually static and unable to dynamically respond or transform within the internal environment of the body. These materials are fabricated ex situ, which involves first printing on a planar substrate and then deploying it to the target surface, thus resulting in a possible mismatch between the printed part and the target surfaces. The emergence of 4D printing addresses some of these drawbacks, opening an attractive path for the biomedical sector. By preprogramming smart materials, 4D printing is able to manufacture structures that dynamically respond to external stimuli. Despite these potentials, 4D printed dynamic materials are still in their infancy of development. The rise of artificial intelligence (AI) could push these technologies forward enlarging their applicability, boosting the design space of smart materials by selecting promising ones with desired architectures, properties, and functions, reducing the time to manufacturing, and allowing the in situ printing directly on target surfaces achieving high-fidelity of human body micro-structures. In this review, an overview of 4D printing as a fascinating tool for designing advanced smart materials is provided. Then will be discussed the recent progress in AI-empowered 3D and 4D printing with open-loop and closed-loop methods, in particular regarding shape-morphing 4D-responsive materials, printing on moving targets, and surgical robots for in situ printing. Lastly, an outlook on 5D printing is given as an advanced future technique, in which AI will assume the role of the fifth dimension to empower the effectiveness of 3D and 4D printing for developing intelligent systems in the biomedical sector and beyond.

Emerging tissue engineering strategies for the corneal regeneration

Cornea as the outermost layer of the eye is at risk of various genetic and environmental diseases that can damage the cornea and impair vision. Corneal transplantation is among the most applicable surgical procedures for repairing the defected tissue. However, the scarcity of healthy tissue donations as well as transplantation failure has remained as the biggest challenges in confront of corneal grafting. Therefore, alternative approaches based on stem-cell transplantation and classic regenerative medicine have been developed for corneal regeneration. In this review, the application and limitation of the recently-used advanced approaches for regeneration of cornea are discussed. Additionally, other emerging powerful techniques such as 5D printing as a new branch of scaffold-based technologies for construction of tissues other than the cornea are highlighted and suggested as alternatives for corneal reconstruction. The introduced novel techniques may have great potential for clinical applications in corneal repair including disease modeling, 3D pattern scheming, and personalized medicine.

3D direct printing of composite bone scaffolds containing polylactic acid and spray dried mesoporous bioactive glass-ceramic microparticles

In this study, a three-dimensional composite scaffold is proposed consisting of polylactic acid and spray dried glass-ceramic microparticles (SGCMs). The compositional and structural characterization showed that the obtained spray dried powder formed as glass-ceramic (GC) with a completely interconnected porosity structure. Before direct printing of scaffolds, the rheological behavior of polylactic acid (PLA) and PLA-GC (PLA matrix containing SGCMs) inks were investigated. The PLA-GC composite ink represents sharper shear-thinning behavior and higher loss and storage modulus comparable to that of pure PLA. Microscopic observations and elemental mapping elements showed that 3D scaffolds had well-defined interconnected porosity and uniform distribution of the glass-ceramic particles. Mechanical tests indicated that compression strength is dependent on the scaffold porosity and the presence of SGCMs. Apatite formation evaluation besides ion release study showed better biomineralization capacity of PLA-GC scaffolds, as larger and denser sediments formed on the PLA-GC scaffolds after 7- and 14-day soaking. The preliminary cell response was studied with primary human mesenchymal stem cells (hMSCs) and revealed that SGCMs improved cell adhesion and viability and ALP activity. The appropriate combination of the biomaterials/methods to fabricate 3D porous constructs and their available bioactivity and biocompatibility, both being important characteristics for bone tissue engineering applications.

A Review on Current Designation of Metallic Nanocomposite Hydrogel in Biomedical Applications

In the past few decades, nanotechnology has been receiving significant attention globally and is being continuously developed in various innovations for diverse applications, such as tissue engineering, biotechnology, biomedicine, textile, and food technology. Nanotechnological materials reportedly lack cell-interactive properties and are easily degraded into unfavourable products due to the presence of synthetic polymers in their structures. This is a major drawback of nanomaterials and is a cause of concern in the biomedicine field. Meanwhile, particulate systems, such as metallic nanoparticles (NPs), have captured the interest of the medical field due to their potential to inhibit the growth of microorganisms (bacteria, fungi, and viruses). Lately, researchers have shown a great interest in hydrogels in the biomedicine field due to their ability to retain and release drugs as well as to offer a moist environment. Hence, the development and innovation of hydrogel-incorporated metallic NPs from natural sources has become one of the alternative pathways for elevating the efficiency of therapeutic systems to make them highly effective and with fewer undesirable side effects. The objective of this review article is to provide insights into the latest fabricated metallic nanocomposite hydrogels and their current applications in the biomedicine field using nanotechnology and to discuss the limitations of this technology for future exploration. This article gives an overview of recent metallic nanocomposite hydrogels fabricated from bioresources, and it reviews their antimicrobial activities in facilitating the demands for their application in biomedicine. The work underlines the fabrication of various metallic nanocomposite hydrogels through the utilization of natural sources in the production of biomedical innovations, including wound healing treatment, drug delivery, scaffolds, etc. The potential of these nanocomposites in relation to their mechanical strength, antimicrobial activities, cytotoxicity, and optical properties has brought this technology into a new dimension in the biomedicine field. Finally, the limitations of metallic nanocomposite hydrogels in terms of their methods of synthesis, properties, and outlook for biomedical applications are further discussed.

Hydrogels for Tissue Engineering: Addressing Key Design Needs Toward Clinical Translation

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Simple and robust 3D bioprinting of full-thickness human skin tissue

Artificial skins have been used as skin substitutes for wound healing in the clinic, and as in vitro models for safety assessment in cosmetic and pharmaceutical industries. The three-dimensional (3D) bioprinting technique provides a promising strategy in the fabrication of artificial skins. Despite the technological advances, many challenges remain to be conquered, such as the complicated preparation conditions for bio-printed skin and the unavailability of stability and robustness of skin bioprinting. Here, we formulated a novel bio-ink composed of gelatin, sodium alginate and fibrinogen. By optimizing the ratio of components in the bio-ink, the design of the 3D model and the printing conditions, a fibroblasts-containing dermal layer construct was firstly fabricated, on the top of which laminin and keratinocytes were sequentially placed. Through air-liquid interface (ALI) culture by virtue of sterile wire mesh, a full-thickness skin tissue was thus prepared. HE and immunofluorescence staining showed that the bio-printed skin was not only morphologically representative of the human skin, but also expressed the specific markers related to epidermal differentiation and stratum corneum formation. The presented easy and robust preparation of full-thickness skin constructs provides a powerful tool for the establishment of artificial skins, holding critical academic significance and application value.

Cell-based dressings: A journey through chronic wound management

The field of regenerative medicine has undergone a paradigm shift in recent decades thanks to the emergence of novel therapies based on the use of living organisms. The development of cell-based strategies has become a trend for the treatment of different conditions and pathologies. In this sense, the need for more adequate, biomimetic and well-planned treatments for chronic wounds has found different and innovative strategies, based on the combination of cells with dressings, which seek to revolutionize the wound healing management. Therefore, the objective of this review is to analyze the current state and the latest advances in the research of cell-based dressings for chronic wounds, ranging from traditional and "second generation" bioengineered living skin equivalents to mesenchymal stem cell dressings; the latter include biopolymeric porous scaffolds, electrospun nanofiber meshes, hydrogels and 3D printed bio-printed dressings. Finally, this review updates the completed and ongoing clinical trials in this field and encourages researchers to rethink these new approaches, manufacturing processes and mechanisms of action, as well as their administration strategies and timings.

In Situ 3D Bioprinting Living Photosynthetic Scaffolds for Autotrophic Wound Healing

Three-dimensional (3D) bioprinting has been extensively explored for tissue repair and regeneration, while the insufficient nutrient and oxygen availability in the printed constructs, as well as the lack of adaptive dimensions and shapes, compromises the overall therapeutic efficacy and limits their further application. Herein, inspired by the natural symbiotic relationship between salamanders and algae, we present novel living photosynthetic scaffolds by using an in situ microfluidic-assisted 3D bioprinting strategy for adapting irregular-shaped wounds and promoting their healing. As the oxygenic photosynthesis unicellular microalga (Chlorella pyrenoidosa) was incorporated during 3D printing, the generated scaffolds could produce sustainable oxygen under light illumination, which facilitated the cell proliferation, migration, and differentiation even in hypoxic conditions. Thus, when the living microalgae-laden scaffolds were directly printed into diabetic wounds, they could significantly accelerate the chronic wound closure by alleviating local hypoxia, increasing angiogenesis, and promoting extracellular matrix (ECM) synthesis. These results indicate that the in situ bioprinting of living photosynthetic microalgae offers an effective autotrophic biosystem for promoting wound healing, suggesting a promising therapeutic strategy for diverse tissue engineering applications.

Recent Advances in Bioengineered Scaffolds for Cutaneous Wound Healing

Wound healing is an evolved dynamic biological process. Though many research and clinical approaches have been explored to restore damaged or diseased skin, the current treatment for deep cutaneous injuries is far from being perfect, and the ideal regenerative therapy remains a significant challenge. Of all treatments, bioengineered scaffolds play a key role and represent great progress in wound repair and skin regeneration. In this review, we focus on the latest advancement in biomaterial scaffolds for wound healing. We discuss the emerging philosophy of designing biomaterial scaffolds, followed by precursor development. We pay particular attention to the therapeutic interventions of bioengineered scaffolds for cutaneous wound healing, and their dual effects while conjugating with bioactive molecules, stem cells, and even immunomodulation. As we review the advancement and the challenges of the current strategies, we also discuss the prospects of scaffold development for wound healing.

Adaptive multi‐degree‐of‐freedom in situ bioprinting robot for hair‐follicle‐inclusive skin repair: A preliminary study conducted in mice

Skin acts as an essential barrier, protecting organisms from their environment. For skin trauma caused by accidental injuries, rapid healing, personalization, and functionality are vital requirements in clinical, which are the bottlenecks hindering the translation of skin repair from benchside to bedside. Herein, we described a novel design and a proof‐of‐concept demonstration of an adaptive bioprinting robot to proceed rapid in situ bioprinting on a full‐thickness excisional wound in mice. The three‐dimensional (3D) scanning and closed‐loop visual system integrated in the robot and the multi‐degree‐of‐freedom mechanism provide immediate, precise, and complete wound coverage through stereotactic bioprinting, which hits the key requirements of rapid‐healing and personalization in skin repair. Combined with the robot, epidermal stem cells and skin‐derived precursors isolated from neonatal mice mixed with Matrigel were directly printed into the injured area to replicate the skin structure. Excisional wounds after bioprinting showed complete wound healing and functional skin tissue regeneration that closely resembling native skin, including epidermis, dermis, blood vessels, hair follicles and sebaceous glands etc. This study provides an effective strategy for skin repair through the combination of the novel robot and a bioactive bioink, and has a promising clinical translational potential for further applications.

Engineering Hydrogels for the Development of Three-Dimensional In Vitro Models

The superiority of in vitro 3D cultures over conventional 2D cell cultures is well recognized by the scientific community for its relevance in mimicking the native tissue architecture and functionality. The recent paradigm shift in the field of tissue engineering toward the development of 3D in vitro models can be realized with its myriad of applications, including drug screening, developing alternative diagnostics, and regenerative medicine. Hydrogels are considered the most suitable biomaterial for developing an in vitro model owing to their similarity in features to the extracellular microenvironment of native tissue. In this review article, recent progress in the use of hydrogel-based biomaterial for the development of 3D in vitro biomimetic tissue models is highlighted. Discussions of hydrogel sources and the latest hybrid system with different combinations of biopolymers are also presented. The hydrogel crosslinking mechanism and design consideration are summarized, followed by different types of available hydrogel module systems along with recent microfabrication technologies. We also present the latest developments in engineering hydrogel-based 3D in vitro models targeting specific tissues. Finally, we discuss the challenges surrounding current in vitro platforms and 3D models in the light of future perspectives for an improved biomimetic in vitro organ system.

In vivo printing of growth factor-eluting adhesive scaffolds improves wound healing

Acute and chronic wounds affect millions of people around the world, imposing a growing financial burden on patients and hospitals. Despite the application of current wound management strategies, the physiological healing process is disrupted in many cases, resulting in impaired wound healing. Therefore, more efficient and easy-to-use treatment modalities are needed. In this study, we demonstrate the benefit of in vivo printed, growth factor-eluting adhesive scaffolds for the treatment of full-thickness wounds in a porcine model. A custom-made handheld printer is implemented to finely print gelatin-methacryloyl (GelMA) hydrogel containing vascular endothelial growth factor (VEGF) into the wounds. In vitro and in vivo results show that the in situ GelMA crosslinking induces a strong scaffold adhesion and enables printing on curved surfaces of wet tissues, without the need for any sutures. The scaffold is further shown to offer a sustained release of VEGF, enhancing the migration of endothelial cells in vitro. Histological analyses demonstrate that the administration of the VEGF-eluting GelMA scaffolds that remain adherent to the wound bed significantly improves the quality of healing in porcine wounds. The introduced in vivo printing strategy for wound healing applications is translational and convenient to use in any place, such as an operating room, and does not require expensive bioprinters or imaging modalities.

3D Bioprinting in Skin Related Research: Recent Achievements and Application Perspectives

In recent years, significant progress has been observed in the field of skin bioprinting, which has a huge potential to revolutionize the way of treatment in injury and surgery. Furthermore, it may be considered as an appropriate platform to perform the assessment and screening of cosmetic and pharmaceutical formulations. Therefore, the objective of this paper was to review the latest advances in 3D bioprinting dedicated to skin applications. In order to explain the boundaries of this technology, the architecture and functions of the native skin were briefly described. The principles of bioprinting methods were outlined along with a detailed description of key elements that are required to fabricate the skin equivalents. Next, the overview of recent progress in 3D bioprinting studies was presented. The article also highlighted the potential applications of bioengineered skin substituents in various fields including regenerative medicine, modeling of diseases, and cosmetics/drugs testing. The advantages, limitations, and future directions of this technology were also discussed.

Personalized and Programmable Microneedle Dressing for Promoting Wound Healing

Microneedle (MN) dressings, with the ability of transdermal drug delivery, have played an essential role in the field of wound healing. However, patients may still feel uncomfortable when sensitive unhealing wounds are pieced by strong needles. Here, inspired by the structure of mosquito mouthparts, which possess a fixation part and a liquid-transferring part, we present a novel MN wound dressing with superfine needle tips, personalized pattern design, programmable needle length, and multiple mechanical strengths for intelligent and painless drug delivery. By simply stretching the silicone rubber (Ecoflex) molds before engraving, superfine MNs can be formed in the restored molds. Meanwhile, by utilizing intelligent image recognition, precise treatment for irregular wounds is achieved. Notably, combined with temperature-responsive N-isopropylacrylamide (NIPAM) hydrogel and inverse opal (IO) photonic crystals (PCs), a controllable drug release system has been achieved on MN dressings. Moreover, the performance of the MN dressing in facilitating wound recovery has been demonstrated by full-thickness skin wounds of a mouse model. These results indicate that novel personalized and programmable MN wound dressings are of considerable value in the field of wound management.

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

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

Emerging Technologies in Multi-Material Bioprinting

Bioprinting, within the emerging field of biofabrication, aims at the fabrication of functional biomimetic constructs. Different 3D bioprinting techniques have been adapted to bioprint cell-laden bioinks. However, single-material bioprinting techniques oftentimes fail to reproduce the complex compositions and diversity of native tissues. Multi-material bioprinting as an emerging approach enables the fabrication of heterogeneous multi-cellular constructs that replicate their host microenvironments better than single-material approaches. Here, bioprinting modalities are reviewed, their being adapted to multi-material bioprinting is discussed, and their advantages and challenges, encompassing both custom-designed and commercially available technologies are analyzed. A perspective of how multi-material bioprinting opens up new opportunities for tissue engineering, tissue model engineering, therapeutics development, and personalized medicine is offered.