3D cell printing of in vitro stabilized skin model and in vivo pre-vascularized skin patch using tissue-specific extracellular matrix bioink: A step towards advanced skin tissue engineering

3D bioprinting for fabricating artificial skin tissue

As an organ in direct contact with the external environment, the skin is the first line of defense against external stimuli, so it is the most vulnerable to damage. In addition, there is an increasing demand for artificial skin in the fields of drug testing, disease research and cosmetic testing. Traditional skin tissue engineering has made encouraging progress after years of development. However, due to the complexity of the skin structures, there is still a big gap between existing artificial skin and natural skin in terms of function. Three-dimensional (3D) bioprinting is an advanced biological manufacturing method. It accurately deposits bioinks into pre-designed three-dimensional shapes to create complex biological tissues. This technology aims to print artificial tissues and organs with biological activities and complete physiological functions, thereby alleviating the problem of tissues and organs in short supply. Here, based on the introduction to skin structure and function, we systematically elaborate and analyze skin manufacturing methods, 3D bioprinting biomaterials and strategies, etc. Finally, the challenges and perspectives in 3D bioprinting skin field are summarized.

A biological functional hybrid scaffold based on decellularized extracellular matrix/gelatin/chitosan with high biocompatibility and antibacterial activity for skin tissue engineering

Nowadays, decellularized extracellular matrix (dECM) has received widespread attention due to its diversity in providing the unique structural and functional components to support cell growth, and finding material with good biocompatibility and anti-infection capability for skin tissue engineering is still a challenge. In this study, a novel dECM/Gel/CS scaffold with appropriate mechanical strength, good antibacterial activity and high biocompatibility was prepared using a one-pot method. The results showed that the immune components such as cells and DNA (about 98.1%) were successfully removed from the porcine skin tissue. The dECM/Gel/CS scaffolds exhibited an interconnected pore structure and had a high porosity (>90%) to promote cell growth. Moreover, the appropriate elastic modulus (≥482.17 kPa) and degradability (≥80.04% for 15 days) of the scaffolds offered stout "houses" for cell proliferation and suitable degradation rate to match the new tissue formation in skin tissue engineering. Furthermore, the addition of chitosan endowed the scaffold with good antibacterial activity, water and protein absorption capacity to avoid wound infection, and maintain the moisture and nutrition balance. In vitro cytocompatibility studies showed that the presence of dECM effectively enhanced the cell proliferation. Overall, the advanced dECM/Gel/CS scaffold has considerable potential to be applied in skin tissue engineering.

Tissue-Specific Decellularized Extracellular Matrix Bioinks for Musculoskeletal Tissue Regeneration and Modeling Using 3D Bioprinting Technology

The musculoskeletal system is a vital body system that protects internal organs, supports locomotion, and maintains homeostatic function. Unfortunately, musculoskeletal disorders are the leading cause of disability worldwide. Although implant surgeries using autografts, allografts, and xenografts have been conducted, several adverse effects, including donor site morbidity and immunoreaction, exist. To overcome these limitations, various biomedical engineering approaches have been proposed based on an understanding of the complexity of human musculoskeletal tissue. In this review, the leading edge of musculoskeletal tissue engineering using 3D bioprinting technology and musculoskeletal tissue-derived decellularized extracellular matrix bioink is described. In particular, studies on in vivo regeneration and in vitro modeling of musculoskeletal tissue have been focused on. Lastly, the current breakthroughs, limitations, and future perspectives are described.

Construction of Tissue-Level Cancer-Vascular Model with High-Precision Position Control via In Situ 3D Cell Printing

During tumor progression, the size and location of the tumor are important factors closely associated with the metastatic potential of the cancer as they largely govern tumor hypoxia and angiogenesis. However, despite the achievements of previous studies, these critical factors are poorly studied, mainly due to the lack of a flexible technique that can readily control 3D tumor mimicking constructs and their spatial relations with vasculature. Here, a novel tissue-level platform consisting of a metastatic cancer unit (MCU) and a perfusable vascular endothelium system (VES) is presented using in situ 3D cell printing. Size-tunable and position-controllable 3D cancer spheroids (500-1000 µm) are directly printed within the established bath bioink with a self-driven perfusable vascular channel. The cancer-vascular interactions are generated through controlling the distance between MCU and VES to investigate metastasis-associated changes at adjacent and distal regions. The result shows that MCU in 600 µm diameter includes hypoxia, invasion, and angiogenetic signaling. The further observations demonstrate that the proximity of MCU to VES augments the epithelial-mesenchymal transition (EMT) in MCU and vascular dysfunction/inflammation in VES, corroborating the positional significance in tumor metastasis. The platform with the precise-positioning control enables the recapitulation of patient's detailed metastatic progression, opening the chance for precision cancer medicine.

A Hybrid Injectable and Self-Healable Hydrogel System as 3D Cell Culture Scaffold

Cell therapies have great potential for the treatment of many different diseases, while the direct application of cells to the targeted location leads to limited therapeutic outcomes due to the low cell engraftment and cell survival rate. Injectable hydrogels have been developed to facilitate cell delivery; however, those currently developed hydrogel systems still face the limited cell survival rate. Here, an injectable and self-healable hydrogel is reported through the combination of hyperbranched PEG-based multi-hydrazide macro-crosslinker (HB-PEG-HDZ) and aldehyde-functionalized hyaluronic acid (HA-CHO), with gelatin added to increase the crosslinking density and cell activity. The hydrogels can be formed only in 7 s due to the relatively high content of the functional end groups. The reversible crosslinking mechanism between the hydrazide and aldehyde groups endows the hydrogel with shear-thinning and self-healing properties. The hydrogels with gelatin exhibit relatively better mechanical properties and cell activity. The hydrogels can improve the survival, attachment, and engraftment of injected cells due to the rapid sol-gel transition, which can promote an enhanced regenerative response.

Diving into 3D (bio)printing: A revolutionary tool to customize the production of drug and cell-based systems for skin delivery

The incorporation of 3D printing technologies in the pharmaceutical industry can revolutionize its R&D, by providing a simple and rapid method to produce tailored one-off batches, each with customized dosages, different compounds, shapes, sizes, and adjusted release rates. Particularly, this type of technology can be advantageous for the development of topical and transdermal drug delivery systems, including patches and microneedles. The use of both systems as drug carriers offers advantages over the oral administration, but the possibility of skin irritation and sensitization, and the high production costs, may hinder the expansion of this market. In this context, 3D printing, a high-resolution technique, allows the design of high quality, personalized, complex and sophisticated structures, thus reducing the production costs and improving the patient compliance. This review covers the 3D printing concept and discusses the relevance of this technology to the pharmaceutical industry, with a special focus on the development of topical and transdermal products - patches and microneedles. The potential of 3D bioprinting for skin applications is also presented, highlighting the development of patch-like skin constructs for wound and burn treatment, and skin equivalents for in vitro research and drug development. Several recent studies were selected to support the relevance of the subjects addressed herein. Additionally, the limitations of these printing technologies are discussed, including regulatory, quality and safety issues.

Mechano-regulation of vascular network formation without branches in 3D bioprinted cell-laden hydrogel constructs

Restoration of a wound is a common surgical procedure in clinic. Currently, the skin required for clinical use is taken from the patient's own body. However, it can be difficult to obtain enough skin sources for large-sized wounds and thus surgeons have started using commercial skin substitutes. The current commercial skin, which includes epidermis substitute, dermis substitute, and bilateral skin substitute, has been popularized in clinic. However, the application is limited by the occurrence of ischemia necrosis after transplantation. Recent studies suggest the use of pre-vascularized skin substitutes for wound healing is a promising area in the research field of skin tissue engineering. Pre-vascularization can be induced by changes in cultivation periods, exertion of mechanical stimuli, or coculture with endothelial cells and various factors. However, few methods could control the formation of vascular branches in engineering tissue in a self-assembly way. In this study, we use three-dimensional (3D) printing technology to confirm that a mechanical force can control the growth of blood vessels in the direction of mechanical stimulation with no branches, and that Yes-associated protein activity is involved in the regulatory progress. In vivo experiments verified that the blood vessels successfully function for blood circulation, and maintain the same direction. Results provide a theoretical basis for products of pre-vascularized skin tissues and other organs created by 3D bioprinting.

Bio-Inspired Hydrogels via 3D Bioprinting

Many soft tissues of the human body such as cartilages, muscles, and ligaments are mainly composed of biological hydrogels possessing excellent mechanical properties and delicate structures. Nowadays, bio-inspired hydrogels have been intensively explored due to their promising potential applications in tissue engineering. However, the traditional manufacturing technology is challenging to produce the bio-inspired hydrogels, and the typical biological composite topologies of bio-inspired hydrogels are accessible completed using 3D bioprinting at micrometer resolution. In this chapter, the 3D bioprinting techniques used for the fabrication of bio-inspired hydrogels were summarized, and the materials used were outlined. This chapter also focuses on the applications of bio-inspired hydrogels fabricated using available 3D bioprinting technologies. The development of 3D bioprinting techniques in the future would bring us closer to the fabrication capabilities of living organisms, which would be widely used in biomedical applications.

Neovascularization of engineered tissues for clinical translation: Where we are, where we should be?

One of the key challenges in engineering three-dimensional tissue constructs is the development of a mature microvascular network capable of supplying sufficient oxygen and nutrients to the tissue. Recent angiogenic therapeutic strategies have focused on vascularization of the constructed tissue, and its integration in vitro; these strategies typically combine regenerative cells, growth factors (GFs) with custom-designed biomaterials. However, the field needs to progress in the clinical translation of tissue engineering strategies. The article first presents a detailed description of the steps in neovascularization and the roles of extracellular matrix elements such as GFs in angiogenesis. It then delves into decellularization, cell, and GF-based strategies employed thus far for therapeutic angiogenesis, with a particularly detailed examination of different methods by which GFs are delivered in biomaterial scaffolds. Finally, interdisciplinary approaches involving advancement in biomaterials science and current state of technological development in fabrication techniques are critically evaluated, and a list of remaining challenges is presented that need to be solved for successful translation to the clinics.

Tissue Engineering for Mimics and Modulations of Immune Functions

In the field of regenerative medicine, advances in tissue engineering have surpassed the reconstruction of individual tissues or organs and begun to work towards engineering systemic factors such as immune objects and functions. The immune system plays a crucial role in protecting and regulating systemic functions in the human body. Engineered immune tissues and organs have shown potential in recovering dysfunctions and aplasia of the immune system and the evasion from immune-mediated inflammatory responses and rejection elicited by engineered implants from allogeneic or xenogeneic sources are also being pursued to facilitate clinical transplantation of tissue engineered grafts. Here, current progress in tissue engineering to mimic or modulate immune functions is reviewed and elaborated from two perspectives: 1) engineering of immune tissues and organs per se and 2) immune evasion of host immunoinflammatory rejection by tissue-engineered implants.

Bioprinting on 3D Printed Titanium Scaffolds for Periodontal Ligament Regeneration

The three-dimensional (3D) cell-printing technique has been identified as a new biofabrication platform because of its ability to locate living cells in pre-defined spatial locations with scaffolds and various growth factors. Osseointegrated dental implants have been regarded as very reliable and have long-term reliability. However, host defense mechanisms against infections and micro-movements have been known to be impaired around a dental implant because of the lack of a periodontal ligament. In this study, we fabricated a hybrid artificial organ with a periodontal ligament on the surface of titanium using 3D printing technology. CEMP-1, a known cementogenic factor, was enhanced in vitro. In animal experiments, when the hybrid artificial organ was transplanted to the calvarial defect model, it was observed that the amount of connective tissue increased. 3D-printed hybrid artificial organs can be used with dental implants, establishing physiological tooth functions, including the ability to react to mechanical stimuli and the ability to resist infections.

A perfusable vascularized full-thickness skin model for potential topical and systemic applications

Vascularization of reconstructed tissues is one of the remaining hurdles to be considered to improve both the functionality and viability of skin grafts and the relevance ofin vitroapplications. Our study, therefore, sought to develop a perfusable vascularized full-thickness skin equivalent that comprises a more complex blood vasculature compared to existing models. We combined molding, auto-assembly and microfluidics techniques in order to create a vascularized skin equivalent representing (a) a differentiated epidermis with a physiological organization and correctly expressing K14, K10, Involucrin, TGM1 and Filaggrin, (b) three perfusable vascular channels with angiogenic sprouts stained with VE-Caderin and Collagen IV, (c) an adjacent microvascular network created via vasculogenesis and connected to the sprouting macrovessels. Histological analysis and immunostaining of CD31, Collagen IV, Perlecan and Laminin proved the integrity of vascular constructs. In order to validate the vascularized skin potential of topical and systemic applications, caffeine and minoxidil, two compounds with different chemical properties, were topically applied to measure skin permeability and benzo[a]pyrene pollutant was systemically applied to evaluate systemic delivery. Our results demonstrated that perfusion of skin reconstructs and the presence of a complex vascular plexus resulted in a more predictive and reliable model to assess respectively topical and systemic applications. This model is therefore aimed at furthering drug discovery and improving clinical translation in dermatology.

A low-temperature-printed hierarchical porous sponge-like scaffold that promotes cell-material interaction and modulates paracrine activity of MSCs for vascularized bone regeneration

Mesenchymal stem cells (MSCs) secrete paracrine trophic factors that are beneficial for tissue regeneration. In this study, a sponge-like scaffold with hierarchical and interconnected pores was developed using low-temperature deposition modeling (LDM) printing. Its effects on the cellular behavior, especially on the paracrine secretion patterns of MSCs, were comprehensively investigated. We found that compared with the scaffolds printed via the fused deposition modeling (FDM) technique, the LDM-printed sponges enhanced the adhesion, retention, survival, and ingrowth of MSCs and promoted cell-material interactions. Moreover, the paracrine functions of the cultured MSCs on the LDM-printed sponges were improved, with significant secretion of upregulated immunomodulatory, angiogenic, and osteogenic factors. MSCs on the LDM-printed sponges exert beneficial paracrine effects on multiple regenerative processes, including macrophage polarization, tube formation, and osteogenesis, verifying the enhanced immunomodulatory, angiogenic, and osteogenic potential. Further protein function assays indicated that focal adhesion kinase (FAK), downstream AKT, and yes-associated-protein (YAP) signaling might participate in the required mechanotransductive pathways, through which the hierarchical porous structures stimulated the paracrine effects of MSCs. In a rat distal femoral defect model, the MSC-laden LDM-printed sponges significantly promoted vascularized bone regeneration. The results of the present study demonstrate that the hierarchical porous biomimetic sponges prepared via LDM printing have potential applications in tissue engineering based on their cell-material interaction promotion and MSC paracrine function modulation effects. Furthermore, our findings suggest that the optimization of biomaterial properties to direct the paracrine signaling of MSCs would enhance tissue regeneration.

Tissue printing for engineering transplantable human parathyroid patch to improve parathyroid engraftment, integration, and hormone secretionin vivo

During thyroid surgery, some parathyroid glands fail to maintain their function, therefore, they are unavoidably detached from the patient. For the purpose of re-preservation of the function, they are minced into small segments and transplanted into the fat or muscle layer. Yet, this method of auto-grafting the parathyroid glands is frequently unsuccessful due to its poor interaction and engraftment with the native tissue, eventually leading to the dysfunction of the parathyroid hormone (PTH) secretion. In this study, we suggest a methodology to restore parathyroid activity through the introduction of the 'tissue printing' concept. Parathyroid glands of patients with secondary hyperparathyroidism were minced into the fragments smaller than 0.5 × 0.5 mm, which is in common with the traditional surgical method. These parathyroid tissues (PTs) were uniformly mixed with the adipose-derived decellularized extracellular matrix (adECM) bioink that protects the PTs from hostilein vivoenvironments and promote initial engraftment. PTs-encapsulated adECM bioink (PTs-adECM) was then printed onto the pre-designed polycaprolactone (PCL) mesh to produce patch-type PTs construct, which functions as a mechanical support to further enhance long-termin vivostability. The engineered patch was transplanted subcutaneously into rats and harvested after 4 weeks.In vivoresults showed that the engineered patches were well engrafted and stabilized in their original position for 4 weeks as compared with PTs only. Immunohistochemistry results further revealed that the concentration of PTH was approximately 2.5-fold greater in rats engrafted in the patch. Taken together, we envision that the novel concept 'tissue printing' over cell printing could provide a closer step towards clinical applications of 3D bioprinting to solve the unmet need for parathyroid surgery method.

Bioinks for 3D Bioprinting: A Scientometric Analysis of Two Decades of Progress

This scientometric analysis of 393 original papers published from January 2000 to June 2019 describes the development and use of bioinks for 3D bioprinting. The main trends for bioink applications and the primary considerations guiding the selection and design of current bioink components (i.e., cell types, hydrogels, and additives) were reviewed. The cost, availability, practicality, and basic biological considerations (e.g., cytocompatibility and cell attachment) are the most popular parameters guiding bioink use and development. Today, extrusion bioprinting is the most widely used bioprinting technique. The most reported use of bioinks is the generic characterization of bioink formulations or bioprinting technologies (32%), followed by cartilage bioprinting applications (16%). Similarly, the cell-type choice is mostly generic, as cells are typically used as models to assess bioink formulations or new bioprinting methodologies rather than to fabricate specific tissues. The cell-binding motif arginine-glycine-aspartate is the most common bioink additive. Many articles reported the development of advanced functional bioinks for specific biomedical applications; however, most bioinks remain the basic compositions that meet the simple criteria: Manufacturability and essential biological performance. Alginate and gelatin methacryloyl are the most popular hydrogels that meet these criteria. Our analysis suggests that present-day bioinks still represent a stage of emergence of bioprinting technology.

3D Bioprinting of Functional Skin Substitutes: From Current Achievements to Future Goals

The aim of this review is to present 3D bioprinting of skin substitutes as an efficient approach of managing skin injuries. From a clinical point of view, classic treatments only provide physical protection from the environment, and existing engineered scaffolds, albeit acting as a physical support for cells, fail to overcome needs, such as neovascularisation. In the present work, the basic principles of bioprinting, together with the most popular approaches and choices of biomaterials for 3D-printed skin construct production, are explained, as well as the main advantages over other production methods. Moreover, the development of this technology is described in a chronological manner through examples of relevant experimental work in the last two decades: from the pioneers Lee et al. to the latest advances and different innovative strategies carried out lately to overcome the well-known challenges in tissue engineering of skin. In general, this technology has a huge potential to offer, although a multidisciplinary effort is required to optimise designs, biomaterials and production processes.

Biogenetic Acellular Dermal Matrix Maintaining Rich Interconnected Microchannels for Accelerated Tissue Amendment

Given that many people suffer from extensive skin damage, wound repair has drawn tremendous attention in research. Among the various assistant dressing materials that promote healing, a porcine acellular dermal matrix (PADM), as a skin substitute, can efficiently accelerate healing by promoting cell migration and proliferation. However, a simple, low-cost preparation process remains a challenge facing PADM development, particularly because of the inferior elasticity. To overcome these drawbacks, a CaCl₂-ethanol-H₂O solution (ternary solution) combined with an additional enzyme treatment was used to obtain a transparent, porous, and elastic PADM that retained the major extracellular matrix composition of the dermis. Our results indicated that alterations in the fiber organization and secondary structural changes in the collagen occurred after treatment. Furthermore, the in vivo wound healing and histological analyses clearly revealed an extremely expedited wound repair process following the application of the biocompatible PADM. In conclusion, this study provides new insights into the development of a transparent PADM with a porous structure and good elasticity that can be used as a skin substitute to accelerate the wound healing process. Moreover, this effective technique could potentially be used to extrapolate other decellularized materials in the future.

Incorporating a structural extracellular matrix gradient into a porcine urinary bladder matrix-based hydrogel dermal scaffold

The increasing prevalence of chronic, nonhealing wounds necessitates the investigation of full-thickness skin substitutes conducive to host integration and wound closure. Extracellular matrix (ECM)-based hydrogel scaffolds mimic the physiological matrix environment of dermal cells, thereby conferring favorable cellular adhesion, infiltration, and proliferation. However, low-concentration ECM hydrogels rapidly lose mechanical strength as they degrade, leaving them susceptible to shrinkage from fibroblast-mediated contraction. Conversely, high-concentration ECM hydrogels are typically too dense to permit nutrient diffusion and cellular migration. This study investigates the design and fabrication of a graded-concentration hydrogel composed of porcine urinary bladder matrix (UBM) as a dermal scaffold for potential use in chronic wound treatment. Our method of UBM isolation and decellularization effectively removed native DNA while preserving matrix proteins. Hydrogels composed of a range of decellularized UBM (dUBM) concentrations were characterized and used to design a three-tiered gradient hydrogel that promoted cellular activity and maintained structural integrity. The gradient dUBM hydrogel showed stability of cross-sectional area during collagenase degradation, despite considerable loss of mass. The gradient dUBM hydrogel also resisted fibroblast-mediated contraction while supporting high surface cell viability, demonstrating the mechanical support provided by denser layers of dUBM. Overall, incorporation of an ECM concentration gradient into a porcine UBM-based hydrogel scaffold capitalizes on the unique advantages of both high and low-concentration ECM hydrogels, and mitigates the structural weaknesses that have limited the efficacy of hydrogel dermal scaffolds for chronic wounds. Our gradient design shows promise for future development of stable, pro-regenerative wound scaffolds with customized architectures using 3D printing.

Engineering of diseased human skin equivalent using 3D cell printing for representing pathophysiological hallmarks of type 2 diabetes in vitro

Despite many significant advances in 3D cell printing for skin, a disease model displaying the pathological processes present in the native skin has not been reported yet. Therefore, we were motivated for modeling a 3D diseased skin tissue with pathophysiological hallmarks of type 2 diabetes in vitro based on 3D cell printing technique. By stimulating epidermal-dermal intercellular crosstalk found in the native skin, it was hypothesized that normal keratinocytes would be differentiated as diabetic epidermis when interacting with the diabetic dermal compartment. To prove this, a novel wounded skin model was successfully devised during tissue maturation in vitro. Interestingly, the slow re-epithelization was observed in our diabetic model, which is a representative hallmark of diabetic skin. Using the versatility of 3D cell printing, the structural similarities and diabetic properties of the model were further augmented by addition of perfusable vascularized diabetic hypodermis. Insulin resistance, adipocyte hypertrophy, inflammatory reactions, and vascular dysfunction, as the typical hallmarks in diabetes, were found under hyperglycemia. Finally, the feasibility of this new disease model for drug development was successfully demonstrated through application of test drugs. We trust that this study provides a pioneering step towards 3D cell printing-based in vitro skin disease modeling.

Extracellular matrices derived from different cell sources and their effect on macrophage behavior and wound healing

A decellularized extracellular matrix (dECM) is an excellent biomaterial in regenerative medicine, due to its biomimetic nature in targeting tissues and organs. In this study, we prepared cell-derived ECMs (CDM) derived from four different cell sources, characterized them individually, and found that intrinsic properties of each CDM were substantially different in terms of the fibrous matrix, total protein, and biochemical factors. Based on such information, we selected two ECM candidates, the human lung fibroblast derived matrix (hFDM) and the umbilical cord-blood mesenchymal stem cell derived matrix (UMDM) for the study of ECM-macrophage interactions in vitro and in vivo. In fact, UMDM was the richer in both total protein and angiogenic-related cytokines than any other CDM. When THP-1 cell-derived macrophages (M0) were seeded onto the UMDM or the hFDM, it showed a mixed cell morphology of macrophage phenotype and the macrophages (M0) preconditioned on UMDM presented more diverse cytokine release profiles. The treatment of conditioned medium obtained from CDM-seeded macrophages showed that UMDM could yield significantly advanced wound closure in 24 h via the human dermal fibroblast scratch model. To investigate the role of ECM on macrophage polarization in vivo, we prepared an ECM hydrogel, a mixture of each CDM and Pluronic F127/hyaluronan, and applied them onto a full-thickness mouse skin wound model for 2 weeks. The therapeutic efficacy as assessed via histology and immunofluorescence staining (α-SMA and CD206) revealed that the UMDM-treated group showed more effective wound healing compared to the other groups, as proven via the thinner epidermal layer, significant recovery of skin appendage, better neovascularization, and higher recruitment of myofibroblasts and larger number of macrophages (M2) at 7 days. The difference between UMDM and hFDM was marginal. Taken together, among the CDMs, UMDM and hFDM are promising resources of ECM, showing a great potential for wound healing. Although the mechanism is not fully understood, bioactive innate factors in UMDM may contribute individually and/or collectively to advance wound healing.

Bioinks-materials used in printing cells in designed 3D forms

Use of materials to activate non-functional or damaged organs and tissues goes back to early ages. The first materials used for this purpose were metals, and in time, novel materials such as ceramics, polymers and composites were introduced to the field to serve in medical applications. In the last decade, the advances in material sciences, cell biology, technology and engineering made 3D printing of living tissues or organ models in the designed structure and geometry possible by using cells alone or together with hydrogels through additive manufacturing. This review aims to give a brief information about the chemical structures and properties of bioink materials and their applications in the production of 3D tissue constructs.

Flexible Adipose-Vascular Tissue Assembly Using Combinational 3D Printing for Volume-Stable Soft Tissue Reconstruction

A new concept, assembling cell-laden tissue modules, is for the first time proposed for soft tissue engineering. Adipose-vascular tissue modules composed of a synthetic polymer-based substructure and customized bioinks using planar 3D cell printing are engineered. Such tissue modules are systematically assembled into a synthetic polymer-based module holder fabricated with rotational 3D printing, resulting in the development of a flexible and volumetric tissue assembly. Whereas most of the previous studies about the construction of adipose tissue are limited to hypoxia, poor vascularization, rapid resorption, and mismatch in mechanical properties, it is aimed to realize the construction of nonhypoxic, flexible, and volume-stable tissue assembly in this study. The significance of engineered tissue assembly is proven through various in vitro and in vivo evaluations. In particular, stable volume and remarkable neovascularization/adipogenesis are observed in the implanted assembly over four weeks. Interestingly, the size of newly formed lipid droplets and the remodeled morphology in the assembly are comparable to those in native adipose tissue. As far as it is known, this work is a first report suggesting a cell printing-based tissue assembly for functional reconstruction of soft tissue.

3D scaffolds in the treatment of diabetic foot ulcers: New trends vs conventional approaches

Diabetic foot ulcer (DFU) is a serious complication of diabetes mellitus, affecting roughly 25% of diabetic patients and resulting in lower limb amputation in over 70% of known cases. In addition to the devastating physiological consequences of DFU and its impact on patient quality of life, DFU has significant clinical and economic implications. Various traditional therapies are implemented to effectively treat DFU. However, emerging technologies such as bioprinting and electrospinning, present an exciting opportunity to improve current treatment strategies through the development of 3D scaffolds, by overcoming the limitations of current wound healing strategies. This review provides a summary on (i) current prevention and treatment strategies available for DFU; (ii) methods of fabrication of 3D scaffolds relevant for this condition; (iii) suitable materials and commonly used molecules for the treatment of DFU; and (iv) future directions offered by emerging technologies.

Recent Advances in the Design of Three-Dimensional and Bioprinted Scaffolds for Full-Thickness Wound Healing

Three-dimensional (3D) printed scaffolds have recently emerged as an innovative treatment option for patients with critical-sized skin wounds. Current approaches to managing life-threatening wounds include skin grafting and application of commercially sourced skin substitutes. However, these approaches are not without several challenges. Limited donor tissue and donor site morbidity remain a concern for tissue grafting, while engineered skin substitutes fail to fully recapitulate the complex native environment required for wound healing. The implementation of 3D printed dermal scaffolds offers a potential solution for these shortcomings. Spatial control over scaffold structure, the ability to incorporate multiple materials and bioactive ingredients, enables the creation of conditions specifically optimized for wound healing. Three-dimensional bioprinting, a subset of 3D printing, allows for the replacement of lost cell populations and secreted active compounds that contribute to tissue repair and recovery. The replacement of damaged and lost cells delivers beneficial effects directly, or synergistically, supporting injured tissue to recover its native state. Despite encouraging results, the promise of 3D printed scaffolds has yet to be realized. Further improvements to current material formulations and scaffold designs are required to achieve the goal of clinical adoption. Herein, we provide an overview of 3D printing techniques and discuss several strategies for healing of full-thickness wounds by using 3D printed acellular scaffolds or bioprinted cellular scaffolds, aimed at translating this technology to the clinical management of skin lesions. We identify the challenges associated with designing and optimizing printed tissue replacements, and discuss the future perspectives of this emerging option for managing patients who present with critical-sized life-threatening cutaneous wounds.

Natural Biomaterials and Their Use as Bioinks for Printing Tissues

The most prevalent form of bioprinting-extrusion bioprinting-can generate structures from a diverse range of materials and viscosities. It can create personalized tissues that aid in drug testing and cancer research when used in combination with natural bioinks. This paper reviews natural bioinks and their properties and functions in hard and soft tissue engineering applications. It discusses agarose, alginate, cellulose, chitosan, collagen, decellularized extracellular matrix, dextran, fibrin, gelatin, gellan gum, hyaluronic acid, Matrigel, and silk. Multi-component bioinks are considered as a way to address the shortfalls of individual biomaterials. The mechanical, rheological, and cross-linking properties along with the cytocompatibility, cell viability, and printability of the bioinks are detailed as well. Future avenues for research into natural bioinks are then presented.

Electrospun porous bilayer nano-fibrous fish collagen/PCL bio-composite scaffolds with covalently cross-linked chitooligosaccharides for full-thickness wound-healing applications

The development of tissue-engineered biodegradable artificial tissue substitutes with extracellular matrix-mimicking properties that govern the interaction between the material and biological environment is of great interest in wound-healing applications. In the present study, novel bilayer nanofibrous scaffolds composed of fish collagen (FC) and poly(ε-caprolactone) (PCL) were fabricated using electrospinning, with the covalent attachment of chitooligosaccharides (COS) via carbodiimide chemistry. The architecture and fiber diameter of the non-cross-linked nanofibrous scaffolds remained consistent irrespective of the polymer ratio under different electrospinning conditions, but the fiber diameter changed after cross-linking in association with the FC content. Fourier-transform infrared spectroscopy analysis indicated that the blend of biomaterials was homogenous, with an increase in COS levels with increasing FC content in the nanofibrous scaffolds. Based on cytocompatibility analysis (i.e., the cellular response to the nanofibrous scaffolds and their interaction), the nanofibrous scaffolds with high FC content were functionally active in response to normal human dermal fibroblast‑neonatal (NHDF-neo) and HaCaT keratinocyte cells, leading to the generation of a very effective tissue-engineered implant for full-thickness wound-healing applications. In addition to these empirical results, an assessment of the hydrophilicity, swelling, and mechanical integrity of the proposed COS-containing FC-rich FC/PCL (FCP) nanofibrous scaffolds confirmed that they have significant potential for use as tissue-engineered skin implants for rapid skin regeneration.

Shape Fidelity of 3D-Bioprinted Biodegradable Patches

There is high demand in the medical field for rapid fabrication of biodegradable patches at low cost and high throughput for various instant applications, such as wound healing. Bioprinting is a promising technology, which makes it possible to fabricate custom biodegradable patches. However, several challenges with the physical and chemical fidelity of bioprinted patches must be solved to increase the performance of patches. Here, we presented two hybrid hydrogels made of alginate-cellulose nanocrystal (CNC) (2% w/v alginate and 4% w/v CNC) and alginate-TEMPO oxidized cellulose nanofibril (T-CNF) (4% w/v alginate and 1% w/v T-CNC) via ionic crosslinking using calcium chloride (2% w/v). These hydrogels were rheologically characterized, and printing parameters were tuned for improved shape fidelity for use with an extrusion printing head. Young's modulus of 3D printed patches was found to be 0.2-0.45 MPa, which was between the physiological ranges of human skin. Mechanical fidelity of patches was assessed through cycling loading experiments that emulate human tissue motion. 3D bioprinted patches were exposed to a solution mimicking the body fluid to characterize the biodegradability of patches at body temperature. The biodegradation of alginate-CNC and alginate-CNF was around 90% and 50% at the end of the 30-day in vitro degradation trial, which might be sufficient time for wound healing. Finally, the biocompatibility of the hydrogels was tested by cell viability analysis using NIH/3T3 mouse fibroblast cells. This study may pave the way toward improving the performance of patches and developing new patch material with high physical and chemical fidelity for instant application.

The Future of Regenerative Medicine: Cell Therapy Using Pluripotent Stem Cells and Acellular Therapies Based on Extracellular Vesicles

The rapid progress in the field of stem cell research has laid strong foundations for their use in regenerative medicine applications of injured or diseased tissues. Growing evidences indicate that some observed therapeutic outcomes of stem cell-based therapy are due to paracrine effects rather than long-term engraftment and survival of transplanted cells. Given their ability to cross biological barriers and mediate intercellular information transfer of bioactive molecules, extracellular vesicles are being explored as potential cell-free therapeutic agents. In this review, we first discuss the state of the art of regenerative medicine and its current limitations and challenges, with particular attention on pluripotent stem cell-derived products to repair organs like the eye, heart, skeletal muscle and skin. We then focus on emerging beneficial roles of extracellular vesicles to alleviate these pathological conditions and address hurdles and operational issues of this acellular strategy. Finally, we discuss future directions and examine how careful integration of different approaches presented in this review could help to potentiate therapeutic results in preclinical models and their good manufacturing practice (GMP) implementation for future clinical trials.

Development of 3D Printed Bruch's Membrane-Mimetic Substance for the Maturation of Retinal Pigment Epithelial Cells

Retinal pigment epithelium (RPE) is a monolayer of the pigmented cells that lies on the thin extracellular matrix called Bruch's membrane. This monolayer is the main component of the outer blood-retinal barrier (BRB), which plays a multifunctional role. Due to their crucial roles, the damage of this epithelium causes a wide range of diseases related to retinal degeneration including age-related macular degeneration, retinitis pigmentosa, and Stargardt disease. Unfortunately, there is presently no cure for these diseases. Clinically implantable RPE for humans is under development, and there is no practical examination platform for drug development. Here, we developed porcine Bruch's membrane-derived bioink (BM-ECM). Compared to conventional laminin, the RPE cells on BM-ECM showed enhanced functionality of RPE. Furthermore, we developed the Bruch's membrane-mimetic substrate (BMS) via the integration of BM-ECM and 3D printing technology, which revealed structure and extracellular matrix components similar to those of natural Bruch's membrane. The developed BMS facilitated the appropriate functions of RPE, including barrier and clearance functions, the secretion of anti-angiogenic growth factors, and enzyme formation for phototransduction. Moreover, it could be used as a basement frame for RPE transplantation. We established BMS using 3D printing technology to grow RPE cells with functions that could be used for an in vitro model and RPE transplantation.

Fabrication and characterization of mucoadhesive bioplastic patch via coaxial polylactic acid (PLA) based electrospun nanofibers with antimicrobial and wound healing application

Polylactic acid (PLA) is the second-highest consumed bioplastic in the world. PVP/PLA-PEO complex nanofibers encapsulating collagen and cefazolin dressing scaffold were fabricated using a coaxial electrospinning method to target the release of the encapsulated compounds. It was observed that in collagen doses of 10 and 20%, the speed of healing showed a significant difference with the control sample, but the dose of 40% caused a decrease in wound healing rate in mice. The nanofibers' morphology and surface roughness were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The mechanical properties and adhesion strength of the scaffolds were investigated. The scaffolds' antimicrobial activity was evaluated by disk diffusion method against the E. coli, S. aureus, and P. aeruginosa. The results indicated a positive effect on the antimicrobial activity of the samples. In this study, we were able to prolong the effect of scaffolds by changing the pattern of release of cefazolin from inside the nanofibers. Possible interactions between the polymers and the encapsulated compounds were investigated using Fourier-transform infrared spectroscopy (FTIR). Finally, in-vivo and histological tests were performed to evaluate the efficacy of the scaffolds in accelerating wound healing.

Extracellular-Matrix-Reinforced Bioinks for 3D Bioprinting Human Tissue

Recent advances in 3D bioprinting allow for generating intricate structures with dimensions relevant for human tissue, but suitable bioinks for producing translationally relevant tissue with complex geometries remain unidentified. Here, a tissue-specific hybrid bioink is described, composed of a natural polymer, alginate, reinforced with extracellular matrix derived from decellularized tissue (rECM). rECM has rheological and gelation properties beneficial for 3D bioprinting while retaining biologically inductive properties supporting tissue maturation ex vivo and in vivo. These bioinks are shear thinning, resist cell sedimentation, improve viability of multiple cell types, and enhance mechanical stability in hydrogels derived from them. 3D printed constructs generated from rECM bioinks suppress the foreign body response, are pro-angiogenic and support recipient-derived de novo blood vessel formation across the entire graft thickness in a murine model of transplant immunosuppression. Their proof-of-principle for generating human tissue is demonstrated by 3D bioprinting human airways composed of regionally specified primary human airway epithelial progenitor and smooth muscle cells. Airway lumens remained patent with viable cells for one month in vitro with evidence of differentiation into mature epithelial cell types found in native human airways. rECM bioinks are a promising new approach for generating functional human tissue using 3D bioprinting.

3D bioprinting dermal-like structures using species-specific ulvan

3D cellularized structures revealing dermal-like properties have been successfully printed using bioinks based on the sulfated polysaccharide ulvan from Australian green seaweed.

Designing Decellularized Extracellular Matrix-Based Bioinks for 3D Bioprinting

3D bioprinting is an emerging technology to fabricate tissues and organs by precisely positioning cells into 3D structures using printable cell-laden formulations known as bioinks. Various bioinks are utilized in 3D bioprinting applications; however, developing the perfect bioink to fabricate constructs with biomimetic microenvironment and mechanical properties that are similar to native tissues is a challenging task. In recent years, decellularized extracellular matrix (dECM)-based bioinks have received an increasing attention in 3D bioprinting applications, since they are derived from native tissues and possess unique, complex tissue-specific biochemical properties. This review focuses on designing dECM-based bioinks for tissue and organ bioprinting, including commonly used decellularization and decellularized tissue characterization methods, bioink formulation and characterization, applications of dECM-based bioinks, and most recent advancements in dECM-based bioink design.

Fabrication of 3D hybrid scaffold by combination technique of electrospinning-like and freeze-drying to create mechanotransduction signals and mimic extracellular matrix function of skin

Fabrication of extracellular matrix (ECM)-like scaffolds (in terms of structural-functional) is the main challenge in skin tissue engineering. Herein, inspired by macromolecular components of ECM, a novel hybrid scaffold suggested which includes silk/hyaluronan (SF/HA) bio-complex modified by PCP: [polyethylene glycol/chitosan/poly(ɛ-caprolactone)] copolymer containing collagen to differentiate human-adipose-derived stem cells into keratinocytes. In followed by, different weight ratios (wt%) of SF/HA (S1:100/0, S2:80/20, S3:50/50) were applied to study the role of SF/HA in the improvement of physicochemical and biological functions of scaffolds. Notably, the combination of electrospinning-like and freeze-drying methods was also utilized as a new method to create a coherent 3D-network. The results indicated this novel technique was led to ~8% improvement of the scaffold's ductility and ~17% decrease in mean pore diameter, compared to the freeze-drying method. Moreover, the increase of HA (>20wt%) increased porosity to 99%, however, higher tensile strength, modulus, and water absorption% were related to S2 (38.1, 0.32 MPa, 75.3%). More expression of keratinocytes along with growth pattern similar to skin was also observed on S2. This study showed control of HA content creates a microporous-environment with proper modulus and swelling%, although, the role of collagen/PCP as base biocomposite and fabrication technique was undeniable on the inductive signaling of cells. Such a scaffold can mimic skin properties and act as the growth factor through inducing keratinocytes differentiation.

Green fabrication of seedbed-like Flammulina velutipes polysaccharides-derived scaffolds accelerating full-thickness skin wound healing accompanied by hair follicle regeneration

A novel seedbed-like scaffold was firstly fabricated by the "frozen sectioning" processing method using Flammulina velutipes as a raw material. The Flammulina velutipes polysaccharides scaffold is composed of a natural structure imitating the "ground" (connected and aligned hollow tubes with porous walls). Meanwhile, its biologically active components include polysaccharides and proteins, mimicking the "plant nutrition" in the seedbed. To further optimize the ground and nutrition components, Flammulina velutipes polysaccharides-derived scaffolds (FPDSs) were fabricated via the treatment of original Flammulina velutipes polysaccharides scaffold (labeled FPS) by NaOH, cysteine (labeled as FPS/NaOH, FPS/Cys, respectively). FPDSs were characterized by SEM, FTIR, XRD, water absorption and retention, and mechanical evaluations. From the results, FPS/NaOH and FPS/Cys lost the characteristic big tubes of original strips and had higher water absorption capacities comparing to FPS. Simultaneously, FPS/NaOH had better ductility, FPS/Cys had showed increased stiffness. Biological activities of FPDSs were tested against different types of bacteria exhibiting excellent anti-bacterial activity, and FPS/NaOH and FPS/Cys had dramatically higher anti-bacterial activity than FPS. The cytocompatibility of FPDSs was evaluated utilizing mouse fibroblast cell line (L929), and all FPDSs showed good cytocompatibility. The FPDSs were further applied to a rat full-thickness skin wound model, and they all exhibited obviously accelerated re-epithelialization, among which FPS/NaOH showed the greatest efficiency. FPS/NaOH could shorten the wound-healing process as evidenced by dynamic alterations of the expression levels of specific stagewise markers in the healing areas. Similarly, FPS/NaOH can efficiently induce hair follicle regeneration in the healing skin tissues. In summary, FPDSs exhibit potential functions as seedbeds to promote the regeneration of the "seed" including hair follicles and injured skin, opening a new avenue for wound healing.

Advances in generation of three-dimensional skin equivalents: pre-clinical studies to clinical therapies

The inability of two-dimensional cell culture systems to adequately map the structure and function of complex organs like skin necessitates the development of three-dimensional (3D) skin models. A diverse range of 3D skin equivalents have been developed over the last few decades for studying complex properties of skin as well as for drug discovery and clinical applications for skin regeneration in chronic wounds, such as diabetic foot ulcers, where the normal mechanism of wound healing is compromised. These 3D skin substitutes also serve as a suitable alternative to animal models in industrial applications and fundamental research. With the emergence of tissue engineering, new scaffolds and matrices have been integrated into 3D cell culture systems, along with gene therapy approaches, to increase the efficacy of transplanted cells in skin regeneration. This review summarizes recent approaches to the development of skin equivalents as well as different models for studying skin diseases and properties and current therapeutic applications of skin substitutes.

Pharmaceutical evaluation of atorvastatin-loaded nanostructured lipid carriers incorporated into the gelatin/hyaluronic acid/polycaprolactone scaffold for the skin tissue engineering

In this study, gelatin/hyaluronic acid (HA) scaffolds containing different amounts of atorvastatin-loaded nanostructured lipid carriers (NLCs) coated entirely with polycaprolactone (PCL) film were fabricated for skin regeneration. 12 atorvastatin-loaded NLCs formulations were synthesized, and particle size, zeta potential, drug entrapment efficiency (EE), and drug release of the formulations were determined. The optimum freeze-dried atorvastatin-loaded NLCs were added in 3 different weight percentages to the gelatin and HA membranous scaffolds. Thereafter, the membranes were coated entirely by a thin layer of the PCL. They were characterized, and then mechanical properties, in vitro degradation and in vitro drug release were assessed. Moreover, human dermal fibroblasts (HDF) were cultured on the prepared nanocomposite scaffolds in order to investigate the cytotoxicity by the MTT assay after the first day, third day, and fifth day. Results revealed that the most favorable atorvastatin-loaded NLCs had 99.54 nm average particle size, -24.30 mV zeta potential, 97.98% EE, and 75.24% drug release within 237 hrs. Mechanical tests indicated that all the three scaffolds had approximately a 90 MPa elastic modulus which was more than two-fold of tensile modulus of normal human skin. The in vitro degradation test demonstrated that the membranes were degraded up to 98% after 5 days, and the scaffolds drug release efficiency (DRE) was in a range of 75-79% during those 5 days. The MTT assay results confirmed the cytocompatibility of the scaffolds. The scaffold containing 54.1 wt% NCLs was the optimum sample (S₃). Scanning Electron Microscopy (SEM) images of the latter one showed the uniform distribution of the NLCs with an average size of 150 nm, and the images of cultured HDF illustrated the good cell attachment. In conclusion, suitable physicochemical and biological properties of the novel gelatin/HA/PCL nanocomposite scaffold containing 54.1 wt% atorvastatin-loaded NLCs (S₃) can be a good candidate for skin regeneration.

Application of 3D Bioprinting Technologies to the Management and Treatment of Diabetic Foot Ulcers

Diabetes mellitus (DM) is a chronic metabolic disease with increasing prevalence worldwide. Diabetic foot ulcers (DFUs) are a serious complication of DM. It is estimated that 15-25% of DM patients develop DFU at least once in their lifetime. The lack of effective wound dressings and targeted therapy for DFUs often results in prolonged hospitalization and amputations. As the incidence of DM is projected to rise, the demand for specialized DFU wound management will continue to increase. Hence, it is of great interest to improve and develop effective DFU-specific wound dressings and therapies. In the last decade, 3D bioprinting technology has made a great contribution to the healthcare sector, with the development of personalized prosthetics, implants, and bioengineered tissues. In this review, we discuss the challenges faced in DFU wound management and how 3D bioprinting technology can be applied to advance current treatment methods, such as biomanufacturing of composite 3D human skin substitutes for skin grafting and the development of DFU-appropriate wound dressings. Future co-development of 3D bioprinting technologies with novel treatment approaches to mitigate DFU-specific pathophysiological challenges will be key to limiting the healthcare burden associated with the increasing prevalence of DM.

3D Cell Printing of Tissue/Organ-Mimicking Constructs for Therapeutic and Drug Testing Applications

The development of artificial tissue/organs with the functional maturity of their native equivalents is one of the long-awaited panaceas for the medical and pharmaceutical industries. Advanced 3D cell-printing technology and various functional bioinks are promising technologies in the field of tissue engineering that have enabled the fabrication of complex 3D living tissue/organs. Various requirements for these tissues, including a complex and large-volume structure, tissue-specific microenvironments, and functional vasculatures, have been addressed to develop engineered tissue/organs with native relevance. Functional tissue/organ constructs have been developed that satisfy such criteria and may facilitate both in vivo replenishment of damaged tissue and the development of reliable in vitro testing platforms for drug development. This review describes key developments in technologies and materials for engineering 3D cell-printed constructs for therapeutic and drug testing applications.

Therapeutic effect of decellularized extracellular matrix-based hydrogel for radiation esophagitis by 3D printed esophageal stent

Radiation esophagitis, the most common acute adverse effect of radiation therapy, leads to unwanted consequences including discomfort, pain, an even death. However, no direct cure exists for patients suffering from this condition, with the harmful effect of ingestion and acid reflux on the damaged esophageal mucosa remaining an unresolved problem. Through the delivery of the hydrogel with stent platform, we aimed to evaluate the regenerative capacity of a tissue-specific decellularized extracellular matrix (dECM) hydrogel on damaged tissues. For this, an esophagus-derived dECM (EdECM) was developed and shown to have superior biofunctionality and rheological properties, as well as physical stability, potentially providing a better microenvironment for tissue development. An EdECM hydrogel-loaded stent was sequentially fabricated using a rotating rod combined 3D printing system that showed structural stability and protected a loaded hydrogel during delivery. Finally, following stent implantation, the therapeutic effect of EdECM was examined in a radiation esophagitis rat model. Our findings demonstrate that EdECM hydrogel delivery via a stent platform can rapidly resolve an inflammatory response, thus promoting a pro-regenerative microenvironment. The results suggest a promising therapeutic strategy for the treatment of radiation esophagitis.

Matrisome Provides a Supportive Microenvironment for Skin Functions of Diverse Species

A biomaterial scaffold is a promising tool employed to drive tissue regeneration. This technology has been successfully applied in human tissue rebuilding, particularly for the skin. Meanwhile, there is still room for further improvement, such as maintaining sufficient functionality of a biomaterial scaffold. Here, we developed a new decellularization method to generate a complete anatomical skin biomatrix scaffold with a preserved extracellular matrix (ECM) architecture. We performed proteomic and bioinformatic analyses of the skin scaffold maps of humans, pigs, and rats and systematically analyzed the interaction between ECM proteins and different cell types in the skin microenvironment. These interactions served to quantify the structure and function of the skin's Matrisome comprising core ECM components and ECM-associated soluble signaling molecules required for the regulation of epidermal development. We primarily found that the properties of the skin ECM were species-specific. For example, the composition and function of the ECM of the human skin were more similar to those of pigs compared with those of rats. However, the skin ECM of the pig was significantly deficient in its enzyme systems and immune regulatory factors compared with that of humans. These findings provide a new understanding of the role of the skin ECM niche as well as an attractive strategy that can apply tissue engineering principles to skin biomatrix scaffold materials, which promises to accelerate and enhance tissue regeneration.

Review of Plastic Surgery Biomaterials and Current Progress in Their 3D Manufacturing Technology

Plastic surgery is a broad field, including maxillofacial surgery, skin flaps and grafts, liposuction and body contouring, breast surgery, and facial cosmetic procedures. Due to the requirements of plastic surgery for the biological safety of materials, biomaterials are widely used because of its superior biocompatibility and biodegradability. Currently, there are many kinds of biomaterials clinically used in plastic surgery and their applications are diverse. Moreover, with the rise of three-dimensional printing technology in recent years, the macroscopically more precise and personalized bio-scaffolding materials with microporous structure have made good progress, which is thought to bring new development to biomaterials. Therefore, in this paper, we reviewed the plastic surgery biomaterials and current progress in their 3D manufacturing technology.

Overview of Current Advances in Extrusion Bioprinting for Skin Applications

Bioprinting technologies, which have the ability to combine various human cell phenotypes, signaling proteins, extracellular matrix components, and other scaffold-like biomaterials, are currently being exploited for the fabrication of human skin in regenerative medicine. We performed a systematic review to appraise the latest advances in 3D bioprinting for skin applications, describing the main cell phenotypes, signaling proteins, and bioinks used in extrusion platforms. To understand the current limitations of this technology for skin bioprinting, we briefly address the relevant aspects of skin biology. This field is in the early stage of development, and reported research on extrusion bioprinting for skin applications has shown moderate progress. We have identified two major trends. First, the biomimetic approach uses cell-laden natural polymers, including fibrinogen, decellularized extracellular matrix, and collagen. Second, the material engineering line of research, which is focused on the optimization of printable biomaterials that expedite the manufacturing process, mainly involves chemically functionalized polymers and reinforcement strategies through molecular blending and postprinting interventions, i.e., ionic, covalent, or light entanglement, to enhance the mechanical properties of the construct and facilitate layer-by-layer deposition. Skin constructs manufactured using the biomimetic approach have reached a higher level of complexity in biological terms, including up to five different cell phenotypes and mirroring the epidermis, dermis and hypodermis. The confluence of the two perspectives, representing interdisciplinary inputs, is required for further advancement toward the future translation of extrusion bioprinting and to meet the urgent clinical demand for skin equivalents.

Instant Gelation System as Self-Healable and Printable 3D Cell Culture Bioink Based on Dynamic Covalent Chemistry

The rapid development of additive manufacturing techniques in the field of tissue regeneration offers unprecedented success for artificial tissue and organ fabrication. However, some limitations still remain for current bioinks, such as the compromised cell viability after printing, the low cross-linking efficiency leading to poor printing resolution and speed due to the relatively slow gelation rate, and the requirement of external stimuli for gelation. To address these problems, herein, a biocompatible and printable instant gelation hydrogel system has been developed based on a designed hyperbranched poly(ethylene glycol) (PEG)-based multihydrazide macro-cross-linker (HB-PEG-HDZ) and an aldehyde-functionalized hyaluronic acid (HA-CHO). HB-PEG-HDZ is prepared by the postfunctionalization of hyperbranched PEG-based multivinyl macromer via thiol-ene chemistry. Owing to the high functional group density of HB-PEG-HDZ, the hydrogel can be formed instantly upon mixing the solutions of two components. The reversible cross-linking mechanism between the hydrazide and aldehyde groups endows the hydrogel with shear-thinning and self-healing properties. The minimally toxic components and cross-linking chemistry allow the resulting hydrogel to be a biocompatible niche. Moreover, the fast sol-to-gel transition of the hydrogel, combining all of the advanced characteristics of this platform, protects the cells during the printing procedure, avoids their damage during extrusion, and improves the transplanted cell survival.

Development of novel microenvironments for promoting enhanced wound healing

PURPOSE OF REVIEW

Nonhealing wounds are a significant issue facing the healthcare industry. Materials that modulate the wound microenvironment have the potential to improve healing outcomes.

RECENT FINDINGS

A variety of acellular and cellular scaffolds have been developed for regulating the wound microenvironment, including materials for controlled release of antimicrobials and growth factors, materials with inherent immunomodulative properties, and novel colloidal-based scaffolds. Scaffold construction methods include electrospinning, 3D printing, decellularization of extracellular matrix, or a combination of techniques. Material application methods include layering or injecting at the wound site.

SUMMARY

Though these techniques show promise for repairing wounds, all material strategies thus far struggle to induce regeneration of features such as sweat glands and hair follicles. Nonetheless, innovative technologies currently in the research phase may facilitate future attainment of these features. Novel methods and materials are constantly arising for the development of microenvironments for enhanced wound healing.