Additive manufacturing of hierarchical injectable scaffolds for tissue engineering

Construction of a Decellularized Multicomponent Extracellular Matrix Interpenetrating Network Scaffold by Gelatin Microporous Hydrogel 3D Cell Culture System

Interface tissue repair requires the construction of biomaterials with integrated structures of multiple protein types. Hydrogels that modulate internal porous structures provide a 3D microenvironment for encapsulated cells, making them promise for interface tissue repair. Currently, reduction of intrinsic immunogenicity and increase of bioactive extracellular matrix (ECM) secretion are issues to be considered in these materials. In this study, gelatin methacrylate (GelMA) hydrogel is used to encapsulate chondrocytes and construct a phase transition 3D cell culture system (PTCC) by utilizing the thermosensitivity of gelatin microspheres to create micropores within the hydrogel. The types of bioactive extracellular matrix protein formation by chondrocytes encapsulated in hydrogels are investigated in vitro. After 28 days of culture, GelMA PTCC forms an extracellular matrix predominantly composed of collagen type II, collagen type I, and fibronectin. After decellularization, the protein types and mechanical properties are well preserved, fabricating a decellularized tissue-engineered extracellular matrix and GelMA hydrogel interpenetrating network hydrogel (dECM-GelMA IPN) consisting of GelMA hydrogel as the first-level network and the ECM secreted by chondrocytes as the second-level network. This material has the potential to mediate the repair and regeneration of tendon-bone interface tissues with multiple protein types.

Stimuli-Responsive Hydrogels: The Dynamic Smart Biomaterials of Tomorrow

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

Embedded 3D Printing of Cryogel-Based Scaffolds

Cryogel-based scaffolds have attracted great attention in tissue engineering due to their interconnected macroporous structures. However, three-dimensional (3D) printing of cryogels with a high degree of precision and complexity is a challenge, since the synthesis of cryogels occurs under cryogenic conditions. In this study, we demonstrated the fabrication of cryogel-based scaffolds for the first time by using an embedded printing technique. A photo-cross-linkable gelatin methacryloyl (GelMA)-based ink composition, including alginate and photoinitiator, was printed into a nanoclay-based support bath. The layer-by-layer extruded ink was held in complex and overhanging structures with the help of pre-cross-linking of alginate with Ca2+ present in the support bath. The printed 3D structures in the support bath were frozen, and then GelMA was cross-linked at a subzero temperature under UV light. The printed and cross-linked structures were successfully recovered from the support bath with an integrated shape complexity. SEM images showed the formation of a 3D printed scaffold where porous GelMA cryogel was integrated between the cross-linked alginate hydrogels. In addition, they showed excellent shape recovery under uniaxial compression cycles of up to 80% strain. In vitro studies showed that the human fibroblast cells attached to the 3D printed scaffold and displayed spread morphology with a high proliferation rate. The results revealed that the embedded 3D printing technique enables the fabrication of cytocompatible cryogel based scaffolds with desired morphology and mechanical behavior using photo-cross-linkable bioink composition. The properties of the cryogels can be modified by varying the GelMA concentration, whereby various shapes of scaffolds can be fabricated to meet the specific requirements of tissue engineering applications.

Highly Efficient Cardiac Differentiation and Maintenance by Thrombin-Coagulated Fibrin Hydrogels Enriched with Decellularized Porcine Heart Extracellular Matrix

Biochemical and biophysical properties instruct cardiac tissue morphogenesis. Here, we are reporting on a blend of cardiac decellularized extracellular matrix (dECM) from porcine ventricular tissue and fibrinogen that is suitable for investigations employing an in vitro 3D cardiac cell culture model. Rapid and specific coagulation with thrombin facilitates the gentle inclusion of cells while avoiding sedimentation during formation of the dECM-fibrin composite. Our investigations revealed enhanced cardiogenic differentiation in the H9c2 myoblast cells when using the system in a co-culture with Nor-10 fibroblasts. Further enhancement of differentiation efficiency was achieved by 3D embedding of rat neonatal cardiomyocytes in the 3D system. Calcium imaging and analysis of beating motion both indicate that the dECM-fibrin composite significantly enhances recovery, frequency, synchrony, and the maintenance of spontaneous beating, as compared to various controls including Matrigel, pure fibrin and collagen I as well as a fibrin-collagen I blend.

In Situ Forming Hydrogel as a Tracer and Degradable Lacrimal Plug for Dry Eye Treatment

Lacrimal plug is an effective and widely therapeutic strategy to treat dry eye. However, almost all commercialized plugs are fixed in a certain design and associated with many complications, such as spontaneous plug extrusion, epiphora, and granuloma and cannot be traced in the long-term. Herein, a simple in situ forming hydrogel is developed as a tracer and degradable lacrimal plug to achieve the best match with the irregular lacrimal passages. In this strategy, methacrylate-modified silk fibroin (SFMA) is served as a network, and a self-assembled indocyanine green fluorescence tracer nanoparticle (FTN) is embedded as an indicator to develop the hydrogel plug using visible photo-crosslinking. This SFMA/FTN hydrogel plug has excellent biocompatibility and biodegradability, which can be noninvasively monitored by near-infrared light. In vivo tests based on dry eye rabbits show that the SFMA/FTN hydrogel plug can completely block the lacrimal passages and greatly improve the various clinical indicators of dry eye. These results demonstrate that the SFMA/FTN hydrogel is suitable as an injectable and degradable lacrimal plug with a long-term tracking function. The work offers a new approach to the development of absorbable plugs for the treatment of dry eye.

Design of an elastic porous injectable biomaterial for tissue regeneration and volume retention

Soft tissue reconstruction currently relies on two main approaches, one involving the implantation of external biomaterials and the second one exploiting surgical autologous tissue displacement. While both methods have different advantages and disadvantages, successful long-term solutions for soft tissue repair are still limited. Specifically, volume retention over time and local tissue regeneration are the main challenges in the field. In this study the performance of a recently developed elastic porous injectable (EPI) biomaterial based on crosslinked carboxymethylcellulose is analyzed. Nearly quantitative volumetric stability, with over 90% volume retention at 6 months, is observed, and the pore space of the material is effectively colonized with autologous fibrovascular tissue. A comparative analysis with hyaluronic acid and collagen-based clinical reference materials is also performed. Mechanical stability, evidenced by a low-strain elastic storage modulus (G') approaching 1kPa and a yield strain of several tens of percent, is required for volume retention in-vivo. Macroporosity, along with in-vivo persistence of at least several months, is instead needed for successful host tissue colonization. This study demonstrates the importance of understanding material design criteria and defines the biomaterial requirements for volume retention and tissue colonization in soft tissue regeneration. STATEMENT OF SIGNIFICANCE: We present the design of an elastic, porous, injectable (EPI) scaffold suspension capable of inducing a precisely defined, stable volume of autologous connective tissue in situ. It combines volume stability and vascularized tissue induction capacity known from bulk scaffolds with the ease of injection in shear yielding materials. By comparative study with a series of clinically established biomaterials including a wound healing matrix and dermal fillers, we establish design rules regarding rheological and compressive mechanical properties as well as degradation characteristics that rationally underpin the volume stability and tissue induction in a high-performance biomaterial. These design rules should allow to streamline the development of new colonizable injectables.

The Role of Interstitial Fluid Pressure in Cerebral Porous Biomaterial Integration

Recent advances in biomaterials offer new possibilities for brain tissue reconstruction. Biocompatibility, provision of cell adhesion motives and mechanical properties are among the present main design criteria. We here propose a radically new and potentially major element determining biointegration of porous biomaterials: the favorable effect of interstitial fluid pressure (IFP). The force applied by the lymphatic system through the interstitial fluid pressure on biomaterial integration has mostly been neglected so far. We hypothesize it has the potential to force 3D biointegration of porous biomaterials. In this study, we develop a capillary hydrostatic device to apply controlled in vitro interstitial fluid pressure and study its effect during 3D tissue culture. We find that the IFP is a key player in porous biomaterial tissue integration, at physiological IFP levels, surpassing the known effect of cell adhesion motives. Spontaneous electrical activity indicates that the culture conditions are not harmful for the cells. Our work identifies interstitial fluid pressure at physiological negative values as a potential main driver for tissue integration into porous biomaterials. We anticipate that controlling the IFP level could narrow the gap between in vivo and in vitro and therefore decrease the need for animal screening in biomaterial design.

Current Advances in 3D Bioprinting for Cancer Modeling and Personalized Medicine

Tumor cells evolve in a complex and heterogeneous environment composed of different cell types and an extracellular matrix. Current 2D culture methods are very limited in their ability to mimic the cancer cell environment. In recent years, various 3D models of cancer cells have been developed, notably in the form of spheroids/organoids, using scaffold or cancer-on-chip devices. However, these models have the disadvantage of not being able to precisely control the organization of multiple cell types in complex architecture and are sometimes not very reproducible in their production, and this is especially true for spheroids. Three-dimensional bioprinting can produce complex, multi-cellular, and reproducible constructs in which the matrix composition and rigidity can be adapted locally or globally to the tumor model studied. For these reasons, 3D bioprinting seems to be the technique of choice to mimic the tumor microenvironment in vivo as closely as possible. In this review, we discuss different 3D-bioprinting technologies, including bioinks and crosslinkers that can be used for in vitro cancer models and the techniques used to study cells grown in hydrogels; finally, we provide some applications of bioprinted cancer models.

Cryogels: recent applications in 3D-bioprinting, injectable cryogels, drug delivery, and wound healing

Cryogels are macroporous polymeric structures formed from the cryogelation of monomers/polymers in a solvent below freezing temperature. Due to their inherent interconnected macroporosity, ease of preparation, and biocompatibility, they are increasingly being investigated for use in biomedical applications such as 3D-bioprinting, drug delivery, wound healing, and as injectable therapeutics. This review highlights the fundamentals of macroporous cryogel preparation, cryogel properties that can be useful in the highlighted biomedical applications, followed by a comprehensive review of recent studies in these areas. Research evaluated includes the use of cryogels to combat various types of cancer, for implantation without surgical incision, and use as highly effective wound dressings. Furthermore, conclusions and outlooks are discussed for the use of these promising and durable macroporous cryogels.

An Injectable Meta-Biomaterial: From Design and Simulation to In Vivo Shaping and Tissue Induction

A novel type of injectable biomaterial with an elastic softening transition is described. The material enables in vivo shaping, followed by induction of 3D stable vascularized tissue. The synthesis of the injectable meta-biomaterial is instructed by extensive numerical simulation as a suspension of irregularly fragmented, highly porous sponge-like microgels. The irregular particle shape dramatically enhances yield strain for in vivo stability against deformation. Porosity of the particles, along with friction between internal surfaces, provides the elastic softening transition. This emergent metamaterial property enables the material to reversibly change stiffness during deformation, allowing native tissue properties to be matched over a wide range of deformation amplitudes. After subcutaneous injection in mice, predetermined shapes can be sculpted manually. The 3D shape is maintained during excellent host tissue integration, with induction of vascular connective tissue that persists to the end of one-year follow-up. The geometrical design is compatible with many hydrogel materials, including cell-adhesion motives for cell transplantation. The injectable meta-biomaterial therefore provides new perspectives in soft tissue engineering and regenerative medicine.

Application of textile technology in tissue engineering: A review

One of the key elements in tissue engineering is to design and fabricate scaffolds with tissue-like properties. Among various scaffold fabrication methods, textile technology has shown its unique advantages in mimicking human tissues' properties such as hierarchical, anisotropic, and strain-stiffening properties. As essential components in textile technology, textile patterns affect the porosity, architecture, and mechanical properties of textile-based scaffolds. However, the potential of various textile patterns has not been fully explored when fabricating textile-based scaffolds, and the effect of different textile patterns on scaffold properties has not been thoroughly investigated. This review summarizes textile technology development and highlights its application in tissue engineering to facilitate the broader application of textile technology, especially various textile patterns in tissue engineering. The potential of using different textile methods such as weaving, knitting, and braiding to mimic properties of human tissues is discussed, and the effect of process parameters in these methods on fabric properties is summarized. Finally, perspectives on future directions for explorations are presented. STATEMENT OF SIGNIFICANCE: Recently, biomedical engineers have applied textile technology to fabricate scaffolds for tissue engineering applications. Various textile methods, especially weaving, knitting, and braiding, enables engineers to customize the physical, mechanical, and biological properties of scaffolds. However, most textile-based scaffolds only use simple textile patterns, and the effect of different textile patterns on scaffold properties has not been thoroughly investigated. In this review, we cover for the first time the effect of process parameters in different textile methods on fabric properties, exploring the potential of using different textile methods to mimic properties of human tissues. Previous advances in textile technology are presented, and future directions for explorations are presented, hoping to facilitate new breakthroughs of textile-based tissue engineering.

Design and Assessment of Biodegradable Macroporous Cryogels as Advanced Tissue Engineering and Drug Carrying Materials

Cryogels obtained by the cryotropic gelation process are macroporous hydrogels with a well-developed system of interconnected pores and shape memory. There have been significant recent advancements in our understanding of the cryotropic gelation process, and in the relationship between components, their structure and the application of the cryogels obtained. As cryogels are one of the most promising hydrogel-based biomaterials, and this field has been advancing rapidly, this review focuses on the design of biodegradable cryogels as advanced biomaterials for drug delivery and tissue engineering. The selection of a biodegradable polymer is key to the development of modern biomaterials that mimic the biological environment and the properties of artificial tissue, and are at the same time capable of being safely degraded/metabolized without any side effects. The range of biodegradable polymers utilized for cryogel formation is overviewed, including biopolymers, synthetic polymers, polymer blends, and composites. The paper discusses a cryotropic gelation method as a tool for synthesis of hydrogel materials with large, interconnected pores and mechanical, physical, chemical and biological properties, adapted for targeted biomedical applications. The effect of the composition, cross-linker, freezing conditions, and the nature of the polymer on the morphology, mechanical properties and biodegradation of cryogels is discussed. The biodegradation of cryogels and its dependence on their production and composition is overviewed. Selected representative biomedical applications demonstrate how cryogel-based materials have been used in drug delivery, tissue engineering, regenerative medicine, cancer research, and sensing.

Injectable Cryogels in Biomedicine

Cryogels are interconnected macroporous materials that are synthesized from a monomer solution at sub-zero temperatures. Cryogels, which are used in various applications in many research areas, are frequently used in biomedicine applications due to their excellent properties, such as biocompatibility, physical resistance and sensitivity. Cryogels can also be prepared in powder, column, bead, sphere, membrane, monolithic, and injectable forms. In this review, various examples of recent developments in biomedical applications of injectable cryogels, which are currently scarce in the literature, made from synthetic and natural polymers are discussed. In the present review, several biomedical applications of injectable cryogels, such as tissue engineering, drug delivery, therapeutic, therapy, cell transplantation, and immunotherapy, are emphasized. Moreover, it aims to provide a different perspective on the studies to be conducted on injectable cryogels, which are newly emerging trend.

Key advances of carboxymethyl cellulose in tissue engineering & 3D bioprinting applications

Carboxymethyl cellulose (CMC) is a water-soluble derivative of cellulose and a major type of cellulose ether prepared by the chemical attack of alkylating reagents on the activated non-crystalline regions of cellulose. It is the first FDA approved cellulose derivative which can be targeted for desired chemical modifications. In this review, the properties along with current advances in the physical and chemical modifications of CMC are discussed. Further, CMC and modified CMC could be engineered to fabricate scaffolds for tissue engineering applications. In recent times, CMC and its derivatives have been developed as smart bioinks for 3D bioprinting applications. From these perspectives, the applications of CMC in tissue engineering and current knowledge on peculiar features of CMC in 3D and 4D bioprinting applications are elaborated in detail. Lastly, future perspectives of CMC for wider applications in tissue engineering and 3D/4D bioprinting are highlighted.

Neurothreads: Development of supportive carriers for mature dopaminergic neuron differentiation and implantation

In this study we present the use of elastic macroporous cryogels for differentiation and transplantation of mature neurons. We develop a coating suitable for long-term neuronal culture, including stem cell differentiation, by covalent immobilization of neural adhesion proteins. In the context of cell therapy for Parkinson's disease, we show compatibility with established dopaminergic differentiation of both immortalized mesencephalic progenitors - LUHMES - and human embryonic stem cells (hESCs). We adjust structural properties of the biomaterial to create carriers - Neurothreads - favourable for cell viability during transplantation. Finally, we show feasibility of preservation of mature neurons, supported by Neurothreads, one month after in-vivo transplantation. Preliminary data suggests that the Neurothread approach could provide more mature and less proliferative cells in vivo.

Triggered micropore-forming bioprinting of porous viscoelastic hydrogels

Cell-laden scaffolds of architecture and mechanics that mimic those of the host tissues are important for a wide range of biomedical applications but remain challenging to bioprint. To address these challenges, we report a new method called triggered micropore-forming bioprinting. The approach can yield cell-laden scaffolds of defined architecture and interconnected pores over a range of sizes, encompassing that of many cell types. The viscoelasticity of the bioprinted scaffold can match that of biological tissues and be tuned independently of porosity and stiffness. The bioprinted scaffold also exhibits superior mechanical robustness despite high porosity. The bioprinting method and the resulting scaffolds support cell spreading, migration, and proliferation. The potential of the 3D bioprinting system is demonstrated for vocal fold tissue engineering and as an in vitro cancer model. Other possible applications are foreseen for tissue repair, regenerative medicine, organ-on-chip, drug screening, organ transplantation, and disease modeling.

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

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

Injectable, scalable 3D tissue-engineered model of marrow hematopoiesis

Modeling the interaction between the supportive stroma and the hematopoietic stem and progenitor cells (HSPC) is of high interest in the regeneration of the bone marrow niche in blood disorders. In this work, we present an injectable co-culture system to study this interaction in a coherent in vitro culture and in vivo transplantation model. We assemble a 3D hematopoietic niche in vitro by co-culture of supportive OP9 mesenchymal cells and HSPCs in porous, chemically defined collagen-coated carboxymethylcellulose microscaffolds (CCMs). Flow cytometry and hematopoietic colony forming assays demonstrate the stromal supportive capacity for in vitro hematopoiesis in the absence of exogenous cytokines. After in vitro culture, we recover a paste-like living injectable niche biomaterial from CCM co-cultures by controlled, partial dehydration. Cell viability and the association between stroma and HSPCs are maintained in this process. After subcutaneous injection of this living artificial niche in vivo, we find maintenance of stromal and hematopoietic populations over 12 weeks in immunodeficient mice. Indeed, vascularization is enhanced in the presence of HSPCs. Our approach provides a minimalistic, scalable, biomimetic in vitro model of hematopoiesis in a microcarrier format that preserves the HSPC progenitor function, while being injectable in vivo without disrupting the cell-cell interactions established in vitro.

Advances in bioprinting using additive manufacturing

Since its conception in the 1980's, several advances in the field of additive manufacturing have led to exploration of alternate as well as combination biomaterials. These progresses have directed the use of 3D printing in wider applications such as printing of dermal layers, cartilage, bone defects, and surgical implants. Furthermore, the incorporation of live and functional cells with or atop biomaterials has laid the foundation for its use in tissue engineering. The purpose of this review is to summarize the advances in 3D printing and bioprinting of several types of tissues such as skin, cartilage, bones, and cardiac valves. This review will address the current 3D technologies used in tissue construction and study the biomaterials being investigated. There are several requirements that need to be addressed, in order to reconstruct functional tissue such as mechanical strength, porosity of the replicate and cellular incorporation. Researchers have focused their studies to answer questions regarding these requirements.

Hydrostatic High-Pressure Post-Processing of Specimens Fabricated by DLP, SLA, and FDM: An Alternative for the Sterilization of Polymer-Based Biomedical Devices

In this work, we assess the effects of sterilization in materials manufactured using additive manufacturing by employing a sterilization technique used in the food industry. To estimate the feasibility of the hydrostatic high-pressure (HHP) sterilization of biomedical devices, we have evaluated the mechanical properties of specimens produced by commercial 3D printers. Evaluations of the potential advantages and drawbacks of Fused Deposition Modeling (FDM), Digital Light Processing (DLP) technology, and Stereolithography (SLA) were considered for this study due to their widespread availability. Changes in mechanical properties due to the proposed sterilization technique were compared to values derived from the standardized autoclaving methodology. Enhancement of the mechanical properties of samples treated with Hydrostatic high-pressure processing enhanced mechanical properties, with a 30.30% increase in the tensile modulus and a 26.36% increase in the ultimate tensile strength. While traditional autoclaving was shown to systematically reduce the mechanical properties of the materials employed and damages and deformation on the surfaces were observed, HHP offered an alternative for sterilization without employing heat. These results suggest that while forgoing high-temperature for sanitization, HHP processing can be employed to take advantage of the flexibility of additive manufacturing technologies for manufacturing implants, instruments, and other devices.

Lyophilized Scaffolds Fabricated from 3D-Printed Photocurable Natural Hydrogel for Cartilage Regeneration

Repair of cartilage defects is highly challenging in clinical treatment. Tissue engineering provides a promising approach for cartilage regeneration and repair. As a core component of tissue engineering, scaffolds have a crucial influence on cartilage regeneration, especially in immunocompetent large animal and human. Native polymers, such as gelatin and hyaluronic acid, have known as ideal biomimetic scaffold sources for cartilage regeneration. However, how to precisely control their structure, degradation rate, and mechanical properties suitable for cartilage regeneration remains a great challenge. To address these issues, a series of strategies were introduced in the current study to optimize the scaffold fabrication. First, gelatin and hyaluronic acid were prepared into a hydrogel and 3D printing was adopted to ensure precise control in both the outer 3D shape and internal pore structure. Second, methacrylic anhydride and a photoinitiator were introduced into the hydrogel system to make the material photocurable during 3D printing. Finally, lyophilization was used to further enhance mechanical properties and prolong degradation time. According to the current results, by integrating photocuring 3D printing and lyophilization techniques, gelatin and hyaluronic acid were successfully fabricated into human ear- and nose-shaped scaffolds, and both scaffolds achieved shape similarity levels over 90% compared with the original digital models. The scaffolds with 50% infill density achieved proper internal pore structure suitable for cell distribution, adhesion, and proliferation. Besides, lyophilization further enhanced mechanical strength of the 3D-printed hydrogel and slowed its degradation rate matching to cartilage regeneration. Most importantly, the scaffolds combined with chondrocytes successfully regenerated mature cartilage with typical lacunae structure and cartilage-specific extracellular matrixes both in vitro and in the autologous goat model. The current study established novel scaffold-fabricated strategies for native polymers and provided a novel natural 3D scaffold with satisfactory outer shape, pore structure, mechanical strength, degradation rate, and weak immunogenicity for cartilage regeneration.