Three-dimensional Printing of Multilayered Tissue Engineering Scaffolds

Towards the development of sensation-enabled skin substitutes

Recent advances in cell and biofabrication technologies have contributed to the development of complex human organs. In particular, several skin substitutes are being generated using tissue engineering and regenerative medicine (TERM) technologies. However, recent studies mainly focus on the restoration of the dermis and epidermis layers rather than the regeneration of a fully functional innervated skin organ. Innervation is a critical step in functional tissue repair which has been overlooked in the current TERM studies. In the current study, we highlight the importance of sensation in the skin as the largest sensory organ in the human body. In large non-healing skin wounds, the skin sensation is severely diminished or completely lost and ultimately lead to chronic pain and wound healing process interruption. Current therapeutics for restoring skin sensation after trauma are limited. Recent regenerative medicine-based studies could successfully induce neural networks in skin substitutes, but the effectiveness of these technologies in enhancing sensory capability needs further investigation.

Zonal Patterning of Extracellular Matrix and Stromal Cell Populations Along a Perfusable Cellular Microchannel

The spatial organization of biophysical and biochemical cues in the extracellular matrix (ECM) in concert with reciprocal cell-cell signaling is vital to tissue patterning during development. However, elucidating the role an individual microenvironmental factor plays using existing in vivo models is difficult due to their inherent complexity. In this work, we have developed a microphysiological system to spatially pattern the biochemical, biophysical, and stromal cell composition of the ECM along an epithelialized 3D microchannel. This technique is adaptable to multiple hydrogel compositions and scalable to the number of zones patterned. We confirmed that the methodology to create distinct zones resulted in a continuous, annealed hydrogel with regional interfaces that did not hinder the transport of soluble molecules. Further, the interface between hydrogel regions did not disrupt microchannel structure, epithelial lumen formation, or media perfusion through an acellular or cellularized microchannel. Finally, we demonstrated spatially patterned tubulogenic sprouting of a continuous epithelial tube into the surrounding hydrogel confined to local regions with stromal cell populations, illustrating spatial control of cell-cell interactions and signaling gradients. This easy-to-use system has wide utility for modeling three-dimensional epithelial and endothelial tissue interactions with heterogeneous hydrogel compositions and/or stromal cell populations to investigate their mechanistic roles during development, homeostasis, or disease.

Vat photopolymerization printing of functionalized hydrogels on commercial contact lenses

Contact lenses are widely used for vision correction and cosmetic purposes. Smart contact lenses offer further opportunities as functionalized non-invasive devices capable of simultaneous vision correction, real-time health monitoring and patient specific drug delivery. Herein, a low-cost vat photopolymerization technique is developed for directly 3D printing functionalized structures on commercially available contact lenses. The process enables controlled deposition of functionalized hydrogels, in customizable patterns, on the commercial contact lens surface with negligible optical losses. Multi-functional contact lenses can also be 3D printed with multiple materials deposited at different regions of the contact lens. Herein, the functionalities of colour blindness correction and real-time UV monitoring are demonstrated, by employing three suitable dyes incorporated into 2-hydroxyethyl methacrylate (HEMA) hydrogel structures printed on contact lenses. The results suggest that 3D printing can pave the way towards simple production of low-cost patient specific smart contact lenses.

Electrohydrodynamic Direct-Writing Micro/Nanofibrous Architectures: Principle, Materials, and Biomedical Applications

Electrohydrodynamic (EHD) direct-writing has recently gained attention as a highly promising additive manufacturing strategy for fabricating intricate micro/nanoscale architectures. This technique is particularly well-suited for mimicking the extracellular matrix (ECM) present in biological tissue, which serves a vital function in facilitating cell colonization, migration, and growth. The integration of EHD direct-writing with other techniques has been employed to enhance the biological performance of scaffolds, and significant advancements have been made in the development of tailored scaffold architectures and constituents to meet the specific requirements of various biomedical applications. Here, a comprehensive overview of EHD direct-writing is provided, including its underlying principles, demonstrated materials systems, and biomedical applications. A brief chronology of EHD direct-writing is provided, along with an examination of the observed phenomena that occur during the printing process. The impact of biomaterial selection and architectural topographic cues on biological performance is also highlighted. Finally, the major limitations associated with EHD direct-writing are discussed.

Hydrogels in Gene Delivery Techniques for Regenerative Medicine and Tissue Engineering

Hydrogels are 3D networks swollen with water. They are biocompatible, strong, and moldable and are emerging as a promising biomedical material for regenerative medicine and tissue engineering to deliver therapeutic genes. The excellent natural extracellular matrix simulation properties of hydrogels enable them to be co-cultured with cells or enhance the expression of viral or non-viral vectors. Its biocompatibility, high strength, and degradation performance also make the action process of carriers in tissues more ideal, making it an ideal biomedical material. It has been shown that hydrogel-based gene delivery technologies have the potential to play therapy-relevant roles in organs such as bone, cartilage, nerve, skin, reproductive organs, and liver in animal experiments and preclinical trials. This paper reviews recent articles on hydrogels in gene delivery and explains the manufacture, applications, developmental timeline, limitations, and future directions of hydrogel-based gene delivery techniques.

Seamless Integration of Conducting Hydrogels in Daily Life: From Preparation to Wearable Application

Conductive hydrogels (CHs) have received significant attention for use in wearable devices because they retain their softness and flexibility while maintaining high conductivity. CHs are well suited for applications in skin-contact electronics and biomedical devices owing to their high biocompatibility and conformality. Although highly conductive hydrogels for smart wearable devices are extensively researched, a detailed summary of the outstanding results of CHs is required for a comprehensive understanding. In this review, the recent progress in the preparation and fabrication of CHs is summarized for smart wearable devices. Improvements in the mechanical, electrical, and functional properties of high-performance wearable devices are also discussed. Furthermore, recent examples of innovative and highly functional devices based on CHs that can be seamlessly integrated into daily lives are reviewed.

3D printed osteochondral scaffolds: design strategies, present applications and future perspectives

Articular osteochondral (OC) defects are a global clinical problem characterized by loss of full-thickness articular cartilage with underlying calcified cartilage through to the subchondral bone. While current surgical treatments can relieve pain, none of them can completely repair all components of the OC unit and restore its original function. With the rapid development of three-dimensional (3D) printing technology, admirable progress has been made in bone and cartilage reconstruction, providing new strategies for restoring joint function. 3D printing has the advantages of fast speed, high precision, and personalized customization to meet the requirements of irregular geometry, differentiated composition, and multi-layered boundary layer structures of joint OC scaffolds. This review captures the original published researches on the application of 3D printing technology to the repair of entire OC units and provides a comprehensive summary of the recent advances in 3D printed OC scaffolds. We first introduce the gradient structure and biological properties of articular OC tissue. The considerations for the development of 3D printed OC scaffolds are emphatically summarized, including material types, fabrication techniques, structural design and seed cells. Especially from the perspective of material composition and structural design, the classification, characteristics and latest research progress of discrete gradient scaffolds (biphasic, triphasic and multiphasic scaffolds) and continuous gradient scaffolds (gradient material and/or structure, and gradient interface) are summarized. Finally, we also describe the important progress and application prospect of 3D printing technology in OC interface regeneration. 3D printing technology for OC reconstruction should simulate the gradient structure of subchondral bone and cartilage. Therefore, we must not only strengthen the basic research on OC structure, but also continue to explore the role of 3D printing technology in OC tissue engineering. This will enable better structural and functional bionics of OC scaffolds, ultimately improving the repair of OC defects.

New insight into biodegradable macropore filler on tuning mechanical properties and bone tissue ingrowth in sparingly dissolvable bioceramic scaffolds

Structural parameters of the implants such as shape, size, and porosity of the pores have been extensively investigated to promote bone tissue repair, however, it is unknown how the pore interconnectivity affects the bone growth behaviors in the scaffolds. Herein we systematically evaluated the effect of biodegradable bioceramics as a secondary phase filler in the macroporous networks on the mechanical and osteogenic behaviors in sparingly dissolvable bioceramic scaffolds. The pure hardystonite (HT) scaffolds with ∼550 & 800 μm in pore sizes were prepared by digital light processing, and then the Sr-doped calcium silicate (SrCSi) bioceramic slurry without and with 30 % organic porogens were intruded into the HT scaffolds with 800 μm pore size and sintered at 1150 °C. It indicated that the organic porogens could endow spherical micropores in the SrCSi filler, and the invasion of the SrCSi component could not only significantly enhance the compressive strength and modulus of the HT-based scaffolds, but also induce osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs). The pure HT scaffolds showed extremely slow bio-dissolution in Tris buffer after immersion for 8 weeks (∼1 % mass decay); in contrast, the SrCSi filler would readily dissolve into the aqueous medium and produced a steady mass decay (>6 % mass loss). In vivo experiments in rabbit femoral bone defect models showed that the pure HT scaffolds showed bone tissue ingrowth but the bone growth was impeded in the SrCSi-intruded scaffolds within 4 weeks; however, the group with higher porosity of SrCSi filler showed appreciable osteogenesis after 8 weeks of implantation and the whole scaffold was uniformly covered by new bone tissues after 16 weeks. These findings provide some new insights that the pore interconnectivity is not inevitable to impede bone ingrowth with the prolongation of implantation time, and such a highly biodegradable and bioactive filler intrusion strategy may be beneficial for optimizing the performances of scaffolds in bone regenerative medicine applications.

Engineering periodontal tissue interfaces using multiphasic scaffolds and membranes for guided bone and tissue regeneration

Periodontal diseases are one of the greatest healthcare burdens worldwide. The periodontal tissue compartment is an anatomical tissue interface formed from the periodontal ligament, gingiva, cementum, and bone. This multifaceted composition makes tissue engineering strategies challenging to develop due to the interface of hard and soft tissues requiring multiphase scaffolds to recreate the native tissue architecture. Multilayer constructs can better mimic tissue interfaces due to the individually tuneable layers. They have different characteristics in each layer, with modulation of mechanical properties, material type, porosity, pore size, morphology, degradation properties, and drug-releasing profile all possible. The greatest challenge of multilayer constructs is to mechanically integrate consecutive layers to avoid delamination, especially when using multiple manufacturing processes. Here, we review the development of multilayer scaffolds that aim to recapitulate native periodontal tissue interfaces in terms of physical, chemical, and biological characteristics. Important properties of multiphasic biodegradable scaffolds are highlighted and summarised, with design requirements, biomaterials, and fabrication methods, as well as post-treatment and drug/growth factor incorporation discussed.

Peptide-Based Biomaterials for Bone and Cartilage Regeneration

The healing of osteochondral defects (OCDs) that result from injury, osteochondritis, or osteoarthritis and bear lesions in the cartilage and bone, pain, and loss of joint function in middle- and old-age individuals presents challenges to clinical practitioners because of non-regenerative cartilage and the limitations of current therapies. Bioactive peptide-based osteochondral (OC) tissue regeneration is becoming more popular because it does not have the immunogenicity, misfolding, or denaturation problems associated with original proteins. Periodically, reviews are published on the regeneration of bone and cartilage separately; however, none of them addressed the simultaneous healing of these tissues in the complicated heterogeneous environment of the osteochondral (OC) interface. As regulators of cell adhesion, proliferation, differentiation, angiogenesis, immunomodulation, and antibacterial activity, potential therapeutic strategies for OCDs utilizing bone and cartilage-specific peptides should be examined and investigated. The main goal of this review was to study how they contribute to the healing of OCDs, either alone or in conjunction with other peptides and biomaterials.

Alginate-waterborne polyurethane 3D bioprinted scaffolds for articular cartilage tissue engineering

Articular cartilage defects comprise a spectrum of diseases associated with degeneration or damage of the connective tissue present in particular joints, presenting progressive osteoarthritis if left untreated. In vitro tissue regeneration is an innovative treatment for articular cartilage injuries that is attracting not only clinical attention, but also great interest in the development of novel biomaterials, since this procedure involves the formation of a neotissue with the help of material support. In this work, functional alginate and waterborne polyurethane (WBPU) scaffolds have been developed for articular cartilage regeneration using 3D bioprinting technology. The particular properties of alginate-WBPU blends, like mechanical strength, elasticity and moistening, mimic the original cartilage tissue characteristics, being ideal for this application. To fabricate the scaffolds, mature chondrocytes were loaded into different alginate-WBPU inks with rheological properties suitable for 3D bioprinting. Bioinks with high alginate content showed better 3D printing performance, as well as structural integrity and cell viability, being most suitable for scaffolds fabrication. After 28 days of in vitro cartilage formation experiments, scaffolds containing 3.2 and 6.4 % alginate resulted in the maintenance of cell number in the range of 104 chondrocytes/scaffold in differentiated phenotypes, capable of synthesizing specialized extracellular matrix (ECM) up to 6 μg of glycosaminoglycans (GAG) and thus, showing a potential application of these scaffolds for in vitro regeneration of articular cartilage tissue.

Empowering Precision Medicine: The Impact of 3D Printing on Personalized Therapeutic

This review explores recent advancements and applications of 3D printing in healthcare, with a focus on personalized medicine, tissue engineering, and medical device production. It also assesses economic, environmental, and ethical considerations. In our review of the literature, we employed a comprehensive search strategy, utilizing well-known databases like PubMed and Google Scholar. Our chosen keywords encompassed essential topics, including 3D printing, personalized medicine, nanotechnology, and related areas. We first screened article titles and abstracts and then conducted a detailed examination of selected articles without imposing any date limitations. The articles selected for inclusion, comprising research studies, clinical investigations, and expert opinions, underwent a meticulous quality assessment. This methodology ensured the incorporation of high-quality sources, contributing to a robust exploration of the role of 3D printing in the realm of healthcare. The review highlights 3D printing's potential in healthcare, including customized drug delivery systems, patient-specific implants, prosthetics, and biofabrication of organs. These innovations have significantly improved patient outcomes. Integration of nanotechnology has enhanced drug delivery precision and biocompatibility. 3D printing also demonstrates cost-effectiveness and sustainability through optimized material usage and recycling. The healthcare sector has witnessed remarkable progress through 3D printing, promoting a patient-centric approach. From personalized implants to radiation shielding and drug delivery systems, 3D printing offers tailored solutions. Its transformative applications, coupled with economic viability and sustainability, have the potential to revolutionize healthcare. Addressing material biocompatibility, standardization, and ethical concerns is essential for responsible adoption.

Two-dimensional printing of nanoparticles as a promising therapeutic method for personalized drug administration

The necessity for personalized patient treatment has drastically increased since the contribution of genes to the differences in physiological and metabolic state of individuals have been exposed. Different approaches have been considered so far in order to satisfy all of the diversities in patient needs, yet none of them have been fully implemented thus far. In this framework, various types of 2D printing technologies have been identified to offer some potential solutions for personalized medication, which development is increasing rapidly. Accurate drug-on-demand deposition, the possibility of consuming multiple drug substances in one product and adjusting individual drug concentration are just some of the few benefits over existing bulk pharmaceuticals manufacture, which printing technologies brings. With inclusion of nanotechnology by printing nanoparticles from its dispersions some further opportunities such as controlled and stimuli-responsive drug release or targeted and dose depending on drug delivery were highlighted. Yet, there are still some challenges to be solved before such products can reach the pharmaceutical market. In those terms mostly chemical, physical as well as microbiological stability concerns should be answered, with which 2D printing technology could meet the treatment needs of every individual and fulfill some existing drawbacks of large-scale batch production of pharmaceuticals we possess today.

Advances and Prospects in Materials for Craniofacial Bone Reconstruction

The craniofacial region is composed of 23 bones, which provide crucial function in keeping the normal position of brain and eyeballs, aesthetics of the craniofacial complex, facial movements, and visual function. Given the complex geometry and architecture, craniofacial bone defects not only affect the normal craniofacial structure but also may result in severe craniofacial dysfunction. Therefore, the exploration of rapid, precise, and effective reconstruction of craniofacial bone defects is urgent. Recently, developments in advanced bone tissue engineering bring new hope for the ideal reconstruction of the craniofacial bone defects. This report, presenting a first-time comprehensive review of recent advances of biomaterials in craniofacial bone tissue engineering, overviews the modification of traditional biomaterials and development of advanced biomaterials applying to craniofacial reconstruction. Challenges and perspectives of biomaterial development in craniofacial fields are discussed in the end.

Bioinspired gradient scaffolds for osteochondral tissue engineering

Repairing articular osteochondral defects present considerable challenges in self-repair due to the complex tissue structure and low proliferation of chondrocytes. Conventional clinical therapies have not shown significant efficacy, including microfracture, autologous/allograft osteochondral transplantation, and cell-based techniques. Therefore, tissue engineering has been widely explored in repairing osteochondral defects by leveraging the natural regenerative potential of biomaterials to control cell functions. However, osteochondral tissue is a gradient structure with a smooth transition from the cartilage to subchondral bone, involving changes in chondrocyte morphologies and phenotypes, extracellular matrix components, collagen type and orientation, and cytokines. Bioinspired scaffolds have been developed by simulating gradient characteristics in heterogeneous tissues, such as the pores, components, and osteochondrogenesis-inducing factors, to satisfy the anisotropic features of osteochondral matrices. Bioinspired gradient scaffolds repair osteochondral defects by altering the microenvironments of cell growth to induce osteochondrogenesis and promote the formation of osteochondral interfaces compared with homogeneous scaffolds. This review outlines the meaningful strategies for repairing osteochondral defects by tissue engineering based on gradient scaffolds and predicts the pros and cons of prospective translation into clinical practice.

Research on Cartilage 3D Printing Technology Based on SA-GA-HA

Cartilage damage is difficult to heal and poses a serious problem to human health as it can lead to osteoarthritis. In this work, we explore the application of biological 3D printing to manufacture new cartilage scaffolds to promote cartilage regeneration. The hydrogel made by mixing sodium alginate (SA) and gelatin (GA) has high biocompatibility, but its mechanical properties are poor. The addition of hydroxyapatite (HA) can enhance its mechanical properties. In this paper, the preparation scheme of the SA-GA-HA composite hydrogel cartilage scaffold was explored, the scaffolds prepared with different concentrations were compared, and better formulations were obtained for printing and testing. Mathematical modeling of the printing process of the bracket, simulation analysis of the printing process based on the mathematical model, and adjustment of actual printing parameters based on the results of the simulation were performed. The cartilage scaffold, which was printed using Bioplotter 3D printer, exhibited useful mechanical properties suitable for practical needs. In addition, ATDC-5 cells were seeded on the cartilage scaffolds and the cell survival rate was found to be higher after one week. The findings demonstrated that the fabricated chondrocyte scaffolds had better mechanical properties and biocompatibility, providing a new scaffold strategy for cartilage tissue regeneration.

Material Extrusion of Multi-Polymer Structures Utilizing Design and Shrinkage Behaviors: A Design of Experiment Study

Material extrusion (ME) is an additive manufacturing technique capable of producing functional parts, and its use in multi-material fabrication requires further exploration and expansion. The effectiveness of material bonding is one of the main challenges in multi-material fabrication using ME due to its processing capabilities. Various procedures for improving the adherence of multi-material ME parts have been explored, such as the use of adhesives or the post-processing of parts. In this study, different processing conditions and designs were investigated with the aim of optimizing polylactic acid (PLA) and acrylonitrile-butadiene-styrene (ABS) composite parts without the need for pre- or post-processing procedures. The PLA-ABS composite parts were characterized based on their mechanical properties (bonding modulus, compression modulus, and strength), surface roughness (Ra, Rku, Rsk, and Rz), and normalized shrinkage. All process parameters were statistically significant except for the layer composition parameter in terms of Rsk. The results show that it is possible to create a composite structure with good mechanical properties and acceptable surface roughness values without the need for costly post-processing procedures. Furthermore, the normalized shrinkage and the bonding modulus were correlated, indicating the ability to utilize shrinkage in 3D printing to improve material bonding.

Peptide-Based Hydrogels: Template Materials for Tissue Engineering

Tissue and organ regeneration are challenging issues, yet they represent the frontier of current research in the biomedical field. Currently, a major problem is the lack of ideal scaffold materials' definition. As well known, peptide hydrogels have attracted increasing attention in recent years thanks to significant properties such as biocompatibility, biodegradability, good mechanical stability, and tissue-like elasticity. Such properties make them excellent candidates for 3D scaffold materials. In this review, the first aim is to describe the main features of a peptide hydrogel in order to be considered as a 3D scaffold, focusing in particular on mechanical properties, as well as on biodegradability and bioactivity. Then, some recent applications of peptide hydrogels in tissue engineering, including soft and hard tissues, will be discussed to analyze the most relevant research trends in this field.

How to Build Live-Cell Sensor Microdevices

There is a lot of discussion on how viruses (such as influenza and SARS-CoV-2) are transmitted in air, potentially from aerosols and respiratory droplets, and thus it is important to monitor the environment for the presence of an active pathogen. Currently, the presence of viruses is being determined using primarily nucleic acid-based detection methods, such as reverse transcription- polymerase chain reaction (RT-PCR) tests. Antigen tests have also been developed for this purpose. However, most nucleic acid and antigen methods fail to discriminate between a viable and a non-viable virus. Therefore, we present an alternative, innovative, and disruptive approach involving a live-cell sensor microdevice that captures the viruses (and bacteria) from the air, becomes infected by them, and emits signals for an early warning of the presence of pathogens. This perspective outlines the processes and components required for living sensors to monitor the presence of pathogens in built environments and highlights the opportunity to use immune sentinels in the cells of normal human skin to produce monitors for indoor air pollutants.

Co-Culture of Mesenchymal Stem Cells and Ligamentocytes on Triphasic Embroidered Poly(L-lactide-co-ε-caprolactone) and Polylactic Acid Scaffolds for Anterior Cruciate Ligament Enthesis Tissue Engineering

Successful anterior cruciate ligament (ACL) reconstructions strive for a firm bone-ligament integration. With the aim to establish an enthesis-like construct, embroidered functionalized scaffolds were colonized with spheroids of osteogenically differentiated human mesenchymal stem cells (hMSCs) and lapine (l) ACL fibroblasts in this study. These triphasic poly(L-lactide-co-ε-caprolactone) and polylactic acid (P(LA-CL)/PLA) scaffolds with a bone-, a fibrocartilage transition- and a ligament zone were colonized with spheroids directly after assembly (DC) or with 14-day pre-cultured lACL fibroblast and 14-day osteogenically differentiated hMSCs spheroids (=longer pre-cultivation, LC). The scaffolds with co-cultures were cultured for 14 days. Cell vitality, DNA and sulfated glycosaminoglycan (sGAG) contents were determined. The relative gene expressions of collagen types I and X, Mohawk, Tenascin C and runt-related protein (RUNX) 2 were analyzed. Compared to the lACL spheroids, those with hMSCs adhered more rapidly. Vimentin and collagen type I immunoreactivity were mainly detected in the hMSCs colonizing the bone zone. The DNA content was higher in the DC than in LC whereas the sGAG content was higher in LC. The gene expression of ECM components and transcription factors depended on cell type and pre-culturing condition. Zonal colonization of triphasic scaffolds using spheroids is possible, offering a novel approach for enthesis tissue engineering.

3D printing of biomaterials for vascularized and innervated tissue regeneration

Neurovascular networks play significant roles in the metabolism and regeneration of many tissues and organs in the human body. Blood vessels can transport sufficient oxygen, nutrients, and biological factors, while nerve fibers transmit excitation signals to targeted cells. However, traditional scaffolds cannot satisfy the requirement of stimulating angiogenesis and innervation in a timely manner due to the complexity of host neurovascular networks. Three-dimensional (3D) printing, as a versatile and favorable technique, provides an effective approach to fabricating biological scaffolds with biomimetic architectures and multimaterial compositions, which are capable of regulating multiple cell behaviors. This review paper presents a summary of the current progress in 3D-printed biomaterials for vascularized and innervated tissue regeneration by presenting skin, bone, and skeletal muscle tissues as an example. In addition, we highlight the crucial roles of blood vessels and nerve fibers in the process of tissue regeneration and discuss the future perspectives for engineering novel biomaterials. It is expected that 3D-printed biomaterials with angiogenesis and innervation properties can not only recapitulate the physiological microenvironment of damaged tissues but also rapidly integrate with host neurovascular networks, resulting in accelerated functional tissue regeneration.

The effect of multi-material architecture on the ex vivo osteochondral integration of bioprinted constructs

Extrusion bioprinted constructs for osteochondral tissue engineering were fabricated to study the effect of multi-material architecture on encapsulated human mesenchymal stem cells' tissue-specific matrix deposition and integration into an ex vivo porcine osteochondral explant model. Two extrusion fiber architecture groups with differing transition regions and degrees of bone- and cartilage-like bioink mixing were employed. The gradient fiber (G-Fib) architecture group showed an increase in chondral integration over time, 18.5 ± 0.7 kPa on Day 21 compared to 9.6 ± 1.6 kPa on Day 1 for the required peak push-out force, and the segmented fiber (S-Fib) architecture group did not, which corresponded to the increase in sulfated glycosaminoglycan deposition noted only in the G-Fib group and the staining for cellularity and tissue-specific matrix deposition at the fiber-defect boundary. Conversely, the S-Fib architecture was associated with significant mineralization over time, but the G-Fib architecture was not. Notably, both fiber groups also had similar chondral integration as a re-inserted osteochondral tissue control. While architecture did dictate differences in the cells' responses to their environment, architecture was not shown to distinguish a statistically significant difference in tissue integration via fiber push-out testing within a given time point or explant region. Use of this three-week osteochondral model demonstrates that these bioink formulations support the fabrication of cell-laden constructs that integrate into explanted tissue as capably as natural tissue and encapsulate osteochondral matrix-producing cells, and it also highlights the important role that spatial architecture plays in the engineering of multi-phasic tissue environments. STATEMENT OF SIGNIFICANCE: Here, an ex vivo model was used to interrogate fundamental questions about the effect of multi-material scaffold architectural choices on osteochondral tissue integration. Cell-encapsulating constructs resembling stratified osteochondral tissue were 3D printed with architecture consisting of either gradient transitions or segmented transitions between the bone-like and cartilage-like bioink regions. The printed constructs were assessed alongside re-inserted natural tissue plugs via mechanical tissue integration push-out testing, biochemical assays, and histology. Differences in osteochondral matrix deposition were observed based on architecture, and both printed groups demonstrated cartilage integration similar to the native tissue plug group. As 3D printing becomes commonplace within biomaterials and tissue engineering, this work illustrates critical 3D co-culture interactions and demonstrates the importance of considering architecture when interpreting the results of studies utilizing spatially complex, multi-material scaffolds.

Bioprinted Hydrogels for Fibrosis and Wound Healing: Treatment and Modeling

Three-dimensional (3D) printing has been used to fabricate biomaterial scaffolds with finely controlled physical architecture and user-defined patterning of biological ligands. Excitingly, recent advances in bioprinting have enabled the development of highly biomimetic hydrogels for the treatment of fibrosis and the promotion of wound healing. Bioprinted hydrogels offer more accurate spatial recapitulation of the biochemical and biophysical cues that inhibit fibrosis and promote tissue regeneration, augmenting the therapeutic potential of hydrogel-based therapies. Accordingly, bioprinted hydrogels have been used for the treatment of fibrosis in a diverse array of tissues and organs, including the skin, heart, and endometrium. Furthermore, bioprinted hydrogels have been utilized for the healing of both acute and chronic wounds, which present unique biological microenvironments. In addition to these therapeutic applications, hydrogel bioprinting has been used to generate in vitro models of fibrosis in a variety of soft tissues such as the skin, heart, and liver, enabling high-throughput drug screening and tissue analysis at relatively low cost. As biological research begins to uncover the spatial biological features that underlie fibrosis and wound healing, bioprinting offers a powerful toolkit to recapitulate spatially defined pro-regenerative and anti-fibrotic cues for an array of translational applications.

Fabrication and properties of interweaved poly(ether ether ketone) composite scaffolds

This paper interweaved scaffolds with poly(ether ether ketone) (PEEK) and poly(lactic acid)/Walnut shell/hydroxypatite (PLA/WS/HA) composites by using fused filament fabrication technology, although there was a huge difference in thermal property term between PLA and PEEK. In order to keep mechanical properties of PEEK scaffold and remedy the stress loss produced by pores, PLA/WS/HA composites were used to fill the pores with gradient form outside-in (0.4-0.8 mm, 0.6-1.0 mm, 0.8-1.2 mm and 1.6-2.0 mm). The thermal stability, tensile and compression properties, tensile fracture surface morphology, cytotoxicity and in vivo experiment were investigated. The results showed: the scaffolds were intact without any flashes and surface destruction, and kept a well thermal stability. Compared with the PEEK porous scaffolds, the tensile fracture stress and strain, compression yield stress and strain of interweaved scaffolds were dramatically enhanced by 24.1%, 438%, 359.1% and 921.2%, respectively, and they climbed to the climax at 8 wt% of WS. In vivo experiment showed that the degradation of PLA/WS/HA composites synchronized with the adhesion, proliferation and ingrowth of bone cells, keeping the stable biomechanical properties of interweaved scaffolds. Those experiments showed that interweaved PEEK-PLA/WS/HA scaffolds had the potential to be used as bone implant in tissue engineering.

Layered scaffolds in periodontal regeneration

Periodontitis is a common inflammatory disease in dentistry that may lead to tooth loss and aesthetic problems. Periodontal tissue has a sophisticated architecture including four sections of alveolar bone, cementum, gingiva, and periodontal ligament fiber; all these four can be damaged during periodontitis. Thus, for whole periodontal regeneration, it is important to form both hard and soft tissue structures simultaneously on the tooth root surface without forming junctional epithelium and ankylosis. This condition makes the treatment of the periodontium a challenging process. Various regenerative methods including Guided Bone/Tissue Regeneration (GBR/GTR) using various membranes have been developed. Although using such GBR/GTR membranes was successful for partial periodontal treatment, they cannot be used for the regeneration of complete periodontium. For this purpose, multilayered scaffolds are now being developed. Such scaffolds may include various biomaterials, stem cells, and growth factors in a multiphasic configuration in which each layer is designed to regenerate specific section of the periodontium. This article provides a comprehensive review of the multilayered scaffolds for periodontal regeneration based on natural or synthetic polymers, and their combinations with other biomaterials and bioactive molecules. After highlighting the challenges related to multilayered scaffolds preparation, features of suitable scaffolds for periodontal regeneration are discussed.

Hydrogel: A Potential Material for Bone Tissue Engineering Repairing the Segmental Mandibular Defect

Free flap surgery is currently the only successful method used by surgeons to reconstruct critical-sized defects of the jaw, and is commonly used in patients who have had bony lesions excised due to oral cancer, trauma, infection or necrosis. However, donor site morbidity remains a significant flaw of this strategy. Various biomaterials have been under investigation in search of a suitable alternative for segmental mandibular defect reconstruction. Hydrogels are group of biomaterials that have shown their potential in various tissue engineering applications, including bone regeneration, both through in vitro and in vivo pre-clinical animal trials. This review discusses different types of hydrogels, their fabrication techniques, 3D printing, their potential for bone regeneration, outcomes, and the limitations of various hydrogels in preclinical models for bone tissue engineering. This review also proposes a modified technique utilizing the potential of hydrogels combined with scaffolds and cells for efficient reconstruction of mandibular segmental defects.

Self-Searching Writing of Human-Organ-Scale Three-Dimensional Topographic Scaffolds with Shape Memory by Silkworm-like Electrospun Autopilot Jet

Bioengineered scaffolds satisfying both the physiological and anatomical considerations could potentially repair partially damaged tissues to whole organs. Although three-dimensional (3D) printing has become a popular approach in making 3D topographic scaffolds, electrospinning stands out from all other techniques for fabricating extracellular matrix mimicking fibrous scaffolds. However, its complex charge-influenced jet-field interactions and the associated random motion were hardly overcome for almost a century, thus preventing it from being a viable technique for 3D topographic scaffold construction. Herein, we constructed, for the first time, geometrically challenging 3D fibrous scaffolds using biodegradable poly(ε-caprolactone), mimicking human-organ-scale face, female breast, nipple, and vascular graft, with exceptional shape memory and free-standing features by a novel field self-searching process of autopilot polymer jet, essentially resembling the silkworm-like cocoon spinning. With a simple electrospinning setup and innovative writing strategies supported by simulation, we successfully overcame the intricate jet-field interactions while preserving high-fidelity template topographies, via excellent target recognition, with pattern features ranging from 100's μm to 10's cm. A 3D cell culture study ensured the anatomical compatibility of the so-made 3D scaffolds. Our approach brings the century-old electrospinning to the new list of viable 3D scaffold constructing techniques, which goes beyond applications in tissue engineering.

Human gelatin-based composite hydrogels for osteochondral tissue engineering and their adaptation into bioinks for extrusion, inkjet, and digital light processing bioprinting

The investigation of novel hydrogel systems allows for the study of relationships between biomaterials, cells, and other factors within osteochondral tissue engineering. Three-dimensional (3D) printing is a popular research method that can allow for further interrogation of these questions via the fabrication of 3D hydrogel environments that mimic tissue-specific, complex architectures. However, the adaptation of promising hydrogel biomaterial systems into 3D-printable bioinks remains a challenge. Here, we delineated an approach to that process. First, we characterized a novel methacryloylated gelatin composite hydrogel system and assessed how calcium phosphate and glycosaminoglycan additives upregulated bone- and cartilage-like matrix deposition and certain genetic markers of differentiation within human mesenchymal stem cells (hMSCs), such as RUNX2 and SOX9. Then, new assays were developed and utilized to study the effects of xanthan gum and nanofibrillated cellulose, which allowed for cohesive fiber deposition, reliable droplet formation, and non-fracturing digital light processing (DLP)-printed constructs within extrusion, inkjet, and DLP techniques, respectively. Finally, these bioinks were used to 3D print constructs containing viable encapsulated hMSCs over a 7 d period, where DLP printed constructs facilitated the highest observed increase in cell number over 7 d (∼2.4×). The results presented here describe the promotion of osteochondral phenotypes via these novel composite hydrogel formulations, establish their ability to bioprint viable, cell-encapsulating constructs using three different 3D printing methods on multiple bioprinters, and document how a library of modular bioink additives affected those physicochemical properties important to printability.

Organ-on-a-chip: A new tool for in vitro research

Organ-on-a-chip (OOC, organ chip) technology can closely simulate the human microenvironment, synthesize organ-like functional units on a fluidic chip substrate, and simulate the physiology of tissues and organs. It will become an increasingly important platform for in vitro drug development and screening. Most importantly, organ-on-a-chip technology, incorporating 3D cell cultures, overcomes the traditional drawbacks of 2D (flat) cell-culture technology in vitro and in vivo animal trials, neither of which generate completely reliable results when it comes to the actual human subject. It is expected that organ chips will allow huge reductions in the incidence of failure in late-stage human trials, thus slashing the cost of drug development and speeding up the introduction of drugs that are effective. There have been three key enabling technologies that have made organ chip technology possible: 3D bioprinting, fluidic chips, and 3D cell culture, of which the last has allowed cells to be cultivated under more physiologically realistic growth conditions than 2D culture. The fusion of these advanced technologies and the addition of new research methods and algorithms has enabled the construction of chip types with different structures and different uses, providing a wide range of controllable microenvironments, both for research at the cellular level and for more reliable analysis of the action of drugs on the human body. This paper summarizes some research progress of organ-on-a-chip in recent years, outlines the key technologies used and the achievements in drug screening, and makes some suggestions concerning the current challenges and future development of organ-on-a-chip technology.

Challenges and tissue engineering strategies of periodontal guided tissue regeneration

Periodontitis is a chronic infectious oral disease with a high prevalence rate in the world, and is a major cause of tooth loss. Nowadays, people have realized that the local microenvironment that includes proteins, cytokines, and extracellular matrix has a key influence on the functions of host immune cells and periodontal ligament stem cells during a chronic infectious disease such as periodontitis. The above pathological process of periodontitis will lead to a defect of periodontal tissues. Through the application of biomaterials, biological agents, and stem cells therapy, guided tissue regeneration (GTR) makes it possible to reconstruct healthy periodontal ligament tissue after local inflammation control. To date, substantial advances have been made in periodontal guided tissue regeneration. However, the process of periodontal remodeling experiences complex microenvironment changes, and currently periodontium regeneration still remains to be a challenging feat. In this review, we summarized the main challenges in each stage of periodontal regeneration, and try to put forward appropriate biomaterial treatment mechanisms or potential tissue engineering strategies that provide a theoretical basis for periodontal tissue engineering regeneration research.

Traction of 3D and 4D Printing in the Healthcare Industry: From Drug Delivery and Analysis to Regenerative Medicine

Three-dimensional (3D) printing and 3D bioprinting are promising technologies for a broad range of healthcare applications from frontier regenerative medicine and tissue engineering therapies to pharmaceutical advancements yet must overcome the challenges of biocompatibility and resolution. Through comparison of traditional biofabrication methods with 3D (bio)printing, this review highlights the promise of 3D printing for the production of on-demand, personalized, and complex products that enhance the accessibility, effectiveness, and safety of drug therapies and delivery systems. In addition, this review describes the capacity of 3D bioprinting to fabricate patient-specific tissues and living cell systems (e.g., vascular networks, organs, muscles, and skeletal systems) as well as its applications in the delivery of cells and genes, microfluidics, and organ-on-chip constructs. This review summarizes how tailoring selected parameters (i.e., accurately selecting the appropriate printing method, materials, and printing parameters based on the desired application and behavior) can better facilitate the development of optimized 3D-printed products and how dynamic 4D-printed strategies (printing materials designed to change with time or stimulus) may be deployed to overcome many of the inherent limitations of conventional 3D-printed technologies. Comprehensive insights into a critical perspective of the future of 4D bioprinting, crucial requirements for 4D printing including the programmability of a material, multimaterial printing methods, and precise designs for meticulous transformations or even clinical applications are also given.

Extracellular matrix dynamics: tracking in biological systems and their implications

The extracellular matrix (ECM) constitutes the main acellular microenvironment of cells in almost all tissues and organs. The ECM not only provides mechanical support, but also mediates numerous biochemical interactions to guide cell survival, proliferation, differentiation, and migration. Thus, better understanding the everchanging temporal and spatial shifts in ECM composition and structure - the ECM dynamics - will provide fundamental insight regarding extracellular regulation of tissue homeostasis and how tissue states transition from one to another during diverse pathophysiological processes. This review outlines the mechanisms mediating ECM-cell interactions and highlights how changes in the ECM modulate tissue development and disease progression, using the lung as the primary model organ. We then discuss existing methodologies for revealing ECM compositional dynamics, with a particular focus on tracking newly synthesized ECM proteins. Finally, we discuss the ramifications ECM dynamics have on tissue engineering and how to implement spatial and temporal specific extracellular microenvironments into bioengineered tissues. Overall, this review communicates the current capabilities for studying native ECM dynamics and delineates new research directions in discovering and implementing ECM dynamics to push the frontier forward.

Biomaterials and biotechnology for periodontal tissue regeneration: Recent advances and perspectives

Periodontal tissues are organized in a complex three-dimensional (3D) architecture, including the alveolar bone, cementum, and a highly aligned periodontal ligament (PDL). Regeneration is difficult due to the complex structure of these tissues. Currently, materials are developing rapidly, among which synthetic polymers and hydrogels have extensive applications. Moreover, techniques have made a spurt of progress. By applying guided tissue regeneration (GTR) to hydrogels and cell sheets and using 3D printing, a scaffold with an elaborate biomimetic structure can be constructed to guide the orientation of fibers. The incorporation of cells and biotic factors improves regeneration. Nevertheless, the current studies lack long-term effect tracking, clinical research, and in-depth mechanistic research. In summary, periodontal tissue engineering still has considerable room for development. The development of materials and techniques and an in-depth study of the mechanism will provide an impetus for periodontal regeneration.

Integrated polycaprolactone microsphere-based scaffolds with biomimetic hierarchy and tunable vascularization for osteochondral repair

Osteochondral lesion potentially causes a variety of joint degenerative diseases if it cannot be treated effectively and timely. Microfracture as the conservative surgical choice achieves limited results for the larger defect whereas cartilage patches trigger integrated instability and cartilage fibrosis. To tackle aforementioned issues, here we explore to fabricate an integrated osteochondral scaffold for synergetic regeneration of cartilage and subchondral bone in one system. On the macro level, we fabricated three integrated scaffolds with distinct channel patterns of Non-channel, Consecutive-channel and Inconsecutive-channel via Selective Laser Sintering (SLS). On the micro level, both cartilage zone and subchondral bone zone of integrated scaffold were made of small polycaprolactone (PCL) microspheres and large PCL microspheres, respectively. Our findings showed that Inconsecutive-channel scaffolds possessed integrated hierarchical structure, adaptable compression strength, gradient interconnected porosity. Cartilage zone presented a dense phase for the inhibition of vessel invasion while subchondral bone zone generated a porous phase for the ingrowth of bone and vessel. Both cartilage regeneration and subchondral bone remodeling in the group of Inconsecutive-channel scaffolds have been demonstrated by histological evaluation and immunofluorescence staining in vivo. Consequently, our current work not only achieves an effective and regenerative microsphere scaffold for osteochondral reconstruction, but also provides a feasible methodology to recover injured joint through integrated design with diverse hierarchy. STATEMENT OF SIGNIFICANCE: Recovery of osteochondral lesion highly depends on hierarchical architecture and tunable vascularization in distinct zones. We therefore design a special integrated osteochondral scaffold with inconsecutive channel structure and vascularized modulation. The channel pattern impacts on mechanical strength and the infiltration of bone marrow, and eventually triggers synergetic repair of osteochondral defect. The cartilage zone of integrated scaffolds consisted of small PCL microspheres forms a dense phase for physical restriction of vascularized infiltration whereas the subchondral bone zone made of large PCL microspheres generates porous trabecula-like structure for promoting vascularization. Consequently, the current work indicates both mechanical adaptation and regional vascularized modulation play a pivotal role on osteochondral repair.

Efficacy of treating segmental bone defects through endochondral ossification: 3D printed designs and bone metabolic activities

Three-dimensional printing (3D printing) is a promising technique for producing scaffolds for bone tissue engineering applications. Porous scaffolds can be printed directly, and the design, shape and porosity can be controlled. 3D synthetic biodegradable polymeric scaffolds intended for in situ bone regeneration must meet stringent criteria, primarily appropriate mechanical properties, good 3D design, adequate biocompatibility and the ability to enhance bone formation. In this study, healing of critical-sized (5 ​mm) femur defects of rats was enhanced by implanting two different designs of 3D printed poly(l-lactide-co-ε-caprolactone) (poly(LA-co-CL)) scaffolds seeded with rat bone marrow mesenchymal stem cells (rBMSC), which had been pre-differentiated in vitro into cartilage-forming chondrocytes. Depending on the design, the scaffolds had an interconnected porous structure of 300-500 ​μm and porosity of 50-65%. According to a computational simulation, the internal force distribution was consistent with scaffold designs and comparable between the two designs. Moreover, the defects treated with 3D-printed scaffolds seeded with chondrocyte-like cells exhibited significantly increased bone formation up to 15 weeks compared with empty defects. In all experimental animals, bone metabolic activity was monitored by positron emission tomography 1, 3, 5, 7, 11 and 14 weeks after surgery. This demonstrated a time-dependent relationship between scaffold design and metabolic activity. This confirmed that successful regeneration was highly reproducible. The in vitro and in vivo data indicated that the experimental setups had promising outcomes and could facilitate new bone formation through endochondral ossification.

3D Printing for Bone-Cartilage Interface Regeneration

Due to the vasculature defects and/or the avascular nature of cartilage, as well as the complex gradients for bone-cartilage interface regeneration and the layered zonal architecture, self-repair of cartilage and subchondral bone is challenging. Currently, the primary osteochondral defect treatment strategies, including artificial joint replacement and autologous and allogeneic bone graft, are limited by their ability to simply repair, rather than induce regeneration of tissues. Meanwhile, over the past two decades, three-dimension (3D) printing technology has achieved admirable advancements in bone and cartilage reconstruction, providing a new strategy for restoring joint function. The advantages of 3D printing hybrid materials include rapid and accurate molding, as well as personalized therapy. However, certain challenges also exist. For instance, 3D printing technology for osteochondral reconstruction must simulate the histological structure of cartilage and subchondral bone, thus, it is necessary to determine the optimal bioink concentrations to maintain mechanical strength and cell viability, while also identifying biomaterials with dual bioactivities capable of simultaneously regenerating cartilage. The study showed that the regeneration of bone-cartilage interface is crucial for the repair of osteochondral defect. In this review, we focus on the significant progress and application of 3D printing technology for bone-cartilage interface regeneration, while also expounding the potential prospects for 3D printing technology and highlighting some of the most significant challenges currently facing this field.

Three-Dimensional Bioprinting of Organs: Modern Trends

The paper presents an overview of some modern technologies for three-dimensional (3D) bioprinting of human tissues and organs. 3D bioprinting of human organs is increasingly used for the production and transplantation of artificial biological organs. Existing technologies of 3D bioprinting using bioink on a special substrate are considered. The features of the production of bioinks using biocompatible polymer compounds, hydrogels are given, some popular modern bioprinters are noted. Modern technologies of bioprinting of tissues of human organs are considered: skin, liver, lungs, heart, brain, existing technological problems in this area are given. Based on the analysis, the future prospects for the development of bioprinting technology for human organs are noted.

A review of recent advances in metal ion hydrogels: mechanism, properties and their biological applications

The mechanisms, common properties and biological applications of different types of metal ion hydrogels are summarized.

Advances in Filament Structure of 3D Bioprinted Biodegradable Bone Repair Scaffolds

Conventional bone repair scaffolds can no longer meet the high standards and requirements of clinical applications in terms of preparation process and service performance. Studies have shown that the diversity of filament structures of implantable scaffolds is closely related to their overall properties (mechanical properties, degradation properties, and biological properties). To better elucidate the characteristics and advantages of different filament structures, this paper retrieves and summarizes the state of the art in the filament structure of the three-dimensional (3D) bioprinted biodegradable bone repair scaffolds, mainly including single-layer structure, double-layer structure, hollow structure, core-shell structure and bionic structures. The eximious performance of the novel scaffolds was discussed from different aspects (material composition, ink configuration, printing parameters, etc.). Besides, the additional functions of the current bone repair scaffold, such as chondrogenesis, angiogenesis, anti-bacteria, and anti-tumor, were also concluded. Finally, the paper prospects the future material selection, structural design, functional development, and performance optimization of bone repair scaffolds.

Fabrication of 3D Printed Poly(lactic acid)/Polycaprolactone Scaffolds Using TGF-β1 for Promoting Bone Regeneration

Our research was designed to evaluate the effect on bone regeneration with 3-dimensional (3D) printed polylactic acid (PLA) and 3D printed polycaprolactone (PCL) scaffolds, determine the more effective option for enhancing bone regeneration, and offer tentative evidence for further research and clinical application. Employing the 3D printing technique, the PLA and PCL scaffolds showed similar morphologies, as confirmed via scanning electron microscopy (SEM). Mechanical strength was significantly higher in the PLA group (63.4 MPa) than in the PCL group (29.1 MPa) (p < 0.01). Average porosity, swelling ratio, and degeneration rate in the PCL scaffold were higher than those in the PLA scaffold. SEM observation after cell coculture showed improved cell attachment and activity in the PCL scaffolds. A functional study revealed the best outcome in the 3D printed PCL-TGF-β₁ scaffold compared with the 3D printed PCL and the 3D printed PCL-Polydopamine (PDA) scaffold (p < 0.001). As confirmed via SEM, the 3D printed PCL- transforming growth factor beta 1 (TGF-β₁) scaffold also exhibited improved cell adhesion after 6 h of cell coculture. The 3D printed PCL scaffold showed better physical properties and biocompatibility than the 3D printed PLA scaffold. Based on the data of TGF-β₁, this study confirms that the 3D printed PCL scaffold may offer stronger osteogenesis.

Multilayered and heterogeneous hydrogel construct printing system with crosslinking aerosol

Microextrusion bioprinting has been used to recreate the complex architecture and composition of a physiological system through the quick and accurate handling of various biomaterials. However, existing techniques are limited in precisely fabricating complex constructs, including multilayers and heterogeneous patterns with distinct regions, because the extruded bioink spreads rapidly upon contact with the substrate and is partially mixed with subsequently printed bioinks. This issue leads to difficulties in accurately and stably constructing multi-material structures with clear interfaces for prolonged printing before gelation. To fabricate multilayered and heterogeneous constructs, a bioprinting system should be able to continuously extrude various biomaterials and simultaneously crosslink the extruded bioink to stabilize the printed construct. In this study, a multiple-bioink printing system was developed by integrating a multibarrel nozzle for extruding multiple bioinks with a nebulizer for simultaneous crosslinking. The crosslinking aerosol sprayed from the nebulizer was able to gelate the various hydrogel bioinks as they were extruded through the multibarrel nozzle. Such aerosol-based crosslinking improved printing resolution and stability. The developed bioprinting system showed the possibility of recapitulating the physiological complex architecture such as a cancer microenvironment with well-defined interfaces between regions of different mechanical properties and cellular compositions. Using the integrated bioprinting system, a multilayered and heterogeneous construct was printed with four bioinks, including three types of cells (breast cancer cells, stromal cells, and vascular endothelial cells). The printed biological model was characterized by analyzing cancer cell migration and vascular network formation. The developed multiple-bioink printing system is expected to be highly efficient in recapitulating complex tissues and their environments with compartmentalized regions.

Recent advancements in decellularized matrix technology for bone tissue engineering

The native extracellular matrix (ECM) provides a matrix to hold tissue/organ, defines the cellular fate and function, and retains growth factors. Such a matrix is considered as a most biomimetic scaffold for tissue engineering due to the biochemical and biological components, 3D hierarchical structure, and physicomechanical properties. Several attempts have been performed to decellularize allo- or xeno-graft tissues and used them for bone repairing and regeneration. Decellularized ECM (dECM) technology has been developed to create an in vivo-like microenvironment to promote cell adhesion, growth, and differentiation for tissue repair and regeneration. Decellularization is mediated through physical, chemical, and enzymatic methods. In this review, we describe the recent progress in bone decellularization and their applications as a scaffold, hydrogel, bioink, or particles in bone tissue engineering. Furthermore, we address the native dECM limitations and the potential of non-bone dECM, cell-based ECM, and engineered ECM (eECM) for in vitro osteogenic differentiation and in vivo bone regeneration.

Extracellular matrix extraction techniques and applications in biomedical engineering

The concept of tissue engineering involves regeneration and repair of damaged tissue and organs using various combinations of cells, growth factors and scaffolds. The extracellular matrix (ECM) forms the integral part of the scaffold to induce cell proliferation thereby leading to new tissue formation. Decellularization technique provides decellularized ECM (dECM), free of cells while preserving the in vivo biomolecules. In this review, we focus on the detailed methodology of diverse decellularization techniques for various organs of different animals, and the biomedical applications employing the dECM. A culmination of different methods of decellularization is optimized, which offers a suitable microenvironment mimicking the native in vivo topography for in vitro organ regeneration. A detailed assessment of the dECM provides information on the microarchitecture, presence of ECM proteins, biocompatibility and cell proliferation. dECM has also been processed as scaffolds and drug-delivery vehicles, and utilized for regeneration.

Preparation, Structure, and Properties of Polystyrene-Microsphere-Reinforced PEG-Based Hydrogels

To improve the mechanical strength and practicability of hydrogels, polystyrene microspheres with core-shell structure were prepared by the soap-free emulsion polymerization, polyethylene glycol hydrogels with polystyrene microspheres by the in-situ polymerization. The structure, morphology, roughness, swelling property, surface energy, and mechanical properties of the microspheres and hydrogels were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, confocal laser microscopy, swelling test, contact angle measurement, and compression test. The results showed that they have certain swelling capacity and excellent mechanical properties, and can change from hydrophobic to hydrophilic surface. The reason is that the hydrophilic chain segment can migrate, enrich, and form a hydration layer on the surface after soaking for a certain time. Introducing proper content of polystyrene microspheres into the hydrogel, the compressive strength and swelling degree improved obviously. Increasing the content of polystyrene microspheres, the surface energy of the hydrogels decreased gradually.

Nanoclay Reinforced Biomaterials for Mending Musculoskeletal Tissue Disorders

Nanoclay-reinforced biomaterials have sparked a new avenue in advanced healthcare materials that can potentially revolutionize treatment of musculoskeletal defects. Native tissues display many important chemical, mechanical, biological, and physical properties that engineered biomaterials need to mimic for optimal tissue integration and regeneration. However, it is time-consuming and difficult to endow such combinatorial properties on materials via feasible and nontoxic procedures. Fortunately, a number of nanomaterials such as graphene, carbon nanotubes, MXenes, and nanoclays already display a plethora of material properties that can be transferred to biomaterials through a simple incorporation procedure. In this direction, the members of the nanoclay family are easy to functionalize chemically, they can significantly reinforce the mechanical performance of biomaterials, and can provide bioactive properties by ionic dissolution products to upregulate cartilage and bone tissue formation. For this reason, nanoclays can become a key component for future orthopedic biomaterials. In this review, we specifically focus on the rapidly decreasing gap between clinic and laboratory by highlighting their application in a number of promising in vivo studies.

3D printed colloidal biomaterials based on photo-reactive gelatin nanoparticles

Biomaterials-based strategies have shown great promise for tissue regeneration. 3D printing technologies can deliver unprecedented control over architecture and properties of biomaterial constructs when combined with innovative material design strategies. Colloidal gels made of polymeric nanoparticles are attractive injectable and self-healing systems, but their use as bio-inks for extrusion-based printing is largely unexplored. Here, we report 3D printing of novel biomaterial constructs with shape memory behavior using photo-reactive gelatin nanoparticles as colloidal building blocks. These nanoparticles are stabilized with intraparticle covalent crosslinks, and also contain pendant methacryloyl groups as photo-reactive moieties. While non-covalent interactions between nanoparticles enable formation of colloidal gel inks that are printable at room temperature, UV-induced covalent interparticle crosslinks based on methacryloyl moieties significantly enhance mechanical properties of printed constructs. Additionally, the UV crosslinking modality enables remarkable control over swelling, degradation, and biomolecule release behavior of 3D constructs. Finally, by exploiting the mechanical properties of colloidal biomaterials after UV crosslinking, 3D constructs can be designed with shape memory properties, returning to their original programmed geometry upon re-hydration. Accordingly, these novel colloidal inks exhibit great potential to serve as bio-inks for 3D printing of biomaterials with shape-morphing features for a wide range of tissue engineering and regenerative medicine applications.

Design Challenges in Polymeric Scaffolds for Tissue Engineering

Numerous surgical procedures are daily performed worldwide to replace and repair damaged tissue. Tissue engineering is the field devoted to the regeneration of damaged tissue through the incorporation of cells in biocompatible and biodegradable porous constructs, known as scaffolds. The scaffolds act as host biomaterials of the incubating cells, guiding their attachment, growth, differentiation, proliferation, phenotype, and migration for the development of new tissue. Furthermore, cellular behavior and fate are bound to the biodegradation of the scaffold during tissue generation. This article provides a critical appraisal of how key biomaterial scaffold parameters, such as structure architecture, biochemistry, mechanical behavior, and biodegradability, impart the needed morphological, structural, and biochemical cues for eliciting cell behavior in various tissue engineering applications. Particular emphasis is given on specific scaffold attributes pertaining to skin and brain tissue generation, where further progress is needed (skin) or the research is at a relatively primitive stage (brain), and the enumeration of some of the most important challenges regarding scaffold constructs for tissue engineering.

Triblock Copolymer Bioinks in Hydrogel Three-Dimensional Printing for Regenerative Medicine: A Focus on Pluronic F127

Three-dimensional (3D) bioprinting is a novel technique applied to manufacture semisolid or solid objects via deposition of successive thin layers. The widespread implementation of the 3D bioprinting technology encouraged scientists to evaluate its feasibility for applications in human regenerative medicine. 3D bioprinting gained much interest as a new strategy to prepare implantable 3D tissues or organs, tissue and organ evaluation models to test drugs, and cell/material interaction systems. The present work summarizes recent and relevant progress based on the use of hydrogels for the technology of 3D bioprinting and their emerging biomedical applications. An overview of different 3D printing techniques in addition to the nature and properties of bioinks used will be described with a focus on hydrogels as suitable bioinks for 3D printing. A comprehensive overview of triblock copolymers with emphasis on Pluronic F127 (PF127) as a bioink in 3D printing for regenerative medicine will be provided. Several biomedical applications of PF127 in tissue engineering, particularly in bone and cartilage regeneration and in vascular reconstruction, will be also discussed.