Engineering complex tissues

Functionally graded biomimetic biomaterials in dentistry: an evidence-based update

Design and development of novel therapeutic strategies to regenerate lost tissue structure and function is a serious clinical hurdle for researchers. Traditionally, much of the research is dedicated in optimising properties of scaffolds. Current synthetic biomaterials remain rudimentary in comparison to their natural counterparts. The ability to incorporate biologically inspired elements into the design of synthetic materials has advanced with time. Recent reports suggest that functionally graded material mimicking the natural tissue morphology can have a more exaggerated response on the targeted tissue. The aim of this review is to deliver an overview of the functionally graded concept with respect to applications in clinical dentistry. A comprehensive understanding of spatiotemporal arrangement in fields of restorative, prosthodontics, periodontics, orthodontics and oral surgery is presented. Different processing techniques have been adapted to achieve such gradients ranging from additive manufacturing (three dimensional printing/rapid prototyping) to conventional techniques of freeze gelation, freeze drying, electrospinning and particulate leaching. The scope of employing additive manufacturing technique as a reliable and predictable tool for the design and accurate reproduction of biomimetic templates is vast by any measure. Further research in the materials used and refinement of the synthesis techniques will continue to expand the frontiers of functionally graded membrane based biomaterials application in the clinical domain.

Pressurized Perfusion System for Obtaining Completely Acellular Pulmonary Valve Scaffolds for Tissue Engineering

Introduction. Xenogeneic tissues decellularization represents the obtaining process of extracellular matrix derived scaffolds. Most antigens being cell based, non-immunogenicity is obtained by cells removal. Scaffolds are temporary structures with biologic and mechanical role. Scaffolds, stem cells and bioreactors represent premise of regenerative medicine, aiming towards the ideal valvular substitute. In previous studies, we decellularized pulmonary valves root by immersion histology revealing cellular residue, requiring a more efficient approach. We hypothesized that immersion is insufficient and thus a pressure gradient was added.

Material and Method. This is part of a grant approved by the UMFTS. Eleven porcine pulmonary valves were included in the study: n=6 underwent immersion decellularization and n=5 were cyclically perfused with a 20-25mmHg pressure gradient during a 10-day protocol. The acellular valves obtained underwent a quality control using DAPI (4′,6-diamidino-2-phenylindol) nuclear staining, histological Haematoxylin-Eosin, DNA extraction and quantification, harvested from different structural levels: arterial wall, sinus, cusp.

Results. Histological assessments highlighted integrity of extracellular matrix in both groups and overall cells absence at the different levels of valvular structures analyzed. Immersion decellularized valves exhibited DAPI positive structures identified as potential residual nucleic material. Comparatively, the perfusion decellularized valves, lacked in those structures, result confirmed by DNA extraction and quantitation procedure.

Conclusions. Perfusion decellularization represents a feasible approach to obtain acellular cardiac valvular scaffolds derived from the extracellular matrix, being superior to immersion decellularization method. Their nonimmunogenic potential is underlined by total absence of nuclei. The process is fast, allowing production of an abundant number of valvular biomaterials in a short time.

Tissue Engineering in Liver Regenerative Medicine: Insights into Novel Translational Technologies

Organ and tissue shortage are known as a crucially important public health problem as unfortunately a small percentage of patients receive transplants. In the context of emerging regenerative medicine, researchers are trying to regenerate and replace different organs and tissues such as the liver, heart, skin, and kidney. Liver tissue engineering (TE) enables us to reproduce and restore liver functions, fully or partially, which could be used in the treatment of acute or chronic liver disorders and/or generate an appropriate functional organ which can be transplanted or employed as an extracorporeal device. In this regard, a variety of techniques (e.g., fabrication technologies, cell-based technologies, microfluidic systems and, extracorporeal liver devices) could be applied in tissue engineering in liver regenerative medicine. Common TE techniques are based on allocating stem cell-derived hepatocyte-like cells or primary hepatocytes within a three-dimensional structure which leads to the improvement of their survival rate and functional phenotype. Taken together, new findings indicated that developing liver tissue engineering-based techniques could pave the way for better treatment of liver-related disorders. Herein, we summarized novel technologies used in liver regenerative medicine and their future applications in clinical settings.

Ciprofloxacin-Modified Degradable Hybrid Polyurethane-Polylactide Porous Scaffolds Developed for Potential Use as an Antibacterial Scaffold for Regeneration of Skin

The aim of the performed study was to fabricate an antibacterial and degradable scaffold that may be used in the field of skin regeneration. To reach the degradation criterion for the biocompatible polyurethane (PUR), obtained by using amorphous α,ω-dihydroxy(ethylene-butylene adipate) macrodiol (PEBA), was used and processed with so-called "fast-degradable" polymer polylactide (PLA) (5 or 10 wt %). To meet the antibacterial requirement obtained, hybrid PUR-PLA scaffolds (HPPS) were modified with ciprofloxacin (Cipro) (2 or 5 wt %) and the fluoroquinolone antibiotic inhibiting growth of bacteria, such as Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus, which are the main causes of wound infections. Performed studies showed that Cipro-modified HPPS, obtained by using 5% of PLA, possess suitable mechanical characteristics, morphology, degradation rates, and demanded antimicrobial properties to be further developed as potential scaffolds for skin tissue engineering.

Multimaterial Dual Gradient Three-Dimensional Printing for Osteogenic Differentiation and Spatial Segregation

In this study of three-dimensional (3D) printed composite β-tricalcium phosphate (β-TCP)-/hydroxyapatite/poly(ɛ-caprolactone)-based constructs, the effects of vertical compositional ceramic gradients and architectural porosity gradients on the osteogenic differentiation of rabbit bone marrow-derived mesenchymal stem cells (MSCs) were investigated. Specifically, three different concentrations of β-TCP (0, 10, and 20 wt%) and three different porosities (33% ± 4%, 50% ± 4%, and 65% ± 3%) were examined to elucidate the contributions of chemical and physical gradients on the biochemical behavior of MSCs and the mineralized matrix production within a 3D culture system. By delaminating the constructs at the gradient transition point, the spatial separation of cellular phenotypes could be specifically evaluated for each construct section. Results indicated that increased concentrations of β-TCP resulted in upregulation of osteogenic markers, including alkaline phosphatase activity and mineralized matrix development. Furthermore, MSCs located within regions of higher porosity displayed a more mature osteogenic phenotype compared to MSCs in lower porosity regions. These results demonstrate that 3D printing can be leveraged to create multiphasic gradient constructs to precisely direct the development and function of MSCs, leading to a phenotypic gradient. Impact Statement In this study, three-dimensional (3D) printed ceramic/polymeric constructs containing discrete vertical gradients of both composition and porosity were fabricated to precisely control the osteogenic differentiation of mesenchymal stem cells. By making simple alterations in construct architecture and composition, constructs containing heterogenous populations of cells were generated, where gradients in scaffold design led to corresponding gradients in cellular phenotype. The study demonstrates that 3D printed multiphasic composite constructs can be leveraged to create complex heterogeneous tissues and interfaces.

Liver Cancer: Current and Future Trends Using Biomaterials

Hepatocellular carcinoma (HCC) is the fifth most common type of cancer diagnosed and the second leading cause of death worldwide. Despite advancement in current treatments for HCC, the prognosis for this cancer is still unfavorable. This comprehensive review article focuses on all the current technology that applies biomaterials to treat and study liver cancer, thus showing the versatility of biomaterials to be used as smart tools in this complex pathologic scenario. Specifically, after introducing the liver anatomy and pathology by focusing on the available treatments for HCC, this review summarizes the current biomaterial-based approaches for systemic delivery and implantable tools for locally administrating bioactive factors and provides a comprehensive discussion of the specific therapies and targeting agents to efficiently deliver those factors. This review also highlights the novel application of biomaterials to study HCC, which includes hydrogels and scaffolds to tissue engineer 3D in vitro models representative of the tumor environment. Such models will serve to better understand the tumor biology and investigate new therapies for HCC. Special focus is given to innovative approaches, e.g., combined delivery therapies, and to alternative approaches-e.g., cell capture-as promising future trends in the application of biomaterials to treat HCC.

Microporous Bio-orthogonally Annealed Particle Hydrogels for Tissue Engineering and Regenerative Medicine

Microporous annealed particle (MAP) hydrogels are an emerging class of biomaterials with the potential to improve outcomes in tissue repair and regeneration. Here, a new MAP hydrogel platform comprising poly(ethylene) glycol (PEG) hydrogel microparticles that are annealed in situ using bio-orthogonal tetrazine click chemistry is reported (i.e., TzMAP hydrogels). Briefly, clickable PEG-peptide hydrogel microparticles with extracellular matrix mimetic peptides to permit cell adhesion and enzymatic degradation were fabricated via submerged electrospraying and stoichiometrically controlled thiol-norbornene click chemistry. Subsequently, unreacted norbornene groups in the microparticles were leveraged for functionalization with bioactive proteins as well as annealing into TzMAP hydrogels via the tetrazine-norbornene click reaction, which is highly selective and proceeds spontaneously without requiring an initiator or catalyst. The results demonstrate that the clickable particles can be easily applied to a tissue-like defect and then annealed into an inherently microporous structure in situ. In addition, the ability to produce TzMAP hydrogels with heterogeneous properties by incorporating multiple types of hydrogel microspheres is demonstrated, first with fluorophore-functionalized hydrogel microparticles and then with protein-functionalized hydrogel microparticles. For the latter, tetrazine-modified alkaline phosphatase was conjugated to PEG hydrogel microparticles, which were mixed with nonfunctionalized microparticles and used to produce TzMAP hydrogels. A biomimetic mineralized/nonmineralized interface was then produced upon incubation in calcium glycerophosphate. Finally, platelet-derived growth factor-BB (PDGF-BB) and human periodontal ligament stem cells (PDLSC) were incorporated into the TzMAP hydrogels during the annealing step to demonstrate their potential for delivering regenerative therapeutics, specifically for periodontal tissue regeneration. In vitro characterization revealed excellent PDGF-BB retention as well as PDLSC growth and spreading. Moreover, PDGF-BB loading increased PDLSC proliferation within hydrogels by 90% and more than doubled the average volume per cell. Overall, these results demonstrate that TzMAP hydrogels are a versatile new platform for the delivery of stem cells and regenerative factors.

A Cellularized Biphasic Implant Based on a Bioactive Silk Fibroin Promotes Integration and Tissue Organization during Osteochondral Defect Repair in a Porcine Model

In cartilage tissue engineering, biphasic scaffolds (BSs) have been designed not only to influence the recapitulation of the osteochondral architecture but also to take advantage of the healing ability of bone, promoting the implant's integration with the surrounding tissue and then bone restoration and cartilage regeneration. This study reports the development and characterization of a BS based on the assembly of a cartilage phase constituted by fibroin biofunctionalyzed with a bovine cartilage matrix, cellularized with differentiated autologous pre-chondrocytes and well attached to a bone phase (decellularized bovine bone) to promote cartilage regeneration in a model of joint damage in pigs. BSs were assembled by fibroin crystallization with methanol, and the mechanical features and histological architectures were evaluated. The scaffolds were cellularized and matured for 12 days, then implanted into an osteochondral defect in a porcine model (n = 4). Three treatments were applied per knee: Group I, monophasic cellular scaffold (single chondral phase); group II (BS), cellularized only in the chondral phase; and in order to study the influence of the cellularization of the bone phase, Group III was cellularized in chondral phases and a bone phase, with autologous osteoblasts being included. After 8 weeks of surgery, the integration and regeneration tissues were analyzed via a histology and immunohistochemistry evaluation. The mechanical assessment showed that the acellular BSs reached a Young's modulus of 805.01 kPa, similar to native cartilage. In vitro biological studies revealed the chondroinductive ability of the BSs, evidenced by an increase in sulfated glycosaminoglycans and type II collagen, both secreted by the chondrocytes cultured on the scaffold during 28 days. No evidence of adverse or inflammatory reactions was observed in the in vivo trial; however, in Group I, the defects were not reconstructed. In Groups II and III, a good integration of the implant with the surrounding tissue was observed. Defects in group II were fulfilled via hyaline cartilage and normal bone. Group III defects showed fibrous repair tissue. In conclusion, our findings demonstrated the efficacy of a biphasic and bioactive scaffold based on silk fibroin and cellularized only in the chondral phase, which entwined chondroinductive features and a biomechanical capability with an appropriate integration with the surrounding tissue, representing a promising alternative for osteochondral tissue-engineering applications.

Development of a 3D Tissue-Engineered Skeletal Muscle and Bone Co-culture System

In vitro 3D tissue-engineered (TE) structures have been shown to better represent in vivo tissue morphology and biochemical pathways than monolayer culture, and are less ethically questionable than animal models. However, to create systems with even greater relevance, multiple integrated tissue systems should be recreated in vitro. In the present study, the effects and conditions most suitable for the co-culture of TE skeletal muscle and bone are investigated. High-glucose Dulbecco's modified Eagle medium (HG-DMEM) supplemented with 20% fetal bovine serum followed by HG-DMEM with 2% horse serum is found to enable proliferation of both C2C12 muscle precursor cells and TE85 human osteosarcoma cells, fusion of C2C12s into myotubes, as well as an upregulation of RUNX2/CBFa1 in TE85s. Myotube formation is also evident within indirect contact monolayer cultures. Finally, in 3D co-cultures, TE85 collagen/hydroxyapatite constructs have significantly greater expression of RUNX2/CBFa1 and osteocalcin/BGLAP in the presence of collagen-based C2C12 skeletal muscle constructs; however, fusion within these constructs appears reduced. This work demonstrates the first report of the simultaneous co-culture and differentiation of 3D TE skeletal muscle and bone, and represents a significant step toward a full in vitro 3D musculoskeletal junction model.

Current Trends in Viral Gene Therapy for Human Orthopaedic Regenerative Medicine

Background

Viral vector-based therapeutic gene therapy is a potent strategy to enhance the intrinsic reparative abilities of human orthopaedic tissues. However, clinical application of viral gene transfer remains hindered by detrimental responses in the host against such vectors (immunogenic responses, vector dissemination to nontarget locations). Combining viral gene therapy techniques with tissue engineering procedures may offer strong tools to improve the current systems for applications in vivo.

Methods

The goal of this work is to provide an overview of the most recent systems exploiting biomaterial technologies and therapeutic viral gene transfer in human orthopaedic regenerative medicine.

Results

Integration of tissue engineering platforms with viral gene vectors is an active area of research in orthopaedics as a means to overcome the obstacles precluding effective viral gene therapy.

Conclusions

In light of promising preclinical data that may rapidly expand in a close future, biomaterial-guided viral gene therapy has a strong potential for translation in the field of human orthopaedic regenerative medicine.

Ultrasonic mixing chamber as an effective tool for the biofabrication of fully graded scaffolds for interface tissue engineering

One of the main challenges of the interface-tissue engineering is the regeneration of diseased or damaged interfacial native tissues that are heterogeneous both in composition and in structure. In order to achieve this objective, innovative fabrication techniques have to be investigated. This work describes the design, fabrication, and validation of a novel mixing system to be integrated into a double-extruder bioprinter, based on an ultrasonic probe included into a mixing chamber. To validate the quality and the influence of mixing time, different nanohydroxyapatite–gelatin samples were printed. Mechanical characterization, micro-computed tomography, and thermogravimetric analysis were carried out. Samples obtained from three-dimensional bioprinting using the mixing chamber were compared to samples obtained by deposition of the same final solution obtained by manually operated ultrasound probe, showing no statistical differences. Results obtained from samples characterization allow to consider the proposed mixing system as a promising tool for the fabrication of graduated structures which are increasingly being used in interface-tissue engineering.

Recent advances in three-dimensional bioprinting of stem cells

In spite of being a new field, three-dimensional (3D) bioprinting has undergone rapid growth in the recent years. Bioprinting methods offer a unique opportunity for stem cell distribution, positioning, and differentiation at the microscale to make the differentiated architecture of any tissue while maintaining precision and control over the cellular microenvironment. Bioprinting introduces a wide array of approaches to modify stem cell fate. This review discusses these methodologies of 3D bioprinting stem cells. Fabricating a fully operational tissue or organ construct with a long life will be the most significant challenge of 3D bioprinting. Once this is achieved, a whole human organ can be fabricated for the defect place at the site of surgery.

Remote Control in Formation of 3D Multicellular Assemblies Using Magnetic Forces

Cell constructs have been utilized as building blocks in tissue engineering to closely mimic the natural tissue and also overcome some of the limitations caused by two-dimensional cultures or using scaffolds. External forces can be used to enhance the cells' adhesion and interaction and thus provide better control over production of these structures compared to methods like cell seeding and migration. In this paper, we demonstrate an efficient method to generate uniform, three-dimensional cell constructs using magnetic forces. This method produced spheroids with higher densities and more symmetrical structures than the commonly used centrifugation method for production of cell spheroids. It was also shown that shape of the cell constructs could be changed readily by using different patterns of magnetic field. The application of magnetic fields to impart forces on the cells enhanced the fusion of these spheroids, which could be used to produce larger and more complicated structures for future tissue engineering applications.

Cellular and Chemical Gradients to Engineer the Meniscus-to-Bone Insertion

Tissue-engineered menisci hold promise as an alternative to allograft procedures but require a means of robust fixation to the native bone. The insertion of the meniscus into bone is critical for meniscal function and inclusion of a soft tissue-to-bone interface in a tissue engineered implant can aid in the fixation process. The native insertion is characterized by gradients in composition, tissue architecture, and cellular phenotype, which are all difficult to replicate. In this study, a soft tissue-to-bone interface is tissue engineered with a cellular gradient of fibrochondrocytes and mesenchymal stem cells and subjected to a biochemical gradient through a custom media diffusion bioreactor. These constructs, consisting of interpenetrating collagen and boney regions, display improved mechanical performance and collagen organization compared to controls without a cellular or chemical gradient. Media gradient exposure produces morphological features in the constructs that appear similar to the native tissue. Collectively, these data show that cellular and biochemical gradients improve integration between collagen and bone in a tissue engineered soft tissue-to-bone construct.

Mechanically robust photodegradable gelatin hydrogels for 3D cell culture and in situ mechanical modification

Highly conjugated, hydrophobically modified gelatin hydrogels were synthesized, polymerized and degraded with orthogonal wavelengths of light.

Cell laden alginate-keratin based composite microcapsules containing bioactive glass for tissue engineering applications

Microcapsules based on alginate-keratin, alginate dialdehyde (ADA)-keratin and ADA-keratin-45S5 bioactive glass (BG) were successfully prepared. The samples were characterized by light microscopy, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The results showed that ADA-based materials possess higher degradation rate compared to alginate-based materials. The incorporation of BG particles (mean particle size: 2.0 µm) improved the bioactivity of the materials. Moreover, the biological properties of the samples were evaluated by encapsulating MG-63 osteosarcoma cells into the microcapsules. The cell viability in all samples increased during 21 days of cultivation. However, the presence of 0.5% BG particle seemed to have initial negative effect on cell growth compared to other samples without BG. On the other hand, the positive effect of CaP formation was visible after 3 weeks in the BG containing samples. The results are relevant to consider the development of cell laden bioinks incorporating inorganic bioactive particles for biofabrication approaches.

Advanced smart biomaterials and constructs for hard tissue engineering and regeneration

Hard tissue repair and regeneration cost hundreds of billions of dollars annually worldwide, and the need has substantially increased as the population has aged. Hard tissues include bone and tooth structures that contain calcium phosphate minerals. Smart biomaterial-based tissue engineering and regenerative medicine methods have the exciting potential to meet this urgent need. Smart biomaterials and constructs refer to biomaterials and constructs that possess instructive/inductive or triggering/stimulating effects on cells and tissues by engineering the material's responsiveness to internal or external stimuli or have intelligently tailored properties and functions that can promote tissue repair and regeneration. The smart material-based approaches include smart scaffolds and stem cell constructs for bone tissue engineering; smart drug delivery systems to enhance bone regeneration; smart dental resins that respond to pH to protect tooth structures; smart pH-sensitive dental materials to selectively inhibit acid-producing bacteria; smart polymers to modulate biofilm species away from a pathogenic composition and shift towards a healthy composition; and smart materials to suppress biofilms and avoid drug resistance. These smart biomaterials can not only deliver and guide stem cells to improve tissue regeneration and deliver drugs and bioactive agents with spatially and temporarily controlled releases but can also modulate/suppress biofilms and combat infections in wound sites. The new generation of smart biomaterials provides exciting potential and is a promising opportunity to substantially enhance hard tissue engineering and regenerative medicine efficacy.

MicroRNAs delivery into human cells grown on 3D-printed PLA scaffolds coated with a novel fluorescent PAMAM dendrimer for biomedical applications

Many advanced synthetic, natural, degradable or non-degradable materials have been employed to create scaffolds for cell culture for biomedical or tissue engineering applications. One of the most versatile material is poly-lactide (PLA), commonly used as 3D printing filament. Manufacturing of multifunctional scaffolds with improved cell growth proliferation and able to deliver oligonucleotides represents an innovative strategy for controlled and localized gene modulation that hold great promise and could increase the number of applications in biomedicine. Here we report for the first time the synthesis of a novel Rhodamine derivative of a poly-amidoamine dendrimer (G = 5) able to transfect cells and to be monitored by confocal microscopy that we also employed to coat a 3D-printed PLA scaffold. The coating do not modify the oligonucleotide binding ability, toxicity or transfection properties of the scaffold that is able to increase cell proliferation and deliver miRNA mimics (i.e., pre-mir-503) into human cells. Although further experiments are required to optimize the dendrimer/miRNA ratio and improve transfection efficiency, we demonstrated the effectiveness of this promising and innovative 3D-printed transfection system to transfer miRNAs into human cells for future biomedical applications.

Construction of vascularized tissue-engineered bone with a double-cell sheet complex

A double-cell sheet (DCS) complex composed of an osteogenic cell sheet and a vascular endothelial cell sheet with osteogenesis and blood vessel formation potential was developed in this study. The osteogenic and vascular endothelial cell sheets were obtained after induced culture of rabbit adipose-derived mesenchymal stem cells. The osteogenic cell sheet showed positive alizarin red, von Kossa, and alkaline phosphatase (ALP) staining. The vascular endothelial cell sheet exhibited visible W-P bodies in the cells, the expression of CD31 was positive, and a vascular mesh structure was spontaneously formed in a Matrigel matrix. The subcutaneous transplantation results for four groups of DCS and DCS-coral hydroxyapatite (CHA) complexes, and the CHA scaffold group in nude mice revealed mineralization of collagen fibers and vascularization in each group at 12 weeks, but the degrees of mineralization and vascularization showed differences among groups. The pattern involving endothelial cell sheets covered with osteogenic cell sheets, group B, exhibited the best results. In addition, the degree of mineralization of the DCS-CHA complexes was more mature than those of the same group of DCS complexes and the CHA scaffold, and the capillary number was greater than those of the same group of DCS complexes and the CHA scaffold. Therefore, the CHA scaffold strengthened the osteogenesis and blood vessel formation potential of the DCS complexes. Meanwhile, the DCS complexes also promoted the osteogenesis and blood vessel formation potential of the CHA scaffold. This study will provide a basis for building vascularized tissue-engineered bone for bone defect therapy.

STATEMENT OF SIGNIFICANCE

This study developed a double-cell sheet (DCS) complex composed of an osteogenic cell sheet and a vascular endothelial cell sheet with osteogenesis and blood vessel formation potential. Osteogenic and vascular endothelial cell sheets were obtained after induced culture of rabbit adipose-derived mesenchymal stem cells. The DCS complex and DCS-CHA complex exhibited osteogenic and blood vessel formation potential in vivo. CHA enhanced the osteogenesis and blood vessel formation abilities of the DCS complexes in vivo. Meanwhile, the DCS complexes also promoted the osteogenesis and blood vessel formation potential of the CHA scaffold. Group B of the DCS complexes and DCS-CHA complexes exhibited the best osteogenesis and blood vessel formation abilities.

Injectable Hyaluronic Acid-co-Gelatin Cryogels for Tissue-Engineering Applications

Polymeric scaffolds such as hydrogels can be engineered to restore, maintain, or improve impaired tissues and organs. However, most hydrogels require surgical implantation that can cause several complications such as infection and damage to adjacent tissues. Therefore, developing minimally invasive strategies is of critical importance for these purposes. Herein, we developed several injectable cryogels made out of hyaluronic acid and gelatin for tissue-engineering applications. The physicochemical properties of hyaluronic acid combined with the intrinsic cell-adhesion properties of gelatin can provide suitable physical support for the attachment, survival, and spreading of cells. The physical characteristics of pure gelatin cryogels, such as mechanics and injectability, were enhanced once copolymerized with hyaluronic acid. Reciprocally, the adhesion of 3T3 cells cultured in hyaluronic acid cryogels was enhanced when formulated with gelatin. Furthermore, cryogels had a minimal effect on bone marrow dendritic cell activation, suggesting their cytocompatibility. Finally, in vitro studies revealed that copolymerizing gelatin with hyaluronic acid did not significantly alter their respective intrinsic biological properties. These findings suggest that hyaluronic acid-co-gelatin cryogels combined the favorable inherent properties of each biopolymer, providing a mechanically robust, cell-responsive, macroporous, and injectable platform for tissue-engineering applications.

Genetic Engineering of Mesenchymal Stem Cells for Differential Matrix Deposition on 3D Woven Scaffolds

Tissue engineering approaches for the repair of osteochondral defects using biomaterial scaffolds and stem cells have remained challenging due to the inherent complexities of inducing cartilage-like matrix and bone-like matrix within the same local environment. Members of the transforming growth factor β (TGFβ) family have been extensively utilized in the engineering of skeletal tissues, but have distinct effects on chondrogenic and osteogenic differentiation of progenitor cells. The goal of this study was to develop a method to direct human bone marrow-derived mesenchymal stem cells (MSCs) to deposit either mineralized matrix or a cartilaginous matrix rich in glycosaminoglycan and type II collagen within the same biochemical environment. This differential induction was performed by culturing cells on engineered three-dimensionally woven poly(ɛ-caprolactone) (PCL) scaffolds in a chondrogenic environment for cartilage-like matrix production while inhibiting TGFβ3 signaling through Mothers against DPP homolog 3 (SMAD3) knockdown, in combination with overexpressing RUNX2, to achieve mineralization. The highest levels of mineral deposition and alkaline phosphatase activity were observed on scaffolds with genetically engineered MSCs and exhibited a synergistic effect in response to SMAD3 knockdown and RUNX2 expression. Meanwhile, unmodified MSCs on PCL scaffolds exhibited accumulation of an extracellular matrix rich in glycosaminoglycan and type II collagen in the same biochemical environment. This ability to derive differential matrix deposition in a single culture condition opens new avenues for developing complex tissue replacements for chondral or osteochondral defects.

Segmental bone replacement via patient-specific, three-dimensional printed bioresorbable graft substitutes and their use as templates for the culture of mesenchymal stem cells under mechanical stimulation at various frequencies

The treatment of large segmental bone defects remains a challenge as infection, delayed union, and nonunion are common postoperative complications. A three-dimensional printed bioresorbable and physiologically load-sustaining graft substitute was developed to mimic native bone tissue for segmental bone repair. Fabricated from polylactic acid, this graft substitute is novel as it is readily customizable to accommodate the particular size and location of the segmental bone of the patient to be replaced. Inspired by the structure of the native bone tissue, the graft substitute exhibits a gradient in porosity and pore size in the radial direction and exhibit mechanical properties similar to those of the native bone tissue. The graft substitute can serve as a template for tissue constructs via seeding with stem cells. The biocompatibility of such templates was tested under in vitro conditions using a dynamic culture of human mesenchymal stem cells. The effects of the mechanical loading of cell-seeded templates under in vitro conditions were assessed via subjecting the tissue constructs to 28 days of daily mechanical stimulation. The frequency of loading was found to have a significant effect on the rate of mineralization, as the alkaline phosphatase activity and calcium deposition were determined to be particularly high at the typical walking frequency of 2 Hz, suggesting that mechanical stimulation plays a significant role in facilitating the healing process of bone defects. Utilization of such patient-specific and biocompatible graft substitutes, coupled with patient's bone marrow cells seeded and exposed to mechanical stimulation of 2 Hz have the potential of reducing significant volumes of cadaveric tissue required, improving long-term graft stability and incorporation, and alleviating financial burdens associated with delayed or failed fusions of long bone defects.

In vitro evaluation of decontamination effects on mechanical properties of fibrin membrane

Background: Tissue engineering has been investigated as a potential method for healing traumatized tissues. Biomaterials are material devices or implants used to repair or replace native body tissues and organs. The present study was conducted to evaluate the effects of decontamination methods on biological/mechanical properties and degradation/adhesion test of the platelet-rich fibrin (PRF) membranes to compare these properties with intact membranes as a biological biomaterial.

Methods: The in vitro degradation tests were conducted by placing the equal sizes of (i) intact PRF membrane, (ii) PRF membrane sterilized by autoclave (iii), ultraviolet (UV), and (iiii) gamma irradiation in phosphate buffer solution on a shaker. The degradation profiles were expressed. Adhesion test was performed by counting adhered mouse fibroblast and sterilized fibrin membrane was compared to normal fibrin membrane by different sterilization methods.

Results: The preliminary findings of sterilized PRF membranes showed that UV exposure (p<0.05) and autoclaved fibrin membranes (p<0.01) have significantly lower degradability compared to normal fibrin membranes. Gamma irradiation is similar to normal membrane in degradability. Cell adherence in all groups of fibrin membrane was significantly lower than the group without membrane, but there was no significant difference between intact and sterilized groups of fibrin membranes.

Conclusion: Sterilization of fibrin membrane with different protocols does not have any adverse effects on cell adhesion; however, cell adherence is naturally very weak even in normal membranes. Also, it seems that ultraviolet ray polymerizes fibrin filaments and merges them to each other and increases the ability of fibrin membrane against degradation. Autoclaved fibrin membrane content proteins are denatured because of pressure and heat and show an increase in hardness and stability against degradation.

Enhancing Biological and Biomechanical Fixation of Osteochondral Scaffold: A Grand Challenge

Osteoarthritis (OA) is a degenerative joint disease, typified by degradation of cartilage and changes in the subchondral bone, resulting in pain, stiffness and reduced mobility. Current surgical treatments often fail to regenerate hyaline cartilage and result in the formation of fibrocartilage. Tissue engineering approaches have emerged for the repair of cartilage defects and damages to the subchondral bones in the early stage of OA and have shown potential in restoring the joint's function. In this approach, the use of three-dimensional scaffolds (with or without cells) provides support for tissue growth. Commercially available osteochondral (OC) scaffolds have been studied in OA patients for repair and regeneration of OC defects. However, some controversial results are often reported from both clinical trials and animal studies. The objective of this chapter is to report the scaffolds clinical requirements and performance of the currently available OC scaffolds that have been investigated both in animal studies and in clinical trials. The findings have demonstrated the importance of biological and biomechanical fixation of the OC scaffolds in achieving good cartilage fill and improved hyaline cartilage formation. It is concluded that improving cartilage fill, enhancing its integration with host tissues and achieving a strong and stable subchondral bone support for overlying cartilage are still grand challenges for the early treatment of OA.

Physiologically based microenvironment for in vitro neural differentiation of adipose-derived stem cells

The limited capacity of nervous system to promote a spontaneous regeneration and the high rate of neurodegenerative diseases appearance are keys factors that stimulate researches both for defining the molecular mechanisms of pathophysiology and for evaluating putative strategies to induce neural tissue regeneration. In this latter aspect, the application of stem cells seems to be a promising approach, even if the control of their differentiation and the maintaining of a safe state of proliferation should be troubled. Here, we focus on adipose tissue-derived stem cells and we seek out the recent advances on the promotion of their neural differentiation, performing a critical integration of the basic biology and physiology of adipose tissue-derived stem cells with the functional modifications that the biophysical, biomechanical and biochemical microenvironment induces to cell phenotype. The pre-clinical studies showed that the neural differentiation by cell stimulation with growth factors benefits from the integration with biomaterials and biophysical interaction like microgravity. All these elements have been reported as furnisher of microenvironments with desirable biological, physical and mechanical properties. A critical review of current knowledge is here proposed, underscoring that a real advance toward a stable, safe and controllable adipose stem cells clinical application will derive from a synergic multidisciplinary approach that involves material engineer, basic cell biology, cell and tissue physiology.

Injectable calcium phosphate scaffold with iron oxide nanoparticles to enhance osteogenesis via dental pulp stem cells

Literature search revealed no systematic report on iron oxide nanoparticle-incorporating calcium phosphate cement scaffolds (IONP-CPC). The objectives of this study were to: (1) use γFe₂O₃ nanoparticles (γIONPs) and αFe₂O₃ nanoparticles (αIONPs) to develop novel IONP-CPC scaffolds, and (2) investigate human dental pulp stem cells (hDPSCs) seeding on IONP-CPC for bone tissue engineering for the first time. IONP-CPC scaffolds were fabricated. Physiochemical properties of IONP-CPC scaffolds were characterized. hDPSC seeding on scaffolds, cell proliferation, osteogenic differentiation and bone matrix mineral synthesis by cells were measured. Our data demonstrated that the osteogenic differentiation of hDPSCs was markedly enhanced via IONP incorporation into CPC. Substantial increases (about three folds) in ALP activity and osteogenic gene expressions were achieved over those without IONPs. Bone matrix mineral synthesis by the cells was increased by two- to three folds over that without IONPs. The enhanced cellular osteogenesis was attributed to: (1) the surface nanotopography of IONP-CPC scaffold, and (2) the cell internalization of IONPs released from IONP-CPC scaffold. Our results demonstrate that the novel CPC functionalized with IONPs is promising to promote osteoinduction and bone regeneration. In conclusion, it is highly promising to incorporate γIONPs and αIONPs into CPC scaffold for bone tissue engineering, yielding substantially better stem cell attachment, spreading and osteogenic differentiation, and much greater bone mineral synthesis by the seeded cells. Therefore, novel CPC scaffolds containing γIONPs and αIONPs are promising for dental, craniofacial and orthopaedic applications to substantially enhance bone regeneration.

Electrospun 3D Scaffolds for Tissue Regeneration

Tissue engineering aims to fabricate and functionalise constructs that mimic the native extracellular matrix (ECM) in the closest way possible to induce cell growth and differentiation in both in vitro and in vivo conditions. Development of scaffolds that can function as tissue substitutes or augment healing of tissues is an essential aspect of tissue regeneration. Although there are many techniques for achieving this biomimicry in 2D structures and 2D cell cultures, translation of successful tissue regeneration in true 3D microenvironments is still in its infancy. Electrospinning, a well known electrohydrodynamic process, is best suited for producing and functionalising, nanofibrous structures to mimic the ECM. A systematic control of the processing parameters coupled with novel process innovations, has recently resulted in novel 3D electrospun structures. This chapter gives a brief account of the various 3D electrospun structures that are being tried as tissue engineering scaffolds. Combining electrospinning with other 3D structure forming technologies, which have shown promising results, has also been discussed. Electrospinning has the potential to bridge the gap between what is known and what is yet to be known in fabricating 3D scaffolds for tissue regeneration.

Three-dimensional fabrication of thick and densely populated soft constructs with complex and actively perfused channel network

One of the fundamental steps needed to design functional tissues and, ultimately organs is the ability to fabricate thick and densely populated tissue constructs with controlled vasculature and microenvironment. To date, bioprinting methods have been employed to manufacture tissue constructs with open vasculature in a square-lattice geometry, where the majority lacks the ability to be directly perfused. Moreover, it appears to be difficult to fabricate vascular tissue constructs targeting the stiffness of soft tissues such as the liver. Here we present a method for the fabrication of thick (e.g. 1 cm) and densely populated (e.g. 10 million cells·mL^-1) tissue constructs with a three-dimensional (3D) four arm branch network and stiffness in the range of soft tissues (1-10 kPa), which can be directly perfused on a fluidic platform for long time periods (>14 days). Specifically, we co-print a 3D four-arm branch using water-soluble Poly(vinyl alcohol) (PVA) as main material and Poly(lactic acid) (PLA) as the support structure. The PLA support structure was selectively removed, and the water soluble PVA structure was used for creating a 3D vascular network within a customized extracellular matrix (ECM) targeting the stiffness of the liver and with encapsulated hepatocellular carcinoma (HepG2) cells. These constructs were directly perfused with medium inducing the proliferation of HepG2 cells and the formation of spheroids. The highest spheroid density was obtained with perfusion, but overall the tissue construct displayed two distinct zones, one of rapid proliferation and one with almost no cell division and high cell death. The created model, therefore, simulate gradients in tissues of necrotic regions in tumors. This versatile method could represent a fundamental step in the fabrication of large functional and complex tissues and finally organs.

STATEMENT OF SIGNIFICANCE

Vascularization within hydrogels with mechanical properties in the range of soft tissues remains a challenge. To date, bioprinting have been employed to manufacture tissue constructs with open vasculature in a square-lattice geometry that are most of the time not perfused. This study shows the creation of densely populated tissue constructs with a 3D four arm branch network and stiffness in the range of soft tissues, which can be directly perfused. The cells encapsulated within the construct showed proliferation as a function of the vasculature distance, and the control of the micro-environment induced the encapsulated cells to aggregate in spheroids in specific positions. This method could be used for modeling tumors and for fabricating more complex and densely populated tissue constructs with translational potential.

Multiscale design and synthesis of biomimetic gradient protein/biosilica composites for interfacial tissue engineering

Continuous gradients present at tissue interfaces such as osteochondral systems, reflect complex tissue functions and involve changes in extracellular matrix compositions, cell types and mechanical properties. New and versatile biomaterial strategies are needed to create suitable biomimetic engineered grafts for interfacial tissue engineering. Silk protein-based composites, coupled with selective peptides with mineralization domains, were utilized to mimic the soft-to-hard transition in osteochondral interfaces. The gradient composites supported tunable mineralization and mechanical properties corresponding to the spatial concentration gradient of the mineralization domains (R5 peptide). The composite system exhibited continuous transitions in terms of composition, structure and mechanical properties, as well as cytocompatibility and biodegradability. The gradient silicified silk/R5 composites promoted and regulated osteogenic differentiation of human mesenchymal stem cells in an osteoinductive environment in vitro. The cells differentiated along the composites in a manner consistent with the R5-gradient profile. This novel biomimetic gradient biomaterial design offers a useful approach to meet a broad range of needs in regenerative medicine.

Scaffold curvature-mediated novel biomineralization process originates a continuous soft tissue-to-bone interface

A myriad of shapes are found in biological tissues, often naturally evolved to fulfill a particular function. In the field of tissue engineering, substrate geometry influences cell behavior and tissue formation in vitro, yet little is known how this translates to an in vivo scenario. Here we investigate scaffold curvature-induced tissue growth, without additional growth factors or cells, in an ovine animal model. We show that soft tissue formation follows a curvature-driven tissue growth model. The highly organized endogenous soft matrix, potentially under mechanical strain, leads to a non-standard form of biomineralization, whereby the pre-existing organic matrix is mineralized without collagen remodeling and without an intermediate cartilage ossification phase. Micro- and nanoscale characterization of the tissue microstructure using histology, backscattered electron (BSE) and second-harmonic generation (SHG) imaging and synchrotron small angle X-ray scattering (SAXS) revealed (i) continuous collagen fibers across the soft-hard tissue interface on the tip of mineralized cones, and (ii) bone remodeling by basic multicellular units (BMUs) in regions adjacent to the native cortical bone. Thus, features of soft tissue-to-bone interface resembling the insertion sites of ligaments and tendons into bone were created, using a scaffold that did not mimic the structural or biological gradients across such a complex interface at its mature state. This study provides fundamental knowledge for biomimetic scaffold design in the fields of bone regeneration and soft tissue-to-bone interface tissue engineering.

STATEMENT OF SIGNIFICANCE

Geometry influences cell behavior and tissue formation in vitro. However, little is known how this translates to an in vivo scenario. Here we investigate the influence of scaffold mean surface curvature on in vivo tissue growth using an ovine animal model. Based on a multiscale tissue microstructure characterization, we show a seamless integration of soft tissue into newly formed bone, resembling the insertion sites of ligaments and tendons into bone. This interface was created using a scaffold without additional growth factors or cells that did not recapitulate the structural or biological gradients across such a complex tissue interface at its mature state. These findings have important implications for biomimetic scaffold design for bone regeneration and soft tissue-to-bone interface tissue engineering.

Tissue engineering toward organ-specific regeneration and disease modeling

Tissue engineering has been recognized as a translational approach to replace damaged tissue or whole organs. Engineering tissue, however, faces an outstanding knowledge gap in the challenge to fully recapitulate complex organ-specific features. Major components, such as cells, matrix, and architecture, must each be carefully controlled to engineer tissue-specific structure and function that mimics what is found in vivo. Here we review different methods to engineer tissue, and discuss critical challenges in recapitulating the unique features and functional units in four major organs-the kidney, liver, heart, and lung, which are also the top four candidates for organ transplantation in the USA. We highlight advances in tissue engineering approaches to enable the regeneration of complex tissue and organ substitutes, and provide tissue-specific models for drug testing and disease modeling. We discuss the current challenges and future perspectives toward engineering human tissue models.

The Top 50 Most Cited Articles in Cartilage Regeneration

The aim of this study was to identify and analyze the top 50 most cited articles in cartilage regeneration. The impact of a scientific journal can be gauged by the total number of citations it has accrued. The top 50 most cited articles involving cartilage regeneration represent the most quoted level of evidence among this new subspecialty. This study aims to identify and analyze the 50 most cited articles in cartilage regeneration. The Web of Science™ citation indexing service was utilized to determine the most frequently cited articles published after 1956 containing “cartilage regeneration” in the “topic” or “title.” The 50 most cited articles were included. The number of citations, year of publication, country of article origin, article institution, journal of publication, publication format, and authorship were then calculated for each article. The span of citations ranged from 1287 to 203 citations, with a mean of 361.02 citations per article in question. The articles originated from 11 countries, with the United States contributing 34 articles, followed by Japan with 5 articles. The articles were distributed across 34 high-impact journals. Biomaterials was the journal with the highest number of publications (seven articles) followed by the Journal of Orthopaedic Research (three articles). Of the 50 articles, 2 were clinical observational studies, 47 concerned basic science, and 1 was review article. The most cited articles involving cartilage regeneration are detected in both experimental and clinical research fields. The high ratio of basic science to clinical articles reflects the infancy of this relatively new specialty and that further clinical research is required in this area.

Functional liver tissue engineering by an adult mouse liver-derived neuro-glia antigen 2-expressing stem/progenitor population

Deaths due to end-stage liver diseases are increasingly registered annually in the world. Liver transplantation is the ultimate treatment for end-stage liver diseases to date, which has been hampered by a critical shortage of organs. The potential of decellularized liver scaffolds (DLS) derived from solid organs as a three-dimensional platform has been evolved as a promising approach in liver tissue engineering for translating functional liver organ replacements, but questions still exist regarding the optimal cell population for seeding in DLS and the preparation of the DLS themselves. The aim of our study was to utilize a sodium dodecyl sulfate decellularization procedure in combination with a low concentration of trypsin (0.005%)-ethylenediaminetetraacetic acid (0.002%) process to manufacture DLS from whole mouse livers and recellularized with hepatic stem/progenitors for use in liver tissue engineering and injured liver treatment. Results showed that the DLS generated with all the necessary microstructure and the extracellular components to support seeded hepatic stem/progenitor cell attachment, functional hepatic cell differentiation. Hepatic differentiation from stem/progenitor cells loaded by DLS was more efficient than that of the stem/progenitor cells in the two-dimensional cell culture model. In summary, the method of DLS loaded by hepatic stem/progenitor cells provided by this study was effective in maintaining DLS extracellular matrix to introduce seeded stem/progenitor cell differentiation, hepatic-like tissue formation and functional hepatic protein production in vitro that promoted functional recovery and survival in a mouse model of dimethylnitrosamine-induced liver cirrhosis after auxiliary heterotopic liver transplantation. Copyright © 2016 John Wiley & Sons, Ltd.

Calcium Silicate/Chitosan-Coated Electrospun Poly (Lactic Acid) Fibers for Bone Tissue Engineering

Electrospinning technology allows fabrication of nano- or microfibrous fibers with inorganic and organic matrix and it is widely applied in bone tissue engineering as it allows precise control over the shapes and structures of the fibers. Natural bone has an ordered composition of organic fibers with dispersion of inorganic apatite among them. In this study, poly (lactic acid) (PLA) mats were fabricated with electrospinning and coated with chitosan (CH)/calcium silicate (CS) mixer. The microstructure, chemical component, and contact angle of CS/CH-PLA composites were analyzed by scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. In vitro, various CS/CH-coated PLA mats increased the formation of hydroxyapatite on the specimens' surface when soaked in cell cultured medium. During culture, several biological characteristics of the human mesenchymal stem cells (hMSCs) cultured on CS/CH-PLA groups were promoted as compared to those on pure PLA mat. Increased secretion levels of Collagen I and fibronectin were observed in calcium silicate-powder content. Furthermore, with comparison to PLA mats without CS/CH, CS10 and CS15 mats markedly enhanced the proliferation of hMSCs and their osteogenesis properties, which was characterized by osteogenic-related gene expression. These results clearly demonstrated that the biodegradable and electroactive CS/CH-PLA composite mats are an ideal and suitable candidate for bone tissue engineering.

WAT-on-a-chip: a physiologically relevant microfluidic system incorporating white adipose tissue

Organ-on-a-chip systems possess a promising future as drug screening assays and as testbeds for disease modeling in the context of both single-organ systems and multi-organ-chips. Although it comprises approximately one fourth of the body weight of a healthy human, an organ frequently overlooked in this context is white adipose tissue (WAT). WAT-on-a-chip systems are required to create safety profiles of a large number of drugs due to their interactions with adipose tissue and other organs via paracrine signals, fatty acid release, and drug levels through sequestration. We report a WAT-on-a-chip system with a footprint of less than 1 mm2 consisting of a separate media channel and WAT chamber connected via small micropores. Analogous to the in vivo blood circulation, convective transport is thereby confined to the vasculature-like structures and the tissues protected from shear stresses. Numerical and analytical modeling revealed that the flow rates in the WAT chambers are less than 1/100 of the input flow rate. Using optimized injection parameters, we were able to inject pre-adipocytes, which subsequently formed adipose tissue featuring fully functional lipid metabolism. The physiologically relevant microfluidic environment of the WAT-chip supported long term culture of the functional adipose tissue for more than two weeks. Due to its physiological, highly controlled, and computationally predictable character, the system has the potential to be a powerful tool for the study of adipose tissue associated diseases such as obesity and type 2 diabetes.

The efficacy of a scaffold-free Bio 3D conduit developed from human fibroblasts on peripheral nerve regeneration in a rat sciatic nerve model

BACKGROUND

Although autologous nerve grafting is the gold standard treatment of peripheral nerve injuries, several alternative methods have been developed, including nerve conduits that use supportive cells. However, the seeding efficacy and viability of supportive cells injected in nerve grafts remain unclear. Here, we focused on a novel completely biological, tissue-engineered, scaffold-free conduit.

METHODS

We developed six scaffold-free conduits from human normal dermal fibroblasts using a Bio 3D Printer. Twelve adult male rats with immune deficiency underwent mid-thigh-level transection of the right sciatic nerve. The resulting 5-mm nerve gap was bridged using 8-mm Bio 3D conduits (Bio 3D group, n = 6) and silicone tube (silicone group, n = 6). Several assessments were conducted to examine nerve regeneration eight weeks post-surgery.

RESULTS

Kinematic analysis revealed that the toe angle to the metatarsal bone at the final segment of the swing phase was significantly higher in the Bio 3D group than the silicone group (-35.78 ± 10.68 versus -62.48 ± 6.15, respectively; p < 0.01). Electrophysiological studies revealed significantly higher compound muscle action potential in the Bio 3D group than the silicone group (53.60 ± 26.36% versus 2.93 ± 1.84%; p < 0.01). Histological and morphological studies revealed neural cell expression in all regions of the regenerated nerves and the presence of many well-myelinated axons in the Bio 3D group. The wet muscle weight of the tibialis anterior muscle was significantly higher in the Bio 3D group than the silicone group (0.544 ± 0.063 versus 0.396 ± 0.031, respectively; p < 0.01).

CONCLUSIONS

We confirmed that scaffold-free Bio 3D conduits composed entirely of fibroblast cells promote nerve regeneration in a rat sciatic nerve model.