Synthetic extracellular matrices for tissue engineering and regeneration

Bioengineering approaches for modelling retinal pathologies of the outer blood-retinal barrier

Alterations of the junctional complex of the outer blood-retinal barrier (oBRB), which is integrated by the close interaction of the retinal pigment epithelium, the Bruch's membrane, and the choriocapillaris, contribute to the loss of neuronal signalling and subsequent vision impairment in several retinal inflammatory disorders such as age-related macular degeneration and diabetic retinopathy. Reductionist approaches into the mechanisms that underlie such diseases have been hindered by the absence of adequate in vitro models using human cells to provide the 3D dynamic architecture that enables expression of the in vivo phenotype of the oBRB. Conventional in vitro cell models are based on 2D monolayer cellular cultures, unable to properly recapitulate the complexity of living systems. The main drawbacks of conventional oBRB models also emerge from the cell sourcing, the lack of an appropriate Bruch's membrane analogue, and the lack of choroidal microvasculature with flow. In the last years, the advent of organ-on-a-chip, bioengineering, and stem cell technologies is providing more advanced 3D models with flow, multicellularity, and external control over microenvironmental properties. By incorporating additional biological complexity, organ-on-a-chip devices can mirror physiologically relevant properties of the native tissue while offering additional set ups to model and study disease. In this review we first examine the current understanding of oBRB biology as a functional unit, highlighting the coordinated contribution of the different components to barrier function in health and disease. Then we describe recent advances in the use of pluripotent stem cells-derived retinal cells, Bruch's membrane analogues, and co-culture techniques to recapitulate the oBRB. We finally discuss current advances and challenges of oBRB-on-a-chip technologies for disease modelling.

Surface Modification of Polycaprolactone Scaffold With Improved Biocompatibility and Controlled Growth Factor Release for Enhanced Stem Cell Differentiation

Polycaprolactone (PCL) has been widely used as a scaffold material for tissue engineering. Reliable applications of the PCL scaffolds require overcoming their native hydrophobicity and obtaining the sustained release of signaling factors to modulate cell growth and differentiation. Here, we report a surface modification strategy for electrospun PCL nanofibers using an azide-terminated amphiphilic graft polymer. With multiple alkylation and pegylation on the side chains of poly-L-lysine, stable coating of the graft polymer on the PCL nanofibers was achieved in one step. Using the azide-alkyne “click chemistry”, we functionalized the azide-pegylated PCL nanofibers with dibenzocyclooctyne-modified nanocapsules containing growth factor, which rendered the nanofiber scaffold with satisfied cell adhesion and growth property. Moreover, by specific immobilization of pH-responsive nanocapsules containing bone morphogenetic protein 2 (BMP-2), controlled release of active BMP-2 from the PCL nanofibers was achieved within 21 days. When bone mesenchyme stem cells were cultured on this nanofiber scaffold, enhanced ossification was observed in correlation with the time-dependent release of BMP-2. The established surface modification can be extended as a generic approach to hydrophobic nanomaterials for longtime sustainable release of multiplex signaling proteins for tissue engineering.

Osteoblast-derived extracellular matrix coated PLLA/silk fibroin composite nanofibers promote osteogenic differentiation of bone mesenchymal stem cells

Poly-L-lactic acid (PLLA) is one of the most commonly used synthetic materials for regenerative medicine, and silk fibroin (SF) is a natural protein with excellent biocompatibility. Combination of PLLA and SF in a proper proportion by electrospinning may generate composite nanofibers that could meet the requirements of scaffolding in bone tissue engineering. The application of PLLA/SF nanofibrous scaffold for osteogenesis is well established in vitro and in vivo. However, PLLA/SF nanofibrous scaffold does not have an ideal ability to promote cell adhesion, proliferation, and differentiation. Extracellular matrix (ECM) plays a critical role in modulating cellular behavior. However, the role of combination of natural ECM with nanofibrous scaffold in regulating osteogenic differentiation is unclear. In this study, we aimed to develop a novel composite PLLA/SF nanofibrous scaffold coated with osteoblast-derived extracellular matrix (O-ECM/PLLA/SF) and analyze the effects of the modified scaffold on osteogenic differentiation of BMSCs. The surface structural features and compositions of the O-ECM/PLLA/SF scaffold were characterized by SEM and immunofluorescence staining. The capacities of the O-ECM/PLLA/SF scaffold to induce osteogenic differentiation of BMSCs were investigated by alkaline phosphatase (ALP) and alizarin red staining (ARS). The results showed BMSCs cultured on O-ECM/PLLA/SF scaffold significantly increased osteogenic differentiation compared with cells cultured individually on a scaffold or O-ECM. Collectively, these findings indicate that O-ECM-coated nanofibrous scaffold can be a promising strategy for osteogenic differentiation of BMSCs, opening a new possibility of utilizing composite scaffolds for bone tissue engineering.

Humanized Mouse Models for Evaluation of PSC Immunogenicity

New human pluripotent stem cell (hPSC)-derived therapies are advancing to clinical trials at an increasingly rapid pace. In addition to ensuring that the therapies function properly, there is a critical need to investigate the human immune response to these cell products. A robust allogeneic (or autologous) immune response could swiftly eliminate an otherwise promising cell therapy, even in immunosuppressed patients. In coming years, researchers in the regenerative medicine field will need to utilize a number of in vitro and in vivo assays and models to evaluate and better understand hPSC immunogenicity. Humanized mouse models-mice engrafted with functional human immune cell types-are an important research tool for investigating the mechanisms of the adaptive immune response to hPSC therapies. This article provides an overview of humanized mouse models relevant to the study of hPSC immunogenicity and explores central considerations for investigators seeking to utilize these powerful models in their research. © 2020 Wiley Periodicals LLC.

Wistar rat dermis recellularization

Skin lesions are normal to all species, regardless of gender or age. The skin, the largest organ of the body, has function as a primary barrier to the chemical, physical and biological aggressions of the environment. In animals, these lesions may be due to fights and/or predations, also as in humans, there is a very common cause of dermal lesions that are caused by burns and carcinomas. Looking for new techniques of tissue bioengineering, studies have been shown promising results for formulations of acellular biological scaffolds from tissue decellularization for the reconstitution of these lesions. The decellularization has its proof by a varied range of tests such as scanning electron microscopy and residual genomic DNA tests. Subsequently the tissue can go through the process of recellularization using cells of interest, even the animal that will receive this tissue, reducing the risks of rejection and improving the response to tissue transplantation. Thus, this manuscript aimed at the decellularization of the tissue with the use of chemical and physical means followed by sterilization and the establishment of a protocol for the recellularization of a decellularized scaffold from the Wistar rat dermis using murine fibroblasts and mesenchymal stem cells from canine adipose tissue for 7 days. After efficacy tests, the tissue recellularization were confirmed by immunofluorescence assays and scanning electron microscopy where the adherence of the cells in the biological scaffold was observed.

Bacteriophage-based biomaterials for tissue regeneration

Bacteriophage, also called phage, is a human-safe bacteria-specific virus. It is a monodisperse biological nanostructure made of proteins (forming the outside surface) and nucleic acids (encased in the protein capsid). Among different types of phages, filamentous phages have received great attention in tissue regeneration research due to their unique nanofiber-like morphology. They can be produced in an error-free format, self-assemble into ordered scaffolds, display multiple signaling peptides site-specifically, and serve as a platform for identifying novel signaling or homing peptides. They can direct stem cell differentiation into specific cell types when they are organized into proper patterns or display suitable peptides. These unusual features have allowed scientists to employ them to regenerate a variety of tissues, including bone, nerves, cartilage, skin, and heart. This review will summarize the progress in the field of phage-based tissue regeneration and the future directions in this field.

Biomaterials Enabled Cell-Free Strategies for Endogenous Bone Regeneration

Repairing bone defects poses a major orthopedic challenge because current treatments are constrained by the limited regenerative capacity of human bone tissue. Novel therapeutic strategies, such as stem cell therapy and tissue engineering, have the potential to enhance bone healing and regeneration, and hence may improve quality of life for millions of people. However, the ex vivo expansion of stem cells and their in vivo delivery pose technical difficulties that hamper clinical translation and commercial development. A promising alternative to cell delivery-based strategies is to stimulate or augment the inherent self-repair mechanisms of the patient to promote endogenous restoration of the lost/damaged bone. There is growing evidence indicating that increasing the endogenous regenerative potency of bone tissues for therapeutics will require the design and development of new generations of biomedical devices that provide key signaling molecules to instruct cell recruitment and manipulate cell fate for in situ tissue regeneration. Currently, a broad range of biomaterial-based deployment technologies are becoming available, which allow for controlled spatial presentation of biological cues required for endogenous bone regeneration. This article aims to explore the proposed concepts and biomaterial-enabled strategies involved in the design of cell-free endogenous techniques in bone regenerative medicine.

Insight on stem cell preconditioning and instructive biomaterials to enhance cell adhesion, retention, and engraftment for tissue repair

Stem cells are a promising solution for the treatment of a variety of diseases. However, the limited survival and engraftment of transplanted cells due to a hostile ischemic environment is a bottleneck for effective utilization and commercialization. Within this environment, the majority of transplanted cells undergo apoptosis prior to participating in lineage differentiation and cellular integration. Therefore, in order to maximize the clinical utility of stem/progenitor cells, strategies must be employed to increase their adhesion, retention, and engraftment in vivo. Here, we reviewed key strategies that are being adopted to enhance the survival, retention, and engraftment of transplanted stem cells through the manipulation of both the stem cells and the surrounding environment. We describe how preconditioning of cells or cell manipulations strategies can enhance stem cell survival and engraftment after transplantation. We also discuss how biomaterials can enhance the function of stem cells for effective tissue regeneration. Biomaterials can incorporate or mimic extracellular function (ECM) function and enhance survival or differentiation of transplanted cells in vivo. Biomaterials can also promote angiogenesis, enhance engraftment and differentiation, and accelerate electromechanical integration of transplanted stem cells. Insight gained from this review may direct the development of future investigations and clinical trials.

Nano/microfibrous polymeric constructs loaded with bioactive agents and designed for tissue engineering applications: a review

Nano/microfibrous polymeric constructs present various inherent advantages, such as highly porous architecture and high surface to volume ratio, making them attractive for tissue engineering purposes. Electrospinning is the most preferred technique for the fabrication of polymeric nanofibrous assemblies that can mimic the physical functions of native extracellular matrix greatly favoring cells attachment and thus influencing their morphology and activities. Different approaches have been developed to apply polymeric microfiber fabrication techniques (e.g. wet-spinning) for the obtainment of scaffolds with a three-dimensional network of micropores suitable for effective cells migration. Progress in additive manufacturing technology has led to the development of complex scaffold's shapes and microfibrous structures with a high degree of automation, good accuracy and reproducibility. Various loading methods, such as direct blending, coaxial electrospinning and microparticles incorporation, are enabling to develop customized strategies for the biofunctionalization of nano/microfibrous scaffolds with a tailored kinetics of release of different bioactive agents, ranging from small molecules, such as antibiotics, to protein drugs, such as growth factors, and even cells. Recent activities on the combination of different processing techniques and loading methods for the obtainment of biofunctionalized polymeric constructs with a complex multiscale structure open new possibilities for the development of biomimetic scaffolds endowed with a hierarchical architecture and a sophisticated release kinetics of different bioactive agents. This review is aimed at summarizing current advances in technologies and methods for manufacturing nano/microfibrous polymeric constructs suitable as tissue engineering scaffolds, and for their combination with different bioactive agents to promote tissue regeneration and therapeutic effects.

Regenerative therapy and tissue engineering for the treatment of end-stage cardiac failure: new developments and challenges

Regeneration of myocardium through regenerative therapy and tissue engineering is appearing as a prospective treatment modality for patients with end-stage heart failure. Focusing on this area, this review highlights the new developments and challenges in the regeneration of myocardial tissue. The role of various cell sources, calcium ion and cytokine on the functional performance of regenerative therapy is discussed. The evolution of tissue engineering and the role of tissue matrix/scaffold, cell adhesion and vascularisation on tissue engineering of cardiac tissue implant are also discussed.

Cardiac fibroblast-derived 3D extracellular matrix seeded with mesenchymal stem cells as a novel device to transfer cells to the ischemic myocardium

PURPOSE

Demonstrate a novel manufacturing method to generate extracellular matrix scaffolds from cardiac fibroblasts (CF-ECM) as a therapeutic mesenchymal stem cell-transfer device.

MATERIALS AND METHODS

Rat CF were cultured at high-density (~1.6×10⁵/cm²) for 10-14 days. Cell sheets were removed from the culture dish by incubation with EDTA and decellularized with water and peracetic acid. CF-ECM was characterized by mass spectrometry, immunofluorescence and scanning electron microscopy. CF-ECM seeded with human embryonic stem cell derived mesenchymal stromal cells (hEMSCs) were transferred into a mouse myocardial infarction model. 48 hours later, mouse hearts were excised and examined for CF-ECM scaffold retention and cell transfer.

RESULTS

CF-ECM scaffolds are composed of fibronectin (82%), collagens type I (13%), type III (3.4%), type V (0.2%), type II (0.1%) elastin (1.3%) and 18 non-structural bioactive molecules. Scaffolds remained intact on the mouse heart for 48 hours without the use of sutures or glue. Identified hEMSCs were distributed from the epicardium to the endocardium.

CONCLUSIONS

High density cardiac fibroblast culture can be used to generate CF-ECM scaffolds. CF-ECM scaffolds seeded with hEMSCs can be maintained on the heart without suture or glue. hEMSC are successfully delivered throughout the myocardium.

Tissue engineering concept in the research of the tumor biology

Tumor is a heterogeneous complex, which lives in a three-dimensional environment flush with biopathophysiological and biomechanical signals. This signaling abundant extracellular milieu co-evolving from cell-cell and cell-host interaction guides the development and the generation of the tumor. There has been a recent surge of interest in studying the tumor biology that more closely mirror what happens in living organisms, especially in cancer research. Incorporating cancer cells in the 3D mimicking environment instead of monolayers is reasonable for maintaining in vivo cancer behaviors in spatial and temporal context. However, 3D culture for cancer still presents a challenge for researchers in this field. Tissue engineering, originally aiming at designing the artificial organs, provided a feasible approach to recreate such complex mechanical and biochemical interplay. Aside from reproducing bionic environment, tissue engineering has been routinely introduced into cancer study to build three dimensional structures not only to develop molecular therapeutics, but also to screen for toxic effects of drugs or radiotherapy sensitivity. In this article, we focused on the recent advances of the well-defined tissue-engineering biomaterials in the application in tumor biology. We also discussed the fabrications of the scaffolds from different materials, which might contribute to future cancer research.

Silver acetate coating promotes early vascularization of Dacron vascular grafts without inducing host tissue inflammation

BACKGROUND

Silver acetate is frequently used as an antimicrobial coating of prosthetic vascular grafts. However, the effects of this coating on the early inflammatory and angiogenic host tissue response still remain elusive. Therefore, the aim of the present in vivo study was to analyze the biocompatibility and vascularization of silver acetate-coated and uncoated vascular grafts during the initial phase after implantation.

METHODS

Two different prosthetic vascular grafts (ie, uncoated Dacron and silver acetate-coated Dacron Silver) were implanted into the dorsal skinfold chamber of C57BL/6 mice (n = 8 per group) to study angiogenesis and leukocytic inflammation at the implantation site by means of repetitive intravital fluorescence microscopy over a 14-day period. At the end of the in vivo experiments, collagen formation, apoptosis, and cell proliferation were analyzed in the newly developed granulation tissue surrounding the implants by histology and immunohistochemistry.

RESULTS

During the initial 14 days after implantation, Dacron Silver exhibited an improved vascularization, as indicated by a significantly increased functional capillary density compared with Dacron. This was not associated with a stronger leukocytic inflammatory host tissue response to the implants. Moreover, silver acetate coating did not affect collagen formation, apoptosis, and cell proliferation at the implantation site.

CONCLUSIONS

Silver acetate coating of prosthetic vascular grafts improves their early vascularization without inducing severe inflammatory side effects. Accordingly, this material modification crucially contributes to an improved incorporation of the implants into the host tissue, which may decrease the risk of vascular graft infection.

The Mechanical Bidomain Model: A Review

The mechanical bidomain model is a new mathematical description of the elastic behavior of cardiac tissue. Its primary advantage over previous models is that it accounts for forces acting across the cell membrane arising form differences in the displacement of the intracellular and extracellular spaces. In this review, I describe the development of the mechanical bidomain model. I emphasize new predictions of the model, such as the existence of boundary layers at the tissue surface where the membrane forces are large, and pressure differences between the intracellular and extracellular spaces. Although the theoretical analysis is quite mathematical, I highlight the types of experiments that could be used to test the model predictions. Finally, I present open questions about the mechanical bidomain model that may be productive future directions for research.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

Improved function and growth of pancreatic cells in a three-dimensional bioreactor environment

Methods of three-dimensional (3D) cell culture have made significant progress in recent years due to a better understanding of cell to cell interactions and the cell's interface with their surrounding environment. We hypothesized that a microgravity 3D culture system would improve upon the growth and function of a pancreatic progenitor cell population. We developed a rotating wall vessel bioreactor and established a culture system using a pancreatic cell line. Cells in the bioreactors showed robust proliferation, enhanced transcriptional signaling, and improved translation of pancreatic genes compared with two-dimensional static culture. Cells also gained the ability to respond to glucose stimulation, which was not observed in the control cultures. These findings suggest that a 3D microgravity bioreactor environment mimics the niche of the pancreas yielding a cell source with potential for cell-based therapy in the treatment of diabetes.

The osteogenic differentiation of human bone marrow MSCs on HUVEC-derived ECM and β-TCP scaffold

Extracellular matrix (ECM) serves a key role in cell migration, attachment, and cell development. Here we report that ECM derived from human umbilical vein endothelial cells (HUVEC) promoted osteogenic differentiation of human bone marrow mesenchymal stem cells (hMSC). We first produced an HUVEC-derived ECM on a three-dimensional (3D) beta-tricalcium phosphate (β-TCP) scaffold by HUVEC seeding, incubation, and decellularization. The HUVEC-derived ECM was then characterized by SEM, FTIR, XPS, and immunofluorescence staining. The effect of HUVEC-derived ECM-containing β-TCP scaffold on hMSC osteogenic differentiation was subsequently examined. SEM images indicate a dense matrix layer deposited on the surface of struts and pore walls. FTIR and XPS measurements show the presence of new functional groups (amide and hydroxyl groups) and elements (C and N) in the ECM/β-TCP scaffold when compared to the β-TCP scaffold alone. Immunofluorescence images indicate that high levels of fibronectin and collagen IV and low level of laminin were present on the scaffold. ECM-containing β-TCP scaffolds significantly increased alkaline phosphatase (ALP) specific activity and up-regulated expression of osteogenesis-related genes such as runx2, alkaline phosphatase, osteopontin and osteocalcin in hMSC, compared to β-TCP scaffolds alone. This increased effect was due to the activation of MAPK/ERK signaling pathway since disruption of this pathway using an ERK inhibitor PD98059 results in down-regulation of these osteogenic genes. Cell-derived ECM-containing calcium phosphate scaffolds is a promising osteogenic-promoting bone void filler in bone tissue regeneration.

A myocardial patch made of collagen membranes loaded with collagen-binding human vascular endothelial growth factor accelerates healing of the injured rabbit heart

Tissue-engineered myocardial patches could be useful in the repair of myocardial injuries. The aim of the present study was to evaluate a collagen targeting delivery system for myocardial repair. A specific peptide collagen-binding domain (CBD) was fused to human vascular endothelial growth factor (VEGF) to enhance the binding of VEGF to collagen. In this study, collagen membranes loaded with CBD-VEGF, natural VEGF, or phosphate-buffered saline are used as cardiac patches to repair the infarcted myocardium in a rabbit model. CBD-VEGF/collagen group could effectively induce more cells to penetrate into the collagen membrane after 4 weeks and promote more vascularization in infarcted myocardium after 12 weeks compared with the other two control groups. Echocardiography and hemodynamic studies both show cardiac function improvement in the CBD-VEGF/collagen group. These results reveal that implantation of CBD-VEGF collagen membrane patch into the infarcted myocardium could effectively improve left ventricle cardiac function and increase the vascular density.

Applications of knitted mesh fabrication techniques to scaffolds for tissue engineering and regenerative medicine

Knitting is an ancient and yet, a fresh technique. It has a history of no less than 1,000 years. The development of tissue engineering and regenerative medicine provides a new role for knitting. Several meshes knitted from synthetic or biological materials have been designed and applied, either alone, to strengthen materials for the patching of soft tissues, or in combination with other kinds of biomaterials, such as collagen and fibroin, to repair or replace damaged tissues/organs. In the latter case, studies have demonstrated that knitted mesh scaffolds (KMSs) possess excellent mechanical properties and can promote more effective tissue repair, ligament/tendon/cartilage regeneration, pipe-like-organ reconstruction, etc. In the process of tissue regeneration induced by scaffolds, an important synergic relationship emerges between the three-dimensional microstructure and the mechanical properties of scaffolds. This paper presents a comprehensive overview of the status and future prospects of knitted meshes and its KMSs for tissue engineering and regenerative medicine.

Growth factor delivery-based tissue engineering: general approaches and a review of recent developments

The identification and production of recombinant morphogens and growth factors that play key roles in tissue regeneration have generated much enthusiasm and numerous clinical trials, but the results of many of these trials have been largely disappointing. Interestingly, the trials that have shown benefit all contain a common denominator, the presence of a material carrier, suggesting strongly that spatio-temporal control over the location and bioactivity of factors after introduction into the body is crucial to achieve tangible therapeutic effect. Sophisticated materials systems that regulate the biological presentation of growth factors represent an attractive new generation of therapeutic agents for the treatment of a wide variety of diseases. This review provides an overview of growth factor delivery in tissue engineering. Certain fundamental issues and design strategies relevant to the material carriers that are being actively pursued to address specific technical objectives are discussed. Recent progress highlights the importance of materials science and engineering in growth factor delivery approaches to regenerative medicine.

Nanomaterials can dynamically steer cell responses to biological ligands

Traditional tissue regeneration approaches to activate cell behaviors on biomaterials rely on the use of extracellular-matrix-based or soluble growth-factor cues. In this article, a novel approach is highlighted to dynamically steer cellular phenomena such as cell motility based on nanoscale substratum features of biological ligands. Albumin-derived nanocarriers (ANCs) with variable nanoscale-size features are functionalized with fibronectin III9-10 matrix ligands, and their effects on primary human keratinocyte activation are investigated. The presentation of fibronectin fragments from ANCs significantly enhances cell migration as compared to free ligands at equivalent concentrations. Notably, cell migration is influenced by the size of the underlying ANCs even for variably sized ANCs covered in comparable levels of fibronectin fragment. For equivalent ligand concentrations, cell migration on the smaller-sized ANCs (30 and 50 nm) is significantly enhanced as compared to that on larger-sized ANCs (75 and 100 nm). In contrast, the enhancement of cell migration on nanocarriers is abolished by the use of immobilized, biofunctionalized ANCs, indicating that "dynamic" nanocarrier internalization events underlie the role of nanocarrier geometry on the differential regulation of cell migration kinetics. Uptake studies using fluorescent ANCs indicate that larger-sized ANCs cause delayed endocytic kinetics and hence could present barriers for internalization during the cell adhesion and motility processes. Motile cells exhibit diminished migration upon exposure to clathrin inhibitors, but not caveolin inhibitors, suggesting the role of clathrin-mediated endocytosis in facilitating cell migratory responsiveness to the nanocarriers. Overall, a monotonic relationship is found between the nanocarrier cytointernalization rate and the cell migration rate, suggesting the possibility of designing biointerfacial features for the dynamic control of cell migration. Thus, the functionalization of a mobile nanocarrier by a biorelevant ligand can be used to sensitize cellular motility activation to the adhesion ligands, and such nanocarrier interfaces can dynamically attune cell migration kinetics by modulating the uptake of the ligand-nanocarrier complex via nanocarrier size.

Cell delivery: from cell transplantation to organ engineering

Cell populations derived from adult tissue and stem cells possess a great expectation for the treatment of several diseases. Great efforts have been made to generate cells with therapeutic impact from stem cells. However, it is clear that the development of systems to deliver such cells to induce efficient engraftment, growth, and function is a real necessity. Biologic and artificial scaffolds have received significant attention for their potential therapeutic application when use to form tissues in vitro and facilitate engraftment in vivo. Ultimately more sophisticated methods for decellularization of organs have been successfully used in tissue engineering and regenerative medicine applications. These decellularized tissues and organs appear to provide bioactive molecules and bioinductive properties to induce homing, differentiation, and proliferation of cells. The combination of decellularized organs and stem cells may dramatically improve the survival, engraftment, and fate control of transplanted stem cells and their ultimate clinical utility, opening the doors to a new era of organ engineering.

Generation of stable co-cultures of vascular cells in a honeycomb alginate scaffold

Scaffold-guided vascular tissue engineering has been investigated as a means to generate functional and transplantable vascular tissue grafts that increase the efficacy of cell-based therapeutic strategies in regenerative medicine. In this study, we employed confocal microscopy and three-dimensional reconstruction to assess the engraftment and growth potential of vascular cells within an alginate scaffold with aligned pores. We fabricated honeycomb alginate scaffolds with aligned pores, whose surface was immobilized with fibronectin and subsequently coated with matrigel. Endothelial cells were seeded into aligned pore scaffolds in the presence and absence of human smooth muscle cells. We showed that endothelial cells seeded into alginate scaffolds attach on the surface of aligned pores in vitro, giving rise to stable co-cultures of vascular cells. Moreover, the three-dimensional alginate depots containing the cells were exposed to laminar flow in order to recapitulate physiological shear stress found in the vasculature in vivo. After the flow exposure, the scaffold remained intact and some cells remained adherent to the scaffold and aligned in the flow direction. These studies demonstrate that alginate scaffolds provide a suitable matrix for establishing durable angiogenic modules that may ultimately enhance organ revascularization.

Controlling stiffness in nanostructured hydrogels produced by enzymatic dephosphorylation

In the present paper, we report on enzyme-initiated self-assembly of Fmoc (fluoren-9-ylmethoxycarbonyl)-tyrosine hydrogels by enzymatic dephosphorylation under physiological conditions and provide evidence for the ability to control the modulus. Upon enzyme action, a self-assembling network of interconnecting fibres is formed, observed by cryo-SEM (scanning electron microscopy) and TEM (transmission electron microscopy). The concentration of alkaline phosphatase added to the Fmoc-tyrosine phosphate ester precursor solution had a direct effect on the gelation time, mechanical properties and molecular arrangements as determined through oscillatory rheology, fluorescence spectroscopy and CD spectroscopy. This highly tuneable cost-effective gel system may have applications in three-dimensional cell culture.

Drug releasing systems in cardiovascular tissue engineering

Heart disease and atherosclerosis are the leading causes of morbidity and mortality worldwide. The lack of suitable autologous grafts has produced a need for artificial grafts; however, current artificial grafts carry significant limitations, including thrombosis, infection, limited durability and the inability to grow. Tissue engineering of blood vessels, cardiovascular structures and whole organs is a promising approach for creating replacement tissues to repair congenital defects and/or diseased tissues. In an attempt to surmount the shortcomings of artificial grafts, tissue-engineered cardiovascular graft (TECVG), constructs obtained using cultured autologous vascular cells seeded onto a synthetic biodegradable polymer scaffold, have been developed. Autologous TECVGs have the potential advantages of growth, durability, resistance to infection, and freedom from problems of rejection, thrombogenicity and donor scarcity. Moreover polymers engrafted with growth factors, cytokines, drugs have been developed allowing drug-releasing systems capable of focused and localized delivery of molecules depending on the environmental requirements and the milieu in which the scaffold is placed. A broad range of applications for compound-releasing, tissue-engineered grafts have been suggested ranging from drug delivery to gene therapy. This review will describe advances in the development of drug-delivery systems for cardiovascular applications focusing on the manufacturing techniques and on the compounds delivered by these systems to date.

Biomaterial technology for tissue engineering applications

Tissue engineering is a newly emerging biomedical technology and methodology to assist and accelerate the regeneration and repairing of defective and damaged tissues based on the natural healing potentials of patients themselves. For the new therapeutic strategy, it is indispensable to provide cells with a local environment that enhances and regulates their proliferation and differentiation for cell-based tissue regeneration. Biomaterial technology plays an important role in the creation of this cell environment. For example, the biomaterial scaffolds and the drug delivery system (DDS) of biosignalling molecules have been investigated to enhance the proliferation and differentiation of cell potential for tissue regeneration. In addition, the scaffold and DDS technologies contribute to develop the basic research of stem cell biology and medicine as well as obtain a large number of cells with a high quality for cell transplantation therapy. A technology to genetically engineer cells for their functional manipulation is also useful for cell research and therapy. Several examples of tissue engineering applications with the cell scaffold and DDS of growth factors and genes are introduced to emphasize the significance of biomaterial technology in new therapeutic and research fields.

Cell delivery mechanisms for tissue repair

Many cell populations, derived from both adult tissues and embryonic stem cells, show promise for the treatment of a variety of diseases. Although the major effort in stem cell therapies in the past has been identifying potentially therapeutic cells, it is now clear that developing systems to deliver these cells and promote their efficient engraftment will provide an equally challenging task. More sophisticated pretransplantation manipulations and material carriers may dramatically improve the survival, engraftment, and fate control of transplanted stem cells and their ultimate clinical utility.

Self-crosslinkable hydrogels composed of partially oxidized hyaluronan and gelatin: in vitro and in vivo responses

Self-crosslinkable hydrogels had been formulated from two precursors, partially oxidized hyaluronan (oHA) and gelatin. The physicochemical properties of the resulting hydrogels have been elucidated by instrumental analyses (FTIR, SEM, and rheometry). These hydrogels were highly porous with an average pore size of 60 microm, and evidently, accommodative to cell infiltration. Increasing the oxidation degree of oHA resulted in corresponding increases in hydrogels' storage moduli and decreases in water uptake. Dermal fibroblasts were used to study the cell-hydrogel interactions in vitro. Both the hydrogels and their degradation byproducts are biocompatible as indicated by long-term cell viability assay. In addition, significant amount of cells migrated into the hydrogels and they aligned into highly organized arrays. When cultured with cells, the hydrogels underwent degradation within 4 weeks depending on composition with obvious loss of cohesiveness over time. The good biocompatibility and biodegradability of oHA/gelatin hydrogel were further demonstrated in mice subdermal implantations. Lastly, in vitro and in vivo depositions of extracellular matrix in hydrogels by cells were demonstrated by SEM analyses.

Developmental biology and tissue engineering

Morphogenesis implies the controlled spatial organization of cells that gives rise to tissues and organs in early embryonic development. While morphogenesis is under strict genetic control, the formation of specialized biological structures of specific shape hinges on physical processes. Tissue engineering (TE) aims at reproducing morphogenesis in the laboratory, i.e., in vitro, to fabricate replacement organs for regenerative medicine. The classical approach to generate tissues/organs is by seeding and expanding cells in appropriately shaped biocompatible scaffolds, in the hope that the maturation process will result in the desired structure. To accomplish this goal more naturally and efficiently, we set up and implemented a novel TE method that is based on principles of developmental biology and employs bioprinting, the automated delivery of cellular composites into a three-dimensional (3D) biocompatible environment. The novel technology relies on the concept of tissue liquidity according to which multicellular aggregates composed of adhesive and motile cells behave in analogy with liquids: in particular, they fuse. We emphasize the major role played by tissue fusion in the embryo and explain how the parameters (surface tension, viscosity) that govern tissue fusion can be used both experimentally and theoretically to control and simulate the self-assembly of cellular spheroids into 3D living structures. The experimentally observed postprinting shape evolution of tube- and sheet-like constructs is presented. Computer simulations, based on a liquid model, support the idea that tissue liquidity may provide a mechanism for in vitro organ building.

Controlled drug delivery in tissue engineering

The concept of tissue and cell guidance is rapidly evolving as more information regarding the effect of the microenvironment on cellular function and tissue morphogenesis become available. These disclosures have lead to a tremendous advancement in the design of a new generation of multifunctional biomaterials able to mimic the molecular regulatory characteristics and the three-dimensional architecture of the native extracellular matrix. Micro- and nano-structured scaffolds able to sequester and deliver in a highly specific manner biomolecular moieties have already been proved to be effective in bone repairing, in guiding functional angiogenesis and in controlling stem cell differentiation. Although these platforms represent a first attempt to mimic the complex temporal and spatial microenvironment presented in vivo, an increased symbiosis of material engineering, drug delivery technology and cell and molecular biology may ultimately lead to biomaterials that encode the necessary signals to guide and control developmental process in tissue- and organ-specific differentiation and morphogenesis.

Fibronectin, vitronectin, and collagen I induce chemotaxis and haptotaxis of human and rabbit mesenchymal stem cells in a standardized transmembrane assay

The mesenchymal stem cell (MSC) is a critical element in tissue repair and regeneration. Its ability to differentiate into multiple connective tissue cell types and to self-renew has made it a prime candidate in regenerative medicine strategies. Currently, the environmental cues responsible for in situ recruitment and control of MSC distribution at repair sites are not entirely revealed and in particular the role of extracellular matrix (ECM) proteins as motogenic factors has not been studied. Here we have used a standardized transmembrane chemotaxis assay to assess the chemotactic and haptotactic potential of fibronectin, vitronectin, and collagen type 1 on MSCs from both rabbit and human origin. The use of both cell types was based in part on the widespread use of rabbit models for musculoskeletal-related tissue engineering and repair models and their unknown correspondence to human in terms of MSC migration. The optimized assay yielded a greatly increased chemotactic response toward known factors such as platelet-derived growth factor-BB (PDGF)-BB compared to previous studies. Our primary finding was that all three ECM proteins tested (fibronectin, vitronectin, and collagen I) induced significant motogenic activity, in both soluble and insoluble forms, for both rabbit and human MSCs. These results suggest that ECM proteins could play roles as significant as cytokines in the recruitment of pluripotential repair cells wound and tissue repair sites. Furthermore, designed ECM coatings of scaffolds or implants could provide a new tool to control both cell influx and outflux from the scaffold post-implantation. Finally, the similarity of motogenic behavior of both rabbit and human cells suggests the rabbit is a reliable model for assessing MSC recruitment in repair and regeneration strategies.

Experimental tissue regeneration by DDS technology of bio-signaling molecules

The medical therapy of tissue regeneration achieved by biomaterial-based tissue engineering has been currently expected as the third option following reconstructive surgery and organ transplantation. The basic idea of this regenerative therapy is to assist the self-healing potentials of body to induce the natural regeneration and repairing of defective or injured tissue. To this end, it is practically important to create a local environment which enables cells to promote their proliferation and differentiation, resulting in the induction of cell-based tissue regeneration. Tissue engineering is a biomedical technology or methodology to build up this regeneration environment by making use of biomaterials. Drug delivery system (DDS) is a biomaterial technology to enhance the in vivo biological functions of bio-signaling molecules (growth factors and genes) for promoted tissue regeneration. This paper overviews the recent status of tissue regeneration therapy based on the DDS technology of bio-signaling molecules.

Engineering the follicle microenvironment

In vitro ovarian follicle culture provides a tool to investigate folliculogenesis, and may one day provide women with fertility-preservation options. The application of tissue engineering principles to ovarian follicle maturation may enable the creation of controllable microenvironments that will coordinate the growth of the multiple cellular compartments within the follicle. Three-dimensional culture systems can preserve follicle architecture, thereby maintaining critical cell-cell and cell-matrix signaling lost in traditional two-dimensional attached follicle culture systems. Maintaining the follicular structure while manipulating the biochemical and mechanical environment will enable the development of controllable systems to investigate the fundamental biological principles underlying follicle maturation. This review describes recent advances in ovarian follicle culture, and highlights the tissue engineering principles that may be applied to follicle culture, with the ultimate objective of germline preservation for females facing premature infertility.

Spatiotemporal control of vascular endothelial growth factor delivery from injectable hydrogels enhances angiogenesis

Therapeutic angiogenesis with vascular endothelial growth factor (VEGF) delivery may provide a new approach for the treatment of ischemic diseases, but current strategies to deliver VEGF rely on either bolus delivery or systemic administration, resulting in limited clinical utility, because of the short half-life of VEGF in vivo and its resultant low and transient levels at sites of ischemia. We hypothesize that an injectable hydrogel system can be utilized to provide temporal control and appropriate spatial biodistribution of VEGF in ischemic hindlimbs. A sustained local delivery of relatively low amounts of bioactive VEGF (3 mug) with this system led to physiologic levels of bioactive VEGF in ischemic murine (ApoE(-/-)) hindlimbs for 15 days after injection of the gel, as contrasted with complete VEGF deprivation after 72 h with bolus injection. The gel delivery system resulted in significantly greater angiogenesis in these limbs as compared to bolus (266 vs. 161 blood vessels mm(-2)). Laser Doppler perfusion imaging showed return of tissue perfusion to normal levels by day 28 with the gel system, whereas normal levels of perfusion were never achieved with saline delivery of VEGF or in control mice. The system described in this article could represent an attractive new generation of therapeutic delivery vehicle for treatment of cardiovascular diseases, as it combines long-term in vivo therapeutic benefit (localized bioactive VEGF for 1-2 weeks) with minimally invasive delivery.

Synthesis and evaluation of injectable, in situ crosslinkable synthetic extracellular matrices for tissue engineering

Simple and effective biocompatible materials that mimic the natural extracellular matrix (ECM) were developed for a variety of uses in regenerative medicine. These synthetic ECMs (sECMs) were designed to recapitulate the minimal composition required to obtain functional ECMs. The sECM components are crosslinkable in situ, and may be seeded with cells prior to injection in vivo, without compromising either the cells or the recipient tissues. Several sECM compositions were evaluated to establish which formulation would be most beneficial for cell growth and tissue remodeling. Three natural ECM macromonomeric building blocks were employed: hyaluronan (HA), chondroitin sulfate (CS), and gelatin (Gtn). The carboxyl-rich glycosaminoglycans and Gtn were each chemically modified to give the corresponding thiolated dithiopropionylhydrazide (DTPH) derivatives (CS-DTPH, HA-DTPH, and Gtn-DTPH). Different compositions of CS-Gtn and HA-Gtn hydrogels were fabricated by crosslinking the thiolated biomacromonomers with polyethylene glycol diacrylate. Each sECM had high water content (>96%), biologically suitable mechanical properties, and a useful gelation time ( approximately 2-6 min). The bioerosion rates for the sECMs were determined, and a given composition could be selected to meet the requirements of a given clinical application. Both the HA-Gtn and CS-Gtn sECM hydrogels supported cell growth and proliferation with cultured murine fibroblasts in vitro. Moreover, subcutaneous injection of a suspension of murine fibroblasts in each of the two sECM hydrogels into nude mice in vivo resulted in the formation of viable and uniform soft tissue in vivo.

Rational design of hydrogels for tissue engineering: Impact of physical factors on cell behavior

When designing suitable biomaterials for tissue-engineering applications, biological and chemical parameters are frequently taken into account, while the equally important physical design variables have often been neglected. For a rational design of biomaterials, however, all variables influencing cell function and tissue morphogenesis have to be considered. This review will stress the development of cross-linked hydrogels and outline the impact of their physical properties on cell function and tissue morphogenesis. In the first part, the principles of cellular mechanosensitivity, as well as the influence of substrate mechanics on cell behavior, will be discussed. Afterwards, methods to characterize the mechanical properties of biomaterials will be presented. The subsequent chapters will address hydrogels that allow for the control of their physical qualities followed by a discussion of their use in tissue-engineering applications.

Role of matrix metalloproteinases in delayed cortical responses after stroke

Matrix metalloproteinases (MMPs) are zinc-endopeptidases with multifactorial actions in central nervous system (CNS) physiology and pathology. Accumulating data suggest that MMPs have a deleterious role in stroke. By degrading neurovascular matrix, MMPs promote injury of the blood-brain barrier, edema and hemorrhage. By disrupting cell-matrix signaling and homeostasis, MMPs trigger brain cell death. Hence, there is a movement toward the development of MMP inhibitors for acute stroke therapy. But MMPs may have a different role during delayed phases after stroke. Because MMPs modulate brain matrix, they may mediate beneficial plasticity and remodeling during stroke recovery. Here, we show that MMPs participate in delayed cortical responses after focal cerebral ischemia in rats. MMP-9 is upregulated in peri-infarct cortex at 7-14 days after stroke and is colocalized with markers of neurovascular remodeling. Treatment with MMP inhibitors at 7 days after stroke suppresses neurovascular remodeling, increases ischemic brain injury and impairs functional recovery at 14 days. MMP processing of bioavailable VEGF may be involved because inhibition of MMPs reduces endogenous VEGF signals, whereas additional treatment with exogenous VEGF prevents MMP inhibitor-induced worsening of infarction. These data suggest that, contrary to MMP inhibitor therapies for acute stroke, strategies that modulate MMPs may be needed for promoting stroke recovery.

Laminin peptide-conjugated chitosan membrane: Application for keratinocyte delivery in wounded skin

Tissue engineering requires the delivery and survival of cells to organ sites needing repair. Previously, we showed that an active laminin peptide (AG73: RKR-LQVQLSIRT)-conjugated chitosan membrane promoted cell adhesion and spreading in vitro. Here, we seeded human keratinocytes onto AG73-chitosan membranes and found that nearly 80% of the cells were attached to the membranes within 2 h. The membranes carrying the keratinocytes were inverted and placed onto exposed muscle fascia on the backs of nude mice. After 3 days, the keratinocytes had migrated from the membrane and established a stratified epidermis-like structure on the fascia. Cells recognize the AG73 through transmembrane proteoglycan syndecans, which recognition system has not previously been tested in tissue engineering applications. We suggest that the AG73-chitosan membrane is useful as a therapeutic formulation and is applicable as a cell delivery system such as delivering keratinocytes to a wound bed.

Significance of release technology in tissue engineering

Regenerative medical therapy has been expected to compensate for the therapeutic disadvantages of reconstructive surgery and organ transplantation, as well as offering a new therapeutic strategy. The objective of regenerative medical therapy is to induce the repair of defective tissues based on the natural healing potential of patients. For successful tissue regeneration, it is indispensable to provide cells with a local environment of artificial extracellular matrix where they can proliferate and differentiate efficiently. Tissue engineering is the key to this regeneration environment; release technology often enhances the in vivo stability of growth factors and related genes and prolongs the maintenance of biological functions for tissue regeneration.