Cell responses to biomimetic protein scaffolds used in tissue repair and engineering

The Construction of Three-Layered Biomimetic Arterial Graft Balances Biomechanics and Biocompatibility for Dynamic Biological Reconstruction

The process of reconstructing an arterial graft is a complex and dynamic process that is subject to the influence of various mechanical factors, including tissue regeneration and blood pressure. The attainment of favorable remodeling outcomes is contingent upon the biocompatibility and biomechanical properties of the arterial graft. A promising strategy involves the emulation of the three-layer structure of the native artery, wherein the inner layer is composed of polycaprolactone (PCL) fibers aligned with blood flow, exhibiting excellent biocompatibility that fosters endothelial cell growth and effectively prevents platelet adhesion. The middle layer, consisting of PCL and polyurethane (PU), offers mechanical support and stability by forming a contractile smooth muscle ring and antiexpansion PU network. The outer layer, composed of PCL fibers with an irregular arrangement, promotes the growth of nerves and pericytes for long-term vascular function. Prioritizing the reconstruction of the inner and outer layers establishes a stable environment for intermediate smooth muscle growth. Our three-layer arterial graft is designed to provide the blood vessel with mechanical support and stability through nondegradable PU, while the incorporation of degradable PCL generates potential spaces for tissue ingrowth, thereby transforming our graft into a living implant.

Bioactive injectable hydrogels for on demand molecule/cell delivery and for tissue regeneration in the central nervous system

Currently there are no potential curative therapies that can improve the central nervous system (CNS) regeneration after traumatic injuries or diseases. Indeed, the regeneration of CNS is greatly impaired by limited drug penetration across the blood brain barrier (BBB), poor drug targeting, deficient progenitor neural cells and limited proliferation of mature neural cells. To overcome these limitations, bioengineered injectable hydrogels in combination with drug and cell therapy have been proposed to mimic the complexity of the CNS microenvironment and architecture. Additionally, to enhance relevant CNS regeneration, proper biophysical and biochemical cues are needed. Recently, great efforts have been devoted to tailor stimuli-responsive hydrogels as novel carrier systems which are able to guide neural tissue regeneration. This review provides an extensive overview on the most promising injectable hydrogels for neural tissue engineering. A special emphasis is made to highlight the ability of these hydrogels to deliver bioactive compounds/cells upon the exposure to internal and external stimuli. Bioactive injectable hydrogels have a broad application in central nervous system's (CNS) regeneration. This review gives an overview of the latest pioneering approaches in CNS recovery using stimuli-responsive hydrogels for several neurodegenerative disorders. STATEMENT OF SIGNIFICANCE: This review summarizes the latest innovations on bioactive injectable hydrogels, focusing on tailoring internal/external stimuli-responsive hydrogels for the new injectable systems design, able to guide neural tissue response. The purpose is to highlight the advantages and the limitations of thermo-responsive, photo responsive, magnetic responsive, electric responsive, ultrasound responsive and enzymes-triggered injectable hydrogels in developing customizable neurotherapies. We believe that this comprehensive review will help in identifying the strengths and gaps in the existing literature and to further support the use of injectable hydrogels in stimulating CNS regeneration.

Flexible, Biocompatible PET Sheets: A Platform for Attachment, Proliferation and Differentiation of Eukaryotic Cells

Transparent, flexible, biaxially oriented polyethylene terephthalate (PET) sheets were modified by bioactive polymer-fibronectin top layers for the attachment of cells and growth of muscle fibers. Towards this end, PET sheets were grafted with 4-(dimethylamino)phenyl (DMA) groups from the in situ generated diazonium cation precursor. The arylated sheets served as macro-hydrogen donors for benzophenone and the growth of poly(2-hydroxy ethyl methacrylate) (PHEMA) top layer by surface-confined free radical photopolymerization. The PET-PHEMA sheets were further grafted with fibronectin (FBN) through the 1,1-carbonyldiimidazole coupling procedure. The bioactive PET-PHEMA-I-FBN was then employed as a platform for the attachment, proliferation and differentiation of eukaryotic cells which after a few days gave remarkable muscle fibers, of ~120 µm length and ~45 µm thickness. We demonstrate that PET-PHEMA yields a fast growth of cells followed by muscle fibers of excellent levels of differentiation compared to pristine PET or standard microscope glass slides. The positive effect is exacerbated by crosslinking PHEMA chains with ethylene glycol dimethacrylate at initial HEMA/EGDA concentration ratio = 9/1. This works conclusively shows that in situ generated diazonium salts provide aryl layers for the efficient UV-induced grafting of biocompatible coating that beneficially serve as platform for cell attachment and growth of muscle fibers.

A collagen-silica-based biocomposite for potential application in bone tissue engineering

Bone is a hierarchical material that has inspired the design of biopolymer-derived biocomposites for tissue engineering purposes. The present study sought to synthesize and perform the physicochemical characterization and biocompatibility of a collagen-silica-based biocomposite for potential application in bone tissue engineering. Ultrastructure, biodegradability, swelling behavior, and biocompatibility properties were analyzed to gain insight into the advantages and limitations to the use of this biomaterial as a bone substitute. Scanning electron microscopy analysis showed a packed-collagen fibril matrix and silica particles in the biocomposite three-dimensional structure. As shown by analysis of in vitro swelling behavior and biodegradability, it would seem that the material swelled soon after implantation and then suffered degradation. Biocompatibility properties were analyzed in vivo 14-days postimplantation using an experimental model in Wistar rats. The biocomposite was placed inside the hematopoietic bone marrow compartment of both tibiae (n = 16). Newly formed woven bone was observed in response to both materials. Unlike the pure-collagen-tissue interface, extensive areas of osseointegration were observed at the biocomposite-tissue interface, which would indicate that silica particles stimulated new bone formation. Agglomerates of finely particulate material with no inflammatory infiltrate or multinucleated giant cells were observed in the bone marrow implanted with the biocomposite. The biocomposite showed good biocompatibility properties. Further studies are necessary to evaluate their biological behavior over time.

'EngNT' - Engineering live neural tissue for nerve replacement

Peripheral nerve injury can result in severe long-term disability and current clinical approaches for repairing large gaps rely on the nerve autograft. Engineered Neural Tissue (EngNT) has been developed to provide living aligned therapeutic cells in a stabilised collagen hydrogel, mimicking the key features of the autograft. This Perspective article will introduce the field and discuss the current stage of translation, highlighting the key opportunities for commercial and clinical development.

Natural Biomaterials as Instructive Engineered Microenvironments That Direct Cellular Function in Peripheral Nerve Tissue Engineering

Nerve tissue function and regeneration depend on precise and well-synchronised spatial and temporal control of biological, physical, and chemotactic cues, which are provided by cellular components and the surrounding extracellular matrix. Therefore, natural biomaterials currently used in peripheral nerve tissue engineering are selected on the basis that they can act as instructive extracellular microenvironments. Despite emerging knowledge regarding cell-matrix interactions, the exact mechanisms through which these biomaterials alter the behaviour of the host and implanted cells, including neurons, Schwann cells and immune cells, remain largely unclear. Here, we review some of the physical processes by which natural biomaterials mimic the function of the extracellular matrix and regulate cellular behaviour. We also highlight some representative cases of controllable cell microenvironments developed by combining cell biology and tissue engineering principles.

In ovariectomy-induced osteoporotic rat models, BMP-2 substantially reversed an impaired alveolar bone regeneration whereas PDGF-BB failed

OBJECTIVES

We previously suggested an ovariectomy (OVX)-induced osteoporotic rat model showing an impaired alveolar bone defect healing. This study aimed to evaluate and compare the effects of recombinant human bone morphogenetic protein-2 (rhBMP-2) and recombinant human platelet-derived growth factor-BB (rhPDGF-BB) on alveolar bone defect healing in OVX-induced osteoporotic rats.

MATERIALS AND METHODS

A total of forty-one female rats were divided into four groups: a collagen group (n=10), a PDGF-BB group (n=11), a BMP-2 group (n=10), and a control group (n=10). Four months after OVX, alveolar bone drill-hole defects were created and grafted with collagen gel, rhPDGF-BB/collagen gel, or rhBMP-2/collagen gel. The defects in the control group were not grafted with any material. Defect healing was evaluated by histological, histomorphometric, and microcomputed tomographic (micro-CT) analyses at 2 and 4 weeks.

RESULTS

According to the micro-CT analysis, the BMP-2 group exhibited the greatest bone volume fraction among all groups, while the PDGF-BB group did not show significant differences compared with the collagen group. The histomorphometric analysis showed a significantly larger amount of new bone area in the BMP-2 group than in the control and collagen groups at 4 weeks; however, the PDGF-BB group did not reach significant superiority compared with the other groups.

CONCLUSIONS

Alveolar bone regeneration was significantly enhanced by the local use of rhBMP-2/collagen gel compared with the use of rhPDGF-BB/collagen gel in OVX-induced osteoporotic rats.

CLINICAL RELEVANCE

A treatment modality using rhBMP-2 may be a promising approach to promote alveolar bone regeneration in patients suffering from postmenopausal osteoporosis.

Kaolin-loaded chitosan/polyvinyl alcohol electrospun scaffold as a wound dressing material: in vitro and in vivo studies

OBJECTIVE

To evaluate the application of a fabricated dressing containing kaolin for skin regeneration in a rat model of excisional wounds.

METHOD

In the present study, kaolin was loaded into electrospun polyvinyl alcohol (PVA)/chitosan polymer blend to develop a composite nanofibrous dressing. To make the yarns, kaolin with weight ratio of 5% was added to PVA/chitosan polymer blend and subsequently formed into nanofibres using the electrospinning method. Scaffolds were evaluated for to their microstructure, mechanical properties, surface wettability, water vapour transmission rate, water-uptake capacity, blood uptake capacity, blood compatibility, microbial penetration test, the number of colonies, and cellular response with the L929 cell line. Rats with full-thickness excisional wounds were treated with kaolin-containing and kaolin-free dressings.

RESULTS

The study showed that rats treated with the kaolin-incorporated mats demonstrated a significant closure to nearly 97.62±4.81% after 14 days compared with PVA/chitosan and the sterile gauze, which showed 86.15±8.11% and 78.50±4.22% of wound closure, respectively. The histopathological studies showed that in the PVA/chitosan/kaolin group, dense and regular collagen fibres were formed, while wounds treated with sterile gauze or PVA/chitosan scaffolds had random and loose collagen fibres.

CONCLUSION

Our results show the potential applicability of PVA/chitosan/kaolin scaffolds as a wound care material.

Radiation cross-linked gelatin/sodium alginate/carboxymethylcellulose sodium hydrogel for the application as debridement glue paste

Autolytic debridement can accelerate wound healing by removing necrotic tissue. A hydrogel was fabricated from an aqueous solution of gelatin, sodium alginate and carboxymethylcellulose sodium by radiation-induced cross-linking at room temperature, which was aiming at the application of debridement glue paste. The swelling ratio of the debridement glue paste is 30 times to its dry weight, when the weight ratio of gelatin/sodium alginate/carboxymethylcellulose sodium was 2:2:2 and the absorbed dose was 20 kGy, with dose rate of 20 Gy/min. The extrusion and compressive assay have confirmed that it possessed stable mechanical strength, and the weight ratio had little effect on the molecular structure by FTIR and TGA. Cell culture experiments demonstrated the debridement glue pastes with the cytotoxicity of grade 0 or 1 (biosafe). The debridement glue paste group could remove the necrotic tissue within 4 days and showed complete wound healing within 18 days; comparatively, the control group without treatment removed the necrotic tissue within 10 days and showed complete wound healing within 26 days in animal experiments using rabbit scald model. Histologic analysis exhibited that more granulation tissue was observed in debridement glue paste. The result of this study suggested that debridement glue pastes had excellent biocompatibility, could selectively remove necrotic tissue, induced granulation tissue formation and accelerated the wound healing.

Distinct differences in hypoxic responses between human oral mucosa and skin fibroblasts in a 3D collagen matrix

The differences between oral mucosa and skin wound healing involving hypoxic responses of fibroblasts are poorly elucidated. In this study, we aimed to study the different hypoxic responses between oral and skin fibroblasts embedded in a three-dimensional (3D) collagen matrix to address the early stage of wound healing. Primary oral mucosa fibroblasts (OMFs) obtained from the retromolar area and skin fibroblasts (SFs) obtained from the abdomen were cultured in the 3D 'floating model' under either 21%, 5% or 1% O₂ for 2 days. Cell viability under hypoxia was higher in the OMFs than in the SFs. Collagen gel contraction was suppressed under hypoxic conditions in both fibroblasts, consistent with the reduction of alpha smooth muscle actin expression, except for SFs under 1% O₂. Subsequently, their gene expression profiles between 21 and 1% O₂ concentrations were compared via microarray technology, and the expression profiles of the extracellular matrix (ECM)-associated proteins, including matrix metalloproteinases and collagens, were evaluated. The OMFs were more susceptible to 1% O₂, and more of their genes were downregulated than the SFs'. Although the production and expression levels of ECM-associated proteins in both fibroblasts diminished under hypoxia, those levels in OMFs were significantly higher than those in SFs. In the case of single origin OMFs and SFs, our findings suggest that OMFs possess a higher baseline production capacity of several ECM-associated proteins than SFs, except type III collagen. The intrinsic hypoxic responses of OMFs may be attributed to a more favourable wound healing in oral mucosa.

Combining in silico and in vitro models to inform cell seeding strategies in tissue engineering

The seeding density of therapeutic cells in engineered tissue impacts both cell survival and vascularization. Excessively high seeded cell densities can result in increased death and thus waste of valuable cells, whereas lower seeded cell densities may not provide sufficient support for the tissue in vivo, reducing efficacy. Additionally, the production of growth factors by therapeutic cells in low oxygen environments offers a way of generating growth factor gradients, which are important for vascularization, but hypoxia can also induce unwanted levels of cell death. This is a complex problem that lends itself to a combination of computational modelling and experimentation. Here, we present a spatio-temporal mathematical model parametrized using in vitro data capable of simulating the interactions between a therapeutic cell population, oxygen concentrations and vascular endothelial growth factor (VEGF) concentrations in engineered tissues. Simulations of collagen nerve repair constructs suggest that specific seeded cell densities and non-uniform spatial distributions of seeded cells could enhance cell survival and the generation of VEGF gradients. These predictions can now be tested using targeted experiments.

[Biocompatibility and osteoinductive properties of collagen and fibronectin hydrogel impregnated with rhBMP-2]

The study aimed to demonstrate the biocompatibility and osteoinductive properties of a hydrogel based on highly purified collagen and fibronectin impregnated with rhBMP-2. In vitro and in vivo experiments have shown that the minimum effective dosage of rhBMP-2 is 10 μg/ml. The cytocompatibility of the collagen-fibronectin gel was determined using MTT test and staining with PKH-26. There was no inflammation reaction when the material was subcutaneously implanted in rats (n=30) in vivo. The collagen-fibronectin hydrogel containing 10 μg/ml rhBMP-2 showed high osteogenic properties. By the end of 28 days 8±4% of its volume was replaced by newly formed bone tissue in case of subcutaneous implantation, 17±10% in intramuscular implantation and 26±11% in intraosseous implantation in the calvarial critical-size. The optimal combination of biocompatible and osteogenic properties of collagen-fibronectin hydrogel impregnated with BMP-2 allows us to consider it as a promising basis for creating the new generation of osteoplastic materials for dentistry.

Magnetic-Assisted Cell Alignment within a Magnetic Nanoparticle-Decorated Reduced Graphene Oxide/Collagen 3D Nanocomposite Hydrogel

Hydrogel scaffolds are particularly interesting for applications in tissue engineering because of their ability to create a favorable environment which mimics in vivo conditions. However, the hierarchically ordered anisotropic structure which is found in many native tissues and cellular components is hard to achieve in 3D scaffolds. In this work, we report the incorporation of magnetic nanoparticle-decorated reduced graphene oxide (m-rGO) within a collagen hydrogel. This magneto-responsive m-rGO aligned within the collagen hydrogel during gelation with the application of a low external magnetic field. This nanocomposite hydrogel with magnetically aligned m-rGO flakes is capable of encapsulating neuroblastoma cells (SH-SY5Y), promoting cell differentiation and inducing oriented cell growth owing to its excellent biocompatibility and electrical conductivity. The directionally oriented and differentiated SH-SY5Y cells within the m-rGO collagen hydrogel showed propagation of calcium signal along the direction of orientation. This method can be applied to creating magnetically responsive materials with potential for various biomedical applications.

Therapeutic strategies for peripheral nerve injury: decellularized nerve conduits and Schwann cell transplantation

In recent years, the use of Schwann cell transplantation to repair peripheral nerve injury has attracted much attention. Animal-based studies show that the transplantation of Schwann cells in combination with nerve scaffolds promotes the repair of injured peripheral nerves. Autologous Schwann cell transplantation in humans has been reported recently. This article reviews current methods for removing the extracellular matrix and analyzes its composition and function. The development and secretory products of Schwann cells are also reviewed. The methods for the repair of peripheral nerve injuries that use myelin and Schwann cell transplantation are assessed. This survey of the literature data shows that using a decellularized nerve conduit combined with Schwann cells represents an effective strategy for the treatment of peripheral nerve injury. This analysis provides a comprehensive basis on which to make clinical decisions for the repair of peripheral nerve injury.

Accelerated Angiogenic Induction and Vascular Integration in a Novel Synthetic Scaffolding Matrix for Tissue Replacement

Purpose

Reduced or delayed neovascularization is a major obstacle with regard to tissue-engineered constructs. The aim of this study was to evaluate the early microvascular response to a novel degradable ε-caprolactone terpolymer matrix.

Methods

ε-caprolactone terpolymer matrices (Suprathel Plus®; Institute of Textile and Process Engineering, Denkendorf, Germany) were implanted into dorsal skinfold chambers of balb/c mice (n=10). Microcirculatory changes were observed by intravital fluorescence microscopy. Scaffolding matrices from PEGT/PBT copolymer were used as controls (n=10).

Results

The formation of de novo vascular networks within both scaffolding matrices was noted throughout the experiment. A vascular ingrowth of perfused microvessels into the matrices up to 600 μm apart from the edge was noted within 10 days of implantation. The earliest signs of neoangiogenesis were visible in ε-caprolactone terpolymer matrices on day 1. In both scaffolds the new developed vessels extended centripetally from the border of the matrices towards the center and anastomosed to form a perfused microvascular network. There was significantly earlier onset of vascularization, increased vascularized area and higher vessel density in ε-caprolactone terpolymer matrices compared to PEGT/PBT copolymer matrices were observed.

Conclusions

The scaffolding matrix from ε-caprolactone terpolymer allowed for an earlier and more intense induction of angiogenesis and displayed the tendency to vascularize more rapidly within a shorter period of time after transplantation compared to PEGT/PBT copolymer scaffolds, thus indicating its potential application for tissue engineering purposes.

VEGF nanoparticles repair the heart after myocardial infarction

Vascular endothelial growth factor (VEGF) is a well-characterized proangiogenic cytokine that has been shown to promote neovascularization in hearts of patients with ischemic heart disease but can also lead to adverse effects depending on the dose and mode of delivery. We investigated whether prolonged exposure to a low dose of VEGF could be achieved by encapsulating VEGF in polylactic coglycolic acid nanoparticles and whether treatment with VEGF-containing nanoparticles improved cardiac function and protected against left ventricular remodeling in the hearts of mice with experimentally induced myocardial infarction. Polylactic coglycolic acid nanoparticles with a mean diameter of ~113 nm were generated via double emulsion and loaded with VEGF; the encapsulation efficiency was 53.5 ± 1.7% (107.1 ± 3.3 ng VEGF/mg nanoparticles). In culture, VEGF nanoparticles released VEGF continuously for at least 31 days, and in a murine myocardial infarction model, VEGF nanoparticle administration was associated with significantly greater vascular density in the peri-infarct region, reductions in infarct size, and improvements in left ventricular contractile function 4 wk after treatment. Thus, our study provides proof of principle that nanoparticle-mediated delivery increases the angiogenic and therapeutic potency of VEGF for the treatment of ischemic heart disease. NEW & NOTEWORTHY Vascular endothelial growth factor (VEGF) is a well-characterized proangiogenic cytokine but has a short half-life and a rapid clearance rate. When encapsulated in nanoparticles, VEGF was released for 31 days and improved left ventricular function in infarcted mouse hearts. These observations indicate that our new platform increases the therapeutic potency of VEGF.

Polymeric scaffolds for three-dimensional culture of nerve cells: a model of peripheral nerve regeneration

Understanding peripheral nerve repair requires the evaluation of 3D structures that serve as platforms for 3D cell culture. Multiple platforms for 3D cell culture have been developed, mimicking peripheral nerve growth and function, in order to study tissue repair or diseases. To recreate an appropriate 3D environment for peripheral nerve cells, key factors are to be considered including: selection of cells, polymeric biomaterials to be used, and fabrication techniques to shape and form the 3D scaffolds for cellular culture. This review focuses on polymeric 3D platforms used for the development of 3D peripheral nerve cell cultures.

Influence of direct or indirect contact for the cytotoxicity and blood compatibility of spider silk

Spider silk became one of the most-researched biomaterials in the last years due to its unique mechanical strength and most favourable chemical composition for tissue engineering purposes. However, standardized analysis of cytocompatibility is missing. Therefore, the aim of this study was to investigate hemolysis, cytotoxicity of native spider silk as well as influences on the cell culture medium. Changes of cell culture medium composition, osmolarity as well as glucose and lactate content were determined via ELISA measurement. Possible hemolysis and cytotoxicity in vitro of spider silk were performed via measurement of hemoglobin release of human red blood cells or relative metabolic activity of L929 fibroblasts, respectively, according to international standard procedures. In ELISA measurement, no significant changes in medium composition could be found in this study. Spider silk was not hemolytic in direct and indirect testing. However, a borderline cytotoxicity according to definitions was found in indirect cytotoxicity testing. Nevertheless, in direct cytotoxicity testing, relative metabolic activity measurement revealed that spider silk is not cytotoxic under these conditions. This is the first study to conduct standardized tests regarding cytotoxicity and hemolysis of native spider silk, which might be considered inert in cell culture. As neither hemolysis nor cytotoxicity was found in direct contact in standardized procedures, safety in biomedical applications may be assumed. The indirect cytotoxicity seems to play a minor role in vivo. However, a borderline toxicity was revealed, suggesting potential leachables not yet identified. Displays one of the weaving frames used in this study after seeding with the single drop technique described herein.

Electrospun Polycaprolactone/Aloe Vera_Chitosan Nanofibrous Asymmetric Membranes Aimed for Wound Healing Applications

Today, none of the wound dressings available on the market is fully capable of reproducing all the features of native skin. Herein, an asymmetric electrospun membrane was produced to mimic both layers of skin. It comprises a top dense layer (manufactured with polycaprolactone) that was designed to provide mechanical support to the wound and a bottom porous layer (composed of chitosan and Aloe Vera) aimed to improve the bactericidal activity of the membrane and ultimately the healing process. The results obtained revealed that the produced asymmetric membranes displayed a porosity, wettability, as well as mechanical properties similar to those presented by the native skin. Fibroblast cells were able to adhere, spread, and proliferate on the surface of the membranes and the intrinsic structure of the two layers of the membrane is capable of avoiding the invasion of microorganisms while conferring bioactive properties. Such data reveals the potential of these asymmetric membranes, in the near future, to be applied as wound dressings.

Biomimetic Heparan Sulfate-Like Coated ePTFE Grafts Reduce In-graft Neointimal Hyperplasia in Ovine Carotids

BACKGROUND

Thrombogenicity and neointimal hyperplasia are major causes for synthetic vascular graft failure. Bioactive coatings like heparin have improved patency by reducing thrombogenicity, but neointimal hyperplasia still remains an unsolved problem. Surface coatings with heparan sulfate (HS), the major component of the glycocalyx of endothelial cells, have shown reduced platelet and cell adhesion in vitro. The aim of the study was to evaluate the in vivo surface properties of expanded ePTFE vascular grafts with a semisynthetic HS-like coating (SSHS).

METHODS

ePTFE vascular grafts (n = 16, diameter 3.5 mm) covalently coated with SSHS were compared with uncoated grafts (n = 16) of the same diameter in a carotid interposition model in 16 sheep. The grafts were harvested at 20 wk for histological and morphometric analysis.

RESULTS

SSHS-coated grafts showed less neointima formation than uncoated grafts (P < 0.001). There was no evidence for cell or protein adhesion to SSHS-coated grafts, whereas the surface of uncoated ePTFE grafts was covered with a confluent circular layer of neointima. No difference was found concerning reactions at the anastomotic site of the genuine carotid vessel, both groups displayed neointimal hyperplasia.

CONCLUSIONS

ePTFE grafts covalently coated with a semisynthetic SSHS-glycosaminoglycan successfully mimicked the endothelial glycocalyx. They displayed excellent antiadhesive properties preventing neointimal formation on the graft surface. The results indicate that a biomimetic SSHS coating may be a useful component of bioengineered grafts and an alternative to synthetic surfaces and endothelial seeding.

Nerve Guidance by a Decellularized Fibroblast Extracellular Matrix

Spinal cord and peripheral nerve injuries require the regeneration of nerve fibers across the lesion site for successful recovery. Providing guidance cues and soluble factors to promote neurite outgrowth and cell survival can enhance repair. The extracellular matrix (ECM) plays a key role in tissue repair by controlling cell adhesion, motility, and growth. In this study, we explored the ability of a mesenchymal ECM to support neurite outgrowth from neurons in the superior cervical ganglia (SCG). Length and morphology of neurites extended on a decellularized fibroblast ECM were compared to those on substrates coated with laminin, a major ECM protein in neural tissue, or fibronectin, the main component of a mesenchymal ECM. Average radial neurite extension was equivalent on laminin and on the decellularized ECM, but contrasted with the shorter, curved neurites observed on the fibronectin substrate. Differences between neurites on fibronectin and on other substrates were confirmed by fast Fourier transform analyses. To control the direction of neurite outgrowth, we developed an ECM with linearly aligned fibril organization by orienting the fibroblasts that deposit the matrix on a polymeric surface micropatterned with a striped chemical interface. Neurites projected from SCGs appeared to reorient in the direction of the pattern. These results highlight the ability of a mesenchymal ECM to enhance neurite extension and to control the directional outgrowth of neurites. This micropatterned decellularized ECM architecture has potential as a regenerative microenvironment for nerve repair.

Viscoelastic, physical, and bio-degradable properties of dermal scaffolds and related cell behaviour

Dermal scaffolds promote healing of debilitating skin injuries caused by burns and chronic skin conditions. Currently available products present disadvantages and therefore, there is still a clinical need for developing new dermal substitutes. This study aimed at comparing the viscoelastic, physical and bio-degradable properties of two dermal scaffolds, the collagen-based and clinically well established Integra(®) and a novel fibrin-based dermal scaffold developed at our laboratory called Smart Matrix(®), to further evaluate our previous published findings that suggested a higher influx of cells, reduced wound contraction and less scarring for Smart Matrix(®) when used in vivo. Rheological results showed that Integra(®) (G'  =  313.74 kPa) is mechanically stronger than Smart Matrix(®) (G'  =  8.26 kPa), due to the presence of the silicone backing layer in Integra(®). Micro-pores were observed on both dermal scaffolds, although nano-pores as well as densely packed nano-fibres were only observed for Smart Matrix(®). Average surface roughness was higher for Smart Matrix(®) (Sa  =  114.776 nm) than for Integra(®) (Sa  =  75.565 nm). Both scaffolds possess a highly porous structure (80-90%) and display a range of pore micro-sizes that represent the actual in vivo scenario. In vitro proteolytic bio-degradation suggested that Smart Matrix(®) would degrade faster upon implantation in vivo than Integra(®). For both scaffolds, the enzymatic digestion occurs via bulk degradation. These observed differences could affect cell behaviour on both scaffolds. Our results suggest that fine-tuning of scaffolds' viscoelastic, physical and bio-degradable properties can maximise cell behaviour in terms of attachment, proliferation and infiltration, which are essential for tissue repair.

Approaches to Peripheral Nerve Repair: Generations of Biomaterial Conduits Yielding to Replacing Autologous Nerve Grafts in Craniomaxillofacial Surgery

Peripheral nerve injury is a common clinical entity, which may arise due to traumatic, tumorous, or even iatrogenic injury in craniomaxillofacial surgery. Despite advances in biomaterials and techniques over the past several decades, reconstruction of nerve gaps remains a challenge. Autografts are the gold standard for nerve reconstruction. Using autografts, there is donor site morbidity, subsequent sensory deficit, and potential for neuroma development and infection. Moreover, the need for a second surgical site and limited availability of donor nerves remain a challenge. Thus, increasing efforts have been directed to develop artificial nerve guidance conduits (ANCs) as new methods to replace autografts in the future. Various synthetic conduit materials have been tested in vitro and in vivo, and several first- and second-generation conduits are FDA approved and available for purchase, while third-generation conduits still remain in experimental stages. This paper reviews the current treatment options, summarizes the published literature, and assesses future prospects for the repair of peripheral nerve injury in craniomaxillofacial surgery with a particular focus on facial nerve regeneration.

The influence of anisotropic nano- to micro-topography on in vitro and in vivo osteogenesis

AIM

Topographically modified substrates are increasingly used in tissue engineering to enhance biomimicry. The overarching hypothesis is that topographical cues will control cellular response at the cell-substrate interface.

MATERIALS & METHODS

The influence of anisotropically ordered poly(lactic-co-glycolic acid) substrates (constant groove width of ~1860 nm; constant line width of ~2220 nm; variable groove depth of ~35, 306 and 2046 nm) on in vitro and in vivo osteogenesis were assessed.

RESULTS & DISCUSSION

We demonstrate that substrates with groove depths of approximately 306 and 2046 nm promote osteoblast alignment parallel to underlined topography in vitro. However, none of the topographies assessed promoted directional osteogenesis in vivo.

CONCLUSION

2D imprinting technologies are useful tools for in vitro cell phenotype maintenance.

Flexible Fabrication of Shape-Controlled Collagen Building Blocks for Self-Assembly of 3D Microtissues

Creating artificial tissue-like structures that possess the functionality, specificity, and architecture of native tissues remains a big challenge. A new and straightforward strategy for generating shape-controlled collagen building blocks with a well-defined architecture is presented, which can be used for self-assembly of complex 3D microtissues. Collagen blocks with tunable geometries are controllably produced and released via a membrane-templated microdevice. The formation of functional microtissues by embedding tissue-specific cells into collagen blocks with expression of specific proteins is described. The spontaneous self-assembly of cell-laden collagen blocks into organized tissue constructs with predetermined configurations is demonstrated, which are largely driven by the synergistic effects of cell-cell and cell-matrix interactions. This new strategy would open up new avenues for the study of tissue/organ morphogenesis, and tissue engineering applications.

Optimising contraction and alignment of cellular collagen hydrogels to achieve reliable and consistent engineered anisotropic tissue

Engineered anisotropic tissue constructs containing aligned cell and extracellular matrix structures are useful as in vitro models and for regenerative medicine. They are of particular interest for nervous system modelling and regeneration, where tracts of aligned neurons and glia are required. The self-alignment of cells and matrix due to tension within tethered collagen gels is a useful tool for generating anisotropic tissues, but requires an optimal balance between cell density, matrix concentration and time to be achieved for each specific cell type. The aim of this study was to develop an assay system based on contraction of free-floating cellular gels in 96-well plates that could be used to investigate cell–matrix interactions and to establish optimal parameters for subsequent self-alignment of cells in tethered gels. Using C6 glioma cells, the relationship between contraction and alignment was established, with 60–80% contraction in the 96-well plate assay corresponding to alignment throughout tethered gels made using the same parameters. The assay system was used to investigate the effect of C6 cell density, collagen concentration and time. It was also used to show that blocking α1 integrin reduced the contraction and self-alignment of these cells, whereas blocking α2 integrin had little effect. The approach was validated by using primary astrocytes in the assay system under culture conditions that modified their ability to contract collagen gels. This detailed investigation describes a robust assay for optimising cellular self-alignment and provides a useful reference framework for future development of self-aligned artificial tissue.

A three-dimensional model of primary bovine endometrium using an electrospun scaffold

Endometrial stromal and epithelial cell function is typically studied in vitro using standard two-dimensional monocultures, but these cultures fail to reflect the complex three-dimensional (3D) architecture of tissue. A 3D model of bovine endometrium that reflects the architectural arrangement of in vivo tissue would beneficially assist the study of tissue function. An electrospun polyglycolide (PGA) scaffold was selected to grow a 3D model of primary bovine endometrial epithelial and stromal cells, that reflects the architecture of the endometrium for the study of pathophysiology. Electrospun scaffolds were seeded with stromal and epithelial cells, and growth was assessed using histological techniques. Prostaglandin E2 and prostaglandin F2α responsiveness of endometrial scaffold constructs was tested using oxytocin plus arachidonic acid (OT + AA) or lipopolysaccharide (LPS). Stromal and epithelial cells growing on the electrospun scaffold had an architectural arrangement that mimicked whole tissue, deposited fibronectin, had appropriate expression of vimentin and cytokeratin and were responsive to OT + AA and LPS, as measured by prostaglandin accumulation. In conclusion, a functional 3D model of stromal and epithelial cells was developed using a PGA electrospun scaffold which may be used to study endometrial pathophysiology.

3D culture model of fibroblast-mediated collagen creep to identify abnormal cell behaviour

Native collagen gels are important biomimetic cell support scaffolds, and a plastic compression process can now be used to rapidly remove fluid to any required collagen density, producing strong 3D tissue-like models. This study aimed to measure the mechanical creep properties of such scaffolds and to quantify any enhanced creep occurring in the presence of cells (cell-mediated creep). The test rig developed applies constant creep tension during culture and measures real-time extension due to cell action. This was used to model extracellular matrix creep, implicated in the transversalis fascia (TF) in inguinal hernia. Experiments showed that at an applied tension equivalent to 15% break strength, cell-mediated creep over 24-h culture periods was identified at creep rates of 0.46 and 0.38%/h for normal TF and human dermal fibroblasts, respectively. However, hernia TF fibroblasts produced negligible cell-mediated creep levels under the same conditions. Raising the cell culture temperature from 4 to 37 °C was used to demonstrate live cell dependence of this creep. This represents the first in vitro demonstration of TF cell-mediated collagen creep and to our knowledge the first demonstration of a functional, hernia-related cell abnormality.

Integrating valve-inspired design features into poly(ethylene glycol) hydrogel scaffolds for heart valve tissue engineering

The development of advanced scaffolds that recapitulate the anisotropic mechanical behavior and biological functions of the extracellular matrix in leaflets would be transformative for heart valve tissue engineering. In this study, anisotropic mechanical properties were established in poly(ethylene glycol) (PEG) hydrogels by crosslinking stripes of 3.4 kDa PEG diacrylate (PEGDA) within 20 kDa PEGDA base hydrogels using a photolithographic patterning method. Varying the stripe width and spacing resulted in a tensile elastic modulus parallel to the stripes that was 4.1-6.8 times greater than that in the perpendicular direction, comparable to the degree of anisotropy between the circumferential and radial orientations in native valve leaflets. Biomimetic PEG-peptide hydrogels were prepared by tethering the cell-adhesive peptide RGDS and incorporating the collagenase-degradable peptide PQ (GGGPQG↓IWGQGK) into the polymer network. The specific amounts of RGDS and PEG-PQ within the resulting hydrogels influenced the elongation, de novo extracellular matrix deposition and hydrogel degradation behavior of encapsulated valvular interstitial cells (VICs). In addition, the morphology and activation of VICs grown atop PEG hydrogels could be modulated by controlling the concentration or micro-patterning profile of PEG-RGDS. These results are promising for the fabrication of PEG-based hydrogels using anatomically and biologically inspired scaffold design features for heart valve tissue engineering.

Directional migration and differentiation of neural stem cells within three-dimensional microenvironments

Harnessing neural stem cells to repair neuronal damage is a promising potential treatment for neuronal diseases. To enable future therapeutic efficacy, the survival, proliferation, migration and differentiation of neural stem/progenitor cells (NPCs) should be accurately studied and optimized in in vitro platforms before transplanting these cells into the body for treatment purposes. Such studies can determine the appropriate quantities of the biochemical and biomechanical factors needed to control and optimize NPC behavior in vivo. In this study, NPCs were cultured within a microfluidic device while being encapsulated within the collagen matrix. The migration and differentiation of NPCs were studied in response to varying concentrations of nerve growth factor (NGF) and within varying densities of collagen matrices. It was shown that the migration and differentiation of NPCs can be significantly improved by providing the appropriate range of NGF concentrations while encapsulating the cells within the collagen matrix of optimal density. In particular, it was observed that within collagen matrices of intermediate density (0.9 mg ml(-1)), NPCs have a higher ability to migrate farther and in a collective manner while their differentiation into neurons is significantly higher and the cells can form protrusions and connections with their neighboring cells. Within collagen matrices with higher densities (1.8 mg ml(-1)), the cells did not migrate significantly as compared to the ones within lower matrix densities; within the matrices with lower collagen densities (0.45 mg ml(-1)) most of the cells migrated in an individual manner. However, no significant differentiation into neurons was observed for these two cases. It was also found that depending on the collagen matrix density, a minimum concentration of NGF caused a collective migration of NPCs, and a minimum concentration gradient of this factor stimulated the directional navigation of the cells. The results of this study can be implemented in designing platforms appropriate for regeneration of damaged neuronal systems.

Peripheral nerve conduits: technology update

Peripheral nerve injury is a worldwide clinical problem which could lead to loss of neuronal communication along sensory and motor nerves between the central nervous system (CNS) and the peripheral organs and impairs the quality of life of a patient. The primary requirement for the treatment of complete lesions is a tension-free, end-to-end repair. When end-to-end repair is not possible, peripheral nerve grafts or nerve conduits are used. The limited availability of autografts, and drawbacks of the allografts and xenografts like immunological reactions, forced the researchers to investigate and develop alternative approaches, mainly nerve conduits. In this review, recent information on the various types of conduit materials (made of biological and synthetic polymers) and designs (tubular, fibrous, and matrix type) are being presented.

Recombinant production and film properties of full-length hornet silk proteins

Full-length versions of the four main components of silk cocoons of Vespa simillima hornets, Vssilk1-4, were produced as recombinant proteins in Escherichia coli. In shake flasks, the recombinant Vssilk proteins yielded 160-330mg recombinant proteinl(-1). Films generated from solutions of single Vssilk proteins had a secondary structure similar to that of films generated from native hornet silk. The films made from individual recombinant hornet silk proteins had similar or enhanced mechanical performance compared with films generated from native hornet silk, possibly reflecting the homogeneity of the recombinant proteins. The pH-dependent changes in zeta (ζ) potential of each Vssilk film were measured, and isoelectric points (pI) of Vssilk1-4 were determined as 8.9, 9.1, 5.0 and 4.2, respectively. The pI of native hornet silk, a combination of the four Vssilk proteins, was 4.7, a value similar to that of Bombyx mori silkworm silk. Films generated from Vssilk1 and 2 had net positive charge under physiological conditions and showed significantly higher cell adhesion activity. It is proposed that recombinant hornet silk is a valuable new material with potential for cell culture applications.

Carriers in cell-based therapies for neurological disorders

There is a pressing need for long-term neuroprotective and neuroregenerative therapies to promote full function recovery of injuries in the human nervous system resulting from trauma, stroke or degenerative diseases. Although cell-based therapies are promising in supporting repair and regeneration, direct introduction to the injury site is plagued by problems such as low transplanted cell survival rate, limited graft integration, immunorejection, and tumor formation. Neural tissue engineering offers an integrative and multifaceted approach to tackle these complex neurological disorders. Synergistic therapeutic effects can be obtained from combining customized biomaterial scaffolds with cell-based therapies. Current scaffold-facilitated cell transplantation strategies aim to achieve structural and functional rescue via offering a three-dimensional permissive and instructive environment for sustainable neuroactive factor production for prolonged periods and/or cell replacement at the target site. In this review, we intend to highlight important considerations in biomaterial selection and to review major biodegradable or non-biodegradable scaffolds used for cell transplantation to the central and peripheral nervous system in preclinical and clinical trials. Expanded knowledge in biomaterial properties and their prolonged interaction with transplanted and host cells have greatly expanded the possibilities for designing suitable carrier systems and the potential of cell therapies in the nervous system.

Synthetic small intestinal scaffolds for improved studies of intestinal differentiation

In vitro intestinal models can provide new insights into small intestinal function, including cellular growth and proliferation mechanisms, drug absorption capabilities, and host-microbial interactions. These models are typically formed with cells cultured on 2D scaffolds or transwell inserts, but it is widely understood that epithelial cells cultured in 3D environments exhibit different phenotypes that are more reflective of native tissue. Our focus was to develop a porous, synthetic 3D tissue scaffold with villous features that could support the culture of epithelial cell types to mimic the natural microenvironment of the small intestine. We demonstrated that our scaffold could support the co-culture of Caco-2 cells with a mucus-producing cell line, HT29-MTX, as well as small intestinal crypts from mice for extended periods. By recreating the surface topography with accurately sized intestinal villi, we enable cellular differentiation along the villous axis in a similar manner to native intestines. In addition, we show that the biochemical microenvironments of the intestine can be further simulated via a combination of apical and basolateral feeding of intestinal cell types cultured on the 3D models.

Building stable anisotropic tissues using cellular collagen gels

Combining cellular self-alignment within tethered collagen gels with stabilization through subsequent removal of interstitial fluid has yielded a new process for the fabrication of aligned cellular biomaterials. This commentary discusses the generation of engineered neural tissue for peripheral nerve repair using this combination of techniques, providing additional insight into the rationale underpinning the approach. By describing the potential benefits of using cell and matrix interactions to organize 3D hydrogels that can be stabilized to form tissue-like constructs, the article aims to highlight the potential for the approach to be used in the generation of a wider range of functional replacement tissues.

Human dental pulp stem cells can differentiate into Schwann cells and promote and guide neurite outgrowth in an aligned tissue-engineered collagen construct in vitro

In the present study, we evaluated the differentiation potential of human dental pulp stem cells (hDPSCs) toward Schwann cells, together with their functional capacity with regard to myelination and support of neurite outgrowth in vitro. Successful Schwann cell differentiation was confirmed at the morphological and ultrastructural level by transmission electron microscopy. Furthermore, compared to undifferentiated hDPSCs, immunocytochemistry and ELISA tests revealed increased glial marker expression and neurotrophic factor secretion of differentiated hDPSCs (d-hDPSCs), which promoted survival and neurite outgrowth in 2-dimensional dorsal root ganglia cultures. In addition, neurites were myelinated by d-hDPSCs in a 3-dimensional collagen type I hydrogel neural tissue construct. This engineered construct contained aligned columns of d-hDPSCs that supported and guided neurite outgrowth. Taken together, these findings provide the first evidence that hDPSCs are able to undergo Schwann cell differentiation and support neural outgrowth in vitro, proposing them to be good candidates for cell-based therapies as treatment for peripheral nerve injury.—Martens, W., Sanen, K., Georgiou, M., Struys, T., Bronckaers, A., Ameloot, M., Phillips, J., Lambrichts, I. Human dental pulp stem cells can differentiate into Schwann cells and promote and guide neurite outgrowth in an aligned tissue-engineered collagen construct in vitro.

Second harmonic generation reveals collagen fibril remodeling in fibroblast-populated collagen gels

Remodeling of collagen fibrils is involved in a variety of physiological and pathological processes including development, tissue repair, and metastasis. Fibroblast-populated collagen gel contraction has been employed as a model system to investigate the collagen fibril remodeling within three-dimensional collagen matrices. Research on collagen gel contraction is also important for understanding the mechanism underlying connective tissue repair, and for design considerations for engineered tissues in regenerative medicine. Second harmonic generation (SHG) is a non-linier optical effect by which well-ordered protein assemblies, including collagen fibrils, can be visualized without any labeling, and used for a noninvasive imaging of collagen fibrils in the skin. Here we demonstrate that the remodeling of collagen fibrils in the fibroblast-populated collagen gel can be analyzed by SHG imaging with a multiphoton microscope. Two models of collagen gel contraction (freely versus restrained contraction) were prepared, and orientation of fibroblasts, density, diameter, and distribution of collagen fibrils were examined by multiphoton fluorescent and SHG microscopy. Three-dimensional construction images revealed vertical and horizontal orientation of fibroblasts in freely and restrained gel contraction, respectively. Quantitative analysis indicated that collagen fibrils were accumulated within the gel and assembled into the thicker bundles in freely but not restrained collagen gel contraction. We also found that actomyosin contractility was involved in collagen fibril remodeling. This study elucidates how collagen fibrils are remodeled by fibroblasts in collagen gel contraction, and also proves that SHG microscopy can be used for the investigation of the fibroblast-populated collagen gel.

Enhanced reconstruction of rat calvarial defects achieved by plasma-treated electrospun scaffolds and induced pluripotent stem cells

Tissue engineering with a combination of stem cells and nanofibrous scaffolds has attracted interest with regard to bone regeneration applications. In the present study, human induced pluripotent stem cells (iPSCs) were cultured on polymeric nanofibrous polyethersulfone (PES) with and without plasma treatment. The capacity of PES and plasma-treated PES (Plasma-PES) scaffolds to support the proliferation and osteogenic differentiation of iPSCs was investigated by MTT assay and for common osteogenic markers such as alkaline phosphatase activity, calcium mineral deposition and bone-related genes. Plasma-PES scaffolds with or without iPSCs were subsequently used to evaluate bone regeneration of critical-size defects in the rat by digital mammography, multislice spiral-computed tomography imaging and histological analysis. The results of in vitro analysis showed that plasma treatment significantly enhanced iPSC proliferation and osteogenesis. After 8 weeks of iPSC-loaded Plasma-PES implantation, no mortality or complication was observed in animals or at the site of surgery. Imaging analysis revealed more extensive bone reconstruction in rats receiving nanofibers compared with untreated control groups. Moreover, Plasma-PES seeded with iPSCs induced the highest regeneration of bone defects among all groups. These findings were confirmed by histological staining. Affective osseointegration was observed in implanted scaffolds. Thus, plasma-treated nanofibrous scaffolds are suitable tissue-engineered matrices for supporting the proliferation and osteogenic differentiation of iPSCs and might also be appropriate for the reconstruction of bone defects.

Direct collagen-material engineering for tissue fabrication

So what is the "big deal" about engineering of collagen materials? It is certainly not enough to produce the familiar line about it being "more physiological." This phrase explains very little of the substance and gets nowhere near to the base of the question. We need to be clear about why this is important enough to justify a major new research investment.

Bioengineering of articular cartilage: past, present and future

The treatment of cartilage defects poses a clinical challenge owing to the lack of intrinsic regenerative capacity of cartilage. The use of tissue engineering techniques to bioengineer articular cartilage is promising and may hold the key to the successful regeneration of cartilage tissue. Natural and synthetic biomaterials have been used to recreate the microarchitecture of articular cartilage through multilayered biomimetic scaffolds. Acellular scaffolds preserve the microarchitecture of articular cartilage through a process of decellularization of biological tissue. Although promising, this technique often results in poor biomechanical strength of the graft. However, biomechanical strength could be improved if biomaterials could be incorporated back into the decellularized tissue to overcome this limitation.