A strategy for the development of tissue engineering scaffolds that regulate cell behavior

Effect of UV Irradiation of Pre-Gel Solutions on the Formation of Collagen Gel Tubes

Hollow collagen gels are promising materials for drug/cell delivery systems to promote tissue regeneration because they may be able to function as carriers for these types of loads. Controlling the cavity size and swelling suppression is essential to expand the applications and improve the usability of such gel-like systems. We investigated the effects of UV-treated collagen solutions as a pre-gel aqueous mixture on the formation and properties of the hollow collagen gels in terms of their preparation range limits, morphology, and swelling ratio. The UV treatment thickened the pre-gel solutions, which allowed hollowing at lower collagen concentrations. This treatment also prevents the over-swelling of the hollow collagen rods in PBS buffer solutions. The UV-treated collagen solutions provided a large lumen space in the prepared collagen hollow fiber rods with a limited swelling ratio, allowing vascular endothelial cells and ectodermal cells to be cultured separately in the outer and inner lumen.

A Paradigm Shift in Tissue Engineering: From a Top–Down to a Bottom–Up Strategy

Tissue engineering (TE) was initially designed to tackle clinical organ shortage problems. Although some engineered tissues have been successfully used for non-clinical applications, very few (e.g., reconstructed human skin) have been used for clinical purposes. As the current TE approach has not achieved much success regarding more broad and general clinical applications, organ shortage still remains a challenging issue. This very limited clinical application of TE can be attributed to the constraints in manufacturing fully functional tissues via the traditional top–down approach, where very limited cell types are seeded and cultured in scaffolds with equivalent sizes and morphologies as the target tissues. The newly proposed developmental engineering (DE) strategy towards the manufacture of fully functional tissues utilises a bottom–up approach to mimic developmental biology processes by implementing gradual tissue assembly alongside the growth of multiple cell types in modular scaffolds. This approach may overcome the constraints of the traditional top–down strategy as it can imitate in vivo-like tissue development processes. However, several essential issues must be considered, and more mechanistic insights of the fundamental, underpinning biological processes, such as cell–cell and cell–material interactions, are necessary. The aim of this review is to firstly introduce and compare the number of cell types, the size and morphology of the scaffolds, and the generic tissue reconstruction procedures utilised in the top–down and the bottom–up strategies; then, it will analyse their advantages, disadvantages, and challenges; and finally, it will briefly discuss the possible technologies that may overcome some of the inherent limitations of the bottom–up strategy.

Orbital seeding of mesenchymal stromal cells increases osteogenic differentiation and bone-like tissue formation

In bone tissue engineering (TE), an efficient seeding and homogenous distribution of cells is needed to avoid cell loss and damage as well as to facilitate tissue development. Dynamic seeding methods seem to be superior to the static ones because they tend to result in a more homogeneous cell distribution by using kinetic forces. However, most dynamic seeding techniques are elaborate or require special equipment and its influence on the final bone tissue-engineered construct is not clear. In this study, we applied a simple, dynamic seeding method using an orbital shaker to seed human bone marrow-derived mesenchymal stromal cells (hBMSCs) on silk fibroin scaffolds. Significantly higher cell numbers with a more homogenous cell distribution, increased osteogenic differentiation, and mineral deposition were observed using the dynamic approach both for 4 and 6 hours as compared to the static seeding method. The positive influence of dynamic seeding could be attributed to both cell density and distribution but also nutrient supply during seeding and shear stresses (0.0-3.0 mPa) as determined by computational simulations. The influence of relevant mechanical stimuli during seeding should be investigated in the future, especially regarding the importance of mechanical cues for bone TE applications. Our results highlight the importance of adequate choice of seeding method and its impact on developing tissue-engineered constructs. The application of this simple seeding technique is not only recommended for bone TE but can also be used for seeding similar porous scaffolds with hBMSCs in other TE fields.

The Thickness of Electrospun Poly (ε-Caprolactone) Nanofibrous Scaffolds Influences Cell Proliferation

Nanofibrous scaffolds have morphological similarities to native extracellular matrix and have been considered as candidate scaffolds in tissue engineering. However, there is no report on the effect of the thickness of nanofibrous scaffold on cell behavior. In this study poly (∊-caprolactone) (PCL) nanofibrous scaffolds with thicknesses of 0.1 and 0.6 mm were fabricated by electrospinning. Properties of PCL nanofibrous scaffolds were measured by contact angle and air permeability measurements while the morphology of the nanofibers was observed by SEM. Mouse embryonal carcinoma stem cells (P19), monkey epithelial kidney cells (Vero), Chinese hamster ovary cells (CHO) and mouse mesenchymal stem cells (MSCs) were seeded on PCL nanofibrous scaffolds with thicknesses of 0.1 and 0.6 mm. Air permeability measurements showed that air permeability decreases with the increase in the thickness of nanofibrous scaffolds, and contact angle measurements revealed a contact angle of 118° for electrospun PCL nanofibers. The MTT assays showed that the proliferation of the cells was influenced by the thickness of the nanofibrous scaffold. Scaffolds with a thickness of 0.6 mm were found to provide a better substrate for cell proliferation, possibly due to more dimensional stability. Therefore, regardless of cell origin, thicker scaffolds provide a better substrate for cell proliferation, possibly due to the higher dimensional stability and tightness of thicker scaffolds.

Fabrication of poly(vinyl alcohol)-Carrageenan scaffolds for cryopreservation: Effect of composition on cell viability

The present investigation reports the fabrication of three dimensional (3D), interconnected, highly porous, biodegradable scaffolds using freeze-gelation technique. The hydrogels prepared with different ratios (5:5, 6:4, 7:3, 8:2 and 9:1) of poly(vinyl alcohol) (PVA) and Carrageenan (Car) was lyophilized to obtain their respective scaffolds. The PVA-Car scaffolds were further characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The prepared scaffolds were found to be biodegradable and highly compatible with hemoglobin. Further, normal keratinocyte (HaCaT) and osteosarcoma (Saos-2) cells seeded on PVA-Car scaffolds were cryopreserved for 15days and their viability was checked at regular interval of 3days (0, 3, 6, 9, 12, 15 days) through MTT assay and fluorescence microscopy. Overall, the collective results indicate the scaffold constructs with 7:3 and 8:2 PVA-Car ratios possess ideal characteristics for tissue engineering applications and for long term cryopreservation of cells.

A multinuclear solid state NMR spectroscopic study of the structural evolution of disordered calcium silicate sol–gel biomaterials

Multinuclear solid state NMR, especially ¹⁷O, shows the structural evolution of calcium silicate sol–gel bioactive glasses during stabilisation and subsequent reaction with simulated body fluid.

Enhancement of renal epithelial cell functions through microfluidic-based coculture with adipose-derived stem cells

Current hemodialysis has functional limitations and is insufficient for renal transplantation. The bioartificial tubule device has been developed to contribute to metabolic functions by implanting renal epithelial cells into hollow tubes and showed a higher survival rate in acute kidney injury patients. In healthy kidney, epithelial cells are surrounded by various types of cells that interact with extracellular matrices, which are primarily composed of laminin and collagen. The current study developed a microfluidic coculture platform to enhance epithelial cell function in bioartificial microenvironments with multiple microfluidic channels that are microfabricated by polydimethylsiloxane. Collagen gel (CG) encapsulated with adipose-derived stem cells (CG-ASC) was injected into a central microfluidic channel for three-dimensional (3D) culture. The resuspended Madin-Darby canine kidney (MDCK) cells were injected into nascent channels and formed an epithelial monolayer. In comparison to coculture different cells using the commercial transwell system, the current coculture device allowed living cell monitoring of both the MDCK epithelial monolayer and CG-ASC in a 3D microenvironment. By coculture with CG-ASC, the cell height was increased with columnar shapes in MDCK. Promotion of cilia formation and functional expression of the ion transport protein in MDCK were also observed in the cocultured microfluidic device. When applying fluid flow, the intracellular protein dynamics can be monitored in the current platform by using the time-lapse confocal microscopy and transfection of GFP-tubulin plasmid in MDCK. Thus, this microfluidic coculture device provides the renal epithelial cells with both morphological and functional improvements that may avail to develop bioartificial renal chips.

Electrospun silk fibroin nanofibers in different diameters support neurite outgrowth and promote astrocyte migration

Nerve tissue engineering has been one of the promising strategies for regenerative treatment in patients suffering from neural tissue loss, but considerable challenges remain before it is able to progress toward clinical application. It has been demonstrated that transplantation of cells in combination with physically or chemically modified biomaterials provides better environments for neurite outgrowth and further promotes axonal regeneration in animal models of spinal cord injury. In this study, neurons and astrocytes were incorporated into 400-nm, 800-nm, and 1200-nm electrospun Bombyx mori silk fibroin (SF) materials to investigate the effects of scaffold-diameter in regulating and directing cell behaviors. β-III-tubulin immunofluorescence analyses reveal that SF nanofibers with smaller diameters are more favorable to the development and maturation of subventricular zone-derived neurons than 1200-nm SF scaffolds. In addition, astrocytes exhibited well-arranged glial fibrillary acidic protein (GFAP) expression on SF scaffolds, and a significant increase in cell-spreading area was observed on 400-nm but not 1200-nm SF scaffolds. Moreover, a significantly enhanced migration efficiency of astrocytes grown on SF scaffolds was verified, which highlights the guiding roles of SF nanofibers to the migratory cells. Overall, our results may provide valuable information to develop effective tissue remodeling substrates and to optimize existing biomaterials for neural tissue engineering applications.

Processing and characterization of innovative scaffolds for bone tissue engineering

A new protocol, based on a modified replication method, is proposed to obtain bioactive glass scaffolds. The main feature of these samples, named "shell scaffolds", is their external surface that, like a compact and porous shell, provides both high permeability to fluids and mechanical support. In this work, two different scaffolds were prepared using the following slurry components: 59 % water, 29 % 45S5 Bioglass(®) and 12 % polyvinylic binder and 51 % water, 34 % 45S5 Bioglass(®), 10 % polyvinylic binder and 5 % polyethylene. All the proposed samples were characterized by a widespread microporosity and an interconnected macroporosity, with a total porosity of 80 % vol. After immersion in a simulated body fluid (SBF), the scaffolds showed strong ability to develop hydroxyapatite, enhanced by the high specific surface of the porous systems. Moreover preliminary biological evaluations suggested a promising role of the shell scaffolds for applications in bone tissue regeneration. As regards the mechanical behaviour, the shell scaffolds could be easily handled without damages, due to their resistant external surface. More specifically, they possessed suitable mechanical properties for bone regeneration, as proved by compression tests performed before and after immersion in SBF.

Spheroid formation and neural induction in human adipose-derived stem cells on a chitosan-coated surface

The application of stem cells appears to have great therapeutic potential to facilitate nerve regeneration in patients with neurodegenerative disease or spinal cord injury. Human adipose-derived stem cells (hADSCs), a subset of multipotent mesenchymal stem cells, possess the great advantages of an abundant amount of cells, less ethical conflict and minimal invasive surgical procedures to obtain the cells. Chitosan, a naturally derived polysaccharide from chitin, has been widely studied to facilitate and guide the direction of nerve regeneration as a biomaterial for the neural tube. Chitosan also serves as a three-dimensional culture substrate to facilitate cellular sphere formation among various cells but is as yet unexplored in hADSCs. In this study, the ability of hADSCs to transdifferentiate from the mesenchymal into the neural lineage by seeding hADSCs on a chitosan-coated surface to form therapeutic cell spheres was investigated. The optimal seeding density (2 × 10(4) cells/cm(2)) and harvesting time (72 h) to obtain sphere formation were determined by cell viability on a chitosan-coated surface. Expression of neural lineage markers was observed by immunofluorescent staining of nestin, neurofilament heavy chain and glial fibrillary acidic protein. The neural induction potentials were also provoked by replating spheres from primary to tertiary passages. The effect of neural induction in hADSCs on a chitosan-coated surface may help to provide cell sources for facilitating nerve regeneration in future clinical applications.

The fabrication and cell culture of three-dimensional rolled scaffolds with complex micro-architectures

Cell cultures for tissue engineering are traditionally prepared on two-dimensional or three-dimensional scaffolds with simple pores; however, this limits mass transportation, which is necessary for cell viability and function. In this paper, an innovative method is proposed for fabricating porous scaffolds with designed complex micro-architectures. Channels devised by computer-aided design were used to simulate features of blood vessels in native rat liver. Rapid prototyping and microreplication were used to produce a negative polydimethylsiloxane mold, and then a planar porous scaffold with predefined microchannel parameters was obtained by freeze-drying a silk fibroin/gelatin solution of an optimized concentration. After seeding with rat primary hepatocytes, the planar scaffold was rolled up to build spatial channels. By reconstructing the three-dimensional channel model in the scaffold in the form of micro-computed topography data and observing the cross-sections of the scroll, we confirmed that the bent channels were still interconnected, with restricted deviations. A comparison of the primary hepatocyte culture in the scaffolds with and without the devised channels proved that our design influenced cell organization and improved cell survival and proliferation. This method can be used for the construction of complex tissues for implantation and for culturing cells in vitro for biological tests and observations.

Crystallinity assessment and in vitro cytotoxicity of polylactide scaffolds for biomedical applications

Bioresorbable polylactides are one of the most important materials for tissue engineering applications. In this work we have prepared scaffolds based on the two optically pure stereoisomers: poly(L: -lactide) (PLLA) and poly(D: -lactide) (PDLA). The crystalline structure and morphology were evaluated by DSC, AFM and X-ray diffraction. PLLA and PDLA crystallized in the α form and the equimolar PLLA/PDLA blend, crystallized in the stereocomplex form, were analyzed by a proliferation assay in contact with mouse L-929 and human fibroblasts and neonatal keratinocytes for in vitro cytotoxicity evaluation. SEM analysis was conducted to determine the cell morphology, spreading and adhesion when in contact with the different polymer surfaces. The preserved proliferation rate showed in MTT tests and the high colonization on the surface of polylactides observed by SEM denote that PLLA, PDLA and the equimolar PLLA/PDLA are useful biodegradable materials in which the crystalline characteristics can be tuned for specific biomedical applications.

A New Highly Bioactive Composite for Scaffold Applications: A Feasibility Study

Hydroxyapatite (HA) has been widely investigated as scaffolding material for bone tissue engineering, mainly for its excellent biocompatibility. Presently, there is an increasing interest in the composites of hydroxyapatite with bioactive glasses, with the aim to obtain systems with improved bioactivity or mechanical properties. Moreover, modifying the ratio between bioactive glass and hydroxyapatite results in the possibility of controlling the reaction rate of the composite scaffold in the human body. However, high temperature treatments are usually required in order to sinter HA-based composites, causing the bioactive glass to crystallize into a glass-ceramic, with possible negative effects on its bioactivity. In the present research work, a glass composition belonging to the Na₂O-CaO-P₂O₅-SiO₂ system, with a reduced tendency to crystallize, is applied to realize HA-based composites. The novel samples can be sintered at a relative low temperature (750 °C) compared to the widely studied HA/45S5 Bioglass^® composites. This fact greatly helps to preserve the amorphous nature of the glass, with excellent effects in terms of bioactivity, according to in vitro tests. As a first application, the obtained composites are also tested to realize highly porous scaffolds by means of the standard burning out method.

Bioactive glass scaffolds for bone regeneration and their hierarchical characterisation

Scaffolds are needed that can act as temporary templates for bone regeneration and actively stimulate vascularized bone growth so that bone grafting is no longer necessary. To achieve this, the scaffold must have a suitable interconnected pore network and be made of an osteogenic material. Bioactive glass is an ideal material because it rapidly bonds to bone and degrades over time, releasing soluble silica and calcium ions that are thought to stimulate osteoprogenitor cells. Melt-derived bioactive glasses, such as the original Bioglass® composition, are available commercially, but porous scaffolds have been difficult to produce because Bioglass and similar compositions crystallize on sintering. Sol-gel foam scaffolds have been developed that avoid this problem. They have a hierarchical pore structure comprising interconnected macropores, with interconnect diameters in excess of the 100 μm that is thought to be needed for vascularized bone ingrowth, and an inherent nanoporosity of interconnected mesopores (2–50 nm) which is beneficial for the attachment of osteoprogenitor cells. They also have a compressive strength in the range of cancellous bone. This paper describes the optimized sol-gel foaming process and illustrates the importance of optimizing the hierarchical structure from the atomic through nano, to the macro scale with respect to biological response.

A Novel Scaffold from Recombinant Spider Silk Protein in Tissue Engineering

As a new biomaterial, recombinant spider silk protein has attracted much attention in tissue engineering. The pNSR-16/ BL21(DE3)pLysS strains fermented and produced the recombinant spider silk protein, which was then cast into scaffolds. NIH-3T3 cells were cultivated with extractions of the scaffolds in vitro. The cytotoxicity of scaffolds was analyzed with a MTT assay. The performances of cells adhesion, growth and expression on the scaffolds were observed with SEM, HE staining and immunohistochemistry. Compared with the control, the extract fluid of materials culturing the NIH-3T3 cells was not apparently different. NIH-3T3 cells could adhere and grow on the scaffolds and secret FGF-2. The pNSR-16 recombinant spider silk protein scaffolds has satisfactory cytocompatibility and the scaffolds are ideal scaffold material for tissue engineering.

Biological cell detachment from poly(N-isopropyl acrylamide) and its applications

Over the past two decades, poly(N-isopropyl acrylamide) (pNIPAM) has become widely used for bioengineering applications. In particular, pNIPAM substrates have been used for the nondestructive release of biological cells and proteins. In this feature article, we review the applications for which pNIPAM substrates have been used to release biological cells, including for the study of the extracellular matrix (ECM), for cell sheet engineering and tissue transplantation, the formation of tumorlike spheroids, the study of bioadhesion and bioadsorption, and the manipulation or deformation of individual cells. The articles reviewed include submissions from our own group as well as from those performing research in the field worldwide.

Synchrotron X-ray microtomography for assessment of bone tissue scaffolds

X-ray microtomography (microCT) is a popular tool for imaging scaffolds designed for tissue engineering applications. The ability of synchrotron microCT to monitor tissue response and changes in a bioactive glass scaffold ex vivo were assessed. It was possible to observe the morphology of the bone; soft tissue ingrowth and the calcium distribution within the scaffold. A second aim was to use two newly developed compression rigs, one designed for use inside a laboratory based microCT machine for continual monitoring of the pore structure and crack formation and another designed for use in the synchrotron facility. Both rigs allowed imaging of the failure mechanism while obtaining stress-strain data. Failure mechanisms of the bioactive glass scaffolds were found not to follow classical predictions for the failure of brittle foams. Compression strengths were found to be 4.5-6 MPa while maintaining an interconnected pore network suitable for tissue engineering applications.

Formation of keratocyte spheroids on chitosan-coated surface can maintain keratocyte phenotypes

Corneal keratocytes have been reported to be able to form spheroids that can preserve their phenotypes after being seeded back onto tissue culture plate in specific culture media. In this study, we found that keratocytes could also form spheroids on a bioengineered material, chitosan-coated surface, with 10% horse serum and Dulbecco's modified Eagle's medium. Under scanning electron microscopy observation, the cells in the spheroids were found to adhere each other tightly, and the cellular boundary could not be distinguished. They could return to a dendritic (keratocyte) morphology and proliferate after they were seeded back onto tissue culture plate. Immunocytochemistry was used to characterize these cells. Reverse transcription-polymerase chain reaction revealed that keratocytes in the spheroids were not from the PAX-6-positive progenitor cells. Further, the results of the seeding density and the number of spheroids formation, cell viability (MTT) assays, negative staining of Ki-67, and Live/Dead assay suggested that the spheroids were from cell aggregation instead of cell proliferation. Cells in the spheroids maintained phenotypes and functions characteristic of keratocytes, as seen by reverse transcription-polymerase chain reaction, collagen gel contraction assay, and challenges of keratocytes with transforming growth factor-beta1. Our results showed that corneal keratocytes could form spheroids on a chitosan-coated surface and maintain a keratocyte phenotype. However, such keratocyte spheroids do not proliferate and cannot withstand transforming growth factor-beta from myofibroblast differentiation.

Growth and osteogenic differentiation of adipose stem cells on PLA/bioactive glass and PLA/beta-TCP scaffolds

The aim of this study was to compare the effects of novel three-dimensional composite scaffolds consisting of a bioactive phase (bioactive glass or beta-tricalcium phosphate [beta-TCP] 10 and 20 wt%) incorporated within a polylactic acid (PLA) matrix on viability, distribution, proliferation, and osteogenic differentiation of human adipose stem cells (ASCs). The viability and distribution of ASCs on the bioactive composite scaffolds was evaluated using Live/Dead fluorescence staining, environmental scanning electron microscopy, and scanning electron microscopy. There were no differences between the two concentrations of bioactive glass and beta-TCP in PLA scaffolds on proliferation and osteogenic differentiation of ASCs. After 2 weeks of culture, DNA content and alkaline phosphatase (ALP) activity of ASCs cultured on PLA/beta-TCP composite scaffolds were higher relative to other scaffold types. Interestingly, the cell number was significantly lower, but the relative ALP/DNA ratio of ASCs was significantly higher in PLA/bioactive glass scaffolds than in other three scaffold types. These results indicate that the PLA/beta-TCP composite scaffolds significantly enhance ASC proliferation and total ALP activity compared to other scaffold types. This supports the potential future use of PLA/beta-TCP composites as effective scaffolds for tissue engineering and as bone replacement materials.

Cytoskeletal role in differential adhesion patterns of normal fibroblasts and breast cancer cells inside silicon microenvironments

In this paper we studied differential adhesion of normal human fibroblast cells and human breast cancer cells to three dimensional (3-D) isotropic silicon microstructures and investigated whether cell cytoskeleton in healthy and diseased state results in differential adhesion. The 3-D silicon microstructures were formed by a single-mask single-isotropic-etch process. The interaction of these two cell lines with the presented microstructures was studied under static cell culture conditions. The results show that there is not a significant elongation of both cell types attached inside etched microstructures compared to flat surfaces. With respect to adhesion, the cancer cells adopt the curved shape of 3-D microenvironments while fibroblasts stretch to avoid the curved sidewalls. Treatment of fibroblast cells with cytochalasin D changed their adhesion, spreading and morphology and caused them act similar to cancer cells inside the 3-D microstructures. Statistical analysis confirmed that there is a significant alteration (P < 0.001) in fibroblast cell morphology and adhesion property after adding cytochalasin D. Adding cytochalasin D to cancer cells made these cells more rounded while there was not a significant alteration in their adhesion properties. The distinct geometry-dependent cell-surface interactions of fibroblasts and breast cancer cells are attributed to their different cytoskeletal structure; fibroblasts have an organized cytoskeletal structure and less deformable while cancer cells deform easily due to their impaired cytoskeleton. These 3-D silicon microstructures can be used as a tool to investigate cellular activities in a 3-D architecture and compare cytoskeletal properties of various cell lines.

Method to analyze three-dimensional cell distribution and infiltration in degradable scaffolds

Effective cell seeding throughout the tissue scaffold often determines the success of tissue-engineering products, although most current methods focus on determining the total number, not the distribution, of the cells associated with tissue-engineering constructs. The purpose of this investigation was to establish a quick, convenient, and efficient method to quantify cell survival, distribution, and infiltration into degradable scaffolds using a combination of fluorescence cell staining and cryosectioning techniques. After cell seeding and culture for different periods of time, seeded scaffolds were stained with a live cell dye and then cryosectioned. Cryosectioned scaffolds were then recompiled into a three-dimensional (3D) image to visualize cell behavior after seeding. To test the effectiveness of this imaging method, four common seeding methods, including static surface seeding, cell injection, orbital shaker seeding, and centrifuge seeding, were investigated for their seeding efficacy. Using this new method, we were able to visualize the benefits and drawbacks of each seeding method with regard to the cell behavior in 3D within the scaffolds. This method is likely to provide useful information to assist the development of novel materials or cell-seeding methods for producing full-thickness tissue grafts.

Quantifying the 3D macrostructure of tissue scaffolds

The need to shift from tissue replacement to tissue regeneration has led to the development of tissue engineering and in situ tissue regeneration. Both of these strategies often employ the use of scaffolds--templates that allow cells to attach and then guide the new tissue growth. There are many design criteria for an ideal scaffold. These criteria vary depending on the tissue type and location in the body. In any application of a scaffold it is vital to be able to characterise the scaffold before it goes into in vitro testing. In vitro testing allows the cell response to be investigated before its in vivo performance is assessed. A full characterisation of events in vitro and in vivo, in three dimensions (3D), is necessary if a scaffold's performance and effectiveness is to be fully quantified. This paper focuses on porous scaffolds for bone regeneration, suggests appropriate design criteria for a bone regenerating scaffold and then reviews techniques for obtaining the vitally important quantification of its pore structure. The techniques discussed will include newly developed methods of quantifying X-ray microtomography (microCT) images in 3D and for predicting the scaffolds mechanical properties and the likely paths of fluid flow (and hence potential cell migration). The complications in investigating scaffold performance in vitro are then discussed. Finally, the use of microCT for imaging scaffolds for in vivo tests is reviewed.

Evaluation of cell affinity on poly(L-lactide) and poly(epsilon-caprolactone) blends and on PLLA-b-PCL diblock copolymer surfaces

An evaluation of cell proliferation and adhesion on biocompatible film supports was performed. A series of films were compression molded from commercially available poly (L-lactide), PLLA, and poly(epsilon-caprolactone), PCL, and from their melt mixed blends (PLLA/PCL blends). These were compared with compression molded films of PLLA-b-PCL model diblock copolymers. The samples were analyzed by differential scanning calorimetry (DSC), contact angle measurements, and scanning force microscopy (SFM). Cell adhesion and proliferation were performed with monkey derived fibroblasts (VERO) and with osteoblastic cells obtained either enzymatically or from explants cultures of Sprague-Dawley rat calvaria. Migration studies were performed with bone explants of the same origin. The results obtained indicate that although all materials tested were suitable for the support of cellular growth, a PLLA-b-PCL diblock copolymer sample with 93% PLLA was significantly more efficient. This sample exhibited a unique surface morphology with long range ordered domains (of the order of 2-3 mum) of edge-on PLLA lamellae that can promote "cell contact guidance." The influence of other factors such as chemical composition, degree of crystallinity, and surface roughness did not play a major role in determining cell preference toward a specific surface for the materials employed in this work.

Inorganic materials for bone repair or replacement applications

In recent years, excipient systems have been used increasingly in biomedicine in reconstructive and replacement surgery, as bone cements, drug-delivery vehicles and contrast agents. Particularly, interest has been growing in the development and application of controlled pore inorganic ceramic materials for use in bone-replacement and bone-repair roles and, in this context, attention has been focused on calcium-phosphate, bioactive glasses and SiO2- and TiO2-based materials. It has been shown that inorganic materials that most closely mimic bone structure and surface chemistry most closely function best in bone replacement/repair and, in particular, if a substance possesses a macroporous structure (pores and interconnections >100µm diameter), then cell infiltration, bone growth and vascularization can all be promoted. The surface roughness and micro/mesoporosity of a material have also been observed to significantly influence its ability to promote apatite nucleation and cell attachment significantly. Pores (where present) can also be packed with pharmaceuticals and biomolecules (e.g., bone morphogenetic proteins [BMPs], which can stimulate bone formation). Finally, the most bio-efficient – in terms of collagen formation and apatite nucleation – materials are those that are able to provide soluble mineralizing species (Si, Ca, PO4) at their implant sites and/or are doped or have been surface-activated with specific functional groups. This article presents the context and latest advances in the field of bone-repair materials, especially with respect to the development of bioactive glasses and micro/mesoporous and macroporous inorganic scaffolds. It deals with the possible methods of preparing porous pure/doped or functionalized silicas or their composites, the studies that have been undertaken to evaluate their abilities to act as bone repair scaffolds and also presents future directions for work in that context.

Insulin and heparin co-immobilized 3D polyester fabrics for the cultivation of fibroblasts in low-serum media

Insulin and/or heparin immobilized/co-immobilized non-woven polyester fabric (NWPF) discs were developed for the cultivation of L929 mouse fibroblasts in low-serum media. At first, NWPF discs were hydrolyzed to obtain a carboxylic acid group-introduced matrix (NWPF-hydrolyzed). Insulin and heparin co-immobilized NWPF (NWPF-insulin-heparin) was prepared by the grafting of PEO onto NWPF-hydrolyzed disc (NWPF-PEO), followed by the reaction first with insulin and then heparin. In the presence of spacer arm, PEO, the amount of immobilized insulin molecules significantly increased from 6.96 to 84.45 microg/cm(2). The amount of heparin bound to the NWPF-PEO (5.93 microg/cm(2)) was higher than that of the insulin immobilized surface (4.59 microg/cm(2)). Insulin and heparin immobilized NWPF discs were observed with fluorescence microscopy by labeling the insulin and heparin with 8-anilino-1-naphthalene sulfonic acid (ANS) or fluorescein isothiocyanate (FITC), respectively. L929 fibroblasts were used to check the cell adhesion and cell growth capabilities of modified NWPF discs in low-serum media (containing 5% fetal bovine serum). Optical photographs showed that after 2nd day of the culture, fibroblastic cells spread along the length of modified fibers, eventually filling the interfiber space. At the end of 6-day growth period, cell yield in the presence of immobilized heparin was a little bit higher than that of the immobilized insulin. Co-immobilized (insulin/heparin) NWPF discs did not accelerate the cell growth as well as insulin or heparin immobilized discs.

Collagen vitrigel membrane useful for paracrine assays in vitro and drug delivery systems in vivo

We previously succeeded in converting a soft and turbid disk of type-I collagen gel into a strong and transparent vitrigel membrane utilizing a concept for the vitrification of heat-denatured proteins and have demonstrated its protein-permeability and advantage as a scaffold for reconstructing crosstalk models between two different cell types. In this study, we observed the nano-structure of the type-I collagen vitrigel membrane and verified its utility for paracrine assays in vitro and drug delivery systems in vivo. Scanning electron microscopic observation revealed that the vitrigel membrane was a dense network architecture of typical type-I collagen fibrils. In the crosstalk model between PC-12 pheochromocytoma cells and L929 fibroblasts, nerve growth factor (NGF) secreted from L929 cells passed through the collagen vitrigel membrane and induced the neurite outgrowth of PC-12 cells by its paracrine effect. Also, the collagen vitrigel membrane containing vascular endothelial growth factor (VEGF) showed sustained-release of VEGF in vitro and its subcutaneous transplantation into a rat resulted in remarkable angiogenesis. These data suggest that the collagen vitrigel membrane is useful for paracrine assays in vitro and drug delivery systems in vivo.

A protein-permeable scaffold of a collagen vitrigel membrane useful for reconstructing crosstalk models between two different cell types

Soft and turbid collagen gel disks were previously converted into strong and transparent gel membranes utilizing a concept for the vitrification of heat-denatured of proteins. This novel stable and transparent gel has been termed 'vitrigel'. By encompassing the collagen vitrigel membrane in a nylon frame, it can be easily handled with tweezers, and functions as an excellent scaffold for three-dimensional cell culture models, as cells can be cultured on both sides. Here, we investigated the molecular permeability of the collagen vitrigel membrane in a time course-dependent manner using glucose and serum proteins. Glucose penetrated through the collagen vitrigel membrane to the opposite side, and concentrations on each side were found to be equilibrated within 24 h. Serum proteins up to a molecular weight >100 kDa also gradually passed through the collagen vitrigel membrane. In addition, human microvascular endothelial cells (HMVECs) were cultured on one surface of the collagen vitrigel membrane with a nylon frame, and human dermal fibroblasts (HDFs) or HT-29 (a human colon carcinoma cell line) cells were cocultured on the opposite surface. Histomorphological observations revealed the formation of three-dimensional crosstalk models composed of HMVECs and HDFs or HMVECs and HT-29 cells. Resulting data suggest that the protein-permeable scaffold composed of the collagen vitrigel membrane is useful for the reconstruction and/or modeling of 'crosstalk' between two different cells types. Hereafter, such crosstalk models in vitro could be applied to research not only of paracrine factors, but also to epithelial- or endothelial-mesenchymal transitions.

Reconstruction of a Hard Connective Tissue Utilizing a Pressed Silk Sheet and Type-I Collagen as the Scaffold for Fibroblasts

A pressed silk sheet is a new biomaterial composed of a network of numerous cocoon filaments and having excellent mechanical strength and shape stability compared to a cotton-gauze sheet. To reconstruct a hard connective tissue using the silk sheet and type-I collagen as the scaffold for fibroblasts, three different three-dimensional floating culture systems were designed. "On sheet" system: fibroblasts were seeded on the silk sheet coated with collagen and the cell-attached sheet was cultured. "In gel" system: fibroblasts and the silk sheet were co-embedded in a collagen gel and the gel was cultured. "On vitrigel" system: fibroblasts were seeded on both sides of a collagen vitrigel involving the silk sheet and the vitrigel was cultured. The fibroblasts in all culture systems grew and formed disk-shaped connective tissue models involving the silk sheet by 14 days of culture. The "on sheet" and "on vitrigel" models retained a maximum elastic load of about 23 kgf and an ultimate tensile load of about 3.6 kgf, which were almost the same as for the individual silk sheet. However, the "in gel" system showed a low value for the tensile load. Cell damage following application of mechanical stress was lowest in the "on vitrigel" system. These data demonstrated the advantage of the "on vitrigel" system in reconstructing hard connective tissues. Such a novel culture method would contribute to a regenerative medicine for the failure of ligaments, tendons, and other connective tissues.

One-step fabrication of porous micropatterned scaffolds to control cell behavior

This paper reports a one-step method to fabricate highly porous micropatterned 2-D scaffold sheets. The scaffold sheets have high glucose diffusion, indicating that the porosity and pore morphology of the scaffolds are viable with respect to nutrient transport, and a micropattern for cell alignment. HUVEC culturing proved that the scaffold sheets are suitable for cell culturing. More extensive culturing experiments with mouse myoblasts, C2C12, and mouse osteoblasts, MC3T3, showed that tissue organization can be controlled; the micropattern design affects the extent of cell alignment and tissue formation. Cells are favorably settled in the micropattern and even at higher confluence levels, when the cells start to overgrow the ridges of the micropattern, these cells align themselves in the direction of the micropattern. Preliminary multi-layer stacking experiments indicate that the 2-D scaffold sheets are very promising as basis for building 3-D scaffolds.

Extracellular matrix formation and mineralization on a phosphate-free porous bioactive glass scaffold using primary human osteoblast (HOB) cells

Sol-gel derived bioactive glasses of the 70S30C (70mol% SiO2, 30mol% CaO) composition have been foamed to produce 3D bioactive scaffolds with hierarchical interconnected pore morphologies similar to trabecular bone. The aim of this study was to investigate primary human osteoblast response to porous bioactive glass scaffolds. The scaffolds supported osteoblast growth and induced differentiation, within the 3-week culture period, as depicted by enhanced ALPase enzymatic activity, without the addition of supplementary factors such as ascorbic acid, beta-glycerophosphate and dexamethasone. This is the first time this has been observed on a bioactive glass that does not contain phosphate. Deposition of extracellular matrix was also confirmed by enhanced production of the extracellular matrix protein collagen type I. SEM showed indications of mineralized bone nodule formation without the addition of growth factors. The 70S30C bioactive glass scaffolds therefore fulfil many of the criteria for an ideal scaffold for bone tissue engineering applications.

Chitosan-phosphorylated chitosan polyelectrolyte complex hydrogel as an osteoblast carrier

To simulate extra-cellular matrix, a novel three-dimensional scaffold of polyelectrolyte complex (PEC) hydrogel as an osteoblast carrier was synthesized. First, chitosan, a natural glycosaminoglycan, was modified by phosphorylation to obtain a water-soluble phosphorylated chitosan (P-content: 10.7 mass%). The PEC hydrogel was then formed from equal volumes of 0.173 mass% phosphorylated chitosan in water and 1 mass% chitosan in 1% (V/V) acetic acid solution. Rat osteoblasts were seeded in the hydrogel. The PEC hydrogel had a three-dimensional hierarchically-porous structure and good cytobiocompatibility for osteoblasts in vitro. It is concluded that the PEC hydrogel is a promising material as an osteoblast carrier.

Incorporation of basic fibroblast growth factor by a layer-by-layer assembly technique to produce bioactive substrates

Basic fibroblast growth factor (bFGF) was immobilized onto quartz slides and collagen films by assembly with chondroitin sulfate (CS) in a layer-by-layer (LBL) manner. First, the LBL-deposition process on the amino-silanized quartz slides was monitored by UV-vis spectroscopy and water contact angle measurement. By substituting the normal bFGF with rhodamine-labeled one (Rd-bFGF), a linear increase of the absorbance versus bilayer number was recorded. The water contact angle oscillated between the odd CS and the even bFGF layers, demonstrating the alternating change of the surface chemistry. Scanning force microscopy (SFM) revealed that the surface topography was altered slightly after multilayer assembly. In vitro incubation of the CS/bFGF multilayers in PBS showed that approximately 30% of the incorporated bFGF was released within 8 days. In vitro cell culture found that the fibroblasts showed star-like morphology with plenty of pseudopods on the bFGF-incorporated collagen film after cultured for 1 day, and the collagen films assembled with bFGF possess improved bioactivity than that of the virgin one and the bFGF control. Since the immobilized growth factors can maximally retain their bioactivity, the LBL assembly would be a potential approach to construct a bioactive substrate for biomedical applications.

New era in health care: tissue engineering

Tissue engineering is a rapidly expanding field, which applies the principles and methods of physical sciences, life sciences and engineering to understand physiological and pathological systems and to modify and create cells and tissues for therapeutic applications. It has emerged as a rapidly expanding ‘interdisciplinary field’ that is a significant potential alternative wherein tissue and organ failure is addressed by implanting natural, synthetic, or semi synthetic tissue or organ mimics that grow into the required functionality or that are fully functional from the start. This review presents in a comprehensive manner the various considerations for the reconstruction of various tissues and organs as well as the various applications of this young emerging field in different disciplines.

Controlling ion release from bioactive glass foam scaffolds with antibacterial properties

Bioactive glass scaffolds have been produced, which meet many of the criteria for an ideal scaffold for bone tissue engineering applications, by foaming sol-gel derived bioactive glasses. The scaffolds have a hierarchical pore structure that is very similar to that of cancellous bone. The degradation products of bioactive glasses have been found to stimulate the genes in osteoblasts. This effect has been found to be dose dependent. The addition of silver ions to bioactive glasses has also been investigated to produce glasses with bactericidal properties. This paper discusses how changes in the hierarchical pore structure affect the dissolution of the glass and therefore its bioactivity and rate of ion delivery and demonstrates that silver containing bioactive glass foam scaffolds can be synthesised. It was found that the rate of release of Si and Ca ions was more rapid for pore structures with a larger modal pore diameter, although the effect of tailoring the textural porosity on the rate of ion release was more pronounced. Bioactive glass scaffolds, containing 2 mol% silver, released silver ions at a rate that was similar to that which has previously been found to be bactericidal but not high enough to be cytotoxic to bone cells.

3D tissue culture substrates produced by microthermoforming of pre-processed polymer films

We describe a new technology based on thermoforming as a microfabrication process. It significantly enhances the tailoring of polymers for three dimensional tissue engineering purposes since for the first time highly resolved surface and bulk modifications prior to a microstructuring process can be realised. In contrast to typical micro moulding techniques, the melting phase is avoided and thus allows the forming of pre-processed polymer films. The polymer is formed in a thermoelastic state without loss of material coherence. Therefore, previously generated modifications can be preserved. To prove the feasibility of our newly developed technique, so called SMART = Substrate Modification And Replication by Thermoforming, polymer films treated by various polymer modification methods, like UV-based patterned films, and films modified by the bombardment with energetic heavy ions, were post-processed by microthermoforming. The preservation of locally applied specific surface and bulk features was demonstrated e.g. by the selective adhesion of cells to patterned microcavity walls.

Three-dimensional characterization of cell clusters using synchrotron-radiation-based micro-computed tomography

Micro-computed tomography with the highly intense, monochromatic X rays produced by the synchrotron is a superior method to nondestructively measure the local absorption in three-dimensional space. Because biological tissues and cells consist mainly of water as the surrounding medium, higher absorbing agents have to be incorporated into the structures of interest. Even without X-ray optics such as refractive lens, one can uncover the stain distribution with the spatial resolution of about 1 mum. Incorporating the stain at selected cell compartments, for example, binding to the RNA/DNA, their density distribution becomes quantified. In this communication, we demonstrate that tomograms obtained at the beamlines BW2 and W2 (HASYLAB at DESY, Hamburg, Germany) and 4S (SLS, Villigen, Switzerland) clearly show that the RNA/DNA-stained HEK 293 cell clusters have a core of high density and a peripheral part of lower density, which correlate with results of optical microscopy. The inner part of the clusters is associated with nonvital cells as the result of insufficient oxygen and nutrition supply. This necrotic part is surrounded by (6 +/- 1) layers of vital cells.

Optimising bioactive glass scaffolds for bone tissue engineering

A 3D scaffold has been developed that has the potential to fulfil the criteria for an ideal scaffold for bone tissue engineering. Sol-gel derived bioactive glasses of the 70S30C (70 mol% SiO2, 30 mol% CaO) composition have been foamed to produce 3D bioactive scaffolds with hierarchical interconnected pore morphologies similar to trabecular bone. The scaffolds consist of a hierarchical pore network with macropores in excess of 500 microm connected by pore windows with diameters in excess of 100 microm, which is thought to be the minimum pore diameter required for tissue ingrowth and vasularisation in the human body. The scaffolds also have textural porosity in the mesopore range (10-20 nm). The scaffolds were sintered at 600, 700, 800 and 1000 degrees C. As sintering temperature was increased to 800 degrees C the compressive strength increased from 0.34 to 2.26 MPa due to a thickening of the pore walls and a reduction in the textural porosity. The compressive strength is in the range of that of trabecular bone (2-12 MPa). Importantly, the modal interconnected pore diameter (98 microm) was still suitable for tissue engineering applications and bioactivity is maintained. Bioactive glass foam scaffolds sintered at 800 degrees C for 2 h fulfill the criteria for an ideal scaffold for tissue engineering applications.