Characterization of Tissue-Engineered Scaffolds Microfabricated with PAM

The control of stem cell morphology and differentiation using three-dimensional printed scaffold architecture

In this work, we investigated the interactions of human mesenchymal stem cells (hMSCs) with three-dimensional (3D) printed scaffolds displaying different scaffold architectures. Pressure-assisted microsyringe system was used to fabricate scaffolds with square (SQR), hexagonal (HEX), and octagonal (OCT) architectures defined by various degrees of curvatures. OCT represents the highest degree of curvature followed by HEX, and SQR is composed of linear struts without curvature. Scaffolds were fabricated from poly(L-lactic acid) and poly(tyrosol carbonate). We found that hMSCs attached and spread by taking the shape of the individual struts, exhibiting high aspect ratios (ARs) and mean cell area when cultured on OCT scaffolds as compared with those cultured on HEX and SQR scaffolds. In contrast, cells appeared bulkier with low AR on SQR scaffolds. These significant changes in cell morphology directly correlate with the stem cell lineage commitment, such that 80 ± 1% of the hMSCs grown on OCT scaffolds differentiated into osteogenic lineage, compared with 70 ± 4% and 62 ± 2% of those grown on HEX and SQR scaffolds, respectively. Cells on OCT scaffolds also showed 2.5 times more alkaline phosphatase activity compared with cells on SQR scaffolds. This study demonstrates the importance of scaffold design to direct stem cell differentiation, and aids in the development of novel 3D scaffolds for bone regeneration.

Novel Electrohydrodynamic Printing of Nanocomposite Biopolymer Scaffolds

In this paper, we uncover a new method for the preparation of biopolymer scaffolds and demonstrate its potential for the development of organs with the aid of tissue engineering. Two novel nanocomposite polymers, a nonbiodegradable polyhedral oligomeric silsesquioxane-poly (carbonate-urea)urethane and a biodegradable polyhedral oligomeric silsesquioxane-polycaprolactone-poly(carbonate-urea)urethane, have been subjected to flow in an electric field. Electrically forced microthreading of the polymers occurs and a three-dimensional print-patterning device was used to deposit fine (<50 µm) threads of polymer according to a predesigned architecture to prepare scaffolds. The technique can offer tremendous potential in the development of organs.

Pressure-activated microsyringe (PAM) fabrication of bioactive glass-poly(lactic-co-glycolic acid) composite scaffolds for bone tissue regeneration

The aim of this work was the fabrication and characterization of bioactive glass-poly(lactic-co-glycolic acid) (PLGA) composite scaffolds mimicking the topological features of cancellous bone. Porous multilayer PLGA-CEL2 composite scaffolds were innovatively produced by a pressure-activated microsyringe (PAM) method, a CAD/CAM processing technique originally developed at the University of Pisa. In order to select the optimal formulations to be extruded by PAM, CEL2-PLGA composite films (CEL2 is an experimental bioactive SiO₂ -P₂ O₅ -CaO-MgO-Na₂ O-K₂ O glass developed at Politecnico di Torino) were produced and mechanically tested. The elastic modulus of the films increased from 30 to > 400 MPa, increasing the CEL2 amount (10-50 wt%) in the composite. The mixture containing 20 wt% CEL2 was used to fabricate 2D and 3D bone-like scaffolds composed by layers with different topologies (square, hexagonal and octagonal pores). It was observed that the increase of complexity of 2D topological structures led to an increment of the elastic modulus from 3 to 9 MPa in the composite porous monolayer. The elastic modulus of 3D multilayer scaffolds was intermediate (about 6.5 MPa) between the values of the monolayers with square and octagonal pores (corresponding to the lowest and highest complexity, respectively). MG63 osteoblast-like cells and periosteal-derived precursor cells (PDPCs) were used to assess the biocompatibility of the 3D bone-like scaffolds. A significant increase in cell proliferation between 48 h and 7 days of culture was observed for both cell phenotypes. Moreover, qRT-PCR analysis evidenced an induction of early genes of osteogenesis in PDPCs. Copyright © 2015 John Wiley & Sons, Ltd.

Rapid prototyping of chitosan-coated alginate scaffolds through the use of a 3D fiber deposition technique

A novel dispensing system based on two coaxial needles is used to fabricate three dimensional, periodic scaffolds by rapid prototyping.

Substrate stiffness modulates gene expression and phenotype in neonatal cardiomyocytes in vitro

Biomaterials to be used as cell delivery systems for cardiac tissue engineering should be able to comply with cardiac muscle contractile activity, while favoring cell survival and neo-angiogenesis in a hostile environment. Biocompatible synthetic materials can be tailored to mimic cardiac tissue three-dimensional organization in the micro- and nanoscales. Nonetheless, they usually display mechanical properties that are far from those of the native myocardium and thus could affect host cell survival and activity. In the present investigation, inert poly-ε-caprolactone planar layers were manufactured to change the surface stiffness (with Young's modulus ranging from 1 to 133 MPa) without changing matrix chemistry. These substrates were challenged with neonatal murine cardiomyocytes to study the possible effect of substrate stiffness on such cell behavior without changing biological cues. Interestingly, softer substrates (0.91±0.08 and 1.53±0.16 MPa) were found to harbor mostly mature cardiomyocytes having assembled sarcomeres, as shown by the expression of alpha actinin and myosin heavy chain in typical striations and the upregulation of sarcomeric actin mRNA. On the other hand, a preferential expression of immature cardiac cell genes (Nkx-2.5) and proteins (GATA-4) in cardiac cells grown onto stiffer materials (49.67±2.56 and 133.23±8.67 MPa) was detected. This result could not be ascribed to significant differences in cell adhesion or proliferation induced by the substrates, but to the stabilization of cardiomyocyte differentiated phenotype induced by softer layers. In fact, cardiac cell electromechanical coupling was shown to be more organized on softer surfaces, as highlighted by connexin 43 distribution. Moreover, a differential regulation of genes involved in extracellular matrix remodeling was detected on soft films (0.91±0.08 MPa) as compared with the stiffest (133.23±8.67 MPa). Finally, the upregulation of a number of genes involved in inflammatory processes was detected when the stiffest polymer is used. These events highlight the differences in cell mechanosensitivity in a heterogeneous cell preparation and are likely to contribute to the differences encountered in cardiac cell phenotype induced by substrate stiffness.

A review of trends and limitations in hydrogel-rapid prototyping for tissue engineering

The combined potential of hydrogels and rapid prototyping technologies has been an exciting route in developing tissue engineering scaffolds for the past decade. Hydrogels represent to be an interesting starting material for soft, and lately also for hard tissue regeneration. Their application enables the encapsulation of cells and therefore an increase of the seeding efficiency of the fabricated structures. Rapid prototyping techniques on the other hand, have become an elegant tool for the production of scaffolds with the purpose of cell seeding and/or cell encapsulation. By means of rapid prototyping, one can design a fully interconnected 3-dimensional structure with pre-determined dimensions and porosity. Despite this benefit, some of the rapid prototyping techniques are not or less suitable for the generation of hydrogel scaffolds. In this review, we therefore give an overview on the different rapid prototyping techniques suitable for the processing of hydrogel materials. A primary distinction will be made between (i) laser-based, (ii) nozzle-based, and (iii) printer-based systems. Special attention will be addressed to current trends and limitations regarding the respective techniques. Each of these techniques will be further discussed in terms of the different hydrogel materials used so far. One major drawback when working with hydrogels is the lack of mechanical strength. Therefore, maintaining and improving the mechanical integrity of the processed scaffolds has become a key issue regarding 3-dimensional hydrogel structures. This limitation can either be overcome during or after processing the scaffolds, depending on the applied technology and materials.

Rapid prototyping composite and complex scaffolds with PAM2

To create composite synthetic scaffolds with the same degree of complexity and multilevel organization as biological tissue, we need to integrate multilevel biomaterial processing in rapid prototyping systems. The scaffolds then encompass the entire range of properties, which characterize biological tissue. A multilevel microfabrication system, PAM(2), has been developed to address this gap in material processing. It is equipped with different modules, each covering a range of material properties and spatial resolutions. Together, the modules in PAM(2) can be used to realize complex and composite scaffolds for tissue engineering, bringing us a step closer to real clinical applications. This chapter describes the PAM(2) system and discusses some of the practical issues associated with scaffold microfabrication and biomaterial processing.

Electrospray deposited fibronectin retains the ability to promote cell adhesion

Scaffolds for tissue engineering require the correct biochemical cues if the seeded cells are to migrate into the scaffold and proliferate. For complex tissues this would require precise patterning of the scaffold structure with the particular biochemical cue required at each location on the scaffold. Electrospray enables the deposition of a wide number of biomolecules onto surfaces and can be used for precise patterning. We assessed the functionality of a key cell-adhesion molecule, fibronectin, after depositing it onto a surface using the electrospray technique. The addition of polypropylene glycol allowed a stable spray to be obtained from solutions with a range of fibronectin concentrations. Immunoassay tests showed that the amount of fibronectin retained on the surface was proportional to that sprayed from the solution. Increasing the surface density of fibronectin deposited onto silicon surfaces enhanced fibroblast attachment. The fibronectin thus appears to have retained its cell attachment functionality after undergoing the electrospray process. Since recent advances allow electrospray to pattern material from solution with micrometre accuracy this may allow materials to be biologically functionalized on a similar scale.

Sensing scaffolds to monitor cellular activity using impedance measurements

Scaffolds are cell adhesive matrices for the realisation of tissue constructs. Here we describe how scaffolds for tissue engineering can also be used as sensors for monitoring cellular activity such as adhesion and spreading. Carbon nanotube polymer composites were fabricated into membranes and scaffolds with electro-conductive properties. Impedance techniques were used to measure the effects of media and cell cultures on composite membranes and the results were analysed using lumped parameter models. We show that protein adhesion can be distinguished from cell adhesion as the impedance changes are much smaller for the latter (5%). In the presence of cells, impedance changes are of the order of 40% and can be correlated with adhesion, spreading and changes in cell density.

Flow-regulated glucose and lipid metabolism in adipose tissue, endothelial cell and hepatocyte cultures in a modular bioreactor

Static cell culture has serious limitations in its ability to represent cellular behaviour within a live organism. In vivo, cells are constantly exposed to the flow of bodily fluids and contact with other cell types. Bioreactors provide the opportunity to study cells in an environment that more closely resembles the in vivo setting because cell cultures can be exposed to dynamic flow in contact with or in proximity to other cell types. In this study we compared the metabolic profile of a dynamic cell culture system to that of a static cell culture in three different cellular phenotypes: adipocytes, endothelial cells and hepatocytes. Albumin, glucose, free fatty acids, glycerol, and lactate were measured over 48 h. We show that all three cell types have increased glucose uptake in the presence of flow; lactate release was also significantly affected. We provide robust evidence that the presence of flow significantly modifies cellular metabolism. While flow provides a more uniform nutrient distribution and increases metabolite turnover, our results indicate that different cell types have specific metabolic responses to flow, suggesting cell-specific flow-regulated activation of metabolite signalling pathways.

Optimization of PAM scaffolds for neural tissue engineering: preliminary study on an SH-SY5Y cell line

Engineering neural tissue is one of the most challenging goals of tissue engineering. Neural tissue is highly complex and possesses an organized three-dimensional (3D) distribution that is essential for tissue function. An optimal scaffold for tissue engineering has to provide this distribution until the cells are able to activate their normal functions and develop neural connections with the host tissue. Different strategies such as gene therapy and cell transplantation particularly in retinal tissue have been tested, but so far they have only induced retinal degeneration in animals. The objective of this work was to study neural cell assembly as a function of scaffold features and surface chemistry for application in retinal tissue engineering using microfabricated patterns with a well-defined geometry. Because retinal neurons are known to be arranged in hexagonal arrays, hexagonal scaffolds of poly(DL-lactide-co-glycolide) acid were fabricated using a pressure-assisted microsyringe (PAM) system. The behavior of a model cell, neuroblastoma originating from human retina (SH-SY5Y), was analyzed after seeding on the scaffolds, measuring cell density as a function of line width and length of the scaffold to identify the optimal hexagonal geometry. We also evaluated the influence of scaffold on cell metabolism using the methyl thiazolyl tetrazolium assay and on neurite extension. As far as two-dimensional scaffolds are concerned, the results show that although metabolic activity per cell remains constant, hexagons with sides of 500 microm and line widths of 20 +/- 5 microm are optimum for neural cell adhesion in terms of cell density. On 3D scaffolds, cell metabolism is about three times higher than controls, and the optimum number of layers in the scaffold is three or four.

HEMET: mathematical model of biochemical pathways for simulation and prediction of HEpatocyte METabolism

Many computer studies and models have been developed in order to simulate cell biochemical pathways. The difficulty of integrating all the biochemical reactions that occur in a cell in a single model is the main reason for the poor results in the prediction and simulation of cell behaviour under different chemical and physical stimuli. In this paper we have translated biochemical reactions into differential equations for the development of modular model of metabolism of a hepatocyte cultured in static and standard conditions (in a plastic multiwell placed in an incubator at 37 degrees C with 5% of CO(2)). Using biochemical equations and energetic considerations a set of non-linear differential equations has been derived and implemented in Simulink. This set of equations mimics some of the principal metabolic pathways of biomolecules present in the culture medium. The software platform developed is subdivided into separate modules, each one describing a different metabolic pathway; they constitute a library which can be used for developing new modules and models to project, predict and validate cell behaviour in vitro.

Criticality of the biological and physical stimuli array inducing resident cardiac stem cell determination

The replacement of injured cardiac contractile cells with stem cell-derived functionally efficient cardiomyocytes has been envisaged as the resolutive treatment for degenerative heart diseases. Nevertheless, many technical issues concerning the optimal procedures to differentiate and engraft stem cells remain to be answered before heart cell therapy could be routinely used in clinical practice. So far, most studies have been focused on evaluating the differentiative potential of different growth factors without considering that only the synergistic cooperation of biochemical, topographic, chemical, and physical factors could induce stem cells to adopt the desired phenotype. The present study demonstrates that the differentiation of cardiac progenitor cells to cardiomyocytes does not occur when cells are challenged with soluble growth factors alone, but requires strictly controlled procedures for the isolation of a progenitor cell population and the artifactual recreation of a microenvironment critically featured by a fine-tuned combination of specific biological and physical factors. Indeed, the scaffold geometry and stiffness are crucial in enhancing growth factor differentiative effects on progenitor cells. The exploitation of this concept could be essential in setting up suitable procedures to fabricate functionally efficient engineered tissues. Disclosure of potential conflicts of interest is found at the end of this article.

Physical characterization of polycaprolactone scaffolds

Films and sponges were prepared from a solution of Poly(epsilon-caprolactone) (PCL) in tetrahydrofuran (THF). The porosity, crystallinity, and mechanical properties of the samples were studied. Porosity of around 15% was obtained for the films produced by evaporation of THF at room temperature. A much more porous structure (50-70%) was found for the sponges obtained by cooling the solution at -30 degrees C and subsequently eliminating the solvent by freeze drying. The porosity of the samples was also observed by scanning electron microscopy (SEM). The crystallinity of the samples was studied by the calorimetric technique (DSC) before and after the compression scans. The mechanical properties of the different samples were determined by compression test, and were compared to those corresponding to the PCL in bulk. The compression scans did not affect the crystallinity of the samples. The variations observed in the results of the different scans were attributed to the differences in porosities and crystallinity.

Quantitative biorelevant profiling of material microstructure within 3D porous scaffolds via multiphoton fluorescence microscopy

This study presents a novel approach, based on fluorescence multiphoton microscopy (MPM), to image and quantitatively characterize the microstructure and cell-substrate interactions within microporous scaffold substrates fabricated from synthetic biodegradable polymers. Using fluorescently dyed scaffolds fabricated from poly(DTE carbonate)/poly(DTO carbonate) blends of varying porosity and complementary green fluorescent protein-engineered fibroblasts, we reconstructed the three-dimensional distribution of the microporous and macroporous regions in 3D scaffolds, as well as cellular morphological patterns. The porosity, pore size and distribution, strut size, pore interconnectivity, and orientation of both macroscale and microscale pores of 3D scaffolds were effectively quantified and validated using complementary imaging techniques. Compared to other scaffold characterizing techniques such as confocal imaging and scanning electron microscopy (SEM), MPM enables the acquisition of images from scaffold thicknesses greater than a hundred microns with high signal-to-noise ratio, reduced bulk photobleaching, and the elimination of the need for deconvolution. In our study, the morphology and cytoskeletal organization of cells within the scaffold interior could be tracked with high resolution within the limits of penetration of MPM. Thus, MPM affords a promising integrated platform for imaging cell-material interactions within the interior of polymeric biomaterials.