A novel poly-l-lactic acid scaffold that possesses a macroporous structure and a branching/joining three-dimensional flow channel network: its fabrication and application to perfusion culture of human hepatoma Hep G2 cells

Fabrication of a Porous Three-Dimensional Scaffold with Interconnected Flow Channels: Co-Cultured Liver Cells and In Vitro Hemocompatibility Assessment

The development of large-scale human liver scaffolds equipped with interconnected flow channels in three-dimensional space offers a promising strategy for the advancement of liver tissue engineering. Tissue-engineered scaffold must be blood-compatible to address the demand for clinical transplantable liver tissue. Here, we demonstrate the construction of 3-D macro scaffold with interconnected flow channels using the selective laser sintering (SLS) fabrication method. The accuracy of the printed flow channels was ensured by the incorporation of polyglycolic acid (PGA) microparticles as porogens over the conventional method of NaCl salt leaching. The fabricated scaffold was populated with Hep G2, followed by endothelization with endothelial cells (ECs) grown under perfusion of culture medium for up to 10 days. The EC covered scaffold was perfused with platelet-rich plasma for the assessment of hemocompatibility to examine its antiplatelet adhesion properties. Both Hep G2-covered scaffolds exhibited a markedly different albumin production, glucose metabolism and lactate production when compared to EC-Hep G2-covered scaffold. Most importantly, EC-Hep G2-covered scaffold retained the antiplatelet adhesion property associated with the perfusion of platelet-rich plasma through the construct. These results show the potential of fabricating a 3-D scaffold with interconnected flow channels, enabling the perfusion of whole blood and circumventing the limitation of blood compatibility for engineering transplantable liver tissue.

Enhanced Functional Maturation of Fetal Porcine Hepatocytes in Three-Dimensional Poly-L-lactic Acid Scaffolds: A Culture Condition Suitable for Engineered Liver Tissues in Large-Scale Animal Studies

To engineer liver tissues with a clinically significant size, in vivo evaluation of performance using large-scale animal studies are necessary before proceeding to human clinical trials. As pigs are the most suitable candidates, the development of culture conditions suitable for porcine hepatocyte progenitors is very important to engineer pig liver tissue equivalents. We therefore investigated the efficacy of poly-L-lactic acid (PLLA) three-dimensional (3D) scaffolds on the functional maturation of fetal porcine hepatocytes in the presence of various combinations of biofactors. Cells were isolated from pig fetuses obtained from a local slaughterhouse, and cultured for 15 days both in monolayer and PLLA scaffolds. Although 15 days of culture resulted in almost the same ratio of proliferation (about fivefold) in both monolayer and 3D PLLA culture, the PLLA culture with hepatocyte growth factor (HGF, 10 ng/ml) and sodium butylate (Sb, 1 mM) remarkably enhanced various liver-specific functions of fetal porcine hepatocytes. The final attained functions based on the numbers of immobilized cells on day 1 compared with those of day 1 monolayers; 191fold increase in albumin secretion, 70.5-fold increase in cytochrome P450 IA1/2 capacity, 20.9-fold increase in ammonia removal, and 18.0-fold increase in urea synthesis were obtained. These functions were 2.0–3.3-fold higher than those obtained by the same period of monolayer culture. In addition, final attained unit cell-based functions on day 15 were almost comparable to the levels reported for cultures of adult porcine hepatocytes in both monolayer and 3D spheroid cultures. These results demonstrate that the use of a biodegradable polymer-based 3D culture with an appropriate combination of biofactors is a promising approach to maximize functional maturation of hepatocyte progenitors from large animals. In addition, the established culture conditions are worth using to engineer large liver tissue equivalents for pigs in large-animal-based preclinical studies.

Novel integrative methodology for engineering large liver tissue equivalents based on three-dimensional scaffold fabrication and cellular aggregate assembly

A novel engineering methodology for organizing a large liver tissue equivalent was established by intergrating both 'top down' and 'bottom up' approaches. A three-dimensional (3D) scaffold was engineered comprising 43 culture chambers (volume: 11.63 cm(3)) assembled in a symmetrical pattern on 3 layers, a design which enables further scaling up of the device to a clinically significant size (volume: 500 cm(3)). In addition, an inter-connected flow channel network was designed and proved to homogenously deliver culture medium to each chamber with the same pressure drop. After fabrication using nylon-12 and a selective laser sintering process, co-cultured cellular aggregates of human hepatoma Hep G2 and TMNK-1 cells were loosely packed into the culture chambers with biodegradable poly-L-lactic acid fibre pieces for 9 days of perfusion culture. The device enabled increased hepatic function and well-maintained cell viability, demonstrating the importance of an independent medium flow supply for cell growth and function provided by the current 3D scaffold. This integrative methodology from the macro- to the micro-scale provides an efficient way of arranging engineered liver tissue with improved mass transfer, making it possible to further scale up to a construct with clinically relevant size while maintaining high per-volume-based physiological function in the near future.

Construction of Large-Volume Tissue Mimics with 3D Functional Vascular Networks

We used indirect stereolithography (SL) to form inner-layered fluidic networks in a porous scaffold by introducing a hydrogel barrier on the luminal surface, then seeded the networks separately with human umbilical vein endothelial cells and human lung fibroblasts to form a tissue mimic containing vascular networks. The artificial vascular networks provided channels for oxygen transport, thus reducing the hypoxic volume and preventing cell death. The endothelium of the vascular networks significantly retarded the occlusion of channels during whole-blood circulation. The tissue mimics have the potential to be used as an in vitro platform to examine the physiologic and pathologic phenomena through vascular architecture.

Biomimetic design and fabrication of porous chitosan–gelatin liver scaffolds with hierarchical channel network

The presence of a hierarchical channel network in tissue engineering scaffold is essential to construct metabolically demanding liver tissue with thick and complex structures. In this research, chitosan–gelatin (C/G) scaffolds with fine three-dimensional channels were fabricated using indirect solid freeform fabrication and freeze-drying techniques. Fabrication processes were studied to create predesigned hierarchical channel network inside C/G scaffolds and achieve desired porous structure. Static in-vitro cell culture test showed that HepG2 cells attached on both micro-pores and micro-channels in C/G scaffolds successfully. HepG2 proliferated at much higher rates on C/G scaffolds with channel network, compared with those without channels. This approach demonstrated a promising way to engineer liver scaffolds with hierarchical channel network, and may lead to the development of thick and complex liver tissue equivalent in the future.

Design and assessment of a microfluidic network system for oxygen transport in engineered tissue

Oxygen and nutrients cannot be delivered to cells residing in the interior of large-volume scaffolds via diffusion alone. Several efforts have been made to meet the metabolic needs of cells in a scaffold by constructing mass transport channels, particularly in the form of bifurcated networks. In contrast to progress in fabrication technologies, however, an approach to designing an optimal network based on experimental evaluation has not been actively reported. The main objective of this study was to establish a procedure for designing an effective microfluidic network system for a cell-seeded scaffold and to develop an experimental model to evaluate the design. We proposed a process to design a microfluidic network by combining an oxygen transport simulation with biomimetic principles governing biological vascular trees. The simulation was performed with the effective diffusion coefficient (D(e,s)), which was experimentally measured in our previous study. Porous scaffolds containing an embedded microfluidic network were fabricated using the lost mold shape-forming process and salt leaching method. The reliability of the procedure was demonstrated by experiments using the scaffolds. This approach established a practical basis for designing an effective microfluidic network in a cell-seeded scaffold.

Enhanced endothelialization for developing artificial vascular networks with a natural vessel mimicking the luminal surface in scaffolds

Large tissue regeneration remains problematic because of a lack of oxygen and nutrient supply. An attempt to meet the metabolic needs of cells has been made by preforming branched vascular networks within a scaffold to act as channels for mass transport. When constructing functional vascular networks with channel patency, emphasis should be placed on anti-thrombogenic surface issues. The aim of this study was to develop a rapid endothelialization method for creating an anti-thrombogenic surface mimicking the natural vessel wall in the artificial vascular networks. Shear stress preconditioning and scaffold surface modification were investigated as effective approaches for promoting biomaterial endothelialization. We found that a transient increase in shear stress at the appropriate time is key to enhancing endothelialization. Moreover, surface modification with bioactive materials such as collagen and recombinant mussel adhesive protein fused with arginine-glycine-aspartic acid peptide (MAP-RGD) showed a synergetic effect with shear stress preconditioning. Platelet adhesion tests demonstrated the anti-thrombogenic potential of MAP-RGD itself without endothelialization. The rapid endothelialization method established in this study can be easily applied to preformed artificial vascular networks in porous scaffolds. Development of artificial vascular networks with an anti-thrombogenic luminal surface will open up a new chapter in tissue engineering and regenerative medicine.

Engineered channels enhance cellular density in perfused scaffolds

Scaffold-based tissue engineering provides cells with an engineered matrix to enhance and direct cell attachment, proliferation and differentiation. One critical limitation to current tissue engineering approaches is the inability to create densely populated constructs thicker than a few 100 μm. We hypothesized that development of porous, channeled scaffolds would increase cell density and uniformity of their spatial distribution through scaffold channel perfusion. Patterned polyurethane sheets were fabricated using a sprayed phase separation technique and laminated together to form 1.5 mm thick channeled scaffolds. Hydraulic permeability testing confirmed the presence of functional channels throughout the multilaminate construct. A continuous flow bioreactor was used to perfuse the construct with medium during the culture period. Cross-sectional cell densities and spatial uniformities were measured in channeled and nonchanneled scaffolds under different seeding and culture conditions. Channeled scaffolds were found to have higher densities of human mesenchymal stem cells than nonchanneled samples. Perfused scaffolds had more uniform spatial distribution of cells within the scaffold compared to statically cultured scaffolds. In conclusion, we have shown the channeled scaffolds to be a promising approach toward creating thick tissue-engineered constructs.

The realistic prediction of oxygen transport in a tissue-engineered scaffold by introducing time-varying effective diffusion coefficients

An adequate oxygen supply is one of the most important factors needed in order to regenerate or engineer thick tissues or complex organs. To devise a method for maximizing the amount of oxygen available to cells, it is necessary to understand and to realistically predict oxygen transport within an engineered tissue. In this study, we focused on the fact that oxygen transport through a tissue-engineered scaffold may vary with time as cells proliferate. To confirm this viewpoint, effective oxygen diffusion coefficients (D(e)(,)(s)) of scaffolds were deduced from experimental measurements and simulations of oxygen-concentration profiles were performed using these D(e)(,)(s) values in a two-dimensional (2-D) perfusion model. The results of this study indicate that higher porosity, hydraulic permeability and interconnectivity of scaffolds with no cells are responsible for the prominent diffusion capability quantified using D(e)(,)(s). On the other hand, the D(e)(,)(s) of scaffolds with cells has a negative linear relationship with cell density. Cell proliferation with time leads to a significant decrease in oxygen concentration in the 2-D perfusion model. This result demonstrates the gradual restriction of oxygen transport in a porous scaffold during cell culture. Therefore, the realistic prediction of oxygen transport using a time-varying D(e)(,)(s) will provide an appropriate basis for designing optimal transport networks within a thick scaffold.

Laser sintering fabrication of three-dimensional tissue engineering scaffolds with a flow channel network

The fabrication of tissue engineering scaffolds for the reconstruction of highly oxygen-dependent inner organs is discussed. An additive manufacturing technology known as selective laser sintering was employed to fabricate a highly porous scaffold with an embedded flow channel network. A porogen leaching system was used to obtain high porosity. A prototype was developed using the biodegradable plastic polycaprolactone and sodium chloride as the porogen. A high porosity of 90% was successfully obtained. Micro x-ray CT observation was carried out to confirm that channels with a diameter of approximately 1 mm were generated without clogging. The amount of residual salt was 930 µg while the overall volume of the scaffold was 13 cm(3), and it was confirmed that the toxicity of the salt was negligible. The hydrophilization of the scaffold to improve cell adhesion on the scaffold is also discussed. Oxygen plasma ashing and hydrolysis with sodium hydroxide, typically employed to improve the hydrophilicity of plastic surfaces, were tested. The improvement of hydrophilicity was confirmed by an increase in water retention by the porous scaffold from 180% to 500%.

Branched Channel Scaffolds Fabricated by SFF for Direct Cell Growth Observations

A β-TCP scaffold with a branched channel system was designed to create a novel micro-device that allowed culture perfusion and direct time observation of the cells attached. The scaffold was made by indirect solid free form fabrication (SFF) technology. The flow channel structure was exposed so that the perfusion of the mesenchymal stem cell (MSC) culture could be viewed directly. The cell-seeded scaffolds were continuously perfused for 7 days in the micro-device; during this time, it was possible to observe the dynamic culture processes with cells adhering to the scaffolds and real time cell growth directly. This concept has great potential for use in bone tissue engineering and for versatile fabrication of enhanced scaffolds.

Design and fabrication of 3D porous scaffolds to facilitate cell-based gene therapy

Biomaterials capable of efficient gene delivery by embedded cells provide a fundamental tool for the treatment of acquired or hereditary diseases. A major obstacle is maintaining adequate nutrient and oxygen diffusion to cells within the biomaterial. In this study, we combined the solid free-form fabrication and porogen leaching techniques to fabricate three-dimensional scaffolds, with bimodal pore size distribution, for cell-based gene delivery. The objective of this study was to design micro-/macroporous scaffolds to improve cell viability and drug delivery. Murine bone marrow-derived mesenchymal stromal cells (MSCs) genetically engineered to secrete erythropoietin (EPO) were seeded onto poly-L-lactide (PLLA) scaffolds with different microporosities. Over a period of 2 weeks in culture, an increase in cell proliferation and metabolic activity was observed with increasing scaffold microporosity. The concentration of EPO detected in supernatants also increased with increasing microporosity level. Our study shows that these constructs can promote cell viability and release of therapeutic proteins, and clearly demonstrates their capacity for a dual role as scaffolds for tissue regeneration and as delivery systems for soluble gene products.

Biological cells on microchips: New technologies and applications

Integration of various chemical devices and complex operations onto a microchip, which is often referred to as a micro total analysis system (mu-TAS) or lab-on-a-chip, creates extremely efficient devices that exploit the advantages of a microspace. Furthermore, as the scale of the fluidic microvolume is roughly proportional to living cell sizes and processing capabilities, cells and micro chemical systems can be combined to develop practical prototypical microdevices. This approach has led to development of tools for investigating cellular functions, biochemical reactors and bioassay systems, as well as hybrid bio/artificial tissue engineered organs. Recently, bio-microactuators exploiting mechanical properties of cells powered without external energy sources have also been reported. This review focuses on new technologies involving cell-based devices on microchips, with a special emphasis on bio-microactuators. Firstly, we review systems to place and handle cells on a microchip. Secondly, we review bio-microactuators developed using single or a few driving cells. Finally, we review bio-microactuators developed using numerous cells or tissue to generate stronger forces. Understanding fundamental concepts behind the distinct features and performance characteristics of these cell-based micro-systems will lead to development of new devices that will be exploited in various fields in the future.

Avidin–biotin binding-based cell seeding and perfusion culture of liver-derived cells in a porous scaffold with a three-dimensional interconnected flow-channel network

To engineer implantable liver tissues, we designed a novel scaffold with a three-dimensional (3D) branching and joining flow-channel network comprising multiple tetrahedral units (4-mm edge length). For the fabrication of this network, biodegradable polycaprolactone (PCL) and 80% (w/w) NaCl salt particles serving as porogen were thoroughly mixed and applied in a selective laser sintering (SLS) process, a technique adapted to rapid prototyping. We thus obtained a scaffold that had high (89%) porosity with a pore size of 100-200 microm and 3D flow channels. To evaluate its biocompatibility, human hepatoma Hep G2 cells were seeded into the scaffold using avidin-biotin (AB) binding and cultured in a perfusion system for 9 days. The results demonstrated that such 3D flow channels are essential to the cells' growth and function. In addition, the AB binding-based seeding remarkably improved the overall performance of the cell-loaded scaffolds. The fabrication of a much finer scaffold, having a 500 cm(3) scale, based on the same design and the use of human hepatocyte progenitors, may, in the near future, lead to the development of an implantable liver tissue equivalent for use in humans.

Polymer carriers for drug delivery in tissue engineering

Growing demand for tissues and organs for transplantation and the inability to meet this need using by autogeneic (from the host) or allogeneic (from the same species) sources has led to the rapid development of tissue engineering as an alternative. Tissue engineering aims to replace or facilitate the regrowth of damaged or diseased tissue by applying a combination of biomaterials, cells and bioactive molecules. This review focuses on synthetic polymers that have been used for tissue growth scaffold fabrication and their applications in both cell and extracellular matrix support and controlling the release of cell growth and differentiation supporting drugs.

The construction of three-dimensional micro-fluidic scaffolds of biodegradable polymers by solvent vapor based bonding of micro-molded layers

It is increasingly important to control cell growth into and within artificial scaffolds. Tissues such as skin, blood vessels, and cartilage have multi-layer structures with different cells in each layer. With the aid of micro-fabrication technology, a novel scaffolding method for biodegradable polymers such as polylactic acid (PLA), polyglycolic acid (PGA), and the copolymers poly(lactide-co-glycolide)(PLGA), was developed to construct three-dimensional multi-layer micro-fluidic tissue scaffolds. The method emphasizes micro-fluidic interconnections between layers within the scaffolds and maintenance of high-resolution geometries during the bonding process for the creation of multi-layered scaffolds. Micro-holes (10-100 microm), micro-channels, and micro-cavities were all created by micro-molding. Solvent-vapor based bonding of micro-molded layers preserved 20 microm sized structures. Sample scaffolds were constructed for purposes such as channel-directed cell growth and size-based cell sorting. Further extension of these techniques to create a micro-vascular network within or between layers is possible. Culturing of human coronary artery endothelial cells (HCAECs) on the sample scaffolds demonstrated the biocompatibility of the developed process and the strong influence of high-resolution micro-geometries on HCAEC growth.

Rapid prototyping in tissue engineering: challenges and potential

Tissue engineering aims to produce patient-specific biological substitutes in an attempt to circumvent the limitations of existing clinical treatments for damaged tissue or organs. The main regenerative tissue engineering approach involves transplantation of cells onto scaffolds. The scaffold attempts to mimic the function of the natural extracellular matrix, providing a temporary template for the growth of target tissues. Scaffolds should have suitable architecture and strength to serve their intended function. This paper presents a comprehensive review of the fabrication methods, including conventional, mainly manual, techniques and advanced processing methods such as rapid prototyping (RP) techniques. The potential and challenges of scaffold-based technology are discussed from the perspective of RP technology.