Preparation of poly(glycolic acid) bonded fiber structures for cell attachment and transplantation

Ectopic neocartilage formation from predifferentiated human adipose derived stem cells induced by adenoviral-mediated transfer of hTGF beta2

Chondrogenic potential of human adipose derived stem cells (hASCs) makes them a possible source of seeding cells for cartilage tissue engineering. In this study, chondrogenic differentiation of hASCs induced by transduction with replication-deficient adenovirus carrying human transforming growth factor beta2 (Ad5-hTGF beta2) was demonstrated by RT-PCR, immunohistochemistry staining, biochemical and western blot analysis. To evaluate if the in vitro differentiated hASCs could keep their chondrocytic phenotype and produce neo-cartilage in vivo, predifferentiated hASCs were seeded in different scaffolds and implanted in subcutaneous pockets on the dorsum of nude mice. After 4 and 12 weeks culture in vivo, specimens were harvested and examined by histological and immunohistochemical analysis, cartilage-like tissue formation was only found in alginate gel and PLGA/alginate compound groups, in PLGA group, fibrous tissues and angiogenesis ingrowth were observed. These findings demonstrated that adenovirus-mediated hTGF beta2 gene transfer could induce hASCs into a chondrogenic lineage in vitro, however, this predifferentiation did not guarantee ectopic cartilage formation in vivo unless appropriate three-dimensional scaffolds were used as the cell carry vehicles.

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.

Low Density Sub-Critical CO2-Blown Solid-State PLA Foams

This paper presents a characterization of the PLA-CO2 system for solid-state foaming process with a particular goal of producing low density foams. The diffusivity of CO2 in PLA increases with increasing saturation pressures. The results show that low relatively density (lower than 10%) can be achieved in a wide processing window. The cell size can vary from tens of microns to several hundreds of microns depending on the solid-state foaming process parameters. There are significant differences in relative density and cell size for 2 and 3 MPa saturation cases, which are likely due to the much higher crystallinity at 3 MPa (26.4%) compared to 2 MPa (9.7%). Foams with cell size of around 300 μm and relative density of 3% were produced at 2 MPa saturation.

It was observed that the higher the gas pressure, the higher the rate of gas-induced crystallization. Further, the gas-induced crystallization as well as the gas concentration in PLA during sorption reach their respective maximum values at approximately the same time for all gas pressures explored. For specimens saturated at 2 MPa, there is a substantial jump in crystallinity (from 5.2% to 23.4%) after foaming.

Influence of the substrate's hydrophilicity on thein vitro Schwann cells viability

A series of polymeric biomaterials including poly (methyl acrylate) (PMA), chitosan (CHT), poly(ethyl acrylate) (PEA), poly(hydroxyethyl acrylate) (PHEA), and a series of random copolymers containing ethyl acrylate and hydroxyethyl acrylate monomeric units were tested in vitro as culture substrates and compared for their impact on the proliferation and expansion of Schwann cells (SCs). Immunocytochemical staining assay and scanning electron microscopy techniques were applied to perform a quantitative analysis to determine the correct maintenance of the cultured glial cells on the different biomaterials. The results strongly suggest that cell attachment and proliferation is influenced by the substrate's surface chemistry, and that hydrophobic biomaterials based on PMA, PEA, and the copolymers PEA and PHEA in a narrow composition window are suitable substrates to promote cell attachment and proliferation of SCs in vitro.

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.

Solvent-assisted room-temperature compression molding approach to fabricate porous scaffolds for tissue engineering

This study investigated the room-temperature compression molding/particle leaching approach to fabricate three-dimensional porous scaffolds for tissue engineering. Scaffolds with anatomical shapes (ear, joint, tube, cylinder) were made from biodegradable poly(D,L-lactide) and poly[(D,L-lactide)-co-glycolide]. The utility of this room-temperature compression approach comes from the effect of solvent assistance, but the tendency for post-molding scaffold shrinkage is a problem unique to this method and is thus examined with emphasis in this paper. Scaffold shrinkage was found to be tolerable under normal fabrication conditions with high salt contents, which is just what the preparation of highly porous scaffolds requires. Furthermore, the resultant porosities after salt leaching were measured as well as the initial scaffold shrinkages after solvent evaporation, and the relation between them was revealed by theoretical analysis and confirmed by comparison with experimental measurements. The pores were interconnected, and porosity can exceed 90%. The effects of porosity on the mechanical properties of porous scaffolds were also investigated. This convenient fabrication approach is a prospective method for the tailoring of porous scaffolds for a variety of possible applications in tissue engineering and tissue reconstruction.

Preparation of 3-D regenerated fibroin scaffolds with freeze drying method and freeze drying/foaming technique

Although three-dimensional fibroin scaffolds have been prepared with freeze drying method, the porosity and pore sizes still can not satisfy the requirement of tissue engineering. In this article, fibroin porous scaffold with high porosity and > 100 microm diameter interconnected pores was firstly prepared with freeze drying method through adjusting fibroin concentration. The morphology of different scaffolds lyophilized from different fibroin concentration was observed by SEM. A novel freeze drying improved method, freeze drying/foaming technique, was also devised to prepare fibroin scaffolds at different fibroin concentrations. Using the said method, the porosity and pore size of fibroin scaffolds prepared from 12% concentration were 85.8 +/- 4% and 109 +/- 20 microm respectively with yield strength up to 450 +/- 6 KPa while the porosity and pore size of fibroin scaffolds prepared from 8% concentration were 96.9 +/- 3.6% and 120 +/- 30 microm respectively with yield strength up to 30 +/- 1 KPa. The freeze drying/foaming technique produced scaffolds with a useful combination of high yield strength, interconnected pores, and pore sizes greater than 100 microm in diameter. Through adjusting fibroin concentration and thawing time, the porosity, pore sizes and mechanical properties could be controlled to satisfy the different requirements of tissue engineering. The results suggested that fibroin scaffolds prepared with the above methods could be formed for utility in biomaterial application.

Future of Regenerative Medicine: Challenges and Hurdles

Tissue regeneration strategies such as tissue engineering, growth factor administration, and stem cell-based therapies have undergone significant development over the past two decades. Most notably, we are much closer to realizing the engineering of whole organs and tissue with complex architecture than we were 5 years ago. A major driving force has been the demand placed by the scientific community at large and the public to go beyond simple engineering of tissues and demonstrate functionality in engineered tissues and functional recovery upon transplantation. Some recent advances include de novo engineering of bone, engineering of fully functional bladder, and vascularization of skeletal muscle constructs. Notwithstanding, several challenges lie ahead in making regenerative medicine a viable science of the future, the key being the evolution of programs and policies that promote a close relationship among government agencies, private sector, and academia, more specifically between materials scientists, biologists, and clinicians.

Angiogenic and inflammatory response to biodegradable scaffolds in dorsal skinfold chambers of mice

For tissue engineering, scaffolds should be biocompatible and promote neovascularization. Because little is known on those specific properties, we herein studied in vivo the host angiogenic and inflammatory response after implantation of commonly used scaffold materials. Porous poly(L-lactide-co-glycolide) (PLGA) and collagen-chitosan-hydroxyapatite hydrogel scaffolds were implanted into dorsal skinfold chambers of balb/c mice. Additional animals received cortical bone as an isogeneic, biological implant, while chambers of animals without implants served as controls. Angiogenesis and neovascularization as well as leukocyte-endothelial cell interaction and microvascular permeability were analyzed over 14 day using intravital fluorescence microscopy. PLGA scaffolds showed a slight increase in leukocyte recruitment compared to controls. This was associated with an elevation of microvascular permeability, which was comparable to that observed in isogeneic bone tissue. Of interest, PLGA induced a marked angiogenic response, revealing a density of newly formed capillaries almost similar to that observed in bone implants. Histology showed infiltration of macrophages, probably indicating resorption of the biomaterial. In contrast, hydrogel scaffolds induced a severe inflammation, as indicated by an approximately 15-fold increase of leukocyte-endothelial cell interaction and a marked elevation of microvascular permeability. This was associated by induction of apoptotic cell death within the surrounding tissue and a complete lack of ingrowth of newly formed microvessels. Histology confirmed adequate engraftment of PLGA and isogeneic bone but not hydrogel within the host tissue. PLGA scaffolds show a better biocompatibility than hydrogel scaffolds and promote vascular ingrowth, guaranteeing adequate engraftment within the host tissue.

Macroporous elastomeric scaffolds with extensive micropores for soft tissue engineering

Macroporous scaffolds are of great value in tissue engineering. We have developed a method to fabricate macroporous scaffolds from a biocompatible and biodegradable elastomer, poly(glycerol sebacate) (PGS). This method is potentially very useful for soft tissue engineering. Our fabrication method produced macroporous scaffolds with extensive micropores. We fabricated flat scaffolds and tubular scaffolds of uniform thickness. This fabrication method demonstrated good control of variables such as pore size, porosity, and pore interconnectivity. Sodium chloride (salt) crystals, which served as solid porogens, were packed into a mold and fused in a humid chamber. PGS was cured while dispersed throughout the fused salt template. Dissolution of the salt and subsequent lyophilization produced elastomer sponges with approximately 90% porosity, interconnected macropores (75-150 microm), and extensive micropores (5-20 microm). The macropores were generated by the salt particles, while the micropores were likely generated by glycerol vapor formed during PGS curing. Such numerous micropores could facilitate cell-cell interactions and mass transport. Fibroblasts adhered to and proliferated well within the PGS scaffolds and formed three-dimensional tissue-engineered constructs within 8 days.

Branched peptide-amphiphiles as self-assembling coatings for tissue engineering scaffolds

An important challenge in regenerative medicine is the design of suitable bioactive scaffold materials that can act as artificial extracellular matrices. We reported previously on a family of peptide-amphiphile (PA) molecules that self-assemble into high-aspect ratio nanofibers under physiological conditions, and can display bioactive peptide epitopes along each nanofiber's periphery. One type of PA displays its epitope at a branched site using a lysine dendron, a molecular feature that improves epitope availability on the nanofiber surface. In this work, we describe the application of these branched PA (b-PA) systems as self-assembling coatings for fiber-bonded poly(glycolic acid) scaffolds. b-PAs bearing variations of the RGDS adhesion epitope from fibronectin were shown by elemental analysis to coat repeatably onto fiber scaffolds. The retention of supramolecular organization after coating on the scaffold was demonstrated through spectroscopic identification of beta-sheet structures and the close association of hydrophobic tails in a model pyrene-containing PA system. Primary human bladder smooth muscle cells demonstrated greater initial adhesion to b-PA-functionalized scaffolds than to bare scaffolds or to those coated with linear PAs. This strategy of molecular design and coating may have potential application in bladder tissue regeneration.

3D scaffolds for bone marrow stem cell support in bone repair

Bone tissue repair is one of the major concerns of regenerative medicine. The current need for tissue replacements has necessitated the development of a new science termed ‘bone tissue engineering’. The basic organization of bone tissue requires the design and fabrication of a porous 3D structure or ‘scaffold’ to contain the bone-forming cells. This scaffold should be formulated from biocompatible, osteoconductive materials that are not immunoreactive. 3D scaffolds provide the necessary support for cells to proliferate and maintain their capacity to differentiate and scaffolds containing bone marrow-derived osteoprogenitors can be employed within implants to enhance bone repair. The complex construct is intended to mimic the native in vivo microenvironment and this demands construction of bioactive scaffolds that are also capable of supporting vascularization as well as cell proliferation and osteogenic differentiation. 3D bioactive scaffolds containing committed osteoprogenitors can provide a promising surgical tool for bone tissue engineering directed at orthopedic and cranio-maxillofacial clinical applications.

The influence of architecture on degradation and tissue ingrowth into three-dimensional poly(lactic-co-glycolic acid) scaffolds in vitro and in vivo

The in vitro and in vivo degradation properties of poly(lactic-co-glycolic acid) (PLGA) scaffolds produced by two different technologies-thermally induced phase separation (TIPS), and solvent casting and particulate leaching (SCPL) were compared. Over 6 weeks, in vitro degradation produced changes in SCPL scaffold dimension, mass, internal architecture and mechanical properties. TIPS scaffolds produced far less changes in these parameters providing significant advantages over SCPL. In vivo results were based on a microsurgically created arteriovenous (AV) loop sandwiched between two TIPS scaffolds placed in a polycarbonate chamber under rat groin skin. Histologically, a predominant foreign body giant cell response and reduced vascularity was evident in tissue ingrowth between 2 and 8 weeks in TIPS scaffolds. Tissue death occurred at 8 weeks in the smallest pores. Morphometric comparison of TIPS and SCPL scaffolds indicated slightly better tissue ingrowth but greater loss of scaffold structure in SCPL scaffolds. Although advantageous in vitro, large surface area:volume ratios and varying pore sizes in PLGA TIPS scaffolds mean that effective in vivo (AV loop) utilization will only be achieved if the foreign body response can be significantly reduced so as to allow successful vascularisation, and hence sustained tissue growth, in pores less than 300 microm.

Tissue-Engineered Lung: An In Vivo and In Vitro Comparison of Polyglycolic Acid and Pluronic F-127 Hydrogel/Somatic Lung Progenitor Cell Constructs to Support Tissue Growth

In this study, we describe the isolation and characterization of a population of adult-derived or somatic lung progenitor cells (SLPC) from adult mammalian lung tissue and the promotion of alveolar tissue growth by these cells (both in vitro and in vivo) after seeding onto synthetic polymer scaffolds. After extended in vitro culture, differentiating cells expressed Clara cell 10kDa protein, surfactant protein-C, and cytokeratin but did not form organized structures. When cells were combined with synthetic scaffolds, polyglycolic acid (PGA) or Pluronic F-127 (PF-127), and maintained in vitro or implanted in vivo, they expressed lung-specific markers for Clara cells, pneumocytes, and respiratory epithelium and organized into identifiable pulmonary structures (including those similar to alveoli and terminal bronchi), with evidence of smooth muscle development. Although PGA has been shown to be an excellent polymer for culture of specific cell types in vitro, in vivo culture in an immunocompetent host induced a foreign body response that altered the integrity of the developing lung tissue. Use of PF-127/cell constructs resulted in the development of tissue with less inflammatory reaction. These data suggest that the therapeutic use of engineered tissues requires both the use of specific cell phenotypes, as well as the careful selection of synthetic polymers, to facilitate the assembly of functional tissue.

A method for solvent-free fabrication of porous polymer using solid-state foaming and ultrasound for tissue engineering applications

Most of the existing fabrication techniques for tissue engineering scaffolds require the use of organic solvents that may never be fully removed even after long leaching hours. The residues of these organic solvents reduce the ability of biological cells to form new tissue. This paper presents an approach toward solvent-free fabrication of tissue engineering scaffolds. Interconnected porous structures were created using solid-state foaming and ultrasound. The material used in this study was polylactic acid (PLA) and the blowing agent was CO(2). In order to determine suitable process conditions, saturation and foaming studies were first conducted. Selected foam samples were then processed using pulsed ultrasound. The microstructures before and after the ultrasound processing were compared. It was shown that the inter-pore connectivity of the solid-state foams was substantially enhanced. The combined solid-state foaming and ultrasound processing provide a way to fabricate porous polymer for potential tissue engineering applications.

Degradation behavior of hydrophilized PLGA scaffolds prepared by melt-molding particulate-leaching method: comparison with control hydrophobic one

Porous PLGA/PVA scaffolds as hydrophilized PLGA scaffolds for tissue engineering applications were fabricated by a novel melt-molding particulate leaching method (non-solvent method). The prepared scaffolds exhibited highly porous and open-cellular pore structures with almost same surface and interior porosities (pore size, 200-300 microm; porosity, about 90%). The in vitro degradation behavior of the PLGA and PLGA/PVA scaffolds was compared at 37 degrees C in PBS (pH 7.4) with and without the solution change everyday to see the effect of solution pH as well as scaffold hydrophilicity on the degradation behavior. The changes in dimension, molecular weight, mechanical properties (maximum load and modulus), and morphology of the scaffolds were examined with degradation time. The degradation behavior of the PLGA and PLGA/PVA scaffolds was further investigated in vivousing a rat model (subcutaneously implantation). It was observed that both PLGA and PLGA/PVA scaffolds in decreasing pH condition (PBS no change) showed faster degradation than those in constant pH condition (PBS change everyday), owing to the enhanced intramolecular depolymerization by the increment of chain hydrophilicity caused by carboxylate groups as well as the autocatalysis of carboxylic acids accumulated in the solution by the cleavage of PLGA backbone ester bonds. The scaffolds in vivo condition also showed faster degradation than those in vitro, probably due to the aid of foreign body giant cells or enzymes. The PLGA/PVA scaffold showed slightly faster degradation than the PLGA scaffold for both in vitro and in vivo conditions.

Preparation of macroporous polymer scaffolds using calcined cancellous bone as a template

A technique for preparing porous polymeric scaffolds using calcined cancellous bone as template has been developed. The scaffolds prepared by this method were morphologically similar to the structure of the demineralized bone matrix (DBM), i.e. scaffolds prepared by this novel method possessed high porosity, interconnected pores and uniform pore distribution. In addition, the mechanical properties of the scaffolds prepared by this novel method were significantly improved as compared to that of the scaffolds prepared by conventional solvent cast-particle leaching (SC-PL) method. The effects of concentration of the polymer solution on the structure, porosity and mechanical properties of the scaffolds were evaluated and the results showed that the mechanical properties of the scaffolds could be controlled by adjusting the concentration of the polymer solution. The advantages mentioned above suggest that this novel method might be a useful technique to prepare polymer scaffolds for tissue engineering.

Fabrication and characterization of poly(DL-lactic-co-glycolic acid)/zirconia-hybridized amorphous calcium phosphate composites

Several minerals, such as hydroxyapatite and beta-tricalcium phosphate, have been incorporated into bioresorbable polyester bone scaffolds to increase the osteoconductivity both in vitro and in vivo. More soluble forms of calcium phosphate that release calcium and phosphate ions have been postulated as factors that increase osteoblast differentiation and mineralization. Recently, a zirconia-hybridized pyrophosphate-stabilized amorphous calcium phosphate (Zr-ACP) has been synthesized allowing controlled release of calcium and phosphate ions. When incorporated into bioresorbable scaffolds, Zr-ACP has the potential to induce osteoconductivity. In this study, 80-90% (w/v) porous poly(DL-lactic-co-glycolic acid) (PLGA) scaffolds were formed by thermal phase separation from dioxane while incorporating Zr-ACP. Scanning electron microscopy revealed a highly porous structure with a pore size ranging from a few microm to about 100 microm, smaller than we had hoped for. Zr-ACP particles were evenly dispersed in the composite structure and incorporated into the pore walls. The amorphous structure of the Zr-ACP was maintained during composite fabrication, as found by X-ray diffraction. Composite scaffolds had larger compressive yield strengths and moduli compared to pure polymer scaffolds. These initial efforts demonstrate that PLGA/Zr-ACP composites can be formed in ways that ultimately serve as promising bone scaffolds in tissue engineering.

Fabrication and characterization of DTBP-crosslinked chitosan scaffolds for skin tissue engineering

Chitosan, the deacetylated derivative of chitin, is a promising scaffold material for skin tissue engineering applications. It is biocompatible and biodegradable, and the degradation products are resorbable. However, the rapid degradation of chitosan and its low mechanical strength are concerns that may limit its use. In this study, chitosan with 80%, 90% and 100% degree of deacetylation (DDA) was crosslinked with dimethyl 3-3, dithio bis' propionimidate (DTBP) and compared to uncrosslinked scaffolds. The scaffolds were characterized with respect to important tissue engineering properties. The tensile strength of scaffolds made from 100% DDA chitosan was significantly higher than for scaffolds made from 80% and 90% DDA chitosan. Crosslinking of scaffolds with DTBP increased the tensile strength. Crosslinking with DTBP had no significant effect on water vapour transmission rate (WVTR) or water absorption but had significant effect on the pore size and porosity of the samples. All samples showed a WVTR and pore size distribution suitable for skin tissue engineering; however, the water absorption and porosity were lower than the optimal values for skin tissue engineering. The biodegradation rate of scaffolds crosslinked with DTBP and glutaraldehyde (GTA) were reduced while no significant effect was observed in biodegradation of the samples made from 100% DDA chitosan whether crosslinked or uncrosslinked after 24 days of degradation.

Microspheres leaching for scaffold porosity control

Scaffold morphology plays a key role in the development of tissue engineering constructs. The control of pore size, shape and interconnection is needed to achieve adequate nutrient transport and cell ingrowth. Several techniques are available for scaffold manufacturing, but none allows easy control of morphology and is, at the same time, applicable to a wide variety of materials. To investigate the possibility of processing a wide range polymers by solvent casting/particulate leaching with accurate control of scaffold morphology, three different porogens (gelatin microspheres, paraffin microspheres and sodium chloride crystals) were used to fabricate scaffolds from commonly employed biodegradable polymers. The outcome of processing was evaluated in terms of scaffold morphology and structure/properties relationships. Highly porous scaffolds were obtained with all porogens and well defined spherical pores resulted from microspheres leaching. Furthermore, scaffolds with spherical pores showed better mechanical performance and lower flow resistance. Cytocompatibility tests performed showed no evidence of processing residuals released from the scaffolds. Solvent casting/microspheres leaching, particularly gelatin microspheres leaching, can be used to process a large number of polymers and enables to tailor scaffold pore size, shape and interconnection, thus providing a powerful tool for material selection and optimization of scaffold morphology.

Biodegradable HA-PLA 3-D porous scaffolds: effect of nano-sized filler content on scaffold properties

Scaffolds comprising poly(lactic acid) and nano-hydroxyapatite (HA) were prepared using the solvent-casting/salt-leaching technique. NaCl was used as the leaching agent. Nano-sized HA was synthesized by a hydrothermal method at 170 degrees C and autogenous pressure. High-resolution TEM imaging revealed that the HA particles were ellipsoidal-shaped with needle-like morphologies. The particles had an average size of approximately 25 nm in width and 150 nm in length with aspect ratios ranging from 6 to 8. As the HA content increased in the scaffold from 0 to 50 wt%, the compression modulus of the scaffolds increased from 4.72+/-1.2 to 9.87+/-1.8 MPa, while the yield strength from 0.29+/-0.03 to 0.44+/-0.01 MPa. Such polymeric scaffolds should be suitable materials for non-load sharing tissue-engineering applications.

A novel culture system for porcine odontogenic epithelial cells using a feeder layer

The growth of cells in vitro can provide useful models for investigating their behaviour and improving our understanding of their function in vivo. Although the developmental regulation of enamel matrix formation has been comprehensively analysed, the detailed cellular characteristics of ameloblasts remain unclear because of the lack of a system of long-term in vitro culture. Therefore, the establishment of odontogenic epithelial cell lines has taken on a new significance. Here, we report on a novel porcine odontogenic epithelial cell-culture system, which has permitted serial culture of these cells. Epithelial cells were harvested from third molar tooth buds in the fresh mandibles of 6-month-old pigs, and seeded on dishes in D-MEM containing 10% FBS. Before the cells reached confluence, the medium was changed to LHC-9 to select the epithelial cells. When trypsinized epithelial cells were plated together with 3T3-J2 cells as a feeder layer, the epithelial cells grew from single cells into colonies. The colonies then expanded and became confluent, and could be sub-cultured for up to 20 passages. The long-term culture cells expressed mRNA for amelogenin and ameloblastin, as well as enamelysin (MMP-20), which is a tissue-specific gene product unique to ameloblasts. These results show that the system is capable of sustaining the multiplication of odontogenic epithelial cells with the characteristics of ameloblasts.

Bioresorbable composites prepared by supercritical fluid foaming

Bone is a natural composite construct, with a gradient structure going from a loose interconnected cellular core to an outer dense wall, thus minimizing bone weight while keeping a high mechanical resistance. Due to this unique and complex structure, bone defects are difficult to replace or repair. Tissue engineering aims at providing artificial bone grafts. Several techniques have been proposed to produce porous structures or scaffolds, but, as yet, with no optimal solutions. This article focuses on bioresorbable ceramic-polymer composite foams obtained by supercritical fluid foaming. This flexible technique enables an adequate morphology and suitable properties for bone tissue engineering to be obtained. Composite scaffolds are biocompatible, allowing cell proliferation and differentiation.

Histological and immunohistochemical studies of tissue engineered odontogenesis

The successful regeneration of complex tooth structures based on tissue-engineering principles was recently reported. The process of this regeneration, however, remains poorly characterized. In this study, we have used histochemistry to examine the regeneration process of tissue engineered teeth in order to determine the cell types that give rise to these engineered tooth structures. Porcine third molar tooth buds were dissociated into single-cell suspensions and seeded onto a biodegradable polyglycolic acid polymer scaffold. Following varying periods of growth in rat hosts, the specimens were evaluated by histology and immunohistochemistry. Aggregates of epithelial cells were first observed 4-6 weeks after implantation. These aggregates assumed three different shapes: a natural tooth germ-like shape, a circular shape, or a bilayer-bundle. Based on the structure of the stellate reticulum in the dental epithelium, the circular and bilayer-bundle aggregates could be clearly classified into two types: one with extensively developed stellate reticulum, and the other with negligible stellate reticulum. The epithelial cells in the circular aggregates differentiated into ameloblasts. The continuous bilayer bundles eventually formed the epithelial sheath, and dentin tissue was evident at the apex of these bundles. Finally, enamel-covered dentin and cementum-covered dentin formed, a process most likely mediated by epithelial-mesenchymal interaction. These results suggest that the development of these engineered teeth closely parallels that of natural odontogenesis derived from the immature epithelial and mesenchymal cells.

PREPARATION AND MORPHOLOGY OF POROUS NANOCALCIUM PHOSPHATE/POLY(L-LACTIC ACID) COMPOSITES

Biodegradable porous materials can serve as a scaffold in tissue engineering. In this work, highly porous nano-calcium phosphate (NCP)/poly(L-lactic acid)(PLLA) composites were prepared by a thermally induced phase separation technique. Five calcium phosphates with different biodegradation rate were selected, i.e. amorphous calcium phosphate, α-tricalcium phosphate, β-tricalcium phosphate and biphasic α/β-tricalcium phosphate. The results showed that the NCP particles could be homogenously incorporated into pore walls; the composites had a porosity of ~90%, and a pore size of ~200 μm. From the point of view of materials science, the obtained porous NCP/PLLA composites demonstrate to have a capability of applying in bone tissue engineering.

Sustained vascular endothelial growth factor delivery enhances angiogenesis and perfusion in ischemic hind limb

PURPOSE

We hypothesized that sustained delivery of vascular endothelial growth factor (VEGF) using a polymer [85:15 poly(lactide-co-glycolide) (PLG)] would enhance angiogenesis and improve perfusion of ischemic tissue.

METHODS

C57BL/6J mice (n = 20/group) underwent unilateral hind limb ischemia surgery and were randomized to groups of no scaffold implantation (0-Implant), unloaded scaffold implantation (Empty-PLG), or implantation of scaffolds incorporating 3 microg of VEGF165 (PLG-VEGF). Endpoints included laser Doppler perfusion imaging (LDPI, ischemic/nonischemic limb, %), local vessel counts, immunohistochemistry for CD31, and alpha-smooth muscle actin. In vitro release kinetics of VEGF from PLG was also measured.

RESULTS

PLG-VEGF resulted in improved lower extremity perfusion vs. controls as measured by LDPI% at 7, 14, 21, and 28 days (p < 0.05). PLG-VEGF was associated with significantly greater percentage of vessels staining for CD31 and alpha-smooth muscle actin compared to the Empty-PLG or 0-Implant (p < 0.05 for both).

CONCLUSIONS

The PLG-VEGF scaffolds resulted in sustained VEGF delivery, improved tissue perfusion, greater capillary density, and more mature vasculature compared to the controls. The sustained-release PLG polymer vehicle is a promising delivery system for therapeutic neovascularization applications.

Biomechanics of cellular solids

Materials with a cellular structure are widespread in nature and include wood, cork, plant parenchyma and trabecular bone. Natural cellular materials are often mechanically efficient: the honeycomb-like microstructure of wood, for instance, gives it an exceptionally high performance index for resisting bending and buckling. Here we review the mechanics of a wide range of natural cellular materials and examine their role in lightweight natural sandwich structures (e.g. iris leaves) and natural tubular structures (e.g. plant stems or animal quills). We also describe two examples of engineered biomaterials with a cellular structure, designed to replace or regenerate tissue in the body.

Electrospun nano- to microfiber fabrics made of biodegradable copolyesters: structural characteristics, mechanical properties and cell adhesion potential

Nano- to micro-structured biodegradable poly(L-lactide-co-epsilon-caprolactone) (PLCL) fabrics were prepared by electrospinning. Electrospun microfiber fabrics with different compositions of PLCL (mol% in feed; 70/30, 50/50, and 30/70), poly(L-lactide) (PLL) and poly(epsilon-caprolactone) (PCL) were obtained using methylene chloride (MC) as a solvent. The PLL microfiber exhibited a nanoscale-pore structure with a pore diameter of approximately 200-800 nm at the surface and subsurface regions, whereas such a surface structure was hardly observed in other polymers containing CL. The microfiber fabric made of PLCL 50/50 was elastomeric. Nanoscale-fiber fabrics with PLCL 50/50 (approx. 0.3 or 1.2 microm in diameter) were electrospun using 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as a solvent. Mercury porosimetry showed that the decrease in the fiber diameter of the fabric decreased porosity, but increased fiber density and mechanical strength. Human umbilical vein endothelial cells (HUVECs) were adhered well and proliferated on the small-diameter-fiber fabrics (0.3 and 1.2 microm in diameter), both of which are dense fabrics, whereas markedly reduced cell adhesion, restricted cell spreading and no signs of proliferation were observed on the large-diameter-fiber fabric (7.0 microm in diameter). The potential biomedical application of electrospun PLCL 50/50 was discussed.

Optimization of 3-D hepatocyte culture by controlling the physical and chemical properties of the extra-cellular matrices

Hepatocytes are anchorage-dependent cells sensitive to microenvironment; the control of the physicochemical properties of the extra-cellular matrices may be useful to the maintenance of hepatocyte functions in vitro for various applications. In a microcapsule-based 3-D hepatocyte culture microenvironment, we could control the physical properties of the collagen nano-fibres by fine-tuning the complex-coacervation reaction between methylated collagen and terpolymer of hydroxylethyl methacrylate-methyl methacrylate-methylacrylic acid. The physical properties of the nano-fibres were quantitatively characterized using back-scattering confocal microscopy to help optimize the physical support for hepatocyte functions. We further enhanced the chemical properties of the collagen nano-fibres by incorporating galactose onto collagen, which can specifically interact with the asialoglycoprotein receptor on hepatocytes. By correlating a range of collagen nano-fibres of different physicochemical properties with hepatocyte functions, we have identified a specific combination of methylated and galactosylated collagen nano-fibres optimal for maintaining hepatocyte functions in vitro. A model of how the physical and chemical supports interplay to maintain hepatocyte functions is discussed.

Fabrication of well-defined PLGA scaffolds using novel microembossing and carbon dioxide bonding

A novel biologically benign technique was developed to produce three-dimensional tissue engineering scaffolds with well-defined structure. Photolithography was used to design and pattern a planar scaffold skeletal structure on a photoresist (SU-8), and a variety of microembossing processes including sacrificial layer embossing and bilayer embossing were developed to transfer the skeletal pattern to the poly(DL-lactide-co-glycolide) substrate as scaffold skeletons. Subcritical carbon dioxide was then introduced to assemble these skeletons to a three-dimensional scaffold at a low temperature. Compared with conventional scaffolds, which have a broad pore size distribution and varying pore geometry, these microfabricated scaffolds have a uniform and well-defined geometry and structure. This uniformity of structural parameters allows for the studies of cell attachment, spreading, and proliferation in scaffolds in a controlled and logical manner. The cytocompatibility of these microfabricated scaffolds was tested by seeding three different cell lines with different morphologies and growth patterns into these scaffolds. All three cell lines attached well to the scaffolds and grew to high densities as observed with scanning electron microscopy. This study demonstrates a controllable method to fabricate tissue scaffolds with a well-defined 3D architecture that can be used to better elucidate the effect of structure parameters such as pore geometry and pore size on tissue growth in 3D scaffolds.

Characterization of Porous PLGA/PLA Microparticles as a Scaffold for Three Dimensional Growth of Breast Cancer Cells

We have designed and evaluated biodegradable porous polymeric microparticles as a scaffold for cell growth. The hypothesis was that microparticles with optimized composition and properties would have better cell adhesion and hence cell growth into a tissue-like structure. Solvent-evaporation method was modified using sucrose as an additive to form large porous microparticles of poly(D,L-lactic-co-glycolic) (PLGA) and polylactide (PLA) polymers. Microparticles containing hydrophilic polymers (poly(vinyl alcohol) and chitosan) incorporated in their internal matrix structure were also formulated. Different formulations of microparticles were evaluated for physical properties, cell adhesion, and cell growth in culture. PLA microparticles containing poly(vinyl alcohol) (PVA) in the matrix structure (PLA-PVA) and treated with serum prior to cell seeding demonstrated better cell adhesion and cell growth than other formulations of microparticles. Cells were seen to grow into clumps, engulfing microparticles completely with time, and forming a 3-D tissue-like structure. Cell density of 1.5 x 10(6) cells per mg of microparticles was achieved in 9 days of culture, which was a 7-fold increase from the initial seeding cell density. The mechanism of better cell growth on PLA-PVA microparticles appears to be due to the PVA associated with the internal matrix structure of microparticles. These microparticles demonstrated better wetting in culture and also cell adhesion. In addition to tissue engineering applications, microparticles with cancer cells grown into a tissue-like structure in vitro can be potentially used as a model system for preclinical evaluation of the cytotoxic effect of anticancer agents.

Polímeros bioreabsorvíveis na engenharia de tecidos

A Engenharia de Tecidos consiste em um conjunto de conhecimentos e técnicas para a reconstrução de novos órgãos e tecidos. Baseada em conhecimentos das áreas de ciência e engenharia de materiais, biológica e médica, a técnica envolve a expansão in vitro de células viáveis do paciente doador sobre suportes de polímeros bioreabsorvíveis. O suporte degrada enquanto um novo órgão ou tecido é formado. Os poli(alfa-hidróxi ácidos) representam a principal classe de polímeros sintéticos bioreabsorvíveis e biodegradáveis utilizados na engenharia de tecidos. No desenvolvimento e na seleção desses materiais, o tempo de degradação é fundamental para o sucesso do implante. Os estudos e os desafios atuais são normalmente direcionados ao entendimento das relações entre composição química, cristalinidade, morfologia do suporte, e o processamento desses materiais. Este artigo faz uma revisão dos trabalhos recentes sobre a utilização dos polímeros sintéticos bioreabsorvíveis como suportes na engenharia de tecidos.

In vitro andin vivo analysis of macroporous biodegradable poly(D,L-lactide-co-glycolide) scaffolds containing bioactive glass

Recent studies have demonstrated the angiogenic potential of 45S5 Bioglass. However, it is not known whether the angiogenic properties of Bioglass remain when the bioactive glass particles are incorporated into polymer composites. The objectives of the current study were to investigate the angiogenic properties of 45S5 Bioglass particles incorporated into biodegradable polymer composites. In vitro studies demonstrated that fibroblasts cultured on discs consisting of specific quantities of Bioglass particles mixed into poly(D,L-lactide-co-glycolide) secreted significantly increased quantities of vascular endothelial growth factor. The optimal quantity of Bioglass particles determined from the in vitro experiments was incorporated into three-dimensional macroporous poly(D,L-lactide-co-glycolide) foam scaffolds. The foam scaffolds were fabricated using either compression molding or thermally induced phase separation processes. The foams were implanted subcutaneously into mice for periods of up to 6 weeks. Histological assessment was used to determine the area of granulation tissue around the foams, and the number of blood vessels within the granulation tissue was counted. The presence of Bioglass particles in the foams produced a sustained increase in the area of granulation tissue surrounding the foams. The number of blood vessels surrounding the neat foams was reduced after 2 weeks of implantation; however, compression-molded foams containing Bioglass after 4 and 6 weeks of implantation had significantly more blood vessels surrounding the foams compared with foams containing no Bioglass at the same time points. These results indicate that composite polymer foam scaffolds containing Bioglass particles retain granulation tissue and blood vessels surrounding the implanted foams. The use of this polymer composite for tissue engineering scaffolds might provide a novel approach for ensuring adequate vascular supply to the implanted device.

Scaffolds, Stem Cells, and Tissue Engineering: A Potent Combination!

Stem cells, either from embryonic or adult sources, have demonstrated the potential to differentiate into a wide range of tissues depending on culture conditions. This makes them prime candidates for use in tissue engineering applications. Current technology allows us to process biocompatible and biodegradable polymers into three-dimensional (3D) configurations, either as solid porous scaffolds or hydrogels, with controlled macro and/or micro spatial geometry and surface chemistry. Such control provides us with the ability to present highly controlled microenvironments to a chosen cell type. However, the precise microenvironments required for optimal expansion and/or differentiation of stem cells are only now being elucidated, and hence the controlled use of stem cells in tissue engineering remains a very young field. We present here a brief review of the current literature detailing interactions between stem cells and 3D scaffolds of varying morphology and chemical properties, concluding with remaining challenges for those interested in tissue engineering using tailored scaffolds and stem cells.

Controlled preparation and properties of porous poly(l-lactide) obtained from a co-continuous blend of two biodegradable polymers

This study prepares porous PLLA from a blend of two biodegradable polymers. This approach is based on a detailed and quantitative morphology control of the blends. Co-continuous blends comprised of poly(L-lactide)/poly(epsilon-caprolactone) PLLA/PCL, were prepared via melt processing. Through a judicious combination of concentration control and a subsequent annealing step it is possible to generate a wide range of sizes for the co-continuous phases. Subsequent extraction of the PCL porogen phase generates a fully interconnected porous PLLA material with a void volume between 50% and 60%. The volume average pore diameter is controlled from 1.5 to 88 microm as measured by mercury intrusion porosimetry. Through static annealing it is also possible to generate porous structures well beyond that upper limit of pore size. The upper limit of pore size reported above is in the range required for scaffolds for tissue engineering. Micrographs of porous polyglycolide and PCL derived from co-continuous blends of PLLA/polyglycolide and PCL/poly(ethylene oxide) are also shown and demonstrate the versatility and wide applicability of this preparation protocol. The porous structures produced from PLLA/PCL blends possess a high level of mechanical integrity and a degree of crystallinity between 25% and 38%. High values of both compressive modulus and strength at 10%-strain are obtained, greater than 190 and 11 MPa, respectively. The compressive modulus is found to be from 10% to 20% of that of the pure PLLA material. A series of loading studies were also carried out and it was shown that under a pressure of 40 atm applied for 1 h, the pores of a 1.5 microm porous PLLA structure were filled to approximately 80% by water. In addition, the loading of an aqueous solution of a model drug compound, bovine serum albumin (BSA), was carried out at 40 atm and the results indicate that large quantities of BSA (up to 25% of the weight of the original porous capsule) can be driven into the pores. These results indicate that the internal porous structure is accessible to aqueous solution and that this material also has potential as a substrate for controlled release applications.

Cell growth on the porous sponges prepared from poly(depsipeptide-co-lactide) having various functional groups

In tissue engineering, excellent biodegradable materials are desired as temporary scaffolds to support cell growth and disappear with the progress of tissue regeneration. We previously synthesized biodegradable poly(depsipeptide-co-lactide), poly[(Glc-Asp)-co-LA] and poly[(Glc-Lys)-co-LA], having reactive side-chain groups. Then, the effects of reactive and ionic side-chain groups on cell attachment and growth were investigated using co-polymer films with various amounts of carboxyl or amino groups. In this study, to evaluate the utility of these co-polymers as functional scaffolds for tissue regeneration, 3-dimensional porous sponges were prepared by freeze-drying method and the effects of reactive and ionic side-chain groups on cell growth and degradation behavior were investigated using co-polymer sponges with various amounts of carboxyl or amino groups. Good cell growth was observed on the co-polymer sponges. During cell culture, the co-polymer sponges exhibited various degradation rates related to the depsipeptide unit content. Three-dimensional biodegradable polymer matrices with reactive surface, controllable degradation behavior and good cell growth were successfully prepared using these co-polymers. Such kinds of co-polymer matrices are good candidate for scaffold for tissue engineering.