Dynamic three-dimensional micropatterned cell co-cultures within photocurable and chemically degradable hydrogels

In this paper we report on the development of dynamically controlled three-dimensional (3D) micropatterned cellular co-cultures within photocurable and chemically degradable hydrogels. Specifically, we generated dynamic co-cultures of micropatterned murine embryonic stem (mES) cells with human hepatocellular carcinoma (HepG2) cells within 3D hydrogels. HepG2 cells were used due to their ability to direct the differentiation of mES cells through secreted paracrine factors. To generate dynamic co-cultures, mES cells were first encapsulated within micropatterned photocurable poly(ethylene glycol) (PEG) hydrogels. These micropatterned cell-laden PEG hydrogels were subsequently surrounded by calcium alginate (Ca-Alg) hydrogels containing HepG2 cells. After 4 days, the co-culture step was halted by exposing the system to sodium citrate solution, which removed the alginate gels and the encapsulated HepG2 cells. The encapsulated mES cells were then maintained in the resulting cultures for 16 days and cardiac differentiation was analysed. We observed that the mES cells that were exposed to HepG2 cells in the co-cultures generated cells with higher expression of cardiac genes and proteins, as well as increased spontaneous beating. Due to its ability to control the 3D microenvironment of cells in a spatially and temporally regulated manner, the method presented in this study is useful for a range of cell-culture applications related to tissue engineering and regenerative medicine. Copyright © 2013 John Wiley & Sons, Ltd.

Engineered cell-laden human protein-based elastomer

Elastic tissue equivalence is a vital requirement of synthetic materials proposed for many resilient, soft tissue engineering applications. Here we present a bioelastomer made from tropoelastin, the human protein that naturally facilitates elasticity and cell interactions in all elastic tissues. We combined this protein's innate versatility with fast non-toxic fabrication techniques to make highly extensible, cell compatible hydrogels. These hydrogels can be produced in less than a minute through photocrosslinking of methacrylated tropoelastin (MeTro) in an aqueous solution. The fabricated MeTro gels exhibited high extensibility (up to 400%) and superior mechanical properties that outperformed other photocrosslinkable hydrogels. MeTro gels were used to encapsulate cells within a flexible 3D environment and to manufacture highly elastic 2D films for cell attachment, growth, and proliferation. In addition, the physical properties of this fabricated bioelastomer such as elasticity, stiffness, and pore characteristics were tuned through manipulation of the methacrylation degree and protein concentration. This photocrosslinkable, functional tissue mimetic gel benefits from the innate biological properties of a human elastic protein and opens new opportunities in tissue engineering.

Carbon-nanotube-embedded hydrogel sheets for engineering cardiac constructs and bioactuators

We engineered functional cardiac patches by seeding neonatal rat cardiomyocytes onto carbon nanotube (CNT)-incorporated photo-cross-linkable gelatin methacrylate (GelMA) hydrogels. The resulting cardiac constructs showed excellent mechanical integrity and advanced electrophysiological functions. Specifically, myocardial tissues cultured on 50 μm thick CNT-GelMA showed 3 times higher spontaneous synchronous beating rates and 85% lower excitation threshold, compared to those cultured on pristine GelMA hydrogels. Our results indicate that the electrically conductive and nanofibrous networks formed by CNTs within a porous gelatin framework are the key characteristics of CNT-GelMA leading to improved cardiac cell adhesion, organization, and cell-cell coupling. Centimeter-scale patches were released from glass substrates to form 3D biohybrid actuators, which showed controllable linear cyclic contraction/extension, pumping, and swimming actuations. In addition, we demonstrate for the first time that cardiac tissues cultured on CNT-GelMA resist damage by a model cardiac inhibitor as well as a cytotoxic compound. Therefore, incorporation of CNTs into gelatin, and potentially other biomaterials, could be useful in creating multifunctional cardiac scaffolds for both therapeutic purposes and in vitro studies. These hybrid materials could also be used for neuron and other muscle cells to create tissue constructs with improved organization, electroactivity, and mechanical integrity.

Directed differentiation of size-controlled embryoid bodies towards endothelial and cardiac lineages in RGD-modified poly(ethylene glycol) hydrogels

Recent advances in stem cell research have demonstrated the importance of microenvironmental cues in directing stem cell fate towards specific cell lineages. For instance, the size of the embryoid body (EB) was shown to play a role in stem cell differentiation. Other studies have used cell adhesive RGD peptides to direct stem cell fate towards endothelial cells. In this study, materials and cell-based approaches are combined by using microwell arrays to produce size-controlled EBs and encapsulating the resulting aggregates in high molecular weight PEG-4 arm acrylate with and without conjugated RGD to study their effect on stem cell differentiation in a 3D microenvironment. Increasing EB size is observed along with a decrease in the total number of EBs in pristine PEG hydrogel, regardless of the initial EB size. In correlation with this aggregation, EBs in PEG show enhanced cardiogenic differentiation compared to RGD-PEG hydrogel. Both aggregation and cardiogenic differentiation are significantly reduced when RGD peptides are introduced to the microenvironment, while endothelial cell differentiation is accelerated by 3 to 5 days, depending on the EB size, and doubled over the course of cell culture for both EB sizes. Presented results indicate that RGD sequence has a dominant effect in driving endothelial cell differentiation in size-controlled EBs, while pristine multi-arm, high molecular weight PEG can induce cardiogenic differentiation, possibly through EB aggregation. The photopatternable nature of the hydrogel used in this study enabled patterning of such domains devoid or abundant of cell attachment sequences. Therefore, these hydrogels can potentially be used for spatially patterned embryonic stem cell differentiation, which may be beneficial for tissue engineering and regenerative medicine applications.

The expanding world of tissue engineering: the building blocks and new applications of tissue engineered constructs

The field of tissue engineering has been growing in the recent years as more products have made it to the market and as new uses for the engineered tissues have emerged, motivating many researchers to engage in this multidisciplinary field of research. Engineered tissues are now not only considered as end products for regenerative medicine, but also have emerged as enabling technologies for other fields of research ranging from drug discovery to biorobotics. This widespread use necessitates a variety of methodologies for production of tissue engineered constructs. In this review, these methods together with their non-clinical applications will be described. First, we will focus on novel materials used in tissue engineering scaffolds; such as recombinant proteins and synthetic, self assembling polypeptides. The recent advances in the modular tissue engineering area will be discussed. Then scaffold-free production methods, based on either cell sheets or cell aggregates will be described. Cell sources used in tissue engineering and new methods that provide improved control over cell behavior such as pathway engineering and biomimetic microenvironments for directing cell differentiation will be discussed. Finally, we will summarize the emerging uses of engineered constructs such as model tissues for drug discovery, cancer research and biorobotics applications.

Directed endothelial cell morphogenesis in micropatterned gelatin methacrylate hydrogels

Engineering of organized vasculature is a crucial step in the development of functional and clinically relevant tissue constructs. A number of previous techniques have been proposed to spatially regulate the distribution of angiogenic biomolecules and vascular cells within biomaterial matrices to promote vascularization. Most of these approaches have been limited to two-dimensional (2D) micropatterned features or have resulted in formation of random vasculature within three-dimensional (3D) microenvironments. In this study, we investigate 3D endothelial cord formation within micropatterned gelatin methacrylate (GelMA) hydrogels with varying geometrical features (50-150 μm height). We demonstrated the significant dependence of endothelial cells proliferation, alignment and cord formation on geometrical dimensions of the patterned features. The cells were able to align and organize within the micropatterned constructs and assemble to form cord structures with organized actin fibers and circular/elliptical cross-sections. The inner layer of the cord structure was filled with gel showing that the micropatterned hydrogel constructs guided the assembly of endothelial cells into cord structures. Notably, the endothelial cords were retained within the hydrogel microconstructs for all geometries after two weeks of culture; however, only the 100 μm-high constructs provided the optimal microenvironment for the formation of circular and stable cord structures. Our findings suggest that endothelial cord formation is a preceding step to tubulogenesis and the proposed system can be used to develop organized vasculature for engineered tissue constructs.

Microfabricated biomaterials for engineering 3D tissues

Mimicking natural tissue structure is crucial for engineered tissues with intended applications ranging from regenerative medicine to biorobotics. Native tissues are highly organized at the microscale, thus making these natural characteristics an integral part of creating effective biomimetic tissue structures. There exists a growing appreciation that the incorporation of similar highly organized microscale structures in tissue engineering may yield a remedy for problems ranging from vascularization to cell function control/determination. In this review, we highlight the recent progress in the field of microscale tissue engineering and discuss the use of various biomaterials for generating engineered tissue structures with microscale features. In particular, we will discuss the use of microscale approaches to engineer the architecture of scaffolds, generate artificial vasculature, and control cellular orientation and differentiation. In addition, the emergence of microfabricated tissue units and the modular assembly to emulate hierarchical tissues will be discussed.

Nanobiomaterials: a review of the existing science and technology, and new approaches

Nanotechnology has made great strides forward in the creation of new surfaces, new materials and new forms which also find application in the biomedical field. Traditional biomedical applications started benefiting from the use nanotechnology in an array of areas, such as biosensors, tissue engineering, controlled release systems, intelligent systems and nanocomposites used in implant design. In this manuscript a review of developments in these areas will be provided along with some applications from our laboratories.

Microfabrication of complex porous tissue engineering scaffolds using 3D projection stereolithography

The success of tissue engineering will rely on the ability to generate complex, cell seeded three-dimensional (3D) structures. Therefore, methods that can be used to precisely engineer the architecture and topography of scaffolding materials will represent a critical aspect of functional tissue engineering. Previous approaches for 3D scaffold fabrication based on top-down and process driven methods are often not adequate to produce complex structures due to the lack of control on scaffold architecture, porosity, and cellular interactions. The proposed projection stereolithography (PSL) platform can be used to design intricate 3D tissue scaffolds that can be engineered to mimic the microarchitecture of tissues, based on computer aided design (CAD). The PSL system was developed, programmed and optimized to fabricate 3D scaffolds using gelatin methacrylate (GelMA). Variation of the structure and prepolymer concentration enabled tailoring the mechanical properties of the scaffolds. A dynamic cell seeding method was utilized to improve the coverage of the scaffold throughout its thickness. The results demonstrated that the interconnectivity of pores allowed for uniform human umbilical vein endothelial cells (HUVECs) distribution and proliferation in the scaffolds, leading to high cell density and confluency at the end of the culture period. Moreover, immunohistochemistry results showed that cells seeded on the scaffold maintained their endothelial phenotype, demonstrating the biological functionality of the microfabricated GelMA scaffolds.

3-D biofabrication using stereolithography for biology and medicine

In this paper, we review our recent work on the potential of stereolithography (SL) for different biomedical applications including tissue engineering, neovessel formation, investigating cell-cell and cell matrix interactions, and development of cellular systems. Also, we show that SL technology can be combined with dielectrophoresis (DEP) to create scaffolds with micro-scale organization, a hallmark of in vivo tissues.

Both sides nanopatterned tubular collagen scaffolds as tissue-engineered vascular grafts

Two major requirements for a tissue-engineered vessel are the establishment of a continuous endothelium and adequate mechanical properties. In this study, a novel tubular collagen scaffold possessing nanopatterns in the form of channels (with a 650 nm periodicity) on both sides was designed and examined after seeding and co-culturing with vascular cells. Initially, the exterior of the tube was seeded with human vascular smooth muscle cells (VSMCs), cultured for 14 days, and then human internal thoracic artery endothelial cells (HITAECs) were seeded on the inside of the tube and cultured for a further week. Microscopy revealed that nano-scale patterns could be reproduced on collagen with high fidelity and preserved during incubation in vitro. The VSMCs were circumferentially orientated with the help of these nanopatterns and formed multilayers on the exterior, while HITAECs formed a continuous layer on the interior, as is the case in natural vessels. Both cell types were observed to proliferate and retain their phenotypes in the co-culture.

Three-dimensional photopatterning of hydrogels using stereolithography for long-term cell encapsulation

Cell-encapsulated hydrogels with complex three-dimensional (3D) structures were fabricated from photopolymerizable poly(ethylene glycol) diacrylate (PEGDA) using modified 'top-down' and 'bottoms-up' versions of a commercially available stereolithography apparatus (SLA). Swelling and mechanical properties were measured for PEGDA hydrogels with molecular weights (M(w)) ranging from 700 to 10 000 Daltons (Da). Long-term viability of encapsulated NIH/3T3 cells was quantitatively evaluated using an MTS assay and shown to improve over 14 days by increasing the M(w) of the hydrogels. Addition of adhesive RGDS peptide sequences resulted in increased cell viability, proliferation, and spreading compared to pristine PEG hydrogels of the same M(w). Spatial 3D layer-by-layer cell patterning was successfully demonstrated, and the feasibility of depositing multiple cell types and material compositions into distinct layers was established.

Nanopatterning of collagen scaffolds improve the mechanical properties of tissue engineered vascular grafts

Tissue engineered constructs with cells growing in an organized manner have been shown to have improved mechanical properties. This can be especially important when constructing tissues that need to perform under load, such as cardiac and vascular tissue. Enhancement of mechanical properties of tissue engineered vascular grafts via orientation of smooth muscle cells by the help of topographical cues have not been reported yet. In the present study, collagen scaffolds with 650, 500, and 332.5 nm wide nanochannels and ridges were designed and seeded with smooth muscle cells isolated from the human saphenous vein. Cell alignment on the construct was shown by SEM and fluorescence microscopy. The ultimate tensile strength (UTS) and Young's modulus of the scaffolds were determined after 45 and 75 days. Alamar Blue assay was used to determine the number of viable cells on surfaces with different dimensioned patterns. Presence of nanopatterns increased the UTS from 0.55 +/- 0.11 to as much as 1.63 +/- 0.46 MPa, a value within the range of natural arteries and veins. Similarly, Young's modulus values were found to be around 4 MPa, again in the range of natural vessels. The study thus showed that nanopatterns as small as 332.5 nm could align the smooth muscle cells and that alignment significantly improved mechanical properties, indicating that nanopatterned collagen scaffolds have the potential for use in the tissue engineering of small diameter blood vessels.

Influence of nanopatterns on endothelial cell adhesion: Enhanced cell retention under shear stress

In this study, nanopatterned crosslinked films of collagen Type I were seeded with human microvascular endothelial cells and tested for their suitability for vascular tissue engineering. Since the films will be rolled into tubes with concentric layers of collagen, nutrient transfer through the collagen films is quite crucial. Molecular diffusivity through the collagen films, cell viability, cell proliferation and cell retention following shear stress were studied. Cells were seeded onto linearly nanogrooved films (groove widths of 332.5, 500 and 650nm), with the grooves aligned in the direction of flow. The nanopatterns did not affect cell proliferation or initial cell alignment; however, they significantly affected cell retention under fluid flow. While cell retention on unpatterned films was 35+/-10%, it was 75+/-4% on 332.5nm patterned films and even higher, 91+/-5%, on 650nm patterned films. The films were found to have diffusion coefficients of ca. 10(-6)cm(2)s(-1) for O(2) and 4-acetaminophenol, which is comparable to that observed in natural tissues. This constitutes another positive asset of these films for consideration as a scaffold material for vascular tissue engineering.

Influence of keratocytes and retinal pigment epithelial cells on the mechanical properties of polyester-based tissue engineering micropatterned films

In this paper the mechanical properties of micropatterned polyester films prepared to serve as tissue engineering scaffolds of cornea were examined. Films were prepared by solvent casting of blends of poly(l-lactide-co-d,l-lactide) and poly(3-hydroxybutyric acid-co-3-hydroxyvaleric acid), on a micropatterned silicon template. They were seeded with keratocytes or retinal pigment epithelial cells and subjected to tensile testing to assess the contribution of cells and the deposited extra-cellular matrix (ECM) to the mechanical properties of the scaffold. In all the tests, the films used were wet and the cells were not fixed. Cell-free scaffolds showed a gradual deterioration in strength upon incubation in the cell culture medium at 37 degrees C; the deterioration rate was highest in the first week and decreased significantly over the second and third weeks. The ultimate strength of the cell-free scaffolds decreased from 0.99 to 0.42N/mm after 21 days of incubation. Cell seeded scaffolds showed a more complicated mechanical strength profile. Their response was found to depend both on the extent of surface coverage and on the cell type. The results were examined after dividing the data into two groups of lower and higher stiffness. For keratocyte seeded scaffolds, the strength of the high stiffness groups continued to increase as the incubation period increased while the lower stiffness groups did not show a distinct change. For the keratocyte seeded scaffolds, tensile strength increased from 0.65N/mm on Day 7 to 0.73N/mm on Day 21. On the other hand, the scaffolds seeded with retinal pigment epithelial cells showed a gradual deterioration over time in both the higher and lower stiffness groups. For epithelial cell seeded scaffolds this was 0.98N/mm on Day 7 and decreased to 0.77N/mm on Day 21 still an improvement over the unseeded scaffolds. This most probably was a result of a lower rate of ECM secretion in comparison to keratocytes and the newly secreted ECM could not compensate for the influence of scaffold degradation on the mechanical properties. It could, therefore, be concluded that cell seeding plays a positive role in strengthening a tissue engineered construct, and cell type has a significant influence on the extent of this improvement.

Cornea engineering on polyester carriers

In this study, biodegradable polyester based carriers were designed for tissue engineering of the epithelial and the stromal layers of the cornea, and the final construct was tested in vitro. In the construction of the epithelial layer, micropatterned films were prepared from blends of biodegradable and biocompatible polyesters of natural (PHBV) and synthetic (P(L/DL)LA) origin, and these films were seeded with D407 (retinal pigment epithelial) cells. To improve cell adhesion and growth, the films were coated with fibronectin. To serve as the stromal layer of the cornea, highly porous foams of P(L/DL)LA-PHBV blends were seeded with 3T3 fibroblasts. Cell numbers on the polyester carriers were significantly higher than those on the tissue culture polystyrene control. The cells and the carriers were characterized scanning electron micrographs showed that the foam was highly porous and the pores were interconnected. 3T3 Fibroblasts were distributed quite homogeneously at the seeding site, but probably because of the high thickness of the carrier ( approximately 6 mm); they could not sufficiently populate the core (central parts of the foam) during the test duration. The D407 cells formed multilayers on the micropatterned polyester film. Immunohistochemical studies showed that the cells retained their phenotype during culturing; D407 cells formed tight junctions characteristic of epithelial cells, and 3T3 cells deposited collagen type I into the foams. On the basis of these results, we concluded that the micropatterned films and the foams made of P(L/DL)LA-PHBV blends have a serious potential as tissue engineering carriers for the reconstruction of the epithelial and stromal layers of the cornea.