Controlled positioning of cells in biomaterials - approaches towards 3D tissue printing

Current tissue engineering techniques have various drawbacks: they often incorporate uncontrolled and imprecise scaffold geometries, whereas the current conventional cell seeding techniques result mostly in random cell placement rather than uniform cell distribution. For the successful reconstruction of deficient tissue, new material engineering approaches have to be considered to overcome current limitations. An emerging method to produce complex biological products including cells or extracellular matrices in a controlled manner is a process called bioprinting or biofabrication, which effectively uses principles of rapid prototyping combined with cell-loaded biomaterials, typically hydrogels. 3D tissue printing is an approach to manufacture functional tissue layer-by-layer that could be transplanted in vivo after production. This method is especially advantageous for stem cells since a controlled environment can be created to influence cell growth and differentiation. Using printed tissue for biotechnological and pharmacological needs like in vitro drug-testing may lead to a revolution in the pharmaceutical industry since animal models could be partially replaced by biofabricated tissues mimicking human physiology and pathology. This would not only be a major advancement concerning rising ethical issues but would also have a measureable impact on economical aspects in this industry of today, where animal studies are very labor-intensive and therefore costly. In this review, current controlled material and cell positioning techniques are introduced highlighting approaches towards 3D tissue printin

Visual mapping of computational shear stresses implies mechanical control of cell proliferation and differentiation in bone tissue engineering cultures

The advantages of longitudinal monitoring techniques are getting more attention in various tissue engineering approaches. They provide consecutive information about one and the same sample over\u3cbr/\u3etime and as such may decrease sample numbers tremendously. These techniques also allow taking the actual environmental status of a tissue into account for predicting future development. Micro-computed\u3cbr/\u3etomography has been previously shown to be suitable to monitor mineralized extracellular matrix deposition in bone tissue engineering cultures. In this study, shear stresses (SS) acting on human mesenchymal stromal cells (hMSC) seeded on silk fibroin scaffolds in a flow perfusion bioreactor were calculated by computational fluid dynamics. Two different flow rates were investigated, mimicking expected loads on cells during early bone repair (0.001 m/s) and during bone remodeling (0.061 m/s), respectively. The threedimensional (3D) distribution of these stresses was then visually mapped to the distribution of the mineralized extracellular matrix deposited by the cells. SS values from 0.55–24 mPa were shown to promote osteogenic differentiation of hMSCs, whereas SS between 0.06 and 0.39 mPa were found to induce cell proliferation. Histological and biochemical analyses have confirmed these findings. In the future, these results may allow predicting the behavior of hMSC in 3D tissue culture. The non-destructive nature of this technique may even allow tight control and adaptation of the mechanical load during culture by taking the present status of the tissue into account

Flow velocity-driven differentiation of human mesenchymal stromal cells in silk fibroin scaffolds:A combined experimental and computational approach

\u3cp\u3eMechanical loading plays a major role in bone remodeling and fracture healing. Mimicking the concept of mechanical loading of bone has been widely studied in bone tissue engineering by perfusion cultures. Nevertheless, there is still debate regarding the in-vitro mechanical stimulation regime. This study aims at investigating the effect of two different flow rates (v\u3csub\u3elow\u3c/sub\u3e = 0.001m/s and v\u3csub\u3ehigh\u3c/sub\u3e = 0.061m/s) on the growth of mineralized tissue produced by human mesenchymal stromal cells cultured on 3-D silk fibroin scaffolds. The flow rates applied were chosen to mimic the mechanical environment during early fracture healing or during bone remodeling, respectively. Scaffolds cultured under static conditions served as a control. Time-lapsed micro-computed tomography showed that mineralized extracellular matrix formation was completely inhibited at v\u3csub\u3elow\u3c/sub\u3e compared to v\u3csub\u3ehigh\u3c/sub\u3e and the static group. Biochemical assays and histology confirmed these results and showed enhanced osteogenic differentiation at v\u3csub\u3ehigh\u3c/sub\u3e whereas the amount of DNA was increased at v\u3csub\u3elow\u3c/sub\u3e. The biological response at v\u3csub\u3elow\u3c/sub\u3e might correspond to the early stage of fracture healing, where cell proliferation and matrix production is prominent. Visual mapping of shear stresses, simulated by computational fluid dynamics, to 3-D micro-computed tomography data revealed that shear stresses up to 0.39mPa induced a higher DNA amount and shear stresses between 0.55mPa and 24mPa induced osteogenic differentiation. This study demonstrates the feasibility to drive cell behavior of human mesenchymal stromal cells by the flow velocity applied in agreement with mechanical loading mimicking early fracture healing (v\u3csub\u3elow\u3c/sub\u3e) or bone remodeling (v\u3csub\u3ehigh\u3c/sub\u3e). These results can be used in the future to tightly control the behavior of human mesenchymal stromal cells towards proliferation or differentiation. Additionally, the combination of experiment and simulation presented is a strong tool to link biological responses to mechanical stimulation and can be applied to various in-vitro cultures to improve the understanding of the cause-effect relationship of mechanical loading.\u3c/p\u3

Tissue engineering of bone

No abstract

Structural and material approaches to bone tissue engineering in powder-based three-dimensional printing

This article reviews the current state of knowledge concerning the use of powder-based three-dimensional printing (3DP) for the synthesis of bone tissue engineering scaffolds. 3DP is a solid free-form fabrication (SFF) technique building up complex open porous 3D structures layer by layer (a bottom-up approach). In contrast to traditional fabrication techniques generally subtracting material step by step (a top-down approach), SFF approaches allow nearly unlimited designs and a large variety of materials to be used for scaffold engineering. Today’s state of the art materials, as well as the mechanical and structural requirements for bone scaffolds, are summarized and discussed in relation to the technical feasibility of their use in 3DP. Advances in the field of 3DP are presented and compared with other SFF methods. Existing strategies on material and design control of scaffolds are reviewed. Finally, the possibilities and limiting factors are addressed and potential strategies to improve 3DP for scaffold engineering are proposed