Page 112 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 5
area for absorbing proteins and o er more binding sites to the receptors of cells. It is known that cells sense and respond to di erent length scales. On micrometer fibers, they attach in a similar fashion as on flat surfaces. The cells flatten and follow the orientation of the fiber, whereas a cell can bind to several nanofibers simultaneously and even spread in-between individual fibers of multiple nanofibers.73 Therefore, to more closely mimic the heart valve ECM structure, multiple fiber diameters need to be integrated into one sca old. Recently developed methods allow electrospinning of various controlled fiber diameters simultaneously and in an intermingled matter.89,90
Next to fiber diameter, porosity (or void space) is important as this favors cell ingrowth, cell proliferation, cell motility and matrix synthesis, relevant for heart valve tissue formation. The addition of fibers with large diameters into nanofibrous sca olds increases the pore size, since nanofibrous sca olds lack pores and void spaces large enough for cells to growth in, resulting in a confluent cell layer on top of the sca old.91 A theoretical model on the porosity in such electrospun architectures illustrates that the mean pore radius in an electrospun sca old directly depends on fiber diameters since the architectures produced by electrospinning are based on stacks of layers of two-dimensional sheets.92 Incorporating fibers of several micrometers directly increases pore and void spaces. A study on
PCL meshes showed an optimum fiber diameter of 12 μm for homogenous three-dimensional cell infiltration with human venous myofibroblasts,93 a cell source commonly used for tissue engineering of heart valves. Natural nanofibers can be included into these meshes by using a fibrin gel as a cell carrier,94 representing an intriguing way to produce bimodal fibrous sca olds, thereby allowing the cells to homogeneously spread along the fibers as well as within the pores. Sca olds with multimodal fiber diameters are only one way to improve the generally limited cellular infiltration. Spinning simultaneous or sequential di erent materials followed by selectively dissolving one material95,96 or using salt leaching methods97 are alternative approaches. These methods include additional steps and processes, which make the mesh production more complicated. Furthermore, these methods have the drawback that they heavily rely on the dependency of pore size with fiber diameter. A promising method independent on fiber diameter is low temperature electrospinning, where embedded ice crystals from the surrounding air act as a pore template resulting in a large increase in porosity with constant fiber diameter.98 The first tissue engineered applications using such meshes showed complete, and homogenous, cell ingrowth.99,100
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