Mechanical behavior of biomimetic oriented cell sheets from a perspective of living materials

When compared to random cell organization, cell sheets with well-organized cell orientation are similar to natural tissues exhibiting better mechanical strength. Furthermore, as living materials, the mechanical strength of cell sheets directly affects their clinical operation and actual applications. However, dynamic mechanics of cell sheets has been ignored. In this work, oriented cell sheets (OCSs) with different degrees of cell orientation were fabricated by culturing cells on self-designed sandwich substrates with patterned surfaces, followed by their photothermal release. Subsequently, the stress-strain curves of OCSs were obtained on a self-made tensile device, the further orientation and deformation of the cells in OCSs under strain were observed, the mechanical relaxation characteristics under constant strain were explored combined with the viscoelastic theory model, and the internal factors of the cell sheets that affect their mechanical behavior were studied. We found that the tensile strength of OCSs can exceed 6 times that of the non-oriented cell sheets (NOCSs) and the stress retention rate of OCS-5 (75.26%) was 2.5 times higher that of NOCSs (30.41%) under high strain (25%). According to the Maxwell-Wiechert model, the viscoelastic model of OCSs was established and their mechanics was attributed to three factors, including the cytoskeleton, cell-cell connection and cell-ECM (extracellular matrix) connection. The key factor for the improvement of mechanical strength is the elastic properties of the cytoskeleton. These conclusions have significance toward understanding the mechanical properties of oriented tissues and guiding the preparation of cell sheets, as well as their practical use for clinical applications.