Figure 1

Illustrations of the four-point bending mechanical loading device. L: distance between two outer pressure points; a: distance between the outer and inner pressure points; t: thickness of the loading plate; d: distance of the pressure point movement; the strain (ϵ) of cells attached to the upper side of the loading plate could be calculated as the following formula: ϵ=td·[a(L-1.33a)]⁻¹.