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In page Anterior cruciate ligament reconstruction:

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In order for MSCs to differentiate into an ACL, they must be placed in a proper scaffold on which to grow, and must be in a bioreactor that maintains a normal physiological environment for the cells to reproduce and proliferate effectively.[1] The scaffold must have the mechanical properties of a healthy ACL to sustain the ligament while it is in its primary form and maintain normal knee movement. Scaffolds that are used for ACL growth include collagen, silk, gelatin, polylactic acid, and glycosaminoglycans.[2] Mechanical properties of the scaffolds are further enhanced through braiding and twisting of the scaffold materials.