is carticel an artificial material and can it be rejected?

No. Carticel is a biologic material. It is a growth of cells from your own body and therefore it cannot be rejected. It is implanted as a suspension of cells, then congeals like gelatin, and then with time slowly hardens. If the rehabilitation process is not done correctly or done too aggressively, it can damage or retard the cell maturation process. Though very rare, programs too aggressive can lead to graft failure. Because the graft is biologic, it must heal and mature like other tissues. A consistent conservative program prescribed by your surgeon may yield more successful results.