Winding and failure of collagen gels during in vitro acupuncture. (A) PLM image of the gel immediately before the onset of tearing. The characteristic '4-leaf clover' pattern of birefringence increases in size up to the point of failure as the gel becomes increasingly aligned due to winding around the needle. (B-E) Development of gel failure at 0.5 sec (0.15 rev) intervals. At the onset of tearing (B), a weakening of the birefringence can be observed near the needle where the dense, circumferentially wound center transitions to radially aligned fibers (arrow). As failure ensues, a hole is observed in the gel (C-E), and the residual stress in the remainder of the gel is enough to bend the needle, as indicated by the shift in needle position, Δ, directed away from the tear. The increasing size of the tear results in a decreasing area of birefringence. (F) Images A-E marked on a plot of the area above a threshold intensity vs. needle revolutions. The peak represents the image taken at maximum alignment immediately prior to the onset of failure. Bar: 1 mm.