Led by Prof. K Jimmy Hsia, a team at Singapore's Nanyang Technological University started out with a micro-patterned hydrogel, the surface of which was seeded with Madin-Darby canine kidney cells. Although these cells are obtained from dog kidneys, they are epithelial cells of the type that are also found in the skin. The researchers proceeded to make numerous cuts in the hydrogel, the cuts simulating wounds and the gel simulating human skin and underlying tissue. Those cuts ranged from 30 to 100 micrometers in width, and were also made in a variety of curving patterns, ranging from quite serpentine to completely straight. Utilizing particle image velocimetry (which is an optical measurement technique for studying fluid flow), the scientists then watched for 64 hours as the cells set about bridging the gaps formed by the cuts. This process is known as re-epithelialisation, and it is the means by which external wounds naturally heal.