Animal experiments in this study were carried out in accordance with the guidelines of the Animals Ordinance (Chapter 340), Department of Health, Hong Kong. All the procedures were approved by the Institutional Animal Care and Use Committee of the Hong Kong Science and Technology Parks Corporation and Animal Experimentation Ethics Committee of the Chinese University of Hong Kong. Four adult pigs (two male and two female) used in the experiment are food-restricted for 24 hours before the procedures. For anesthesia, the pig is first sedated by injecting with a mixture of atropine, xylazine, and ketamine. Then, anesthesia is maintained by 4% pentobarbitone. During the surgical procedures, a 1:1 mixture of 2% isoflurane to oxygen and nitrous oxide is inhaled at a rate of 5 liters/min.