For several years after installing a pet door for his cat, Flipper, Nick Hill had a problem: Stray cats would wander into his house through the rubber flap to eat Flipper’s food, start fights with him, and pee on the furniture.In 2005, Hill, who has a PhD in quantum physics from Cambridge University in England, began devising a mechanical cat flap that would open only when activated by the microchip implanted in Flipper’s back to identify him if he got lost. Two years and eight patents later, Hill had a prototype working in his Cambridge home. Flipper, age 10, “was chief product tester,” says Hill, 38.In the past four years he has sold more than 100,000 of the mechanical doors, called the SureFlap, at 80 pounds a piece, mostly in Europe. Hill’s company of the same name grossed about 2 million pounds ($3.1 million) last year, and he’s projecting growth of 25 percent to 50 percent this year. Now Hill’s trying to sell the device in the U.S., which has more pet cats (86 million) than Britain has people (63 million).Gaining traction may be a challenge. Americans are less likely to let house cats roam outside because of predators, such as coyotes, that Europe doesn’t have. Nearly half of British cats have microchips, which are often required to move pets across borders in Europe, while only 12 percent of American cats have the implants, according to surveys by pet industry groups in both countries.SureFlap isn’t the first “selective pet door.” Others activated by magnets or radio frequency identification (RFID) chips attached to pet collars retail for less than the $150 that SureFlap sells for in the U.S. Hill initially tried using a door activated by a collar, but Flipper refused to wear it.Still, technology that reduces the burden of caring for animals is trendy. Products such as automatic feeding machines and time-release watering bowls are among the fastest-growing segments of the $50 billion Americans spend on pets each year, says Bob Vetere, president of the American Pet Products Assn. Baby Boomers and younger professionals want to be able to go away or spend all day at work and know that their pets will be fine on their own.SureFlap’s plastic cat door is roughly 5½ inches square and requires a drill and jigsaw to install. A sensor powered by four AA batteries unlocks the door when it reads the signal from a cat’s RFID chip, which is about the size of a grain of rice and generally implanted between the shoulder blades. To program the SureFlap, owners press a button and the sensor records the chip in the cat as it passes through the device. SureFlap works with most existing cat microchips and will unlock only when approached by cats it has been programmed to remember (up to 32 felines).Kitty Elder, a merchandising manager for a jewelry company in Liberty, N.C., rescues cats that are slated to be killed at shelters and places them with new owners. She uses two SureFlap doors inside to separate the most timid of her nine foster cats from the more aggressive ones. One in particular, Reilly, a Manx born with very mild brain damage, stays in a spare room and can mingle only with certain cats. “The SureFlap was perfect for that, because I could pick the gentle and the easy-going and the ones that could be trusted with him, and let them go back and forth,” Elder says.When he conceived SureFlap, Hill was developing touchscreen sensors for a Cambridge company called NXT, and he saw his stray cat problem as an engineering challenge. Pet microchip scanners are typically held close to the fur, but he would need one to work as far as 12 inches away, sturdy enough to withstand rain and snow, and able to function even in a door made of metal, which can interfere with RFID signals.Hill left his job at NXT at the end of 2006 to work on SureFlap full time. He mortgaged his house for seed money and in 2007 got a 20,000 pound grant from a U.K. economic development group. The 12-employee company, which contracts with manufacturers in China to make SureFlaps, has not taken outside investment. SureFlap sells through pet supply stores and veterinary offices in Europe, but so far North American sales have been limited to online. (It was recently among the top five best-selling items in the “cat flap” category on Amazon.com (AMZN), which includes 224 products.) Hill says the business is profitable.Although he’s focused on making pet doors and creating a SureFlap for small dogs as well, Hill has an eye on how SureFlap’s technology could improve RFID in industrial settings where metal and moisture currently make the chips impractical. If there’s a broader market, Hill has Flipper to thank for sparking the idea: “It was something out there,” he says, “that was waiting to be discovered.”