a somewhat renegade use of the technology, Washington state entrepreneur Amal Graafstra unlocks his home and car and logs on to his computer using a chip he bought online and had implanted near his thumb.

All three say putting radio-frequency identification chips under the skin can improve people's lives. An implant is like having a set of keys, or an I D card, that can't be lost, they say. Graafstra jokes that he could end up naked in the alley outside his house and still get inside using the electronic key embedded in his hand.

“People ask me why I don't just carry an RFID card in my wallet,” Graafstra said. “I don't want to have to remember whether I have my card or my keys with me. I can leave my house and not carry anything with me.”

Privacy advocates say today's voluntary use is a step toward a future in which employers or the government mandate implants.

“It's creepy,” said Beth Givens, director of the Privacy Rights Clearinghouse in San Diego. “People realize in their gut that if we require implanted chips, we've become the kind of society where people can be tracked by their government.”

Good or bad, the technology is having a breakout year in the United States.

Last month, Darks' security video company, Citywatcher.com, became the first in the nation to use RFID implants to control who has access to a restricted area.

Nationwide, about 70 hospitals – none in San Diego – are developing or have begun programs to make the implants available to patients and to put RFID scanners in emergency rooms to scan all unconscious patients.

Those applications use the only radio ID chip approved by the federal Food and Drug Administration for implanting in humans, a product from Florida-based VeriChip that's ab out the size of a grain of rice. Doctors use a syringe and a local anesthetic to insert them under the skin.

Critics say the devices offers the government, employers or corporations a potentially nefarious tool to track citizens. There are several types of RFID, but the technology available from VeriChip and the versions used by the do-it-yourself crowd don't provide a signal that can be tracked.

The chips don't use batteries or any other power source. To work, they must be held within a few inches of a scanner. Through a process called induction, the scanner temporarily powers the chip by generating a magnetic field that passes through the skin. While it has power, the chip transmits a signal that's picked up by the scanner.

VeriChip says about 70 people in the United States have been implanted with its chips, which cost about $ 200, including doctors' fees.

In addition, an estimated 80 people have had unauthorized “hobbyist” chips implanted. Like Graafstra, they buy them over the Internet to experiment with the technology, which has been used for years to track lost pets. The technology enthusiasts describe themselves as the “do-it-yourself tagged community.”

The United States lags other countries in adopting radio ID implants. In Mexico in 2004, more than 100 employees in the organized-crime division of the Attorney General's Office received implants giving them access to secure areas.

That same year in Spain, the Baja Beach Club in < ST1:CITY w:st="on">Barcelona launched a VIP zone for patrons with radio ID implants. VIP members use them to authorize credit-card payments for their drinks. The club's owners have expanded the program to a bar they own in the Netherlands.

At Citywatcher.com, which operates security cameras and stores video for the Cincinnati Police Department, Darks wanted to beef up security for the area where the video is stored.

Biometric systems, which measure unique physical characteristics such as fingerprints or facial structure, were too expensive for his small company, Darks said. So he decided to use the VeriChip system.

He had a radio ID chip placed in his tricep and gave his employees the option of getting chipped.

Three volunteered. Two others carry RFID cards.

“It was completely voluntary,” Darks said. “I wouldn't ask my employees to do something that I wouldn't do myself.”

The implanted device is essentially just an unseen key card, he said.

“I'm not worried about the government or anyone else tracking me through the chip in my arm,” Darks said. “If they wanted to, they could use the GPS information from my cell phone or the trail of places where I've used my credit card. That's much more of a threat.”

Schmidig, the New Jersey police chief, got a VeriChip implant for other reasons. He said a friend's daughter had an episode of diabetic shock and was unable to speak, which delayed medical treatment. At about the same time, he heard about a nearby hospital implementing the VeriChip system.

So Schmidig decided to have a medical ID chip implanted in his arm.

“I have a vacation home in Florida, and there are hospitals down there using this system,” he said. “All my medical records are up here. If something happens to me in Florida, this could speed up access to my medical records.”

Schmidig said he has no concerns about privacy as a result of his implant. His chip doesn't contain any personal information, only an ID number for a medical database.

“I'm not a Big Brother fanatic,” he s aid. “This is not GPS that can be used to follow me around.”

Graafstra of Bellingham, Wash., is a technology enthusiast and author of “RFID Toys: 11 Cool Projects for Home, Office and Entertainment,” published this year.

A year ago, he decided to take his interest in radio ID technology to a new level, becoming the founding member of the do-it-yourself RFID world. Graafstra bought a chip on the Internet and had a doctor insert it in the tissue between his thumb and index finger.

Graafstra said it's unlikely that anyone would go to the trouble of trying to hack his chip to get the code to his front door. It would be easier to force the door open, he said. “There's very little possibility that anyone could sneak up and get within a couple of inches of my tag to read it,” he said.

While he's comfortable having the code to open his front door and car in a radio transponder in his hand, Graafstra says the technology may not be secure enough to protect credit-card information or access to sensitive government offices.

Although difficult, it's still possible to record and clone the signal from an implanted chip, he said.

Critics say the practice opens a door that would be best left closed.

“RFID has the potential to produce some wonderful applications,” said Givens, the San Diego privacy advocate. “It also has the potential to be a technology with which a government-issued ID number can be read promiscuously.

“It's being rushed to the marketplace without understanding the consequences,” she said. “The privacy implications have not been thoughtfully explored.”

Liz McIntyre, co-author of “Spychips: How Major Corporations and Government Plan to Track Your Every Move with RFID,” said she isn't swayed by technical arguments that implanted chips are benign.

“There may be limits on what the technology can do today, but we don't know what the technology will be capable of tomorrow,” McIntyre said. “Yes, it's a step on that slippery slope. You wouldn't walk down the street with your Social Security number printed on your shirt. Why would you want an RFID chip capable of transmitting an identification number?”