Retirement slammed Carole Jacko. Raising two grandchildren, she's too young for Medicare and too strapped to pay $600 a month for health insurance. So when a trip to the emergency room ended with a diagnosis of diabetes, Jacko found a creative solution. She became a medical guinea pig, offering herself to science in exchange for free medication, free doctor's visits and even a modest payment. With the economy careening and millions uninsured, some doctors and researchers believe the lure of volunteering for medical research is growing - and so are potential ethical pitfalls. Sometimes desperation leads people to be poor shoppers," to gloss over risks or grasp at imagined benefits, said Kevin Weinfurt, a Duke University professor who focuses on medical decision-making and ethics. No regulations limit how much a person can be paid to take part in medical research. Researchers do not agree on how much money it takes to cross the line and exert "undue influence" or coercion to get someone to enroll in a study. That's something federal regulations do forbid. "This is the most complicated issue in research ethics, and it's still an unsettled question," Weinfurt said. It has lingered for more than 100 years, since an Army surgeon named Walter Reed paid volunteers at a Cuban outpost $100 in gold to risk being infected with yellow fever. The men got another $100 if they contracted the disease, payable to themselves - or any designated survivor.