By Julie Steenhuysen PEORIA, Illinois (Reuters) – By all accounts, Sandy Wright of Mackinaw, Illinois, is a challenging patient. "I'm a very hard case." Now, patients like Wright are at the forefront of an experiment, under way in Peoria, Illinois, and hundreds of other U.S. cities, that could transform the way doctors, nurses and hospitals deliver care to patients. Amid the barrage of criticism over the rollout of Obamacare, groups known as Accountable Care Organizations (ACOs) are quietly going about the business of testing the potential for healthcare reform. The efforts, born of President Barack Obama's Affordable Care Act, are part of the biggest experiment yet to fix the costly and error-plagued U.S. healthcare system.