Building on wireless technology that has the potential to interfere with pain, scientists have developed flexible, implantable devices that can activate — and, in theory, block — pain signals in the body and spinal cord before those signals reach the brain. The researchers, at Washington University School of Medicine in St. Louis and the University of Illinois at Urbana-Champaign, said the implants one day may be used in different parts of the body to fight pain that doesn’t respond to other therapies.