Though not a new idea, urgent care centers have never been popular in Massachusetts. But that is changing. Since 2012, centers open nights and weekends have been expanding across the state, as health insurers have begun to usher patients toward the clinics, and away from more expensive ERs, for non-life-threatening conditions. Most insurers no longer require patients to get a referral from their doctor for an urgent care visit to be covered — a rule that was impractical during non-business hours. Insurers also have established urgent care copayments — typically $20 to $40 — that are far lower than the $50 to $200 copayment for an emergency room visit. Some health plans now list the fees on members' insurance cards to highlight the difference.