Smoke Alarms Hard-wire or not to Hard-wire

In the Fine Homebuilding email I just received today was an article about Smoke Alarms. In the on-line article it seemed to indicate that current code requirements are that Smoke Alarms must be hard-wired into the home’s electrical system – not just battery-powered. There are tons of battery-powered Smoke Alarms at big-box stores and on-line. Before I go spending a bunch on running electric cables and adding electric boxes all over my house (apparently 6 are required for my house) – do they really need to be powered from 110V house wiring? And are they really required to be inter-connected (meaning they all go off when only 1 detects smoke or CO)? I’ll do that, if needed, just want to be sure I’m not wasting a couple thousand bucks for a licensed electrician. I’m in south-east Texas – about an hour south of Houston. (Even if it turns out I don’t need hard-wired devices, I appreciated the article, lots of good info.)
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That’s code, hardwired, interconnected with battery backup. It’s a pain to retrofit into a house and many people don’t like it, but it’s code. When I lived in Detroit if I pulled a permit to pass inspection I had to hardwire smokes throughout the house.