AUSTIN, Texas -- As a plane with a Grumpy Cat flag flew overhead, courtesy of Friskies, the Technorati flooded into panel discussions and happy hour spots at the annual tech festival South by Southwest in Austin, Texas, on Sunday.

Top tech influencers pondered immortality and mind cloning. FireChat, an app that lets smartphone users connect via mobile chat even without a cellular connection, was another hot topic. Here's a look at the most notable trending topics Sunday at the tech jamboree.

Off-the-grid mobile chat

No cell service? No problem.

An app called FireChat uses phone signals such as Wi-Fi and Bluetooth to connect to other users' phones and enable chats without any network connection. The app, created by a San Francisco startup called Open Garden, debuted in 2012 and was a hit last August at the Burning Man festival in Black Rock, Nevada, where cell phone service is scarce.

It links people via what it calls a "peer-to-peer mesh network," connecting through phone signals rather than a network. The range is about 90 feet but the connection can jump from phone to phone if there's a crowd. It's software-only, says co-founder and CEO Micha Benoliel. Currently the app supports public group chats and hashtags; private messaging is coming.

The app, which is a finalist for South by Southwest's innovation awards, has 5 million users and has been used by tens of thousands of people in India and the Philippines at political protests. As a new startup, Benoliel says his first time at South by Southwest has been positive. "The best surprise has been going to parties and having people asking how they can use FireChat for their event," he says.

Transgender and beyond in tech

United Therapeutics CEO Martine Rothblatt, who considers herself a "transhumanist," discussed advances in "mind cloning" in a keynote Sunday. She said she believes people will one day be able to clone their cognitive functions, and detailed her biotech company's advances in cloning organs and making the process of transferring organs from donor to recipient more efficient.

Rothblatt urged everyone to question authority and noted that in other eras she might not have survived as a transgender person.

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Hottest tech at South by Southwest: FireChat, 'mind cloning'