Seattle, Washington - Starting Wednesday, customers at the Whole Foods store in Seattle's Capitol Hill neighborhood were able to pay for their groceries with their palms.

Amazon, which acquired the grocery chain in 2017, has installed its Amazon One palm-scanning devices at the Broadway and Madison Avenue site.

The company plans to expand the contactless technology to seven additional Seattle-area Whole Foods stores in coming months, Amazon physical retail executive Dilip Kumar wrote in a blogpost.

Amazon, which often uses Seattle as a laboratory to test technologies, unveiled the scanners at two Amazon Go stores in South Lake Union in the fall.

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Since then, they've also been installed in 10 more Seattle-area Amazon stores, including the Amazon Go Grocery in Redmond, Amazon Books in University Village and the Amazon 4-Star in Southcenter.

So far, thousands of customers have signed up to use the terminals, Kumar said.

Users register by scanning their palm and connecting it to a credit card or Amazon account, then scanning their palm again to pay. Their biometric payment profile follows them to every store with Amazon One cashiers.

While Amazon is the only company using the palm scanners for now, the company envisions marketing Amazon One technology to other retailers, which could give Amazon access to data on consumers’ shopping habits outside its online and physical footprint.

Civil-liberties advocates, meanwhile, have raised concerns about the expansion of biometric data collection, saying it poses privacy concerns.