Where to Stash That Cryptocash? This Man Put It Under His Skin. Martijn Wismeijer keeps his Bitcoin handy -- right between his thumb and index finger.

You've heard of hiding money under your mattress. How about stashing it under your skin?

That's what Martijn "Mr. Bitcoin" Wismeijer just did. Apparently the early Bitcoin adopter likes to keep his cryptocash, ahem, close at hand.

The virtual currency diehard had a pair of xNT near-field communication (NFC) microchips injected into the backs of his hands last week, right into the fleshy webbing between the thumb and index finger. So, yeah, he's officially a cyborg now, a walking crypto-geek fantasy (and promotional machine) come true.

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RT @paulbuitink: coffee with cyborg @MrBitc0in who has chip implants inhands to store #bitcoins under the skin :) pic.twitter.com/avO1Qfs44I

— Martijn Wismeijer (@twiet) November 12, 2014

The handlebar-mustached Dutch programmer, who runs a global Bitcoin ATM collective, told CoinDesk he's using the miniscule skin-deep caches not to store just Bitcoin. He's holding other cryptocurrencies in it, too, including Darkcoin, Dogecoin and Litecoin. Because of course he is.

The chips are compatible with devices containing an NFC antenna, like smartphones, tablets and other gadgets. They can be used to wirelessly share contact details, log into computers, unlock doors -- and, yes, to store cryptocurrency. Right inside your paws. Um, how handy is that?

Tinier than a single grain of rice (2 mm x 12 mm) and encased in a glass cylinder, the radio frequency identification-enabled (RFID) chips are a bit like the implants pet owners have veterinarians insert into their furry friends to follow their whereabouts. Like pet trackers, they're powered by the gadgets they're paired with, so no batteries are required.

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But there's one big difference: Wismeijer's implants are dangerous. They put him at risk of infection and his body could reject them altogether. He knew that going in, though. He bought the chips for $99 each from a freaky online biohacking/DIY body modification store aptly called DangerousThings. The site sells all kinds of freaky, potentially injurious non-FDA approved "biohacks."

The embedded cryptocurrency chips -- yep, actual dangerous things -- come complete with injection syringes and other "medical" supplies that no licensed doctor would probably touch. That's why body piercing "artist" Tom van Oudenaarden was called in for the job. You can watch the whole crazy injection go down in Wismeijer's video below. Heads-up: It's not for the squeamish.

Even if it's not as painful as the injection part, storing cyptocurrency on the 888-byte capacity chip sounds like a pain. First, to set up a password on the device, users have to download a free, Android-only companion app. Then, to enter their private Bitcoin keys into the chip, one long line at a time, they need to download NXP's TagWriter app. And it gets more complicated from there.

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You'd think, for all that work, embedded people should also be able to use the chips to make and take cyptocash payments in the wave of a hand. Not so. At least not yet.

Wismeijer told CoinDesk that he thinks some 3,000 people are already implanted with a chip "like his." How about you? Would you wear one on your insides?

If you are brave enough to biohack your bod with one (or two or more), Wismeijer says you should resist the urge to try the chip too soon, even if he didn't.

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"Don't be like me," he told CoinDesk. "I wanted to try it out even before the blood dried up. Everyone wants to try it out right away...but you should leave it alone until it is no longer swollen and healed."

Blood. Swelling. Risk of infection and rejection. No thanks, we're out.