A bitcoin entrepreneur and ‘biohacking’ enthusiast has found an unlikely place to store digital currency – under his skin.

Martijn Wismeijer, also known as ‘Mr Bitcoin’, had two NFC (near-field communication) chips injected into the back of his hands during a biohacking event organised by Permanent Beta last week. 

Speaking to CoinDesk, Wismeijer said he has so far experimented with storing litecoin, darkcoin, dogecoin and bitcoin on the tiny devices, which can hold up to 888 bytes of data each.

“As long as you can export your private key, you can write that into your hands," he added.

Although these tests have proved successful, Wismeijer will not be using the implants as a long-term offline store for his cryptocurrency. This is because the media coverage he has received is likely to compromise their security, he said.

However, Wismeijer stressed that the chips are a viable, stealthy storage option for users who are a little more camera-shy:

“If you are a bit more discreet about your implant, nobody will ever know it's there. Apparently the chips don't show up in a full-body scanner at airports ... totally secret agent style, available to consumers everywhere."

After stumbling across bitcoin in November 2010, Wismeijer said he became fascinated with the blockchain. "[It] gave me the same 'wow' effect as the world-wide-web in the early '90s," he added.

Four years on, the Dutch native has gone from mining bitcoins on an old laptop and tinkering with APIs to running an international ATM collective that helps operators with everything from compliance to general repairs.

@gavinandresen Thanks for the Kodak moment at #Bitcoin2014 pic.twitter.com/con95yR63N

— MrBitcoin (@MrBitc0in) May 16, 2014

Storing the currency inside his body is simply another experiment for Wismeijer, albeit an extreme one. DangerousThings, the website selling the $99 gadget, warns it hasn't yet been cleared for use in humans.

Indeed, with doctors out of the question, there is only one technician in the Netherlands known to install DangerousThings' pre-loaded injection kit – Tom van Oudenaarden, an Ultrecht-based 'body manipulation artist' who specialises in 3D implants and body suspension.

There are risks too. Once the pain is gone – Wismeijer said this takes around a day or so – you can start programming the chip. But testing the device any sooner could be dangerous, he cautioned, explaining:

Not following this advice, he continued, might risk infection and ultimately the body could reject the implant entirely.

The implant itself is a tiny glass capsule 2x12mm in size that contains a radio frequency identification tag (RFID) – a smart, programmable barcode historically associated with tracking pets and retail products.

Like a pet’s microchip, Wismeijer’s tag is biocompatible and requires no battery. Instead, it receives power and data from paired devices (smartphones, tablets) via its NFC antenna.

"[There's] no charging required, no toxic batteries inside. The chip can be written to about 100,000 times before it needs replacement so in theory it should last a lifetime," Wismeijer said.

Storing cryptocurrency on the device is a relatively pain-free process, installation aside. First, you disable the lock bits so your chip can't be made read-only. Next, using an NFC app like NXP Tagwriter, you write in your private keys as 'text' tags – one private key per line – and program this into your device.

888 bytes of data are equal to 888 characters of text. Thus, according to Wismeijer’s calculations, the device can store 26 different mini private keys around 33 characters long. Since not all users will need this capacity, Wismeijer recommends securing private keys using BIP-38 encryption, which extends each private key to 58 characters.

Although there is no wear and tear when scanning, each chip only stores data for about a decade. For this reason, users will need to rewrite, or 'refresh' their tag every 10 years or so to avoid loss.

Though the implant's functionality is currently limited by the amount of data it can store, bigger, better upgrades may be on the horizon.

"I met with Amal Graafstra, the engineer behind this biohacking implant and he is working on exciting new models with more memory but these will be larger and more difficult to install as they will probably have to be unfolded under the skin as opposed to just be injected," he said.

In a field such as this, specialists experimenting with the technology are few and far between. However, Wismeijer predicts that there are now around 3,000 people with an implant like his.

Indeed, the pre-loaded injection kit raised 383% of its $8,000 goal during its launch on crowdfunding site Indiegogo last December. New uses are still being discovered too, including those that already impact Wismeijer's day-to-day life. For example, the entrepreneur plans to test out his NFC tags on a wireless door lock for his home and as an off-switch for his alarm clock.

“I have real-world uses for it today, my phones and tablets are all compatible (Android only, sorry iOS users) and have been for the past two years," he said, adding: "I personally feel that by supporting these biohacking developments we can learn what works and what doesn't."