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Payment Implant (Purewrist flex conversion)

Hey all,

I located a prepaid card issuer in the US called Purewrist. Their terms are not ideal, but there's really nothing else in the US so I decided to give it a try. It turned out that Amal had an appropriate antenna available so we converted it into the flex form factor, and I had it installed yesterday.

It works really well, and I'm currently running in vivo testing with a variety of reader topologies. The flex implants may be available as a regular item on the DT store sometime in the future, but for now you can order a "card" from Purewrist and send it to Amal to be converted just like a microcard. Here's me paying with the implant immediately after installation:

Pictures attached


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  1. What about the terms makes it non ideal?

  2. There's no fee to load the card, but there is a $4.95 per month fee charged to the account if you spend less than $250/month. Since most users won't spend that much, it's basically a $59.40 per year fee to use the implant. I deemed it worth it, but others may not.

    Since I had it installed, I've done extensive testing with at least 8 different types of readers. I've been able to get a reliable read on every one without much difficulty. The only ones that require a bit of trial and error at first are the ones with the antenna behind an LCD screen. Usually the middle of the top of the screen works well.

  3. @Satur9 did you get the wristband? I don't see any card on their website

  4. Yup, just buy the wristband. They send you the card and the silicone wristband (you can just toss the band)

  5. How did you find that their cards would work as an implant? What was the criteria for it to be implantable?

  6. Specifically it's much easier to convert a card if it's a payment wearable because they often use MOB chip packages (pictured above) that have convenient "wing" contacts that are not bound to a contact interface. Dual interface cards have a tiny silicon die encased in resin which is not easy to connect an antenna to (see first attachment)

    I'm currently exploring conversion of Coil on Module (CoM) cards, which are much more common, especially in the US. You can see in the second and third attachment the type I'm talking about. If you shine a light through you can see the internal antenna, and most have a little antenna trace visible on the outside (that line at the bottom of the right-middle contact).

    I've created some little resonant repeater boards as a proof of concept, and they work pretty well. I'll keep you posted as I improve on the design until it's viable as an implant for any CoM module.

  7. Sent it off to Amal for encapsulation in his biopolymer. It will be nice to have my real debit card with a longer expiration date and no fees after all these Purewrist shenanigans. I wish there weren't so many players standing in the way of implantable payment...

  8. How did it work out? It definitely seems like in order for this to be viable, especially when covered by our meat, it needs to have a stronger signal and evidently, that's something you've been focusing on. Any luck or is it an abandoned project?

  9. I am curious to how it might work. I might have to put the chip in the biopolymer myself
    Right now I am using rfid in my phone and it didn't work on one of the grocer's payment module

  10. The design worked well, but it was difficult for Amal to encapsulate because the z-height of the tuning capacitors was too great. My next step is to revise the design so that the tuning capacitor is two plates in the copper of the PCB. That's likely going to require two revisions to get right and ordering flex PCBs is expensive (~200USD). I just purchased a CNC pick and place, so until I can start getting returns on that investment I've put this project and the bodybytes on the back burner.

    On a semi-related note, I'll shamelessly shill for myself here. I do RFID antenna design, other types of PCB design, and soon medium-batch board population runs as contract work if anyone is interested in that.

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