This page looks at smart card systems. In essence, they authenticated a transaction over a network and the individual card did not embody a monetary value. A value, typically somewhere between twenty and two hundred dollars, would be loaded onto the card - the card itself served as currency, rather than as authentication for money held in a separate account. The relentless fall in the cost of microchips encouraged some developers to add them to cards.
Most German citizens have a health care smart card.
We are in the process of adding pointers to digital cash, smart cards and online payment systems. For the moment an excellent introduction is provided by the electronic money bibliography at Exeter University. The US Smart Card Forum is an industry body concerned with credit card sized devices that act as rechargeable electronic purses and, in more sophisticated versions, can hold digital signatures, medical records and other data.