A patient monitoring system (100, 200), comprising: a plurality of implantable, untethered sensing devices (102, 102a, 102b, 102c, 202, 402) configured to be disposed on or near coronary stents implanted in a patient's body, each untethered sensing device comprising: an antenna (106, 306, 406) configured to receive wireless power from a power source external to the patient; at least one electrical sensor or electrode (110, 310) configured to measure a resistance of blood inside a said coronary stent; and electronics (108, 308) coupled to the antenna and the at least one sensor or electrode and configured to communicate the measured blood resistance to a device (104, 204) external to the patient, wherein the electronics comprise an energy harvesting and power management block and a digital controller and wherein the controller is configured to manage the localized device operation, process commands from the external device, and packetize and send out sensed data back to the external device; and at least one said external device (104, 204) containing the power source and configured to provide wireless power to the antennae of the sensing devices at an operating frequency ranging from 100 MHz to 5 GHz and configured to receive the measured blood resistance from the sensing devices and enable monitoring of the effectiveness and lifetime of the coronary stents implanted in the patient's body; wherein the sensing devices are configured to be powered only when receiving wireless power from the external device and to be turned on only when their antennae are in the presence of a magnetic field formed by said at least one external device. a plurality of implantable, untethered sensing devices (102, 102a, 102b, 102c, 202, 402) configured to be disposed on or near coronary stents implanted in a patient's body, each untethered sensing device comprising: an antenna (106, 306, 406) configured to receive wireless power from a power source external to the patient; at least one electrical sensor or electrode (110, 310) configured to measure a resistance of blood inside a said coronary stent; and electronics (108, 308) coupled to the antenna and the at least one sensor or electrode and configured to communicate the measured blood resistance to a device (104, 204) external to the patient, wherein the electronics comprise an energy harvesting and power management block and a digital controller and wherein the controller is configured to manage the localized device operation, process commands from the external device, and packetize and send out sensed data back to the external device; and an antenna (106, 306, 406) configured to receive wireless power from a power source external to the patient; at least one electrical sensor or electrode (110, 310) configured to measure a resistance of blood inside a said coronary stent; and electronics (108, 308) coupled to the antenna and the at least one sensor or electrode and configured to communicate the measured blood resistance to a device (104, 204) external to the patient, wherein the electronics comprise an energy harvesting and power management block and a digital controller and wherein the controller is configured to manage the localized device operation, process commands from the external device, and packetize and send out sensed data back to the external device; and at least one said external device (104, 204) containing the power source and configured to provide wireless power to the antennae of the sensing devices at an operating frequency ranging from 100 MHz to 5 GHz and configured to receive the measured blood resistance from the sensing devices and enable monitoring of the effectiveness and lifetime of the coronary stents implanted in the patient's body; wherein the sensing devices are configured to be powered only when receiving wireless power from the external device and to be turned on only when their antennae are in the presence of a magnetic field formed by said at least one external device.