An actively-powered temperature data logger patch with wireless data communication includes a sealed, flexible battery comprising a printed electrochemical cell with an anode and a cathode, and a flexible circuit including a microprocessor, a temperature sensor configured to sense a temperature of a target subject, a wireless communication transceiver and an antenna. In one example, the flexible battery and flexible circuit are arranged in a covering, stacked arrangement between first and second substrate layers, and the patch configured to conform to a curved surface of the target subject. In another example, the patch is used in an actively-powered medical system for monitoring a body temperature of a patient, and includes an adhesive configured to be removably applied to skin of the patient. An external computing device is capable of two way communication with the wireless communication transceiver of the patch via an electromagnetic field.