Various examples of methods, systems, and apparatus are provided for monitoring using an improved mouth guard apparatus. In one example, a battery-free diagnostic mouth guard includes a biting force-voltage transducer comprising a piezoelectric film a compact resonance tank comprising a wireless sensor and a transmitting antenna for transmitting sensing data. A piezo-voltage from the biting force-voltage transducer can bias a varactor diode loaded on the wireless sensor whose response frequency is tuned due to a capacitance change of the varactor diode. In some implementations, an external processing equipment can wirelessly detect a frequency shift of the varactor diode integrated resonator. In another example, a method includes detecting a biting force via a biting force-voltage transducer biasing a varactor diode with a voltage from the transducer tuning a response frequency of a split ring resonator using a capacitance change of the varactor diode and emitting sensing data at the response frequency.
