| CPC A61B 5/0031 (2013.01) [A61B 5/076 (2013.01); A61B 5/279 (2021.01); A61B 5/686 (2013.01); A61B 6/4258 (2013.01); A61B 8/0808 (2013.01); A61B 8/085 (2013.01); A61B 8/0875 (2013.01); A61B 8/48 (2013.01); A61N 1/3605 (2013.01); A61N 1/37205 (2013.01); A61N 1/3787 (2013.01); B06B 1/06 (2013.01); G01N 27/327 (2013.01); A61B 2505/05 (2013.01); A61B 2560/0219 (2013.01); A61B 2562/028 (2013.01); A61B 2562/12 (2013.01); A61M 39/0208 (2013.01); A61M 2205/04 (2013.01); A61M 2205/825 (2013.01); A61N 1/0534 (2013.01); A61N 1/372 (2013.01); A61N 5/1071 (2013.01); A61N 2005/1087 (2013.01); A61N 2007/0021 (2013.01)] | 20 Claims |

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1. A system comprising one or more implantable devices and an interrogator comprising one or more ultrasonic transducers configured to transmit ultrasonic waves to the one or more implantable devices or receive ultrasonic backscatter from the one or more implantable devices, the one or more implantable devices comprising a body 5 mm or less in length in the longest dimension, comprising:
(a) an ultrasonic transducer configured to emit an ultrasonic backscatter encoding information relating to an impedance characteristic of a tissue at a plurality of different signal source frequencies based on a modulated current flowing through the ultrasonic transducer; and
(b) an integrated circuit comprising:
(i) a variable frequency signal source circuit electrically connected to a first electrode and a second electrode, wherein the first electrode and the second electrode are configured to be in electrical connection with each other through the tissue, and wherein the variable frequency signal source circuit is configured to apply a current or voltage through the tissue at the plurality of different signal source frequencies;
(ii) a signal detector configured to detect, at the plurality of different signal source frequencies, an impedance, voltage, or current in a circuit comprising the variable frequency signal source circuit, the first electrode, the second electrode, and the tissue; and
(iii) a modulation circuit configured to modulate the current flowing through the ultrasonic transducer to cause the ultrasonic transducer to emit the ultrasonic backscatter encoding the information comprising the impedance characteristic of the tissue at the plurality of different signal source frequencies.
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