Patent ID: 8162839

Claim:
A protected, implantable passive ultrasonically excitable sensor consisting essentially of: (a) a housing; (b) one or more passive ultrasonically excitable sensor units, each sensor unit of said one or more sensor units includes a substrate and at least one vibratable membrane overlying at least one first chamber defined between said substrate and said vibratable membrane, said first chamber being a sensor unit chamber, wherein said vibratable membrane has a resonance frequency that varies as a function of a physical variable in a measurement environment in which said sensor is implanted in response to an interrogating ultrasound beam; and (c) a second chamber defined between at least part of said housing, said at least one vibratable membrane and a compliant member attached to said housing to protect said vibratable membrane, said second chamber including a substantially non-compressible medium disposed therein, wherein said medium is in contact with (i) said at least one vibratable membrane, (ii) said compliant member, and (iii) at least part of said housing, wherein said second chamber is fluidically isolated from said at least one first chamber; wherein said compliant member has a first side and a second side, said first side is in contact with said measurement environment to form a first interface when said sensor is implanted into said measurement environment and said second side is in contact with said medium to form a second interface, and wherein said compliant member has a structure that limits reflection of said interrogating ultrasound beam at said first interface, and said non-compressible medium has a structure that limits reflection of said interrogating ultrasound beam at said second interface.