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A novel, wireless, batteryless, implantable blood pressure monitoring microsystem for small laboratory animals is developed for advanced biological and system biology research. The system employs an instrumented elastic circular cuff, wrapped around a blood vessel, for real-time blood pressure monitoring. The elastic circular cuff is made of soft bio-compatible silicone material, which is filled with bio-compatible insulating fluid with an immersed microelectromechanical systems (MEMS) pressure sensor and integrated electronic system to detect a down-scaled vessel blood pressure waveform. This technique avoids vessel penetration and substantially minimizes vessel restriction due to the soft cuff elasticity, thus attractive for long-term monitoring. A large-model engineering experiment is first developed to verify and demonstrate the concept. A miniature prototype monitoring cuff is then fabricated and implanted in two laboratory rats to evaluate its functionality. A wireless and batteryless monitoring microsystem is then implanted and characterized in a laboratory rat. The measured in vivo blood pressure waveform by the microsystem and a reference waveform recorded by a commercial catheter-tip transducer are closely matched in shape with a constant scaling factor, demonstrating a blood pressure signal with high fidelity can be wirelessly obtained by the implantable monitoring microsystem. The overall implant dissipates 300 Â¿W, which is powered by an external adaptive RF powering source.