Ultrasound-driven triboelectric nanogenerators (TENGs) were recently proposed as an energy solution technology for a sustainable lifespan of implantable medical devices (IMDs). While the improvement of ultrasound transmission is crucial for achieving high energy generation, research on the material properties of ultrasound-driven TENGs is still in its initial stage. In this work, multifunctional, biocompatible 2-hydroxyethyl methacrylate (HEMA) is suggested as both an encapsulation and triboelectric layer for an implantable, modulus-tunable ultrasound-driven TENG (IMU-TENG). By adjusting the acoustic impedance of HEMA to be suitable for the surrounding environment, the ultrasonic transmission coefficient is about 10 times higher than that of a titanium (Ti) plate. Under in vivo conditions, the IMU-TENG generates sufficient energy to charge a 100 μF capacitor 3.7 times faster than the case with a Ti plate. This strategy of using multifunctional HEMA for high-performance ultrasound-driven TENGs could be a promising energy solution for low-powered IMDs.