Introduction: Diastolic Heart Failure (DHF) is estimated to account for 30% to 55% of heart failure cases, resulting in significant mortality and morbidity. Management of DHF is essentially empirical, limited, and disappointing. We used a mathematical model and in-vivo studies to evaluate the feasibility of a new device-based approach for DHF, directed towards reducing LV chamber stiffness, utilizing a passive mechanical device which stores energy during systole and releases it in a recoiling force during diastole. Methods: The cardiovascular system and device were simulated using equivalent electronic circuits. Cardiac chambers were modeled using time-varying elastance, and blood vessels using two-elementWindkessel. The device, comprising spiral attachment screws and elastic elements, was implanted off-pump in five healthy sheep (Figure 1). Echocardiography to evaluate LV function and fluoroscopy to evaluate device energy transfer to the LV chamber, determined by elastic elements strain, were conducted up to 90 days post implantation. Results: Simulated LV pressure-volume loops in DHF patients resulted in increased LV volumes, stroke volume and cardiac output and reduced LV diastolic pressure, pulmonary venous pressure and left atrial pressure. The device attachment procedure was technically simple, all animals exhibited good clinical recuperation, Ejection Fraction was preserved up to 90 days follow-up (N=5) and angiography demonstrated good coronary flow. Elastic elements average strain and average device elastic energy transfer was consistent from implantation and up to 90 days follow-up. Conclusion: This study demonstrates for the first time that a passive mechanical device can consistently transfer energy to the LV during diastole without significantly hampering systolic function, and is safe for up to 3 months follow-up. Additional research is needed to validate the safety and efficacy of the device.