1.1 Aims: Proof-of-concept to determine the direct biomechanical effects of cardiac contractility modulation (CCM) on living myocardial slices (LMS) from patients with end-stage heart failure (HF). 1.2 Methods and results: Left ventricular LMS from patients with end-stage HF were produced and cultured in a biomimetic system with mechanical loading and electrical stimulation. CCM stimulation (80 mA, 40 ms delay, 21 ms duration) enhanced maximum contractile force (CCM: 1229 µN (587 – 2658) vs. baseline: 1066 µN (529 – 2128), p = 0.05) and area under the contractile curve (CCM: 297 (151 – 562) vs. baseline: 243 (129 – 464), p = 0.05), but did not significantly impact contractile duration, time to peak or time to relaxation. Increasing CCM stimulation delay, duration and amplitude resulted in a higher fraction of LMS with a positive inotropic response. Furthermore, CCM attenuated the negative force-frequency relationship in HF-LMS. 1.3 Conclusion: CCM stimulation enhanced contractile force in HF-LMS. The fraction of LMS exerting a positive inotropic response to CCM increased with increasing delay, duration and amplitude settings, suggesting that personalizing stimulation parameters could optimize the beneficial effects of CCM. 1.4 Translational perspective CCM is a novel device-based therapy that may improve contractile function, ejection fraction, functional outcomes and quality of life in patients with heart failure. However, continuous efforts are needed to identify true responders to CCM therapy, understand the exact mechanisms and to optimize the contractile response to CCM stimulation. The present study revealed that CCM enhanced contractile force of HF-LMS in a stimulation setting dependent manner, reaching a larger fraction of the myocardium while increasing delay, duration and amplitude. This understanding may contribute to the individualization of CCM stimulation settings.