Heart failure is a rapidly growing problem affecting an estimated 38 million people worldwide in 2010 and causing over 9 million deaths worldwide in 2015. Heart transplant, the gold standard treatment for patients with heart failure, is burdened by the scarcity of available donor hearts. Ventricular assist devices provide an addition to heart transplant, but the high cost of ventricular assist devices results in poor costeffectiveness when used as a short-term bridging solution, thus a low-cost alternative is desirable. The intra-aortic balloon pump has been one of the most commonly used mechanical devices worldwide in acute heart failure due to its ease of implantation, low cost and high accessibility compared to other mechanical circulatory support devices. However, the poor haemodynamic assistance in cases of Severe Heart Failure (SHF) limits its use. Based on the same volume-displacement concept, IntraVentricular Balloon Pumps (IVBPs) were of research interest from the early 1970’s to late 1990’s. These early IVBP studies used pre-existing balloon shapes (e.g. spherical) and were limited to animal models. No specific reason was given for cessation of further IVBP research. Uptake of the intra-aortic balloon pump in medical centres worldwide and access to improved design methods and experimental equipment justify revisiting the IVBP concept. At commencement of this Ph.D project, the last published IVBP study dated from 1996. Notwithstanding, within the last two years (2018-2019), two other research groups presented further development of the IVBP showing the regained interest and the importance in developing the IVBP. The present study aimed to develop an IVBP for short-term circulatory support. The study aimed to evaluate the effect of balloon actuation timing on the degree of cardiac support provided to a simulated in vitro SHF patient and to analyse the effect of the balloon actuation on the intraventricular flow dynamics of the simulated SHF patient. It was hypothesised that if an IVBP can avoid interactions with the subvalvular apparatus, has an optimised actuation timing, and results in physiological intraventricular flow dynamics it could be a potential treatment or recovery option for patients with SHF. A silicone IVBP was designed to avoid contact with internal Left Ventricular (LV) features (papillary muscles, chordae, aortic, and mitral valve) based on LV computed tomography data of ten SHF patients with dilated cardiomyopathy. The haemodynamic effects of varying balloon inflation and deflation timing parameters (inflation duty [D] and end-inflation point [σ]) were evaluated in a purpose-built systemic mock circulatory loop. Three IVBP actuation timing categories were defined: co-, transitional-, and counter-pulsation with respect to ventricular systole. To evaluate the effect of the IVBP on intraventricular flow dynamics, the Mock Circulatory Loop (MCL) was adapted to reproduce physiological flow dynamic features and to enable particle image velocimetry acquisition. Velocity maps, fluid pulsatility and circulation throughout a plane of the LV were analysed with and without the IVBP to identify risks of thrombosis. Compared to the SHF baseline, co-pulsation increased aortic flow from 3.5 to 5.2 L/min, mean arterial pressure from 72.1 to 94.8 mmHg and ejection fraction from 14.4% to 21.5%, while mean left atrial pressure decreased from 14.6 to 10 mmHg. Transitional and counter-pulsation resulted in a double ventricular pulse and extended the duration of increased ventricular pressure, potentially impeding diastolic filling and coronary perfusion. Ideal synchronisation of balloon actuation with LV systole (i.e. co-pulsation) provided constructive energy to increase fluid momentum during both LV ejection and LV filling without generating appreciably high intraventricular velocity when compared to the SHF baseline. Positive haemodynamic support, presence of medium to high pulsatility and circulation throughout the in vitro LV, indicated that IVBP co-pulsation could be a beneficial actuation mode for supporting a SHF patient. Counter-pulsation of the balloon counteracted the native fluid momentum during LV filling but generated an additional intraventricular filling flow at balloon deflation, thus providing some degree of support. The counteraction to the natural LV flow; the generation of a second beat during LV diastole; and the large increase in LV end-diastolic volume, indicated in vitro that IVBP counter-pulsation was not recommended as a beneficial actuation mode for supporting a SHF patient. This in vitro study presented three novel key findings about the IVBP concept. 1- The IVBP could be designed based on anatomical fitting to avoid interaction between the balloon and the sub-valvular apparatus, potentially preventing development or worsening of mitral regurgitation. 2- There are ideal IVBP actuation timings (σ = 25% and D < 30%) that increased the haemodynamic support restoring the haemodynamic balance of a simulated SHF patient to healthy levels. 3- When intraventricular flow dynamics were tested in a plane of a simulated SHF patient, the ideal IVBP actuation timing did not result in reduced intraventricular pulsatility, blood washout or increased risk of blood thrombosis when compared to baseline. These interesting in vitro findings demonstrate the feasibility of the device and encourage the development of the IVBP to identify its potential for use as short-term ventricular support. Future work must focus on developing better control and sensing systems with respect to the electrical activity of the ventricle, evaluating the effect of the IVBP on pathological mechanisms, and developing biocompatible balloons with appropriate mechanical properties.