Project Summary Atrial fibrillation (AF) is the most common arrhythmia that increases the risk of a stroke while impairing the patient’s quality of life and costing US around $6.65 billion per year 6. Despite advances in the field, the current treatments have limited success rate especially in patients with longstanding AF with enlarged atria that requires multiple procedures with higher costs, inconvenience, and separate recovery of 2 time-consuming procedures that do not guarantee fully transmural lesions (arrhythmia treatment through the tissue thickness) since the epicardial (outside surface of the heart) and endocardial (inside surface of the heart) lesions are performed at 2 separate sittings and not well-aligned. Our proposed approach uses a self-aligning bipolar ablation system that includes a thoracoscopically guided epicardial delivery system and a flexible endocardial catheter following it across myocardium. This allows for simultaneous epicardial and endocardial tissue contact for the purpose of continuous bipolar ablation in a single procedure in a shorter time and higher treatment success rate. Therefore, we have developed a self-aligning technology which allows epicardial and endocardial ablation to be performed simultaneously in a single procedure, increasing transmurality, and decreasing the time and expense of the 2 procedures. Thus far, we have demonstrated 1- smooth catheter movement across the tissue. 2- successful transmural lesion sets in our ex vivo and in vivo open chest studies across the porcine atrial wall experiments. Phase I funding will enable us to optimize and prototype this paired endo- and epi- catheter system that is inserted via a conventional transseptal and thoracoscopic approach, respectively. This minimally invasive system will be capable of smooth movement, consistent contact force with tissue along the ablation path independent of tissue thickness, and transmural ablation along the atrial wall.