Leadless cardiac pacemakers (LCPs) represent a revolutionary leap forward in cardiac pacing technology because they circumvent transvenous leads. The current size of lithium-carbon mono-fluoride (Li/CFX) batteries results in an overall LCP device size of ~1 cc, and which only has a 10-year lifetime. LCPs are currently limited to single-chamber pacing, representing only 10-20% of current pacemaker users. To achieve dual-chamber and multi-chamber leadless pacing, a size reduction of the LCP is required. However, smaller batteries are required to shrink the LCP. This Phase 2 effort will result in a betavoltaic battery for LCPs that is one-sixth the size of Li/CFX batteries, enabling LCPs with a size of less than half the current LCP. Additionally, this new battery technology will have greater than twice the energy capacity compared to Li/CFX batteries, and also will have double the LCP lifespan, to a 20-year lifetime. This size reduction and increased longevity will allow for 2-3 implants over a patient’s lifetime, with minimal invasive overhead, facilitating mainstream use of LCPs, while challenging traditional pacemakers. Dual or multi-chamber LCPs will dramatically increase the use of LCPs but will require a ~0.1cc battery providing a consistent ≥3.8 microwatts for 20 years. Chemical-based batteries do not have the energy density or reliability to meet this requirement, which the City Labs NanoTritiumTM betavoltaic medical implant battery will effectively address. In Phase 2 the construction of the LCP betavoltaic battery will comprise stacking ultrathin III-V betavoltaic cells that utilize a new, high beta-flux, tritium metal hydride film. The betavoltaic battery will be ~0.1 cc with ≥14.9 microwatts power at beginning-of- life and have a 20-year projected life at ≥ 3.8 microwatts. Phase 1 data shows that the target power density for a ~ 0.1 cc LCP battery will be reached, resulting in a power density sufficient to meet pacemaker manufacturers’ performance goals, resulting in continuous power output for 20 years within a ~0.1 cc form- factor. Tritium betavoltaic technology is a solid-state power source that does not lose energy density with decreasing size, which occurs with lithium batteries. Its principles of operation are similar to a solar cell, but in place of photons impinging on the semiconductor cell, the electrons from the radioisotope’s beta decay are utilized. In the NHLBI SBIR Phase 2, City Labs will construct a tritium betavoltaic battery consisting of ultra-thin stacked betavoltaic cell layers in a cylindrical form factor for easy insertion into a leadless pacemaker package and deliver it to a pacemaker manufacturer to assess overall system performance. Post Phase 2, integrated FDA testing and certification of the joint LCP and betavoltaic will be performed by the pacemaker manufacturer through a Premarket Approval. A bioinert package with electrical feedthroughs will be constructed. Packaging of the cell stack, sealing, and electr...