An additively manufactured heat exchanger assembly formed of a lattice structure with a plurality of shell structure unit cells. The plurality of shell structure unit cells each include at least one junction and a plurality of connectors coupled to the junction. The junction and the plurality of connectors form an integral surface. The heat exchanger assembly further includes a fluid boundary wall defined within the lattice structure to define an active heat exchanger portion. The active heat exchanger portion includes at least one working fluid contained within the fluid boundary wall. Further, at least one of the plurality of shell structure unit cells extends through and beyond the fluid boundary wall. Further, the shell structure unit cells may be isotropic.