A core-pin assembly composed of a primary core-pin and at least one secondary core-pin. The primary core-pin has a primary core-pin body defining at least one element for coupling with a mating end of at least one secondary core-pin. The secondary core-pin has a mating end and a secondary core-pin body. The mating end is configured to fit with the element defined in the primary core-pin body such that the primary core-pin and the secondary core-pin(s) reversibly join together to form a branching structure. The assembly may further include at least one tertiary core-pin and the secondary core-pin body may define at least one element for coupling with a mating end of at least one tertiary core-pin. The mating end of the tertiary core pin is configured to fit with the secondary core-pin body such that the primary core-pin, secondary and tertiary core-pin(s) reversibly join together forming a branching structure.