A group of tensile microstructures are modelled that exhibit negative Poisson's ratios. These microstructures are a combination of anisotropic or isotropic particles, tensile springs and topologically constraining rods or strings. A range of structures are modelled, one of which contains an internal rotational degree of freedom. In all cases topological interactions occur between first- and second-nearest neighbour particles to produce cooperative transverse expansions under longitudinal loading.