A biocompatible seed for implantation in tissue of a patient includes an elongated body sized and shaped to be at least partially inserted into the tissue of the patient, in which the body includes a negative Poisson's ratio (NPR) material having a Poisson's ratio of between 0 and 1. The seed can be a thermal seed configured to generate heat responsive to exposure to a magnetic field. The seed can be a seed for brachytherapy that includes an inner layer including a radioactive material and an outer layer including the NPR material.