In this study, we propose an efficient aggregate signcryption scheme to maximize the security of data in a kind of wireless medical network named the disconnected or unattended wireless sensor network (applied in medical systems). These networks address patients who need to be monitored for a long time. The main challenge of these networks that are usually implanted on the patient's clothing and established in sensitive conditions is that the server (station) visits sensors continuously. Moreover, the sensors must retain data for long enough time to off-load to the station as they have limited capacity and batteries. This disconnected nature gives adversaries the power to read and modify target data without being detected or disclose private medical data related to a patient. In this paper, we address these security problems and improve the first study of identity-based aggregate signcryption in UWSNs to achieve both key privacy and invisibility. Our improved approach is at the same time efficient in terms of space and communication overload. Moreover, the proposed scheme allows servers to efficiently verify and unsigncrypt all the related data accumulated by sensors. We further show that the proposed scheme has resistance against reading and modifying attacks. We compare our scheme with the best alternative works in the literature.