Advances in wireless sensor technology have introduced a new dimension in healthcare computing. With miniaturized sensor devices, continuous medical monitoring of patients to detect transient life threatening conditions from daily activities has been made possible. This phenomenon will certainly improve the quality of life for a majority of people. However, the growing variety of sensor devices in the market, has made application developments very challenging. In light of this view, we are proposing SLIM, a lightweight middleware that shields the underlying differences in sensor technology from the application layer. SLIM is designed to simplify/accelerate application development while not simply providing common middleware functions (i.e. data acquisition and plug-and-play capability) but also others like security and interactive features. To support both indoor and outdoor activities, SLIM is built to reside on mobile devices. At the end of the paper, we will show the prototype of the proposed middleware including its functionalities when used with a mobile application.