Using miniature wearable devices, it is possible to follow the health of individuals from outside the hospital in a convenient and easy way that does not restrict their movements or their daily activities. The wearable biomedical sensors or the Wireless Body Area Network (WBAN) systems is about to become an important tool to match the express way of life nowadays. This paper reviews the technological developments in wireless sensor nodes for biomedical applications during the last decade. The review covers the electronics hardware design, challenges, safety and ending by the standards required to implement those systems. The designer of the wearable medical sensor should be aware of the standards and the safety issues at the acquisition and storage level, wireless transmission protocol level, powering level, security and privacy level, and overall safety level. Finally, this paper suggests some of the recent technologies that can be used to enhance the future WBAN systems.