## 1 Introduction  
The budget for health care now accounts for a significant portion of several economies around the world. In the United States (U.S.) alone, $2.5 trillion dollars were dedicated to health care in 2009, amounting to approximately 17% of the gross domestic product (GDP) [1]. The 17% of GDP spent by the U.S on health care is almost double that of the GDP spent by much of the rest of the world (approximately 9%), even though the U.S utilizes fewer doctors and nurses per person [2]. Of each dollar spent on health care in the U.S., approximately 40% of it is consumed by hospital care and nursing homes [3]. There are several negative factors that contribute to the high costs of provisioning health care in hospitals and nursing homes, such as poor doctor and nurse to patient efficiency and the inability to constantly monitor a patient's health. These factors not only contribute to rising health care costs, but may also play a role in health care related preventable deaths, which account for approximately 44,000 to 98,000 deaths per year in the U.S., as estimated by the Institute of Medicine (IOM) [4]. Technology has always presented ways of improving and enhancing our lives. Thanks to the continued realization of [PERSON]'s law, innovation in the area of sensor technology has allowed the integration of tiny, low-power, and wearable smart medical sensor devices (e.g., pulse oximeters [5], electrocardiographs [6], and accelerometers [7]) into commonly used wireless sensor devices. These sensor-equipped wireless devices form wireless sensor networks (WSN) that today, offer a potential solution to the inefficiencies that plague the health care industry. In recent years, WSN-based health care systems have been deployed for applications such as home monitoring for chronic and elderly patients [8], real-time continuous patient monitoring in hospitals [9], automated vital sign analysis to reduce the incidents of medical accidents due to human error [10], and emergency situations [11]. For most applications requiring the design, implementation, and deployment of WSNs, the following challenges inherent to almost all WSN applications must be overcome: low computational power, poor communication bandwidth, congested wireless medium, and limited energy budget. However, health care related applications for nursing homes, emergency scenarios, or hospitals require more specific requirements to make the integration of WSNs successful. There are five main requirements that the architecture for a health care WSN (HCWSN) must satisfy: (1) reliability--The ability to transmit accurate and diverse data while meeting stringent quality of service (QoS) requirements, in terms of high packet delivery ratio (PDR) and low end-to-end latency, is of paramount importance in medical settings. (2) Energy Efficiency--One key draw of WSNs to health care applications is that many sensors do not require external power (_i.e._, they use battery power), so it is important to extend the lifetime of these devices by minimizing energy consumption. (3) Routing--The routing of data can directly impact the reliability, fault tolerance, and scalability of a HCWSN, and also the energy required by the system for communication. Furthermore,the ability to support multiple receivers and multicast technology is also important, especially in scenarios where a single query is issued (e.g., by a doctor or nurse) to a group of subscribed receivers (e.g., patients). (4) Node Mobility--Both patients and caregivers have the ability to move around and thus exhibit mobility, requiring that the implemented communication layer protocol adapt rapidly to broken routes and fluctuating transmission link quality, and (5) timeliness--On-time delivery of transmitted information is critical, especially in emergency situations.