## 1 Introduction
### Closed-Loop Medical Devices  
Recently, the so-called closed-loop medical devices [7] have been highlighted [8]. The closed-loop medical devices can deliver a therapy autonomously with the feedback from bio-sensors monitoring the physiological signals. A pace-maker is a well-known closed-loop medical device that is capable of fibrillating or defibrillating the heart when the heart beat of a patient becomes slower or faster, respectively. In this case, the pace-maker has a heart-rate sensor attached to the heart which is hardwired together with the pacing leads.  
Modern closed-loop medical devices are not only capable of providing therapies for heart diseases but also for more complex diseases. For example, in the case of Parkinson's Disease, Deep Brain Stimulator (DBS) can autonomously adjust the stimulation parameters to stabilize the tremors in real-time. This is achieved by continuously sensing the tremors in a closed-loop fashion. The symptoms of Parkinson's patients are not present in close proximity to the therapeutic location. In most cases, the sensor is placed in the limbs of the patient to monitor the tremor. In such a case, a wired connection is not possible between the DBS and sensor nodes. Other closed-loop medical devices, such as 'neural bypass' which connects the brain directly to the muscles, by-passing the spinal cord in case of the paralytic patients [9], artificial limbs, drug delivery devices, nerve stimulators have also recently been explored [10; 11; 12].  
Those implantable medical devices that monitor symptoms of the diseases using a wired sensor in close proximity of the therapeutic device do not require a wireless communication, while a wireless connection is inevitable for medical devices that monitor symptoms of the diseases that are not present in close proximity of the therapeutic location.  
Due to the fact that the number of sensors required to accurately prognose a disease is increasing, currently more than one medical device is used to deliver a therapy. An energy-efficient and reliable wireless communication mechanism is mandatory for the real-time flow of data between these heterogeneous medical sensor nodes. The closed-loop medical systems inherit the properties of a traditional closed-loop control system. The prime characteristics of such systems are presented in Table 2.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Characteristics** & **Body-Worn Sensor Networks** & **Implantable Body Sensor Networks** \\ \hline Energy harvesting & Energy harvesting is easier from mechanical energy, solar energy, and recharging is also an option & Energy harvesting is very limited, temperature change in the body, chemical changes from glucose and heart vibrations are possible. However, the amount of energy that can be harvested is much lower than the energy harvested from outside the body \\ \hline Access to nodes & Can be very easily accessed by people and without the help of doctors & Cannot be accessed without the physician and small surgery \\ \hline Bio-compatibility & Bio-compatibility is not required & Bio-compatibility is of prime importance, since the nodes are planted inside the body \\ \hline Context-awareness & Not always required & Required \\ \hline Wireless medium & Mostly air, and surface of the human body & Complex layers of muscle tissues, bones and conducting heterogeneous medium \\ \hline Connectivity & Should be connected to the Internet & Primarily connected to the base station placed in close proximity \\ \hline Duty cycling & Very low & Dynamic depending on the application \\ \hline Interference & Shared with ISM band & Dedicated frequency band for medical applications \\ \hline \hline \end{tabular}
\end{table}
Table 1: _Cont._