## 2 Challenging Open Issues
### Bioeffects  
The unique characteristic of WBSNs is that the nodes are located inside, on or around the human body. The node operation will produce heat and cause temperature rise in its vicinity. When the node's power consumption is very low or the node is not actively sending data, it might not generate significant heat. However, when the node is operating continuously, transmitting and receiving the data in a considerable period of time, the heat generated by the node cannot be neglected. This concern becomes even bigger when dealing with _in vivo_ sensor nodes (_i.e._, implanted inside the human body). The human body has a thermoregulatory mechanism to balance the heat around the body. However, when the heat received rate is larger than the thermoregulatory mechanism rate, the temperature will rise and, in turn, damage the human tissue.  
[PERSON] [6] conducted a study about the thermal effects of bioimplants. There are two main sources of temperature rise when sensor nodes are implemented in the human body. They are the power dissipated by the implanted sensor nodes and the electromagnetic fields induced in the human body. The power dissipation itself can be divided into three sources: the power dissipated by the implanted microchip, by the implanted telemetry coil and by the stimulating electrodes. When the node operates, it will consume energy, and there is some energy portion that is dissipated. The dissipated energy will be converted into heat and increase the temperature of its vicinity. The longer the node operates and transmits/receives data, the more energy will be dissipated and turned into heat.  
The sensor nodes implanted inside the human body can be safely assumed to be using a wireless system to transmit and receive the data. The human tissue will absorb the radiation energy and convert it into heat, which in turn, will increase the temperature. The well known parameter used by most international standards regarding the electromagnetic safety toward the human body is specific absorption rate (SAR). It can be defined as a measure of the rate at which energy is absorbed by the body when exposed to a radio frequency electromagnetic field, expressed in W/kg. By looking at its unit, we can also say that SAR shows the power dissipated per unit mass of tissue. The value of SAR is determined by these four factors: tissue density, conductivity and electric field amplitude at a point of location in human tissue. Therefore, based on where the SAR value is calculated, the value can be different depending on those factors. However, the IEEEstandard recommends the value of 1.6 W/kg averaged over one gram of tissue as the acceptable value of SAR. This value is also adopted by FCC (Federal Communications Commission) to regulate the SAR level of mobile phones sold in the United States. The readers are encouraged to read [6] to get a more detailed explanation about the thermal effects of bioimplants.  
The effects of these factors actually can be reduced by a good hardware design. A node and its antenna can be engineered and designed to consume as low energy as possible, which, therefore, reduce the temperature rise. In addition, a properly designed routing protocol used in the network also plays an important role in reducing the bioeffects, and this is what the routing protocols presented in this paper are trying to accomplish.