# A Review of Medication Adherence Monitoring Technologies +
## 5 Challenges and Future Trends
### Challenges
#### 5.1.2 Energy Consumption and Lifetime  
A medication adherence monitoring system can be battery-powered, for example, in the case of sensor networks and mobile device based systems, such as smartwatches, mobile phones, and tablets. This poses a challenge as the battery has limited energy budget [147]. From a system point of view, it is anticipated that a sufficient amount of electric current is being fed to the system to ensure its functionality. At the same time, from a user point of view, it is expected that the system lifetime lasts for as long as possible as application developers must either frequently replace batteries or use rechargeable batteries. This would likely be inadequate for user's acceptance and costly [148; 149; 150; 151].  
It is worth mentioning that, for some kinds of technologies, energy consumption may not be an essential concern as they are either passive or not powered from a battery source. Strictly speaking, among various technologies, sensor-based systems have a bottleneck of being sensitive to energy consumption as they should be powered by small size batteries with limited capacities [152; 153]. This is due to the fact that [PERSON]'s law does not apply to battery manufacturing [154] and, thus, the battery components continue to be constrained. Moreover, typical sensor nodes are compact and as such there is not enough room for large batteries to be used, which places constraints on the energy cost [155].  
Even though only rare studies focused on the energy consumption of medication adherence monitoring systems, this is still central in this context as it can severely affect the performance and efficiency of the system [149; 156]. This can be imagined by taking wearable systems powered by non-rechargeable batteries as an example. In general, the battery is a complex system that can behave unpredictably when affected by several factors and conditions, including the temperature and the applied load [157; 158]. High-fidelity motion sensors are utilized within wearable devices for accurately sensing and quantifying the motion associated with medication taking activity. However, there is a trade-off between energy consumption and data fidelity. On the one hand, the sensor device should be operating continuously and sampling data frequently. On the other hand, even if temperature conditions are perfect, enabling the sensor(s) for frequent data sampling results in increasing the internal resistance of the battery and affecting its chemical and physical properties [159]. Consequently, the battery will not be put to rest for a sufficient time and recover its rated voltage. As a result, the battery voltage will continually drop with time until it reaches the cut-off voltage. Operating the battery under such timing and intensity conditions will not enable it to provide voltage at a sufficient level that operates the connected device correctly, even with a considerable amount of unused charge being left.  
As a consequence of the experienced discharge behavior, the system's lifetime is directly affected. Generally speaking, users prefer systems with longer lifetime, compared to those with shorter lifetimes. As such, wise battery usage is required [160]. Wisely using the battery requires techniques such as collaborative sensing to be employed for minimizing energy depletion in such systems. Once the energy consumption issue achieves notable progress, battery-powered systems such as wearable and portable systems can be used more widely in the area of adherence monitoring applications.