## 6 Discussion  
In this section, we identify and discuss specific areas for further research to close the gaps that have been identified in this paper. Researchers have taken a major step towards enhancing the performance of HCWSNs in terms of reliability, mobility awareness, and timeliness, with the overall goal of minimizing the persisting gaps. However, future work is required to specifically analyze the energy efficiency of HCWSNs, without applying the common assumption that a certain number of sensor nodes in HCWSNs have access to external power supplies. While this assumption normally holds true in environments such as hospitals and nursing homes, the same cannot be said for scenarios where triage is immediately required at disaster sites. Therefore, experiments are required to determine specifically how much energy fully operational battery-driven biomedical sensors consume, and the resulting lifetime duration of the HCWSNs. MAC protocols are typically employed to ensure energy efficiency and fair access to the transmission medium. However, MAC protocols must also consider the different traffic types that are characteristic of sensor-equipped patients experiencing various degrees of medical distress (e.g., life threatening conditions _vs._ minor illness). In a HCWSN, special consideration must be afforded to data traffic of higher priority. Thus, a MAC protocol scheme where a sensor node is granted access to the wireless medium based on traffic prioritization must be further investigated. It is pertinent that the MAC protocols implementing traffic prioritization achieve the desired performance objectives such as good energy efficiency and low packet transmission latency while evaluated under different operational scenarios such as different scalability and mobility characteristics. Data aggregation is a common technique utilized by routing protocols for the purpose of reducing transmissions and increasing throughput. However, in most cases, routing protocols that adopt data aggregation schemes are either organized in clusters or hierarchies where a node (e.g., cluster head) is designated as an aggregation point [37]. In HCWSNs that are modeled using a flat network architecture, an aggregation point is typically not designated _a priori_. Therefore, a routing protocol that adopts a scheme to dynamically identify candidate nodes in the network that present an opportunity for in-network aggregation would be beneficial for the following reason: nodes that remain static for long periods of time in the network (e.g., a group of patients recovering from similar intensive operations such as blood transfusions) periodically broadcast unchanged routing information. As such, the routing information from the group of similar acting nodes can be aggregated and propagated as a single transmission. The benefit of cross-layer design (CLD), applied to conventional WSNs, in terms of QoS support without a loss of energy efficiency has been demonstrated in previous works [38,39]. We conjecture that CLD will also enable HCWSN QoS requirements (e.g., \(>\)90% PDR and 5 s latency [21]) for health care applications to be met. In the context of the three protocol layers reviewed in this paper, information must be exchanged between and across the MAC, routing and transport layers. What and how information should be exchanged along with the benefit-to-cost ratio of cross-layer design in the context of HCWSNs remain an open issue for further investigations. The use of network coding to enhance reliability in HCWSNs has not been investigated, to our knowledge. Assessing the effectiveness of network coding in a HCWSN requires further detailed investigation. Minimum cost multicast algorithms [40, 41] that adopt network coding have been proposed for wireless networks. Based on this work, future work can be performed to assess the effectiveness of the aforementioned algorithms when adapted to the characteristics of a wireless sensor network.