Abstract-Self-monitoring the sensor statuses such as liveness, node density and residue energy is critical for maintaining the normal operation of the sensor network. When building the monitoring architecture, most existing work focuses on minimizing the number of monitoring nodes. However, with less monitoring points, the false alarm rate may increase as a consequence. In this paper, we study the fundamental tradeoff between the number of monitoring nodes and the false alarm rate in the wireless sensor networks. Specifically, we propose fully distributed monitoring algorithms, to build up a poller-pollee based architecture with the objective to minimize the number of overall pollers while bounding the false alarm rate. Based on the established monitoring architecture, we further explore the hop-by-hop aggregation opportunity along the multihop path from the polee to the poller, with the objective to minimize the monitoring overhead. We show that the optimal aggregation path problem is NP-hard and propose an opportunistic greedy algorithm, which achieves an approximation ratio of . As far as we know, this is the first proved constant approximation ratio applied to the aggregation path selection schemes over the wireless sensor networks.

Abstract-In many surveillance scenarios, there are some known critical locations where the events of concern are expected to occur. A common goal in such applications is to use sensors to monitor these critical locations with sufficient quality of surveillance within a designated period. However, with limited sensing resources, the coverage and lifetime requirement may not be satisfied at the same time. Thus, sometimes the sensor needs to reduce its duty cycle in order to satisfy the stringent lifetime constraint. In this paper, we model the critical location coverage problem using a point coverage model with the objective of scheduling sensors to maximize the event detection probability while meeting the network lifetime requirement. We show that this problem is NP-hard and propose a distributed algorithm with a provable approximation ratio of 0.5. Extensive simulations show that the proposed distributed algorithm outperforms the extensions of several state-of-the-art schemes with a significant margin while preserving the network lifetime requirement.

Abstract-Mission-driven sensor networks usually have special lifetime requirements. However, the density of the sensors may not be large enough to satisfy the coverage requirement while meeting the lifetime constraint at the same time. Sometimes, coverage has to be traded for network lifetime. In this paper, we study how to schedule sensors to maximize their coverage during a specified network lifetime. Unlike sensor deployment, where the goal is to maximize the spatial coverage, our objective is to maximize the spatialtemporal coverage by scheduling sensors' activity after they have been deployed. Since the optimization problem is NP-hard, we first present a centralized heuristic whose approximation factor is proved to be 1 2 , and then, propose a distributed parallel optimization protocol (POP). In POP, nodes optimize their schedules on their own but converge to local optimality without conflict with one another. Theoretical and simulation results show that POP substantially outperforms other schemes in terms of network lifetime, coverage redundancy, convergence time, and event detection probability.

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A root canal sealer with antibacterial activity can be efficacious in preventing reinfection that results from residual microorganisms and/or the leakage of microorganisms. In the present study, a series of injectable, self-curing polyurethane (PU)-based antibacterial sealers with different concentrations of silver phosphate (Ag 3 PO 4 ) were fabricated. Subsequently, their physicochemical properties, antibacterial abilities, and preliminary cytocompatibilities were evaluated. The results indicated that the fabricated PU-based sealers can achieve a high conversion rate in a short amount of time. More than 95% of the isocyanate group of PU sealers with 3 wt% (PU3) and 5 wt% (PU5) concentrations of Ag 3 PO 4 were included in the curing reaction after 7 hours. With the exception of those for film thickness for PU5, the results of setting time, film thickness, and solubility were able to meet the requirements of the International Organization for Standardization. The antibacterial tests showed that PU3 and PU5 exhibit stronger antimicrobial effects than that achieved with 1 wt% Ag 3 PO 4 (PU1) and AH Plus (positive control) against Streptococcus mutans . The cytocompatibility evaluation revealed that the PU1 and PU3 sealers possess good cytocompatibility and low cytotoxicity. These results demonstrate that the PU3 sealer offers good physicochemical and antimicrobial properties along with cytocompatibility, which may hold great application potential in the field of root canal fillings.