Temperature-Responsive and Passive Wireless Mechanical Metamaterial Sensors

Abstract

Knowing the location and environmental conditions of sailors that have fallen overboard can greatly facilitate open water search and rescue operations. Currently, active sensors that trigger an alarm when a sailor falls overboard exist, and it works by detecting a loss of signal from a transmitter. However, their applications have been limited because of the dependence on a working battery, as well as the fact that sailor location cannot be tracked beyond the one-time loss-of-signal event. In this study, a passive, person-overboard sensing concept is proposed, and the focus is to demonstrate monitoring of seawater conditions. The passive sensor was designed by incorporating a temperature-responsive hydrogel with an auxetic star pattern unit cell geometry, which transforms from a two- to three-dimensional geometry when immersed in low temperature liquid. First, the passive sensor was formed by incorporating a temperature-responsive hydrogel at opposite ends of the star pattern. The time-dependent swelling and de-swelling behavior of the copolymer hydrogel was characterized to understand the mechanical strains induced after exposure to low temperature water. Second, the design of the star pattern geometry was optimized, and artificial imperfections in the form of notches were employed to allow shape transformation to take place at appropriate and minimal applied strains. The vision is that these shape morphing structures with integrated antennas can serve as passive sensors, which, when interrogated by a remote reader, can be used to monitor how long overboard sailors have been in a low-temperature open sea environment.