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All terrestrial animals must find a proper level of moisture to ensure their health and survival. However, the molecular basis for sensing humidity is unknown in most animals. We used the model nematode Caenorhabditis elegans to uncover a novel mechanism to sense humidity. We found that C. elegans displayed a strong preference, orienting to gradients as shallow as 0.03% relative humidity per mm. Cell-specific ablation and rescue experiments demonstrate that orientation to humidity in C. elegans requires the obligatory combination of distinct mechanosensitive and thermosensitive pathways. The mechanosensitive pathway requires a conserved DEG/ENaC/ASIC mechanoreceptor complex in the FLP neuron pair. Because humidity levels influence the hydration of the worm’s cuticle, our results suggest that FLP may convey humidity information by reporting the degree that subcuticular dendritic sensory branches of FLP neurons are stretched by hydration. The thermosensitive pathway requires cGMP-gated channels in the AFD neuron pair. Because humidity levels affect evaporative cooling, AFD may convey humidity information by reporting thermal flux. Thus, humidity sensation arises as a metamodality in C. elegans that requires the integration of parallel mechanosensory and thermosensory pathways. This hygrosensation strategy, first proposed by Thunberg over 100 years ago, may be conserved because the underlying pathways have cellular and molecular equivalents across a wide range of species including insects and humans. Whereas well-fed worms raised in non-crowded conditions prefer high humidity levels starved worms prefer lower humidity ranges. Our results suggest a model in which social cues from pheromone signaling, along with the worm’s feeding state, influence cyclic guanosine monophosphate (cGMP), peptide, and protein kinase C signaling to produce opposite hygrotaxis behavior.

The advent of new optical fibre technologies in recent years have revolutionized the telecommunications and sensing field. In particular the rapid development of microstructured fiber, also known as photonic crystal fiber (PCF), brings about remarkable features such as air guided mechanism, unique geometric structure and single material fibre structure that could overcome many limitations intrinsic to conventional standard optical fibers. This thesis reports my research and original results from the development and application of microstuctured fibre as fiber humidity sensors. My work covers the design and fabrication of the PCF-based sensors, development of experimental systems with related data generation and acquisition capabilities, novel sensing schemes and techniques as well as proof of concept experiments. I proposed and experimentally demonstrated several novel fibre humidity sensor schemes which do not require coating of any hygroscopic or sensitive film while most of previous reported fiber humidity sensors need the hygroscopic films to sense humidity. I also proposed and demonstrated a microstructured optical fibre based humidity sensor for respiration monitoring for possible application of PCF-based fiber sensors in the bio-medical engineering field.

Abstract: Thin films are produced from layers of materials ranging from nanometres to micrometres in height. They are increasingly common and are being used in integrated circuit design, optical coatings, protective coatings, and environmental sensing. Thin films can be fabricated using a variety of methods involving chemical reactions or physical transport of matter. Glancing angle deposition (GLAD) thin films are produced using physical vapour deposition techniques under high vacuum conditions where exploitation of the geometric conditions between the source and the substrate causes enhanced atomic self shadowing to produce structured thin films. This work deals with the modification of these films, \emph{in situ} by altering growing conditions through substrate temperatures control, or post-deposition through reactive ion etching (RIE). The first part of the thesis deals with the modification of TiO$_2$ GLAD humidity sensors using RIE with CF$_4$. The data presented demonstrates improved response times to step changes in humidity. Characterization revealed response times of better then 50~ms (instrument-limited measurement). An etch recipe for complete removal of TiO$_2$ was also demonstrated with shadow masking to transfer patterns into GLAD films. The subsequent chapter focuses on modification of thin film growth conditions by increasing adatom mobility. A radiative heating system was designed and implemented with the ability to achieve chuck temperatures of 400$^\circ$C. Capping layers on top of GLAD films were grown to demonstrate effects of \emph{in situ} heating, and a quantitative analysis of crack reduction with increased temperatures is presented. Lithographic pattern transfer onto a capped GLAD film was demonstrated. Opposite to the goal of the preceding chapter, the focus of the final experimental chapter was to limit adatom mobility. A LN$_2$-based cooling system was designed and implemented for the purpose of studying the growth by GLAD of lower melting point materials, which under regular growth conditions do not form well-defined structures. Chuck temperatures of $-60$$^\circ$C can be achieved during deposition while still allowing substrate rotation. The growth of helical copper films was used to demonstrate the effects of \emph{in situ} substrate cooling.