Development of Graphene-Based Multi-Modal Piezoresistive Sensors for Human Health and Fruit Growth Monitoring

Abstract

The modern world requires a modern health monitoring approach, which necessitates the requirement of smart wearable devices. Among wearable devices, flexible piezoresistive sensors have gained immense attention due to their broad applicability as wearable sensors for monitoring physical activities and physiology. The applicability of flexible piezoresistive sensors extends beyond wearables, encompassing a diverse array of applications, including smart farming, robotics, structural health monitoring, man-machine interface, etc. However, monitoring physical activities and physiology requires highly sensitive, long-run electromechanical stability, high flexibility or stretchability, and body-conformal piezoresistive sensors. To achieve the desired characteristics requires highly conductive and mechanically robust nanomaterials. Graphene has remarkable electrical, mechanical, and thermal properties, making it an ideal candidate for developing high-performance flexible piezoresistive sensors. In view of the importance of flexible piezoresistive sensors, the primary objective of this thesis is to develop highly sensitive, durable, stretchable, non-invasive, body conformal, eco-friendly, wearable piezoresistive sensors for monitoring human physical activities, physiology, and fruit growth. For the development of wearable strain sensors, graphene has been synthesized with the aim of reducing or eliminating the use of hazardous chemicals or strong acids in synthesizing graphene. In this study, graphene flakes have been synthesized through an electrochemical exfoliation process utilizing various electrolytes, including H2SO4, Pirahna, and CuSO4.5H2O, as intercalating agents. Initially, reduced graphene oxide (rGO) was synthesized by exfoliating pencil lead in H2SO4 electrolyte. The synthesized rGO was characterized using various analytical instruments, including XRD, TEM, FESEM, Raman spectroscopy, EDX, and SAED. Utilizing the prepared rGO, highly sensitive, durable, stretchable piezoresistive sensors have been developed by depositing rGO over silicone sealant using drop casting methodology. The fabricated rGO-sealant sensors have shown a gauge factor greater than 4000, durability longer than 1600 cycles at 100% strain and detection capability over a wide range of strains (0 - 120%). The developed rGO-sealant sensors demonstrated their ability to monitor physical activities and detect physical touch. Further, microsized multilayer graphene flakes (μG) were synthesized on a large scale by exfoliating graphite rods in piranha solution. The synthesized multi-layer graphene flakes were characterized using various methodologies which exhibit high-quality graphene flakes. Using the synthesized μG and silicone elastomer (SiE), a screen printable composite (SiE-μG) was prepared to fabricate supersensitive antibacterial piezoresistive sensors. The sensors were fabricated by depositing the SiE-μG composite on an orthopaedic bandage. The fabricated sensors have shown supersensitivity of 18300 at 35% strain, durability of 50000 cycles, and minimum detection limit of 0.4% strain. The developed strain sensors show antibacterial properties against Escherichia coli (E. coli). Moreover, the sensors have shown all the other significant characteristics required by a reliable wearable health monitoring device, such as ultra-conformality, skin-friendly, eco-friendly, fabrication simplicity, and cost-effectiveness. Owing to the above merits, the SiE-μG@Bandage sensors have demonstrated multifunctional applications in real-time monitoring of physical activities, physiological signals, joint movements, yoga postures, and meditation. Moving further, a lightweight, hydrophobic, and highly stretchable strain sensor using a synthesized graphene silicone-based screen printable conductive paste (GSiCP) for remote monitoring of fruit growth in real-time was fabricated. For the fabrication of GSiCP sensors, Few-layer graphene (FLG) was obtained by electrochemical exfoliation of graphite rod using a suspension of CuSO4.5H2O as electrolytes to eliminate the use of strong acids. The GSiCP printed hydrophobic strain sensor (GSiCP) shows high sensitivity, stretchability, and cyclic durability of 2050, 125%, and 5000 cycles, respectively. The continuous real-time on-field fruit (brinjal) growth was monitored remotely using a GSiCP sensor with the Internet of Things (IoTs). Further, these sensors have shown their capability to monitor various physiological signals, physical activities and orthopaedic joint movements. The objective of the thesis was extended towards the development of degradable strain sensors. The degradable strain sensors were developed by depositing A prepared composite of multilayer graphene flakes, Arabic gum, and cellulose acetate on a printing paper sheet. The developed sensors are water degradable. The developed sensors were used to monitor joint movements. The present research work addresses the new perspective for the development of wearable strain sensors for real-time human health monitoring using economically synthesized graphene flakes.