Graphene conductive inks for an effective textile based respiratory sensor system

Abstract

This thesis provides an examination of the development and optimization of graphene-based conductive inks for textile-integrated respiratory sensors, addressing the critical and rising demand for seamless, non-invasive health monitoring solutions embedded within everyday garments. With the proliferation of wearable technology, especially following the COVID-19 pandemic, there is a heightened focus on integrating health monitoring systems into daily life without disrupting user comfort or activities. Positioned at the crossroads of material science, electronics, and healthcare innovation, this research has been performed to advance the wearable health monitoring field. Being part of the Scientific and Technological Research Council of Türkiye (TUBİTAK) project, which consists of several nano-inks, the primary aim of this research is to fabricate a reliable respiratory sensor system embedded within textiles, harnessing the superior electrical and mechanical properties of graphene. Graphene, a material renowned for its outstanding conductivity, flexibility, and robustness, was chosen due to its unique ability to sustain consistent electrical performance under the repetitive stresses typical of wearable applications. These qualities make graphene particularly well-suited for applications requiring durable, comfortable, and inconspicuous health monitoring solutions, essential for prolonged, daily wear. Moreover, the focus on materials inherently compatible with textile substrates facilitates seamless integration into everyday clothing, enhancing the practicality and accessibility of these health-monitoring devices. The motivation behind this study is rooted in the growing need for unobtrusive, real-time health monitoring systems that eliminate the requirement for bulky, traditional equipment or uncomfortable skin contact sensors. Such devices are often cumbersome, especially in everyday scenarios, limiting their appeal and accessibility. By embedding sensors directly into textile substrates, this research aims to create a health monitoring solution that is as unobtrusive as possible. Such a development could pave the way for greater user compliance, as these wearable sensors are easily incorporated into garments and thus require minimal behavioral adjustments from the user. The research methodology outlined in this thesis begins with the analysis of graphene-based conductive inks and extends through to their application on textile substrates, followed by a rigorous evaluation of sensor performance. The graphene inks used in this research were sourced from Versarien® and were manufactured through a microfluidization process, which was critical to achieving inks with consistent and controllable rheological properties. The selection and optimization of ink properties, such as viscosity and shear-thinning behavior, were central to this research, as these characteristics directly influence the ink's compatibility with textile printing practical techniques like screen printing. The rheological analysis allows for fine-tuning of the ink's properties, ensuring that the inks can be printed onto textiles efficiently without compromising the mechanical integrity of the fabric. In the fabrication phase, graphene-based inks were applied to polyamide and cotton textile substrates, using screen printing. To optimize the screen-printing process, various mesh openings were tested for identifying the ideal conditions for achieving uniform and durable conductive patterns on the textile surfaces. A variety of printed patterns, including line, serpentine, strain gauge, and omnidirectional designs, were explored to assess the mechanical and electrical performance of the sensors under different conditions. Mechanical testing, in particular, was the cornerstone of this analysis, with samples subjected to extensive stretching and flexing to evaluate their durability and performance stability. This testing was essential in confirming that the sensors could withstand the demands of real-world use while maintaining conductivity and sensitivity. Electromechanical testing further underscored the effectiveness of these graphene-based inks in creating resilient, sensitive respiratory sensors for textiles. The sensors exhibited mechanical stability, consistently maintaining both sensitivity and conductivity even after repeated mechanical stresses. Such resilience is vital for wearable applications, where sensors must endure the rigors of daily life without failing or requiring frequent recalibration. The research highlighted the critical role of rheological properties in determining the quality and functionality of the printed sensors, emphasizing the need for precise control over ink viscosity and deposition methods to achieve optimal performance. One of the most promising aspects of the developed sensors is their sensitivity to strain, which enables them to accurately monitor respiratory rates and detect irregular breathing patterns. This functionality is particularly valuable in medical applications, as it could provide early warnings of respiratory distress or other health issues that require immediate attention. The sensors generally demonstrated a linear response to changes in strain, allowing for reliable tracking of respiratory activity over time. However, the thesis also identifies several technical challenges that must be addressed in future work. For instance, achieving precise alignment of the sensor patterns in relation to the textile fibers proved challenging, affecting the sensors' responsiveness to directional forces. Additionally, converting the fabricated sensors into testing specimens presented certain difficulties, especially when testing sensors with layered configurations. The testing procedure involved both 4x5 and 5x5 layered sensors, each subjected to controlled strains of 5% and 10%. To investigate the effects of pre-strain and increase the robustness of the data, the testing protocol was adjusted in such a way that specimens initially intended for 5% strain measurements were subsequently tested at 10% strain and vice versa. The results showed that pre-straining the sensors generally had a beneficial impact on their performance, as the initial strain appeared to "settle" the sensor structure, yielding smoother and more consistent measurement patterns in subsequent tests. Furthermore, it has been understood that geometry and measurement area are intertwined with a trade-off between sensitivity and stability. For example, for applications that require high sensitivity, line geometry seems to be a suitable candidate. However, their consistency over time can be in question. In contrast, if a more stable and long-term measurement is required, strain gauge sensors might be a better solution. Serpentine and omnidirectional patterns proved both balanced sensitivity and stability, and they are more likely to be used in various areas of the body for respiration sensing.