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Title: Packaging of fabric sensing network with flexible and stretchable electronic components
Authors: Li, Qiao
Degree: Ph.D.
Issue Date: 2014
Abstract: To enable electronics intimately wearable on curvilinear human bodies with various gesture and motion, it requires electronic devices to be breathable, foldable and elastic, as well as reliable and durable. To date, technologies based on in-plane or out-of-plane buckled thin films on compliant silicones or plastics, net-shaped integrated circuits, as well as organic stretchable conductors have all contributed to the development of flexible and stretchable electronics. Yet it is uncomfortable when elastic rubbers make an intimate contact with human skins for a long time. Also, the reliability and long-term durability of the stretchable electronics needs to be investigated. Without relying on elastic rubbers, this thesis conducts a systematical investigation into the packaging of a highly sensitive network integrated on a permeable, elastic, thin, and lightweight knitted substrate. The fabric sensing network was achieved by physically linking distributed fabric sensors with stretchable knitted interconnects in a manner through helical connections. First, flexible sensors with identical electro-mechanical behavior were packaged from knitted conductive fabrics coated by carbon nano-particle (CNP)/ silicone elastomer (SE)/ dimethyl silicone oil (SO) mixture pastes. Next, stretchable knitted interconnect routes were realized by an intarsia pattern, electrically and mechanically connecting the distributed sensors in a predetermined configuration. Then, helical connections were established from the sensor electrodes to the knitted interconnects, without degrading the stretchability of the entire fabric sensitive network. Finally, a prototype was developed, consisting of an optimized electrical adapter, for an in-situ ballistic impact measurement, where reliability of the fabric sensing network was investigated. With respect to the flexible sensor packaging technology, sensor electrodes were integrated into a sensing element by sewing twisted conductive yarns as the bottom threads in a double-needle sewing machine, where a geometrical match between the electrode and the sensing element was established and mechanically fastened. Also, to protect the packaged sensor from external (chemical) attack and mechanical stress in largely repeated deformations, a thin layer of silicone elastomer was encapsulated on the sensitive area where stretchability was necessary and a non-elastic woven fabric was bonded around the connection regions where mechanical stability was required. The packaged sensor was flexible, bendable and can be repeatedly stretched to 60% strain in its wale direction with more than 100000 cycles. The electrical contact resistance between the sensor electrode and the sensing element was stable with applied pressure and tensile strain (60%). The reliability was also discussed from the factors such as environmental, mechanical and fabrication conditions.
Like island-bridge combination, identical packaged sensors were connected by knitted stretchable interconnects, which were created by integrating fine conductive wires into a knitted structure with three-dimensional loop configurations. The electro-mechanical behavior was investigated and optimized by relevant parameters, such as the diameters of the conductive wire, the amount of elastic filaments, and Young{174}s modulus of the knitted substrate, in both experimental and theoretical analysis. The optimized knitted interconnect was capable of maintaining its electrical integrity until being stretched beyond 300% strain in a tensile test and over an average elongation of 200% in a three-dimensional punching measurement. Also, the average fatigue life of the optimized knitted interconnect was 1787 cycles at 160% membrane strain in the three-dimensional punching test. In addition, the knitted interconnect was porous and washable. Therefore, the combination of considerable flexibility, stretchability with electrical integrity, and bio-compatible micro-structure with human skins makes the fabric sensitive network intimately wearable on soft, curvilinear and movable human bodies. Instead of stiffening the connection between the sensor electrode and the knitted interconnect, a helical structure with a compliant encapsulation enables the connection electrically integral while being stretched over 300% strain or three-dimensionally punched with 200% average membrane strain. The geometrical change of the helical connection was optically observed during the stretching process, demonstrating the total length of the helical connection did not change due to the adjustments of its geometrical parameters, such as in-plane radius, radius angle, as well as pitch between two adjacent loops. The average fatigue life of the helical connection was investigated and compared with those of the knitted stretchable interconnect and the conventional soldering method, suggesting the helical connection was promising for stretchable electronics in terms of maximum stretchability and repeatability. Additionally, the helical connection was washable. The packaged fabric sensing network, in which distributed sensors were connected by knitted interconnect routes through helical connections, is highly sensitive, flexible, stretchable, drape-able in a two-curvilinear fashion as well as durable so that it can be used in the applications where large deformation occurs within a fraction of mini-second. To further optimize the packaging technology and to investigate the reliability of the fabric sensing network in such applications, three versions of prototypes were designed and fabricated, then tested in in-situ ballistic impact measurements. The results demonstrated (1) the possibility of the packaged sensor in a high-speed impact measurement and (2) the reliability of the stretchable knitted interconnects and helical connections in such high-speed impact applications.
Subjects: Textile fabrics -- Technological innovations.
Smart materials.
Hong Kong Polytechnic University -- Dissertations
Pages: viii, 275 pages : color illustrations ; 30 cm
Appears in Collections:Thesis

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