New Designs for Wearable Technologies: Stretchable e-Textiles and e-Skin

This dissertation comprises research efforts in addressing the challenges of integration of different materials with mechanical mismatches in stretchable e-textiles and e-skin, with a major focus on the design and fabrication of stretchable e-textiles. Chapter 2 describes the solution-based metallization of a knitted textile that conformally coats individual fibers with gold, leaving the void structure intact. The resulting gold-coated textile is highly conductive, with a sheet resistance of 1.07 ohm/sq in the course direction. The resistance decreases by 80% when the fabric is stretched to 15% strain and remains at this value to 160% strain. This outstanding combination of stretchability and conductivity is accompanied by durability to wearing, sweating, and washing. Low-cost screen printing of a wax resist is demonstrated to produce patterned gold textiles suitable for electrically connecting discrete devices in clothing. The fabrication of electroluminescent fabric by depositing layers of device materials onto the gold-coated textile is furthermore demonstrated, intimately merging device functionality with textiles for imperceptible wearable devices. Chapter 3 presents a new textile-centric design paradigm in which we use the textile structure as an integral part of wearable device design. Coating the open framework structure of an ultrasheer knitted textile with a conformal gold film using solution-based metallization forms gold-coated ultrasheer electrodes that are highly conductive (3.6 ± 0.9 ohm/sq) and retain conductivity to 200% strain with R/R0 \u3c 2. The ultrasheer electrodes produce wearable, highly stretchable light-emitting e-textiles that function to 200% strain. Stencil printing a wax resist provides patterned electrodes for patterned light emission; furthermore, incorporating soft-contact lamination produces light-emitting textiles that exhibit, for the first time, readily changeable patterns of illumination. Chapter 4 demonstrates the strategic use of a warp-knitted velour fabric in an “island-bridge” architectural strain-engineering design to prepare stretchable textile-based lithium ion battery (LIB) electrodes. The velour fabric consists of a warp-knitted framework and a cut pile. We integrate the LIB electrode into this fabric by solution-based metallization to create the warp-knitted framework current collector “bridges”, followed by selectively deposition of the brittle electroactive material CuS on the cut pile “islands”. As the textile electrode is stretched, the warp-knitted framework current collector elongates, while the electroactive cut pile fibers simply ride along at their anchor points on the framework, protecting the brittle CuS coating from strain and subsequent damage. The textile-based stretchable LIB electrode exhibited excellent electrical and electrochemical performance with a current collector sheet resistance of 0.85 ± 0.06 ohm/sq and a specific capacity of 400 mAh/g at 0.5 C for 300 charging-discharging cycles, as well as outstanding rate capability. The electrical performance and charge-discharge cycling stability of the electrode persisted even after 1000 repetitive stretching-releasing cycles, demonstrating the protective functionality of the textile-based island-bridge architectural strain-engineering design. Chapter 5 demonstrates the engineering of metal cracking patterns using the topography from acid-oxidized PDMS. Oxidizing the surface of PDMS with aqueous acid mixture created hierarchical topographies. Coating the surface of acid-oxidized PDMS with copper using electroless deposition produced stretchable conductors with a sheet resistance of ~1.2 ohm/sq. The cracking patterns of copper films with strain were tuned by simply adjusting the composition of acid mixture to change the topography of PDMS, which affects the resistance change of copper films with strain. The Cu films with an optimal cracking pattern on acid-treated PDMS remain conductive to 85% strain with R/R0 less than 20