No / This paper presents a novel method to prepare stretchable conductors and pressure sensors based on the gold/polydimethylsiloxane (PDMS) system. The gold films were sputtered onto structured PDMS surfaces produced with a photolithographic surface treatment with the aim of reducing tensile strains in the gold film. Scanning electron microscopy (SEM) and atomic force microscopy analyses showed that these 3D patterns reduce cracks and delaminations in the gold film. Electrical measurements indicate that the patterns also protect the films against repeated tensile cycling, although the un-patterned samples remained conducting as well after the completion of 120 cycles. The extrapolated resistivity value of the patterned sample (4.5 × 10−5 Ωcm) compares well with previously published data. SEM micrographs indicate that the pattern features deflect the cracks and therefore toughen the gold film. However, x-ray photoelectron spectroscopy and contact angle analyses indicate that the patterning process also slightly modifies the surface chemistry. This patterning method was used to prepare capacitive strain gauges with pressure sensitivity (ΔZ/Z)/P of 0.14 kPa−1 in the sub-kPa regime. Such stretchable and potentially conformal low-pressure sensors have not been produced before and could prove advantageous for many smart fabric applications.

<p>By using flexible and stretchable materials in place of fixed components, soft robots can materially adapt or change to their environment, providing built-in safeties for robotic operation around humans or fragile, delicate objects. And yet, building a robot out of only soft and flexible materials can be a significant challenge depending on the tasks that the robot needs to perform, for example if the robot were to need to exert higher forces (even temporarily) or self-report its current state (as it deforms unexpectedly around external objects). Thus, the appeal of multifunctional materials for soft robots, wherein the materials used to build the body of the robot also provide actuation, sensing, or even simply electrical connections, all while maintaining the original vision of environmental adaptability or safe interactions. Multifunctional material systems are explored throughout the body of this dissertation in three ways: (1) Sensor integration into high strain actuators for state estimation and closed-loop control. (2) Simplified control of multifunctional material systems by enabling multiple functions through a single input stimulus (<i>i.e.</i>, only requiring one source of input power). (3) Presenting a solution for the open challenge of controlling both well established and newly developed thermally-responsive soft robotic materials through an on-body, high strain, uniform, Joule-heating energy source. Notably, these explorations are not isolated from each other as, for example, work towards creating a new material for thermal control also facilitated embedded sensory feedback. The work presented in this dissertation paves a way forward for multifunctional material integration, towards the end-goal of full-functioning soft robots, as well as (more broadly) design methodologies for other safety-forward or adaptability-forward technologies.</p>

Inkjet printing has attracted much attention in recent years as a versatile manufacturing tool, suitable for printing functional materials. This facile, low-cost printing technique with high throughput and accuracy is considered promising for a wide range of applications including but not limited to optical and electronic devices, sensors, solar cells, biochips, and displays. The performance of such functional devices is significantly influenced by the deposit morphology and printing resolution. Therefore, fabrication functional devices with precise footprints by inkjet printing requires deep understanding of ink properties, material interactions, and material self-assembly. In conventional inkjet printing process, where sessile droplets are directly printed on substrates, particle depositions are usually associated with the well-known, undesirable coffee-ring effect due to the high solvent evaporation rate at the edges of the printed droplets. Such particle accumulation phenomenon in vicinity of the three-phase contact lines of sessile droplets is considered detrimental to inkjet printing applications. This study investigates the material interactions and self-assembly of colloidal inks in inkjet printing applications at different length scales. The potential of inkjet printing has been exploited through employing the dual-droplet inkjet printing of colloidal particles to investigate the self-assembly of colloidal nanoparticles at the air-liquid interface and at the three-phase contact line of sessile droplets, which provide better understanding of the particle deposition morphologies after solvent evaporation. Different from conventional inkjet printing, the dual-droplet printing involves jetting wetting droplets, containing colloidal nanoparticles dispersed in solvents with high vapor pressure, over supporting droplets composed of water only. By tuning the surface tensions and controlling the jetting parameters of the jetted droplets, monolayers with closely-packed deposition of colloidal nanoparticles are demonstrated. Various solutions are proposed to totally suppress or mitigate the coffee-ring effect in inkjet printing applications through tuning the pH value of the supporting droplets in the dual-droplet inkjet printing to control the multibody interactions (i.e., particle-particle, particle-interface, and particle-substrate interactions) or by applying magnetic field to direct the self-assembly of colloidal particles in conventional inkjet printing. In addition, the influence of various forces such as drag force, van der Waals force, electrotactic force, and capillary force on the particle deposition and assembly in vicinity of the three-phase contact line area were investigated for both the conventional and dual-droplet inkjet printing techniques. Finally, fabrication of functional devices such as stretchable conductors have also been demonstrated by inkjet printing of silver nanowires into elastomer substrate, where the viscous liquid elastomer layer shaped the printed silver wire lines into tens of micrometers in dimeter. The silver nanowires align along the printing direction during solvent evaporation, resulting in wires with good mechanical stability and electrical performance. The printing techniques and the outcomes presented in this study can be harnessed in engineering and manufacturing a wide range of technological applications ranging from high-performance optical and electronic devices to stretchable conductors and sensors.