abstract: Gallium-based liquid metals are of interest for a variety of applications including flexible electronics, soft robotics, and biomedical devices. Still, nano- to microscale device fabrication with these materials is challenging because of their strong adhesion to a majority of substrates. This unusual high adhesion is attributed to the formation of a thin oxide shell; however, its role in the adhesion process has not yet been established. In the first part of the thesis, we described a multiscale study aiming at understanding the fundamental mechanisms governing wetting and adhesion of gallium-based liquid metals. In particular, macroscale dynamic contact angle measurements were coupled with Scanning Electron Microscope (SEM) imaging to relate macroscopic drop adhesion to morphology of the liquid metal-surface interface. In addition, room temperature liquid-metal microfluidic devices are also attractive systems for hyperelastic strain sensing. Currently two types of liquid metal-based strain sensors exist for inplane measurements: single-microchannel resistive and two-microchannel capacitive devices. However, with a winding serpentine channel geometry, these sensors typically have a footprint of about a square centimeter, limiting the number of sensors that can be embedded into. In the second part of the thesis, firstly, simulations and an experimental setup consisting of two GaInSn filled tubes submerged within a dielectric liquid bath are used to quantify the effects of the cylindrical electrode geometry including diameter, spacing, and meniscus shape as well as dielectric constant of the insulating liquid and the presence of tubing on the overall system's capacitance. Furthermore, a procedure for fabricating the two-liquid capacitor within a single straight polydiemethylsiloxane channel is developed. Lastly, capacitance and response of this compact device to strain and operational issues arising from complex hydrodynamics near liquid-liquid and liquid-elastomer interfaces are described. / Dissertation/Thesis / Masters Thesis Materials Science and Engineering 2015

Stretchable Electronics is an emerging class of electronics that allow electronics to be bent, conform, ex and stretch while still retaining its full functionality. Other than bending, existing and conforming, adding stretchability to electronic systems can open up a new frontier for a myriad of applications. Especially in the medical sector, these stretchable devices can increase the scope of monitoring and ease and comfort of the patient. All kinds of wearable devices can be based on these technologies to augment our daily lives. With the kind of state of art technology available to the common man today, the bar has already been set for the performance of such devices. Hence, its imperative that these stretchable devices perform at this level and should be capable of adapting to the market to serve the mass requirement. Hence, it becomes inevitable to use metal interconnects to provide very low resistance and easy adhesion to commercial electronic components. Another aspect of such devices is an adhesion ability with which we can attach it to various kinds of surfaces. In this thesis, we propose a new multi-layered PDMS structure approach to bring stretchability in the device. For all kinds of adhesion requirements, various ratios of PDMS: Cross-linker have been used. These different ratios of PDMS: Cross-linker changes the mechanical and adhesive properties of the cured PDMS. Hence, the same material can be used as the stretchable substrate as well as to serve various adhesion requirements. A soft adhesion allows us to attach it to the human body/other surfaces. The adhesion can be tailored to be quite conformal and strong, yet its removal is quite gentle to the skin. A higher curing ratio makes the PDMS very sticky and soft. Aluminum/Copper foils can be directly stuck upon it and tracks can be then etched out to get a printed circuit. Since this adhesive layer is quite soft, it acts as a cushion and reduces the amount of stress transferred to the metal interconnects. Hence, stretchable circuits with metal interconnects can be realized. The electronic components can be then attached upon it via normal soldering techniques/using conductive ink. Various devices that can be built with the proposed techniques have been coined the term CAS (Conformal Active Sheets) to allow easy reference to such kind of devices. Since the substrate is soft, physical handling of such devices becomes an issue as one tries to transfer the circuit pattern. Hence, direct etching of the metal foil was explored via high pulsed current discharge technique. A CNC machine was also designed to try various ways of direct etching of the metal foil in an accurate and repeatable fashion.

abstract: Deformable heat exchangers could provide a multitude of previously untapped advantages ranging from adaptable performance via macroscale, dynamic shape change (akin to dilation/constriction seen in blood vessels) to enhanced heat transfer at thermal interfaces through microscale, surface deformations. So far, making deformable, ‘soft heat exchangers’ (SHXs) has been limited by the low thermal conductivity of materials with suitable mechanical properties. The recent introduction of liquid-metal embedded elastomers by Bartlett et al1 has addressed this need. Specifically, by remaining soft and stretchable despite the addition of filler, these thermally conductive composites provide an ideal material for the new class of “soft thermal systems”, which is introduced in this work. Understanding such thermal systems will be a key element in enabling technology that require high levels of stretchability, such as thermoregulatory garments, soft electronics, wearable electronics, and high-powered robotics. Shape change inherent to SHX operation has the potential to violate many conventional assumptions used in HX design and thus requires the development of new theoretical approaches to predict performance. To create a basis for understanding these devices, this work highlights two sequential studies. First, the effects of transitioning to a surface deformable, SHX under steady state static conditions in the setting of a liquid cooling device for thermoregulation, electronics and robotics applications was explored. In this study, a thermomechanical model was built and validated to predict the thermal performance and a system wide analysis to optimize such devices was carried out. Second, from a more fundamental perspective, the effects of SHXs undergoing transient shape deformation during operation was explored. A phase shift phenomenon in cooling performance dependent on stretch rate, stretch extent and thermal diffusivity was discovered and explained. With the use of a time scale analysis, the extent of quasi-static assumption viability in modeling such systems was quantified and multiple shape modulation regime limits were defined. Finally, nuance considerations and future work of using liquid metal-silicone composites in SHXs were discussed. / Dissertation/Thesis / Doctoral Dissertation Engineering 2020

Next-generation soft robots, wearable health monitoring devices, and human-machine interfaces require electronic systems that can maintain their performance under deformations. Thus, researchers have been developing materials and methods to enable high-performance soft electronic systems in diverse applications. While a variety of solutions have been presented, development of stretchable materials with a combination of high stretchability, electrical conductivity, cyclic stability, and manufacturability is still an open challenge. Throughout this dissertation, gallium-based<br>liquid metal alloy is used as the conductive material, leveraging its high conductivity and intrinsic stretchability for maintained performance under deformations. This dissertation presents both new liquid metal-based conductive materials and scalable manufacturing methods for the development of a diverse range of flexible and stretchable electronic circuits. First, a laser sintering method was developed to coalesce liquid metal micro/nanoparticles into soft, conductive structures enabled by oxide rupturing. The fast, non-contact, and maskless laser sintering technique, in combination with large-area spray-printing deposition, and high-throughput emulsion processing, provided a methodology to create different physical manifestations of liquid metal-based soft, stretchable, and reconfigurable electronics. Second, a liquid metal-based biphasic material was created using a thermal processing technique, yielding a printable, mechanically stable, and extremely stretchable conductor. This material’s compatibility with existing scalable manufacturing methods, robust interfaces with off-the-shelf electronic components, and electrical/mechanical cyclic stability enabled direct conversion of established circuit board assemblies to stretchable forms. The work presented in this dissertation paves the way for future mass-manufacturing of<br>soft, stretchable circuits for direct integration into smart garments or soft robots. <br>

The thesis examines the adhesive interaction between graphite layers and atomically thin MoS2 crystals. Vertical van der Waals(vdW) heterostructures are fabricated by stacking different two-dimensional (2D) materials on top of each other. Blister test is used to measure the adhesive interactions between 2D heterostructures and their transferred substrates and between the layers themselves. This adhesive interaction is important in maintaining the mechanical integrity of the device during mechanical loadings and its understanding will help pave the way to the design and fabrication of micromechanical device from 2D heterostructures. Furthermore, applying controlled strains can be used to alter the electrical and optical properties thereby improving efficiency and performance. At first, we grew MoS2 and graphene by CVD and stacked the layers on top of each other using a dry transfer method. The MoS2/graphene heterostructure was then transferred onto pre-etched cavities on a silicon wafer. The blister test was used for controllably introducing strain into the heterostructure. Atomic Force Microscopy was used for measuring the shape of the deformed blister and Raman and Photoluminescence(PL) measured the optical response. The strain mismatch between the biaxial strain and a PL-converted strain suggests crumpling of the graphene layer and a substantial softening of the mechanical response. Lastly, we created graphite holes with photolithography to measure the work of separation between an atomically smooth graphite surface and MoS2. We found this value to be at least 320mJ/m2 which is higher than the MoS2/SiOx areas that was previously studied. / 2023-07-02T00:00:00Z

Significant developments in flexible/stretchable electronics are needed due to the increasing demand for stretchable sensors in soft robotics, prostheses, and human-machine interfaces. Stretchable strain sensors must be extremely sensitive to the applied strain in order to be used in monitoring human movement, tracking pulses, and identifying sounds. Percolated networks based on nanomaterials with intrinsic stretchability are primarily used to create large stretchable strain sensors with high sensitivity and stretchability. However, sensitivity and stretchability are two opposite faces of a coin, and these sensors face limited sensitivity both in tension and compression.The aforementioned drawbacks limit application such as large-scale deformable surface monitoring and effective e-skins for monitoring complex strain states. Pollution from strain, on the other hand, is a problem that must be avoided for other types of stretchable sensors. Strain-insensitive sensors are mostly based on the geometrical design with a complicated fabrication. New methods for developing strain-insensitive sensors based on percolated networks are urgently needed to simplify the fabrication process. Four objectives are listed to solve the problems as mentioned above: to develop a method to balance the stretchability and sensitivity; to design a stretchable strain sensor with whole range working ability; to create a strain insensitivity sensor different from the geometry design; to investigate the physical mechanism of the new method. In Chapter 2, a laser engraving method was used to increase the crack density in CNT paper, which successfully improved the stretchability while maintaining the high sensitivity. Then, in Chapter 3, a pre-stretching/releasing method was used to partially open the cracks in CNT paper in order to achieve sensitivity in both positive and negative strain. The Seebeck effect of percolated networks was then used to develop a strain-insensitive temperature sensor in Chapter 4. Finally, in Chapter 5, we performed a theoretical analysis to reveal the physical mechanism of the Seebeck coefficient’s stability in percolated networks.

A stretchable silver ink was screen printed to TPU sheets, then tensile coupons of the TPU, both bare and laminated to cotton, Denim and spandex fabric, were subjected to 1000 cycles of 20% uniaxial strain. In-situ resistance measurements of printed traces were processed to generate datasets of maximum and minimum resistance per cycle. A mechanistic fit model was used to predict the resistance behavior of the ink across TPU/fabric levels. The results show that traces strained on TPU laminated to spandex (polyester) fibers had an average rate of increase in resistance significantly lower than that of traces strained on bare TPU. The variation in predicted resistance was significantly lower in the spandex group than in the TPU group. Trace width was not found to have a significant effect on the resistance behavior across TPU/fabric groups. More testing is required to understand the effect of lamination to high elasticity fabrics on resistance behavior as it relates to the viscoelastic properties of the fibers and weave structure.

No description available.

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.

Profound sensorineural hearing loss is at the present time a major worldwide health concern, affecting over 5% of the worlds' population. Through cochlear implants (CI), treatment of sensorineural hearing loss now offers the possibility to restore hearing function through electrical stimulation of auditory nerves. Treatment is based on the surgical implantation of a thin, flexible array of microelectrodes into the cochlea. Nevertheless, availability of the treatment is limited due to high costs, and surgical insertion is associated with a high risk of trauma to the fragile soft tissue of the cochlea. At the heart of this thesis lies the proposition that these two problems may be addressed by the development of a novel type of cochlear implant founded on batch-producible, stretchable printed circuit board (PCB) technology. As an alternative to conventional cochlear implant fabrication, this thesis presents a fabrication process based on batch-producible stretchable PCB, featuring liquid alloy microchannels in place of solid metallic wire conductors. A series of proof-of-concept prototypes were designed, fabricated and evaluated. According to results obtained from evaluation of the prototypes, certain steps in the fabrication process were later revisited and improved upon. Preliminary prototype fabrication yielded batches of thin flexible cone-shaped electrode arrays designed for in-vivo evaluation in guinea-pig cochleae. In-vitro evaluation in 3D-printed cochlea models revealed that the prototypes were sufficiently thin and compliant for insertion 23 mm deep into a human cochlea and 4-6 mm into a guinea-pig cochlea, comparable to commercially available counterparts. Characterization of prototype test devices by optical microscopy, optical interferometry and resistance measurements revealed a high inherent variability in the developed fabrication process. In order to ensure consistently adequate quality, further improvement must be done. In particular, results of this work suggest that the deposition of liquid alloy involved in stretchable PCB fabrication should be automated to minimize uncertainty in the deposited liquid alloy thickness and thus enable further miniaturization of the stretchable PCB. Future efforts to successfully produce and integrate electrodes from soft materials, e.g. conductive polymer, liquid alloy or conductive hydrogels are highly recommended to further reduce implant stiffness.