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Thermally Drawn Elastomer Nanocomposites for Soft Mechanical Sensors

Chaoqun Dong, Stella Andréa Françoise Laperrousaz, Andreas Leber, Yunpeng Qu, Inès Richard, Fabien Sorin

Stretchable and conductive nanocomposites are emerging as important constituents of soft mechanical sensors for health monitoring, human-machine interactions, and soft robotics. However, tuning the materials' properties and sensor structures to the targeted mode and range of mechanical stimulation is limited by current fabrication approaches, particularly in scalable polymer melt techniques. Here, thermoplastic elastomer-based nanocomposites are engineered and novel rheological requirements are proposed for their compatibility with fiber processing technologies, yielding meters-long, soft, and highly versatile stretchable fiber devices. Based on microstructural changes in the nanofiller arrangement, the resistivity of the nanocomposite is tailored in its final device architecture across an entire order of magnitude as well as its sensitivity to strain via tuning thermal drawing processing parameters alone. Moreover, the prescribed electrical properties are coupled with suitable device designs and several fiber-based sensors are proposed aimed at specific types of deformations: i) a robotic fiber with an integrated bending mechanism where changes as small as 5 degrees are monitored by piezoresistive nanocomposite elements, ii) a pressure-sensing fiber based on a geometrically controlled resistive signal that responds with a sub-newton resolution to changes in pressing forces, and iii) a strain-sensing fiber that tracks changes in capacitance up to 100% elongation.

WILEY2023

Thermally Drawn CNT-Based Hybrid Nanocomposite Fiber for Electrochemical Sensing

Inès Richard, Fabien Sorin

Nowadays, bioelectronic devices are evolving from rigid to flexible materials and substrates, among which thermally-drawn-fiber-based bioelectronics represent promising technologies thanks to their inherent flexibility and seamless integration of multi-functionalities. However, electrochemical sensing within fibers remains a poorly explored area, as it imposes new demands for material properties-both the electrochemical sensitivity and the thermomechanical compatibility with the fiber drawing process. Here, we designed and fabricated microelectrode fibers made of carbon nanotube (CNT)-based hybrid nanocomposites and further evaluated their detailed electrochemical sensing performances. Carbon-black-impregnated polyethylene (CB-CPE) was chosen as the base material, into which CNT was loaded homogeneously in a concentration range of 3.8 to 10 wt%. First, electrical impedance characterization of CNT nanocomposites showed a remarkable decrease of the resistance with the increase in CNT loading ratio, suggesting that CNTs notably increased the effective electrical current pathways inside the composites. In addition, the proof-of-principle performance of fiber-based microelectrodes was characterized for the detection of ferrocenemethanol (FcMeOH) and dopamine (DA), exhibiting an ultra-high sensitivity. Additionally, we further examined the long-term stability of such composite-based electrode in exposure to the aqueous environment, mimicking the in vivo or in vitro settings. Later, we functionalized the surface of the microelectrode fiber with ion-sensitive membranes (ISM) for the selective sensing of Na+ ions. The miniature fiber-based electrochemical sensor developed here holds great potential for standalone point-of-care sensing applications. In the future, taking full advantage of the thermal drawing process, the electrical, optical, chemical, and electrochemical modalities can be all integrated together within a thin strand of fiber. This single fiber can be useful for fundamental multi-mechanistic studies for biological applications and the weaved fibers can be further applied for daily health monitoring as functional textiles.

MDPI2022

Ultimate Feature Sizes in Thermally Drawn Fibers: from Fundamental Analysis to Novel Functional Fibers

Inès Richard

The fiber thermal drawing process has emerged as a simple and scalable technique for the fabrication of multifunctional and flexible electronics. The integration of materials with various physical and functional properties in well-defined architectures opened a wide range of applications. However, the trend of increasing number of functionalities, which requires a reduction in size of each fiber constituent, revealed the importance of instability mechanisms that need to be better understood and overcome. In this thesis, we address several challenges associated with the process-microstructure relationship that govern the achievable feature sizes. We first tackle the problem of thermal reflow of polymeric textures induced by the Laplace pressure. We develop a reflow model that can be applied to periodic textures during isothermal annealing and extend it to the thermal drawing process. After validating our model with experimental data, we show that the reflow driving force can be significantly reduced by codrawing two materials of low interfacial tension. We demonstrate this finding by drawing sub-100 nm textures on polycarbonate fibers. The second part focuses on the influence of the drawing parameters on the degree of polymer chain alignment which induces serious shrinkage upon heating. We reveal that the drawing stress, which depends on the drawing speed and temperature, controls the shrinkage stress. Furthermore, we show that the degree of chain orientation increases linearly with the stress at low thermal drawing stress, and then saturates, which correlates well with the amount of shrinkage observed. We then highlight the use of this process-dependent alignment to tune the mechanical properties of the fibers and the bending behavior of multi-material fibers. Finally, a heat treatment is proposed for reducing the chain alignment to increase the dimensional stability of fiber devices such as temperature sensors. In the third part, we switch our attention to inorganic materials and discuss the long-lasting challenge of the thermal drawing of bulk metallic glasses (BMGs) which enables us to circumvent the size limitation of crystalline metal electrodes due to capillary instabilities. We first demonstrate the production of well-ordered Pt57.5Cu14.7Ni5.3P22.5 (Pt-MG) ribbons within a polyetherimide matrix with uniform features down to 40 nm. We reveal via transmission electron microscopy analyses a crystallization-induced break up. Furthermore, our approach enables us to study the influence the process on the crystallization kinetics of the Pt-MG ribbons. We show that the latter is enhanced by the increase in deformation and decrease in size both at the micro- and nanoscale. We then demonstrate the ability of thermal drawing of another BMG, Au49Ag5.5Pd2.3Cu26.9Si16.3, by selecting a poly(methylmetacrylate) cladding with matching thermo-mechanical and rheological properties. The ultimate feature size, caused by the onset of crystallization, is a few hundred nanometers. Finally, we show two examples of devices highlighting the novel functionalities enabled by the thermal drawing of BMGs. We produce and test in vivo, in collaboration with colleagues in the bio-engineering department, neural probes allowing electrical stimulation, recording and precise drug delivery, and electrochemical sensors integrating the three-electrode system in a single fiber.

EPFL2021