Soft functional fibers for mechanical sensing and actuation

The measurement and generation of mechanical deformations is a key functionality in health monitoring, human-machine interaction, and soft robotics. However, current methods typically rely on small and hard transducers, which result in poor performance, cumbersome implementation, and incompatibility with the human body. An alternative approach consists of imparting functionality within long and soft fibers. However, the multi-material assemblies, which are required for advanced functionalities, are challenging to realize in fibers. Additionally, the 1-dimensional geometry of fibers, which stands in stark contrast to traditional 0-dimensional devices, requires novel schemes to extract meaningful information and induce targeted movements.In this Thesis, four distinct innovations to fiber-based transducers are made, which are composed of soft materials and fabricated by the thermal drawing technique. The first three works are each aimed at one of the three conceptual elements of a sensor: the structural design, the constituting materials, and the signal processing. In the fourth work, actuation is coupled with sensing to develop robotic fibers.(i) Considering the design element of a sensor first, fibers with an asymmetric elastomer architecture are developed, featuring several electrodes separated by an air gap. Compressive loads on the meter-long fibers result in the selective contacting of electrodes within the deformed fiber structure, triggering pronounced electrical signals at distinct pressure levels. This developed sensing concept enables the facile functionalization of large surfaces, demonstrated by a fiber-augmented gymnastic mat for the monitoring of human body position, posture, and motion. (ii) Focusing on the materials next, a novel polymer nanocomposite with carefully engineered rheological, mechanical, and electrical properties is introduced. In particular, controlled changes to the nanoparticle network and the intertwined electrical conduction of the material are explored - induced either by viscous flow during thermal drawing fiber fabrication or elastic deformation in applications. The nanocomposite represents a platform upon which a family of fiber devices can be developed. (iii) As the third pillar of sensing, signal processing is addressed by employing electrical reflectometry within microstructured elastomeric fibers that integrate tens of liquid metal conductors. Reflected electrical waves, triggered by multimodal deformations of the fiber structure, are investigated experimentally and theoretically. This unprecedented physical mechanism enables multiplexed measurements of the mode, magnitude, and position of multiple simultaneous pressing and stretching events. By integrating a single fiber with a single interface port into a larger fabric, the technique can be used to create an electronic textile that can decipher convoluted mechanical stimulation. (iv) Finally, both actuation and sensing schemes are combined, resulting in robotic fibers with closed-loop control. Thanks to three integrated tendons pulling on the distal fiber end, bending with two degrees of freedom is achieved. Envisioned as steerable catheters, the fibers can autonomously avoid obstacles to minimize tissue damage, scan the environment to execute the best navigation path, and accurately deliver mechanical tools, fluids, as well as optical and electrical stimulation.

Thermal Drawing of Polymer Nano-composites: Fluid Dynamic Analysis and Application to Novel Functional Fibers

Alexis Gérald Page

The thermal drawing process is emerging as a versatile platform for the development of multimaterial fiber devices and advanced textiles for sensing, energy harvesting or medical applications, owing to the possibility to integrate insulators, conductors and various types of functional materials together in complex architectures. This field of research remains relatively recent and still has unexploited potential and unanswered questions. In particular, the lack of transparent conducting material compatible with thermal drawing has so far limited the development of optoelectronic fiber devices. It has indeed hindered the ability to fabricate the types of optoelectronic devices that require a transparent electrode, most notably photovoltaic cells. Furthermore, conductive polymer composites based on carbon black have been utilized for years as an alternative to crystalline metals that can maintain finer geometries, crystalline metals being limited by capillary instabilities due to their low viscosity in the melted state during thermal drawing. However, the observation that the conductivity of composites depends on drawing conditions has thus far not been explained through an understanding of the underlying mechanism. In this thesis, we investigate the production and thermal drawing of thermoplastic nanocomposites based on carbon black, but also on high aspect ratio conductive fillers such as silver nanowires and carbon nanotubes. The latter are indeed suitable to fabricate transparent conducting films, and we show that we can find appropriate compositions and drawing conditions to integrate them in multimaterial fibers. After looking into the thermal drawing of such composites in more details, we propose an advanced fluid dynamic analysis that accurately models the thermal drawing process, with new insights on the radial dependency of the axial velocity. We design a way to visualize the deformation in the neckdown region and show that our model compares well with the experimental observation. We use this model to further develop the theoretical approach to account for the influence of thermal drawing on the conductivity of nanocomposites. The impact of both the draw ratio and the radial position observed experimentally is explained quantitatively by the results of the model. Our approach can potentially be applied to other types of materials whose properties change under the deformation due to thermal drawing. From these findings, we demonstrate a variety of novel composite-based fiber devices. We fabricated a functional photodetecting fiber using a transparent electrode made of a carbon nanotube composite, paving the way towards the development of novel optoelectronic fiber devices that integrate transparent electrodes. Moreover, we present a new type of touch sensing fiber device that relies on a freely moving conductive domain made of a carbon black nanocomposite. Such type of electronic fiber device is capable of both detecting a pressure applied and localizing it along its entire length. We can envision applications in soft electronics or healthcare with individual fiber devices or by embedding them in smart textiles.

EPFL2019

Design and Fabrication of Stretchable Photonic Fibers

Nicola Bartolomei

Optical fibers have reshaped the technological landscape, from optical networks and high-speed data communication to in situ imaging and non-invasive surgery methods. The revolution allowed by these fibers has been made possible by its fabrication method, the thermal drawing process, which combines high scalability and nanoscale fabrication accuracy. Despite tremendous successes, silica step-index and photonic crystal fibers have held intrinsic limitations due to their mechanical rigidity and fragility, limiting their possible applications . There is however an increasing demand for optical fibers that could guide light efficiently while sustaining large deformations in fields like robotics, smart textiles and the automotive world. Other fields in sensing, helath care or advanced textiles could benefit from soft fibers that could display tunable optical effects. Techniques like lithography and molding have proven valuable to fabricate complex elastomeric photonic fibers, but their limited scalability intrinsically limits their use in several fields of application. On the other hand, scalable fabrication techniques like extrusion guarantee the desired throughput but fail to deliver the architectural complexity required to realize softfibers advanced optical properties. In this thesis, we propose to exploit the thermal drawing process to realize different types of soft and highly stretchable optical fibers, from the classic step-index architecture to 1D photonic crystal fibers, liquid core fibers and 2D photonic crystal fibers. To achieve this overall objective, we first establish a theoretical framework to evaluate the material requirements from the rheological, optical and mechanical point of view. After selecting the materials, we develop a step-by-step fabrication process that starts from the material pellets and ends with the fabrication of meters of elastomeric photonic fiber via thermal drawing. We then move on to fully characterize the optical and mechanical properties of the fibers and design several fiber-based devices with specific optical effects. More specifically in Chapter 2, we demonstrate the materials selection, innovative fabrication process, and the optical and mechanical characterizations to realize a soft optical wave-guide based on a step-index design. To highlight its optical and mechanical performance, we demonstrate a strain sensing sport gear that integrates this newly developed fiber. In chapter 3, we go one step further in the control over feature size by going about a similar demonstration for a 1D photonic crystal fibers based on all-elastomeric materials, with feature sizes down to sub-100 nm. We show in particular a mechanochromic fiber that can reversibly change color upon stretching. In Chapter 4, we propose a series of other fiber architectures based on our scientific and technological achievements that include a multicore stretchable step-index fiber that form the basis of an advanced demonstrator of a soft fiber with multiple channels and functionalities found in endoscopes. We also present preliminary results on a liquid-core step index fiber, and a stretchable 2D photonic crystal fiber, that pave the way towards highly complex and high-performance soft optical fiber devices.

EPFL2021

A new versatile hybrid MEMS technology for high sensitivity, fluid proof sensing applications.

Matthias Neuenschwander

The field of micro electromechanical systems (MEMS) evolved from the microelectronic industry and the technologies developed to fabricate integrated circuits. As a result, MEMS are commonly fabricated on silicon wafers. The development of MEMS has been driven by three main merits: miniaturisation, microelectronics integration, and parallel fabrication with high precision. Many operational properties scale well with smaller sizes. In addition, integrated electronic circuitry allows embedding MEMS with computing or networking capabilities, while parallel manufacturing enables the fabrication of many identical devices on a single wafer, reducing the unit cost. The main MEMS application is transducers which transform signals from one form of energy to another, and can be used for perception (sensors) or to produce actions (actuators). Silicon and other microelectronic materials like metals have enabled very high-performance MEMS transducers because these materials can be used for a range of very effective actuating and sensing principles.More recently, polymers have started to be used in MEMS because of their unique electrical, physical and chemical properties which include biocompatibility, viscoelasticity and mechanical shock tolerance. They are also much softer than silicon or metals, and they can be processed using many techniques that allow unique low-cost, batch-style fabrication and packaging. However, polymers have relatively low glass-transition and melting temperatures. As a result, the advantages of polymers cannot easily be combined with high-performance actuating and sensing elements as these are based on silicon technology and require high-temperature fabrication steps. Here, a hybrid-MEMS fabrication process is used to address this issue. The developed devices are based on a trilayer structure, where a thick polymer core is sandwiched between two hard thin films, while electronic layers are embedded within in a fluid-compatible way.The objective of this thesis is to turn hybrid-MEMS into a benchmark MEMS technology. This involves many different aspects. First, sensing and actuation elements are integrated into trilayer devices, and their performance is analysed. Then, some more unique possibilities offered by hybrid-MEMS are exploited. A way to fabricate electronics on multiple layers is developed, which allows different electronic features to be integrated in parallel. In addition, three-dimensional bulk features fabricated at several levels are demonstrated. This is possible because the hybrid-MEMS process is based on the bonding of multiple wafers.All these new fabrication features are demonstrated in atomic force microscopy (AFM) applications. Self-actuated trilayer AFM cantilevers with sharp silicon tips are used to boost imaging speeds in the off-resonance tapping (ORT) mode, thanks to an order of magnitude faster surface probing rates. Furthermore, fully integrated ORT imaging is demonstrated with self-actuated piezoresistive cantilevers. Next, trilayer cantilevers with shielded conductive tips are introduced for electrical AFM modes. Finally, ongoing research projects are touched upon, including approaches to fabricate hybrid-MEMS with single crystal silicon piezoresistors for improved sensitivity, and an analysis of reproducibility of fabricated trilayer cantilevers.

EPFL2022