3D Stretchable Arch Ribbon Array Fabricated via Grayscale Lithography.

Microstructures with flexible and stretchable properties display tremendous potential applications including integrated systems, wearable devices and bio-sensor electronics. Hence, it is essential to develop an effective method for fabricating curvilinear and flexural microstructures. Despite significant advances in 2D stretchable inorganic structures, large scale fabrication of unique 3D microstructures at a low cost remains challenging. Here, we demonstrate that the 3D microstructures can be achieved by grayscale lithography to produce a curved photoresist (PR) template, where the PR acts as sacrificial layer to form wavelike arched structures. Using plasma-enhanced chemical vapor deposition (PECVD) process at low temperature, the curved PR topography can be transferred to the silicon dioxide layer. Subsequently, plasma etching can be used to fabricate the arched stripe arrays. The wavelike silicon dioxide arch microstructure exhibits Young modulus and fracture strength of 52 GPa and 300 MPa, respectively. The model of stress distribution inside the microstructure was also established, which compares well with the experimental results. This approach of fabricating a wavelike arch structure may become a promising route to produce a variety of stretchable sensors, actuators and circuits, thus providing unique opportunities for emerging classes of robust 3D integrated systems

Materials, Mechanics, and Patterning Techniques for Elastomer-Based Stretchable Conductors

Stretchable electronics represent a new generation of electronics that utilize soft, deformable elastomers as the substrate or matrix instead of the traditional rigid printed circuit boards. As the most essential component of stretchable electronics, the conductors should meet the requirements for both high conductivity and the capability to maintain conductive under large deformations such as bending, twisting, stretching, and compressing. This review summarizes recent progresses in various aspects of this fascinating and challenging area, including materials for supporting elastomers and electrical conductors, unique designs and stretching mechanics, and the subtractive and additive patterning techniques. The applications are discussed along with functional devices based on these conductors. Finally, the review is concluded with the current limitations, challenges, and future directions of stretchable conductors

Design, characterization and validation of integrated bioelectronics for cellular studies: from inkjet-printed sensors to organic actuators

Mención Internacional en el título de doctorAdvances in bioinspired and biomimetic electronics have enabled coupling engineering devices to biological systems with unprecedented integration levels. Major efforts, however, have been devoted to interface malleable electronic devices externally to the organs and tissues. A promising alternative is embedding electronics into living tissues/organs or, turning the concept inside out, lading electronic devices with soft living matters which may accomplish remote monitoring and control of tissue’s functions from within. This endeavor may unleash the ability to engineer “living electronics” for regenerative medicine and biomedical applications. In this context, it remains a challenge to insert electronic devices efficiently with living cells in a way that there are minimal adverse reactions in the biological host while the electronics maintaining the engineered functionalities. In addition, investigating in real-time and with minimal invasion the long-term responses of biological systems that are brought in contact with such bioelectronic devices is desirable. In this work we introduce the development (design, fabrication and characterization) and validation of sensors and actuators mechanically soft and compliant to cells able to properly operate embedded into a cell culture environment, specifically of a cell line of human epithelial keratinocytes. For the development of the sensors we propose moving from conventional microtechnology approaches to techniques compatible with bioprinting in a way to support the eventual fabrication of tissues and electronic sensors in a single hybrid plataform simultaneously. For the actuators we explore the use of electroactive, organic, printing-compatible polymers to induce cellular responses as a drug-free alternative to the classic chemical route in a way to gain eventual control of biological behaviors electronically. In particular, the presented work introduces inkjet-printed interdigitated electrodes to monitor label-freely and non-invasively cellular migration, proliferation and cell-sensor adhesions of epidermal cells (HaCaT cells) using impedance spectroscopy and the effects of (dynamic) mechanical stimulation on proliferation, migration and morphology of keratinocytes by varying the magnitude, frequency and duration of mechanical stimuli exploiting the developed biocompatible actuator. The results of this thesis contribute to the envision of three-dimensional laboratory-growth tissues with built-in electronics, paving exciting avenues towards the idea of living smart cyborg-skin substitutes.En los útimos años los avances en el desarrollo de dispositivos electrónicos diseñados imitando las propiedades de sistemas vivos han logrado acoplar sistemas electrónicos y órganos/tejidos biológicos con un nivel de integración sin precedentes. Convencionalmente, la forma en que estos sistemas bioelectrónicos son integrados con órganos o tejidos ha sido a través del contacto superficial entre ambos sistemas, es decir acoplando la electrónica externamente al tejido. Lamentablemente estas aproximaciones no contemplan escenarios donde ha habido una pérdida o daño del tejido con el cual interactuar, como es el caso de daños en la piel debido a quemaduras, úlceras u otras lesiones genéticas o producidas. Una alternativa prometedora para ingeniería de tejidos y medicina regenerativa, y en particular para implantes de piel, es embeber la electrónica dentro del tejido, o presentado de otra manera, cargar el sistema electrónico con células vivas y tejidos fabricados por ingeniería de tejidos como parte innata del propio dispositivo. Este concepto permitiría no solo una monitorización remota y un control basado en señalizaciones eléctricas (sin químicos) de tejidos biológicos fabricados mediante técnicas de bioingeniería desde dentro del propio tejido, sino también la fabricación de una “electrónica viva”, biológica y eléctricamente funcional. En este contexto, es un desafío insertar de manera eficiente dispositivos electrónicos con células vivas sin desencadenar reacciones adversas en el sistema biológico receptor ni en el sistema electrónico diseñado. Además, es deseable monitorizar en tiempo real y de manera mínimamente invasiva las respuestas de dichos sistemas biológicos que se han añadido a tales dispositivos bioelectrónicos. En este trabajo presentamos el desarrollo (diseño, fabricación y caracterización) y validación de sensores y actuadores mecánicamente suaves y compatibles con células capaces de funcionar correctamente dentro de un entorno de cultivo celular, específicamente de una línea celular de células epiteliales humanas. Para el desarrollo de los sensores hemos propuesto utilizar técnicas compatibles con la bioimpresión, alejándonos de la micro fabricación tradicionalmente usada para la manufactura de sensores electrónicos, con el objetivo a largo plazo de promover la fabricación de los tejidos y los sensores electrónicos simultáneamente en un mismo sistema de impresión híbrido. Para el desarrollo de los actuadores hemos explorado el uso de polímeros electroactivos y compatibles con impresión y hemos investigado el efecto de estímulos mecánicos dinámicos en respuestas celulares con el objetivo a largo plazo de autoinducir comportamientos biológicos controlados de forma electrónica. En concreto, este trabajo presenta sensores basados en electrodos interdigitados impresos por inyección de tinta para monitorear la migración celular, proliferación y adhesiones célula-sustrato de una línea celular de células epiteliales humanas (HaCaT) en tiempo real y de manera no invasiva mediante espectroscopía de impedancia. Por otro lado, este trabajo presenta actuadores biocompatibles basados en el polímero piezoeléctrico fluoruro de poli vinilideno y ha investigado los efectos de estimular mecánicamente células epiteliales en relación con la proliferación, migración y morfología celular mediante variaciones dinámicas de la magnitud, frecuencia y duración de estímulos mecánicos explotando el actuador biocompatible propuesto. Ambos sistemas presentados como resultado de esta tesis doctoral contribuyen al desarrollo de tejidos 3D con electrónica incorporada, promoviendo una investigación hacia la fabricación de sustitutos equivalentes de piel mitad orgánica mitad electrónica como tejidos funcionales biónicos inteligentes.The main works presented in this thesis have been conducted in the facilities of the Universidad Carlos III de Madrid with support from the program Formación del Profesorado Universitario FPU015/06208 granted by Spanish Ministry of Education, Culture and Sports. Some of the work has been also developed in the facilities of the Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (IZM) and University of Applied Sciences (HTW) in Berlin, under the supervision of Prof. Dr. Ing. H-D. Ngo during a research visit funded by the Mobility Fellows Program by the Spanish Ministry of Education, Culture, and Sports. This work has been developed in the framework of the projects BIOPIELTEC-CM (P2018/BAA-4480), funded by Comunidad de Madrid, and PARAQUA (TEC2017-86271-R) funded by Ministerio de Ciencia e Innovación.Programa de Doctorado en Ingeniería Eléctrica, Electrónica y Automática por la Universidad Carlos III de MadridPresidente: José Antonio García Souto.- Secretario: Carlos Elvira Pujalte.- Vocal: María Dimak

Fabrication and light management of microscale solar cells

Photovoltaic (PV) technology holds great promises to become one of the renewable alternatives that can eventually replace the depleting fossil fuel reserves. Challenges, however, remain in various disciplines to achieve a performance-to-cost ratio that can stay economically competitive against traditional energy sources. This dissertation highlights efforts that tackle such challenges from different perspectives, using lightweight microscale semiconductor membranes with unconventional form factors. We start with the fabrication of second-generation silicon solar microcells, with enhanced processing robustness and energy conversion efficiency by utilizing a thermally grown SiO2 material, which serves as both an etching/doping mask and a passivation/anti-reflection layer. Combined with a backside-reflector and a polymer waveguide, these ribbon-like miniature semiconductor membranes demonstrate performance merits that are comparable to commercial silicon solar cells, albeit with significantly less active material consumption. The inherent low optical absorption of these ultrathin devices can be effectively improved by either creating nanocone structures on the device surface that elongate the photon propagation path within the cell, or converting the polymer waveguide to a luminescent solar concentrator (LSC) with luminophores that actively down-converts incident sunlight and redirects it to the embedded microcells. Strategies explored in this work to improve the performance of such LSC devices include the use of core-shell quantum dots with tunable bandgaps and minimum reabsorption losses, the design of a luminescence-trapping photonic mirror with photon recycling effects and the assembly of a multilayer construct with expanded spectral coverage. The low-cost microcell concept can be extended from Si to III-V PV materials, which have much higher efficiency due to their direct bandgap structure and the ability to form multi-junction architectures that minimize both absorption and carrier thermalization losses. Their high material cost due to the epitaxy growth process is usually compensated by use of concentrating optics, which then leads to performance constraints that include the optical losses from the geometric lenses and the inability to capture diffuse solar radiation. In the last section of this work, novel nanoporous optical materials and hybrid module architectures are created for a commercial concentration photovoltaics (CPV) module that employs triple-junction III-V microcells, with significantly reduced Fresnel losses and added capability of utilizing diffuse sunlight

Organic Semiconductors-Based Devices Electrical Reliability to Environmental Stress

In this thesis, I report on the characterisation of the response of organic semiconductor based devices, namely organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic diodes (OPVDs) to environmental stress factors such as electrostatic discharge (ESD) and neutrons irradiation. The ESD stress was obtained by means of a transmission line-pulsing (TLP), responsible to generate current pulses with an increasing amplitude and a duration of few tens of nanoseconds. The exposure to neutron irradiation was obtained in the pulsed neutron and muon source at ISIS part of the Rutherford Appleton Laboratories (RAL). The tested devices were: P3HT (poly(3-hexylthiophene)):PCBM ([6,6]- phenyl C61 butyric acid methyl ester) bulk heterojunction solar cells; PBTTT (poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2- b]thiophene) and P3HT OFETs; F8BT (poly(9,9'-dioctylfluorene-alt-benzothiadiazole)) OLEDs. An analysis of both electrical (IV and JV curves, Electroluminescence (EL)) and optical (photoluminescence (PL), Raman Spectroscopy) characteristics of tested devices prior and following the exposure to various degrees of ESD, neutron irradiation or both is reported. For each tested device I obtained the respective TLP parameters (the leakage current (ILEAK), the TLP current (ITLP), the TLP voltage (VTLP), the TLP resistance (RTLP)) and the correlation of these with parameters extracted by means of their electrical/optical characterisation, namely: (i) the charge mobility, the threshold voltage (VTH) and the on/off ratio of OFETs; (ii) the current density (Jsc), the open-circuit voltage (Voc), the fill factor (FF) and the power conversion efficiency (η) of OPVs; (iii) the turn-on voltage (Von), the external quantum efficiency (EQE) and the EL maximum wavelength emission (λmax) of OLEDs. Importantly, the activity carried out in this thesis gives novel insights about the response of conjugated polymer-based devices with respect to the stressing stimuli (ESD events, cosmic rays) they are exposed to in their most suitable application fields (space, medicine, robotics), such as the energy necessary to cause a total or partial failure during ESD events, the requirements necessary to design electrical protections, the expected loss of device figures after a decade of exposure to cosmic rays. Interestingly, the results in this thesis reported point out, in most of the cases, an excellent robustness of these devices to both ESD and cosmic rays stress. In fact, whilst technology silicon-based is found to suffer a permanent failure in most of the cases for an applied TLP power lower than 400 W, polymer-based technology was found to withstand up to 800 W (OPVs and OLEDs) without suffering permanent damages. As regards the stress correlated to the same dose of neutrons irradiation, optoelectronic devices based on inorganic semiconductors suffer of a 90% reduction of their figures of merit (JSC, h), whilst the same figures are reduced of only 20% in polymer-based devices. Although previous works are reported in literature, the work reported in this thesis, at the best of my knowledge, is the first work reporting a systematic quantitative TLP characterisation of organic devices along with a qualitative description of the effects on the organic materials within these devices because of the conditions imposed by the TLP test (high-frequency, high-voltage). Therefore, this thesis opens a new scenario proposing an investigating tool aimed both at measuring parameters useful for the design of the devices and at highlighting organic materials properties that can lead organic electronics to gain its definitive momentum