Electronics can be made on elastically stretchable “skin.” Such skins conform to irregularly curved surfaces and carry arrays of thin-film devices and integrated circuits. Laypeople and scientists ...intuitively grasp the concept of electronic skins; material scientists then ask “what materials are used?” and “how does it work?” Stretchable circuits are made of diverse materials that span more than 12 orders of magnitude in elastic modulus. We begin with a brief overview of the materials and the architecture of stretchable electronics, then we discuss stretchable substrates, encapsulation, interconnects, and the fabrication of devices and circuits. These components and techniques provide the tools for creating new concepts in biocompatible circuits that conform to and stretch with living tissue. They enable wireless energy transfer via stretchable antennas, stretchable solar cells that convert sunlight to electricity, supercapacitors, and batteries that store energy in stretchable electronic devices. We conclude with a brief outlook on the technical challenges for this revolutionary technology on its road to functional stretchable electronic systems.
Gold films on poly(dimethylsiloxane) (PDMS) have applications in stretchable electronics, tunable diffraction gratings, soft lithography and as neural interfaces. The electrical and optical ...properties of these films depend critically on the morphology of the gold. Therefore, we examine qualitatively and quantitatively the factors that affect the morphology of the gold film. Three morphologies can be produced controllably: microcracked, buckled, and smooth. Which morphology a gold film will adopt depends on the film stress and the growth mode of the film. The factors that affect the film stress and growth mode, and thus the morphology, are as follows: deposition temperature, film thickness, elastic modulus, adhesion layer thickness, surface properties of the PDMS, and mechanical prestrain applied during deposition. We discuss how the different components of the film stress and growth mode of the film affect the morphology.
This paper is an informal transcript of the Mott Lecture given at ICANS 23 in Utrecht. While the writer was fortunate to still have seen and heard Sir Nevill Mott at conferences, he came to know him ...mostly through stories and comments that told of his enthusiasm, his wide‐ranging interests, and the pleasure of interacting with him. Nevill Mott was the guide in preparing this brief account of the success story that amorphous silicon has become, and of what still may come of it. What would he have liked to learn about it today? The Lecture and the present account also reflect the writer's research interest in electronic materials. He wants to convey his opinion that hydrogenated forms of thin‐film silicon, having become the foundation of a large industry, still hold tremendous promise for research and for application.
Damage significantly influences response of a strain sensor only if it occurs in the proximity of the sensor. Thus, two-dimensional (2D) sensing sheets covering large areas offer reliable early-stage ...damage detection for structural health monitoring (SHM) applications. This paper presents a scalable sensing sheet design consisting of a dense array of thin-film resistive strain sensors. The sensing sheet is fabricated using flexible printed circuit board (Flex-PCB) manufacturing process which enables low-cost and high-volume sensors that can cover large areas. The lab tests on an aluminum beam showed the sheet has a gauge factor of 2.1 and has a low drift of 1.5 μ ϵ / d a y . The field test on a pedestrian bridge showed the sheet is sensitive enough to track strain induced by the bridge's temperature variations. The strain measured by the sheet had a root-mean-square (RMS) error of 7 μ ϵ r m s compared to a reference strain on the surface, extrapolated from fiber-optic sensors embedded within the bridge structure. The field tests on an existing crack showed that the sensing sheet can track the early-stage damage growth, where it sensed 600 μ ϵ peak strain, whereas the nearby sensors on a damage-free surface did not observe significant strain change.
The classical SiO2/Si interface, which is the basis of integrated circuit technology, is prepared by thermal oxidation followed by high temperature (>800 °C) annealing. Here we show that an interface ...synthesized between titanium dioxide (TiO2) and hydrogen-terminated silicon (H:Si) is a highly efficient solar cell heterojunction that can be prepared under typical laboratory conditions from a simple organometallic precursor. A thin film of TiO2 is grown on the surface of H:Si through a sequence of vapor deposition of titanium tetra(tert-butoxide) (1) and heating to 100 °C. The TiO2 film serves as a hole-blocking layer in a TiO2/Si heterojunction solar cell. Further heating to 250 °C and then treating with a dilute solution of 1 yields a hole surface recombination velocity of 16 cm/s, which is comparable to the best values reported for the classical SiO2/Si interface. The outstanding performance of this heterojunction is attributed to Si–O–Ti bonding at the TiO2/Si interface, which was probed by angle-resolved X-ray photoelectron spectroscopy. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) showed that Si–H bonds remain even after annealing at 250 °C. The ease and scalability of the synthetic route employed and the quality of the interface it provides suggest that this surface chemistry has the potential to enable fundamentally new, efficient silicon solar cell devices.
This article reviews a grounding in thin-film science and technology, an interest in combining materials science with applied physics and electrical engineering, and the active pursuit of ...collaborations with experts in other disciplines. That basis has enabled participation in the beginnings of integrated-circuit technology, the invention of new solar cells, the understanding of hydrogenated amorphous silicon for solar cells and thin-film transistors, the development of the principles of flexible, conformable, and stretchable electronics, and the devising and demonstration of large-area electronic systems.
A high‐resolution elastically stretchable microelectrode array (SMEA) for interfacing with neural tissue is described. The SMEA consists of an elastomeric substrate, such as poly(dimethylsiloxane) ...(PDMS), elastically stretchable gold conductors, and an electrically insulating encapsulating layer in which contact holes are opened. We demonstrate the feasibility of producing contact holes with 40 μm × 40 μm openings, show why the adhesion of the encapsulation layer to the substrate is weakened during contact hole fabrication, and provide remedies. These improvements result in greatly increased fabrication yield and reproducibility. An SMEA with 28 microelectrodes was fabricated. The contact holes (100 μm × 100 μm) in the encapsulation layer are only ∼10% the size of the previous generation, allowing a larger number of microelectrodes per unit area, thus affording the capability to interface with a smaller neural population per electrode. This new SMEA is used to record spontaneous and evoked activity in organotypic hippocampal tissue slices at 0% strain before stretching, at 5% and 10% equibiaxial strain, and again at 0% strain after relaxation. Stimulus–response curves at each strain level are measured. The SMEA shows excellent biocompatibility for at least two weeks.
Spontaneous and evoked neural activity of a hippocampal tissue slice are recorded using a high‐resolution stretchable microelectrode array (SMEA). The SMEA is capable of recording and stimulating neural activity at large biaxial strains and completely recovers after relaxation.
This paper presents a large-area image sensing and detection system that integrates, on glass, sensors and thin-film transistor (TFT) circuits for classifying images from sensor data. Large-area ...electronics (LAE) enables the formation of millions of sensors spanning physically large areas; however, to perform processing functions, thousands of sensor signals must be interfaced to CMOS ICs, posing a critical limitation to system scalability. This work presents an approach whereby image detection of shapes is performed using simple circuits in the LAE domain based on amorphous silicon (a-Si) TFTs. This reduces the interfaces to the CMOS domain. The limited computational capability of TFT circuits as well as high variability and high density of process defects affecting TFTs and sensors is overcome using a machine-learning algorithm known as error-adaptive classifier boosting (EACB) to form embedded weak classifiers. Through EACB, we show that high-dimensional sensor data from a-Si photoconductors can be reduced to a small number of weak-classifier decisions, which can then be combined in CMOS to achieve strong-classifier performance. For demonstration, a system classifying five shapes achieves performance of >85%/>95% true-positive (tp)/true-negative (tn) rates near the level of an ideal software-implemented support vector machine (SVM) classifier, while the total number of signals from 36 sensors in the LAE domain is reduced by <inline-formula><tex-math notation="LaTeX">3.5\text{-}9\times</tex-math></inline-formula>.
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