This review comprises a detailed survey of ongoing methodologies for soft actuators, highlighting approaches suitable for nanometer- to centimeter-scale robotic applications. Soft robots present a special design challenge in that their actuation and sensing mechanisms are often highly integrated with the robot body and overall functionality. When less than a centimeter, they belong to an even more special subcategory of robots or devices, in that they often lack on-board power, sensing, computation, and control. Soft, active materials are particularly well suited for this task, with a wide range of stimulants and a number of impressive examples, demonstrating large deformations, high motion complexities, and varied multifunctionality. Recent research includes both the development of new materials and composites, as well as novel implementations leveraging the unique properties of soft materials.

Soft Actuators for Small-Scale Robotics.

Gallium and several of its alloys are liquid metals at or near room temperature. Gallium has low toxicity, essentially no vapor pressure, and a low viscosity. Despite these desirable properties, applications calling for liquid metal often use toxic mercury because gallium forms a thin oxide layer on its surface. The oxide interferes with electrochemical measurements, alters the physicochemical properties of the surface, and changes the fluid dynamic behavior of the metal in a way that has, until recently, been considered a nuisance. Here, we show that this solid oxide “skin” enables many new applications for liquid metals including soft electrodes and sensors, functional microcomponents for microfluidic devices, self-healing circuits, shape-reconfigurable conductors, and stretchable antennas, wires, and interconnects.

Emerging Applications of Liquid Metals Featuring Surface Oxides

All commercial forms of electronic/optoelectronic technologies use planar, rigid substrates. Device possibilities that exploit bio-inspired designs or require intimate integration with the human body demand curvilinear shapes and/or elastic responses to large strain deformations. This article reviews progress in research designed to accomplish these outcomes with established, high-performance inorganic electronic materials and modest modifications to conventional, planar processing techniques. We outline the most well developed strategies and illustrate their use in demonstrator devices that exploit unique combinations of shape, mechanical properties and electronic performance. We conclude with an outlook on the challenges and opportunities for this emerging area of materials science and engineering.

Stretchable, curvilinear electronics based on inorganic materials.

Paper has been present in the world of analytical chemistry for centuries, but it seems that just a few years back it was rediscovered as a valuable substrate for sensors. We can easily list some of the countless advantages of this simple cellulosic substrate, including mechanical properties, three-dimensional fibrous structure, biocompatibility and biodegradability, easiness of production and modification, reasonable price, and availability all over the world. Those characteristics make paper a first-choice substrate for disposable sensors and integrated sensing platforms. Nowadays, numerous examples of paper-based sensors are being presented in the literature. This review describes some of the most prominent examples classifying them by type of detection: optical (colorimetric, fluorescence, surface-enhanced Raman spectroscopy, and transmittance methods) and electrochemical (voltammetric, potentiometric, and conductivity-based methods). We take a closer look at recent advances in immunoassays fabricated on paper, excluding simple lateral flow tests assembled on nitrocellulose. This review also summarizes the main advantages and disadvantages of the use of paper as a substrate for sensors, as well as its impact on their performance and application, presents a short history of paper in analytical chemistry, and discusses fabrication methods and available sources of paper.

Sensing approaches on paper-based devices: a review

The authors acknowledge generous support of the King Abdullah University of Science and Technology (KAUST). The authors acknowledge Dr. Joanna M. Nassar, Dr. Galo A. Torres, Dr. Mohamed T. Ghoneim, Andres A. Aguirre-Pablo, Davide Priante, Dr. Jhonathan P. Rojas, Sigurdur T. Thoroddsen, and Prof. Boon S. Ooi who contributed to the “pause-embed-resume” data. The authors thank Kelly Rader for proof reading this manuscript.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/aisy.202000128

Soft Actuators for Soft Robotic Applications: A Review

This paper reports a biaxial stretchable interconnect on an elastomeric substrate. To increase the stretchability of interconnects, a 2-D diamond-shaped geometry of gold on a polydimethylsiloxane substrate was adopted in which the potentially breakable points were covered with room temperature liquid alloy. Finite element model simulations were performed to identify the most vulnerable points subjected to stress concentration and optimize the design process. Simulations also indicated an optimum gold thickness and linewidth that result in a minimum stress when the substrate is stretched. Four different geometries were designed, fabricated, and characterized. These included: 1) 2-D diamond-shaped gold lines connected at circular junctions with an intersection angle of 90deg; 2) 2-D diamond-shaped gold lines connected at circular junctions with intersection angles of 120deg and 60deg; 3) 2-D diamond-shaped gold lines separated at circular junctions with an intersection angle of 90deg; and 4) 2-D diamond-shaped gold lines separated at circular junctions with intersection angles of 120 deg and 60deg. A maximum stretchability (DeltaL/L) of ~ 60% was achieved for the design in which the lines and circles were separated and had intersection angles of 120deg and 60deg. A resistance variation of (DeltaR/R) ~ 30% was measured for this configuration.

/pdf/a-biaxial-stretchable-interconnect-with-liquid-alloy-covered-2zvulk06gd.pdf

A Biaxial Stretchable Interconnect With Liquid-Alloy-Covered Joints on Elastomeric Substrate

In this paper, we report on an inexpensive method of fabricating miniature magnetic actuators using ferrofluid impregnated paper. Different types of papers (including soft tissue paper, cleanroom paper, Whatman-1 filter paper, printer paper, and newspaper) were loaded with oil-based ferrofluid, microma-chined by a CO2 laser and coated with a thin layer of parylene-C. The soaking capability of the different papers was investigated, with the soft tissue paper having the highest loading capacity, being able to absorb ferrofluid by as much as six times its original weight. Cantilever actuators fabricated from cleanroom and filter papers were able to generate the largest force (>; 40-mg equiva lent force), whereas the soft-tissue-paper cantilevers provided the greatest deflection (40° tip angle).

Ferrofluid-Impregnated Paper Actuators

In this paper, we present a stretchable microelectrode array for studying cell behavior under mechanical strain. The electrode array consists of gold-plated nail-head pins (250 µm tip diameter) or tungsten micro-wires (25.4 µm in diameter) inserted into a polydimethylsiloxane (PDMS) platform (25.4 × 25.4 mm2). Stretchable interconnects to the outside were provided by fusible indium-alloy-filled microchannels. The alloy is liquid at room temperature, thus providing the necessary stretchability and electrical conductivity. The electrode platform can withstand strains of up to 40% and repeated (100 times) strains of up to 35% did not cause any failure in the electrodes or the PDMS substrate. We confirmed biocompatibility of short-term culture, and using the gold pin device, we demonstrated electric field pacing of adult murine heart cells. Further, using the tungsten microelectrode device, we successfully measured depolarizations of differentiated murine heart cells from embryoid body clusters.

Stretchable microelectrode array using room- temperature liquid alloy interconnects

In this paper, we present a stretchable electrode array for studying cell behavior subjected to mechanical strain. The electrode array consists of four gold nail-head pins (250?m tip diameter and 1.75mm spacing) inserted into a polydimethylsiloxane (PDMS) platform (25.4×25.4mm2). Fusible indium alloy (liquid at room temperature) filled microchannels are used to connect the electrodes to the outside, thus providing the required stretchability. The electrode platform is biocompatible and can withstand strains of up to 40%. We tested these electrodes by repeatedly (100 times) subjecting them to 35% strain and did not notice any failure. We also successfully cultured mice cardiomyocytes onto the platform and performed electrical pacing.

A Stretchable Cell Culture Platform with Embedded Electrode Array

This paper describes a new method of fabricating 3D microstructure by using the self-folding of hydrogel-parylene bilayer. The environmental responsive properties of the hydrogel allows for automated bending 3D microstructures upon exposure to a chemical or physical stimuli. For a highly cross-linked pH-sensitive hydrogel, bending of the bilayer structure can be completed in 6 seconds when pH value is changed from 2.0 to 9.0.

Self-Folding Smart 3D Microstructures Using a Hydrogel-Parylene Bilayer

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