The compliant, continuum, and configurable robotics field in general has gained growing interest in the past years especially with the exciting advances in artificial intelligence technology, [1] which could enable various valuable applications ranging from manufacturing to safety and healthcare. [2,3] Soft robots are of notable interest because, unlike their rigid counterpart, they can easily deform while being mechanically resilient, [4,5] adapt to the outer environment without harm to humans, [6] and finally, enable low-cost manufacturing. [7] For robots to interact with the outer environment and complete tasks, a set of sensors and actuators need to be integrated into the system. Soft robots, in specific, present additional challenges because their sensing and actuation devices are generally highly integrated within the body of the robot and its whole functionality. These challenges become even more critical when the soft robot is scaled down to sub-centimeter size as the sensing, power, and data analysis units are moved off-board. As a result, miniaturized soft actuators that respond to various stimuli and show large deformations in addition to mechanical resilience are crucial. These would be particularly promising for application in artificial muscles, microrobots, and micro-manipulators. [8-10] Active and soft materials are promising for this task as they can be actuated through various external stimuli, such as photons, thermal, magnetic and/or electric field. Such materials range from particles, to polymers (either electroactive or shape memory), papers, fluids, shape memory alloys (SMAs), liquid metals, hydrogels, 2D materials, or a combination of these. [6-25] Nevertheless, some materials can be more suitable for a specific set of applications than others; for instance, materials stimulated by the near-infrared (NIR) spectrum are promising for biomedical applications, whereas sunlight-stimulated materials are suitable for nature-inspired soft robots used in outside environments. Various useful metrics are generally used to assess the performance of the actuators; these include the generated stress and strain, Young's modulus or measured stiffness, in addition to their power, work, energy, and force density. In this Review article, however, we focus on the application of the soft actuators in soft robotics where the reported metrics include mode and speed of actuation (or locomotion), power, voltage, current (of the driving signal), lifting force, and weight among others. In this Review article, different active materials that have been developed and used in soft actuators for soft robotics are discussed and grouped by the stimulus that generates the actuation response as shown in Figure 1. The physics of operation, resulting deformations, mechanical resilience, and their pros and cons are presented with a focus on the applications of the different soft

Soft Robotics In article number http://doi.wiley.com/10.1002/aisy.202000128, Muhammad Mustafa Hussain and co‐workers present a comprehensive review on soft robots which are paving the way towards a wide range of vital applications such as drug delivery, among others. Various state‐of‐the‐art soft actuators which respond to different stimuli, including light, heat, and applied electric field with a focus on their various applications in soft robotics, are discussed.

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Optoelectronic devices are advantageous in in-memory light sensing for visual information processing, recognition, and storage in an energy-efficient manner. Recently, in-memory light sensors have been proposed to improve the energy, area, and time efficiencies of neuromorphic computing systems. This study is primarily focused on the development of a single sensing-storage-processing node based on a two-terminal solution-processable MoS2 metal–oxide–semiconductor (MOS) charge-trapping memory structure—the basic structure for charge-coupled devices (CCD)—and showing its suitability for in-memory light sensing and artificial visual perception. The memory window of the device increased from 2.8 V to more than 6 V when the device was irradiated with optical lights of different wavelengths during the program operation. Furthermore, the charge retention capability of the device at a high temperature (100 °C) was enhanced from 36 to 64% when exposed to a light wavelength of 400 nm. The larger shift in the threshold voltage with an increasing operating voltage confirmed that more charges were trapped at the Al2O3/MoS2 interface and in the MoS2 layer. A small convolutional neural network was proposed to measure the optical sensing and electrical programming abilities of the device. The array simulation received optical images transmitted using a blue light wavelength and performed inference computation to process and recognize the images with 91% accuracy. This study is a significant step toward the development of optoelectronic MOS memory devices for neuromorphic visual perception, adaptive parallel processing networks for in-memory light sensing, and smart CCD cameras with artificial visual perception capabilities.

Microfluidic actuators based on thermally-induced actuation are gaining intense attraction due to their usage in disease diagnosis and drug release-related devices. These devices use a thermally-expandable polymer called Expancel that expands once its temperature exceeds a particular threshold value. Achieving such devices that are cost-effective and consume low input power is crucial for attaining efficacy. Therefore, the need for a low-energy consuming actuator necessitates the improved configurations of microheaters that provide the required heat. We report a novel topology of a copper-based microheater called square-wave meander, exhibiting a 44% higher output temperature, showing high actuation efficiency, as compared to the conventionally used meander design. The reason for increased temperature with low input energy is attributed to increased resistance by a jagged structure while maintaining the same surface area, i.e., without changing the effective thickness of the microheater. Numerical modeling demonstrates the comparison of temperature and electric potential contours for reported and conventionally used microheaters. We reveal the merit of the reported design by comparing the volumetric thermal strains for both designs. We experimentally demonstrate the increased expansion of 25% for the reported design at the same applied current of 200 mA and faster operation time. Later, we show the microfluidic actuator device integrated into the microheater and PDMS-Expancel, controlling the operation/actuation of a fluid through a microchannel. This work might improve the performance of the advanced microfluidic-based drug release and other fluid-based applications.

Carbon nanotubes (CNTs) are a form of carbon that is allotropic with a high electric conductivity, stability, and mechanical flexibility making them ideal for application in electronics, sensors, thin-film transistors, and storage devices.1 More specifically, CNTs are a promising channel material in thin-film transistors (TFT) with high-performance, high mobility, and low-cost processing due to their one-dimensional nature and excellent electrical properties.2-3 Various methods have been reported to coat/deposit CNTs on various substrates, such as 1) filtration which can form uniform films but using a relatively complicated process, 2) dip coating which is a simple process but lacks controllability, 3) transfer printing but the process is also complicated due to the CNTs small diameters and high adherence to the substrate, 4) ink-jet printing which produces a lot of material waste, 5) spray coating/spin coating which are simple processes but lack uniformity and generate material waste, and 6) drop-casting which is nonuniform and requires stamping to create dense and uniformly distributed CNTs.1-3 In this work, we propose a new CNTs-solution process flow based on drop casting on a shrinkable polymer which allows us to create a dense CNTs mesh with reduced porosity, and thus improve the resulting film conductivity using a single droplet of the solution. Thus, there will be no need to use multiple coating steps or a stamping mold to achieve higher density of CNTs, which suggests lower material waste and lower cost. Figure 1a shows the process flow which was performed on a 1 cm by 1 cm plastic heat-shrinkable plastic. First, a Kapton tape-based mask was applied and patterned using a CO2 laser to create square shaped holes for contacts deposition using sputtering. Next, CNTs were mixed with Isopropyl Alcohol (IPA) and sonicated to disperse the CNTs in the solution. Using a pipette, one drop (4 µL) of the solution is placed on the masked shrinkable paper and left to dry at room temperature. Finally, the mask is removed and heat is applied to create the shrinking effect. The approach is promising towards 4D printing as the shrinkable polymer and CNTs can be 3D printed and an external stimulus can induce the shrinkage effect. Using different temperatures for different durations, the effect of the substrate shrinking on the CNTs mesh is analyzed. Figure 1b shows that as the heating duration is increased at a fixed temperature, the resistance of the CNTs channel decreases. The reduction in the resistance is due to the decrease in the porosity of the mesh as confirmed by scanning electron microscopy (SEM) images depicted in Figure 1c. In fact, using SEM images, the sheet is found to shrink by 15% and 19% when heated at 110°C for 1 min and 130°C for 3 min, respectively. In addition, as a result of the shrinking of the substrate, CNTs experience compressive strain which increases the charge density in the material and thus further improves the mesh conductivity.4 The compression effect is observed using Raman spectroscopy using a 532 nm laser, where the initial G peak of the CNTs mesh4 was located at ~1571 cm-1 and after 1 min of heating at 110°C, the peak shifted to ~1575 cm-1 as shown in Figure 1d. In conclusion, we have demonstrated a new fabrication technique that results in high conductivity with a single drop in a single step by using the shrinking effect of the polymer. The results show that the conductivity will increase with increasing the time for a specific temperature due to stress and reduction of porosity. This will open the door for the fabrication of devices using less amount of material, and less waste at a lower cost. References [1] Z. Dong et al., “Carbon nanotubes in perovskite-based optoelectronic devices,” Matter, vol. 5, no. 2, pp. 448–481, Feb. 2022. [2] Q. N. Thanh et al., “Transfer-Printing of As-Fabricated Carbon Nanotube Devices onto Various Substrates,” Advanced Materials, vol. 24, no. 33, pp. 4499–4504, Jun. 2012. [3] N. Qaiser et al., “A Robust Wearable Point‐of‐Care CNT‐Based Strain Sensor for Wirelessly Monitoring Throat‐Related Illnesses,” Advanced Functional Materials, vol. 31, no. 29, p. 2103375, May 2021. [4] C. Androulidakis et al., “Non-Eulerian behavior of graphitic materials under compression,” Carbon, vol. 138, pp. 227–233, Nov. 2018. Figure 1. Fabrication process flow and characterization of the CNTs. a) Detailed fabrication process flow. b) The relative change in the resistance at different heating temperatures and durations. c) SEM images showing a reduction in CNT size before (left) and after shrinking (right). d) Raman spectroscopy of CNTs before and after shrinking. Figure 1

Actuators are the main components of any machine due to their responsibility in providing the desired action of control. As recent studies have been developing new actuation methodologies and remote clinical palpation tools that provide more complex and enhanced functionalities, this review paper aims to provide researchers interested in the actuation techniques and remote clinical palpation technologies with a reference guide about the recent actuation techniques including, shape deformation, moisture, and temperature actuators along with remote clinical palpation mechanisms. In addition, this review paper outlines comprehensive comparisons between their characteristics.