Significance Efficient thermal transport is critical for applications ranging from electronics and energy to advanced manufacturing and transportation; it is essential in emerging domains like wearable computing and soft robotics, which require thermally conductive materials that are also soft and stretchable. However, heat transport within soft materials is limited by the dynamics of phonon transport, which results in a trade-off between thermal conductivity and compliance. We overcome this by engineering an elastomer composite embedded with elongated inclusions of liquid metal (LM) that function as thermally conductive pathways. These composites exhibit an extraordinary combination of low stiffness (<100 kPa), high strain limit (>600%), and metal-like thermal conductivity (up to 9.8 W⋅m−1⋅K−1) that far exceeds any other soft materials. Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal−mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young’s modulus < 100 kPa), and the capability to undergo extreme deformations (>600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m−1⋅K−1) over the base polymer (0.20 ± 0.01 W⋅m−1·K−1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m−1·K−1) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal−mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.

/pdf/high-thermal-conductivity-in-soft-elastomers-with-elongated-3wdtjxvzco.pdf

High thermal conductivity in soft elastomers with elongated liquid metal inclusions.

The evolution of intelligent manufacturing has had a profound and lasting effect on the future of global manufacturing. Industry 4.0 based smart factories merge physical and cyber technologies, making the involved technologies more intricate and accurate; improving the performance, quality, controllability, management, and transparency of manufacturing processes in the era of the internet-of-things (IoT). Advanced low-cost sensor technologies are essential for gathering data and utilizing it for effective performance by manufacturing companies and supply chains. Different types of low power/low cost sensors allow for greatly expanded data collection on different devices across the manufacturing processes. While a lot of research has been carried out with a focus on analyzing the performance, processes, and implementation of smart factories, most firms still lack in-depth insight into the difference between traditional and smart factory systems, as well as the wide set of different sensor technologies associated with Industry 4.0. This paper identifies the different available sensor technologies of Industry 4.0, and identifies the differences between traditional and smart factories. In addition, this paper reviews existing research that has been done on the smart factory; and therefore provides a broad overview of the extant literature on smart factories, summarizes the variations between traditional and smart factories, outlines different types of sensors used in a smart factory, and creates an agenda for future research that encompasses the vigorous evolution of Industry 4.0 based smart factories.

https://www.mdpi.com/1424-8220/20/23/6783/pdf

Advances in Sensor Technologies in the Era of Smart Factory and Industry 4.0.

Self-Shaping Soft Electronics Based on Patterned Hydrogel with Stencil-Printed Liquid Metal

Once merely ancient arts, origami (i.e., paper folding) and kirigami (i.e., paper cutting) have in recent years also become popular for building mechanical metamaterials and now provide valuable design guidelines. By means of folding and cutting, two-dimensional thin-film materials are transformed into complex three-dimensional structures and shapes with unique and programmable mechanical properties. In this review, mechanical metamaterials based on origami and/or kirigami are categorized into three groups: (i) origami-based ones (with folding only), (ii) kirigami-based ones (with cutting only), and (iii) hybrid origami–kirigami-based ones (with both folding and cutting). For each category, the deformation mechanisms, design principles, functions, and applications are reviewed from a mechanical perspective.

Mechanical metamaterials based on origami and kirigami

Wireless and highly sensitive flexible strain sensors would have widespread application across a number of different fields. Here, the novel combination of two different metamaterials, one mechanic...

Kirigami-Enabled Microwave Resonator Arrays for Wireless, Flexible, Passive Strain Sensing.

DOI: 10.1002/admt.201800683 compact, and energy-efficient deformation sensors.[13] Additionally, implementation into wearable sensors for human monitoring and soft robotic systems demand significant extents of deformation.[14–20] Furthermore, wireless monitoring in these systems can reduce system complexity by eliminating or reducing wiring components and can enable more compliant materials by removing semirigid wiring and connection points.[21,22] Thus, wireless monitoring of deformation with passive elements can provide a path forward for deformation sensing in soft bodies for energy-efficient reporting and control.[23] Wired soft sensors can provide a solution when integrated with wireless communication circuitry (Wi-Fi, Bluetooth, or cellular).[24] These include various designs of stretchable strain sensors, such as dipole, serpentine, spiral, and helical geometries composed of strain-dependent resistive elements such as carbon-impregnated rubbers or compliant microfluidic channels filled with liquid metals.[25–27] In each of these cases, the extra wiring and power consumption of the communication circuitry can limit the utility of untethered applications. Progress is being made on integrating more flexible, wireless sensors for feedback and control of untethered systems. A common solution is imaging, in which motion and position of the device are monitored externally by a camera.[28,29] This works well in structured environments; however, in unstructured environments, where a camera may be blocked or unavailable, this can be a significant challenge. Another approach is the use of resonant sensors to wirelessly report a positional change of soft materials. Resonant sensors are a long-standing class of passive, wireless sensors that use radio frequency electromagnetic radiation to wirelessly interrogate the scattering parameters of an inductor–capacitor–resistor (LCR) circuit.[30] The resonant circuit responds to changes in the local dielectric (changes circuit capacitance) which has been exploited for measuring physical parameters such as fluid level, pressures, temperature, and biocatalyst activity.[31–39] Flexible LCR sensors have been used to wirelessly measure the strain of compliant materials, such as inductors composed of serpentine copper traces formed as planar[40] or helical coils.[41] In the case of the planar coils, the force is set coplanar to the resonator, and A passive resonant sensor with kirigami patterning is presented to wirelessly report material deformation in closed systems. The sensors are fabricated from copper-coated polyimide by etching a conductive Archimedean spiral and then laser cutting kirigami patterns. The sensor response is defined as the resonant frequency in the transmission scattering parameter signal (S21), which is captured via a benchtop vector network analyzer. The sensors are tested over a 0–22 cm range of extension and show a significant shift in resonant frequency (e.g., 90 MHz shift for 10 cm stretch). Furthermore, the effect of resonator coil pitch on the extension sensor gain (MHz cm−1) and linear span of the sensor is studied. The repeatability of the sensor gain is confirmed by performing hysteresis cycles. The sensors is coated with polydimethylsiloxane films to protect from electrical shorting in aqueous environments. The coated resonators are placed in a pipe to report flow rates. The sensor with 1 mm coating is found to have the largest gain (0.17 MHz⋅s mL−1) and linear span (10–100 mL s−1). Thus, flexible resonant sensors with kirigami-inspired patterns can be tuned via geometric and coating considerations to wirelessly report a large range of extension lengths for potential uses in health monitoring, motion tracking, deformation detection, and soft robotics.

https://par.nsf.gov/servlets/purl/10096738

Kirigami‐Enabled, Passive Resonant Sensors for Wireless Deformation Monitoring

In this work, four square, planar resonators with unique frequency windows were used to form a 2 by 2 array for wireless position determination and normalization of position-dependent, embedded resonant sensors. First, a master table of S|21| gain and phase data was collected at 8100 positions. Automated scripts extracted the characteristic gain and phase peaks and used cubic interpolation to expand the master table to 7,157,160 unique angle and coordinate positions. An unknown position is then determined by comparing its S|21| measurements to this table. To further improve the position accuracy, multiple measurements are collected on linear flyby trajectories. The average and standard deviation of predicted position offset from true value using this method were 3.2 and 2.3 mm, respectively. To test normalization of a position dependent sensor, a spiral resonant sensor was placed underneath the square array. The sensor signal was modulated using varying amounts of water on the sensor surface. A corrected reading was determined using four different flyby trajectories using the position array data to adjust the signal based on position. We found that average errors of the normalized signals were between 0.04 to 0.15 MHz at lower water volume (0.5 mL) and -0.53 to -0.74 MHz at higher water volume (2.0 mL). In its current state, the positional array can be used for asset tracking or feedback control and the sensor normalization can be used to improve the measurement accuracy of embedded sensors. This technique can be further improved by collecting more accurate master calibration data using an automated system.

/pdf/wireless-position-sensing-and-normalization-of-embedded-3stmwkgo15.pdf

Wireless position sensing and normalization of embedded resonant sensors using a resonator array

Effects of fabrication materials and methods on flexible resonant sensor signal quality

Here we detail an inductively coupled extender (ICE) and resonant (LC) sensor to monitor soil moisture using a portable reader. Significant advances of this ICE-LC design are extending typical LC sensor read range over a meter and reducing positional alignment sensitivity between reader and sensor. An analytical model validates the working principle and feasibility of the ICE-LC system. Prototypes of the ICE-LC sensor were built and optimized in terms of sensitivity and power transfer (single and four turns for ICE top and bottom coils, respectively). Soil moisture tests validated the ICE-LC improvements on minimized positional alignment sensitivity and extended read range, transducing a decrease in resonant frequency with increasing soil moisture. When calibrating with existing wired approaches, the ICE-LC sensor had a reproducible, linear sensor gain of 4.52%moisture content/MHz with an R2 of 0.745 and RMSE of 2.41%. A smaller, planar form factor of the ICE-LC sensor was also tested and exhibited reduced positional alignment sensitivity between reader and sensor at shorter read ranges. This initial study demonstrates the feasibility of the ICE-LC resonant sensor as a cost-effective method to monitor soil moisture content throughout the growing season at many field locations.

Positionally-independent and Extended Read Range Resonant Sensors Applied to Deep Soil Moisture Monitoring

Spiral coil-based inductor–capacitor (LC) resonant sensors have seen many applications in agriculture, healthcare, biomanufacturing, and consumer electronics. Traditional techniques to interrogate such sensors using vector network analyzers (VNAs) are expensive and often not portable, whereas custom readouts suffer from high cost, low range of usable interrogation distances, or low frequency range. This article proposes a new simple, low-cost, and portable readout design based on the technique of coherent demodulation. A complete theoretical analysis examining how the interrogation distance is related to other circuit parameters is presented. Finally, a complete readout system was implemented using printed circuit board technology and commercially available off-the-shelf components. The system operates between 1 and 100 MHz and the fabricated system consumes 1.26 W. Measurements show that the system operates reliably and repeatably with interrogation distances up to 5 cm.

Yee Jher Chan

Papers

Kirigami‐Enabled, Passive Resonant Sensors for Wireless Deformation Monitoring

Wireless position sensing and normalization of embedded resonant sensors using a resonator array

Effects of fabrication materials and methods on flexible resonant sensor signal quality

Positionally-independent and Extended Read Range Resonant Sensors Applied to Deep Soil Moisture Monitoring

Low-Cost Portable Readout System Design for Inductively Coupled Resonant Sensors