Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

The use of liquid metals based on gallium for soft and stretchable electronics is discussed. This emerging class of electronics is motivated, in part, by the new opportunities that arise from devices that have mechanical properties similar to those encountered in the human experience, such as skin, tissue, textiles, and clothing. These types of electronics (e.g., wearable or implantable electronics, sensors for soft robotics, e-skin) must operate during deformation. Liquid metals are compelling materials for these applications because, in principle, they are infinitely deformable while retaining metallic conductivity. Liquid metals have been used for stretchable wires and interconnects, reconfigurable antennas, soft sensors, self-healing circuits, and conformal electrodes. In contrast to Hg, liquid metals based on gallium have low toxicity and essentially no vapor pressure and are therefore considered safe to handle. Whereas most liquids bead up to minimize surface energy, the presence of a surface oxide on these metals makes it possible to pattern them into useful shapes using a variety of techniques, including fluidic injection and 3D printing. In addition to forming excellent conductors, these metals can be used actively to form memory devices, sensors, and diodes that are completely built from soft materials. The properties of these materials, their applications within soft and stretchable electronics, and future opportunities and challenges are considered.

Stretchable and Soft Electronics using Liquid Metals.

A comparative analysis of various reconfigurable andmultiband antenna concepts is presented. In order to satisfy the requirements for the advanced systems used in modern wireless and radar applications, different multiband and reconfigurable antennas have been proposed and investigated in the past years. In this paper, these design concepts have been classified into three basic approaches: tunable/switchable antenna integration with radio-frequency switching devices, wideband or multiband antenna integration with tunable filters, and array architectures with the same aperture utilized for different operational modes. Examples of each design approach are discussed along with their inherent benefits and challenges.

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Recent Developments in Reconfigurable and Multiband Antenna Technology

Combined advances in material science, mechanical engineering, and electrical engineering form the foundations of thin, soft electronic/optoelectronic platforms that have unique capabilities in wireless monitoring and control of various biological processes in cells, tissues, and organs. Miniaturized, stretchable antennas represent an essential link between such devices and external systems for control, power delivery, data processing, and/or communication. Applications typically involve a demanding set of considerations in performance, size, and stretchability. Some of the most effective strategies rely on unusual materials such as liquid metals, nanowires, and woven textiles or on optimally configured 2D/3D structures such as serpentines and helical coils of conventional materials. In the best cases, the performance metrics of small, stretchable, radio frequency (RF) antennas realized using these strategies compare favorably to those of traditional devices. Examples range from dipole, monopole, and patch antennas for far-field RF operation, to magnetic loop antennas for near-field communication (NFC), where the key parameters include operating frequency, Q factor, radiation pattern, and reflection coefficient S11 across a range of mechanical deformations and cyclic loads. Despite significant progress over the last several years, many challenges and associated research opportunities remain in the development of high-efficiency antennas for biointegrated electronic/optoelectronic systems.

Flexible and Stretchable Antennas for Biointegrated Electronics.

This communication presents a new reconfigurable origami bifilar helical antenna. This antenna can change its operating frequencies by changing its height. Also, analytical equations for the design of such antennas are derived based on an equivalent model of a standard helical antenna. An origami bifilar helical antenna is designed and its performance is verified using simulations and measurements.

An Origami Reconfigurable Axial-Mode Bifilar Helical Antenna

A frequency-reconfigurable microstrip patch antenna is implemented using a stretchable silicone TC5005 substrate. The stretchable patch is fabricated by injecting liquid metal alloy Galinstan into a square reservoir fabricated in the silicone elastomer substrate. An aperture-coupled feeding method is presented to enhance the strain by separating the fixed feed element from the stretched radiating element. The implemented prototype is demonstrated to be stretchable by a strain of up to 300% (and the material has the potential for more). The electrical length of the patch antenna varies with the stretching, and we demonstrate frequency tuning from 1.3 to 3 GHz. A maximum radiation efficiency of 80% is measured for the antenna. The fabrication process of the antenna system and experimental results for impedance and radiation pattern are presented.

A Reconfigurable Patch Antenna Using Liquid Metal Embedded in a Silicone Substrate

Reconfigurable structures based on smart materials offer a potential solution to realize adaptive antennas for emerging communication devices. In this paper, a reconfigurable axial mode helix antenna is studied. A shape memory alloy spring actuator is used to adjust the height of a helix antenna. With the total length of the helix wire fixed, the pitch spacing and pitch angle are varied as the height is varied. This in turn can alter the antenna pattern in order to adjust to altered operating conditions. In order to undertake the design, the Kraus equations for the axial mode helix are compared with simulation results, and their range of applicability is clarified. It is shown that based on these equations, antenna gain variation is possible by varying the height of the antenna, while keeping its conductor length fixed. We then show that a pattern can be reconfigured using a two-helix structure. Finally, a proof-of-concept helix antenna is implemented using a shape memory alloy actuator. Measurement results confirm that the pattern can reconfigure while maintaining a reasonable impedance match.

Reconfigurable Axial-Mode Helix Antennas Using Shape Memory Alloys

Methods are suggested and tested to measure and optimize the wireless energy transfer efficiency for mid-range (10–100cm) inductive coils with relatively low profile using magnetic resonance. These coils can be used to provide energy for wireless sensors and battery-operated devices. It is shown that for every system, a resonance frequency can be identified where the wireless energy transfer efficiency is optimal. Several prototypes are developed and tested as a proof of validity of the proposed technique. It is also shown that by tuning to the optimum resonant frequency and designing proper matching circuitry, an efficiency of about 25% for moderate profiles can be achieved.

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Mid-range wireless energy transfer using inductive resonance for wireless sensors

A pattern reconfigurable ring-patch antenna is presented. It comprises a bendable parasitic plate located in the middle of a fixed square ring. The fixed ring supports the feed, and the plate is effectively a parasitic structure. The plate's bending action is from a bisecting hinge between the rectangular halves (flaps) that can be actuated separately. The actuation is by extending a shape memory alloy spring. The restoration is by decompressing the shape memory spring action using a parallel standard spring. The parasitic plate thus reconfigures with rectangular flaps which bend up, level, and down, relative to the ring plane. As a result of the reconfiguration, the pattern of the antenna is changed while the impedance match is maintained. Simulated and experimental results are presented for the pattern and impedance of a 100 mm × 100 mm square ring patch antenna prototype operating at 1.15 GHz. The effects of some geometric parameters on the pattern and impedance are investigated in order to analyze the pattern changing mechanism. A two-element array of the reconfigurable ring-patch antenna is presented that provides further de-correlation of the patterns.

Square Ring Antenna With Reconfigurable Patch Using Shape Memory Alloy Actuation

Mechanically reconfigurable antennas have the potential to provide a large range of antenna reconfiguration and to have a lower cost than other reconfigurable technologies. Solid state devices in the RF signal path can cause distortion and power limiting, but such issues can be averted by keeping them out of the direct RF signal path. In this paper, new smart material technologies, namely electro-active polymers and shape memory alloy actuators, are presented as potential candidates to implement mechanically reconfigurable antennas. A review of some proof-of-concept reconfigurable antenna prototypes using these technologies is presented, including the advantages and disadvantages for the antenna applications.

Shahrzad Jalali Mazlouman

Papers

A Reconfigurable Patch Antenna Using Liquid Metal Embedded in a Silicone Substrate

Reconfigurable Axial-Mode Helix Antennas Using Shape Memory Alloys

Mid-range wireless energy transfer using inductive resonance for wireless sensors

Square Ring Antenna With Reconfigurable Patch Using Shape Memory Alloy Actuation

A review of mechanically reconfigurable antennas using smart material actuators