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.

Reconfigurable antennas, with the ability to radiate more than one pattern at different frequencies and polarizations, are necessary in modern telecommunication systems. The requirements for increased functionality (e.g., direction finding, beam steering, radar, control, and command) within a confined volume place a greater burden on today's transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. This paper discusses the different reconfigurable components that can be used in an antenna to modify its structure and function. These reconfiguration techniques are either based on the integration of radio-frequency microelectromechanical systems (RF-MEMS), PIN diodes, varactors, photoconductive elements, or on the physical alteration of the antenna radiating structure, or on the use of smart materials such as ferrites and liquid crystals. Various activation mechanisms that can be used in each different reconfigurable implementation to achieve optimum performance are presented and discussed. Several examples of reconfigurable antennas for both terrestrial and space applications are highlighted, such as cognitive radio, multiple-input-multiple-output (MIMO) systems, and satellite communication.

Reconfigurable Antennas for Wireless and Space Applications

Millimeter wave (mmWave) communication has raised increasing attentions from both academia and industry due to its exceptional advantages. Compared with existing wireless communication techniques, such as WiFi and 4G, mmWave communications adopt much higher carrier frequencies and thus come with advantages including huge bandwidth, narrow beam, high transmission quality, and strong detection ability. These advantages can well address difficult situations caused by recent popular applications using wireless technologies. For example, mmWave communications can significantly alleviate the skyrocketing traffic demand of wireless communication from video streaming. Meanwhile, mmWave communications have several natural disadvantages, e.g., severe signal attenuation, easily blocked by obstacles, and small coverage, due to its short wavelengths. Hence, the major challenge is how to overcome its shortcomings while fully utilizing its advantages. In this paper, we present a taxonomy based on the layered model and give an extensive review on mmWave communications. Specially, we divide existing efforts into four categories that investigate: physical layer, medium access control (MAC) layer, network layer, and cross layer optimization, respectively. First, we present an overview of some technical details in physical layer. Second, we summarize available literature in MAC layer that pertains to protocols and scheduling schemes. Third, we make an in-depth survey of related research work in network layer, providing brain storming and methodology for enhancing the capacity and coverage of mmWave networks. Fourth, we analyze available research work related to cross layer allocation/optimization for mmWave communications. Fifth, we make a review of mmWave applications to illustrate how mmWave technology can be employed to satisfy other services. At the end of each section described above, we point out the inadequacy of existing work and identify the future work. Sixth, we present some available resources for mmWave communications, including related books about mmWave, commonly used mmWave frequencies, existing protocols based on mmWave, and experimental platforms. Finally, we have a simple summary and point out several promising future research directions.

Millimeter Wave Communication: A Comprehensive Survey

The aggressive scaling of the CMOS technology and the convergence of CMOS and MEMS technologies along the “More Than Moore” axis of the International Technology Roadmap for Semiconductors, paves the way to the realization of millimeter-waves front-end circuits with integrated three-dimensional antennas. In this work we present a short conical helical antenna built applying MEMS micro-fabrication techniques whose dimensions can scale down to 100 μm (for operation in the THz region) and that can be integrated on silicon chips for communication and imaging purpose. Antenna design rules and methodology, electromagnetic characteristics and actual measurement are presented.

U-Helix: On-chip short conical 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

A pattern reconfigurable square ring patch antenna comprising a movable parasitic plate within the ring patch is presented. The plate is actuated using a hemispherical dielectric elastomer actuator. Small variations of the height of the parasitic plate (i.e. below, level, or above the ring patch) can significantly alter the antenna radiation pattern while maintaining reasonable impedance matching. Experimental results are presented for the radiation pattern and the return loss for different parasitic plate positions.

Pattern reconfigurable square ring patch antenna actuated by hemispherical dielectric elastomer

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 partially-transparent antenna is described that may be utilized to wirelessly transmit or receive data. In one or more implementations, an apparatus includes an antenna and one or more modules implemented at least partially in hardware. The antenna is formed from a material that is at least partially transparent. The one or more modules are communicatively coupled to the antenna to use the antenna to wirelessly transmit or receive data.

Alireza Mahanfar

Papers

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

Reconfigurable Axial-Mode Helix Antennas Using Shape Memory Alloys

Pattern reconfigurable square ring patch antenna actuated by hemispherical dielectric elastomer

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

Partially Transparent Antenna