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May 1, 2012 - Index Terms—EGaIn antenna, flexible microstrip patch an- tenna, liquid metal ... dium 25% (EGaIn), have been utilized for dipole antennas.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 5, MAY 2012

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Flexible Liquid Metal Alloy (EGaIn) Microstrip Patch Antenna Gerard J. Hayes, Member, IEEE, Ju-Hee So, Amit Qusba, Michael D. Dickey, and Gianluca Lazzi, Fellow, IEEE

Abstract—This paper describes a flexible microstrip patch antenna that incorporates a novel multi-layer construction consisting of a liquid metal (eutectic gallium indium) encased in an elastomer. The combined properties of the fluid and the elastomeric substrate result in a flexible and durable antenna that is well suited for conformal antenna applications. Injecting the metal into microfluidic channels provides a simple way to define the shape of the liquid, which is stabilized mechanically by a thin oxide skin that forms spontaneously on its surface. This approach has proven sufficient for forming simple, single layer antenna geometries, such as dipoles. More complex fluidic antennas, particularly those featuring large, co-planar sheet-like geometries, require additional design considerations to achieve the desired shape of the metal. Here, a multi-layer patch antenna is fabricated using specially designed serpentine channels that take advantage of the unique rheological properties of the liquid metal alloy. The flexibility of the resulting antennas is demonstrated and the antenna parameters are characterized through simulation and measurement in both the relaxed and flexed states. Index Terms—EGaIn antenna, flexible microstrip patch antenna, liquid metal antenna, multi-layer, polydimethylsiloxane (PDMS).

I. INTRODUCTION IQUID metal alloys, such as eutectic gallium 75%—indium 25% (EGaIn), have been utilized for dipole antennas [1] and single-layer antennas with co-planar ground planes formed through a series of meshed micro-fluidic channels [2]. Injecting the liquid metal into a single layer of microchannels in an elastomeric substrate (e.g., polydimethylsiloxane, PDMS) has proven to be a simple method of fabricating these radiating structures. Unlike conventional antennas composed of rigid solid metal components, these fluidic antennas adopt the mechanical properties of the encasing material (e.g., PDMS) and are therefore flexible and mechanically durable. Fig. 1 demonstrates the flexibility of this emerging class of antennas. These properties are important for conformal antennas, which are motivated by the new applications that emerge when electronics can be integrated into flexible substrates, such as textiles

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Manuscript received October 28, 2010; revised September 13, 2011; accepted November 10, 2011. Date of publication April 12, 2012; date of current version May 01, 2012. This work was funded by NSF Award ECCS-0925797. G. J. Hayes and A. Qusba are with the Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, NC 27695 USA (e-mail: [email protected]; [email protected]). J.-H. So and M. D. Dickey are with the Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695 USA (e-mail: [email protected]; [email protected]). G. Lazzi is with the Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2012.2189698

Fig. 1. Photographs of a multi-layer, flexible microstrip patch antenna composed of liquid metal encased in elastomer.

or electronic paper. There are, however, challenges associated with shaping the fluid metal into more complex geometries that go beyond simple dipoles (such as the co-planar, sheet-like geometry of a patch or aperture tocoupled slot antenna) due to non-uniform filling of wide microchannels. Here, we describe a novel fabrication procedure that harnesses the unusual rheological properties of EGaIn to create multi-layer antenna structures with non-meshed, broad-area components composed entirely of the liquid metal alloy embedded in elastomer. We demonstrate the flexibility of these structures and characterize and simulate their spectral properties both in the native (i.e., relaxed) and the flexed state. II. BACKGROUND AND MOTIVATION Flexible and conformal antennas have mechanical properties that allow them to be used in applications such as wearable electronics and implantable medical devices [3]–[5]. A conformal antenna must be able to bend or flex in concert with its local environment. When subjected to flexing, conventional metallic structures can crease, fatigue, and eventually fail mechanically with a catastrophic loss of conductivity and functionality. Unlike solid metals, liquid metals and liquid metal alloys (such as EGaIn) that are encapsulated in an elastomeric substrate (such as PDMS) can flow in response to stress and are therefore not prone to fatigue or cracking. Furthermore, these materials have been shown to ‘self-heal’ in response to sharp cuts through the metal due to the fluid nature of the metal [1]. Injecting liquid metal into microfluidic channels offers a simple route to shaping the metal into useful structures, such as antennas. PDMS is often used as an encasing material for the metal because it is commercially available, easy to process, and elastomeric [6], [7]. Because the metal is a low viscosity liquid (similar to water), the antennas adopt the mechanical properties of the PDMS microchannels; consequently, the antennas return to their original shape after being deformed. Although the

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Fig. 2. Photograph of a partially filled patch antenna taken as the liquid metal fills the antenna in a serpentine fashion to ensure uniform and complete filling of the patch geometry. (right) A close-up micrograph of the leading edge of the liquid.

melting point of EGaIn is 15.5 , it can be supercooled significantly and can therefore stay in the liquid state well below its melting point [8]. Mercury, a common liquid metal, is not well suited for microfluidics because it tends to bead up to minimize its surface energy and therefore withdraws spontaneously from the channels. Unlike mercury, EGaIn forms a thin oxide skin that provides mechanical stability to the liquid after it is injected into microchannels [9]. It is straightforward to fill microfluidic channels with simple geometries, such as those used to make dipole antennas. In our experience, it is significantly more challenging to shape the metal into channels with low-aspect ratios (height: width) because of the tendency of the metal to fill unevenly or trap air bubbles. Here, we show it is possible to fill these geometries with liquid metal by using specially designed microfluidic channels that take advantage of the rheological properties of the metal. As a demonstration, we have chosen microstrip patch antennas because it is particularly challenging to use microfluidics to shape the liquid metal into the requisite sheet-like, co-planar geometry. III. FABRICATION OF LIQUID METAL PATCH ANTENNAS The microstrip patch antenna consists of two parallel conductive planes (i.e., the radiating element and ground plane) separated by a thin film of PDMS. We used soft lithographic processes to fabricate two PDMS microchannels that define the shape of the radiating element and ground plane [1]. For each layer of the antenna, photolithography defined a “master” pattern into a film of negative photoresist (SU-8, Microchem) coated on a silicon wafer. Curing the PDMS pre-polymer against this topographically patterned substrate (i.e., replica molding) produced an inverse replica of the master. We used 4 mm thick PDMS to facilitate handling, although thinner layers could also be used to improve the flexibility. Sealing this layer of elastomer against another thin, flat sheet of elastomer (i.e., the spacer layer) produced microfluidic channels. A syringe injected the liquid metal into an inlet hole punched in the PDMS (Fig. 2) to fill the microfluidic channels that define the radiating element. We repeated this process to produce the ground plane and sealed it to the radiating element to complete the antenna. A pair of copper tabs immersed in the EGaIn provided electrical connectivity to an external SMA connector for testing. Antennas with radiating elements that have narrow cross sections (such as a dipole) have proven to be straight forward to fabricate reliably by simply injecting the metal into microchannels

Fig. 3. Photograph of a complete, evenly filled patch antenna and ground plane. (left) A close-up micrograph of a representative region of the radiating element shows the PDMS posts and the evenly filled liquid metal.

shaped with the desired geometry. In contrast, the conductive elements (i.e., radiating element and ground layer) of the patch antenna resemble a large , flat rectangle. Channels with cross-sections that have small aspect ratios (i.e., a small height relative to the width, or 1:500 in this design) present at least two challenges. First, the channels tend to collapse because of the low modulus of the PDMS, which is an important attribute of the polymer that allows it to flex easily. Second, we found it challenging to inject the metal uniformly into low aspect ratio channels without creating pockets of air bubbles or uneven filling. This problem is exacerbated by the critical yield stress behavior of EGaIn; that is, it flows readily only when the pressure is large enough to rupture the skin and therefore does not necessarily flow evenly in every direction. We overcame these challenges by designing microchannel geometries that guide the metal to fill the wide, rectangular geometry of the patch antenna uniformly by taking advantage of the rheological properties of EGaIn. The ability to inject EGaIn into a microfluidic channel depends on the pressure applied to the metal at the inlet of the channel. The minimum pressure required to induce flow, (critical pressure), is inversely proportional to the cross-sectional dimensions of the channel (width, W, and height, H) according to the following equation based on Young-Laplace equation

We designed a serpentine pathway of posts with constant height (H, 100 ) in which the distance between two neighboring posts ( , 100 ) is much smaller than the width of the serpentine channel ( , 1000 ). The critical pressure reis therefore quired to inject the metal through the channel less than that required to inject it through the posts . In this implementation, the ratio of the two critical pressures is approximately 2:1; that is, it takes nearly twice as much pressure to force the metal between the posts than through the channels. The liquid metal therefore flows selectively through the serpentine-shaped channel (rather than between the posts) and thereby fills the entire area of the antenna, as shown in Fig. 2. The posts also prevent the channel from collapsing. Once the metal fills the serpentine pathway completely, the pressure is increased (e.g., by simply pressing on the top of the antenna by hand) to force the metal between the posts to merge, as shown in Fig. 3. We used a commercially available Finite Integration Technique (FIT) to assess the impact of the periodic PDMS posts on antenna performance by comparing antenna models with

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and without the posts. The results showed no significant differences between the two simulation models for the frequency range from 3.0 to 4.0 GHz. Measurements of the prototype antenna further confirmed the simulation results. We designed the multi-layer, microstrip patch antenna to operate at a center frequency, , using the following dimensions:

The spacer thickness represents the separation between the radiating element and the ground plane. The microstrip feed is also 1 mm above the ground plane. IV. ANALYSIS AND SIMULATION Dipole and loop antennas composed of liquid metal have been shown to have high overall efficiencies ( 90%) at microwave frequencies [1], [2] despite the relatively low conductivity of EGaIn (compared to copper) and the relatively high loss tangent of PDMS (compared to Teflon or Duroid). The measured radiation efficiencies for these antennas, however, cannot be extended readily to other classes of high-Q factor antennas, such as multi-layered microstrip patch antennas and telemetry coils in which electric field densities underneath or within the radiating structure are relatively high. The presence of additional material losses within the high electric field density regions of these structures reduces the radiation efficiency when compared to equivalent antennas that are fabricated using lower loss microwave materials, such as copper on Duroid or copper on alumina. Simulation and measurement provide a more complete characterization of the loss mechanisms (with respect to radiation efficiency) of microstrip patch antennas composed of liquid metal. This class of fluidic antennas has two primary loss mechanisms: (i) EGaIn has a lower conductivity compared to copper [1], and (ii) PDMS (relative permittivity of to 3.00) has a reported loss tangent [2] at 100 kHz. Using a coaxial dielectric probe (Agilent Technologies, Model 85070E), we determined that our PDMS samples had a relative permittivity of and a loss tangent, at 3.45 GHz. The measured permittivity ranged from to 3.0 and the measured loss tangent ranged from to 0.05 over the frequency range of 1.0 GHz to 5.0 GHz. To characterize the loss mechanisms, we simulated four combinations of conductive materials and dielectric substrates that span from ideal material properties to realistic material properties. For the metal components (i.e. radiating element and ground plane), we considered both EGaIn and perfectly electrically conductive (PEC) materials. Similarly, for the substrate, we evaluated both PDMS with loss and ideal, lossless materials. All four of the simulated combinations of materials produced identical radiation patterns with a numerically calculated directivity (neglecting impedance mismatch) of 6.46 dBi. Fig. 4

Fig. 4. Simulated radiation patterns for PEC/Lossless-PDMS microstrip patch antenna configuration: Elevation Plane Pattern (i) and Azimuth Plane Pattern (ii).

TABLE I COMPARISON OF SIMULATED RADIATION EFFICIENCY

shows representative radiation patterns of the azimuth and elevation planes for the ideal (lossless) configuration. Table I is a summary of the simulation results for the reflection coefficient and radiation efficiency, which excludes losses due to impedance mismatch. The loss due to the PDMS substrate degrades the radiation efficiency by 1.02 to 1.40 dB when compared to the lossless configurations. Similarly, the loss due to the EGaIn material degrades the radiation efficiency by 0.97 to 1.35 dB when compared to the PEC configurations. As mentioned previously, these effects have not been significant on previously reported dipole and monopole antennas [1], [2] which have a much lower Q factor.

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Fig. 5. Measured (solid) and simulated (dotted) reflection coefficient for the EGaIn/PDMS microstrip patch antenna.

Fig. 7. Measured (solid) and simulated (dotted) radiation efficiency for the EGaIn/PDMS microstrip patch antenna excluding impedance mismatch.

Fig. 6. Measured (solid) and simulated (dotted) total efficiency for the EGaIn/ PDMS microstrip patch antenna including impedance mismatch.

The microstrip antenna requires impedance matching to improve the return loss from 3.79 dB in the ideal case. As seen in Table I, the reflection coefficient improves to 6.56 dB due to the additional losses attributed to the PDMS substrate and the EGaIn metal. V. MEASURED RESULTS We measured the antenna properties of the microstrip patch and compared the results to simulations. Fig. 5 compares the measured frequency response of the refection coefficient from 3.0 GHz to 4.0 GHz with the simulated response for the full-loss configuration. Fig. 6 compares the simulated total efficiency (including losses due to impedance mismatch) to the measured total efficiency over the frequency range from 3.36 GHz to 4.36 GHz. Fig. 7 compares the simulated radiation efficiency (excluding mismatch) to the measured radiation efficiency (adjusted for mismatch) over the same frequency range. Fig. 8 compares the simulated elevation plane pattern (realized gain) to the measured elevation plane pattern at 3.45 GHz. Figs. 5 through 8 show that the measured results agree very well with the simulations. The differences between mea-

Fig. 8. Measured (solid) and simulated (dotted) radiation patterns (realized gain) for the EGaIn/PDMS microstrip patch antenna.

surement and simulation are well within the measurement uncertainty. The efficiency requirements for many conventional patch antenna applications are typically 72–91% [10]–[12]. Neglecting impedance mismatch losses, the radiation efficiency of the EGaIn/PDMS microstrip patch antenna prototype is 60%. For a flexible, conformal application, this efficiency may be acceptable. An elastomer with a lower loss tangent could further improve the efficiency. To demonstrate the conformal mechanical capabilities of the flexible patch antenna, we evaluated a second prototype with a thicker (2 mm) microstrip feed formed by the copper tab near the SMA connector to improve the impedance mismatch and measured its frequency response under three conditions: static (flat and relaxed), curved around a low dielectric mandrel with a radius of 12.7 mm and curved around a similar mandrel with a radius of 25.4 cm. Fig. 9 illustrates these three configurations. After measuring the reflection coefficient for each state, we again measured the response in the static state (flat and relaxed).

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Fig. 9. Cross-sectional view of the conformal configurations: (a) static (flat, relaxed), (b) curved to a 25.4 mm radius, and (c) curved to a 12.7 mm radius. The antenna feed is indicated by the red triangle. Fig. 11. Simulated reflection coefficient of the EGaIn/PDMS microstrip patch antenna when flat (solid) and flexed under two conditions (dotted and dashed).

Fig. 10. Measured reflection coefficient of the EGaIn/PDMS microstrip patch antenna when flat (solid) and flexed under two conditions (dotted and dashed).

After deformation, the return loss of the antenna returned to its initial (i.e., pre-deformation) state without any signs of hysteresis. The resonant frequency remained essentially constant under all of the conditions of flexure. Fig. 10 presents the measured frequency response of the refection coefficient for each configuration. We simulated the antenna configurations to further analyze the performance when flexed. Fig. 11 presents the simulated frequency responses of the refection coefficient for each condition. The simulations agreed very well with the measurements. Fig. 12 presents the simulated antenna patterns for each of the conditions. The radiation pattern remains relatively unaffected by the curvature of the antenna. VI. CONCLUSIONS We demonstrated a novel, multi-layer microstrip patch antenna composed of liquid metal that is mechanically flexible. Injecting the metal into a microchannel featuring a serpentine pathway of posts forced the metal into the shape of the microstrip patch, which is a low-aspect ratio, rectangular geometry that is otherwise difficult to fill uniformly. The performance of the resulting flexible antennas did not vary significantly during flexing and showed no hysteretic behavior.

Fig. 12. Simulated radiation patterns (realized gain) for the EGaIn/PDMS microstrip patch antenna when flat (solid), curved to a 25.4 mm radius (dotted), and curved to a 12.7 mm radius (dashed).

Unlike previously published dipole and monopole antenna applications, the losses from the PDMS substrate and conductor losses can dominate for high-Q factor antenna applications. Based on the results of this investigation, a multi-layer EGaIn/PDMS structure is achievable and may provide acceptable performance for many flexible and conformal antenna applications that require large surface areas of conductive materials (such as microstrip patch and aperture coupled slot antennas). Significant improvements to radiation efficiency can be realized by investigating alternative flexible substrates with improved loss tangents and alternative metal alloys with improved conductivity. In this application, we have presented microfluidic channels that are inherently planar (2D). More complex, 3D structures with interconnecting vias should be possible using this approach by carefully aligning multiple microfluidic channels. We demonstrated that the patch antennas could be flexed without significant change in performance. Because the antennas are built in PDMS (an elastomer), it should also be

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possible to stretch the antennas and alter the reflection coefficient depending on the amount of inter-layer spatial deformation. ACKNOWLEDGMENT The authors thank K. Rutkowski and Satimo in Kennesaw, GA for providing the farfield antenna pattern measurements of the prototype patch antenna. REFERENCES [1] J.-H. So, J. Thelen, A. Qusba, G. J. Hayes, G. Lazzi, and M. D. Dickey, “Reversibly deformable and mechanically tunable fluidic antennas,” Adv. Funct. Mater., vol. 19, pp. 3632–3637, 2009. [2] S. Cheng, A. Rydberg, K. Hjort, and Z. Wu, “Liquid metal stretchable unbalanced loop antenna,” Appl. Phys. Lett., vol. 94, p. 144103, 2009. [3] J. C. G. Matthews and G. Pettitt, “Development of flexible, wearable antennas,” presented at the Antennas and Propagation, EuCAP, 2009. [4] J. C. G. Matthews et al., “Wide-band body wearable antennas,” presented at the Institute of Engineering and Technology Seminar on Wideband, Multiband Arrays for Defence or Civil Applications, Mar. 2008. [5] L. Yang, L. Martin, D. Staiculescu, C. P. Wong, and M. M. Tentzeris, “Conformal magnetic composite RFID for wearableRF and bio-monitoring applications,” IEEE Trans. Microw. Theory Tech., vol. 56, no. 12, pp. 3223–3229, Dec. 2008. [6] H. Cong and T. Pan, “Microfabrication of conductive PDMS on flexible substrates for biomedical applications,” in Proc. 4th IEEE Int. Conf. on Nano/Micro Engineered and Molecular Systems, Jan. 5–8, 2009, pp. 731–734. [7] S. Hage-Ali, N. Tiercelin, P. Coquet, R. Sauleau, V. Preobrazhensky, and P. Pernod, “Millimeter-wave patch array antenna on ultra flexible micromachined polydimethylsiloxane (PDMS) substrate,” presented at the Antennas and Propagation Society Int. Symp., APSURSI’09, Jun. 2009. [8] I. A. Sheka, I. S. Chaus, and T. T. Mitiureva, The Chemistry of Gallium. New York: Elsevier, 1966, p. 302. [9] M. D. Dickey, R. C. Chiechi, R. J. Larsen, E. A. Weiss, D. A. Weitz, and G. M. Whitesides, “Eutectic gallium-indium (EGaIn): A liquid metal alloy for the formation of stable structures in microchannels at room temperature,” Adv. Funct. Mater., vol. 18, pp. 1097–1104, 2008. [10] T. Milligan, “Bandwidth and efficiency of a microstrip patch antenna,” in Proc. Antennas and Propagation Society Int. Symp., Jun. 1980, vol. 18, pp. 585–588. [11] A. Bhattacharyya and R. Garg, “Effect of substrate on the efficiency of an arbitrarily shaped microstrip patch antenna,” IEEE Trans. Antennas Propag., vol. 34, no. 10, pp. 1181–1188, Oct. 1986. [12] D. M. Pozar and B. Kaufman, “Comparison of three methods for the measurement of printed antenna efficiency,” IEEE Trans. Antennas Propag., vol. 36, no. 1, pp. 136–139, Jan. 1988.

Gerard J. Hayes (M’90) received the B.S. degree in electrical engineering (1987) from North Carolina State University, Raleigh, the M.S. degree in electrical engineering (1989) from the Ohio State University, and the M.S. degree in applied mathematics from North Carolina State University (1993), were he is currently working toward the doctorate. His research interests include antenna design, computational electromagnetism, bioelectromagnetics, and microwave circuit design.

Ju-Hee So received the B.S. (2004) and M.S. (2006) degrees in chemical engineering from Seoul National University, Seoul, South Korea. Currently, she is working toward the doctorate at North Carolina State University, Raleigh. Her research interests include nano-and micro fabrication, soft matter electronics, microfluidics, surface chemistry and rheology.

Amit Qusba received the B.Tech. degree in electronics and communication engineering from the Indian Institute of Technology, Guwahati, India, in 2006. Currently, he is working toward the doctorate at North Carolina State University, Raleigh. His research interests include mixed-signal circuit design, computational electromagnetism, bioelectromagnetics, digital communications.

Michael D. Dickey received the B.S. degree in chemical engineering (1999) from the Georgia Institute of Technology and the Ph.D. degree in chemical engineering from the University of Texas (2006) under his advisor, Prof. C. Grant Willson. He was a Postdoctoral Fellow in the laboratory of Professor George M. Whitesides at Harvard University (2006–2008). He is currently an Assistant Professor in chemical and biomolecular engineering at NC State. His research interests include unconventional nanofabrication, micro-and nanotechnology, and materials science. Prof. Dickey received the NSF CAREER award in 2010, Sigma Xi Faculty Research Award in 2011, and the NCSU Outstanding Teacher Award in 2012.

Gianluca Lazzi (S’94–M’95–SM’99–F’08) received the Dr.Eng. degree in electronics from the University of Rome “LaSapienza,” Rome, Italy, in 1994, and the Ph.D. degree in electrical engineering from the University of Utah, Salt Lake City, in 1998. He is currently a USTAR Professor and Department Chair at the Department of Electrical and Computer Engineering, The University of Utah, Salt Lake City. Since 2007, he was a Professor with the Department of Electrical and Computer Engineering, North Carolina State University (NCSU), Raleigh, where he was an Assistant Professor from 1999 to 2003 and an Associate Professor from 2003 to 2006. He has been a Visiting Researcher with the Italian National Board for New Technologies, Energy, and Environment (ENEA) (1994), a Visiting Researcher with the University of Rome “La Sapienza” (1994–1995), and a Research Associate (1995–1998) and Research Assistant Professor (1998–1999) with the University of Utah. He has authored or coauthored over 100 international journal papers or conference presentations on implantable devices, medical applications of electromagnetic fields, antenna design, FDTD modeling, dosimetry, and bioelectromagnetics. Dr. Lazzi was the Chair of Commission K (Electromagnetics in Biology and Medicine) of the U.S. National Committee of the International Union of Radio Science (URSI) (2006–2008). He was the recipient of the 1996 Curtis Carl Johnson Memorial Award for the best student paper presented at the 18th Annual Technical Meeting of the Bioelectromagnetics Society (BEMS), a 1996 International Union of Radio Science (URSI) Young Scientist Award, a 2001 Whitaker Foundation Biomedical Engineering Grant for Young Investigators, a 2001 National Science Foundation (NSF) CAREER Award, a 2003 NCSU Outstanding Teacher Award, the 2003 NCSU Alumni Outstanding Teacher Award, the 2003 ALCOA Foundation Engineering Research Award, the 2006 H.A. Wheeler award from the IEEE Antennas and Propagation Society for the best application paper published in IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION in 2005, and a 2008 best paper award at the IEEE conference GlobeCom, and the 2009 ALCOA Foundation Distinguished Engineering Research Award. He has been an Associate Editor for the IEEE ANTENNAS AND PROPAGATION LETTERS (2001–2007) and served as a Guest Editor for the Special Issue on Biological Effects and Medical Applications of RF/Microwaves of the IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES in 2004. Since January 2008, he has been the Editor-in-Chief of IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS.