Harnessing metamaterials for efficient wireless power transfer for implantable medical devices.

Wireless power transfer (WPT) within the human body can enable long-lasting medical devices but poses notable challenges, including absorption by biological tissues and weak coupling between the transmitter (Tx) and receiver (Rx). In pursuit of more robust and efficient wireless power, various innovative strategies have emerged to optimize power transfer efficiency (PTE). One such groundbreaking approach stems from the incorporation of metamaterials, which have shown the potential to enhance the capabilities of conventional WPT systems. In this review, we delve into recent studies focusing on WPT systems that leverage metamaterials to achieve increased efficiency for implantable medical devices (IMDs) in the electromagnetic paradigm. Alongside a comparative analysis, we also outline current challenges and envision potential avenues for future advancements.

High-Efficiency Wireless-Power-Transfer System Using Fully Rollable Tx/Rx Coils and Metasurface Screen.

This work presents a high-efficiency reconfigurable wireless-power-transfer (WPT) system using fully rollable Tx/Rx coils and a metasurface (MS) screen working at 6.78 MHz, for the first time. The MS screens are placed between the Tx and Rx to magnify the power-transfer efficiency (PTE) of the WPT system. The proposed MS-based WPT can be rolled down or rolled up as required, which allows end-users to use the space more flexibly. In the measurement results, the PTE of the WPT is improved from 13.32% to 32.49% at a power-transfer distance (PTD) of 40 cm with one MS screen, 5.42% to 42.25% at a PTD of 50 cm with two MS screens, 1.78% to 49% at a PTD of 60 cm with three MS screens, 0.85% to 46.24% at a PTD of 70 cm with four MS screens. The measured PTE results indicate that the demonstrated MS screens are greatly effective for magnifying the PTE and the PTD of the WPT. In addition, the measured PTE results in the misaligned condition verify that the MS screens also help increase the PTE of the WPT even in the misalignment condition.

Preparation of a ZnO Nanostructure as the Anode Material Using RF Magnetron Sputtering System.

In this study, a four-inch zinc oxide (ZnO) nanostructure was synthesized using radio frequency (RF) magnetron sputtering to maximize the electrochemical performance of the anode material of a lithium-ion battery. All materials were grown on cleaned p-type silicon (100) wafers with a deposited copper layer inserted at the stage. The chamber of the RF magnetron sputtering system was injected with argon and oxygen gas for the growth of the ZnO films. A hydrogen (H2) reduction process was performed in a plasma enhanced chemical vapor deposition (PECVD) chamber to synthesize the ZnO nanostructure (ZnO NS) through modification of the surface structure of a ZnO film. Field emission scanning electron microscopy and atomic force microscopy were performed to confirm the surface and structural properties of the synthesized ZnO NS, and cyclic voltammetry was used to examine the electrochemical characteristics of the ZnO NS. Based on the Hall measurement, the ZnO NS subjected to H2 reduction had a higher electron mobility and lower resistivity than the ZnO film. The ZnO NS that was subjected to H2 reduction for 5 min and 10 min had average roughness of 3.117 nm and 3.418 nm, respectively.

Growth Properties of Carbon Nanowalls on Nickel and Titanium Interlayers.

This research is conducted in order to investigate the structural and electrical characteristics of carbon nanowalls (CNWs) according to the sputtering time of interlayers. The thin films were deposited through RF magnetron sputtering with a 4-inch target (Ni and Ti) on the glass substrates, and the growth times of the deposition were 5, 10, and 30 min. Then, a microwave plasma-enhanced chemical vapor deposition (PECVD) system was used to grow CNWs on the interlayer-coated glass substrates by using a mixture of H2 and CH4 gases. The FE-SEM analysis of the cross-sectional and planar images confirmed that the thickness of interlayers linearly increased according to the deposition time. Furthermore, CNWs grown on the Ni interlayer were taller and denser than those grown on the Ti interlayer. Hall measurement applied to measure sheet resistance and conductivity confirmed that the electrical efficiency improved significantly as the Ni or Ti interlayers were used. Additionally, UV-Vis spectroscopy was also used to analyze the variations in light transmittance; CNWs synthesized on Ni-coated glass have lower average transmittance than those synthesized on Ti-coated glass. Based on this experiment, it was found that the direct growth of CNW was possible on the metal layer and the CNWs synthesized on Ni interlayers showed outstanding structural and electrical characterizations than the remaining interlayer type.

Metamaterial-Integrated High-Gain Rectenna for RF Sensing and Energy Harvesting Applications.

This paper presents a metamaterial (MTM)-integrated high-gain rectenna for RF sensing and energy harvesting applications that operates at 2.45 GHz, an industry, science, medicine (ISM) band. The novel MTM superstrate approach with a three-layered integration method is firstly introduced for rectenna applications. The integrated rectenna consists of three layers, where the first layer is an MTM superstrate consisting of four-by-four MTM unit cell arrays, the second layer a patch antenna, and the third layer a rectifier circuit. By integrating the MTM superstrate on top of the patch antenna, the gain of the antenna is enhanced, owing to its beam focusing capability of the MTM superstrate. This induces the increase of the captured RF power at the rectifier input, resulting in high-output DC power and high entire end-to-end efficiency. A parametric analysis is performed in order to optimize the near-zero property of the MTM unit cell. In addition, the effects of the number of MTM unit cells on the performance of the integrated rectenna are studied. A prototype MTM-integrated rectenna, which is designed on an RO5880 substrate, is fabricated and characterized. The measured gain of the MTM-integrated rectenna is 11.87 dB. It shows a gain improvement of 6.12 dB compared to a counterpart patch antenna without an MTM superstrate and a maximum RF-DC conversion efficiency of 78.9% at an input RF power of 9 dBm. This results in the improvement of the RF-DC efficiency from 39.2% to 78.9% and the increase of the output DC power from 0.7 mW to 6.27 mW (a factor of 8.96 improvements). The demonstrated MTM-integrated rectenna has shown outstanding performance compared to other previously reported work. We emphasize that the demonstrated MTM-integrated rectenna has a low design complexity compared with other work, as the MTM superstrate layer is integrated on top of the simple patch antenna and rectifier circuit. In addition, the number of MTM units can be determined depending on applications. It is highly envisioned that the demonstrated MTM-integrated rectenna will provide new possibilities for practical energy harvesting applications with improved antenna gain and efficiency in various IoT environments.

Hybrid Electrospun Polycaprolactone Mats Consisting of Nanofibers and Microbeads for Extended Release of Dexamethasone.

PURPOSE: We designed electrospun polycaprolactone mats consisting of nanofibers and microbeads for extended delivery of dexamethasone. METHODS: Thin flexible dexamethasone loaded polycaprolactone mats were prepared by electrospinning. The solvents, polymer loading, voltage and tip-to-collector distance were varied to explore the effects on microstructure of the mats. The microstructure was determined by scanning electron microscope imaging; drug transport was measured and modeled, and X-ray diffraction was used to gauge the crystallinity. Drug transport and X-ray diffraction studies were also conducted with a spin cast film for comparison. RESULTS: Thin mats, about 10 µm in thickness, were prepared by electrospinning. By controlling the voltage and tip-to-collector distance, we achieved a hybrid structure comprising of nanorods (nanofibers) and microbeads. The release profiles were fitted to the diffusion equation to obtain the diffusivities in the spheres and the rods. The diffusivity in the electrospun nanofibers was significantly lower compared to the casted films due to increased crystallinity, which was estimated from X-ray diffraction analysis. The electrospun hybrid mats sustained drug release for the desired duration of a month, in spite of the small thickness of about 10 µm. By comparison, a ten-fold thicker cast film sustains release for about the same duration suggesting about 100-fold decrease in diffusivity in the electrospun mats due to increased crystallinity. CONCLUSIONS: Electrospun polycaprolactone mats are optimal for achieving long release durations due to increased crystallinity. Designing a hybrid structure by controlling the electrospinning parameters can be a useful approach to increase the release durations.

Scale of Carbon Nanomaterials Affects Neural Outgrowth and Adhesion.

Carbon nanomaterials have become increasingly popular microelectrode materials for neuroscience applications. Here we study how the scale of carbon nanotubes and carbon nanofibers affect neural viability, outgrowth, and adhesion. Carbon nanotubes were deposited on glass coverslips via a layer-by-layer method with polyethylenimine (PEI). Carbonized nanofibers were fabricated by electrospinning SU-8 and pyrolyzing the nanofiber depositions. Additional substrates tested were carbonized and SU-8 thin films and SU-8 nanofibers. Surfaces were O2-plasma treated, coated with varying concentrations of PEI, seeded with E18 rat cortical cells, and examined at 3, 4, and 7 days in vitro (DIV). Neural adhesion was examined at 4 DIV utilizing a parallel plate flow chamber. At 3 DIV, neural viability was lower on the nanofiber and thin film depositions treated with higher PEI concentrations which corresponded with significantly higher zeta potentials (surface charge); this significance was drastically higher on the nanofibers suggesting that the nanostructure may collect more PEI molecules, causing increased toxicity. At 7 DIV, significantly higher neurite outgrowth was observed on SU-8 nanofiber substrates with nanofibers a significant fraction of a neuron's size. No differences were detected for carbonized nanofibers or carbon nanotubes. Both carbonized and SU-8 nanofibers had significantly higher cellular adhesion post-flow in comparison to controls whereas the carbon nanotubes were statistically similar to control substrates. These data suggest a neural cell preference for larger-scale nanomaterials with specific surface treatments. These characteristics could be taken advantage of in the future design and fabrication of neural microelectrodes.

Subwavelength direct laser patterning of conductive gold nanostructures by simultaneous photopolymerization and photoreduction.

This article presents a new method for fabricating highly conductive gold nanostructures within a polymeric matrix with subwavelength resolution. The nanostructures are directly written in a gold precursor-doped photoresist using a femtosecond pulsed laser. The laser energy is absorbed by a two-photon dye, which induces simultaneous reduction of gold in the precursor and polymerization of the negative photoresist. This results in gold nanoparticle-doped polymeric lines that exhibit both plasmonic effects, due to the constituent gold nanoparticles, and relatively high conductivity (within an order of magnitude of the bulk metal), due to the high density of particles within these lines. Line widths from 150 to 1000 nm have been achieved with this method. Various optically functional structures have been prepared, and their structural and optical properties have been characterized. The influence of laser intensity and scan speed on feature size have been studied and found to be in agreement with predictions of a mathematical model of the process.

Large-area, near-infrared (IR) photonic crystals with colloidal gold nanoparticles embedding.

A polymeric composite material composed of colloidal gold nanoparticles (<10 nm) and SU8 has been utilized for the fabrication of large-area, high-definition photonic crystal. We have successfully fabricated near-infrared photonic crystal slabs from composite materials using a combination of multiple beam interference lithography and reactive ion etching processes. Doping of colloidal gold nanoparticles into the SU8 photopolymer results in a better definition of structural features and hence in the enhancement of the optical properties of the fabricated photonic crystals. A 2D air hole array of triangular symmetry with a hole-to-hole pitch of approximately 500 nm has been successfully fabricated in a large circular area of 1 cm diameter. Resonant features observed in reflectance spectra of our slabs are found to depend on the exposure time, and can be tuned over a range of near-infrared frequencies.

An electrically active microneedle array for electroporation.

We have designed and fabricated a microneedle array with electrical functionality with the final goal of electroporating skin's epidermal cells to increase their transfection by DNA vaccines. The microneedle array was made of polymethylmethacrylate (PMMA) by micromolding technology from a polydimethylsiloxane (PDMS) mold, followed by metal deposition, patterning using laser ablation, and electrodeposition. This microneedle array possessed sufficient mechanical strength to penetrate human skin in vivo and was also able to electroporate both red blood cells and human prostate cancer cells as an in vitro model to demonstrate cell membrane permeabilization. A computational model to predict the effective volume for electroporation with respect to applied voltages was constructed from finite element simulation. This study demonstrates the mechanical and electrical functionalities of the first MEMS-fabricated microneedle array for electroporation, designed for DNA vaccine delivery.

Double-layer fabrication scheme for large-area polymeric photonic crystal membrane on silicon surface by multibeam interference lithography.

We report on the fabrication of two-dimensional polymeric photonic crystal membranes on the surface of silicon using visible-light multibeam interference lithography. The structures are created by the interference of three beams of a green laser. A polymer buffer layer doped with a Rhodamine B laser dye, interlaid between the lithography layer and the silicon substrate, suppresses the effects of strong reflection and nonradiative absorption of silicon on the interference pattern. Large-area defect-free photonic crystal membranes are experimentally realized on silicon surface.

Tapered conical polymer microneedles fabricated using an integrated lens technique for transdermal drug delivery.

Administration of protein and DNA biotherapeutics is limited by the need for hypodermic injection. Use of micron-scale needles to deliver drugs in a minimally invasive manner provides an attractive alternative, but application of this approach is limited by the need for suitable microneedle designs and fabrication methods. To address this need, this paper presents a conical polymer microneedle design that is fabricated using a novel integrated lens technique and analyzed for its ability to insert into the skin without mechanical failure. Microneedle master structures were fabricated using microlenses etched into a glass substrate that focused light through SU-8 negative epoxy resist to produce sharply tapered structures. Microneedle replicates were fabricated out of biodegradable polymers by micromolding. Because microneedle mechanical properties are critical to their insertion into the skin, we theoretically modeled two failure modes (axial mode and transverse mode), and analytical models were compared with measured data showing general agreement. Guided by this analysis, polymer microneedles were designed and demonstrated to insert to different depths into porcine skin in vitro. "Long" polymer microneedles were also demonstrated in human subjects to insert deeply without failure.

Polymer particle-based micromolding to fabricate novel microstructures.

Conventional micromolding provides rapid and low-cost methods to fabricate polymer microstructures, but has limitations when producing sophisticated designs. To provide more versatile micromolding techniques, we developed methods based on filling micromolds with polymer microparticles, as opposed to polymer melts, to produce microstructures composed of multiple materials, having complex geometries, and made using mild processing conditions. Polymer microparticles of 1 to 30 microm in size were made from PLA, PGA and PLGA using established spray drying and emulsion techniques either with or without encapsulating model drug compounds. These polymer microparticles were filled into PDMS micromolds at room temperature and melted or bonded together to form microstructures according to different protocols. Porous microstructures were fabricated by ultrasonically welding microparticles together in the mold while maintaining the voids inherent in their packing structure. Multi-layered microstructures were fabricated to have different compositions of polymers and encapsulated compounds located in different regions of the microstructures. More complex arrowhead microstructures were fabricated in a two-step process using a single mold. To assess possible applications, microstructures were designed as microneedles for minimally invasive drug delivery. Multi-layer microneedles were shown to insert into cadaver tissue and, according to design, detach from their base substrate and remain embedded in the tissue for controlled release drug delivery over time. We conclude that polymer particle-based micromolding can encapsulate compounds within microstructures composed of multiple materials, having complex geometries, and made using mild processing conditions.

Dynamic sonography of external snapping hip syndrome.

OBJECTIVE: To evaluate the dynamic sonographic findings of external snapping hip syndrome. METHODS: Five patients with 7 cases of painful external snapping hip (3 male and 2 female; age range, 14-32 years; mean, 19 years) were examined with sonography. Two patients had bilateral snapping. Dynamic sonographic examinations of hips were performed with a linear 5- to 12-MHz transducer during hip motion. RESULTS: Dynamic sonographic studies of the affected hip revealed causes of the external snapping hip in all cases. It was elicited by an abnormal jerky movement of the iliotibial band overlying the greater trochanter in 5 of 7 cases and of the gluteus maximus muscle in 2 cases. The iliotibial band over the greater trochanter was hypoechoic in 3 of the 5 cases and thickened in 1 case. Dynamic sonography showed good correlations between the jerky movements of the iliotibial band and the gluteus maximus muscle and the painful snapping reported by the patients. CONCLUSIONS: Dynamic sonography was helpful in the diagnosis of external snapping hip syndrome; it showed real-time images of sudden abnormal displacement of the iliotibial band or the gluteus maximus muscle overlying the greater trochanter as a painful snap during hip motion.

Significance of 99mTc-ECD SPECT in acute and subacute ischemic stroke: comparison with MR images including diffusion and perfusion weighted images.

99mTc-ECD SPECT is valuable for the evaluation of cell viability and function. The purpose of the present study was to evaluate the significance of 99mTc-ECD brain SPECT in ischemic stroke. We compared 99mTc-ECD brain SPECT with perfusion and diffusion weighted images (PWI, DWI). Ten patients with acute and early subacute ischemic stroke were included in this prospective study. T2-weighted images (T2WI), DWI, PWI and 99mTc-ECD SPECT were obtained during both the acute/early subacute and late subacute stages. In the case of PWI, time to peak (TTP) and regional cerebral blood volume (rCBV) maps were obtained. The rCBV map and 99mTc-ECD SPECT images were compared in 8 lesions using DeltaAI. The asymmetry index (AI) was calculated as (Ci - Cc) X 200 / (Ci + Cc); where Ci is the mean number of pixel counts of an ipsilateral lesion and Cc is the mean number of pixel counts of the normal contralateral hemisphere. DeltaAI was defined as AIacute - AIsubacute in the ischemic core and periphery. PWI and 99mTc-ECD SPECT detected new lesions of the hyperacute stage or of evolving stroke more accurately than T2WI and DWI. 99mTc-ECD SPECT was able to localize the infarct core and peri-infarct ischemia in all lesions in both the acute and the subacute stages. DeltaAI was higher in the rCBV map than in the 99mTc-ECD SPECT images in the ischemic core (p = 0.063) and in the periphery (p = 0.091). In the 99mTc-ECD SPECT images, DeltaAI was higher in the ischemic core than in the periphery (p = 0.028). During the subacute stage, 99mTc-ECD SPECT detected all the lesions without the pseudonormalization seen in the MR images of 5/11 lesions. Based on this study, 99mTc-ECD SPECT is comparable to PWI in terms of its ability to detect acute stroke and is more useful than PWI in the case of subacute infarction.