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Liquid metal in stretchable tubes: A wearable 4-channel knee array1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland, 2Institute for Electronics, ETH Zurich, Zurich, Switzerland
Synopsis
MR detectors that conform to individual patients and anatomy are a long-standing goal dating back to the 1980s. Rigid-adjustable and flexible coil designs have been proposed. The highest degree of wearability and adaptation is arguably achieved with stretchable coils. In this work, we present highly stretchable and lightweight receive coils realized by containing eutectic Gallium Indium liquid metal in silicone tubes. Particular emphasis is placed on assessing stretching behavior in terms of Q, SNR and integrity of liquid metal containment. Practical utility is explored with a 4-channel array implementation used to image flexion of the human knee at 3T.
Introduction
MR detectors that conform to individual patients and anatomy are a long-standing goal dating back to the 1980s. Rigid-adjustable1,2,3 and flexible4-10 coil designs have been proposed. The highest degree of wearability and adaptation is arguably achieved with stretchable coils. One type of stretchable designs is based on braided11 or meandering12 copper wire, which achieves good overall conductance but requires carefully balanced, reproducible restoring forces exerted by the coil substrate. In another design, a stretchable RF conductor has been formed by liquid metal (eutectic Gallium Indium, eGaIn), stencil-printed onto neoprene foam13. Used to form thin metal layers, this approach has accomplished somewhat lower Q values so far. It also poses remaining challenges in terms of containing and sealing the eGaIn phase.
In the present work, we explore an alternative liquid metal implementation that aims to reconcile high conductance with a simple sealing solution and large stretching range. Detector coils are formed by containing eGaIn in polymer tubes as previously proposed for forming flexible conductors4,5,14. To deploy this concept for a stretchable coil design, thin, highly elastic silicone tube with suitable strain-volume characteristic is employed. Particular emphasis is placed on assessing stretching behavior in terms of Q, SNR and integrity of liquid metal containment. Practical utility is explored with a 4-channel array implementation used to image flexion of the human knee at 3T.
Methods
Liquid Metal Coil
eGaIn is liquid at temperatures above 15.7°C/60.3°F, has low vapor pressure and low toxicity15. Its resistivity is ρeGaIn=29.4·10-8 Ωm - 17 times higher than that of copper ρCu=1.68·10-8 Ωm. However, at 128 MHz, the skin depth of eGaIn is 24μm – four times larger compared to 6μm for copper. Therefore, the effective resistance of an eGaIn conductor is only 4-fold4. To contain the eGaIn liquid metal, thin silicone tube with stretchability up to 500% was chosen (Fig. 1). At a Poisson ratio of 0.5, silicone ensures isovolumetric behavior under strain, which results in favorable pressure conditions for containing liquid in sealed tubes16. Electrical contacts are formed by inserting copper litz wire into the end of the silicone tube (Fig. 1). Unlike other metals copper is not subject to corrosion or oxidation in contact with eGaIn15. For sealing instant adhesive and heat shrink tube are applied. Q and resonance frequency were evaluated under stretch (Fig. 2).
Array Construction
To form a stretchable array, four 0.8mm tube coils were sewn onto a common knee bandage (Fig. 3). Coil overlap and high-Z preamplifiers serve for approximate geometric and preamplifier decoupling. A π-matching network provides sufficient bandwidth to accommodate resonance frequency shifts under stretch1.
Imaging
Images were obtained on a Philips 3T Ingenia system with full Fourier encoding and Roemer combination17. SNR maps and noise correlation1 were calculated for phantoms of three different diameters (Fig. 4). In-vivo knee images were acquired at two different flexion angles (Fig 5).
Results
The coils’ unloaded Q was assessed at 62 to 164 for increasing tube diameter (Fig. 2). Offering most stable Q under strain (116-130), the 0.8mm tube was selected for the rest of the study. Loading by placement on a volunteer’s thigh yielded a Qunloaded/Qloaded ≈ 10 - well sufficient for SNR-efficient detection. Q values were rechecked after several weeks with no degradation found, indicating no performance limitations through corrosion or oxidation. Coil resonance frequencies decreased by 28% for maximum stretch, as expected due to increasing inductance (Fig. 2). Figure 4 shows SNR maps derived from phantom imaging with an SPGR sequence, confirming consistent performance under strain, with some SNR penalty at large diameter as expected from Biot-Savart’s law. Noise correlation coefficients show little variation indicating well-maintained decoupling (Fig. 4). In-vivo images confirm sensitivity and coverage at different flexion angles (Fig. 5).Discussion/Conclusion
According to the results of this study, receiver coils formed by eGaIn contained in silicone tubes are feasible and offer ample stretchability without any significant expense in sensitivity. In particular, the proposed tube coils achieve adequate mechanical and electrical robustness as well as sealing performance. They do not appear to suffer from chemical deterioration, such as oxidation and corrosion. Besides stretchability, the tube solution is also extremely flexible and lightweight, which makes it highly suitable for coil frontends in wearable detection18-22. A potential limitation of the current implementation is NMR signal from silicone, which will be depicted in MR images. When of concern, silicone could be either deuterated or replaced by fluoroelastomers, of which many varieties are available, albeit with lower stretchability. Resonance frequency shifts and variable loading, managed here by π-matching, could be addressed more fully by automatic tuning and matching systems23,24.
Acknowledgements
No acknowledgement found.References
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Figures
Figure 3: Four segmented coils of 13.5 cm diameter are made from 0.8 mm inner diameter silicone tube and attached to a knee bandage by sewing. a) The assembled coil array is attached to a phantom containing copper sulfate solution (770 mg/L CuSO4·5(H2O), 2000 mg/L NaCl). b) For in-vivo imaging the array is put on like a piece of clothing. The size of the array was selected to fit average knee sizes.
