Micro-scale inductively coupled radiofrequency resonators on fluidic platforms for wireless nuclear magnetic resonance spectroscopy

Introduction Micro-scale inductively coupled rf resonators enable wireless investigation of small sample volumes in a conventional NMR spectrometer. The inductively coupled approach has the advantage of focussing the sensitivity and rf power on the small sample volume, without the need for any connections to the spectrometer. Preceding research demonstrated that inductively coupled coils can indeed rival the performance of directly connected ones [1]. The achieved resolution is currently about 2Hz, limited by B0 inhomogeneity. The hitherto fabricated microcoils were composed of a planar inductor coil and an integrated tuning capacitor, which required an out-of-plane connection (wire bonding). We also present planar resonators that are tuned to selfresonate at the rf frequency of 600MHz and thus spare the out-of-plane connection. That can be accomplished by adapting the resonator geometry (spacing and number of turns) respectively. Their B0 homogeneity can be improved by filling the coil area homogenously with metal tracks, thus avoiding susceptibility mismatches at the coil surface. We investigated the influence of Eddy currents on the performance of those filled coils. Fabrication Conveniently, the fabrication includes mainly standard MEMS processes. The microfluidic sample chambers are produced by conventional lithography and glass etch. A second glass substrate covers the fluidic network, assembled by thermal fusion bonding (see Fig. 1). The spiral gold tracks are patterned on the sample chamber side of the glass sandwich. Subsequently, the resonators are covered by a dielectric SU-8 layer. The spiral gold tracks are patterned via lithography and electroplating and finally covered by a dielectric SU-8 layer. Fig. 2 shows a completed resonator with integrated capacitor, whereas the microcoil in Fig. 3 is self-resonant at 600MHz without the need of bond wires.