Microfabricated solenoids and Helmholtz coils for NMR spectroscopy of mammalian cells

NMR-microprobes based on solenoids and Helmholtz coils have been microfabricated and NMR-spectra of mammalian cells have successfully been taken. The microfabrication technology developed for these probes consists of three electroplated copper levels for low resistance coils and three SU-8 layers for the integration of microchannels. This technology allows fabricating solenoids, Helmholtz and planar coils on the same wafer. The coils have inner diameters in the range of 160 to 400 mm and detection volumes of 5 to 22 nL. The solenoid and Helmholtz coils show improved RF-field characteristics compared to a planar coil fabricated with the same process. The fabricated solenoid has a particularly low resistance of only 0.46 ohm at 300 MHz. Moreover, it is very sensitive and has a very uniform RF-field, but shows large line width. The Helmholtz coils are slightly less sensitive, but display a far narrower line width, and are therefore a good compromise. With a Helmholtz coil, a SNR of 620 has been measured after one scan on 9 nL pure water. An NMR-microprobe based on a Helmholtz coil has also been used to take spectra of CHO cells that have been concentrated in the sensitive region of the coil with a mechanical filter integrated into the channel.