Microneedles with integrated magnetoresistive sensors: A precision tool in biomedical instrumentation

This work describes how needle shape silicon probes with integrated magnetoresistive sensing elements can be used in neurosciences. The fabricated probes allow simultaneous electric and magnetic recording with a micrometric spatial resolution, therefore offering great advantages for the detection of neural magnetic fields. In this paper we present results obtained upon insertion of high sensitivity magnetoresistive sensors in a mouse hippocampus brain slice submerged by Krebs solution. To perform these in-vitro experiments, an electric pulsed stimulus was applied to the CA3 region of the hippocampus while the needle shape probe was placed at CA1 to measure the field created by the ionic currents. The magnetoresistive sensor presented an output signal with amplitude of 130 nV, recorded 9 ms after the stimulation. Considering the signal has a pure magnetic nature, the detected magnetic field would be of 33 nT. However, cumulative parasitic effects arising from capacitive couplings can induce an electric component in the recorded signal. In this case, and considering a minimum recording frequency of 5 Hz (detectivity of 54 nT/Hz
^1/2
), a capacitance ~ 10 nF was estimated through the probe's oxide passivation layer.