Real-time measurement of biomagnetic vector fields in functional syncytium using amorphous metal

Abstract

Magnetic field detection of biological electric activities would provide a non-invasive and aseptic estimate of the functional state of cellular organization, namely a syncytium constructed with cell-to-cell electric coupling. In this study, we investigated the properties of biomagnetic waves which occur spontaneously in gut musculature as a typical functional syncytium, by applying an amorphous metal-based gradio-magneto sensor operated at ambient temperature without a magnetic shield. The performance of differentiation was improved by using a single amorphous wire with a pair of transducer coils. Biomagnetic waves of up to several nT were recorded ~1 mm below the sample in a real-time manner. Tetraethyl ammonium (TEA) facilitated magnetic waves reflected electric activity in smooth muscle. The direction of magnetic waves altered depending on the relative angle of the muscle layer and magneto sensor, indicating the existence of propagating intercellular currents. The magnitude of magnetic waves rapidly decreased to ~30% by the initial and subsequent 1 mm separations between sample and sensor. The large distance effect was attributed to the feature of bioelectric circuits constructed by two reverse currents separated by a small distance. This study provides a method for detecting characteristic features of biomagnetic fields arising from a syncytial current.

Figure 1. Schematic diagram of a gradio-magneto sensor system.

(a) A gradio-magneto sensor device was composed of a single amorphous metal (Am) wire (50 mm in length) with a pair of detector coils (10 mm in length; 300 turns) mounted at both ends. MS1 was placed below a recording chamber, while MS2 was placed ~30 mm apart from MS1 in the same direction. MS1 and MS2 received driving electric pulses (P_e). The intermediate part of the wire (30 mm) was electrically shunted. (b) The Am wire was placed in a plastic bobbin (~1 mm diameter) surrounded by a transducer coil. The sample was separated by a cover glass (100 μm thick). (c) A pulse gate IC (PG) triggered by a clock IC. The same clock IC also triggers sample-and-hold detectors (SH1, SH2) to measure the voltage of the transducer coils in MS1 and MS2. A fast operation amplifier (d-AMP) differentiates the voltage in SH2 from that in SH1. Output signals were filtered by high and low-cut filters (H/LPF: 0.5 Hz and 20 Hz) and stored in computer memory via an analog-to-digital converter (ADC). (d) P_e (100 ns, 5 V) applied at 2 μs intervals. (e) Pickup coil potentials in MS1 and MS2 (E_coil1 and E_coil2) measured upon application of P_e without a sample.

Figure 2. Detection efficacy of the improved gradio-magneto sensor system with a continuous Am wire.

(a,b) A linear cable (30 cm in length, 0.5 mm in radius: R_c) in which a current generator provides oscillating sine waves of 3 Hz. Various amplitudes of the linear cable current (I_L) were applied, and the linear cable was raised with various gaps. (c,d) Changes in the output voltage of the magneto sensor amplifier (E_ms) by applying various I_L with no gaps. (e,f) Changes in E_ms by shifting the I_L gap distance by 476 μA.

Figure 3. Quasi-real-time measurements of biomagnetic fields from an ileal musculature.

(a) The sample was fixed in a recording chamber on MS1. MS1 detects biomagnetic fields along with environmental magnetic fields, while MS2 detects only the latter. Environmental magnetic fields were canceled by subtracting the MS2 signal from MS1 signal. (b) An example of a real-time measurement of biomagnetic activity in an ileal musculature sample in normal solution. The sample was mounted with the longitudinal muscle layer down, and perpendicular to the MS1. (c) Spontanoous biomagnetic activity was applified by application of TEA (0.5 mM). A band elimination filter (BEF) was applied in off-line analysis in (b) and (c). (d) Background noise traces with and without BEF. (E) Linear spectra of b, c and d. The yellow line represents the frequency range of BEF applied in (b–d).

Figure 4. Biomagnetic fields characterized by gap distance (a–i) and direction of musculature (j–m).

(a–d) Gap distance between the cover glass and MS1 magneto sensor was changed from 0 to 1 and 2 mm, in the presence of a K⁺ channel blocker (0.5 mM TEA). (e–g) Linear spectra for the magnetic field recordings B to D. Note that increases in gap distance largely reduced the signals < ~1.2 Hz, while the magneto sensor noise remains at around 1.5 Hz. (h,i) Sum of linear spectrum amplitude in the frequency range indicated in (e) (thick red line) is plotted as biomagnetic activity against gap distance, relative to that without a gap, in the absence and presence of TEA. Thin red lines represent the average of experiments. (j,k) Biomagnetic field measurements from the same ileal musculature that crossed the MS1 magneto sensor in anal-to-oral (A → O) and oral-to-anal (O → A) ends, respectively. Gap = 0. The schema indicates the bottom view of the sample and magneto sensor. (l,m) Four expanded biomagnetic waves in (j) and (k) are superimposed (left) and averaged (right) in (l) and (m), respectively. Biomagnetic waves used are indicated by color bars in (j) and (k).

Figure 5. Practical model of a biomagnetic field.

(a,b) A pair of linear insulated electric cables (30 cm in length, 0.5 mm in radius: R_c) are piled on top of an MS1 magneto sensor and separated by a cover glass plate (100 μm thick). The distance between the two cables (D_L2) is ~1.1 mm. Various amplitudes of current (I_L2) were applied to the paired cables, which were raised with various gaps. (c,d) Changes in the output voltage of the magneto sensor amplifier (E_ms) by applying various I_L2 with no gaps. (e,f) Changes in E_ms by shifting the I_L2 gap distance by 1904 μA. The results of single cable (Fig. 2 d and f: I_L = 476 μA) are reproduced to show the difference. Also, in (f), changes in E_ms with an I_L2 of 476 μA are shown.

Figure 6. Computer simulation of a biomagnetic field.

(a) An illustration of cellular organizations with intercellular propagating current and extracellular return current. The return current may consist of intercellular currents towards surface cells as well as extracellular surface currents. (b) Simplified circuits for simulation of a biomagnetic field. Five electric circuits (L₋₂ to L₂) are combined and a 1 μA current is conducted in each circuit. In brackets, x, y and z coordinates of points in the circuit are indicated in mm. The distance of current propagation corresponds to ~60 cells in a longitudinal direction, assuming a cell length of 150 μm. (c) Biomagnetic field maps of L_z = 0.2, 0.5 and 1 mm. P_S1 – P_S4 are the four corners of the map. Note the amplification of the magnetic field by increasing the L_z distance of the circuit.