Automated manufacturing process for sustainable prototyping of nuclear magnetic resonance transceivers

Abstract

Additive manufacturing has enabled rapid prototyping of components with minimum investment in specific fabrication infrastructure. These tools allow for a fast iteration from design to functional prototypes within days or even hours. Such prototyping technologies exist in many fields, including three-dimensional mechanical components and printed electric circuit boards (PCBs) for electrical connectivity, to mention two. In the case of nuclear magnetic resonance (NMR) spectroscopy, one needs the combination of both fields; we need to fabricate three-dimensional electrically conductive tracks as coils that are wrapped around a sample container. Fabricating such structures is difficult (e.g., six-axis micro-milling) or simply not possible with conventional methods. In this paper, we modified an additive manufacturing method that is based on the extrusion of conductive ink to fast-prototype solenoidal coil designs for NMR. These NMR coils need to be as close to the sample as possible and, by their shape, have specific inductive values. The performance of the designs was first investigated using electromagnetic field simulations and circuit simulations. The coil found to have optimal parameters for NMR was fabricated by extrusion printing, and its performance was tested in a 1.05 $T$ imaging magnet. The objective is to demonstrate reproducible rapid prototyping of complicated designs with high precision that, as a side effect, hardly produces material waste during production.

Figure 1

(a) Computational setup with different domains. The computational domain ( $\mathrm{\Omega }$ ) was truncated by a perfect electric boundary condition. The lumped port was connected to the coil's terminal, where the S parameters were calculated to be in the range of 1–3 $\mathrm{GHz}$ , and the magnetic field produced by the coil was set at 45 $\mathrm{MHz}$ , i.e., the Larmor frequency of ${}^{\mathrm{1}}\mathrm{H}$ at 1.05 $\mathrm{T}$ . (b–d) Reactance of the coil simulated from the S parameters, with different pitch $p$ and number of turns $N$ . The frequency at which the reactance is zero is reported as the coil's self-resonance.

Figure 2

(a) The PCB used for the simulation, where the geometry was determined from the PCB which was used for the measurement from the VNA. The height of the PCB was 49 mm, and the width was 15 mm. Next to it is a schematic of a solenoidal coil. (b–d) Reactance of the coil mounted on the PCB shown in (a), with different pitch $p$ and number of turns $N$ . The frequency at which the reactance is zero is reported as the setup's self-resonance.

Figure 3

Extrusion-printing system for cylindrical substrates. The main components are the granite table (A), gantry (B), printhead (C), $xybz$  stage, goniometer stage (D) with a rotary stage (E), flexible shaft coupling (F), bearing block (G), and optical systems (H) and (I).

Figure 4

Extrusion-printed solenoid-coil variants on glass tubes.

Figure 5

A solenoid coil with the parameters $p=\mathrm{0.3}$ $\mathrm{mm}$  and $N=\mathrm{5}$ mounted on a PCB (A), with a detailed image of the contact (B). Microscopic image (C) and LSM height profile (D) of a winding section.

Figure 6

Coils' reactance $X$ over frequency $f$ of three solenoid coils with $N=\mathrm{6}$ and $p=\mathrm{0.75}\mathrm{mm}$ and of three coils with $N=\mathrm{7}$ and $p=\mathrm{0.3}\mathrm{mm}$ .

Figure 7

(a) The glass capillary with the printed coil used for the acquisition with the sample inside. (b) Homonuclear ${}^{\mathrm{1}}\mathrm{H}$ NMR spectrum of pure ethanol acquired in a 1.05 $\mathrm{T}$ imaging magnet (RF excitation power of 0.1 $\mathrm{W}$ , $\mathit{\pi }/\mathrm{2}$ -pulse duration of 27 $\mathrm{\mu }\mathrm{s}$ ). (c–f) MR images with the coil. Panels (c) and (d), with a completely filled capillary, show a sagittal and an axial image, respectively. Panels (e) and (f) also shows a sagittal and an axial image but with a Teflon tube with a 0.5 $\mathrm{mm}$ outer diameter placed in the capillary and with the sample filled around it. Since Teflon ( ${\mathrm{C}}_{\mathrm{2}}{\mathrm{F}}_{\mathrm{4}}$ ) has no ${}^{\mathrm{1}}\mathrm{H}$ nuclei, it has no ${}^{\mathrm{1}}\mathrm{H}$ NMR signal. This creates a defined border and highlights the spatial resolution achievable.