the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Jan 2025
Automated manufacturing process for sustainable prototyping of NMR transceivers
Abstract. Additive manufacturing has enabled rapid prototyping of components with minimum investment in specific fabrication infrastructure. These tools allow a fast iteration from design to functional prototypes within days or even hours. Such prototyping technologies exist in many fields, from 3-dimensional mechanical components, or 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. 6 axes 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 fastprototype 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 EM-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.
Competing interests: J.G.K. is is a member of the editorial board of Magnetic Resonance and a shareholder in Voxalytic GmbH, a company that produces NMR equipment. S.W. is an employee at Voxalytic GmbH. The other authors declare no conflict of interest.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: final response (author comments only)
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RC1: 'Comment on mr-2024-22', Anonymous Referee #1, 07 Feb 2025
The manuscript reports on the design, fabrication, and electrical characterization of mm-size solenoidal coils. Results of NMR spectroscopy and imaging experiments at 1 T with the realized solenoidal coils are reported. The original contribution of this work is in the technique used to realize the solenoidal microcoils, called "extrusion printing". With this technique the authors demonstrated the fabrication of 1.5 diameter solenoidal coils having a wire pitch from 300 um to 750 um, a wire width of about 60 um, and a wire thickness of 8 um. The "printed" material consist of 82.3 wt-% of Ag nanoparticles of 12 nm (a commercial product). The resistivity of the "printed" material is not mentioned, but from previous publications of the same authors it might be only two times worse that bulk Ag, which is the material with the lowest resistivity at room temperature. The low resistivity of the printed material and the "automated manufactoring process" makes this approach potentially interesting for the fabrication of solenoidal coils (and most probably other coil geometries) for NMR applications. As mentioned below, I would appreciated a more detailed discussion of the foreseen advantages and limitations of the proposed technique for the fabrication of NMR coils.
Here my suggestions/remarks:
1. Previous literature for the "scalable" fabrication of solenoidal microcoils should be included and compared with the approach proposed here. For example: Rogers, John A., et al. "Using microcontact printing to fabricate microcoils on capillaries for high resolution proton nuclear magnetic resonance on nanoliter volumes." Applied physics letters 70.18 (1997): 2464-2466.
2. The measured electrical resistivity of the realized solenoidal microcoil should be specifically mentioned.
3. LINE 75: I think that the phrase "since for the same magnitude of current flowing through the coil, the temperature rise will be higher for a coil with larger resistance and hence the noise" should be re-phrased. To increase the noise significantly the temperature should increase dramatically (at 600 K the thermal noise would be only 1.4 times larger than at 300 K). On the other hand, it is certainly important to reduce the value of the resistance because this affects directly the thermal noise and the RF efficiency (B1/sqrt(power)).
4. TABLE 1: The unit of the RF efficiency are, most probably, kHz/sqrt(W) and not kHz/W. I would express this quantity in both kHz/sqrt(W) and T/sqrt(W).
5. TABLE 1: It is not clear to me why in this table the RF efficiency is in the order of 1 kHz/sqrt(W) whereas later (LINE 205) the RF efficiency is about 30 kHz/sqrt(Hz). Maybe the authors refer to a different frequency or to a different circuitry ?
6. LINE 88: (detail) The field B is given but in the equations 2-4 only the field H is given explicitly.
7. In several places in the manuscript the authors use the term "measured" when they probably refers to simulation results (e.g., in the caption of Figure 1, I guess that "measured" should be replaced by "computed").
8. TABLE 2: The meaning of "...of the electrical circuit" is not very clear. I guess that this refers to the fact that in one case the coil is simulated stand-alone whereas in the other case it is simulated when mounted on the PCB. I guess this explain why the self-resonance frequency values reported in TABLE 1 are not the same as in TABLE 2. I would propose to merge these two table and make more clear the meaning of each quantity. I would add also the values of the inductance and the resistance for each of the 9 coils (since the self-resonance frequency is much larger than the operating frequency of 45 MHz, a series L-R model of the coil should be appropriated). And I would definitely also add the width and thickness of the "wire" used for the simulation of each coil (I guess similar to the values reported at LINE 183 of about 60 um and about 8 um, respectively)
9. LINE 110: Is the Q-factor measured or computed ?
10. LINE 115: (detail) Are the required values of the capacitors for tuning and matching with the other coils somehow "problematic" ? Maybe explain the choice of the (N=5, pitch=0.525 mm) coil in a different way.
11. FIGURE 5, CAPTION: (typo) I guess it is "...with p=0.3 mm and N=5 mounted..." instead of "...with p=0.3 mm N=5) mounted..." .
12. FUGURE 5: I would add scale bars also in A and B.
13. FIGURE 7: (typo) "Since the Teflon tube show no MRI signal...". (maybe mention that Teflon is based on tetrafluoroethylene (C₂F₄) and hence does not contains 1H nuclei and gives no 1H NMR signal).
14: LINE 193: Why the "contact resistance" has been measured at 4.98 MHz ? It seems to me an arbitrary frequency not related to the operating NMR frequency, or maybe it is just the same in a very broad frequency range.
15: I think that somewhere in the paper the limitations of the proposed printing technique for the fabrication of NMR coils should be discussed more in details. For example: Can the pitch be reduced while keeping the same wire width ? Can the wire width be increased while keeping the same pitch ? Can the wire thickness be increased while keeping the same width ? Which difficulties are foreseen if the solenoidal coil diameter is reduced or increased ? Could we imagine to use this technique for a solenoidal coil having a diameter of 150 um (scaling down also pitch, widht, and thickness) ? Is the larger (and positive) magnetic susceptibility of silver with respect to copper a potential problem for high resolution NMR ? Have the 12 nm silver nanoparticles the same susceptibility of bulk silver ?
Citation: https://doi.org/10.5194/mr-2024-22-RC1 - AC1: 'Reply on RC1', Dario Mager, 28 Mar 2025
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RC2: 'Comment on mr-2024-22', Anonymous Referee #2, 13 Feb 2025
The manuscript reports on the the design, manufacturing and validation of a novel additive manufacturing process to produce microcoils for NMR applications. The novelty of the shown approach consists in the specific type of additive manufacturing used that is based on extrusion-printing (EP) of high-viscosity silver nanoparticle-filled ink. The authors designed a range of 1.5 mm ID solenoid coils with varying turn count (3-7) and pitch (0.3 - 0.75 mm) and simulated their electrical properties. The coils were then fabricated using the EP process and characterized in bench-top electrical measurements. A 5-turn / 0.525 mm pitch coil was then used to highlight its properties in NMR-spectroscopy and NMR-imaging experiments.
The scientific work is carefully carried out and the claims are supported with the experimental data. Also, the manuscript is well-written and structured. Yet, I propose the following changes to improve the reader’s understanding and to better delineate its scope.- Please provide an additional paragraph that better delineates the scope and limitations of the novel technique, both in terms of technical limitations (how small can the solenoids get, are there limitations in the pitch/width, etc) and applications (what are the advantages/disadvantages of these new coils compared to the „traditional“ ways, etc)
- For the readers unfamiliar with additive coil manufacturing, a direct comparison of the main electrical properties to a „traditional“ coil wound from an (enameled) silver or copper wire would be beneficial.
- According to Figs 4/5 the terminals of the coils are connected to the PCB via a long „wire“ running along the capillary. It looks like this section of the conductor is comparable in length to the actual coil conductor:
- Was this part of the simulation (according to Fig 1 it doesn’t look like)? And if it wasn’t, could this explain some of the reported discrepancies?
- Please describe what the effects of these connection lines are (I expect these to significantly and negatively affect the electrical properties; also the field produced by these sections of the conductor will leak into the lumen of the capillary and might have a negative effect on the spectral and imaging performance of the setup). - In various sections of the manuscript it remains somewhat unclear what quantities were simulated and what quantities were actually measured (e.g. „measured“ in figure caption 1 or „measure“ in line 82). Please make sure the wording is clear and consistent throughout the manuscript.
- Line 73-75: It seems this argument is incomplete. The primary reason to favor low R's is because it directly affects the thermal noise via V_rms ∝ sqrt(R). This effect is irrespective of a temperature increase and impacts the SNR even if the temperature of the conductor is constant. A tempature increase due to higher resistance worsens the SNR only as a secondary effect.
- In the context of the above, the authors could also discuss the relevance of the skin effect:
- For 45 MHz the skin depth of (pure) silver is around 10 um (and thus about the thickness of the conductor). What is the significance of the skin effect onto the presented resulted and what does it mean for higher target frequencies?
- What is the bulk resistance of the cured silver ink compared to pure silver?
- What is the surface roughness of the structure and does it impact the skin effect? - Table 1: Please make sure to use the correct units for the RF-efficiency, i.e. Hz/sqrt(W) or T/sqrt(W) instead of Hz/W or T/W respectively. Does the use of inanccurate units explain the mismatch between the results reported in table 1 vs Line 205?
- Line 147: I guess the PLC TC1260 TwinCAT 2 software was used to automatically control the manufacturing process after manual adjustment of the substrate placement using the the joystick. Please rephrase.
Citation: https://doi.org/10.5194/mr-2024-22-RC2 - AC2: 'Reply on RC2', Dario Mager, 28 Mar 2025
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RC3: 'Comment on mr-2024-22', Anonymous Referee #3, 11 Mar 2025
The manuscript by Wadhwa et al. reports on a new approach, based metallic paste based extrusion printing (EP), to fabricate micro-coils for NMR and MRI applications. The authors designed, simulated and fabricated a range of 1.5 mm ID solenoidal coils with different number of turns (5, 6, 7) and pitch value (0.3, 0.525, 0.75 mm). Moreover, all coils, connected to a PCB and tuned (matched) to 45 MHz (50 Ohm) are tested using a network analyser for Q-factor assessment. Moreover, driven by a convenient balance between, RF-efficacy, quality factor and tuning range, the coil with 5 turns and a pitch of 0.525 mm was used for acquiring a a spectrum and an image of glass capillary filled with ethanol.
The paper is well written and conclusions are well supported by results. I only have some minor comments:
1- for non expert readers, it would be nice in the introduction to add a paragraph about why traditional methods for micro-coils fabrication fall short. Is it hard to control B1 homogeneity? There is a paragraph about B0 homogeneity that in my opinion is not the most important factor when the sample volume is small
2- specify what is the aspect ratio of a printed track and why is so important
3- correct units for RF-efficacy in table 1 (kHz/√W)
4- would be nice to add a figure with the Q-factor of the different coils (measured and calculated)
5- how many averages were used to acquire the spectrum?
3- specify
Citation: https://doi.org/10.5194/mr-2024-22-RC3 - AC3: 'Reply on RC3', Dario Mager, 28 Mar 2025
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