Exploration\nof Materials for Three-Dimensional NMR\nMicrocoil Production via CNC Micromilling and Laser Etching

The excellent versatility of 5-axis computer numerical\ncontrol\n(CNC) micromilling has led to its application for prototyping NMR\nmicrocoils tailored to mass-limited samples (reducing development\ntime and cost). However, vibrations during 5-axis milling can hinder\nthe creation of complex 3D volume microcoils (i.e., solenoids and\nsaddle coils). To address these limitations, a high-resolution NSCNC\nELARA 4-axis milling machine was developed with the extra precision\nrequired for making complex 3D volume microcoils. Upon investigating\nthe performance of resonators made with various copper-coated dielectrics,\nresonators with poly(methyl methacrylate) (PMMA) provided the best\nSNR/line shape. Thus, complex 1.7 mm microcoil designs were machined\nfrom Cu-coated PMMA. A milled 6.4 mm solenoid also provided 6.6×\nthe total carbon signal for a <sup>13</sup>C-labeled broccoli seed\ncompared to a commercial inverse 5 mm NMR probe (demonstrating potential\nfor larger coil designs). However, the manufacture of coils <1.7\nmm with copper-coated PMMA rods was challenging as ∼0.5 mm\nof remaining PMMA was needed to retain their structural integrity.\nTo manufacture smaller microcoils, both a solenoid and saddle coil\n(both with 1 mm O.D., 0.1 mm thick walls) were etched from Cu-coated\nglass capillaries using a UV picosecond laser that was mounted onto\nan NSCNC 5-axis MiRA7L. Both resonators showed excellent signal and\nidentified a wide range of metabolites in a <sup>13</sup>C-labeled\nalgae extract, while the solenoid was further tested on two copepod\negg sacs (∼4 μg of total sample). In summary, the flexibility\nto prototype complex microcoils in-house allows laboratories to tailor\nmicrocoils to specific mass-limited samples while avoiding the costs\nof cleanrooms.