Nils Spengler

Microscale nuclear magnetic resonance: a tool for soft matter research

Nuclear magnetic resonance spectroscopy (NMR) and imaging (MRI) are important non-destructive investigative techniques for soft matter research. Continuous advancements have not only lead to more sensitive detection, and new applications, but have also enabled the shrinking of the detectable volume of sample, and a reduction in time needed to acquire a spectrum or image. At the same time, advances in microstructuring and on-chip laboratories have also continued unabated. In recent years these two broad areas have been productively joined into what we term micro nuclear magnetic resonance (μMR), an exciting development that includes miniaturized detectors and hyphenation with other laboratory techniques, for it opens up a range of new possibilities for the soft matter scientist. In this paper we review the available miniaturization technologies for NMR and MRI detection, and also suggest a way to compare the performance of the detectors. The paper also takes a close look at chip-laboratory augmented μMR, and applications within the broad soft matter area. The review aims to contribute to a better understanding of both the scientific potential and the actual limits of μMR tools in the various interdisciplinary soft matter research fields.

Micro-fabricated Helmholtz coil featuring disposable microfluidic sample inserts for applications in nuclear magnetic resonance

In this study, we report on a novel, multi-use, high-resolution NMR/MRI micro-detection probe for the screening of flat samples. It is based on a Helmholtz coil pair in the centre of the probe, built out of two 1.5?mm diameter wirebonded copper coils, resulting in a homogeneous distribution of the magnetic field. For liquids and suspensions, custom fabricated, disposable sample inserts are placed inside the pair and aligned automatically, preventing the sensor and the samples from contamination. The sensor was successfully tested in a 500?MHz (11.7 T) spectrometer where we achieved a linewidth of 1.79?Hz (3.58?ppb) of a water phantom. Nutation experiments revealed an overall B1-field uniformity of 92% (ratio in signal intensity at flip angles of 810?/90?), leading to a homogeneous excitation of concentration limited samples. To demonstrate the imaging capabilities of the detector, we acquired images of a solid and a liquid sample?of a piece of leaf, directly inserted into the probe and of a sample insert, filled with a suspension of 50 ?m diameter polymer beads and deionized water, with in-plane resolutions of 20 ? 20 ??m2 and 10 ? 10 ??m2, respectively.

Heteronuclear Micro-Helmholtz Coil Facilitates µm-Range Spatial and Sub-Hz Spectral Resolution NMR of nL-Volume Samples on Customisable Microfluidic Chips.

We present a completely revised generation of a modular micro-NMR detector, featuring an active sample volume of ∼ 100 nL, and an improvement of 87% in probe efficiency. The detector is capable of rapidly screening different samples using exchangeable, application-specific, MEMS-fabricated, microfluidic sample containers. In contrast to our previous design, the sample holder chips can be simply sealed with adhesive tape, with excellent adhesion due to the smooth surfaces surrounding the fluidic ports, and so withstand pressures of ∼2.5 bar, while simultaneously enabling high spectral resolution up to 0.62 Hz for H2O, due to its optimised geometry. We have additionally reworked the coil design and fabrication processes, replacing liquid photoresists by dry film stock, whose final thickness does not depend on accurate volume dispensing or precise levelling during curing. We further introduced mechanical alignment structures to avoid time-intensive optical alignment of the chip stacks during assembly, while we exchanged the laser-cut, PMMA spacers by diced glass spacers, which are not susceptible to melting during cutting. Doing so led to an overall simplification of the entire fabrication chain, while simultaneously increasing the yield, due to an improved uniformity of thickness of the individual layers, and in addition, due to more accurate vertical positioning of the wirebonded coils, now delimited by a post base plateau. We demonstrate the capability of the design by acquiring a 1H spectrum of ∼ 11 nmol sucrose dissolved in D2O, where we achieved a linewidth of 1.25 Hz for the TSP reference peak. Chemical shift imaging experiments were further recorded from voxel volumes of only ∼ 1.5nL, which corresponded to amounts of just 1.5 nmol per voxel for a 1 M concentration. To extend the micro-detector to other nuclei of interest, we have implemented a trap circuit, enabling heteronuclear spectroscopy, demonstrated by two 1H/13C 2D HSQC experiments.

High-performance, 3D-microtransformers on multilayered magnetic cores

We present the fabrication of 3D-microtransformers combining a new type of multilayered magnetic core and coil winding with an automatic wirebonder. For the magnetic cores we stapled up to 30 layers of 20 μm thick amorphous metal layers with an industrial laminator. Intermediate layers of 10 μm thick double-sided sticky tape provided adhesion and electrical insulation. Electrical discharge machining was used to precisely cut these magnetic stacks to sub-millimeter cubes. To flip the cubes by 90° and assemble them onto a wafer for coil winding, we produced a receptor wafer providing magnetic landing sites. Subsequently, to wind a primary and secondary coil, one on top of the other, an automatic wirebonder was employed with 25 μm thick insulated Gold wire. A fabricated transformer with a core size of 0.9*0.8*1 mm3 yielded an inductance of 1412 nH and a coupling factor of 97%. The maximum transformer efficiency of 73% was measured at a load of 50 Ω.

3-D Microtransformers for DC–DC On-Chip Power Conversion

We address the miniaturization of power converters by introducing novel 3-D microtransformers with magnetic core for low-megahertz frequency applications. The core is fabricated by lamination and microstructuring of Metglas 2714A magnetic alloy. The solenoids of the microtransformers are wound around the core using a ball-wedge wirebonder. The wirebonding process is fast, allowing the fabrication of solenoids with up to 40 turns in 10 s. The fabricated devices yield the high inductance per unit volume of 2.95 μH/mm3 and energy per unit volume of 133 nJ/mm3 at the frequency of 1 MHz. The power efficiency of 64-76% is measured for different turns ratio with coupling factors as high as 98%. To demonstrate the applicability of our passive components, two PWM controllers were selected to implement an isolated and a nonisolated switch-mode power supply. The isolated converter operates with overall efficiency of 55% and maximum output power of 136 mW; then, we experimentally demonstrate how we increased this efficiency to 71% and output power to 408 mW. The nonisolated converter can deliver an overall efficiency of 81% with a maximum output power of 515 mW. Finally, we benchmarked the results to underline the potential of the technology for power on-chip applications.

Magnetic resonance imaging reveals functional anatomy and biomechanics of a living dragon tree

Magnetic resonance imaging (MRI) was used to gain in vivo insight into load-induced displacements of inner plant tissues making a non-invasive and non-destructive stress and strain analysis possible. The central aim of this study was the identification of a possible load-adapted orientation of the vascular bundles and their fibre caps as the mechanically relevant tissue in branch-stem-attachments of Dracaena marginata. The complex three-dimensional deformations that occur during mechanical loading can be analysed on the basis of quasi-three-dimensional data representations of the outer surface, the inner tissue arrangement (meristem and vascular system), and the course of single vascular bundles within the branch-stem-attachment region. In addition, deformations of vascular bundles could be quantified manually and by using digital image correlation software. This combination of qualitative and quantitative stress and strain analysis leads to an improved understanding of the functional morphology and biomechanics of D. marginata, a plant that is used as a model organism for optimizing branched technical fibre-reinforced lightweight trusses in order to increase their load bearing capacity.

Ink-jet printing technology enables self-aligned mould patterning for electroplating in a single step

We present a new self-aligned, mask-free micro-fabrication method with which to form thick-layered conductive metal micro-structures inside electroplating moulds. Seed layer patterning for electroplating was performed by ink-jet printing using a silver nano-particle ink deposited on SU-8 or Ordyl SY permanent resist. The silver ink contact angle on SU-8 was adjusted by oxygen plasma followed by a hard bake. Besides functioning as a seed layer, the printed structures further served as a shadow mask during patterning of electroplating moulds into negative photoresist. The printed silver tracks remained in strong adhesion to the substrate when exposed to the acidic chemistry of the electroplating bath. To demonstrate the process, we manufactured rectangular, low-resistivity planar micro-coils for use in magnetic resonance microscopy. MRI images of a spring onion with an in-plane resolution down to 10 µm × 10 µm were acquired using a micro-coil on an 11.7 T MRI scanner.

Magnetic Lenz lenses improve the limit-of-detection in nuclear magnetic resonance

A high NMR detection sensitivity is indispensable when dealing with mass and volume-limited samples, or whenever a high spatial resolution is required. The use of miniaturised RF coils is a proven way to increase sensitivity, but situations may arise where space restrictions could prevent the use of a small resonant coil, e.g., in the interior of the smallest practicable micro-coils. We present the use of magnetic lenses, denoted as Lenz lenses due to their working principle, to focus the magnetic flux of an RF coil into a smaller volume and thereby locally enhance the sensitivity of the NMR experiment—at the expense of the total sensitive volume. Besides focusing, such lenses facilitate re-guiding or re-shaping of magnetic fields much like optical lenses do with light beams. For the first time we experimentally demonstrate the use of Lenz lenses in magnetic resonance and provide a compact mathematical description of the working principle. Through simulations we show that optimal arrangements can be found.

Wire bonded MEMS-scale on-chip transformers

We present a novel, wafer-level fabrication method of 3D solenoidal microtransformers using an automatic wire bonder. Automatic wire bonders allow to precisely shape 25 μm diameter wire around prefabricated yokes or magnetic cores within seconds. Former reports of wire bonded microcoils treated individual solenoids showing low mutual inductances, whereas in this study transformers with strongly coupled microsolenoids are presented. The process is fully compatible with standard microelectronic manufacturing, therefore, enables the direct integration of transformers into a given electronic circuit. Two different prototypes are presented here. A non-magnetic core transformer with a footprint of 1 mm2 and 20 windings in both the primary and the secondary coil, yields a power efficiency of 67% and a coupling factor of 94%. A magnetic core transformer with a footprint of 0.64 mm2 and 18 windings in both the primary and the secondary coil, yields more than 1 μH inductance and 74% power efficiency with a coupling factor of 98%. The feasibility of both prototypes with respect to the power conversion in miniaturized circuits is evaluated. Finally, the microtransformers are benchmarked to underline the potential of the wire bonded microtransformers compared to other state-of-the-art publications.

A comparison of Lenz lenses and LC resonators for NMR signal enhancement

Abstract High signal‐to‐noise ratio (SNR) of the NMR signal has always been a key target that drives massive research effort in many fields. Among several parameters, a high filling factor of the MR coil has proven to boost the SNR. In case of small‐volume samples, a high filling factor and thus a high SNR can be achieved through miniaturizing the MR coil. However, under certain circumstances, this can be impractical. In this paper, we present an extensive theoretical and experimental investigation of the inductively coupled LC resonator and the magnetic Lenz lens as two candidate approaches that can enhance the SNR in such circumstances. The results demonstrate that the narrow‐band LC resonator is superior in terms of SNR, while the non‐tuned nature of the Lenz lens makes it preferable in broadband applications.