Magnetic particle imaging (MPI) is an emerging biomedical imaging modality that enables non-invasive quantification of superparamagnetic iron oxide nanoparticle (SPION) tracers by comparing signal in a region of interest (ROI) to that of reference samples of known tracer mass. However, despite growing interest and use of MPI, recent studies have raised concern over quantification accuracy as a function of the volume over which the tracer is distributed. Here, a constant mass dilution series of the tracer Ferucarbotran contained in well-defined sample geometries was used to evaluate the effect of relative reference and object sample volumes on MPI quantification accuracy. MPI images of this series were acquired under various scan modes, segmented using two commonly used methods, and iron mass quantified relative to two reference sample sets with different volumes. The maximum signal and percent error relative to ground truth were compared amongst these sample volumes, scan modes, analysis methods, and reference volumes. Tracer concentration influenced signal distribution and, in some cases, influenced iron estimation accuracy. Reference samples that were similar in volume to the volume of interest were found to provide the most accurate results across scan modes. The observed behavior is attributed to the partial volume effect, which arises due to the finite resolution in MPI. These results are discussed within the current scope of the field and suggest that care must be taken when selecting reference sample volume relative to test sample volume for quantitative MPI studies.

Magnetic particle imaging (MPI) is an emerging imaging modality with exciting biomedical applications, such as cell tracking, blood pool imaging, and image-guided magnetic hyperthermia. MPI is unique in that signal is generated entirely by synthetic nanoparticle tracers, motivating precise engineering of magnetic nanoparticle properties including size, shape, composition, and coating to address the needs of specific applications. However, success in many applications and in clinical transition requires development of high-sensitivity and high-resolution tracers, for which there is considerable room for improvement. This review summarizes recent advancements in MPI tracer synthesis and compares reported tracers in terms of sensitivity and resolution. In making these comparisons, we point out inconsistencies in reporting of MPI tracer properties. To overcome this challenge, we propose a list of properties to standardize characterization and reporting of new MPI tracers and improve communication within the field.

Objective.Magnetic particle imaging (MPI) is an emerging tomographic 'hot spot' imaging modality with potential to visualize superparamagnetic iron oxide nanoparticle tracer distributions with high sensitivity and quantitative accuracy. MPI shares many similarities with positron emission tomography (PET), where the partial volume effect (PVE) can result in signal under- and over-quantification due to spill-over of signal arising from limited resolution. While the PVE has been alluded to in the MPI literature it has not been previously studied nor characterized. The objective of this study was to systematically characterize this PVE in MPI.Approach.This contribution characterizes the PVE using models of varying size and shape filled with a uniform concentration of tracer. The effect of object size on signal distribution was analyzed after application of a new image post-processing filter.Main results.As object size increased, signal distribution increased to a maximum signal value independent of object geometry and proportional to tracer concentration. Furthermore, for small objects with characteristic dimensions below the resolution of the tracer at the scanning conditions used, signal suppression was observed. These results are consistent with foundational observations of PVE in PET, suggesting that approaches to overcome the PVE in PET may be applicable to MPI.Significance.This finding has significant impact on the MPI field by demonstrating the presence of the PVE phenomenon that can directly influence imaging results.

One of the inherent limitations associated with laser-induced breakdown spectroscopy (LIBS) in the identification of elements lies in the strength of the emission signals. Several approaches exist to enhance the emission capacity of LIBS. In this particular investigation, our focus was on amplifying the signal intensity of LIBS through the utilization of two techniques. These techniques include the application of a low-power electric field within the zone where plasma is formed, in conjunction with the utilization of nanoparticles on the surface of the sample. Specifically, our analysis involved the examination of samples consisting of metallic Zn powder as the matrix element, with the incorporation of small quantities of Ca in the form of CaCO3. The combination of these two methods resulted in unprecedented outcomes, demonstrating a 3.5-fold increase in samples containing 0.05

The increased clinical application of cell-based therapies has resulted in a parallel increase in the need for non-invasive imaging-based approaches for cell tracking, often through labeling with nanoparticles. An ideal nanoparticle for such applications must be biologically compatible as well as readily internalized by cells to ensure adequate and stable cell loading. Surface coatings have been used to make nanoparticle trackers suitable for these purposes, but those currently employed tend to have cytotoxic effects. Zwitterionic ligands are known to be biocompatible and antifouling; however, head-to-head evaluation of specific zwitterionic ligands for cell loading has not yet been explored. Magnetic particle imaging (MPI) detects superparamagnetic iron oxide nanoparticles (SPIONs) using time-varying magnetic fields. Because MPI can produce high-contrast, real-time images with no tissue depth limitation, it is an ideal candidate for in vivo cell tracking. In this work, we have conjugated hard (permanently charged) and soft (pKa-dependently charged) biomimetic zwitterionic ligands to SPIONs and characterized how these ligands changed SPION physicochemical properties. We have evaluated cellular uptake and subcellular localization between zwitterions, how the improvement in cell uptake generated stronger MPI signal for smaller numbers of cells, and how these cells can be tracked in an animal model with greater sensitivity for longer periods of time. Our best-performing surface coating afforded high cell loading within 4 h, with full signal retention in vivo over 7 days.

Rapid and accurate assessment of conditions characterized by altered blood flow, cardiac blood pooling, or internal bleeding is crucial for diagnosing and treating various clinical conditions. While widely used imaging modalities such as magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound offer unique diagnostic advantages, they fall short for specific indications due to limited penetration depth and prolonged acquisition times. Magnetic particle imaging (MPI), an emerging tracer-based technique, holds promise for blood circulation assessments, potentially overcoming existing limitations with reduction in background signals and high temporal and spatial resolution, below the millimeter scale. Successful imaging of blood pooling and impaired flow necessitates tracers with diverse circulation half-lives optimized for MPI signal generation. Recent MPI tracers show potential in imaging cardiovascular complications, vascular perforations, ischemia, and stroke. The impressive temporal resolution and penetration depth also position MPI as an excellent modality for real-time vessel perfusion imaging via functional MPI (fMPI). This review summarizes advancements in optimized MPI tracers for imaging blood circulation and analyzes the current state of pre-clinical applications. This work discusses perspectives on standardization required to transition MPI from a research endeavor to clinical implementation and explore additional clinical indications that may benefit from the unique capabilities of MPI.

Magnetic hyperthermia holds significant therapeutic potential, yet its clinical adoption faces challenges. One obstacle is the large-scale synthesis of high-quality superparamagnetic iron oxide nanoparticles (SPIONs) required for inducing hyperthermia. Robust and scalable manufacturing would ensure control over the key quality attributes of SPIONs, and facilitate clinical translation and regulatory approval. Therefore, we implemented a risk-based pharmaceutical quality by design (QbD) approach for SPION production using flame spray pyrolysis (FSP), a scalable technique with excellent batch-to-batch consistency. A design of experiments method enabled precise size control during manufacturing. Subsequent modeling linked the SPION size (6-30 nm) and composition to intrinsic loss power (ILP), a measure of hyperthermia performance. FSP successfully fine-tuned the SPION composition with dopants (Zn, Mn, Mg), at various concentrations. Hyperthermia performance showed a strong nonlinear relationship with SPION size and composition. Moreover, the ILP demonstrated a stronger correlation to coercivity and remanence than to the saturation magnetization of SPIONs. The optimal operating space identified the midsized (15-18 nm) Mn0.25Fe2.75O4 as the most promising nanoparticle for hyperthermia. The production of these nanoparticles on a pilot scale showed the feasibility of large-scale manufacturing, and cytotoxicity investigations in multiple cell lines confirmed their biocompatibility. In vitro hyperthermia studies with Caco-2 cells revealed that Mn0.25Fe2.75O4 nanoparticles induced 80% greater cell death than undoped SPIONs. The systematic QbD approach developed here incorporates process robustness, scalability, and predictability, thus, supporting the clinical translation of high-performance SPIONs for magnetic hyperthermia.

Magnetic particle imaging (MPI) is a novel biomedical imaging modality that allows non-invasive, tomographic, and quantitative tracking of the distribution of superparamagnetic iron oxide nanoparticle (SPION) tracers. While MPI possesses high sensitivity, detecting nanograms of iron, it does not provide anatomical information. Computed tomography (CT) is a widely used biomedical imaging modality that yields anatomical information at high resolution. A multimodal imaging agent combining the benefits of MPI and CT imaging would be of interest. Here we combine MPI-tailored SPIONs with CT-contrast hafnium oxide (hafnia) nanoparticles using flash nanoprecipitation to obtain dual-imaging MPI/CT agents. Co-encapsulation of iron oxide and hafnia in the composite nanoparticles was confirmed via transmission electron microscopy and elemental mapping. Equilibrium and dynamic magnetic characterization show a reduction in effective magnetic diameter and changes in dynamic magnetic susceptibility spectra at high oscillating field frequencies, suggesting magnetic interactions within the composite dual imaging tracers. The MPI performance of the dual imaging agent was evaluated and compared to the commercial tracer ferucarbotran. The dual-imaging agent has MPI sensitivity that is ∼3× better than this commercial tracer. However, worsening of MPI resolution was observed in the composite tracer when compared to individually coated SPIONs. This worsening resolution could result from magnetic dipolar interactions within the composite dual imaging tracer. The CT performance of the dual imaging agent was evaluated in a pre-clinical animal scanner and a clinical scanner, revealing better contrast compared to a commercial iodine-based contrast agent. We demonstrate that the dual imaging agent can be differentiated from the commercial iodine contrast agent using dual energy CT (DECT) imaging. Furthermore, the dual imaging agent displayed energy-dependent CT contrast arising from the combination of SPION and hafnia, making it potentially suitable for virtual monochromatic imaging of the contrast agent distribution using DECT.

Purpose Magnetic particle imaging (MPI) is being explored in biological contexts that require accurate and reproducible quantification of superparamagnetic iron oxide nanoparticles (SPIONs). While many groups have focused on improving imager and SPION design to improve resolution and sensitivity, few have focused on improving quantification and reproducibility of MPI. The aim of this study was to compare MPI quantification results by two different systems and the accuracy of SPION quantification performed by multiple users at two institutions. Procedures Six users (3 from each institute) imaged a known amount of Vivotrax+ (10 μg Fe), diluted in a small (10 μL) or large (500 μL) volume. These samples were imaged with or without calibration standards in the field of view, to create a total of 72 images (6 users x triplicate samples x 2 sample volumes x 2 calibration methods). These images were analyzed by the respective user with two region of interest (ROI) selection methods. Image intensities, Vivotrax+ quantification, and ROI selection was compared across users, within and across institutions. Results MPI imagers at two different institutes produce significantly different signal intensities, that differ by over 3 times for the same concentration of Vivotrax+. Overall quantification yielded measurements that were within ± 20% from ground truth, however SPION quantification values obtained at each laboratory were significantly different. Results suggest that the use of different imagers had a stronger influence on SPION quantification compared to differences arising from user error. Lastly, calibration conducted from samples in the imaging field of view gave the same quantification results as separately imaged samples. Conclusions This study highlights that there are many factors that contribute to the accuracy and reproducibility of MPI quantification, including variation between MPI imagers and users, despite pre-defined experimental set up, image acquisition parameters, and ROI selection analysis.

Covalent adaptable networks (CANs) combine the mechanicaland chemicalstability of thermosets with the reprocessability of thermoplasticsthrough the incorporation of stimuli-responsive dynamic crosslinks.To allow for processing through induction heating, we have createdassociative CANs that include fillers in the polymer matrix for efficientheat transfer. While the inclusion of inorganic fillers often decreasesflow rate in CANs and complicates reprocessing of the material, thepresence of Fe3O4 nanoparticles had no detrimentaleffect on flow behavior in a vinylogous urethane vitrimer, an observationwe attribute to the catalytic nature of nanoparticles on the dynamicexchange chemistry. We employed two methods of nanoparticle incorporation:blending bare nanoparticles and crosslinking chemically modified nanoparticles.The vitrimers with covalently crosslinked nanoparticles exhibiteda decreased relaxation time compared to those with blended nanoparticles.The magnetic character of the Fe3O4 nanoparticlesenabled self-healing of the vitrimer composite materials upon exposureto an alternating electromagnetic field during induction heating.

Particle characterization. (A) Specific Absorption Rate (SAR) values as a function of magnetic field intensity. Particles were heated from 25 to 800 ºC in air using a TA Instruments 2950. Specific absorption rate (SAR) was determined from the heat balance equation using a particle concentration of approximately 1.6mg core/ml suspended in water. The magnetic field was increased and applied for 200s in an Easy Heat ( Ambrell). The temperature was measured with a Luxtron Fluorometric Thermoprobe (LumaSense Technology). (B) Particle size distribution as determiend by dynamic light scattering. Particle hydrodynamic diameter was measured with particles suspended in deionized water at a concentration of 0.01 mg/ml in a Brookhaven Instruments BI-90 Plus Particle Size Analyzer (Holtsville, NY, USA). A thermo-gravimetric analysis was performed to determine the amount of CMDx attached to the magnetic core of the nanoparticles.

The size tunable formulation of magnetic alginate microparticles (MAMs) using a 3D flow‐focusing microfluidic device is reported. The droplet phase consists of iron oxide nanoparticles (IONPs) in an alginate solution and the continuous phase consists of fluorocarbon oils. The stability of IONP colloids in alginate and calcium ethylenediamine tetraacetic acid solutions using optical microscopy, dynamic light scattering, and zeta potential measurements is studied. These studies suggest that IONPs coated with polyethylene glycol (PEG) are most stable. MAMs using the PEG‐coated IONP colloid are then formulated and it is studied how MAM the average size and coefficient of variance vary as a function of droplet and continuous phase flow rates and viscosities. Droplet and MAM size decrease when the carrier flow rate or viscosity increases, and droplet and MAM size increase when droplet flow rate or viscosity increases. Crosslinking and drying of droplets result in MAMs whose diameter is ≈44% less than the original droplets while maintaining a population coefficient of variance below 8%. Conditions are identified that enable fabrication of MAMs with diameters between 30 and 60 µm with coefficients of variance of ≈6–7%. These results may guide future work exploring the role of MAM size on various applications.

Every year, there are approximately 500 000 peripheral nerve injury (PNI) procedures due to trauma in the US alone. Autologous and acellular nerve grafts are among current clinical repair options; however, they are limited largely by the high costs associated with donor nerve tissue harvesting and implant processing, respectively. Therefore, there is a clinical need for an off-the-shelf nerve graft that can recapitulate the native microenvironment of the nerve. In our previous work, we created a hydrogel scaffold that incorporates mechanical and biological cues that mimic the peripheral nerve microenvironment using chemically modified hyaluronic acid (HA). However, with our previous work, the degradation profile and cell adhesivity was not ideal for tissue regeneration, in particular, peripheral nerve regeneration. To improve our previous hydrogel, HA was conjugated with fibrinogen using Michael-addition to assist in cell adhesion and hydrogel degradability. The addition of the fibrinogen linker was found to contribute to faster scaffold degradation via active enzymatic breakdown, compared to HA alone. Additionally, cell count and metabolic activity was significantly higher on HA conjugated fibrinogen compared previous hydrogel formulations. This manuscript discusses the various techniques deployed to characterize our new modified HA fibrinogen chemistry physically, mechanically, and biologically. This work addresses the aforementioned concerns by incorporating controllable degradability and increased cell adhesivity while maintaining incorporation of hyaluronic acid, paving the pathway for use in a variety of applications as a multi-purpose tissue engineering platform.

Nanoparticles are a promising approach for improving intra-articular drug delivery and tissue targeting. How-ever, techniques to non-invasively track and quantify their concentration in vivo are limited, resulting in an inadequate understanding of their retention, clearance, and biodistribution in the joint. Currently, fluorescence imaging is often used to track nanoparticle fate in animal models; however, this approach has limitations that impede long-term quantitative assessment of nanoparticles over time. The goal of this work was to evaluate an emerging imaging modality, magnetic particle imaging (MPI), for intra-articular tracking of nanoparticles. MPI provides 3D visualization and depth-independent quantification of superparamagnetic iron oxide nanoparticle (SPION) tracers. Here, we developed and characterized a polymer-based magnetic nanoparticle system incor-porated with SPION tracers and cartilage targeting properties. MPI was then used to longitudinally assess nanoparticle fate after intra-articular injection. Magnetic nanoparticles were injected into the joints of healthy mice, and evaluated for nanoparticle retention, biodistribution, and clearance over 6 weeks using MPI. In par-allel, the fate of fluorescently tagged nanoparticles was tracked using in vivo fluorescence imaging. The study was concluded at day 42, and MPI and fluorescence imaging demonstrated different profiles in nanoparticle retention and clearance from the joint. MPI signal was persistent over the study duration, suggesting NP retention of at least 42 days, much longer than the 14 days observed based on fluorescence signal. These data suggest that the type of tracer -SPIONs or fluorophores - and modality of imaging can affect interpretation of nanoparticle fate in the joint. Given that understanding particle fate over time is paramount for attaining insights about therapeutic profiles in vivo, our data suggest MPI may yield a quantitative and robust method to non-invasively track nanoparticles following intra-articular injection on an extended timeline.

(A) Lysosome permeability when cells were exposed to PES for 24 hours in 35mm dish plates. Permeability was determined by flow cytometry using acridine orange. Flow cytometry measurements were performed with an Accuri C6 flow cytometer (BD Biosciences) and a 488nm argon laser. 15,000 events were measured. A positive control consisted of cells treated with hydrogen peroxide (Sigma-Aldrich) 0.3 µM for 30 min. (B) Cathepsin B release when SKOV3 cells were exposed to PES for 24 hours. Nucleus was stained with Hoescht (blue) and Cathepsin B was stained with with Magic Red (red). cells were dyed with Magic Red{trade mark, serif} Cathepsin B Assay Kit in in glass bottom dishes (Mattek). (C) Representative histograms showing percent of pale cells as a result of exposure to PES at 10, 20 and 30 uM for 24 hours. (D) Cell viability of HeyA8 cells when exposed to MFH at 43˚C at various times and PES concentrations. (E) Cell viability when ovarian cancer cell lines were exposed to MFH at 41˚C (left) and 43˚C (Right) over time.

Hafnium oxide (hafnia) nanoparticles have recently attracted attention for their application as computed tomography (CT) contrast agents due to their high X-ray attenuation and low cost. Here we employ flash nanoprecipitation (FNP) to formulate hafnia nanoclusters (HNCs) consisting of a core of precipitated individual hafnia nanoparticles and polylactic acid homopolymer and an outer coating of the block copolymer polylactic acid-b-polyethylene glycol (PLA-b-PEG). The HNCs can be produced with controlled hydrodynamic size in the range of 100–300 nm using a simple 3D-printed FNP mixer. The CT performance of the HNCs was evaluated in a pre-clinical small animal scanner and in a clinical scanner. In both systems the HNCs displayed ~1.5× greater contrast compared to the commercial iodinated molecular contrast agent Omnipaque used clinically.

(A) Representative temperature profile of one mouse with HeyA8 subcutaneous tumor models exposed to MFH. (B) Final relative tumor volume of HeyA8 subcutaneous tumor model (C) HeyA8 positive staining for cleaved caspase-3 in MFH treated tumors siControl (left), siControl+MFH (right), from tumors collected at day 19. The primary antibody was cleaved caspase-3 (Biocare Medical, 1:1000) was incubated overnight and incubated with HPR-conjugated secondary antibody, visualized with 3,3'-diaminobenzidine chromogen and counterstained with Gill's hematoxylin.

Supplementary protocol, figure legends and tables. Supplementary Table S1. Cumulative Equivalent Minutes; Supplementary Table S.2 The top 20 genes upregulated by MFH at 43ËšC and 30 min; Supplementary Table S.3 The top 10 genes downregulated by MFH at 43ËšC and 30 min; Supplementary Tables S.4 IC50 values of PES in ovarian cancer cell lines; Supplementary Table S.5 Combination index parameters from CompSyn; Supplementary Table S.6 Combination index values for HeyA8 cells treated in combination with various concentrations of PES and various exposure times of MFH at 43ËšC; Supplementary Table S.7 HSPA6 RNA interference and MFH decreased subcutaneous HeyA8 P values from relative tumor volume. The P values were calculated using the nonparametric test Mann-Whithey.

Adoptive cell transfer (ACT) therapies are growing in popularity due to their ability to interact with diseased tissues in a specific manner. Disc-shaped particles, or "backpacks", that bind to cellular surfaces show promise for augmenting the therapeutic potential of adoptively transferred cells by resisting phagocytosis and locally releasing drugs to maintain cellular activity over time. However, many ACTs suffer from limited tumor infiltration and retention and lack a method for real-time spatial analysis. Therefore, we have designed biodegradable backpacks loaded with superparamagnetic iron oxide nanoparticles (SPIONs) to improve upon current ACT strategies by (i) controlling the localization of cell-backpack complexes using gradient magnetic fields and (ii) enabling magnetic particle imaging (MPI) to track complexes after injection. We show that magnetic backpacks bound to macrophages and loaded with a proinflammatory drug, resiquimod, maintain anticancer phenotypes of carrier macrophages for 5 days and create cytokine "factories" that continuously release IL-12. Furthermore, we establish that forces generated by gradient magnet fields are sufficient to displace cell-backpack complexes in physiological settings. Finally, we demonstrate that MPI can be used to visualize cell-backpack complexes in mouse tumors, enabling a potential strategy to track the biodistribution of ACTs in real time.