Since 2005, Magnetic Particle Imaging (MPI) is handled as a key technology with great potential in medical applications as an imaging method . The superparamagnetic iron oxide nanoparticles (SPIONs) which are already used as a tracer in MPI, combined with various polymers, are being investigated in order to enhance this potential. A combination of polymers such as polyethylene (PE) and polyurethane (PU) and SPIONs could be used as a coating for medical devices, or added to semi-rigid polyurethane for the production of surgical instruments . This would be of great interest, since the method provides high sensitivity with simultaneous high spatial resolution and three-dimensional imaging in real time. Therefore various superparamagnetic coatings were developed, tested and characterized. Finally SPIONs and various polymers were combined directly and used for MPI-compatible models.
Currently many different medical imaging methods for a variety of situations are available. MPI ranks among the younger imaging methods and measures the spatial distribution of SPIONs . As the technological development has not been completed yet, MPI is not established in clinical practice. Nevertheless, the potential of MPI in medical applications is considered very high. The relevant SPIONs have already been used as contrast agents for magnetic resonance imaging (MRI) scans. In addition, the use as a coating for medical devices can be another approach in medical application. Above all, the use of catheters and stents would be interesting. Currently catheter laboratories use digital subtraction angiography to controll the application of catheters by X-ray guidance. However, due to the risks arising from the ionising radiation exposure, permanent recordings are not possible . When catheters and stents or endoscopes are coated with SPIONs, they can be inserted under MPI-observation. Also surgical instruments, produced out of a direct combination of polymers and SPIONs could allow operations under MPI-monitoring. Here, five different superparamagnetic coatings were developed, tested and characterized. Also SPIONs and different polymers were combined directly and used to create MPI-compatible models of catheters.
2 Material and methods
2.1 Development of superparamagnetic coatings
To produce the superparamagnetic coatings two basic components were used. The first component consists of commercially available varnish. Overall a total of five different coatings from five different varnishes was prepared. Two of them are based on polyacrylatepolyurethane dispersions (“Schöner Wohnen” DurAcryl Professional Weißlack, “Schöner Wohnen” ProfiDur Bunt-lack hochglänzend), two are based on acrylic dispersions (“Swingcolor” Klarlack 2 in 1 seidenmatt, “Schöner Wohnen” ProfiDur Buntlack hochglänzend) and one is based on a state oil paint (“Kreidezeit” Standölfarbe weiß). Considering medical application of the coatings it has been attempted to use biocompatible products. The second component for the superparamagnetic coatings are the SPIONs. The SPIONs were synthesized at the Institute of Medical Engineering at the Universität zu Lübeck. Varnish and SPIONs were mixed with a ratio of 1:1 and homogenized by vigorous stirring.
2.2 Development of a SPION-Polyurethane-Elastomer
The SPION-polyurethane-elastomer was prepared from two basic components. The first is the elastomer Vulkollan® from Bayer. This elastomer consists of two different components, the prepolymer Desmodur® 15S27 and the crosslinking agent 1,4-Butanediol Baytec® XL B, which are merged in a fluid state. This mixture is then subsequently cured at 100°C for 24 h and solidifies to polyurethane .
Before curing, the SPIONs are added as the second component. The primary goal is to prevent foaming of the polyurethane, caused by elimination and chain termination. The foaming occurs when a certain amount of water is added during the bonding process of the two liquid components . The SPIONs should be ideally in a dry state when they are added. Therefore, two different kinds of SPIONs were used. First, a nanoparticle powder from Sigma Aldrich and additionally SPIONs which were synthesized and concentrated at the Institute of Medical Engineering of the Universität zu Lübeck. For the concentration process of the SPIONs an absorbing granulate (Spectra/Gel® Absorbent) was used to substract a considerable amount of water and to reduce foaming. For the SPION-polyurethane-elastomer different molds have been tested, which can be seen in Figure 1. In Figure 1a) a sample cup was used, in Figure 1b) - Figure 1d) disposable pipettes were used, which were coated with silicon grease in order to prevent the inherence of the polyurethane. In Figure 1e) a polyoxymethylene (POM) mold, which was also coated with silicon grease, was used.
2.3 Characterization of the surface structure
The superparamagnetic coatings were tested in terms of their surface properties and their adhesion to various substrates. For this purpose the coatings were examined with atomic force microscopy (AFM). Thin PE films were coated with a 2:1 mixture of varnish and SPIONs. These films are intended to imitate the surface of potential applications, for example catheters. The films were prepared differently before applying the coating. One film was abraded with sandpaper, the other film remained unchanged. The superparamagnetic coatings were applied to the different films with a spatula. This experiment was also repeated with the pure varnishes that were applied exactly as the superparamagnetic coatings. The surface properties and the atomic topography of the coatings and varnishes were then examined with an AFM (DME, Denmark).
|Field strength||20 mT|
2.4 Characterization with magnetic particle spectroscopy
The detected magnetization of the coatings is intended to provide information on the MPI-compatibility. For this purpose the PE films, which were previously coated for the characterization by means of AFM, were used. Two 10 μl samples from each varnish were placed in Eppendorf cups and analyzed, as well as two 10 μl samples of the coatings and two 10 μl samples of the SPIONs. For each coating, a sample was cut from the PE films, folded and placed in Eppendorf cups. Subsequently, the MPS measurements followed.
Also the SPION-polyurethane-elastomer was examined with MPS. The different types of models that were tested can be seen in Figure 2.
2.5 Characterization with magnetic particle imaging
The Magnetic Particle Imaging (MPI) compatibility of the superparamagnetic coatings was also directly detected by MPI measurements. Therefore, the coatings were applied on a carrier material to imitate a catheter’s surface and form. The coated material consisted of two different laboratory tubes, which should serve as model catheters. These two tubes have different sizes and shore hardness values and are inserted into each other. The inner tube was coated on its outer surface and the outer tube therefore covers and protects the coating. The outer polyvinyl chloride (PVC) tube with a shore A hardness of 60° covers a rigid polytetrafluoroethylene (PTFE) tube with a shore A hardness of 72°. The outer PVC tube is transparent and has an inner diameter of 12 mm and an outer diameter of 16 mm. The PTFE tube has an outside diameter of 12 mm and an inside diameter of 10 mm. These model catheters were coated with the five different superparamagnetic coatings. The surface of the PTFE tubes was processed with sandpaper. Subsequently, six layers of the superparamagnetic coatings were applied with a brush on the PTFE tubes and then dried for 24 h. Afterwards, the PVC tube was pushed over the painted PTFE tube and MPI measurements were executed. The MPI measurements were performed with a FFL scanner. The measurement data is recorded at a frequency of 25 kHz with a gradient of 1.08 T m−1 with 1000 repetitions. The field of view (FOV) has a size of 25 mm2 .
Here, in Figure 4 the results of the AFM measurements of a particle-free varnish and superparamagnetic coatings are shown. It can be seen that varnish contains many small evenly distributed irregularities. Although coating on roughened surface shows less irregularities, most of them appear larger. The most regular topography can be seen in Figure 4c).
Figure 5 shows the comparison of MPS measurements of the pure SPIONs, the particle-free varnish and the coatings on different surfaces. Here, the MPS amplitude spectrum of the measurements is shown. It is normalized to the third harmonic and only the odd harmonics appear. The yellow line shows the spectrum of the pure SPIONs, the black line represents the blank measurement and the red line represents the particle-free varnish. The varnish has a similar spectrum as the blank measurement with strong noise. The SPIONs however, show a linearly decaying spectrum. The blue line represents the spectrum of a 2:1 mixture of varnish and SPIONs, which is nearly identical to the spectrum of the pure SPIONs. Also the coating on roughenend surface and the coating on untreated surface show satisfying results. Therefore, the coatings on both surfaces can be stated as MPI-compatible, due to their magnetization response during the MPS measurements. These results are comparable to the MPS measurements of the SPION-polyurethane-elastomer in figure 6. The MPS amplitude spectrum shows a satisfying response of the samples. Figure 7 shows the MPI measurements of the model catheter tube coated with the superparamagnetic coating. Figure 7a) shows the two-dimensional point spread function (PSF) and b) the three-dimensional PSF. The light gray or yellow areas represent the SPIONs signal. The black and the red areas display the area without a SPION signal detection. The coatings show an even boundary to the outer PVC tube. In both pictures, the coated area can clearly be distinguished from the laboratory tubes, which is a very satisfying result that clearly demonstrates their MPI-compatibility.
The superparamagnetic coatings were characterized regarding their surface texture and adhesion, its magnetization and their MPI-compatibility. One coating out of five provides satisfying results for all tests and characterization measurements and fulfills all of the requirements. Although the other four superparamagnetic coatings can be described as MPI-compatible, they are having drawbacks in adhesion and their regularity of the topography. It can also be stated, that none of the produced superparamagnetic coatings is constraining the superparamagnetic properties of the SPIONs that were added. This proves that the combination of SPIONs and synthethic polymers, such as varnish or solid polyurethane, is possible, which indicates a great potential for medical applications. To increase this potential of superparamagnetic coatings and SPION-polyurethane-elastomers, further research will be needed. One aspect that needs to be examined, is the biocompatibility of varnishes and polymers, which are combined with the SPIONs. In addition there are different ways to improve the coating process to create a smoother and more even surface. Furthermore, the foaming of the polyurethane needs to be reduced to a minimum. Not only MPI but also MRI could be considered as an application field for superparamagnetic coatings and SPION-polyurethane-elastomers, since SPIONs are already used as contrast agents for MRI.
Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent has been obtained from all individuals included in this study. Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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