4. Magnetic-Assisted Medical Applications
The size of nanomaterials is a critical factor for their behavior and distribution in the body. Small particles in the blood tend to aggregate or to be decorated with a protein corona and, because of their charge, create an electrical double layer. In this sense, the total size of particles characterized by the hydrodynamic diameter (d
H) can be measured i.e., using dynamic light scattering (DLS)
[67]. Both the hydrodynamic and physical size of MNPs determine their magnetic properties. In circulation, systemically injected MNPs circulate in the lumen of blood vessels, interacting with macrophages of the reticuloendothelial system (RES). Smaller USPIONs with d
H > 10 nm were characterized by longer blood half-life than SPIONs d
H > 50 nm
[63]. Depending on the hydrodynamic size as well as the electrical charge and other properties, pharmacokinetics and biodistribution of MNPs are rendered
[68]. MNPs with a d
H below than 15 nm are filtered by the kidney; MNPs with a d
H less than 100 nm accumulate in the liver in hepatocytes and Disse space; a d
H of 100–150 nm leads to the primary accumulation in liver that is based on the uptake by Kupffer cells and these larger hydrodynamic particles can also be trapped by splenic macrophages.
Since the liver and spleen are primary targets for MPN accumulation, they can also be targeted with these particles. The hydrodynamic size of 10–50 nm seems optimal for longer circulation time, but it should be stressed that not only size matters, but properties of the surface such as zeta-potential and hydrophilic/hydrophobic properties are critical factors for biodistribution
[63]. Non-specific biodistribution of MNPs was used for MRI enhancement of liver disease
[68][69]. Ferrucci and Starkre reported that 80% of intravenously injected non-specific SPIONs were internalized by Kupffer cells. Thereby, the MNPs create an MR-contrast that enables us to trace hepatic neoplasms
[69].
Four non-exhaustive areas in which magnetic materials are important (
Table 1). The first area includes the biggest success story for using magnetic fields for binding-mediated cell capturing. The chimeric antigen receptor (CAR) T cells have significantly advanced tumor therapies. This technology is based on the isolation of T cells from autologous donors and these cells are genetically engineered to target a specific antigen. The isolation of T cells is currently approved for treatment of B cell lymphoma, and it is expected that T cell engineering will enrich many other fields as well; clinical studies are ongoing in liver cancer. The group of Michael Sadelain pioneered T cell engineering and they have recently shown that macrophages play a key role in mediating the side effects of CAR T cell therapy
[70]. Other groups also attempt to manipulate other immune cells, such as natural killer cells
[71]. Microfluidics that facilitate binding-mediated cell capturing and release is a key technology for sorting cells in closed systems
[72][73][74]. The microfluidic concept for manufacturing lab-on-a-chip devices use MNPs and these allow for a quick analysis and an automated composition of an individual treatment. Currently, the CAR T cell therapies are mainly focused on B cell malignancies, but other types of cancer are being explored in clinical trials.
Table 1. Selected areas of applications for magnetic nanoparticles.
Role of MNP1 |
Area of Biomedical Application |
Literature |
Binding-mediated cell capturing |
Cell isolation and separation |
[70][72][73][74] |
Cell and tissue engineering |
[75][76][77] |
Cell patterning and concentration |
[78][79][80][81][82] |
Mechanical cell control |
Low-frequency magnetic field for cell destruction and induction of apoptosis |
[7][84][85] |
Differentiation of stem cells, modulation of cell division and motility |
[86][87][88] |
Fundamental study of macromolecules and cell‘s mechanical properties |
[89][90] |
Drug delivery |
Magnetic fluid hyperthermia of cancer |
[64][91][92] |
On-demand release of drugs via thermosensitive polymers or azo molecules from hybrid nanoplatforms |
[62][93][94] |
Targeting or delivery of drug or genes immobilized on surfaces |
[95][96][97][98] |
Imaging applications |
Reduction of T1 and T2 relaxation time of the water protons for the MRI-contrast |
[53][99] |
Imaging and detection via a non-linear magnetic signal |
[101][102] |
Improved detection of magnetic signals, imaging of liver fibrosis |
[103][104] |
Tissue engineering has been given further opportunities by magnetic fields, i.e., by facilitating the arrangement of different cellular layers
[75][76][77]. Magnetic fields have been used to establish three-dimenstional cell culture arrays, to enable cell patterning for the evaluation of the effect of fibroblasts on their capability of infiltration
[78][79][80]. Rieck et al. recently demonstrated that magnetic nanocarriers can be localized in specific areas of the body using magnetic fields. Their approach was done using complexes of lentivirus and MNPs in combination with magnetic fields. The group highlighted that using this method in the murine embryonic stem cell system offers the opportunity to site-specifically downregulate protein tyrosine phosphatase SHP2 by RNAi technology in selected areas with the pathogenic vessel formation
[81]. Using this principle would allow us to selectively transmit a specific payload to immune cells based on phagocytic uptake to diseased sites in the liver. Muthana et al. demonstrated that even magnetic resonance imaging machines might be usable to control the spatial concentration of MNPs
[82].
Here, the aim is to get control over the mechanical properties of cells. In the near future, the technology may be used to measure the stiffness of biological molecules or cellular organelles in vivo. Magnetic tweezers are used as a simple tool to study mechanical forces of biological molecules and cells
[83]. Low-frequency magnetic fields are applicable for directed cell destruction and induction of apoptosis
[7][84][85]. The concept of mechanical cell control has already been used to function as a cellular remote control, to modulate the stem cells differentiation, or to modulate the behavior of single cells
[86][87][88]. Magnetic micromanipulation based on the application of magnetic tweezers is a potent biophysical technique that is applicable for single-molecule unfolding, rheology measurements, and analyses of force-regulated processes in living cells
[89][90].
The third type of application is drug delivery. MNPs are a powerful tool for therapy to target killing of injured or infected cells, which may be achieved by, for example, native toxicity of the MNPs, thermomagnetic effect in magnetic hyperthermia, or targeted drug delivery and release
[64][91][92]. In particular, the field of drug delivery can potentially be highly enriched by magnetism-based applications. In addition to the above-mentioned application of MRI scanners to spatially concentrate magnetic particles, magnetic fluids, meaning magnetic particles in a dispersion, might be controlled magnetically. The release of drugs, for example, mediated by magnetism-enhanced thermosensitive polymers, might also be enriched by magnetic actions, for example, to trigger the release of different drugs from a single carrier from porous materials, remote-controlled drug release using azo-functionalized iron oxide nanoparticles, or in order to trigger heat and drug release from magnetically controlled nanocarriers
[62][93][94]. In addition to particulate or crystal-based systems, surface immobilized iron might be controlled using noninvasive magnetic stimulation
[95][96][97][98].
The fourth area in which magnetism has been used intensively is applications based on imaging. For instance, MRI-based imaging of liver fibrosis can be enabled by visualizing fibrosis based on non-invasive analysis of collagen or elastin. Fibrosis might even be visualized by labeling HSC, i.e., by monitoring a specific surface marker such as the folate-receptor, that has already been used for this purpose. Folate receptor-targeted particles have also led to improved specificity of tissue binding
[53][99]. In order to enable an early therapy of fibrosis, it is critical that it is also diagnosed at early stages. However, there is currently no established early stage marker. In the future, improved early recognition techniques such as circulating collagen fragments, i.e., the N-terminal propeptide of collagen III (Pro-C3) will very likely enable to perform “liquid biopsies” for early detection of fibrosis. Screening of Pro-C3 in plasma of patients has already been done in hepatitis C patients and has been demonstrated to be able to predict fibrosis progression
[99]. Other biomarkers might be circulating micro-RNA, and other types of RNA, such as long non-coding RNA (LncRNA), epigenetic analyses, and microbiome studies. Usually, biomarkers work best if they are combined to calculate a specific score such as the NAFLD fibrosis score, the fibrosis 4 (FIB-4) index, or the aspartate aminotransferase to platelet ratio index (APRI) score.
Non-invasive imaging techniques have high potential to confirm the findings from biomarker screening. Consequently, there is a high unmet need for the development of improved techniques for non-invasive diagnosis of liver fibrosis
[45]. In ref.
[45], the molecular MRI was used for distinguishing different fibrosis stages in a CCI
4-based rat model. Enhanced accuracy of
[53]detection was achieved by using a targeted USPIO-based contrast agent for MRI. The MRI contrast depends on the time of relaxation of protons that alters in the presence of a magnetic field generated by MNPs and it depends on the degree of interaction of the MNPs with protons. Furthermore, direct imaging of the distribution of MNPs is possible by magnetic particle imaging (MPI) introduced in 2005 and based on measurements of the nonlinear magnetic signal of MNPs
[101][102]. Potentially, MPI can enable an improved spatial and temporal resolution than the resolution of the MRI and because of the native biodistribution of iron oxide-based MNPs, the use of this technique for liver seems promising. Improved detection methodologies may also further improve the usage of magnetic particles as biosensors
[103]. Imaging probes such as those monitoring Elastin will very likely also improve imaging of liver fibrosis at later stages of the disease, as Elastin appears in late-stage fibrosis
[104].
The tripeptide arginine-glycine-aspartic acid (RGD) binds to integrin αvβ3 expressed on HSCs
[53]. Conjugation with RGD significantly improved targeting of administrated USPIONs
[53]. This allowed the authors to differentiate various liver fibrosis stages with MRI. The relaxivity of developed nanoparticles was higher than, for example, the earlier reported collagen-specific contrast agent based on Gd
3+. Further improvement was done by multimodal imaging with nanohybrids
[52]. In this study, it was suggested that the conjugation of USPIONs-SiO
2 with indocyanine green (ICG) dye may further improve near-infrared fluorescence imaging and RGD for targeting. This combination of imaging modalities enables it to perform multimodal imaging (NIR and MRI). To establish theranostic platforms, an additional drug should be attached
[105][106].
In particular, cell sorting technology which is currently enabling breakthroughs in cancer therapy is largely based on magnetic forces to sort cells, i.e., the magnetic-assisted cell sorting technology (MACS). These technologies are based on magnetic fields and enable sorting and manipulate cells in closed systems. Mechanical cell control is still in its infancy, but may significantly enrich single cell analysis methods. The field of drug delivery may assumingly expect high potential for breakthroughs due to the potential localized targeting of tissues or cells in vivo, to reduce side effects on non-target cells with enhanced specificity. Imaging applications are already profiting from steadily improved probes, improved methods for signal detection, and better biological targets. In fibrosis imaging, non-invasive assessment of Collagen and Elastin might in future enable significantly improved and more specific assessment of liver fibrosis (Figure 1).
Figure 1. Applications of magnetic nanoparticles in medicine and biotechnology. Four main fields of application for magnetic materials were highlighted and have chosen some representative schemes for each field of application.