Magnetic Nanoparticle-Mediated Heating for Biomedical Applications

Abstract

Magnetic nanoparticles, especially superparamagnetic nanoparticles (SPIONs), have attracted tremendous attention for various biomedical applications. Facile synthesis and functionalization together with easy control of the size and shape of SPIONS to customize their unique properties, have made it possible to develop different types of SPIONs tailored for diverse functions/applications. More recently, considerable attention has been paid to the thermal effect of SPIONs for the treatment of diseases like cancer and for nanowarming of cryopreserved/banked cells, tissues, and organs. In this mini-review, recent advances on the magnetic heating effect of SPIONs for magnetothermal therapy and enhancement of cryopreservation of cells, tissues, and organs, are discussed, together with the non-magnetic heating effect (i.e., high Intensity focused ultrasound or HIFU-activated heating) of SPIONs for cancer therapy. Furthermore, challenges facing the use of magnetic nanoparticles in these biomedical applications are presented.

Graphical Abstract

Introduction

Superparamagnetic iron oxide nanoparticles (SPIONs) have attracted a great deal of interest due to their unique properties such as their large surface area to volume ratio, long-term stability, easy manipulation with a magnet, high magnetization, and high saturation field [1–6]. The stability of SPION is affected by the balance of different attractive interactions such as magnetic, dipolar, and van der Waals interactions [7]. Therefore, bare SPIONs tend to aggregate together which affects their magnetic properties. To prevent this aggregation, they are often surface modified with protective shell(s) to stabilize them by either electrostatic or steric repulsion [8,9]. Therefore, SPIONs for biomedical applications often consist of a magnetic core and an outer organic layer which facilitates convenient functionalization to form SPION conjugates [10]. It is worth noting that magnetic nanoparticles can also perform as an energy transfer mediator and as a mechanical force vector. Therefore, through repeated alignments of magnetic spins and relaxations via processes of Néel rotation and Brownian motion in response to the alternating magnetic field, thermal energy can be generated from the magnetic nanoparticles [3,11–14].

SPIONs are mainly made of magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) [15,16]. Different transition metal ions such as manganese[17], nickel [18], copper [19], and cobalt [20] can be mixed with the iron oxides to form SPIONs. The unique magnetic characteristics of SPIONs give them the ability to be rapidly heated upon exposure to an alternating-current (AC) magnetic field, achieving a uniform heating throughout a large sample due to the large wavelength (at least a few meters) of the magnetic wave [21,22]. Therefore, magnetic induction heating (MIH) is advantageous to traditional heating methods such as microwave oven or water bath heating for which the heating is often heterogeneous and relies on thermal conduction [23–25].

This process, in which SPIONs are heated by an AC magnetic field (AMF) that is created by a few hundred (100 to 400 kHz) frequency AC passing through a solenoid coil, has quickly become an attractive method for various technological and biomedical applications [26,27]. When SPIONs are exposed to AMF inside the solenoid coil, there is a conversion of electromagnetic energy into thermal energy through the mechanisms of Brownian relaxation, Néel relaxation, and hysteresis losses [28–30]. The SPION heating efficiency is affected by the applied magnetic field frequency, nanoparticle size, nanoparticle anisotropy, and their collective behavior [31]. With their unique magnetic properties, SPIONs have been used in magnetic-based targeted drug delivery [32,33], cell differentiation [34,35], magnetic resonance imaging (as contrast agents) [36,37], biosensing [38–40], biological separation [41,42], and tissue engineering [43]. In this mini-review, we will focus on the most recent use of SPIONs for heating in biomedical applications specifically in magnetothermal therapy [44], nanowarming of cells, tissues, and organs [45–47], and high intensity focused ultrasound-mediated heating [48,49].

Magnetothermal Therapy

Magnetic induction heating is a non-contact method, for which an external magnetic field is applied throughout the sample that is placed within a solenoid coil for heating [24,27]. This magnetic field is generated at frequencies typically below 30 MHz where the dielectric losses are negligible compared to resistive losses [50]. A transformer/generator is used for generating the desired AC current that is input into a solenoid coil to create an AC magnetic field in the coil (Fig. 1a) [14,51]. Under the AC magnetic field, heat may be generated within the SPIONs through mainly Néel and Brownian relaxations, together with hysteresis losses (Fig. 1b) [28–30,52].

Fig. 1. Magnetic heating of SPIONs for magnetothermal therapy.

a), the Induction heater consists of a transformer/generator to input an alternating current (AC) into a solenoid coil to generate an AC magnetic field (AMF) for heating SPIONs. b), a schematic illustration of the mechanisms of AMF heating of SPIONS. SPIONs perform as an energy transfer mediator and as a mechanical force vector. Through repeated alignments of magnetic spins and relaxations via Néel (spin rotation) and Brownian (particle rotation) relaxations, in response to the AC magnetic field, thermal energy can be generated from the magnetic nanoparticles. Heat can also be generated due to the hysteresis loss. Reproduced with a permission from Ref. [14] (Copyright 2011 American Chemical Society), with modification.

Magnetothermal therapy is a type of thermal therapies, which uses the aforementioned magnetic induction heating for treating diseases like cancer. With this approach, a diseased tissue (e.g., tumor) is exposed to a magnetic field for heating to a high temperature (usually > 43 °C) in the presence of magnetic nanoparticles, to kill diseased cells/tissues [53–56]. Studies have shown that high temperatures can result in irreversible damage to cancer cells in less than a minute to a few tens of minutes dependent on specific temperature and cell type [57–63]. The increase in temperature leads to inactivation and denaturation of cellular proteins which lead to the dysfunction of cellular activities and eventually cell death [64–66]. Traditionally, cancer cells and tumors can be fatally heated through microwave (high-frequency electromagnetic wave) irradiation with [67–69], optical laser irradiation [70,71], or ohmic heat generation [72–75]. However, due to the nonspecific nature of heating (for microwave and ohmic heating) and the limited tissue penetration depth of laser, these techniques are difficult to use for thermal therapy of deep-seated tumors, especially within the principal body cavities such as in the abdomen, thorax, and skull, often yielding recurrent tumor growth [24]. Another major disadvantage of traditional electromagnetic heating techniques is their nonspecific nature: both the diseased tissue and the neighboring healthy tissues may be heated directly [68,76], which may cause undesired dismal side effects if the healthy tissue performs critical functions. As a result, research has been focused on the use of targeted/specific thermal therapy methods to eliminate the cancerous cells by heating while minimizing damage to the surrounding healthy tissue.

One way to address the aforementioned challenge associated with conventional thermal therapies is magnetothermal therapy using SPIONs to convert the magnetic energy carried by the AC magnetic field into thermal energy, which can then increase the temperature in a well-defined area of the tumor containing the SPIONs [77,78]. This has been a major focus of studies regarding SPIONs for magnetothermal therapy of cancer. For example, Kossatz et al. synthesized functionalized SPIONs for targeted magnetothermal therapy in a breast cancer xenograft model [79]. Through a combination of a tumor-specific cell internalization moiety (N6L) and an anti-cancer drug (doxorubicin: DOX) on the SPIONs surface, MDA-MB-231 breast cancer cells internalized the SPIONs and were then subcutaneously injected on the rear backside of the nude mice for hyperthermic treatment. The mice were exposed to a 435 kHz frequency AMF, generating hyperthermic temperatures in the tumor, which led to a 40% reduction in tumor volume, compared to the untreated control. However, despite the uniformity of the SPIONs in the tumor, maintaining hyperthermia inside the tumor remains a challenge due to different thermoregulatory conditions of the human body [80]. Therefore, factors that play important roles in SPIONs-mediated magnetic heating, such as the structural and magnetic properties of the SPIONs and the applied magnetic field, need to be further optimized [81].

A clinical study conducted by Dutz et al. concluded that the solenoid coil size can be optimized to generate an external magnetic field customized for the study [82]. When a small coil of 10 cm in diameter is used to generate an external magnetic field, patients were found to withstand the treatment for more than one hour without any major discomfort if the product of AC amplitude and frequency of 5·10⁹ Am⁻¹ s⁻¹ was not exceeded during magnetothermal therapy to minimize the eddy currents in healthy and/or in tumor tissue (which heats the normal and diseased tissue directly). A few years earlier, Johannsen et al., using an AMF with a frequency of 100 kHz and a variable field strength ranging from 2.5–18 kA/m, tested SPIONs in 10 prostate cancer patients [83]. Magnetothermal heating of SPIONs allowed the tissue to reach temperatures of up to 55 °C, resulting in a tumor reduction of up to 70%. However, this high temperature in the tumor due to the magnetic heating caused nonspecific heating to neighboring tissues (as a result of thermal diffusion) and localized discomfort in patients during magnetothermal treatments. SPIONs biodistribution studies can also provide information on the effective design of a better system for magnetic heating. Pham et al. investigated the ex vivo biodistribution of two block copolymer-coated SPIONs in several organs of both healthy and sarcoma transplanted Swiss mice delivered via an intravenous injection [84]. They showed that many of the SPIONs accumulated in the liver while the least was found in the kidney after 24 hours of SPION injection. The heating of the tumor was accomplished using a 5 kW power transformer or generator with an AC magnetic field of a 170–240 kHz, showing that a better heating is correlated to a higher SPION concentration.

Previous work has also focused on the modification of the physiochemical, structural, size, composition, and surface properties of the SPIONs to control the biodistribution, pharmacokinetics, and enhancing their heating efficiency at the tumor site [85–87]. Giustini et al. observed up to ~91% intracellular uptake of SPIONs after coating the nanoparticles with dextran [88]. Using sub-5 nm SPIONS, Wang et al. improved both the delivery and intratumoral distribution of these small SPIONs, demonstrating that they can easily extravasate from the tumor vasculature and readily diffuse into the tumor tissue [89]. This was followed by self-assembling in the acidic tumor microenvironment to limit their possibility of re-entering the blood circulation. In this study, it was found that the improved passive targeting, intratumoral delivery, and increased tumor retention of SPIONs are due to their easy extravasation into the tumor and restricted intravasation of clustered SPIONs. More recently, there has been an increasing interest in developing SPIONs that can be used for both magnetothermal therapy and magnetic resonance imaging of the tumor [90]. Du et al. developed an image-guided magnetothermal therapy using SPIONs to generate a well-distributed and high enough temperature to kill malignant tumor cells [91].

It is worth noting that intratumoral injection of SPIONs during magnetothermal therapy requires multiple injections at various sites of the tumor to increase SPION concentration and homogeneity of SPION distribution [92]. SPIONs were prepared and functionalized with CREKA (Cys-Arg-Glu-Lys-Ala), a pentapeptide that presents high affinity to fibrin-fibronectin complexes in the tumor, through a standard 1-ethyl-3-(3-dimethylamino propyl) carbodiimide hydrochloride/ sulfo N-Hydroxysuccinimide (EDC/sulfo-NHS)-coupling reaction [91]. SPIONs were synthesized and then intratumorally injected into orthotopic 4T1 tumor-bearing mice and placed under the AMF for hyperthermic treatment. The magnetic heating treatment was performed by applying an AMF at 400 kHz and 20 A for 10 min every 2 days for a total of 3 treatments. The functionalized SPIONs showed an improved targeting and SPION delivery uniformity, which resulted in a uniform increase of temperature and the total destruction of tumor. Intratumoral injection has been a preferred method for delivering SPIONs due to its ability to achieve a higher concentration of nanoparticles in tumor [93]. However, to improve tumor accumulation/retention and specificity, antibody/ligand conjugated SPIONs have been used to target various cancer-specific antigens for magnetothermal therapy [94,95]. For example, hyaluronic acid [96], Trastuzumab [97], Herceptin [94], transferrin, and TAT (YGRKKRRQRRR) peptide [98] conjugated SPIONs have been used to target various receptors on tumor cells. Interestingly, magnetic heating of SPIONs is useful for not only hyperthermic applications but also in the low-temperature field as discussed in more details below.

Nanowarming

Long-term organ and tissue banking would revolutionize current approaches to transplantation and regenerative medicine by providing critical improvement to donor-to-donor organ supply and transport that would improve short- and long-term graft function [99–101]. Cryopreservation, a method used to preserve cell and tissues in a living state of suspended animation at cryogenic temperatures (below −60 °C) for a considerable period of time [59,102], may provide better cell, tissue and organ transplantation outcomes as well as an extended window for organ/tissue assessment, recovery, and allocation [99,101,103,104].

Fundamentally, cryopreservation can be categorized into (1) slow freezing, in which the biological samples are loaded into a low temperature-resistant container and cooled at 1–2 °C/min to a low temperature usually between −40°C and - 80°C before transferring into a liquid nitrogen (at −196°C) tank for long-term storage [101,105,106], and (2) vitrification, in which biological samples are rapidly transformed from a liquid state into an amorphous glassy state through non-equilibrium cooling to minimize or eliminate ice formation [101,107,108]. Unlike slow freezing, the vitrification process requires a high cryoprotective agent (CPAs) concentration (5–8 M) and/or a high cooling rate (as high as 10⁶ °C/min depending on the CPA concentration) [108–110]. It is worth noting that to facilitate cryopreservation while minimizing different adverse effects that come with freezing (e.g., ice formation), CPAs like dimethylsulfoxide (DMSO) that affects the rate of water transport, ice nucleation, and ice crystal growth are commonly used [111], [104].

Vitrification has garnered much attention as a better alternative to slow freezing due to its ability to bypass the formation of ice crystals which may reduce/eliminate freezing injury and improved cell survival [46,112,113]. For effective vitrification, the critical warming rate (CWR), needed to thaw the organ for transplantation, must exceed the respective critical cooling rate (CCR), the lowest rate that allows a solution to vitrify, by one to three orders of magnitude [114–116]. This is to ensure vitrification during cooling and avoid devitrification (which can lead to cell injury) during warming [117–119]. Unfortunately, it is difficult to achieve a high warming rate. For example, the overall warming rates achievable by the most commonly used heating in 37 °C water bath are only 10–100 °C min⁻¹ depending on the sample size. Furthermore, the conventional water bath-based heating relies on thermal conduction from the boundary into the sample, which means nonuniform heating with large temperature gradient that creates thermal stress inside the sample, a major cause of mechanical damage to cryopreserved tissues and organs [120–122].

One potential solution to the challenges associated with the warming step of cryopreservation is SPION-based nanowarming [119]. SPIONs can be used to achieve a relatively uniform distribution within tissues and organs due to their small nanoscale size, compared to micro- and macro-scale tissues [123,124]. A combination of SPIONs and AMF of hundreds of kHz has generated a great interest in cryopreservation because it may provide uniform and rapid heating, which may reduce thermal-mechanical stresses and prevent recrystallization during the warming phase of cryopreservation [46,121,125]. It is worth noting that SPIONs heating under AMF is attributed to the dominant Néel relaxation mode and ferromagnetic hysteresis losses (Fig. 1b) [29,126].

Etheridge et al. presented a new approach for rapidly and uniformly warming a cryopreserved artery tissue sample of 1 ml via magnetic heating of SPIONs with an AMF of 20 kA/m at 360 kHz and 1 kW generated using a 2.75-turn water-cooled copper coil [21]. They demonstrated that heating rates as high as 300 °C/min could be generated with their SPION-based heating approach, which may minimize devitrification in the cryopreserved biospecimen. Moreover, Wang et al. looked at the effect of magnetic heating of SPIONs on cryopreservation of 200 μL of suspension of human umbilical cord matrix mesenchymal stem cells loaded in a plastic straw by vitrification (Fig. 2a) [125]. They found that magnetic heating of SPIONs can improve the heating rate during warming the cell sample, resulting in significantly improved cell viability post-warming. Further studies have shown that the cryopreserved cells can retain their stemness and functional capability of multi-lineage differentiation after warming. Of note, it is essential to employ SPIONs that are colloidally stable to maintain their heating ability. To improve stability and minimize aggregation of SPIONs in suspension, Gao et al. synthesized silica coated SPIONs [127]. In this study, the authors introduced 10 mg Fe mL⁻¹ of SPIONs suspended in the VS55 solution, a widely used vitrification solutions with high concentration of CPAs for vitrification, in a stepwise manner into the rat kidney through the kidney infrarenal aorta. The vitrified biospecimen in a 1.75 ml Eppendorf tube was magnetically warmed in a 2.75-turn copper coil at 20 kA m⁻¹, 360 kHz, and 1 kW, to create a warming rate of 130 °C min⁻¹. The results showed a 85.3% kidney cell viability after the magnetic nanowarming despite a heterogeneous distribution of the SPIONs in the biospecimen.

Fig. 2. Magnetic heating of SPIONs for nanowarming.

a), a schematic illustration of the procedure of preparing human umbilical cord matrix mesenchymal stem cells (hUCM-MSC) for cryopreservation by vitrification using a plastic straw with SPION-mediated magnetic induction heating for warming, b), a schematic illustration of using hydrogel microencapsulation of cells to eliminate direct contact between cells and SPIONs for warming the cryopreserved hydrogel-cell constructs, and c) a schematic illustration of the hot spot generated around a SPION during magnetic heating, due to ballistic thermal transport within the nanoparticle and Fourier diffusive heating in the adjacent medium. The sketches in a) and b) are reproduced with permission from Refs. [125] (Copyright 2016 Acta Materialia Inc. by Elsevier) and [134] (Copyright 2018 American Chemical Society), respectively, with modification.

Subsequently, Manuchehrabadi et al. designed a system that demonstrates that SPIONs-mediated heating can improve the viability of tissue and prevent physical failure during cryopreservation of 1 to 80 ml of samples of cryopreserved porcine arterial and heart valve tissues [46]. Coupling of a relatively low radiofrequency (RF) AC (60 kA/m alternating at 175 kHz on a 15 kW RF system) magnetic field and mesoporous silica-coated SPIONs, was employed to rapidly heat porcine carotid tissue samples after vitrification. In this study, silica-coated SPIONs were dispersed in CPA solutions before the AMF heating was used to enable highly uniform and rapid heating of the vitrified sample. After cooling and warming back, the viabilities of the sample with nanowarming (~90 °C/min) were found to be much higher than that with conventional water-bath heating (~7 °C/min). To achieve a high energy conversion while reducing the SPION dosages, Pan et al. designed an electromagnetic warming system in which low concentrations of SPIONs were added into the CPA solution [128]. They reported the addition of SPIONs generated rapid warming of the bulk cryopreserved sample at >200 °C min⁻¹. Chiu-Lam et al. synthesized polyethylene glycol (PEG)-coated SPIONs that are stable in the VS55 solution for cryopreservation by vitrification and used them for nanowarming of vitrified whole rat hearts [103]. In their study, they reported that these PEG-coated SPIONs can achieve high warming rates of up to 321 °C/min under an AMF of 42.5 kA/m alternating at 278 kHz. The SPIONs can be successfully perfused uniformly into the whole rat heart and be efficiently washed-out using histidine-tryptophan-ketoglutarate (HTK) solution after cooling, cryostorage in liquid nitrogen, and nanowarming. It is worth noting that despite efficient heating of the SPIONs, cells that are in their proximity may have the risk of overheating [83,129–131], and some studies have reported on the toxicity of high concentrations of SPIONs to the neighboring cells [132,133].

To overcome this challenge, recent studies have focused on developing systems in which cells do not have any direct physical contact with the SPIONs [134,135]. In these studies, SPIONs are suspended in the CPA solution outside the alginate hydrogel-cell constructs (Fig. 2b). Stem cells can be encapsulated inside the alginate hydrogel construct and avoid physical contact with the SPIONs to eliminate their potential toxicity, if any, to the cells. After nanowarming with SPIONs under an AMF generated by a 6-turn coil at 375 kHz, >80% cell survival was observed for encapsulated stem cells, due to the protection of the cells from ice crystal formation by the alginate hydrogel and nanowarming. This was in contrast to the < 27% cell survival for non-encapsulated cells where most of the cells were killed by ice formation (i.e., devitrification) during conventional warming with a water bath. It is worth noting that the mechanisms of nanowarming on inhibiting devitrification/ice recrystallization during warming may be attributed to both the global (due to Fourier diffusive heating in the cryopreservation solution) and local (due to ballistic heating within and in the vicinity of the SPIONs) inhibition mechanisms (as illustrated in Fig. 2c). The former is due to the enhanced overall warming rate of the entire sample to surpass the CWR for inhibition of devitrification (via suppression of nucleation) and/or ice recrystallization (via suppression of both nucleation and growth) globally in the entire sample during warming. The latter is due to the thousands to millions and billions (depending on the concentration of SPIONs in the sample) discrete hot spots adjacent to the SPIONs in the sample, which may inhibit ice recrystallization (mainly via suppression of ice nucleation) locally since ice nucleation is also a discrete stochastic event [101]. Magnetic heating of the SPIONs to generate hotspots in the vicinity of the SPIONs is possible, because the size of the size of the SPIONs is similar to the mean free path of the materials in them, leading to ballistic thermal transport in the nanoscale particles that is much less efficient as the Fourier condition/diffusion of heat in bulk materials [136–141].

SPIONs-embedded hydrogels can also be used to suppress ice recrystallization with both enhanced warming and microscale thermal disturbance. Cao et al. made alginate-based hydrogel constructs with embedded graphene oxide (GO)-Fe₃O₄ nanocomposites capable of inhibiting ice recrystallization by infrared irradiation that generates heat via GO and magnetic field for generating heat via Fe₃O₄ [142]. They evaluated both the photothermal and magnetothermal effects of stem cell-laden hydrogel constructs under near-infrared laser and AMF. Further studies showed that after warming, stem cells inside the hydrogel retain their original structure with high viability (> 80%) after long-term culture (7 days).

SPION-based nanowarming has demonstrated the potential to be a revolutionary technology for the field of cryopreservation, which is invaluable for cell, tissue, and organ banking. The SPIONs-based magnetothermal effect has also been utilized in other biomedical areas such as glucose regulation [143,144]. For instance, Stanley et al. decorated a modified temperature-sensitive channel, TRPV1, with antibody-coated SPIONs which were heated with a low-frequency magnetic field to regulate the protein production [145]. In this study, the heating of SPIONs led to an increase in temperature rise at the TRPV1 calcium gate, which subsequently led to the synthesis and release of insulin to lower the blood glucose level in mice. Interestingly, SPIONs not only can be heated by magnetic field, but also others like the acoustic field with mechanical wave as summarized below.

Enhancement of HIFU Heating

High-intensity focused ultrasound (HIFU) is a minimally invasive ablation technique that uses both heat and acoustic cavitation to destroy diseased cells and tissues including tumors [146–148]. HIFU-mediated heating is comprised of two major elements as shown in Fig. 3: an ultrasound generator and a piezoelectric transducer [149]. The operating mechanism centers around the generation of a focused ultrasound field by the piezoelectric transducer. This transducer is able to generate ultrasound waves with a frequency ranging from 1 to 7 MHz [150]. The acoustic energy is generated and detected using piezoelectric crystals that function as an interface between electrical and mechanical energy [151]. These ultrasound waves can travel through the tissue and converge at one focal point, where their energy is converted into thermal energy. The temperature at the focal point could reach more than 60–95 °C within seconds, causing instantaneous cell death [48]. It is worth noting that an additional coupling medium such as water is placed in between the transducer and the target tissue surface to minimize acoustic loss [152]. The HIFU beam focusing minimizes the possibility of thermal damages to non-targeted tissue [153]. For this reason, it has emerged as a promising technology that can be employed as either a standalone treatment method or an adjuvant method that can enhance the effectiveness of other available treatments due to its noninvasive nature [154,155].

Fig. 3. HIFU heating of SPIONS-laden biospecimen.

A high intensity focusing ultrasound (HIUF) system is comprised of a function generator, a gain-variable power amplifier, a transducer, a water tank, an HIFU imaging probe, and a thermal sensor. The transducer creates an ultrasound beam which is focused to a focal point in the target tissue. The kinetic energy of the ultrasound beam is converted into thermal energy, which may cause instantaneous damage to diseased cells and tissues. This heating process may be enhanced by using SPIONs delivered in the biospecimen.

However, despite its great potential in clinical treatments of various cancers (e.g., breast [156], kidney [157], liver [158], and prostate [159] cancers, together with soft tissue sarcoma [160], there have been many challenges in using acoustic energy for tumor treatment [48,161]. The assumption that there is a linear attenuation of the acoustic energy in the soft tissues between the transducer and the target might not always be accurate because the soft tissues are heterogeneous and might attenuate acoustic energy differently [162,163]. This has led to unablated cells in certain areas of the tumor as well as damage to normal tissue due to high acoustic power [164]. Furthermore, due to the high level of acoustic energy required during HIFU for ablating a large tumor, skin burns and other adverse effects stemming from high acoustic energy and long sonication time such as overheating of surrounding healthy tissue, have been observed in certain treatments [165,166].

To overcome these challenges, various exogenous absorbers such as porphyrin, xanthene, and microbubbles that can enhance the thermal ablation by increasing the attenuation and dissipation of acoustic energy, leading to lower ultrasound power, have been explored [161,167]. More recently, SPIONs have emerged as effective sonosensitizers for focused ultrasound therapy [30]. SPIONs increase the attenuation of the acoustic wave within the local tumor area, which may lead to the heating by the ultrasound while sparing the surrounding healthy tissues [168]. For this reason, they have been used to facilitate and enhance HIFU thermal ablation.

Ho et al. investigated the advantages of SPIONs in combination with HIFU to destroy HeLa multicellular tumor spheroids [169]. The HIFU transducer was operated with a frequency of 1.1 MHz, pulse repetition frequency of 1.67 kHz, and peak negative focal pressure of 7.2 MP for 10 sec. After exposing the cells to both SPIONs and HIFU, there was a significant increase in the HIFU induced inertial cavitation which led to the enhancement of the rate of destruction of tumor spheroids. Subsequently, Ahmad et al. used SPIONs to locally enhance heating at low powers during HIFU ablation [170]. In this study, the transducer with a focal length of 6.26 cm, an inner diameter of 2.2 cm, and an outer diameter of 6.4 cm operating at a frequency of 1.025 MHz was used to test various SPIONs concentrations (0% as control, 1%, and 3% w/v) at three distinct acoustic powers (5.2, 9.2, and 14.3 W), which resulted in higher thermal doses for lower acoustic power. They found that when SPIONs are used, the required power to obtain an efficient thermal dose that can cause cell necrosis in tumors can be reduced substantially.

More recently, Devarakonda et al. examined the temperature rise, thermal dose, and the lesion volume of tumor after HIFU with/without SPIONs [164]. Using different SPION concentrations (0% for control, 0.0047%, and 0.047% w/v) that are at least 20 times lower than those previously used in the literature [170], they found that by using only 0.047% SPIONs to enhance HIFU ablation, it was possible to decrease the power required to obtain a tumor lesion volume of 13 mm³ by half. Kaczmarek et al. assessed the utility of SPIONs in ultrasound hyperthermia by enhanced heating in low-power ultrasound [168,171]. SPIONs with the various concentration (0.26–0.35% w/w) were used. The acoustic power of the transducer was set to 2.5 W with a frequency of 1 and 3.5 MHz. It was found that the simultaneous use of the SPIONs and the HIFU lead to a synergistic increase of temperature during ultrasound heating and enabled more precise control over the heating process. In an effort to investigate the heating process of SPION-enhanced HIFU, Sadeghi-Goughari et al. used the principle of conservation of energy for heat transfer mechanism to derive a set of HIFU equations that govern the temperature variation during thermal ablation [161]. In this study, a numerical model was developed to simulate the absorption mechanism of HIFU in the presence of SPIONs which leads to the temperature rise during the sonication period. Moreover, a series of experiments were performed to verify the accuracy of the model. They found that the temperature rise during HIFU sonication emanates from the transport process that takes place at the boundaries between SPIONS and the surrounding medium. Additionally, they reported that the effects of SPION heating can be improved by amplifying the acoustic power and the SPION concentration.

With their multifunctional properties, SPIONs can also be used as contrast agents for both HIFU and magnetic resonance imaging [172]. Sun et al. successfully integrated SPIONs into poly(lactic-co-glycolic acid) (PLGA) microcapsules to simultaneously enhance ultrasound cancer imaging and the HIFU ablation in a rabbit-bearing breast cancer model [173]. Shortly after, Zhou et al. developed PLGA-SPION microspheres for dual imaging and HIFU ablation of liver tissue in rabbits [174]. The focal length of the piezoelectric transducer in this study was 145 mm with a diameter of 220 mm and an operating frequency of 0.94 MHz while a 3.5–5 MHz ultrasound transducer was applied for imaging. The PLGA-SPION microspheres were introduced through intravascular injection followed by HIFU ablation. The targeted tissue was then subjected to pathological examination to determine its structural changes and showed that PLGA-iron oxide microsphere could enhance ultrasound imaging and efficiently enhance HIFU ablation of liver tissue in rabbits. Given these findings, the use of SPIONs during HIFU tumor ablation has the potential to reduce damage to healthy tissue and reduce the HIFU power needed to destroy tumors. Moreover, magnetic resonance imaging, when combined with HIFU, would provide image guidance by identifying tumors for targeting and for tracking the ablation lesion with greater resolution.

Summary and Outlook

In this mini-review, different biomedical applications of SPION-based heating and their respective mechanisms are discussed. A summary of the various types of SPIONs together with the methods of their synthesis and the major results of their use for the three different biomedical applications discussed in this work, is given in Table 1. The future of SPION-based magnetic induction heating in biomedical applications truly holds great potential, especially in cancer therapy and in nanowarming/cryopreservation of cells, tissues, and organs. To date, magnetic heating has been explored to treat many different malignancies [175,176]. However, despite progress made to achieve efficient magnetic heating of SPIONs and use it in both therapeutic and cryopreservation applications, there is still more work that needs to be done to fully integrate these methods in a clinical setting. Different studies and reports have shown that magnetic heating of SPIONs depends not only on their size and other physical properties but also on the frequency and the heating power used during the process. For these reasons, it is imperative to standardize and establish a clinically acceptable SPION size, composition, shape, and surface-functionalization that can be acceptable for clinical applications. Moreover, the nonuniformity of the magnetic field and the difference in its absorption and dissipation by different tissues in organs have proven to be a hurdle in generating a uniform warming [127]. Therefore, a standardized frequency and heating power appropriate for efficient magnetic heating need to be established.

Table 1.

A summary of the various types of SPIONs together with the methods of their synthesis and the major results of their use for the three biomedical applications discussed in this review

Type	Synthesis method	Major result	Ref.
Magnetothermal therapy
MF66 MNP	Co-precipitation of Fe2+ and Fe3+ followed by dimercaptosuccinic acid stabilization	Magnetic hyperthermia of breast cancer led to a 40% tumor reduction	[79]
Magforce NPs	Magforce AG company	Thermal therapy of cancer tumor	[83]
Copolymer coated Fe3O4 nanoparticles	Co-precipitation of Fe2+ and Fe3+ followed by polystyrene-co-polyacrylic acid, polylactide acid and polyethylene glycol coating	Magnetic inductive heating of organs of mouse models	[84]
mPEG coated Fe3O4nanoparticles	Solution-phase thermal decomposition of Fe(acac)3 in oleic acid and benzyl ether	high performance magnetic hyperthermia	[85]
Anionic Iron Oxide Nanomagnets	Alkaline coprecipitation of iron (III) and iron salts followed by citrate stabilization	Colloidal Mediators for Magnetic Hyperthermia	[86]
Oxide Nano-octopods	Nonhydrolytic thermal decomposition of Fe(acac)3 in the presence of oleic acid and oleylamine	Magnetic hyperthermia treatment	[87]
Cys-Arg-Glu-Lys-Ala modified MFNPs	High-temperature thermal decomposition of Fe(acac)3 in the presence of oleic acid	Combined hyperthermia and MRI/MPI of malignant tumor	[91]
Magnetic multicore nanoparticles (MCNP)	co-precipitation of Fe2+ and Fe3+ followed by carboxymethyl dextran coating	Tumor heating within 60 seconds	[194]
Water-Dispersible Sugar-Coated Iron Oxide Nanoparticle	Thermal decomposition of Fe(acac)3 followed by sugar coating	Relaxometry and magnetic hyperthermia	[195]
AEH-Fe2O3 nanomagnetic beads	Magnetic iron oxide particles encapsulated within a coating formed from a polyester of valeric and butyric acids	Treated tumors decreased in volume by 50 to 94%	[196]
PVP coated Magneto-plasmonic nanoparticles (MagPlasNPs)	Co-precipitation of Fe2+ and Fe3+ followed by gold seeding	Photothermia with magnetic hyperthermia of cancer	[197]
CoFe2O4@MnFe2O4 MnFe2O4@ CoFe2O4	Thermal decomposition of MnFe2O4 onto the surface of CoFe2O4	Antitumor therapeutic heating	[198]
Zn0.4Fe2.6O4MNP	Magnetic nanoparticles are coated with SiO2 and then amine-functionalized with geldanamycin	Resistance-Free Apoptotic Hyperthermia	[199]
Fe3O4 nanoparticles	Oxidation of pentacarbonyl iron followed by purification process	Selective inductive heating of lymph nodes	[200]
Biomimetic magnetic nanoparticles (BMNPs)	The precipitation of inorganic magnetite, followed by an oxidation of a strong base (NaOH)	Targeted magnetic hyperthermia	[201]
Magnetosome chains	Extracted from magnetotactic bacteria	Efficient penetration and maximum cell destruction	[202]
DOX/PLGA-Fe MNP	Dispersion of Fe powder into DOX/PLGA solution by stirring	Chemo- magnetic-hyperthermia- induced synergistic tumor eradication	[203]
Oleic acid functionalized Fe3O4	Co-precipitation of FeSO4 and FeCl3 followed by NH4OH	Tumor growth inhibition by apoptosis and Hsp90/AKT modulation	[204]
mAb-guided bioprobes	Polyethylene glycol-iron oxide-impregnated dextran nanoparticles functionalized with dodecanetetraacetic acid	Thermoablative therapy for human Breast cancer in mice results in tumor reduction	[205]
PEGylated Mn-Zn ferrite nanocrystals	Thermal decomposition of Fe(acac)3 in presence of Zn(acac)2 and manganese (II) acetylacetonate followed by oleylamine coating	Induce the apoptosis of tumor cells, inhibit the angiogenesis of tumor vessels, and suppress the tumor growth	[206]
Iron oxide nanocubes	Thermal decomposition of Fe(acac)3	Magnetic hyperthermia and photothermal bimodal treatment leading to a complete apoptosis-mediated cell death	[207]
Poly (D, L-lactic-co-glycolic acid) encapsulated SPIONs	Chemical coprecipitation of Fe3+ and Fe2+ in ammoniacal medium followed by solvent evaporation for encapsulation	Cancer destruction within a short period of time (120 minutes) by initiating early and late apoptosis	[208]
MIONs	Thermal decomposition of Fe(acac)3 in a mixture of oleic acid, oleylamine, and long acyl chain diols in benzyl ether	Effectively heat tumor tissues at a minimal dose	[209]
Nanowarming
Ferrotec EMG-308 solution	Fe3O4 nanoparticles coated with an anionic surfactant in aqueous suspension	Thawing of a cryopreserved artery tissue sample	[21]
Silica coated EMG308, Ferrotec/sIONPs	EMG308, Ferrotec nanoparticles coated with mesoporous silica	Thawing cryopreserved porcine arterial and heart valve tissues with improved viability	[46]
Polyethylene glycol (PEG)-coated SPIONs	chemical coprecipitation of Fe3+ and Fe2+ followed by PEG coating	Successful perfusion of vitrified whole rat hearts	[103]
Fe3O4 nanoparticles	Chemical coprecipitation of Fe3+ and Fe2+ followed by aqueous ammonia mixture	Significantly facilitates rewarming and improves the cryopreservation outcome of human umbilical cord matrix mesenchymal stem cells	[125]
Amine group functionalized Fe3O4	Fe3O4 nanoparticles purchased from Ocean Nanotech LLC, San Diego, CA, USA	Rewarming of bulk sample	[128]
Fe3O4 NPs	Chemical coprecipitation of Fe3+ and Fe2+	Low-Cryoprotectant Vitrification of Stem Cell-Alginate Hydrogel Construct	[134]
Fe3O4 NPs	Chemical co-precipitation of Fe2+ and Fe3+ ions	Massive-Volume Vitrification of Stem Cells with Low-Concentration Cryoprotectants	[135]
Graphene oxide (GO)-Fe3O4 nanocomposites	GO is added to the mixture of acetate stabilized Fe3O4	inhibit ice recrystallization by infrared irradiation that generates heat via GO and magnetic field for generating heat via Fe3O4	[142]
DP6+sIONP	Fe3O4 nanoparticles coated with a silica layer and functionalized with polyvinyl pyrrolidone	Warming of cryopreserved sample	[210]
Mesoporous silica coated Fe3O4 nanoparticles	PVP coated nanoparticles are coated with silica shell followed by stabilization of PEG-TMS	Nanowarming of a cryopreserved rat kidney infrarenal aorta with preserved morphology and good viability at the cellular level	[211]
CP-DMSA- MNPs	Chemical co-precipitation of Fe2+ and Fe3+ ions followed by dimercaptosuccinic acid coating	Multi-Hot-Spot Induction and Sequential Regulation	[212]
TD-PMAO- MNPs	Thermal decomposition of Fe(acac)3 followed by polymaleic anhydride-alt-1-octadecene coating	Multi-hot-spot induction and Sequential Regulation
OP-PAA-MNPs	Oxidative precipitation of FeSO4 by NaOH followed by a coating of polyacrylic acid	Multi-hot-spot induction and Sequential Regulation
Magnetoliposomes	Surrounding the iron-oxide nanoparticles (Fe3O4) with phospholipid bilayer	Magnetic Fluid Hyperthermia Efficacy on Pancreatic Tumor Cell reached 95% tumor cell death	[213]
HIFU-activated heating
MNPs	Not available (purchased from U.S. Research Nanomaterials, Inc.)	NPs play the major role in the temperature rise during HIFU sonication	[161]
Magnetic nanoparticles (mNPs)	Purchased as EMG705 series, Ferrotec (USA)	Reduced damage to healthy tissue, and reduced the procedure time, during tumor ablation using HIFU	[164]
Magnetite (Fe3O4) nanoparticle agglomerates	Chemical co-precipitation of Fe2+ and Fe3+ ions with ammonia solution	Magnetite nanoparticle agglomerates enhance the efficacy of HIFU in destruction of tumor spheroids	[169]
mNPs (Fe3O4))	Purchased as EMG705 series, Ferrotec (USA)	significantly reduce the time for HIFU thermal ablation	[170]
SPION	Chemical coprecipitation using ferric and ferrous salts in alkali medium followed by sodium oleate coating	The presence of SPION increases the absorption of ultrasound energy leading to increased temperature	[171]
Multifunctional PFH/DOX@PLGA/Fe3O4-FA nanocomposites	Double-emulsion	Demonstrated to efficiently suppress the tumor growth based on the enhanced and synergistic chemotherapy and HIFU ablation	[172]
Superparamagnetic PLGA-iron oxide microcapsule	Double emulsion (water/oil/water) evaporation process	Dual-modality US/MR imaging and high intensity focused US breast cancer ablation	[173]
uSPIO/PLGA microspheres	A double emulsion evaporation method was used to synthesize ultraminiature superparamagnetic PLGA–iron oxide microcapsules	Significantly enhance dual-modality US/MR imaging and HIFU synergistic therapy with an intravenous administration method	[174]

AEH: arterial embolization hyperthermia, mPEG: methoxy polyethylene glycol, acac: acetylacetonate, PVP: polyvinyl pyrrolidone, GO: graphene oxide, TMS: trimethyl (TM) and succinimide ester, DMSA: dimercaptosuccinic acid, PMAO: polymaleic anhydride-alt-1-octadecene, PAA: polyacrylic acid, CP: Chemical co-precipitation, TD: thermal decomposition, OP: oxidative precipitation

Furthermore, there is still difficulty in synthesizing SPIONs with special properties such as negligible cellular uptake and cellular association, which would allow their maximum removal after nanowarming for cryopreservation applications. Hence, further studies are needed to generate SPIONs that are colloidally stable to maintain their heating potential while being able to be perfused in and out of the vasculature of the tissue/organ conveniently. Certain hurdles such as the overheating of SPIONs that lead to the destruction of neighboring tissues have also been raised [76]. However, this can be avoided by functionalizing the SPIONs with targeting moieties which allow them to bind to the target only for increasing the SPIONs localization to the targeted tissues/cells before applying the magnetic field [16,78].

Additional technologies in which external energy (e.g., near-infrared laser radiation) is coupled to the SPIONs to increase intratumoral temperatures should be explored more [177]. The combination of therapy and diagnostics (theragnostic) including the combination of HIFU and SPIONs has garnered significant attention due to their ability to introduce a targeted treatment and monitor the response to the therapy. Moreover, the simultaneous use of the SPIONs and HIFU has proven very efficient in reducing the amount of acoustic power necessary for the tissue targeting while enhancing the heating at the local region of the tumor and allowing more precise control over the heating process.

Despite this feat, the use of HIFU-SPIONs mediated heating has been developing relatively slower than other applications such as MRI, or magnetothermal therapy. Even though HIFU is a U.S. Food and Drug Administration (FDA)-approved method for the treatment of various diseases [178–181], there are still challenges that are associated with the use of SPIONs. These include the effective delivery of stable SPIONs into the target area of the tumor. The tumor microenvironment contains different barriers, such as a thick stroma, high interstitial fluid pressure, and macrophage uptake that might hinder the efficient and uniform accumulation of the SPIONs into the tumor [182,183]. Additionally, unlike the leaky vasculature in mouse models, the vasculature of human tumor has fewer openings with a slower growth rate, which might limit SPION accumulation into the tumor [184]. Therefore, clinical studies on humans are needed to advance this technology. Other new techniques and methods that combine both magnetic heating and imaging should be explored to facilitate more targeted heating via guidance by imaging for therapeutic applications [91,185,186]. Recently, new emerging technologies such as artificial intelligence may be used to predict the efficiency of SPIONs heating based on their size, composition, and frequency before the actual use [187]. This technology may also be used to design and optimize different types of SPIONs, or other magnetic nanoparticles customized for different medical uses [188,189].

It is worth noting that with the increasing biomedical applications of SPIONs, there has been also an increase in public concerns about their biosafety, long-term biodistribution, and clearance from the body. However, these concerns have been assuaged by a few recent FDA approvals of SPION-based nanomedicines for human use. These include Feraheme® (ferumoxytol), a non-stoichiometric poly glucose sorbitol carboxymethyl ether capped SPION used for the treatment of iron deficiency associated with chronic kidney disease [190], together with Ferumoxtran-10, Ferucarbotran (Resovist®), and Feridex® (ferumoxides) which are approved for use as magnetic resonance imaging contrast agents [191]. These recent FDA approvals cement the biosafety of SPIONs and their efficacy in treating different diseases. It is anticipated that more SPIONs formulations will be developed and used soon for different biomedical applications. This may include SPION-based treatment of diseases in hard-to-reach deep organs (e.g., glioblastoma multiforme and Parkinson’s and Alzheimer’s diseases in the brain) [192,193]. Furthermore, SPION-based nanowarming studies so far have been focused on smaller samples. Scaling this technology to larger samples like organs (e.g., heart, liver, and lung) would provide a game-changing and lifesaving technology in the organ transplantation field.