Magneto-aerotactic bacteria deliver drug-containing nanoliposomes to tumour hypoxic regions.

Oxygen-depleted hypoxic regions in the tumour are generally resistant to therapies. Although nanocarriers have been used to deliver drugs, the targeting ratios have been very low. Here, we show that the magneto-aerotactic migration behaviour of magnetotactic bacteria, Magnetococcus marinus strain MC-1 (ref. 4), can be used to transport drug-loaded nanoliposomes into hypoxic regions of the tumour. In their natural environment, MC-1 cells, each containing a chain of magnetic iron-oxide nanocrystals, tend to swim along local magnetic field lines and towards low oxygen concentrations based on a two-state aerotactic sensing system. We show that when MC-1 cells bearing covalently bound drug-containing nanoliposomes were injected near the tumour in severe combined immunodeficient beige mice and magnetically guided, up to 55% of MC-1 cells penetrated into hypoxic regions of HCT116 colorectal xenografts. Approximately 70 drug-loaded nanoliposomes were attached to each MC-1 cell. Our results suggest that harnessing swarms of microorganisms exhibiting magneto-aerotactic behaviour can significantly improve the therapeutic index of various nanocarriers in tumour hypoxic regions.

Simultaneous steering and imaging of magnetic particles using MRI toward delivery of therapeutics.

Magnetic resonance navigation (MRN) offers the potential for real-time steering of drug particles and cells to targets throughout the body. In this technique, the magnetic gradients of an MRI scanner perform image-based steering of magnetically-labelled therapeutics through the vasculature and into tumours. A major challenge of current techniques for MRN is that they alternate between pulse sequences for particle imaging and propulsion. Since no propulsion occurs while imaging the particles, this results in a significant reduction in imaging frequency and propulsive force. We report a new approach in which an imaging sequence is designed to simultaneously image and propel particles. This sequence provides a tradeoff between maximum propulsive force and imaging frequency. In our reported example, the sequence can image at 27 Hz while still generating 95% of the force produced by a purely propulsive pulse sequence. We implemented our pulse sequence on a standard clinical scanner using millimetre-scale particles and demonstrated high-speed (74 mm/s) navigation of a multi-branched vascular network phantom. Our study suggests that the magnetic gradient magnitudes previously demonstrated to be sufficient for pure propulsion of micron-scale therapeutics in magnetic resonance targeting (MRT) could also be sufficient for real-time steering of these particles.

Biocompatible Pressure Sensing Skins for Minimally Invasive Surgical Instruments.

This paper presents 800-µm thick, biocompatible sensing skins composed of arrays of pressure sensors. The arrays can be configured to conform to the surface of medical instruments so as to act as disposable sensing skins. In particular, the fabrication of cylindrical geometries is considered here for use on endoscopes. The sensing technology is based on polydimethylsiloxane synthetic silicone encapsulated microchannels filled with a biocompatible salt-saturated glycerol solution, functioning as the conductive medium. A multi-layer manufacturing approach is introduced that enables stacking sensing microchannels, mechanical stress concentration features, and electrical routing via flexcircuits in a thickness of less than 1 mm. The proposed approach is inexpensive and does not require clean room tools or techniques. The mechanical stress concentration features are implemented using a patterned copper layer that serves to improve sensing range and sensitivity. Sensor performance is demonstrated experimentally using a sensing skin mounted on a neuroendoscope insertion cannula and is shown to outperform previously developed non-biocompatible sensors.

Assessment of navigation control strategy for magnetotactic bacteria in microchannel: toward targeting solid tumors.

This paper presents a Magnetotactic Bacteria (MTB) navigation and aggregation technique that allows targeting without prior knowledge of the exact architecture of the vessels network. The MTB's active motility combined with magnetotaxism (their ability to swim following the magnetic field direction) offer new possibilities for the delivery of drugs to tumors. Many tumor microenvironment parameters such as the malformed angiogenesis capillaries, the heterogeneous blood flow, and the high interstitial pressure affect the delivery of blood-borne drugs to the tumor. Microorganisms used as microcarriers might be helpful in bypassing these limitations while helping to uniformly distribute the drug in the targeted area. Since the angiogenesis network of blood vessels that the tumors recruit is highly disorganized and unpredictable, the magnetic control method adopted account for these parameters to achieve targeting. We demonstrate the effectiveness of the proposed method using a microchannel network offering a complex pattern considered as a worst-case navigation situation. Besides targeted drug delivery to tumor sites using bacterial carrier, aggregation of microorganisms is required for micromanipulation and microassembly.

Effect of the chain of magnetosomes embedded in magnetotactic bacteria and their motility on magnetic resonance imaging.

This paper investigates the influence of the magnetosome's chain, the motility, and the bacterial cell of MC-1 magnetotactic bacteria (MTB) on the Magnetic Resonance imaging (MRI) contrast. Because of its embedded magnetic nanoparticles, that allow magnetic guidance and imaging contrast generation under MRI, magnetotactic bacteria are being considered for therapeutic drug delivery to tumors. In order to separately investigate the different potential sources of contrast in MRI, we used three samples of MC-1 MTB. The first sample was constituted of magnetic bacteria that successfully synthesize magnetic nanoparticles. MC-1 bacteria that do not synthesize magnetosomes form the second sample while the third sample is constituted from dead MC-1 magnetic bacteria containing magnetic nanoparticle. T(2)-weighted magnetic resonance images were obtained for multiple echo times. T(2) was then estimated by fitting the signal intensity data for different echo time values to a monoexponential decay curve. It is found that nanoparticles synthesized by MC-1 MTB are the predominant source of contrast in MRI over motion and the cell body.

Magnetotactic bacteria penetration into multicellular tumor spheroids for targeted therapy.

Preliminary experiments showed that MC-1 magnetotactic bacteria (MTB) could be used for the delivery of therapeutic agents to tumoral lesions. Each bacterium can provide a significant thrust propulsion force generated by two flagella bundles exceeding 4pN. Furthermore, a chain of single-domain magnetosomes embedded in the cell allows computer directional control and tracking using a magnetic resonance imaging (MRI) system. Although these embedded functionalities suggest that MTB when under the influence of an external computer could be considered as biological microrobots with the potential of targeting tumors, little is known about their level of penetration in tumoral tissues. In this paper, in vitro experiments were performed to assess the capability of these bacteria to penetrate tumor tissue for the delivery of therapeutic agents. Multicellular tumor spheroids were used since they reproduce many properties of solid tumors. The results show the ability of these MTB when submitted to a directional magnetic field to penetrate inside a 3D multicellular tumor spheroid through openings present in the tissue.

Flagellated Magnetotactic Bacteria as Controlled MRI-trackable Propulsion and Steering Systems for Medical Nanorobots Operating in the Human Microvasculature.

Although nanorobots may play critical roles for many applications in the human body such as targeting tumoral lesions for therapeutic purposes, miniaturization of the power source with an effective onboard controllable propulsion and steering system have prevented the implementation of such mobile robots. Here, we show that the flagellated nanomotors combined with the nanometer-sized magnetosomes of a single Magnetotactic Bacterium (MTB) can be used as an effective integrated propulsion and steering system for devices such as nanorobots designed for targeting locations only accessible through the smallest capillaries in humans while being visible for tracking and monitoring purposes using modern medical imaging modalities such as Magnetic Resonance Imaging (MRI). Through directional and magnetic field intensities, the displacement speeds, directions, and behaviors of swarms of these bacterial actuators can be controlled from an external computer.

MRI-based Medical Nanorobotic Platform for the Control of Magnetic Nanoparticles and Flagellated Bacteria for Target Interventions in Human Capillaries.

Medical nanorobotics exploits nanometer-scale components and phenomena with robotics to provide new medical diagnostic and interventional tools. Here, the architecture and main specifications of a novel medical interventional platform based on nanorobotics and nanomedicine, and suited to target regions inaccessible to catheterization are described. The robotic platform uses magnetic resonance imaging (MRI) for feeding back information to a controller responsible for the real-time control and navigation along pre-planned paths in the blood vessels of untethered magnetic carriers, nanorobots, and/or magnetotactic bacteria (MTB) loaded with sensory or therapeutic agents acting like a wireless robotic arm, manipulator, or other extensions necessary to perform specific remote tasks. Unlike known magnetic targeting methods, the present platform allows us to reach locations deep in the human body while enhancing targeting efficacy using real-time navigational or trajectory control. The paper describes several versions of the platform upgraded through additional software and hardware modules allowing enhanced targeting efficacy and operations in very difficult locations such as tumoral lesions only accessible through complex microvasculature networks.

A computer-assisted protocol for endovascular target interventions using a clinical MRI system for controlling untethered microdevices and future nanorobots.

The possibility of automatically navigating untethered microdevices or future nanorobots to conduct target endovascular interventions has been demonstrated by our group with the computer-controlled displacement of a magnetic sphere along a pre-planned path inside the carotid artery of a living swine. However, although the feasibility of propelling, tracking and performing real-time closed-loop control of an untethered ferromagnetic object inside a living animal model with a relatively close similarity to human anatomical conditions has been validated using a standard clinical Magnetic Resonance Imaging (MRI) system, little information has been published so far concerning the medical and technical protocol used. In fact, such a protocol developed within technological and physiological constraints was a key element in the success of the experiment. More precisely, special software modules were developed within the MRI software environment to offer an effective tool for experimenters interested in conducting such novel interventions. These additional software modules were also designed to assist an interventional radiologist in all critical real-time aspects that are executed at a speed beyond human capability, and include tracking, propulsion, event timing and closed-loop position control. These real-time tasks were necessary to avoid a loss of navigation control that could result in serious injury to the patient. Here, additional simulation and experimental results for microdevices designed to be targeted more towards the microvasculature have also been considered in the identification, validation and description of a specific sequence of events defining a new computer-assisted interventional protocol that provides the framework for future target interventions conducted in humans.

Adapting the clinical MRI software environment for real-time navigation of an endovascular untethered ferromagnetic bead for future endovascular interventions.

A dedicated software architecture for a novel interventional method allowing the navigation of ferromagnetic endovascular devices using a standard real-time clinical MRI system is shown. Through a specially developed software environment integrating a tracking method and a real-time controller algorithm, a clinical 1.5T Siemens Avanto MRI system is adapted to provide new functionality for potential automated interventional applications. The proposed software architecture was successfully validated through in vivo controlled navigation inside the carotid artery of a swine. Here we present how this MRI-upgraded software environment could also be used in more complex vasculature models through the real-time navigation of a 1.5 mm diameter chrome steel bead in two different MR-compatible phantoms with flowless and quiescent flow conditions. The developed platform and software modules needed for such navigation are also presented. Real-time tracking achieved through a dedicated positioning method based on an off-resonance excitation technique has also been successfully integrated in the software platform while maintaining adequate real-time performance. These preliminary feasibility experiments suggest that navigation of such devices can be achieved using a similar software architecture on other conventional clinical MRI systems at an operational closed-loop control frequency of 32 Hz.

Interventional procedure based on nanorobots propelled and steered by flagellated magnetotactic bacteria for direct targeting of tumors in the human body.

Flagellated bacteria used as bio-actuators may prove to be efficient propulsion mechanisms for future hybrid medical nanorobots when operating in the microvasculature. Here, we briefly describe a medical interventional procedure where flagellated bacteria and more specifically MC-1 Magnetotactic Bacteria (MTB) can be used to propel and steer micro-devices and nanorobots under computer control to reach remote locations in the human body. In particular, we show through experimental results the potential of using MTB-tagged robots to deliver therapeutic agents to tumors even the ones located in deep regions of the human body. We also show that such bacterial nanorobots can be tracked inside the human body for enhanced targeting under computer guidance using MRI as imaging modality. MTB can not only be guided and controlled directly towards a specific target, but we also show experimentally that these flagellated bacterial nanorobots can be propelled and steered in vivo deeply through the interstitial region of a tumor. The targeting efficacy is increased when combined with larger ferromagnetic micro-carriers being propelled by magnetic gradients generated by a MRI platform to carry and release nanorobots propelled by a single flagellated bacterium near the arteriocapillar entry. Based on the experimental data obtained and the experience gathered during several experiments conducted in vivo with this new approach, a general medical interventional procedure is briefly described here in a biomedical engineering context.

Medical and technical protocol for automatic navigation of a wireless device in the carotid artery of a living swine using a standard clinical MRI system.

A 1.5 mm magnetic sphere was navigated automatically inside the carotid artery of a living swine. The propulsion force, tracking and real-time capabilities of a Magnetic Resonance Imaging (MRI) system were integrated into a closed loop control platform. The sphere was released using an endovascular catheter approach. Specially developed software is responsible for the tracking, propulsion, event timing and closed loop position control in order to follow a 10 roundtrips preplanned trajectory on a distance of 5 cm inside the right carotid artery of the animal. Experimental protocol linking the technical aspects of this in vivo assay is presented. In the context of this demonstration, many challenges which provide insights about concrete issues of future nanomedical interventions and interventional platforms have been identified and addressed.

Magnetic resonance imaging of Fe3O4 nanoparticles embedded in living magnetotactic bacteria for potential use as carriers for in vivo applications.

MC-1 Magnetotactic Bacteria (MTB) are studied for their potential use as bio-carriers for drug delivery. The exploitation of the flagella combined with nanoparticles magnetite or magnetosomes chain embedded in each bacterium and used to change the swimming direction of each MTB through magnetotaxis provide both propulsion and steering in small diameters blood vessels. But for guiding these MTB towards a target, being capable to image these living bacteria in vivo using an existing medical imaging modality is essential. Here, it is shown that the magnetosomes embedded in each MTB can be used to track the displacement of these bacteria using an MRI system. In fact, these magnetosomes disturb the local magnetic field affecting T1 and T2-relaxation times during MRI. MR T1-weighted and T2-weighted images as well as T2-relaxivity of MTB are studied in order to validate the possibility of monitoring MTB drug delivery operations using a clinical MR scanner. This study proves that MTB affect much more the T2-relaxation than T1-relaxation rate and can be though as a negative contrast agent. The signal decay in the T2-weighted images is found to change proportionally to the bacterial concentration. These results show that a bacterial concentration of 2.2x10(7) cells/mL can be detected using a T2-weighted image, which is very encouraging to further investigate the application of MTB for in vivo applications.

Magnetic field mapping by selective equipotential excitation.

A new magnetic field mapping method in MRI is presented. This technique is ideal for severe inhomogeneities where plane warp cannot be ignored. The present study employs a ferromagnetic ball to create a perturbation within the imaged volume. The magnetic moment and position of the device are acquired experimentally with a new technique that excites magnetic equipotentials within a volume. A three dimensional perturbation field is then reconstructed from which an accurate field map is acquired for any slice within the volume. This method is compared with phase imaging, which is commonly used to map the magnetic field perturbation. Preliminary investigations show that this method is accurate and provides field maps that do not suffer from distortion in the slice select and read direction. This method can help in the correction of susceptibility artifacts by providing an accurate map of the perturbing field generated by magnetic markers on medical instruments.