Multifield and inverse-contrast switching of magnetocaloric high contrast ratio MRI labels.

PURPOSE: Demonstrating multifield and inverse contrast switching of magnetocaloric high contrast ratio MRI labels that either have increasing or decreasing moment versus temperature slopes depending on the material at physiological temperatures and different MRI magnetic field strengths. METHODS: Two iron-rhodium samples of different purity (99% and 99.9%) and a lanthanum-iron-silicon sample were obtained from commercial vendors. Temperature and magnetic field-dependent magnetic moment measurements of the samples were performed on a vibrating sample magnetometer. Temperature-dependent MRI of different iron-rhodium and lanthanum-iron-silicon samples were performed on 3 different MRI scanners at 1 Tesla (T), 4.7T, and 7T. RESULTS: Sharp, first-order magnetic phase transition of each iron-rhodium sample at a physiologically relevant temperature (~37°C) but at different MRI magnetic fields (1T, 4.7T, and 7T, depending on the sample) showed clear image contrast changes in temperature-dependent MRI. Iron-rhodium and lanthanum-iron-silicon samples with sharp, first-order magnetic phase transitions at the same MRI field of 1T and physiological temperature of 37°C, but with positive and negative slope of magnetization versus temperature, respectively, showed clear inverse contrast image changes. Temperature-dependent MRI on individual microparticle samples of lanthanum-iron-silicon also showed sharp image contrast changes. CONCLUSION: Magnetocaloric materials of different purity and composition were demonstrated to act as diverse high contrast ratio switchable MRI contrast agents. Thus, we show that a range of magnetocaloric materials can be optimized for unique image contrast response under MRI-appropriate conditions at physiological temperatures and be controllably switched in situ.

Self-Assembled Origami Neural Probes for Scalable, Multifunctional, Three-Dimensional Neural Interface.

Flexible intracortical neural probes have drawn attention for their enhanced longevity in high-resolution neural recordings due to reduced tissue reaction. However, the conventional monolithic fabrication approach has met significant challenges in: (i) scaling the number of recording sites for electrophysiology; (ii) integrating of other physiological sensing and modulation; and (iii) configuring into three-dimensional (3D) shapes for multi-sided electrode arrays. We report an innovative self-assembly technology that allows for implementing flexible origami neural probes as an effective alternative to overcome these challenges. By using magnetic-field-assisted hybrid self-assembly, multiple probes with various modalities can be stacked on top of each other with precise alignment. Using this approach, we demonstrated a multifunctional device with scalable high-density recording sites, dopamine sensors and a temperature sensor integrated on a single flexible probe. Simultaneous large-scale, high-spatial-resolution electrophysiology was demonstrated along with local temperature sensing and dopamine concentration monitoring. A high-density 3D origami probe was assembled by wrapping planar probes around a thin fiber in a diameter of 80∼105 µm using optimal foldable design and capillary force. Directional optogenetic modulation could be achieved with illumination from the neuron-sized micro-LEDs (µLEDs) integrated on the surface of 3D origami probes. We could identify angular heterogeneous single-unit signals and neural connectivity 360° surrounding the probe. The probe longevity was validated by chronic recordings of 64-channel stacked probes in behaving mice for up to 140 days. With the modular, customizable assembly technologies presented, we demonstrated a novel and highly flexible solution to accommodate multifunctional integration, channel scaling, and 3D array configuration.

Autonomous Microrobotic Manipulation Using Visual Servo Control.

This describes the application of a visual servo control method to the microrobotic manipulation of polymer beads on a two-dimensional fluid interface. A microrobot, actuated through magnetic fields, is utilized to manipulate a non-magnetic polymer bead into a desired position. The controller utilizes multiple modes of robot actuation to address the different stages of the task. A filtering strategy employed in separation mode allows the robot to spiral from the manipuland in a fashion that promotes the manipulation positioning objective. Experiments demonstrate that our multiphase controller can be used to direct a microrobot to position a manipuland to within an average positional error of approximately 8 pixels (64 µm) over numerous trials.

Physical justification for negative remanent magnetization in homogeneous nanoparticles.

The phenomenon of negative remanent magnetization (NRM) has been observed experimentally in a number of heterogeneous magnetic systems and has been considered anomalous. The existence of NRM in homogenous magnetic materials is still in debate, mainly due to the lack of compelling support from experimental data and a convincing theoretical explanation for its thermodynamic validation. Here we resolve the long-existing controversy by presenting experimental evidence and physical justification that NRM is real in a prototype homogeneous ferromagnetic nanoparticle, an europium sulfide nanoparticle. We provide novel insights into major and minor hysteresis behavior that illuminate the true nature of the observed inverted hysteresis and validate its thermodynamic permissibility and, for the first time, present counterintuitive magnetic aftereffect behavior that is consistent with the mechanism of magnetization reversal, possessing unique capability to identify NRM. The origin and conditions of NRM are explained quantitatively via a wasp-waist model, in combination of energy calculations.