Nickel foil microcantilevers for magnetic manipulation and localized heating.

Cellular manipulation has been investigated by a number of techniques. In this manuscript nickel foil microcantilevers were used for magnetophoresis and manipulation of microparticles and magnetically labeled HeLa cells. The cantilevers were also used for localized heating in liquid, reaching biologically relevant temperatures. This work aims to develop cantilevers for sample enrichment, manipulation, and thermal applications, offering an inexpensive and versatile solution compatible with standard tools in research and clinical diagnostic testing, such as microwell plates.

Self-assembled magnetic bead biosensor for measuring bacterial growth and antimicrobial susceptibility testing.

Bacterial antibiotic resistance is one of the major concerns of modern healthcare worldwide, and the development of rapid, growth-based, antimicrobial susceptibility tests is key for addressing it. The cover image shows a self-assembled asynchronous magnetic bead rotation (AMBR) biosensor developed for rapid detection of bacterial growth. Using the biosensors, the minimum inhibitory concentration of a clinical E. coli isolate can be measured within two hours, where currently tests take 6-24 hours. A 16-well prototype is also constructed for simple and robust observation of the self-assembled AMBR biosensors.

Asynchronous magnetic bead rotation microviscometer for rapid, sensitive, and label-free studies of bacterial growth and drug sensitivity.

The long turnaround time in antimicrobial susceptibility testing (AST) endangers patients and encourages the administration of wide spectrum antibiotics, thus resulting in alarming increases of multidrug resistant pathogens. A method for faster detection of bacterial proliferation presents one avenue toward addressing this global concern. We report on a label-free asynchronous magnetic bead rotation (AMBR) based viscometry method that rapidly detects bacterial growth and determines drug sensitivity by measuring changes in the suspension's viscosity. With this platform, we observed the growth of a uropathogenic Escherichia coli isolate, with an initial concentration of 50 cells per drop, within 20 min; in addition, we determined the gentamicin minimum inhibitory concentration (MIC) of the E. coli isolate within 100 min. We thus demonstrated a label-free, microviscometer platform that can measure bacterial growth and drug susceptibility more rapidly, with lower initial bacterial counts than existing commercial systems, and potentially with any microbial strains.

Nanoparticle induced cell magneto-rotation: monitoring morphology, stress and drug sensitivity of a suspended single cancer cell.

Single cell analysis has allowed critical discoveries in drug testing, immunobiology and stem cell research. In addition, a change from two to three dimensional growth conditions radically affects cell behavior. This already resulted in new observations on gene expression and communication networks and in better predictions of cell responses to their environment. However, it is still difficult to study the size and shape of single cells that are freely suspended, where morphological changes are highly significant. Described here is a new method for quantitative real time monitoring of cell size and morphology, on single live suspended cancer cells, unconfined in three dimensions. The precision is comparable to that of the best optical microscopes, but, in contrast, there is no need for confining the cell to the imaging plane. The here first introduced cell magnetorotation (CM) method is made possible by nanoparticle induced cell magnetization. By using a rotating magnetic field, the magnetically labeled cell is actively rotated, and the rotational period is measured in real-time. A change in morphology induces a change in the rotational period of the suspended cell (e.g. when the cell gets bigger it rotates slower). The ability to monitor, in real time, cell swelling or death, at the single cell level, is demonstrated. This method could thus be used for multiplexed real time single cell morphology analysis, with implications for drug testing, drug discovery, genomics and three-dimensional culturing.

Asynchronous magnetic bead rotation (AMBR) biosensor in microfluidic droplets for rapid bacterial growth and susceptibility measurements.

Inappropriate antibiotic use is a major factor contributing to the emergence and spread of antimicrobial resistance. The long turnaround time (over 24 hours) required for clinical antimicrobial susceptibility testing (AST) often results in patients being prescribed empiric therapies, which may be inadequate, inappropriate, or overly broad-spectrum. A reduction in the AST time may enable more appropriate therapies to be prescribed earlier. Here we report on a new diagnostic asynchronous magnetic bead rotation (AMBR) biosensor droplet microfluidic platform that enables single cell and small cell population growth measurements for applications aimed at rapid AST. We demonstrate the ability to rapidly measure bacterial growth, susceptibility, and the minimum inhibitory concentration (MIC) of a small uropathogenic Escherichia coli population that was confined in microfluidic droplets and exposed to concentrations above and below the MIC of gentamicin. Growth was observed below the MIC, and no growth was observed above the MIC. A 52% change in the sensor signal (i.e. rotational period) was observed within 15 minutes, thus allowing AST measurements to be performed potentially within minutes.

Experimental system for one-dimensional rotational brownian motion.

We present here an experimental, strictly one-dimensional rotational system, made by using single magnetic Janus particles in a static magnetic field. These particles were half-coated with a thin metallic film, and by turning on a properly oriented external static magnetic field, we monitor the rotational brownian motion of single particles, in solution, around the desired axis. Bright-field microscopy imaging provides information on the particle orientation as a function of time. Rotational diffusion coefficients are derived for one-dimensional rotational diffusion, both for a single rotating particle and for a cluster of four such particles. Over the studied time domain, up to 10 s, the variation of the angle of rotation is strictly brownian; its probability distribution function is gaussian, and the mean squared angular displacement is linear in time, as expected for free diffusion. Values for the rotational diffusion coefficients were also determined. Monte Carlo and hydrodynamic simulations agree well with the experimental results.

Monitoring the growth and drug susceptibility of individual bacteria using asynchronous magnetic bead rotation sensors.

Continuous growth of individual bacteria has been previously studied by direct observation using optical imaging. However, optical microscopy studies are inherently diffraction limited and limited in the number of individual cells that can be continuously monitored. Here we report on the use of the asynchronous magnetic bead rotation (AMBR) sensor, which is not diffraction limited. The AMBR sensor allows for the measurement of nanoscale growth dynamics of individual bacterial cells, over multiple generations. This torque-based magnetic bead sensor monitors variations in drag caused by the attachment and growth of a single bacterial cell. In this manner, we observed the growth and division of individual Escherichia coli, with 80-nm sensitivity to the cell length. Over the life cycle of a cell, we observed up to a 300% increase in the rotational period of the biosensor due to increased cell volume. In addition, we observed single bacterial cell growth response to antibiotics. This work demonstrates the non-microscopy limited AMBR biosensor for monitoring individual cell growth dynamics, including cell elongation, generation time, lag time, and division, as well as their sensitivity to antibiotics.

High frequency asynchronous magnetic bead rotation for improved biosensors.

Biosensors with increasingly high sensitivity are crucial for probing small scale properties. The asynchronous magnetic bead rotation (AMBR) sensor is an emerging sensor platform, based on magnetically actuated rotation. Here the frequency dependence of the AMBR sensor's sensitivity is investigated. An asynchronous rotation frequency of 145 Hz is achieved. This increased frequency will allow for a calculated detection limit of as little as a 59 nm change in bead diameter, which is a dramatic improvement over previous AMBR sensors and further enables physical and biomedical applications.

Sudden breakdown in linear response of a rotationally driven magnetic microparticle and application to physical and chemical microsensing.

In this work, sensing magnetic microparticles were used to probe both the local pH and the viscosity-dependent nonlinear rotational behavior of the particles. The latter resulted from a critical transition marking a driven particle's crossover from phase-locking to phase-slipping with an externally rotating magnetic field, i.e., a sudden breakdown in its linear response that can be used to measure a variety of physical quantities. The transition from simple rotation to wobbling is described both theoretically and experimentally. The ability to measure both chemical and physical properties of a system could enable simultaneous monitoring of chemical and physical interactions in biological or other complex fluid microsystems.