Abstract
In this work, we present a single-pole magnetic tweezers (MT) device designed for integration with substrate deformation tracking microscopy and/or traction force microscopy experiments intended to explore extracellular matrix rheology and human epidermal keratinocyte mechanobiology. Assembled from commercially available off-the-shelf electronics hardware and software, the MT device is amenable to replication in the basic biology laboratory. In contrast to conventional solenoid current-controlled MT devices, operation of this instrument is based on real-time feedback control of the magnetic flux density emanating from the blunt end of the needle core using a cascade control scheme and a digital proportional-integral-derivative (PID) controller. Algorithms that compensate for a spatially non-uniform remnant magnetization of the needle core that develops during actuation are implemented into the feedback control scheme. Through optimization of PID gain scheduling, the MT device exhibits magnetization and demagnetization response times of less than 100ms without overshoot over a wide range of magnetic flux density setpoints. Compared to current-based control, magnetic flux density-based control allows for more accurate and precise magnetic actuation forces by compensating for temperature increases within the needle core due to heat generated by the applied solenoid currents. Near field calibrations validate the ability of the MT device to actuate 4.5 μm-diameter superparamagnetic beads with forces up to 25 nN with maximum relative uncertainties of ±30% for beads positioned between 2.5 and 40 µm from the needle tip.
Highlights
As motivation for this work, we propose that a magnetic tweezers (MT) device, integrated with substrate deformation tracking microscopy (DTM) and traction force microscopy (TFM), can be used to investigate the biophysical mechanisms of immunobullous skin disease
While integration and application of MT with DTM/TFM is presented in a companion paper,20 here we have presented key findings on the design, assembly, operation, and calibration of an MT device that is amenable to replication in any biology laboratory
We have employed feedback control of the magnetic flux density emanating from the blunt end of the soft ferromagnetic needle core as an alternative to conventional devices that are based on feedforward or feedback control of solenoid current
Summary
Despite advances in our understanding of the pathophysiology of congenital and acquired human blistering skin diseases, the biophysical mechanisms by which cell–cell and cell–matrix anchoring junctions endow epidermal keratinocytes with such an innate mechanical resilience are not completely understood. during the past several decades, cell mechanics has been the subject of intense exploration among biologists, physicists, and engineers. Numerous methodologies have been developed, which can be used to apply mechanical loads and deformations to individual cells via cell–cell and cell–matrix anchoring junction proteins, including magnetic tweezers, optical tweezers, and magnetic twisting cytometry, among others. the well-established techniques of deformation tracking microscopy (DTM) and cell traction force microscopy (TFM) can be used to quantify substrate deformations and traction stresses present at cell–matrix junctions that individual cells and multicellular sheets use to attach to model biological surfaces. As the overarching goal of this work, we set out to demonstrate how a scientific apparatus that integrates magnetic tweezers (MT) and substrate deformation tracking (DTM)/traction force microscopy (TFM) can be employed to explore the mechanobiology of cell–cell and cell–matrix anchoring junctions within human epidermal keratinocytes cultured in vitro as a model for investigating immunobullous skin diseases. As the overarching goal of this work, we set out to demonstrate how a scientific apparatus that integrates magnetic tweezers (MT) and substrate deformation tracking (DTM)/traction force microscopy (TFM) can be employed to explore the mechanobiology of cell–cell and cell–matrix anchoring junctions within human epidermal keratinocytes cultured in vitro as a model for investigating immunobullous skin diseases. In contrast to conventional MT devices that achieve magnetic actuation forces through feedforward or feedback control of solenoid currents, our MT device is unique in that it is based on feedback control of the magnetic flux density emanating from the needle core. Potential advantages of this control configuration are demonstrated
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