An Atomic Force Microscope with Dual Actuation Capability for Biomolecular Experiments

We report a modular atomic force microscope (AFM) design for biomolecular experiments. The AFM head uses readily available components and incorporates deflection-based optics and a piezotube-based cantilever actuator. Jetted-polymers have been used in the mechanical assembly, which allows rapid manufacturing. In addition, a FeCo-tipped electromagnet provides high-force cantilever actuation with vertical magnetic fields up to 0.55 T. Magnetic field calibration has been performed with a micro-hall sensor, which corresponds well with results from finite element magnetostatics simulations. An integrated force resolution of 1.82 and 2.98 pN, in air and in DI water, respectively was achieved in 1 kHz bandwidth with commercially available cantilevers made of Silicon Nitride. The controller and user interface are implemented on modular hardware to ensure scalability. The AFM can be operated in different modes, such as molecular pulling or force-clamp, by actuating the cantilever with the available actuators. The electromagnetic and piezoelectric actuation capabilities have been demonstrated in unbinding experiments of the biotin-streptavidin complex

Embedded Microbubbles for Acoustic Manipulation of Single Cells and Microfluidic Applications.

Acoustically excited microstructures have demonstrated significant potential for small-scale biomedical applications by overcoming major microfluidic limitations. Recently, the application of oscillating microbubbles has demonstrated their superiority over acoustically excited solid structures due to their enhanced acoustic streaming at low input power. However, their limited temporal stability hinders their direct applicability for industrial or clinical purposes. Here, we introduce the embedded microbubble, a novel acoustofluidic design based on the combination of solid structures (poly(dimethylsiloxane)) and microbubbles (air-filled cavity) to combine the benefits of both approaches while minimizing their drawbacks. We investigate the influence of various design parameters and geometrical features through numerical simulations and experimentally evaluate their manipulation capabilities. Finally, we demonstrate the capabilities of our design for microfluidic applications by investigating its mixing performance as well as through the controlled rotational manipulation of individual HeLa cells

Nanomechanics on FGF-2 and Heparin Reveal Slip Bond Characteristics with pH Dependency

Fibroblast growth factor 2 (FGF-2), an important paracrine growth factor, binds electrostatically with low micromolar affinity to heparan sulfates present on extracellular matrix proteins. A single molecular analysis served as a basis to decipher the nanomechanical mechanism of the interaction between FGF-2 and the heparan sulfate surrogate, heparin, with a modular atomic force microscope (AFM) design combining magnetic actuators with force measurements at the low force regime (1 × 101 to 1 × 104 pN/s). Unbinding events between FGF-2–heparin complexes were specific and short-lived. Binding between FGF-2 and heparin had strong slip bond characteristics as demonstrated by a decrease of lifetime with tensile force on the complex. Unbinding forces between FGF-2 and heparin were further detailed at different pH as relevant for (patho-) physiological conditions. An acidic pH environment (5.5) modulated FGF-2–heparin binding as demonstrated by enhanced rupture forces needed to release FGF-2 from the heparin-FGF-2 complex as compared to physiological conditions. This study provides a mechanistic and hypothesis driven model on how molecular forces may impact FGF-2 release and storage during tissue remodeling and repair

Magnetometry of individual polycrystalline ferromagnetic nanowires

Ferromagnetic nanowires are finding use as untethered sensors and actuators for probing micro- and nanoscale biophysical phenomena, such as for localized sensing and application of forces and torques on biological samples, for tissue heating through magnetic hyperthermia, and for micro-rheology. Quantifying the magnetic properties of individual isolated nanowires is crucial for such applications. We use dynamic cantilever magnetometry to measure the magnetic properties of individual sub-500nm diameter polycrystalline nanowires of Ni and Ni80Co20 fabricated by template-assisted electrochemical deposition. The values are compared with bulk, ensemble measurements when the nanowires are still embedded within their growth matrix. We find that single-particle and ensemble measurements of nanowires yield significantly different results that reflect inter-nanowire interactions and chemical modifications of the sample during the release process from the growth matrix. The results highlight the importance of performing single-particle characterization for objects that will be used as individual magnetic nanoactuators or nanosensors in biomedical applications

Controlled three-dimensional rotation of single cells using acoustic waves

The ability to precisely control the three-dimensional orientation of micrometer-sized biological samples is critical for its phenotypic investigation. We develop an acoustic wave-based microfluidic device that can be used for the trapping and rotational manipulation of single plant cells. Resonant acoustic excitation of air-filled microbubbles generates localized vortices that can be used for the controlled three-dimensional rotation of single cells. We compare the rotational capabilities of microbubble-generated vortices with that of vortices generated by vibration of solid microstructures. We demonstrate the rotational capabilities of the device using single plant cells, the pollen grain.ISSN:2212-827

Measuring cytomechanical forces on growing pollen tubes

Cytomechanical measurements are important to unravel the influence of the biochemical composition of the plant cell wall on growth, morphogenesis, and stability. Agronomical research has a great interest in cell wall mechanics because in an ideal situation, crop plants grow as fast and large as possible without loosing the strength to withstand destabilizing environmental influences. Pollen tubes provide a convenient system to study major aspects of cytomechanics. They grow extremely fast but expansion is restricted to the tip region, providing a cellular model where both biochemical and mechanical properties vary spatio-temporally along the cell. The path of the pollen tube from the stigma to the ovary is full of obstacles, which the pollen tube has to overcome to reach the ovule and achieve fertilization. Once an obstruction is sensed, it can be either circumvented or penetrated, which involves mechanosensing, signal transduction, internal physiological changes, and adaptation of the mechanical properties of the pollen tube. As a result, the pollen tube changes its growth direction or increases the pushing force, both of which are controlled by a fine-tuned interplay between turgor pressure and cell wall extensibility. In this chapter, we provide an overview of state-of-the-art methods to measure those two parameters, as well as an outlook on novel technical developments that will allow the precise evaluation of the mechanical properties of the cell wall along the length of the pollen tube

Magnetic microrobots with addressable shape control

Shape shifting soft microrobots are generated from self-folding hydrogel bilayer structures. The folding conditions are analyzed to develop an optimal strategy for producing desired three-dimensional shapes. We present two different methods for programming magnetization in these microrobots that are variant and invariant to folding. The microrobots can be navigated through user-defined trajectories using rotating magnetic fields, and the morphing in response to temperature changes can be tuned for adaptive behavior. On-demand modulation of the mobility of individual microrobots is demonstrated by morphing their shape using selective near infrared light (NIR) exposure