Simon Muntwyler

Fibronectin forms the most extensible biological fibers displaying switchable force-exposed cryptic binding sites

Rather than maximizing toughness, as needed for silk and muscle titin fibers to withstand external impact, the much softer extracellular matrix fibers made from fibronectin (Fn) can be stretched by cell generated forces and display extraordinary extensibility. We show that Fn fibers can be extended more than 8-fold (>700% strain) before 50% of the fibers break. The Youngs modulus of single fibers, given by the highly nonlinear slope of the stress-strain curve, changes orders of magnitude, up to MPa. Although many other materials plastically deform before they rupture, evidence is provided that the reversible breakage of force-bearing backbone hydrogen bonds enables the large strain. When tension is released, the nano-sized Fn domains first contract in the crowded environment of fibers within seconds into random coil conformations (molten globule states), before the force-bearing hydrogen bond networks that stabilize the domains secondary structures are reestablished within minutes (double exponential). The exposure of cryptic binding sites on Fn type III modules increases steeply upon stretching. Thus fiber extension steadily up-regulates fiber rigidity and cryptic epitope exposure, both of which are known to differentially alter cell behavior. Finally, since stress-strain relationships cannot directly be measured in native extracellular matrix (ECM), the stress-strain curves were correlated with stretch-induced alterations of intramolecular fluorescence resonance energy transfer (FRET) obtained from trace amounts of Fn probes (mechanical strain sensors) that can be incorporated into native ECM. Physiological implications of the extraordinary extensibility of Fn fibers and contraction kinetics are discussed.

A Six-Axis MEMS Force–Torque Sensor With Micro-Newton and Nano-Newtonmeter Resolution

This paper describes the design of a six-axis microelectromechanical systems (MEMS) force-torque sensor. A movable body is suspended by flexures that allow deflections and rotations along the x-, y-, and z-axes. The orientation of this movable body is sensed by seven capacitors. Transverse sensing is used for all capacitors, resulting in a high sensitivity. A batch fabrication process is described as capable of fabricating these multiaxis sensors with a high yield. The force sensor is experimentally investigated, and a multiaxis calibration method is described. Measurements show that the resolution is on the order of a micro-Newton and nano-Newtonmeter. This is the first six-axis MEMS force sensor that has been successfully developed.

Toward targeted retinal drug delivery with wireless magnetic microrobots

Retinal vein occlusion is an obstruction of blood flow due to clot formation in the retinal vasculature, and is among the most common causes of vision loss. Currently, the most promising therapy involves injection of t-PA directly into small and delicate retinal vessels. This procedure requires surgical skills at the limits of human performance. In this paper, targeted retinal drug delivery with wireless magnetic microrobots is proposed. We focus on four fundamental issues involved in the development of such a system: biocompatible coating of magnetic microrobots, diffusion-based drug delivery, characterization of forces needed to puncture retinal veins, and wireless magnetic force generation. We conclude that targeted drug delivery with magnetic microrobots is feasible from an engineering perspective, and the idea should now be explored for clinical efficacy.

Three-axis micro-force sensor with sub-micro-Newton measurement uncertainty and tunable force range

The first three-axis micro-force sensor with adjustable force range from ±20 µN to ±200 µN and sub-micro-Newton measurement uncertainty is presented. The sensor design, the readout electronics, the sensor characterization and an uncertainty analysis for the force predictions are described. A novel microfabrication process based on a double silicon-on-insulator (SOI) substrate has been developed enabling a major reduction in the fabrication complexity of multi-axis sensors and actuators.

Design and calibration of a MEMS sensor for measuring the force and torque acting on a magnetic microrobot

This paper presents the design, fabrication and calibration of a multi-axis micro force–torque sensor. The sensor and its readout electronics are specifically designed to simultaneously measure two force components and one torque component. The load is measured by capacitive comb drives which provide high sensitivity. The sensor is applied to measure micro-Newton level forces and nano-Newton-meter level torques on a magnetically actuated microrobot. This microrobot is assembled from microfabricated nickel parts and is designed for directed drug delivery inside the human body. The precise knowledge of the forces and torques will help design magnetic position controllers as well as understand the magnetic properties of the electroplated microparts.

Monolithically Integrated Two-Axis Microtensile Tester for the Mechanical Characterization of Microscopic Samples

This paper describes the first monolithically integrated two-axis microtensile tester and its application to the automated stiffness measurement of single epidermal plant cells. The tensile tester consists of a two-axis electrostatic actuator with integrated capacitive position sensors and a two-axis capacitive microforce sensor. It is fabricated using a bulk silicon microfabrication process. The actuation range is +/-16 m along both axes with a position resolution of 20 nm. The force sensor is capable of measuring forces up to +/-60 N with a resolution down to 60 nN. The position-feedback sensors as well as the force sensor are calibrated by direct comparison with reference standards. A complete uncertainty analysis through the entire calibration chain based on the Monte Carlo method is presented. The functionality of the tensile tester is demonstrated by the automated stiffness measurement of the elongated cells in plant hairs (trichomes) as a function of their size. This enables a quantitative understanding and a model-based simulation of plant growth based on actual measurement data.

Real-time automated characterization of 3D morphology and mechanics of developing plant cells

In this article, we introduce the real-time cellular force microscope (RT-CFM), a high-throughput microrobotic platform for mechanical stimulation and characterization of single cells. We developed computer vision algorithms that fully automate the positioning of target cells and localization of the sensor tip. The control and acquisition architecture dramatically increases the accuracy, speed, and reliability of force measurements. Pollen tubes provide an ideal model system for the study of plant mechanics at the single-cell level. To quantitatively obtain the physical properties of the plant cell wall, we generated topography and stiffness measurements from 3D scans of living, growing pollen tubes. We report techniques for real-time monitoring and analysis of intracellular calcium fluxes during mechanical intervention. Our platform is compatible with various imaging systems and enables a powerful screening technology to facilitate biomechanical and morphological characterization of developing cells.

A multi-axis MEMS force-torque sensor for measuring the load on a microrobot actuated by magnetic fields

This paper presents the design of a multi-axis micro force-torque sensor. The sensor is able to measure forces along two axes and a torque perpendicular to these forces. The load is measured by capacitive comb drives which provide a high sensitivity. The microfabrication process, the sensor readout electronics as well as the calibration procedure are presented. The sensor was used to measure the force and torque on a magnetically actuated microrobot. This microrobot is assembled from microfabricated nickel parts for directed drug delivery inside the human body. Precise knowledge load on the microrobot is required for accurate positioning and control of the robot. The three-axis micro sensor is used to simultaneously measure the forces and torques acting on the microrobot in a magnetic field and thus provides valuable data for magnetic control methods of microrobots.

The cellular force microscope (CFM): A microrobotic system for quantitating the growth mechanics of living, growing plant cells in situ

As the field of biology becomes a more quantitative and predictive natural science, an increasing need for investigation and quantification of the mechanics of growth at individual cellular levels arises. This paper describes a microrobotic force-feedback based system and its application to the mechanical characterization of living, growing plant cells. The Cellular Force Microscope (CFM) is capable of performing the automated mechanical characterization of living plant cells in situ as these cells proliferate and grow. The microrobotic measurement system employs a single-axis capacitive MEMS microforce sensor capable of resolving forces down to 20 nN (1σ, at 10Hz). A multi-axis positioning system with 5 nm resolution position feedback is integrated into a complete system with a high-resolution optical microscope and a custom user interface for guiding an automated force-based measurement process. The CFM has been applied to characterize the mechanical properties of 20µm wide Lilium pollen tubes while they grow at a rate of about 10 µm/min in growth medium. For the mechanical characterization of pollen tubes, loads up to 400 nN are applied that cause indentations up to 300 nm. The force-deformation data acquired show an increase in the observed stiffness from the tip to the apex demonstrating that CFM is a promising tool for better understanding the changing mechanics of living plant cell growth.

Automated stiffness characterization of living tobacco BY2 cells using the Cellular Force Microscope

Understanding the process of cellular morphogenesis requires the characterization of local mechanical properties of living cells in situ. For this purpose, an automated microrobotic system, the Cellular Force Microscope (CFM) has been developed. Its suitability for single cell characterization has been reported in previous work and extensive use on characterization of both single cells and tissues has been demonstrated. A simple experimental method for the determination of the CFMs accuracy, based on the comparison of the measured stiffness with the true stiffness of a calibrated SI-traceable reference artifact, is demonstrated. This method is a practical alternative to the complex complete uncertainty analysis of all the sensors involved in the stiffness measurement based on an SI-traceable reference artifact. We then use the CFM to characterize the stiffness of living Tobacco BY2 cells. The apparent stiffness is determined from the force indentation curves collected from living cells at forces ranging from 10 microNewtons to forces in which the cell slips away or ruptures. The results show a nonlinear increase in the apparent cell stiffness as the applied load increases. This observation combined with the reference measurements done on the calibrated stiffness standard dictate that the stiffness variation stems from the biological organism and not from the CFM.