H. Kahn

Nano measurements with micro-devices: mechanical properties of hydrated collagen fibrils

The mechanical response of a biological material to applied forces reflects deformation mechanisms occurring within a hierarchical architecture extending over several distinct length scales. Characterizing and in turn predicting the behaviour of such a material requires an understanding of the mechanical properties of the substructures within the hierarchy, the interaction between the substructures, and the relative influence of each substructure on the overall behaviour. While significant progress has been made in mechanical testing of micrometre to millimetre sized biological specimens, quantitative reproducible experimental techniques for making mechanical measurements on specimens with characteristic dimensions in the smaller range of 10–1000 nm are lacking. Filling this void in experimentation is a necessary step towards the development of realistic multiscale computational models useful to predict and mitigate the risk of bone fracture, design improved synthetic replacements for bones, tendons and ligaments, and engineer bioinspired efficient and environmentally friendly structures. Here, we describe a microelectromechanical systems device for directly measuring the tensile strength, stiffness and fatigue behaviour of nanoscale fibres. We used the device to obtain the first stress–strain curve of an isolated collagen fibril producing the modulus and some fatigue properties of this soft nanofibril.

The TiNi shape-memory alloy and its applications for MEMS

The shape-memory effect is a solid state phenomenon which exploits a reversible phase transformation to repeatedly achieve an initial shape, even after some deformation of the material. Numerous metal alloys exhibit this effect. One of the most widely used shape-memory alloys is TiNi, due to its large range of recoverable deformations and its relative ease of processing. In bulk and wire form, TiNi has been applied to a number of applications, and as a thin film, TiNi is an excellent material for use as a microactuator in microelectromechanical systems (MEMS), due to its large recovery forces and high recoverable strains. Several TiNi-actuated MEMS devices have already been reported.

Stress-strain experiments on individual collagen fibrils.

Collagen, a molecule consisting of three braided protein helices, is the primary building block of many biological tissues including bone, tendon, cartilage, and skin. Staggered arrays of collagen molecules form fibrils, which arrange into higher-ordered structures such as fibers and fascicles. Because collagen plays a crucial role in determining the mechanical properties of these tissues, significant theoretical research is directed toward developing models of the stiffness, strength, and toughness of collagen molecules and fibrils. Experimental data to guide the development of these models, however, are sparse and limited to small strain response. Using a microelectromechanical systems platform to test partially hydrated collagen fibrils under uniaxial tension, we obtained quantitative, reproducible mechanical measurements of the stress-strain curve of type I collagen fibrils, with diameters ranging from 150-470 nm. The fibrils showed a small strain (epsilon < 0.09) modulus of 0.86 +/- 0.45 GPa. Fibrils tested to strains as high as 100% demonstrated strain softening (sigma(yield) = 0.22 +/- 0.14 GPa; epsilon(yield) = 0.21 +/- 0.13) and strain hardening, time-dependent recoverable residual strain, dehydration-induced embrittlement, and susceptibility to cyclic fatigue. The results suggest that the stress-strain behavior of collagen fibrils is dictated by global characteristic dimensions as well as internal structure.

Electrostatically actuated failure of microfabricated polysilicon fracture mechanics specimens

Polysilicon fracture mechanics specimens have been fabricated using standard microelectromechanical systems (MEMS) processing techniques, and thus have characteristic dimensions comparable with typical MEMS devices. These specimens are fully integrated with simultaneously fabricated electrostatic actuators which are capable of providing sufficient force to ensure catastrophic crack propagation from blunt notches produced using micromachining. Thus, the entire fracture experiment takes place on–chip, without any external loading source. Fracture has been initiated using both monotonic and cyclic resonance loading. A reduction in the nominal toughness under cyclic loading is attributed to subcritical growth of sharp cracks from the micromachined notches in the fracture mechanics specimens. Fatigue fracture has been observed in specimens subjected to as many as 109 cycles, and environmental corrosion is implicated in at least some aspects of the fatigue.

Fracture toughness of polysilicon MEMS devices

Abstract Polysilicon fracture mechanics specimens have been fabricated using standard microelectro-mechanical systems (MEMS) processing techniques, with characteristic dimensions comparable to typical MEMS devices. These specimens are fully integrated with simultaneously fabricated electrostatic actuators that are capable of providing sufficient force to ensure catastrophic crack propagation. Thus, the entire fracture experiment takes place on-chip, eliminating the difficulties associated with attaching the specimen to an external loading source. The specimens incorporate atomically sharp cracks created by indentation, and fracture is initiated using monotonic electrostatic loading. The fracture toughness values are determined using finite element analysis (FEA) of the experimental data, and show a median value of 1.1 MPa m 1/2 .

Novel method for mechanical characterization of polymeric nanofibers

A novel method to perform nanoscale mechanical characterization of highly deformable nanofibers has been developed. A microelectromechanical system (MEMS) test platform with an on-chip leaf-spring load cell that was tuned with the aid of a focused ion beam was built for fiber gripping and force measurement and it was actuated with an external piezoelectric transducer. Submicron scale tensile tests were performed in ambient conditions under an optical microscope. Engineering stresses and strains were obtained directly from images of the MEMS platform, by extracting the relative rigid body displacements of the device components by digital image correlation. The accuracy in determining displacements by this optical method was shown to be better than 50 nm. In the application of this method, the mechanical behavior of electrospun polyacrylonitrite nanofibers with diameters ranging from 300 to 600 nm was investigated. The stress-strain curves demonstrated an apparent elastic-perfectly plastic behavior with elastic modulus of 7.6+/-1.5 GPa and large irreversible strains that exceeded 220%. The large fiber stretch ratios were the result of a cascade of periodic necks that formed during cold drawing of the nanofibers.

A new technique for producing large-area as-deposited zero-stress LPCVD polysilicon films: the MultiPoly process

Polysilicon films deposited by low-pressure chemical vapor deposition (LPCVD) exhibit tensile or compressive residual stresses, depending on the deposition temperature. Polysilicon films composed of alternating tensile and compressive layers can display any overall stress value between those of the individual layers, including a state of zero overall residual stress, depending on the relative thickness of each layer. The residual stress gradient can be similarly controlled by the layer thicknesses and distribution. This has been demonstrated with a ten-layer near-zero stress (<10 MPa), near-zero stress gradient (/spl les/0.2 MPa//spl mu/m) polysilicon film, containing flat cantilever beams whose length-thickness ratios exceed 150. Using multilayer deposition to control the stresses and stress gradients of polysilicon films is termed the MultiPoly process.

The Fracture Toughness of Polysilicon Microdevices: A First Report

Polysilicon microfracture specimens were fabricated using surface micromachining techniques identical to those used to fabricate microelectromechanical systems (M devices. The nominal critical J-integral (the critical energy release rate) for crack initiation, Jc, was determined in specimens whose characteristic dimensions were o the same order of magnitude as the grain size of the polysilicon. Jc values ranged from 16 to 62 Nym, approximately a factor of four larger than Jc values reported for single crystal silicon.

A robust co-sputtering fabrication procedure for TiNi shape memory alloys for MEMS

Co-sputtering has been used to fabricate equiatomic thin films of TiNi, a shape memory alloy which form the basis of microactuators with many applications in MEMS. Methods for overcoming the difficulties involved in obtaining equiatomic TiNi thin films with high transformation temperatures, and a robust procedure suitable for batch fabrication in a production environment, are described.

Mechanical deformation and failure of electrospun polyacrylonitrile nanofibers as a function of strain rate

The mechanical deformation of 12μm long electrospun polyacrylonitrile (PAN) nanofibers with diameters of 300–600nm was investigated. The nanofibers were subjected to cold drawing in atmospheric conditions and at strain rates between 10−2 and 10−4s−1. The ultimate strain of the PAN nanofibers was 60%–130% varying monotonically with the strain rate. On the contrary, the fiber tensile strength, ranging between 30 and 130MPa, varied nonmonotonically with the slowest drawing rate resulting in the largest ductilities and fiber strengths. At the two faster rates, the large fiber ductilities originated in the formation of a cascade of ripples (necks), while at the slowest strain rate, the nanofibers deformed homogeneously allowing for the largest engineering strengths and extension ratios.