Quantification of strain induced damage in medial collateral ligaments.

In the past years, there have been several experimental studies that aimed at quantifying the material properties of articular ligaments such as tangent modulus, tensile strength, and ultimate strain. Little has been done to describe their response to mechanical stimuli that lead to damage. The purpose of this experimental study was to characterize strain-induced damage in medial collateral ligaments (MCLs). Displacement-controlled tensile tests were performed on 30 MCLs harvested from Sprague Dawley rats. Each ligament was monotonically pulled to several increasing levels of displacement until complete failure occurred. The stress-strain data collected from the mechanical tests were analyzed to determine the onset of damage and its evolution. Unrecoverable changes such as increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region of the stress-strain curves indicated the occurrence of damage. Interestingly, these changes were found to appear at two significantly different threshold strains (P<0.05). The mean threshold strain that determined the increase in ligament's elongation at preload was found to be 2.84% (standard deviation (SD) = 1.29%) and the mean threshold strain that caused the decrease in the tangent modulus of the linear region was computed to be 5.51% (SD = 2.10%), respectively. The findings of this study suggest that the damage mechanisms associated with the increase in ligament's elongation at preload and decrease in the tangent modulus of the linear region in the stress-strain curves in MCLs are likely different.

Probabilistic constitutive law for damage in ligaments.

A new constitutive equation is presented to describe the damage evolution process in parallel-fibered collagenous tissues such as ligaments. The model is formulated by accounting for the fibrous structure of the tissues. The tissue's stress is defined as the average of the collagen fiber's stresses. The fibers are assumed to be undulated and straightened out at different stretches that are randomly defined according to a Weibull distribution. After becoming straight, each collagen fiber is assumed to be linear elastic. Damage is defined as a reduction in collagen fiber's stiffness and occurs at different stretches that are also randomly defined by a Weibull distribution. Due to the lack of experimental data, the predictions of the constitutive equation are analyzed by varying the values of its structural parameters. Moreover, the results are compared with the available stress-strain data in the biomechanics literature that evaluate damage produced by subfailure stretches in rat medial collateral ligaments.

Uncooled infrared imaging device based on optimized optomechanical micro-cantilever array.

It is a major issue to improve the thermo-mechanical sensitivity of uncooled optomechanical focal plane arrays (FPAs) for infrared imaging. This work presents an optimized multi-fold interval metallized leg (IML) configuration to increase the thermo-mechanical sensitivity of an uncooled optomechanical bi-material micro-cantilever array. The inclination angle changes of the cantilever elements are measured in the IR imaging system using an optical readout with a knife-edge filtering operation in the spectrum plane. The multi-fold IML configuration consists of alternately connected unmetallized and metallized legs. With the optimized fold number, the thermo-mechanical sensitivity of a micro-cantilever array can be amplified to two times of one-fold IML for a 120 microm x 120 microm element with 1 microm thick SiNx/0.2 microm thick Au films. Room temperature objects are imaged with the fabricated FPA containing 160 x 160 elements and a 12-bit CCD. Further modeling analysis shows that the experimental results are well accordant with the theoretical calculation. An important practical feature of the implemented approach is its straightforward fabrication for a large FPA, without growing complexity and cost.

Optical readout method for microcantilever array sensing and its sensitivity analysis.

An optical readout platform using a knife-edge filter for detecting the bending of a bimaterial microcantilever array is established, and the influence of stress-induced micromirror deformation on the optical detection sensitivity is discussed. The influence of deformation is modeled theoretically and validated experimentally. Analysis shows that the optical detection sensitivity will decrease by 50% when the mirror has a deformation of lambda/5 (lambda is the wavelength of readout light). Finally, an infrared image is obtained by the platform.

Uncooled IR imaging using optomechanical detectors.

In this study, we present an uncooled infrared imaging detector using knife-edge filter optical readout method. The tilt angle change of each cantilever in a focal plane array (FPA) can be simultaneously detected with a resolution of 10(-5) degrees. A deformation magnifying substrate-free microcantilever unit is specially designed. The multi-fold legs of microcantilever are interval metal coated to form a thermal deformation magnifying structure. Thermal and thermomechanical performance of this microcantilever unit are modeled and analyzed. An FPA with 100 x 100 pixels is fabricated and thermal images of human body are obtained by this detector.