Biomedical subjects

Stephen M Hsu

Publications and source records attributed to Stephen M Hsu.

3 recordsLinked to original sources

PubMed2006

Measuring the size dependence of Young's modulus using force modulation atomic force microscopy.

The dependence of the local Young's modulus of organic thin films on the size of the domains at the nanometer scale is systematically investigated. Using atomic force microscopy (AFM) based imaging and lithography, nanostructures with designed size, shape, and functionality are preengineered, e.g., nanostructures of octadecanethiols inlaid in decanethiol self-assembled monolayers (SAMs). These nanostructures are characterized using AFM, followed by force modulation spectroscopy and microscopy measurements. Young's modulus is then extracted from these measurements using a continuum mechanics model. The apparent Young's modulus is found to decrease nonlinearly with the decreasing size of these nanostructures. This systematic study presents conclusive evidence of the size dependence of elasticity in the nanoregime. The approach utilized may be applied to study the size-dependent behavior of various materials and other mechanical properties.

Journal Article↗

PubMed2004

Molecular orientation of ultrahigh molecular weight polyethylene induced by various sliding motions.

Wear and wear debris of ultrahigh molecular weight polyethylene (UHMWPE) in joint replacements have been recognized as one of the major contributors to the failure of orthopedic implants. The detailed wear mechanism of polyethylene under biomechanic motions is not well understood. In simulation wear bench tests, it was found that unidirectional sliding produces the least amount of wear, reciprocating motion increases wear significantly, and cross-shear motion (similar to hip and knee joint motion in the human body) produces the highest amount of wear. Conventional wear theories are inadequate to explain this observation. This study utilizes resonant absorption of linearly polarized soft X-rays at a synchrotron radiation beam line to measure the molecular orientation of a UHMWPE surface layer subjected to different wear motions. Carbon-K-edge partial-electron-yield X-ray absorption measurements were done on the worn UHMWPE samples. X-ray absorption measurements show conclusively that the molecular chains of UHMWPE align preferentially parallel to the direction of sliding. Examination under various wear motions showed that unidirectional shear produced the maximum chain orientation, whereas cross-shear wear motions produced the least amount of orientation. When polymeric chains align, the surface layer tends to be more brittle and hard, thus resisting wear. When they do not align, loose chains may be subjected to both Mode I and Mode II fracture, hence increasing the wear rate. This molecular alignment observation may offer an explanation of why different wear motions have different wear characteristics.

Humans↗

PubMed2003

Generation of narrowly distributed ultra-high-molecular-weight polyethylene particles by surface texturing techniques.

Ultra-high-molecular-weight polyethylene (UHMWPE) wear particles have been recognized as the cause of aseptic loosening in total joint replacement. Macrophage phagocytosis of wear particles induces human biological/physiological responses which eventually lead to bone resorption and osteolysis. However, the dependence of these reactions on the size and shape of the particles has not been elucidated and is not understood. This article describes a procedure to generate narrowly distributed UHMWPE particles of controlled size and shape through surface texturing by microfabrication. The textured surface is then used to rub against the polymer pins to produce wear particles in water. The surface texture produces elongated particles or equiaxed particles by design. The distribution of the particles, due to the surface-texture control, is quite narrow as compared with randomly produced surfaces. With this technique, we are able to generate UHMWPE wear particles of different size and shape within phagocytosable and nonphagocytosable populations for biological response studies.

Biocompatible Materials↗