Hee C. Lim

Measurement of skin stretch via light reflection

A noninvasive technique for measuring the stretch of skin is described. The technique utilizes changes in the reflectivity of polarized light intensity as a monitor of skin stretch. Measurements of in vitro pigskin and in vivo human skin show that the reflectivity of polarized light intensity increases linearly with stretch. The changes in diffusive reflectivity properties of skin result from the alterations that take place in the roughness across the thickness of the skin layers due to stretch. Conceptually, as the roughness of a layer decreases with stretch, a smoother reflecting media is produced, resulting in a proportional increase in the specular reflection. Results can be easily extended to a real-time stretch analysis of large tissue areas that would be applicable for mapping the stretch of skin.

Noninvasive light-reflection technique for measuring soft-tissue stretch

A novel, to our knowledge, sensor for measuring the stretch in soft tissues such as skin is described. The technique, which is a modification of two-dimensional polarization imaging, uses changes in the reflectivity of polarized light as a monitor of skin stretch. Measurements show that the reflectivity increases with stretch. Measurements were made on guinea pig skin and on nonbiological materials. The changes in reflectivity result from the changes that take place in the interface roughness between skin or material layers and the consequential changes in the diffuse reflective characteristics of the skin. Conceptually, as the roughness of an interface decreases, a smoother reflecting interface is produced, resulting in a commensurate increase in specular reflection. A simple roughness model correctly predicts the main experimental results. Results can be extended easily to real-time stretch analysis of large tissue areas that would be applicable for predicting stresses in skin during and after the surgical closure of wounds.

Terahertz response of microfluidic-jetted three-dimensional flexible metamaterials

We demonstrate the fabrication and characterization of three-dimensional (3D) metamaterials in the terahertz (THz) range using the microfluidic-jetted technique. This technique has proven a convenient technique to fabricate metamaterial structures at the micrometer scale. The metamaterials are fabricated using dodecanethiol functionalized gold nanoparticles on flexible polyimide substrates. The metamaterials consist of alternate layers of single split-ring resonator and microstrip arrays that are stacked to form a 3D metamaterial medium. The fabricated metamaterials, with lattice sizes of 180 microm, are characterized using THz time-domain spectroscopy within 0.1 to 2 THz in the transmission mode. Numerical simulation is performed to calculate the effective metamaterials parameter.

Terahertz response of microfluidic-jetted fabricated 3D multilayer metamaterials

The metamaterials (MTMs) are defined as artificial materials which will achieved extraordinary electromagnetic properties that are not available in natural material. Due to their embryonic nature, MTMs evolve in rapid speed in this few years. The science of MTMs, particularly left-handed metamaterials (LH MTMs), has been well established theoretically, numerically, and experimentally in the literature.

Microfabricated implantable pressure sensor for flow measurement

A RF wireless capacitive pressure sensor is developed. The sensor has integrated inductor with the pressure sensitive capacitor as LC circuit. The resonant frequency of the sensor changes as the capacitance changes with applied pressure. The sensor uses LPCVD silicon nitride as sensitive membrane and the residual stress of the membrane has been measure as 139MPa. The sensor has size of 10 mm × 4 mm × 0.5 um. The sensor presents a pressure sensitivity of 1.65 MHz/cmH2O in pressure range of 0-20 cmH2O. The deflection of different shape of membranes is discussed. The deflection of square membrane is 130% to circular membrane under same applied pressure.

Microfabricated implantable flow sensor for medical applications

A RF wireless capacitive flow sensor is developed. The sensor has integrated inductor with the flow sensitive capacitors as LC circuit. The resonant frequency of the sensor changes as the capacitance changes with applied flow. The sensor uses LPCVD silicon nitride as sensitive membrane and the residual stress of the membrane has been measure as 139 MPa. The sensor has size of 10 mm × 4 mm × 0.5 μm. The sensor integrated two pressure sensors together and designed related to flow 5-20ml/hour. The deflection of different shape of membranes and the parameters of flow sensor sensitivity are discussed. The deflection of square membrane is 130% to circular membrane under same applied pressure.

Cyclic thin film flexible pressure sensor testing

The meantime between failures of the thin film strain sensor is a critical indicator for future U.S. Army field sensing application [1]. This accelerated lifetime aging test would characterize the existing flexible strain sensor for repeated load response/application. A typical industrial maximum number of testing cycles used are about 10x106 cycles [2].

Resistance to cracking of a stretchable semiconductor: Speed of crack propagation for varying energy release rate

We have measured and calculated the propagation velocity of successive cracks in a single sample of amorphous SiNx as a function of energy release rate. We have obtained the conditions for controlled, repetitive crack formation by using a Substrate of compliant plastic that survives the cracking of a thin film formed on it. We have recorded the crack velocity curves using high-speed micro-photography using dark field illumination. Under uniform, uniaxial tensile strain, the films crack in an array of essentially straight, parallel lines, if the increase of the strain density is slow. We find reasonable agreement in the comparison of theory and experiment and find a linear relationship between the initial velocity and energy release rate threshold. Consequently, in cases where the theoretical agreement with the data is reasonable, the successive cracks show velocity curves that scale with each other.