Ultrathin conformal devices for precise and continuous thermal characterization of human skin.

Precision thermometry of the skin can, together with other measurements, provide clinically relevant information about cardiovascular health, cognitive state, malignancy and many other important aspects of human physiology. Here, we introduce an ultrathin, compliant skin-like sensor/actuator technology that can pliably laminate onto the epidermis to provide continuous, accurate thermal characterizations that are unavailable with other methods. Examples include non-invasive spatial mapping of skin temperature with millikelvin precision, and simultaneous quantitative assessment of tissue thermal conductivity. Such devices can also be implemented in ways that reveal the time-dynamic influence of blood flow and perfusion on these properties. Experimental and theoretical studies establish the underlying principles of operation, and define engineering guidelines for device design. Evaluation of subtle variations in skin temperature associated with mental activity, physical stimulation and vasoconstriction/dilation along with accurate determination of skin hydration through measurements of thermal conductivity represent some important operational examples.

Nonlinear finite element simulations to elucidate the determinants of perforator patency in propeller flaps.

The propeller-type flap design is increasingly used in reconstructive surgery for various regions of the body. To date, determinants of perforator patency when subjected to twisting have not been elucidated. We propose a simulation model to study parameters affecting perforator patency under such conditions. Nonlinear finite element procedure was used to simulate a perforator consisting of an artery and a vein with both ends fixed. A rigid body was attached to the top of the perforator for applying prescribed angular displacement. The effect of the following parameters on the pedicle patency was determined: (1) increasing angle of twist, (2) vessel stiffness, (3) vessel length, (4) diameter, (5) intraluminal pressure, and (6) the presence or absence of blood flow during twisting. Simulation results were reported in effective stress and strain on the twisted pedicle. In the context of perforator patency, effective strain, which is a measure of vessel deformation or collapse, is the more relevant outcome. The vein was more prone to occlusion because of its weaker wall and lower intraluminal pressure. Four factors that affected perforator patency were identified: angle of twist, intraluminal blood pressure, and perforator diameter and length. There was no significant difference whether twisting was performed prior to or after restoration of blood flow (P > 0.05). Therefore, to optimize condition for maintaining perforator patency, the angle of twist should be kept <180 degrees, perioperative blood pressure should be kept stable (avoiding periods of hypotension), and the selected perforator should be approximately 1 mm in diameter and >30 mm in length. We found that the propeller flap is a feasible design. This study defined the determinants of perforator patency and will serve as a useful guide when performing such flaps.