Extravehicular activity (EVA) suits are designed to protect astronauts exposed to the low temperatures, low pressures, and lack of oxygen in space. Astronauts engaged in extravehicular activities have reported discomfort in their fingertips. The precise cause of this discomfort is not known, but is speculated to be reduced blood flow to the fingertips. Therefore, there is a critical need for monitoring blood circulation in astronaut extremities during missions. The data collected could be used to develop better countermeasures to help the astronauts avoid further discomfort and perhaps injury to their fingers during EVA. One way to quantify blood circulation is to measure the hemodynamics of the capillaries in the fingertips. Laser Doppler flowmetry (LDF) technology has significant potential for this purpose.
LDF is a noninvasive technique that allows the flux of moving red blood cells (RBCs) to be measured in the microvasculature of a tissue (e.g., in a fingertip). The technique is based on the Doppler effect, which describes the frequency shift that a wave undergoes when emitted from an object that is moving away from or toward an observer. Laser light scattered by moving particles, such as RBCs, is shifted in frequency by amounts proportional to the speeds of the particles. This scattered light is optically mixed with a reference signal and is detected by a photodetector. The detected signal is then analyzed to obtain the LDF parameters (speed, volume, and flow).
Prototype LDF sensor.
The goal of this work is to develop a miniaturized LDF system for portable use during EVAs, with a fiber-optic probe that could be outfitted in an astronaut’s glove. As a first step, a prototype LDF system (see the photograph) for the measurement of blood flow in the capillaries of the distal fingertip was developed. A laser source with a wavelength of 780 nm and an output power of 5 mW is used to transmit the light through an optical fiber to the fingertip probe. Scattered light is collected into a second fiber that transmits the light to the detector. The detected light signal is acquired with an NI-DAQ card (National Instruments Corporation data-acquisition card), and the LDF calculations are made with a LabVIEW (National Instruments Corporation’s Laboratory Virtual Instrumentation Engineering Workbench) program.
Ground-based experiment results.
Ground-based experiments were performed on volunteer subjects by measuring the blood flow while increasing the pressure in the arm using a pressure cuff or by cooling the fingertip.1 The results showed good linearity in all measured parameters. A typical data set is shown in the preceding graphs. When the pressure in the arm was increased from 0- to 80-mm mercury, the LDF volume increased by 11.6 percent, the LDF speed decreased by 50.4 percent, and the LDF flow decreased by 44.3 percent. When the fingertip was cooled from 43 to 20 °C, the LDF volume increased by 9 percent, the LDF speed decreased by 68.1 percent, and the LDF flow decreased by 65.7 percent. Further work is being done to integrate the probe into an astronaut’s glove and to miniaturize the LDF instrument to the size of a wrist watch, as shown in the following photographs.
Future design of miniaturized LDF system.
Find out more about Glenn’s microgravity combustion science research: http://microgravity.grc.nasa.gov/combustion/
Glenn contact: Rafat R. Ansari, 216-433-5008, Rafat.R.Ansari@nasa.govLast updated: December 14, 2007
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