An in-shoe laser Doppler sensor for assessing plantar blood flow in the diabetic foot

Increased pressure due, to sensory neuropathy, is important in the development of plantar ulceration in type II diabetes. However, additional factors are thought to pre-dispose the skin tissue to ulceration. Autonomic neuropathy and microangiopathy are the basis for the capillary steal theory and the haemodynamic hypothesis, developed to explain the aetiology of this type of ulcer, in terms of microvascular complications. The aim of the present study was to develop a system to allow assessment of blood flow at prevalent sites of ulceration. Previous studies have been limited to assessment of the bare foot under rest conditions. The new system allows measurements to be made in-shoe, during static and dynamic loading. The system comprises a laser Doppler sensor, a load sensor, measurement shoe, instrumentation and analysis software. The measurement shoe was designed to minimise movement artefact and provide thermal insulation for the foot. A simple flow rig was used to characterise the sensor. The blood flux response was linear (<5% deviation from ideal) for particle concentrations up to 0.25% and for mean particle velocities up to 8mm s(-1). The worst case drift in the response over a six-month period was 3.7%. Device to device repeatability varied by 12.5% over five devices.