DYNAMIC WATERBED WITH PRESSURE MODULATION BY TURBULENT JETS

This work is part of a multidisciplinary project designated "Sense4me - Monitoring and control system of discomfort, for disabled persons and surgical patients". When an individual is confined to prolonged recumbent or sitting positions, a process of ulceration is likely to occur at sites with prominent bony structures, called pressure-sore or pressure-ulcer. Pressure-sores are an increasing problem within our society, with high prevalence amongst hospitalized patients or physically handicapped persons, often leading to premature deaths [1]. In the USA the overall annual cost of dealing with this problem is estimated to be between 5 to 9 billion dollars [2]. The main objective of the study is to analyze the viability of a novel active mattress system, based on turbulent water-jets impinging onto the mattress-cover, which will allow the control of pressure and temperature on the contact-surface with the human skin, thus decreasing the risk of pressure-ulcer formation. A small fluid-mattress prototype was designed and is being constructed. It consists on a chamber with a rectangular cover-area of 500 by 500 mm and a deepness of 100 mm. The bottom surface is a metal plate with equally-spaced orifices forming a matrix. The fluid jets, emerging from these orifices, impinge onto the pliable membrane-cover. The fluid is then collected from the sides and directed to a A leveling tank and to a water-pump. This pump discharges the fluid into a plenum chamber, located behind the main plate with the orifices, thus completing the hydraulic circuit. The relevant physical and geometric parameters for the prototype definition (orifice diameter and matrix spacing, distance plate/membrane, plenum pressures, minimum body sustainable pressure, total jet momentum, etc) are described and studied. This included the realization of static pressure measurements with an inflatable mattress supporting a human body in different positions, as well as essays with a high velocity air-jet impinging on a surface. Preliminary calculations were made for the prototype running with a selected pump. The results indicate that the additional pressure exerted on the membrane will lead to a force in excess of 150 N, for a flow-rate between 20 and 40 m(3)/h, and a plenum pressure from 1 to 4 bar, depending on orifice diameter. The work currently follows with tests involving weights located on the membrane surface, to verify the calculations and the soundness of the jet impingement principle. Further work will entail the division of the plenum in 16 sub-chambers with inlet flow valves, thus allowing the local modulation of membrane pressure by variation of the jets exit velocities, and temperature through sub-chamber individual control of water supply temperature.